Trickling Filters, Rotating Biological Contactors

Transcription

Chapter 21
Trickling Filters, Rotating
Biological Contactors,
and Combined Processes
Introduction
21-2
Distribution Systems
21-13
Concepts
21-4
Filter Media
21-16
Load Variations
21-4
Underdrain System
21-16
pH and Alkalinity
21-5
Containment Structure
21-18
Toxicity
21-5
Filter Pump Station
21-18
Nutrients
21-5
Secondary Clarifier
21-18
Temperature
21-5
Process Control
21-19
Dissolved Oxygen
21-6
Flow Patterns
21-19
Microorganisms
21-7
Distribution Rates
21-19
21-7
Clarifier Operation
21-21
Trickling Filters and Biotowers
Alternatives
21-8
Troubleshooting
21-22
Low-Rate Filters/Biotowers
21-10
Planned Maintenance
21-28
Intermediate-Rate Filters
21-11
Distributor Bearings
21-28
High-Rate Filters
21-12
Safety
21-28
Roughing Filters
21-11
Rotating Biological Contactors
21-31
Description of Process
21-12
Alternatives
21-32
Description of Equipment
21-13
Description of Process
21-35
21-1
Copyright © 2007 Water Environment Federation.
21-2
Operation of Municipal Wastewater Treatment Plants
21-38
Alternatives
21-52
Tankage
21-38
Activated Biofilter
21-53
Baffles
21-38
Trickling Filter Solids Contact
21-53
Filter Media
21-39
21-57
Covers
21-40
Roughing Filter Activated
Sludge
Rotating Biological Contactor
Drives
21-40
Biofilter Activated Sludge
21-58
21-58
Influent and Effluent Lines
and Valves
21-43
Trickling Filter Activated
Sludge
Instrumentation
21-43
Process Control
Description of Processes
21-58
21-59
21-43
Trickling Filter or Biotower
21-59
Other Processes
21-44
Filter Pump Station
21-59
Staging and Trains
21-45
21-59
Supplemental Aeration
21-45
Contact Channel or Aeration
Basin
Step Feeding or Enlarged
First Stage
21-46
Aeration Equipment
21-60
Clarification
21-61
Recirculation
21-46
Process Control
21-61
Rotational Speed
21-47
Process Changes
21-61
Secondary Clarifier
21-47
Biotower
21-61
Nitrification
21-47
21-61
Troubleshooting
21-48
Contact Channel or Aeration
Basin
Planned Maintenance
21-48
Clarification
21-62
Mechanical Drive Systems
21-50
Troubleshooting
21-62
Air-Drive Systems
21-50
Planned Maintenance
21-62
Combined Processes
21-52
References
21-64
INTRODUCTION
Tricking filters, biotowers, and rotating biological contactors (RBCs) are generally
known as fixed-film treatment processes. Of these three processes, the trickling filter
process predates biotowers, RBCs, and combined fixed-film and suspended growth
(FF/SG) processes.
Trickling Filters, Rotating Biological Contactors, and Combined Processes
In fact, trickling filters predate most of the treatment methods considered in
other chapters of this manual, and they are still a viable process. New types of filter
media are now used; therefore, rock media systems are labeled trickling filters, and
plastic media systems are labeled biotowers. Trickling filters are being incorporated
into wastewater facilities using new methods or processes, and many rock filters are
being refurbished for continued use. This chapter helps both operators and engineers
grasp the operations and maintenance requirements of trickling filters as used in existing and new systems.
Although their performance has been good, RBC equipment operation has been
plagued by failures. This chapter discusses many of the changes that occurred in both
design and operation. It includes discussions of predicting operating problems or plant
overload and emphasizes methods of upgrading or improving RBC operations.
Combined processes [i.e., trickling filters, biotowers, or RBCs (fixed-film) coupled
with suspended-growth (activated sludge) processes] now number several hundred in
the United States. Combined FF/SG processes are designed to take advantage of the
strengths and minimize the weaknesses of each process. In many cases, the practice is
used to reduce construction costs by avoiding the need for additional tankage. In some
industrial or high-strength-waste applications, the FF/SG processes have helped eliminate shock loads to the activated sludge process. The coupling of biological processes
has solved many problems, but also produced new control criteria or concerns. This
chapter addresses operations and maintenance concerns associated with the coupling
or combining of biological processes.
Fixed-film biological processes remove dissolved organics and finely divided organic solids from wastewater. Removal occurs primarily by converting soluble and
colloidal material into a biological film that develops on the filter media. Raw domestic and industrial wastewater typically contains settleable solids, floatable materials, and other debris. Failure to remove these solids before the wastewater enters
the fixed-film reactors can interfere with their oxygen-transfer capabilities, plug the
filter media, result in high solids yield, or create other problems. Therefore, both
fixed-film and combined-growth processes are typically preceded by screenings and
the grit removal process. Primary treatment processes should be used to reduce the
fixed-film process load.
The fixed-film or biological media removes soluble biochemical oxygen demand
(BOD) and produces biological solids. Most often, the process removes carbonaceous
BOD; however, sometimes the fixed-film systems can be loaded so slow-growing,
nitrogen-converting autotrophic bacteria (nitrifiers) can compete with the more rapidgrowing heterotrophic bacteria used for carbonaceous BOD removal; therefore, the
fixed-film process can be used to nitrify the wastewater.
21-3
21-4
In combined FF/SG processes, the second-stage process may remove varying
amounts of soluble BOD (SBOD), depending largely on the loading and performance
of the first-stage process. Combined processes are given different names, depending on
which stage removes the most SBOD and at which point biological solids are introduced to the treatment scheme.
To ensure that treatment is successful, the following principles must be considered
in both the design and operation of fixed-film and combined processes.
CONCEPTS. Removal of organic materials through the use of fixed and combined
reactors is accomplished by means of a biological film on the fixed-film media. This
film—a viscous, jellylike slime—typically is composed of a large and diverse population of living organisms (e.g., bacteria, protozoa, algae, fungi, worms, and even insect
larvae). Most of the mass of this population is aerobic organisms. An aerobic organism
requires oxygen to function properly.
As the slime layer thickens, the adsorbed organic matter is metabolized before it
can reach the microorganisms near the media face. As a result, the microorganisms
near the media face enter into an endogenous phase of growth and lose their ability to
cling to the media surface (Metcalf and Eddy, 1979). This allows the flowing wastewater to scour the slime from the filter media—a process is known as sloughing—and a
new slime growth begins. The sloughing process continues at various stages throughout the filter. However, sloughing can be encouraged either by increased hydraulic
loading, supplemental aeration, temperature changes, other operator-induced changes,
or environmental conditions.
The removal of soluble organic material is a relatively rapid process. Good removal of soluble organics can typically be achieved at low to moderate loading of the
fixed-film reactors. However, the stabilization or breakdown of biological solids generated in removing the soluble organics is a longer process. The time required for completion of this process will vary, depending on the type of filter media being used, rate
of organic loading to the fixed-film process, hydraulic shear, temperature, and other
factors (Water Pollution Control Federation, 1988).
LOAD VARIATIONS. Fixed-film reactors (trickling filters or RBCs) vary in their
ability to absorb either seasonal or shock industrial wastewater loads. For either process, extremes may cause a bleed-through of BOD or even severe sloughing or biological kill. Recycled filter effluent is often used to dilute incoming raw waste and add
oxygen to the trickling filters, biotowers, or first stages of the RBCs. Using recycle techniques, treatment of wastewater with 5-day BOD (BOD5) concentrations greater than
10 000 mg/L is possible. This occurs especially in the treatment of food processing
waste, for which trickling filters are often used.
pH AND ALKALINITY. Bacteria associated with fixed-film reactors typically
thrive in a pH range between 5.5 and 9.0; the best pH range is between 6.8 and 7.2. Often, treatment problems result from rapid changes in pH values rather than from extreme long-term average values.
Although low pH may result from the discharge of industrial wastes or poorly
buffered natural waters, a drop in pH may also occur in the treated wastewater if the
fixed-film or combined reactors are lightly loaded or when temperatures are moderate.
This indicates that nitrification is occurring. Nitrification oxidizes ammonia, resulting
in the loss of alkalinity and a corresponding pH drop.
TOXICITY. Both fixed-film and combined-growth processes are typically less susceptible to toxicity or shock loads than suspended-growth (activated sludge) systems.
However, complex organic substances, heavy metals, pesticides, inorganic solids, and
even surges of disinfectants (e.g., chlorine) could either greatly reduce the performance
of, or cause a biological kill within, both fixed-film and combined processes. The toxicity causes either poor treatment performance or massive sloughing. Source control
through industrial waste management is necessary if industrial discharges are causing
the toxicity problem.
NUTRIENTS. Wastewater that is primarily from domestic sources typically has more
than sufficient nutrients to ensure that bacterial growth is not inhibited because of the
lack of essential nutrients. However, some industrial wastes (especially food processing) lack sufficient nutrients to promote normal bacterial growth. Mixtures of domestic
and industrial waste can also be nutrient deficient when the industrial portion dominates the municipal plant’s organic load.
The most commonly deficient nutrients are nitrogen (N) and phosphorus (P). To be
available for bacterial growth, nitrogen must be in the form of soluble ammonia, and phosphorus must occur in the orthophosphate form. Empirical ratios based on the amounts of
nutrients needed for producing biological cells suggest that for each 45 kg (100 lb) of BOD5,
2.3 kg (5 lb) of nitrogen and 0.5 kg (1 lb) of phosphorus must be available for proper cell
growth. In equation form, this ratio is 90 BOD5⬊4.6 N⬊1 P (100 BOD5⬊5 N⬊1 P).
TEMPERATURE. Biological activity (hence BOD removal) in all treatment systems
declines as temperatures decrease. However, fixed-film bacteria appear to be more
sensitive to the temperature drop than the bacteria in suspended-growth (activated
21-5
21-6
sludge) systems. For example, a drop in temperature from 26 to 20 °C (78 to 68 °F) in a
suspended-growth system will typically halve the coefficient of removal. The same
temperature drop in a fixed-film reactor could decrease the removal rate coefficient by
two-thirds or more. As a result of reduced efficiency during winter months, several literature sources indicate that filters should be sized 25% larger in the northern United
States than in southern areas. Attempting to conserve heat in the wastewater as it proceeds through treatment may sometimes reduce the adverse effects of cold temperatures. Following are some ideas on how this can be accomplished:
•
•
•
•
•
•
•
•
•
•
•
Removing a unit from service,
Covering the fixed-film process,
Reducing recirculation,
Using forced rather than natural ventilation,
Reducing the settling tank’s hydraulic retention time,
Operating in parallel rather than in series,
Adjusting orifice and splash plates to reduce spray,
Constructing windbreaks to reduce wind effects,
Intermittently dosing the filter,
Opening dump gates or removing splash plates from distributor arms, and
Covering open sumps and transfer structures.
DISSOLVED OXYGEN. Dissolved oxygen is needed to sustain the aerobic microorganisms in the fixed-film process. As water flows over the fixed-film media, oxygen
transfers to the water. When water is recycled in the fixed-film process, the presence of
a high concentration of dissolved oxygen in the fixed-film underflow or treated effluent does not necessarily mean that this same concentration is available in the RBC’s
first stage, the trickling filter’s interior, or the biotower’s various levels. Oxygen deficiencies with trickling filter and biotower media have been less troublesome than those
with RBC media.
Biotowers with high-rate (plastic or redwood) media are frequently designed for
BOD5 loadings between 320 and 480 kg BOD5/100 m3d (200 and 300 lb BOD5/1000 cu
ft/day) before concerns are raised about low dissolved oxygen in the filter underflow.
However, some biotowers are designed for loadings less than 240 kg BOD5/100 m3d
(150 lb BOD5/1000 cu ft/day) to prevent odor problems. Loadings with rock filter media are often maintained at less than 90 kg BOD5/100 m3d (50 lb BOD5/1000 cu ft/day)
to ensure adequate dissolved oxygen and low odor potential. A survey of highly loaded
trickling filters or biotowers indicates that odors result less frequently from high organic loading or low dissolved oxygen in the filter underflow than from constituents in
the wastewater (e.g., high industrial loading). Some odors always emanate from trickling filters and biotowers; the proximity and sensitivity of nearby residents will often
determine whether the odor is a nuisance.
Dissolved oxygen can be a limiting factor in RBC performance, if improperly designed. Organic loadings to RBCs should be limited to 2.9 to 3.9 kg BOD5/100 m2d (6.0
to 8.0 lb BOD5/1000 sq ft/day) or 1.2 to 2.0 kg SBOD5/100 m2d (2.5 to 4.0 lb SBOD5/
1000 sq ft/day) for the first-stage units in service. First-stage units are typically loaded
at three to four times the recommended BOD5 load of the total units in service.
To reduce the oxygen limitations of RBC facilities, a number of plants have been
upgraded by preceding or combining preaeration, trickling filter, activated sludge, or
solids contact with RBC reactors. Alternatively, supplemental aeration has been added
to the RBC units in some facilities. These approaches to RBC upgrading are discussed
in the Combined Processes section of this chapter.
MICROORGANISMS. The treatment of wastewater by fixed-film processes produces a biological (zoogleal) slime that coats the surface of the media. When fixed-film
reactors are used for BOD removal, the microbial population consists of various species
of heterotrophic bacteria with smaller populations of protozoa and fungi. If these reactors are used for nitrification, autotrophic nitrifying microorganisms predominate,
with smaller numbers of heterotrophic bacteria.
Under conditions of low dissolved oxygen, nutrient deficiencies, or low pH values, organisms that either remove BOD slowly or exhibit poor settling characteristics
can dominate the fixed-film reactor. These organisms are predominately filamentous
bacteria and sometimes fungi and are a nuisance.
Eliminating filamentous bacteria typically involves identifying the source of the
nuisance and eliminating the condition that allows them to dominate the system. In
addition, the recycle of suspended-growth bacteria over the fixed-film reactor has often
reduced the presence of filamentous bacteria. This mode resembles the selector activated sludge process described in Chapter 20. This approach for fixed-film reactors is
discussed in more detail in the Combined Processes section of this chapter.
TRICKLING FILTERS AND BIOTOWERS
Trickling filters attempt to duplicate the natural purification process that occurs
when polluted wastewater enters a receiving stream and trickles over a rock bed or
rocky river bottom. In the natural purification process, bacteria in the rock bed remove the soluble organic pollutants and purify the water. For more than 100 years
(since the late 1880s), trickling filters have been considered a principal method of
21-7
21-8
wastewater purification. The principle of using a rock bed for purification was applied in filter design, with the rock beds typically ranging from 0.9 to 2.4 m (3 to 8 ft)
deep. After declining use in the late 1960s and early 1970s, trickling filters regained
popularity in the late 1970s and early 1980s, primarily because of new media types. The
new high-rate media were typically preferred over rock media because they offer more
surface area for biological growth and improved treatment efficiency. The advent of
high-rate media minimized many of the rock media problems (e.g., plugging, uncontrolled sloughing, odors, and filter flies). Consequently, almost all trickling filters constructed in the late 1980s use high-rate media (Water Pollution Control Federation,
1988); they are called biotowers.
This section focuses on the operation and maintenance of both types of trickling filters. Their operations and maintenance needs may vary greatly, depending on the type
of filter media used and how the filter was designed (Albertson and Eckenfelder, 1984).
ALTERNATIVES. There are four basic categories of filter design, based on the
organic loading of the trickling filter/biotower. In the first three categories—low-,
intermediate-, and high-rate filters—the filter removes all or essentially all of the BOD
applied (Table 21.1). In the fourth category (the roughing filter), the filter is typically
combined with another biological treatment step (typically activated sludge, RBC, or
another filter), where a substantial amount of BOD removal occurs.
The categories of trickling filters/biotowers are typically based on BOD5 loading
to the filter divided by the volume of filter media, calculated as follows:
Organic (BOD 5 ) load Where
BOD5 applied
Volume of media
BOD 5 applied, lb/d
Volume of media, 1000 cu ft
kg primary effluent BOD5/d;
(primary effluent BOD5, mg/L)(flow, ML/d) and
2
horizontal (plan) area, m media depth, m
100
Or (in U.S. customary units)
BOD5 applied
lb primary effluent BOD5/d;
(primary effluent BOD5, mg/L)(flow, mgd)
(8.34 lb/gal); and
Volume of media
(21.1)
horizontal (plan) area, sq ft media depth, ft
1000
TABLE 21.1
Trickling filter categories.
Trickling filter categories
Operating characteristics
Low
Intermediate
High
Roughing
Organic loading,
lb BOD/d/1000 cu ft/day
25
25–40
40–100
100–300
Filter mediaa
Rock or
high rate
Rock or
high rate
Rock or
high rate
High rate
Yes
Partial
Unlikely
No
—For secondary treatment
No
Unlikely
Likely
Yes
—For tertiary treatment
Yes
Yes
Yes
Yes
TF/SC
TF/SC
TF/SC
TF/AS
ABF
ABF
TF/RBC
BF/AS
2-stage
filters
RF/AS
2-stage
filters
Nitrificationb
Combined process required
Type typically used
c
d
High rate plastic or redwood.
At 26 °C (78 °F) and without a second-stage or combined process.
c
Indicates if combined or dual process is typically used.
d
TF/SC trickling filter–solids contact; TF/AS trickling filter–activated sludge; ABF activated
biofilter; TF/RBC trickling filter–rotating biological contactor; BF/AS biofilter–activated sludge; and
RF/AS roughing filter–activated sludge.
a
b
Although filters/biotowers are typically not classified by hydraulic loading, the hydraulic or wetting rate is a useful loading parameter and is calculated as follows:
Hydraulic load Total flow (including recycle), pumped, gal/min
wetting rate
Horizontal (plan) area, sq ft
Where
Total flow
incoming filter recycle flow;
sum of filter pump capacity, gal/min; and
Horizontal plan area
3.14 (diameter 2 ) , for circular unit
4
length width, for rectangular unit.
(21.2)
21-9
21-10
Example 21.1.
Calculate the organic and hydraulic loadings of a filter.
Given:
Q (Flow)
Primary effluent BOD5
Filter diameter
Media depth
Number of filters
Total filter pumping capacity
5 mgd,
120 mg/L,
110 ft,
8 ft,
1, and
7000 gal/min.
Solution:
Horizontal plan area
2
3.14 (110)
4
9499 sq ft
Filter volume
BOD5 applied
Organic load filter
Hydraulic load
(9499 sq ft)(8 ft)
1000
76(1000 cu ft)
(120 mg/L)(5 mgd)(8.34)
5004 lb BOD5/d
5400 lb BOD 5 /d
7 6 (1000 cu ft)
65.8 lb BOD5/d/1000 cu ft
70000 gal/min
9499 sq ft)
0.74 gpm/sq ft
Organic loading, typically expressed as total BOD5 in the primary effluent, is often referred to as total organic loading (TOL). Another common way to evaluate BOD loading
depends on the amount of soluble filter/biotower BOD. This loading is referred to as
the soluble organic loading (SOL).
Low-Rate Filters/Biotowers. Low-rate filters/biotowers typically include rock trickling
filter media. At loadings of less than 40 kg BOD5/100 m3 d (25 lb BOD5/d/1000 cu ft),
fewer problems from filter flies, odors, or media plugging (ponding) are expected than
with filters operating at higher loading rates.
Low-rate trickling filters with rock media range in depth from 0.9 to 2.4 m (3 to
8 ft). Most low-rate filters are circular with rotary distributors. However, a number of
rectangular rock filters remain in operation. With rock media, low-rate filters are typically not hydraulically limited; application rates ranging from 0.01 to 0.04 L/m2 s (0.02
to 0.06 gpm/sq ft) are common. Both circular and rectangular filters are sometimes
equipped with dosing siphons or periodic pumping to provide a high wetting rate for
short intervals between rest periods.
With high-rate plastic filter media, a minimum wetting rate (accomplished via
recirculation or dosing) is typically maintained to prevent the media from drying out
and to ensure good contact. A rate of 0.4 L/m2s (0.7 gpm/sq ft) is typically considered
good practice. Potential loss in BOD removal performance should be evaluated at
lower rates.
Sloughed solids from a low-rate filter are typically well-digested, so these filters
yield less solids than higher rate filters. Solids yields of 0.5 kg total suspended solids
(TSS)/kg (0.5 lb TSS/lb) secondary influent BOD5 are not uncommon, especially with
rock media.
To provide tertiary treatment or polished effluent, combined processes [e.g., trickling filter/solids contact (TF/SC) or activated biofilter (ABF)] may be used. Combined
processes are typically not required to achieve effluent of conventional secondary treatment quality. Secondary quality effluent is readily attainable if the low-rate trickling
filter design incorporates filter media with bioflocculation capabilities or good secondary clarification (Harrison et al., 1984). For more information, see the Combined
Processes section of this chapter.
Intermediate-Rate Filters. Intermediate filters may be loaded up to 64 kg BOD5/
100 m3d (40 lb BOD5/d/1000 cu ft). Recirculation of trickling filter/biotower effluent
is typically practiced to ensure good distribution and thorough blending of filter and
secondary effluents to prevent bleed-through or short-circuiting of BOD with the
treated effluent.
Biological solids that slough from an intermediate trickling filter are not as welldigested as those from a low-rate unit. Yields ranging from 0.6 to 0.8 kg TSS/kg (0.6 to
0.8 lb TSS/lb BOD5) are common, depending on the filter media type.
Nitrifying bacteria have difficulty competing with heterotrophic bacteria, and ammonia removal via nitrification is typically incomplete. However, carbonaceous BOD
removal is nearly complete. Therefore, with good clarification following the filter, use
of a combined process to achieve secondary treatment is almost never necessary. Both
the TF/SC and ABF processes can be used to improve effluent quality, as described in
the Combined Processes section of this chapter.
High-Rate Filters. The maximum organic removal of most filter media ranges from
48 to 96 kg SBOD5/100 m3d (30 to 60 lb SBOD5/d/1000 cu ft), depending on temCopyright © 2007 Water Environment Federation.
21-11
21-12
perature, wastewater characteristics, and other conditions. High-rate filters, typically
loaded at their maximum organic loading capabilities, receive total BOD5 loadings
ranging from 64 to 160 kg BOD5/100 m3d (40 to 100 lb BOD5/d/1000 cu ft). Achieving
secondary effluent quality with high-rate filters reliably without a second-stage
process is less predictable than with low- or intermediate-rate filters. Therefore, highrate trickling filters are typically used with combined processes (Table 21.1).
With the recirculation typically used with high-rate filters, hydraulic loading rates
typically range from 0.3 to 0.4 L/m2s (0.5 to 0.7 gpm/sq ft), depending on the type of
filter media used.
Roughing Filters. Roughing filters are typically designed to allow a substantial amount
of SBOD to bleed through the trickling filter. An RBC or second, smaller trickling filter
follows the first-stage filter to complete the BOD oxidation. The second stage of treatment is typically 30 to 50% of the size required without a roughing filter. An activated
sludge process that follows a roughing filter typically has to assimilate the sloughed
solids and remaining BOD; therefore, a significant load has not been reduced.
Roughing filters typically have a design load ranging from 160 to 480 kg BOD5/
100 m3d (100 to 300 lb BOD5/day/1000 cu ft). A further description of the process
modes follows in the Combined Processes section of this chapter.
DESCRIPTION OF PROCESS. Regardless of the type of trickling filter/biotower
used, the pollutant-removal mechanisms remain the same. Microorganisms cover a filter consisting of rock (river or crushed aggregate), plastic, or redwood media. The
wastewater enters the filter medium at a controlled rate (trickled), causing intimate
contact between waste, the air, microorganisms, and other organisms.
The term filter is misleading, because it suggests physical separation of the solids
from the liquid via straining action. This does not occur, even with closely packed rock
media, and certainly not with the more open high-rate media used in biotowers. Instead,
treatment occurs when the microorganisms absorb and use dissolved organics for their
growth and reproduction as the wastewater cascades randomly through the voids
(spaces between the media).
The complex population of microorganisms is predominately aerobic. It absorbs
oxygen from air circulating through the media. Circulation can be enhanced by a
forced ventilation system consisting of a series of fans and an air-distribution system.
However, most trickling filters/biotowers rely solely on natural ventilation to supply
the oxygen necessary for aerobic treatment.
High-rate biotower media offer more surface area than rock media for microbial
attachment per cubic meter (cubic foot) of media. Also, biotowers have more void
space than rock media, allowing sloughed solids to exit the biotower and improving
air circulation.
The ability to control the unit process will vary greatly, depending on the facilities
and equipment provided by the design engineer. Also, the operating strategy used for
process control may result in significant changes in the process’ character and its ability to remove pollutants. These issues are considered in the following two sections.
DESCRIPTION OF EQUIPMENT. The structure, distribution, and support system used with the media are collectively named either a trickling filter or a biotower. The
term trickling filter typically applies to filters that use rock media and are relatively shallow [1.2 to 3.0 m (4 to 10 ft) deep]; processes that use plastic or redwood media with
depths greater than 3 m (10 ft) are typically referred to as biological towers or biotowers.
A similar term, biofilter, sometimes refers to filter towers in which biological solids
from an activated sludge system are recycled over the media.
The following six basic components are common to all trickling filter and biotower
systems:
•
•
•
•
•
•
Distribution system,
Filter media,
Underdrain system,
Containment structure,
Filter pump station or dosing siphon, and
Secondary clarifiers.
These basic components are illustrated in Figure 21.1. The purpose of these parts is
described in Table 21.2. A more detailed description of the basic components follows.
Distribution Systems. The two basic types of distribution systems are fixed-nozzle
and rotary distributors. Fixed-nozzle distributors were frequently used during the
early to mid-1900s, but their use on new trickling filters is limited. Fixed-nozzle distributors consist of a piping system, often supported slightly above the top of the trickling filter media, that feeds wastewater and recycled wastewater from a pumping station or siphon box through spray nozzles. A number of advancements in fixed-nozzle
design include springs, balls, or other mechanisms to evenly distribute wastewater at
various flows. Even with these improvements, obtaining even distribution with a fixednozzle distribution system is more difficult than with rotary distribution systems. Fixeddistributor systems have also declined in use because of difficult access to the nozzles
for cleaning raising safety concerns. Rotary distributors consist of a center well (typi-
21-13
21-14
FIGURE 21.1 Trickling filter parts. (Copyrighted material from Operation of Wastewater
Treatment Plants, Volume 1, 6th Edition, Chapter 6, “Trickling Filters”; reproduced by
permission of the Office of Water Programs, California State University, Sacramento.)
cally metal) mounted on a distributor base or pier. The distributor typically has two or
more arms that carry the pumped or siphoned wastewater to varying sized orifices for
distribution over the media surface. The thrust of the water spray drives the filter arms
forward. Speed-retardant back-spray orifices are often used to adjust the distributor’s
rotational speed, while maintaining the desired flowrate to the filter.
Recently, some rotary distributors have been equipped with motorized drive units
to precisely control the wastewater flow distribution speed. Distributors may be set up
TABLE 21.2
Parts of a trickling filter (California State, 1988).
Part
Purpose
Inlet pipe
Conveys wastewater to trickling filter
Distributor base
Supports rotating distributor arms
Distributor bearings
Allows distributor arms to rotate
Distributor arm
Conveys wastewater to outlet orifices located along the arms
Outlet orifice
Controls flow to filter media; adjustable to provide even
distribution of wastewater to each square metre (square foot) of
filter media
Speed-retarder orifice
Regulates speed of distributor arms
Splash plate
Distributes flow from orifices evenly over filter media
Arm dump gate
Drains distributor arm and controls filter flies along filter
retaining wall; also used for flushing distributor arm to remove
accumulated debris that might block outlet orifices
Filter media
Provides a large surface area on which the biological slime grows
Support grill
Keeps filter media in place and out of underdrainage system
Underdrain system
Collects treated wastewater from under filter media and conveys
it to the underdrain channel; also permits air flow through media.
Underdrain channel
Drains filter effluent to the outlet box
Outlet box
Collects filter effluent before it flows to the next process
Outlet valve
Regulates flow of filter effluent from outlet box into outlet pipe;
closed when filter is to be flooded
Outlet pipe
Conveys filter effluent to next treatment process
Retaining wall
Holds filter media in place
Ventilation port
Allows air to flow through the media
Stay rod
Supports distributor arm
Turnbuckle on stay rod
Permits adjusting and leveling of distributor arm to produce an
even distribution of wastewater over the media
Center well
Provides for higher water head to maintain equal flow to
distributor arms; typically a head of 45 to 60 cm (18 to 24 in.) is
maintained on the orifices
Splitter box
Divides flow to the trickling filters for recirculation or to the
secondary clarifiers
Recirculation pump
Returns or recirculates flow to the trickling filters
21-15
21-16
to be mechanically driven at all times or just when stalled. These operating provisions
are aimed at selecting a distributor speed to increase biomass sloughing. Decreasing
distributor speed may prevent plugging, decreased performance, and odors, particularly in heavily loaded filters. Adding motorized drives also can increase the performance of an exiting tricking filter or biotower.
The distributor support bearings are either at the top of the mast or at the bottom
of the turntable. Both types of bearings are widely used.
Another newer method of more precisely controlling the wastewater flow distribution to the trickling filter is a system of pneumatically controlled gates to open and
close the orifices on both sides of the distributor arms. As flow to the trickling filter
varies, the speed is maintained by automatically adjusting the gates over the orifices.
Filter Media. Of the many types of media materials used to support biological growth,
the most common types are shown in Figure 21.2. Media are typically classified as
either high-rate (high surface area and void ratio) or standard rock media.
Filter media types made of plastic sheets include vertical, 60-degree crossflow, and
45-degree crossflow. Random media are open-webbed plastic shapes. For carbonaceous BOD removal, their surface area typically ranges from 89 to 105 m2/m3 (27 to 32
sq ft/cu ft) of media, and their void percentage is between 92 and 97% (open-space percentage of unit volume). Filter media for nitrification (post-BOD removal) are available,
with surface areas in excess of 131 m2/m3 (40 sq ft/cu ft). Based on numerous studies to
compare trickling filter media, the present consensus is that cross-flow media may offer
better flow distribution than other media, especially at low organic loads. Compared
with 60-degree cross-flow media, vertical media provide nearly equal distribution and
may better avoid plugging, especially at higher organic loadings.
Rock media may consist of either graded material from natural river beds or
crushed stone. Most rock media provide approximately 149 m2/m3 (15 sq ft/cu ft) of
surface area and less than 40% void space.
A significant difference between rock media and plastic media is that most loose
stone aggregates have a dry weight of approximately 1282 kg/m3 (80 lb/cu ft) compared with a density of 32 to 48 kg/m3 (2 to 3 lb/cu ft) for plastic media. Additional
provisions required for plastic media include UV protective additives on the exposed
top layer of plastic media filters; thicker plastic walls for media packs installed in the
lower sections of the filter, where loads increase; and, under certain conditions, a
means for shielding the top layer from the effects of the distributor’s hydraulic force.
Underdrain System. The underdrain system supporting rock media typically consists
of precast blocks laid over the entire sloping filter floor. Underdrain and support
FIGURE 21.2
Trickling filter media (Harrison and Daigger, 1987).
systems for high-rate media typically consist of a network of concrete piers and support stringers placed with their centers 0.3 to 0.6 m (1 to 2 ft) apart. Redwood or pressure-treated wood is also used as underdrain material.
Underdrains for plastic or high-rate filter media are typically 0.3 to 0.6 m (1 to
2 ft) deep to allow air movement to the interior of the filter. Floors typically slope
downward to a collection trough that carries wastewater to an outlet structure. The
collection trough also serves as an air conduit to the interior of the trickling filter. AcCopyright © 2007 Water Environment Federation.
21-17
21-18
cess to the filter underdrain system should be available at the outlet box to allow periodic inspection. The confined space entry procedures described in Chapter 5 govern
the inspection.
Containment Structure. The housing for rock media typically consists of poured-inplace concrete. Filter towers are lightweight containment structures consisting of precast concrete, fiberglass panels, or other materials. These structures are used with highrate media that are self-supporting (exert no wall pressure).
Ventilation ports, typically located at the base of the filter tower, are designed to
prevent the filter tower from being stained by the heavy splash of distributed wastewater cascading to the filter floor. Closed louvers, if available, allow the vents to be
closed during cold weather. The filter structure may include low-pressure fans and air
ducts (typically fiberglass) to distribute air in the filter underdrain.
The wall of the containment structure often extends 1.2 to 1.5 m (4 to 5 ft) above
the top of the filter media. This prevents spray from staining the sides of the filter
tower, reduces wind effects that may reduce wastewater temperatures or stall distributors, and provides a structural base for domed covers.
Filter Pump Station. As an integral part of the trickling filter or biotower system, the
pumping station typically lifts the primary effluent and the recirculated filter effluent,
if any, to the top of the media. Sometimes a siphon dosing tank or gravity flow feeds
the distributor. The filter/biotower feed pumps most typically used are vertical-turbine
units mounted above a wet well. Submersible pumps and dry-pit centrifugal pumps
may also be used in the filter pump station.
The trickling filter/biotower is typically elevated so the hydraulic grade line allows gravity flow to the secondary clarifier or other downstream treatment units. If recirculation is used, the downstream treatment unit or clarifier typically controls the
water level in the pumping station wet well, so a control valve is not necessary to modulate the amount of underflow returning to the pumps.
Secondary Clarifier. In the past, clarifier design often received insufficient attention.
Although wastewater treatment professionals typically recognize the need for improved clarifier design criteria associated with suspended-growth or activated sludge
plants, this need also exists with the design and operation of secondary clarifiers used
in the fixed-film trickling filter/biotower process.
Performance of the trickling filter/biotower process is typically not limited by
SBOD removal, but by the secondary clarifier’s ability to separate suspended solids
from treated wastewater. This is especially true for low-, intermediate-, and high-rate
processes that remove most of the SBOD. Therefore, effluent quality depends largely
on the particulate BOD associated with solids remaining in the clarifier effluent.
With the trickling filter/biotower process, past practices have resulted in secondary clarifiers with high hydraulic overflow rates and shallow sidewater depths
[2.4 to 3.0 m (8 to 10 ft)]. Corresponding suspended-growth systems were often designed with clarifiers having much lower hydraulic overflow rates and sidewater
depths of 3.0 to 3.7 m (10 to 12 ft). As trickling filter/biotower plants are now required
to achieve secondary or even higher treatment levels, the clarifier sidewater depth
must be larger to provide a greater separation zone for solids removal. Likewise, reduced overflow rates may be needed to achieve the required effluent quality (Tekippe
and Bender, 1987).
PROCESS CONTROL. Many operating problems may be avoided by changing
one or more of the following process control variables: flow patterns, distribution rates,
and clarifier operation.
Flow Patterns. Although operators do not control the arrangement of the major treatment units, opportunities may exist to take units offline, operate in stages (parallel or
series filters), or recycle settled biological solids over high-rate filter media. For example, staging or operating filters in series sometimes increases the overall system’s BOD
removal because of increased efficiency in the higher-loaded first stage. If both rock
and high-rate filters are available, operators should consider operating the rock filter
in the first stage to achieve a high reduction in produced biological solids. Then, the
second-stage filter would be operated in a polishing mode, taking advantage of the
high-rate filter media’s bioflocculation capabilities.
With little or no modification, many existing trickling filters/biotowers with highrate media may incorporate small amounts of biological solids recycle over the filter
media to enhance the flocculation of particulate solids. Recycled solids may be returned to the filter underflow of the rock filter media, as is done in trickling filter and
solids combined processes.
Distribution Rates. As a principal process control measure, operators can control the
rates at which wastewater and filter effluent are distributed to the filter media. Recirculation can serve several purposes, as follows:
• Reduce the strength of the wastewater being applied to the filter;
• Increase the hydraulic load to reduce flies, snails, or other nuisances;
• Maintain distributor movement during low flows;
21-19
21-20
•
•
•
•
•
Produce hydraulic shear to encourage solids sloughing and prevent ponding;
Dilute toxic wastes, if present;
Reseed the filter’s microbial population;
Provide uniform flow distribution; and
Prevent filters from drying out.
The most common recirculation patterns used for trickling filters/biotowers are shown
in Figure 21.3. If odors emanate from the primary clarifier or headworks, recycling filter effluent to either location may help control them. When the recirculated water
passes through either the primary or secondary clarifier, operators need to prevent excessive hydraulic loading of the clarifier.
A typical control strategy for highly loaded or roughing filters is to frequently
(once per week) maintain the maximum pumping rate possible for a 2- to 3-hour period.
This encourages sloughing, so solids buildup is less likely, and uncontrolled sloughing
is minimized.
Another approach is to slow the distributor arm using back-spray nozzles, so the
media receives a greater instantaneous flush. When a trickling filter/biotower accumulates excess solids, aerobic surface area decreases, which, in turn, reduces oxygen transfer. Because oxygen does not penetrate more than 1 to 1.5 mm of film thickness, there is
no benefit with more than 0.76 mm (0.03 in.) biomass on the media (Albertson, 1989).
A German process parameter that has been considered in the United States is
Spulkraft flushing intensity (SK), which is defined by the following equation:
SK Where
SK
qr
a
n
1.0 m3/m2h
(q r )(1000 mm/m)
( a)(n)(60 min/h)
(21.3)
flushing intensity, mm/pass of arm;
total hydraulic rate, m3/m2h;
number of distributor arms;
rotational speed, rev/min; and
0.41 gpm/sq ft.
Some recommended SK values for design and flushing flowrates are given in Table 21.3.
Compared with the activated sludge treatment process, trickling filters can use
30 to 50% less energy if the pumping rate is optimized. Thoughtful consideration needs
to be given to balancing the need to maintain a minimum wetting rate versus the potential energy savings of lower recirculation rates.
FIGURE 21.3
Recirculation patterns for trickling filters.
Clarifier Operation. The manner in which secondary clarifiers are operated can significantly affect trickling filter performance. Although clarifier operation with fixedfilm reactors is not as critical as that with suspended-growth systems, operators must
still pay close attention to final settling.
Sludge must be removed quickly from the final settling tank before gasification occurs or denitrification causes solids to rise. Use of the secondary clarifier as a principal
means of thickening (rather than simply for solids settling) may not produce the best
21-21
21-22
effluent quality, especially during summer months, when denitrification is likely to occur. The sludge blanket depth in the secondary clarifier should be limited to 0.3 to 0.6 m
(1 to 2 ft). Continuous pumping or intermittent pumping with automatic timer control
are used to accomplish solids wasting.
TROUBLESHOOTING. Even though the trickling filter/biotower process is considered one of the most trouble-free means of secondary treatment, the potential for
operating problems exists (Table 21.4). The source of mechanical problems is often obvious. However, less obvious causes of problems may stem from operations, design
overload, influent characteristics, and other non-equipment-related items.
Good records and data associated with the trickling filter are essential in locating,
identifying, and applying the proper corrective measure to solve problems. Tracking
SBOD, suspended solids, pH, temperature, and other parameters may be necessary to
recognize trends that result in adverse trickling filter/biotower effects.
Common operating problems may result from increased growth, changes in wastewater characteristics, improper design, or equipment failures. Regardless of the source,
these problems eventually become categorized into either operation or maintenance
areas. In summary, the problems addressed in Table 21.4 are as follows:
Operations:
• Increase in secondary clarifier effluent suspended solids,
• Increase in secondary clarifier effluent BOD,
• Objectionable odors from filter,
• Ponding on filter media,
• Filter flies, and
• Icing.
TABLE 21.3 Design and flushing Spulkraft (SK) values for distributors (revised, higher
SK values reflect new, post-publication data) (Albertson, 1989).
BOD5 loading (lb/d/cu fta)
25
50
75
100
150
200
Design SK (mm/pass)
25–75
50–150
75–225
100–300
150–450
200–600
BOD biochemical oxygen demand; lb/d/cu ft 16.02 kg/m3d.
a
Flushing SK (mm/pass)
100
150
225
300
450
600
TABLE 21.4
21-23
Troubleshooting guide for trickling filters (continued on next page).
Problem/possible cause
Corrective action
Operations
Increase in secondary clarifier effluent suspended solids
Clarifier hydraulically overloaded
Check clarifier surface overflow rate; if possible,
reduce flow to clarifier to less than 35 m3/m2d
(900 gal/d/sq ft) by reducing recirculation or
putting another clarifier into service
Expand plant
Denitrification in clarifier
Increase clarifier sludge withdrawal rate
Increase loading on trickling filter to prevent
nitrification; skim floating sludge from entire surface
of clarifier or use water sprays to release nitrogen
gas from sludge so sludge will resettle
Excessive sloughings from biofilter because of
changes in wastewater
Increase clarifier sludge withdrawal rate
Check wastewater for toxic materials, changes in pH,
temperature, BOD, or other constituents
Identify and eliminate source of wastewater causing
the upset
Enforce sewer-use ordinance
Equipment malfunction in secondary clarifier
Check for broken sludge-collection equipment and
repair or replace broken equipment
Short-circuiting of flow through secondary clarifier
Level effluent weirs
Install clarifier center pier exit, baffles, effluent weir
baffles, or other baffles to prevent short-circuiting
Increase in secondary clarifier effluent biochemical oxygen demand (BOD)
Increase in effluent suspended solids
Excessive organic loads on filter
See corrective actions for “Increase in secondary
clarifier effluent suspended solids”
Calculate loading
Reduce loading by putting more biofilters in service
Increase BOD removal in primary settling tanks by
using all tanks available and minimizing storage in
primary sludge tanks
Eliminate high-strength sidestreams in plant
Expand plant
Undesirable biological growth on media
Perform microscopic examination of biological
growth
Chlorinate filter to kill off undesirable growth
21-24
TABLE 21.4
Troubleshooting guide for trickling filters (continued on next page).
Corrective action
Objectionable odors from filter
Excessive organic load causing anaerobic
decomposition in filter
Calculate loading
Reduce loading by putting more biofilters in service
Increase BOD removal in primary settling tanks by
using all tanks available and minimizing storage or
primary sludge in tanks
Encourage aerobic conditions in treatment units
ahead of the biofilter by adding chemical oxidants
(e.g., chlorine, potassium permanganate, or hydrogen
peroxide) or by preaerating, recycling plant effluent,
or increasing air to aerated grit chambers
Enforce industrial waste ordinance, if industry is
source of excess load
Scrub biofilter offgases
Replace rock media with plastic media
Expand plant
Insufficient ventilation
Increase hydraulic loading to wash out excess
biological growth
Remove debris from filter effluent channels and
underdrains
Remove debris from top of filter media
Unclog vent pipes
Reduce hydraulic loading if underdrains are flooded
Install fans to induce draft through filter
Check for filter plugging caused by breakdown of
media
Ponding on filter media
Excessive biological growth
Reduce organic loading
Slow down distributor to increase 5K value
Increase hydraulic loading to increase sloughing
Flush filter surface with high-pressure stream of water
Chlorinate filter influent for several hours; maintain
1 to 2 mg/L residual chlorine on the filter
Flood filter for 24 hours
Shut down filter until media dries out
Enforce industrial waste ordinance if industry is
source of excess load
TABLE 21.4
Troubleshooting guide for trickling filters (continued
21-25
on next page).
Corrective action
Poor media
Replace media
Poor housekeeping
Remove debris from filter surface, vent pipes,
underdrains, and effluent channels
Filter flies (psychoda)
Insufficient wetting of filter media (a continually
wet environment is not conducive to filter fly
breeding and a high wetting rate will wash fly
eggs from the filter)
Increase hydraulic loading
Filter environment nodule conducive to filter
fly breeding
Flood filter for several hours each week during fly
season
Unplug spray orifices or nozzles
Use orifice opening at end of rotating distributor
arms to spray filter walls
Chlorinate the filter for several hours each week
during fly season; maintain a 1- to 2-mg/L chlorine
residual on the filter
Poor housekeeping
Keep area surrounding filter mowed; remove weeds
and shrubs
Icing
Low wastewater temperature
Decrease recirculation
Remove ice from orifices, nozzles, and distributor
arms with a high-pressure stream of water
Reduce number of filters in service, provided
effluent limits can still be met
Reduce retention time in pretreatment and primary
treatment units
Construct windbreak or covers.
Maintenance
Rotating distributor slows down or stops
Insufficient flow to turn distributor
Increase hydraulic loading
Close reversing jets
Clogged arms or orifices
Flush out arms by opening end plates; flush out
orifices; remove solids from influent wastewater
Clogged distributor arm vent pipe
Remove material from vent pipe by rodding or
flushing
Remove solids from influent wastewater
Bad main bearing
Replace bearing
Distributor arms not level
Adjust guy wires at tie rods
21-26
TABLE 21.4
Troubleshooting guide for trickling filters (continued
on next page).
Corrective action
Distributor rods hitting media
Level media
Remove some media
Dirt in main bearing lube oil
Worn bearing dust seal
Replace seal
Worn turntable seal or seal plate
Replace seal; inspect seal plate and replace if worn
Condensate not drained regularly or oil level too low
Check oil level, drain condensate, and refill if needed
Water leaking from distributor base
Worn turntable seal
Replace seal
Leaking expansion joint between distributor and
influent piping
Repair or replace expansion joint
Broken top media
Foreign material
Flush out with a high-pressure stream of clean water
Rod out with wire or hook
Disassemble and clean
Secondary clarifier sludge collector stopped
Torque overload setting exceeded
Reduce sludge blanket; withdraw excess sludge
Check if skimmer portion of collector hung up on
scum trough; free and repair or adjust skimmer
Drain tank and remove foreign objects
Loss of power
Reset drive unit circuit breaker if tripped (after cause
for trip is identified and corrected)
Reset drive unit, motor control center, or plant main
circuit breakers as necessary when power is restored
to plant after interruption
Check drive motor for excessive current draw; if
current excessive, determine reason
Check drive motor overload relays; replace if bad or
undersized
Failure of drive unit
Check drive chains and shear pins; replace as
necessary and use proper size shear pin, or damage
will occur
Check and replace worn gears, couplings, speed
reducers, or bearings as needed; lubricate and
provide preventive maintenance for units as per
manufacturer’s instruction
Recirculation pumps delivering insufficient flow
Excessive head
Open closed or throttled valves
Unplug distributor arms, headers, and laterals
TABLE 21.4
21-27
Troubleshooting guide for trickling filters.
Corrective action
Unplug distributor nozzles and orifices
Unplug distributor vent lines
Pump malfunction
Adjust or replace packing or mechanical seals
Adjust impeller to casing clearance
Replace wear rings if worn excessively
Replace or resurface worn shaft sleeves
Check impeller for wear and entangled solids;
remove debris; replace impeller if necessary
Check pump casing for air lock
Release trapped air
Lubricate bearings as per manufacturer’s instructions
Replace worn bearings
Pump drive motor failure
Lubricate bearings as per manufacturer’s instructions
Replace worn bearings
Keep motor as clean and dry as possible
Pump and motor misalignment; check vibration and
alignment
Redesign as needed
Burned windings; rewind or replace motor
Check drive motor for excessive current draw; if
current draw is excessive, determine reason
Check drive motor overload relays; replace if bad or
undersized
Reset drive motor, motor control centers, or plant
main circuit breakers after cause for trip is identified
and corrected, or when power is restored after
interruption
Maintenance:
• Rotating distributor slowing down or stopping,
• Dirt in main bearing lube oil,
• Water leaking from distributor base,
• Nozzle or orifices plugged,
• Top media broken,
• Secondary clarifier sludge collector stopped, and
• Recirculation pumps delivering insufficient flow.
21-28
PLANNED MAINTENANCE. Planned maintenance will vary from plant to plant,
depending on unique design features and equipment installed. Although this chapter
cannot address all of these items, a summary of the most common and important maintenance tasks follows.
Table 21.5 is a guide to planned maintenance for the following:
•
•
•
•
•
•
•
•
Rotary distributors,
Fixed-nozzle distributors,
Filter media,
Underdrains,
Media containment structure,
Filter pumps,
Secondary clarifiers, and
Appurtenant equipment.
The information provided in Table 21.5 is not equipment- or plant-specific. Therefore,
both the manufacturer’s literature and engineer’s operating instructions should be
consulted and followed. The frequency of maintenance procedures depends on sitespecific conditions. However, until operating experience is gained, frequent plant inspections and maintenance should continue. Maintenance schedules should consider
the increased performance of trickling filters in warm weather months, which may reduce the effect of removing process units from service.
DISTRIBUTOR BEARINGS. Distributor bearings typically ride on removable
races (tracks) in a bath of oil (Figure 21.4). The oil, typically specified by the manufacturer, is selected to prevent oxidation and corrosion and to minimize friction. Because
the oil level and condition are crucial to the life of the equipment, they need regular
checking in accordance with the manufacturer’s recommendations (typically weekly).
A common procedure is to check the oil by draining approximately 0.6 L (1 pt) into a
clean container. If the oil is clean and free of water, it is returned to the unit. If the oil is
dirty, it is drained and refilled with a mixture of approximately one part oil and three
parts solvent (e.g., kerosene). Then, the distributor is operated for a few minutes, the
mixture is drained, and the distributor is filled with clean oil.
If water is found in the oil, then either the seal fluid is low or the gasket in the
mechanical seals requires replacement.
SAFETY. Work on distributors may proceed only after the arms have been stopped
and locked in place, and the distributor pump’s or control valve’s electrical switch has
TABLE 21.5
Planned maintainance for trickling filters (continued on next page).
Rotary distributors
• Observe the distributor daily. Make sure the rotation is smooth and that spray nozzles
are not plugged.
• Lubricate the main support bearings and any guide or stabilizing bearings according to
the manufacturer’s instructions. Change lubricant periodically, typically twice a year. If
the bearings are oil-lubricated, check the oil level, drain condensate weekly, and add oil
as needed.
• Time the rotational speed of the distributor at one or more flow rates. Record and file
the results for future comparison. A change in speed at the same flow rate indicates
bearing trouble.
• Flush distributor arms monthly by opening end shear gates or blind flanges to remove
debris. Drain the arms if idle during cold weather to prevent damage via freezing.
• Clean orifices weekly with a high-pressure stream of water or with a hooked piece of wire.
• Keep distributor arm vent pipes free of ice, grease, and solids. Clean in the same manner
as the distributor arm orifices. Air pockets will form if the vents are plugged. Air
pockets will cause uneven hydraulic loading in the filter, and nonuniform load and
excessive wear of the distributor support bearing.
• Make sure distributor arms are level. To maintain level, the vertical guy wire should be
taken up during the summer and let out during the winter by adjusting the guy wire tie
rods. Maintain arms in the correct horizontal orientation by adjusting horizontal tie rods.
• Periodically check distributor seal and, if applicable, the influent pipe to distributor
expansion joint for leaks. Replace as necessary. When replacing, check seal plates for
wear and replace if wear is excessive. Some seals should be kept submerged even if the
filter is idle or their life will be severely shortened.
• Remove ice from distributor arms. Ice buildup causes nonuniform loads and reduces
main bearing life.
• Paint the distributor as needed to guard against corrosion. Cover bearings when sandblasting to protect against contamination. Check oil by draining a little oil through a nylon
stocking after sandblasting. Ground the distributor arms to protect bearings if welding on
distributor and lock out the drive mechanism at the main electrical panel. Adjust secondary
arm overflow weirs and pan test wastewater distribution on filter as needed.
Fixed nozzle distributors
• Observe spray pattern daily. Unplug block nozzles manually or by increasing hydraulic
loading. Flush headers and laterals monthly by opening end plates. Adjust nozzle spring
tension as needed.
Filter media
• Observe condition of filter media surface daily. Remove leaves, large solids and plastics,
grease balls, broken wood lath or plastic media, and other debris. If ponding is evident,
find and eliminate the cause. Keep vent pipes open, and remove accumulated debris.
Store extra plastic media out of sunlight to prevent damage via ultraviolet rays. Observe
media for settling. After they are installed, media settle because of their own weight
21-29
21-30
TABLE 21.5
Planned maintainance for trickling filters.
and the weight of the biofilm and water attached to its surface. Settling should be
uniform and should stabilize after a few weeks. Total settling is typically less than 0.3 m
(1 ft) for random plastic media, less for plastic sheet media, and nearly zero for rock. If
settling is nonuniform or excessive, remove some of the media for inspection.
• Observe media for hydraulic erosion, particularly in regions where reversing jets hit the
media.
Underdrains
• Flush out periodically if possible. Remove debris from the effluent channels.
Media containment structure
• Maintain spray against inside wall of filter to prevent filter fly infestation and to prevent
ice buildup in winter.
• Practice good housekeeping. Keep fiberglass, concrete, or steel outside walls clean and
painted, if applicable. Keep grass around structures cut, and remove weeds and tall
shrubs to help prevent filter fly and other insect infestations. Remember, using
insecticides around treatment units may have adverse effects on water quality or the
biological treatment units.
Filter pumps
• Check packing or mechanical seals for leakage daily. Adjust or replace as needed.
Lubricate pump and motor bearings as per manufacturer’s instructions. Keep pump
motor as clean and dry as possible. Periodically check shaft sleeves, wearing rings,
and impellers for wear; repair or replace as needed. Perform speed reducer, coupling,
and other appurtenant equipment maintenance according to manufacturer’s
instructions.
Secondary clarifier
• Lubricate drive motor bearings, speed-reducing gear, drive chains, work and spur
gears, and the main support bearing for the solids-collection equipment according to the
manufacturer’s instructions. Flush scum troughs and grease wells daily. Maintain solidswithdrawal equipment. Clean effluent wells and baffles at least weekly. Paint or
otherwise protect equipment from corrosion as needed.
Appurtenant equipment
• Maintain piping, valves, forced draft blowers, and other appurtenant equipment
according to the manufacturer’s instructions.
been disengaged and locked out on the electrical panel. The filter medium should not
be walked on, because it will be slippery. Plastic grating is often placed as a permanent
walking surface to provide safe access to the distributor.
Covered trickling filters have special safety considerations, because they are considered confined spaces. The possibility exists for the atmosphere under the dome to
FIGURE 21.4 Trickling filter bearing (Copyrighted material from Operation of Wastewater Treatment Plants, Volume 1, 6th Edition, Chapter 6, “Trickling Filters”; reproduced
with the permission of the Office of Water Programs, California State University,
Sacramento).
contain little oxygen or much hydrogen sulfide or ammonia. Maintenance in these
areas must include proper confined space entry procedures. Chapter 5 discusses other
safety considerations.
ROTATING BIOLOGICAL CONTACTORS
A rotating biological contactor’s filter media consist of plastic discs mounted on a
long, horizontal, rotating shaft (Figure 21.5). A biological slime similar to that of the
trickling filter/biotower grows on the media. However, rather than being stationary, the
filter media rotate into the settled wastewater and then emerge into the atmosphere,
where the microorganisms receive oxygen that helps them consume organic materials
in the wastewater.
Rotating biological contactors have been extensively used at hundreds of locations
in the United States to treat municipal and industrial wastewater. It is estimated that
more than 600 RBC plants are now used for industrial and municipal wastewater treatment. Most of the plants are designed and used for BOD5 removal and a few for both
BOD5 and nitrogen removal.
When RBCs were initially introduced for wastewater treatment during the late
1970s and early 1980s, mechanical problems and organic overloading occurred frequently. By the mid-1980s, both equipment manufacturers and consulting engineers
had developed standards that minimized most of the problems, but some systems are
21-31
21-32
FIGURE 21.5
Rotating biological contactor shaft and media.
still mechanically unsound. This section discusses the use of RBCs principally for carbonaceous BOD5 removal and ammonia nitrification.
ALTERNATIVES. The flow pattern for RBC treatment of wastewater resembles that
for most other biological systems, because good preliminary and primary treatment
are essential to remove solids that would otherwise interfere with RBC performance
(Figure 21.6).
A secondary clarifier must be provided to remove sloughed solids from the
treated wastewater. Solids that settle in the secondary clarifier can either be recycled to
the primary clarifier for cosettling or pumped directly to a solids-handling system (Figure 21.6).
The term shaft typically is used to describe both the metal support and the filter media discs. The discs are made of high-density circular plastic sheets, typically 3.6 m (12 ft)
in diameter (although larger sizes are available from some manufacturers). The sheets,
bonded and assembled onto the horizontal shafts, are typically 7.6 m (25 ft) long. Each
shaft typically provides approximately 9300 m2 (100 000 sq ft) of surface area for microCopyright © 2007 Water Environment Federation.
FIGURE 21.6
Rotating biological contactor process flow schematic.
organism attachment. Lower density media are typically used for carbonaceous BOD5
removal, and higher density media are typically used for ammonia nitrification.
Alternative selection associated with RBCs consists primarily of the number and
arrangement of shafts (Figure 21.7) that support the RBC discs (Zickefoose, 1984). A
common arrangement includes a separate shaft for each stage, especially when the
flow is perpendicular to the shafts, as shown in plan A of Figure 21.7. A single shaft can
be divided into two or more stages by adding a baffle at one or more sections along the
flow pattern (plan B of Figure 21.7). This arrangement typically applies when the
wastewater flow pattern parallels the shaft. A stage may also be eliminated by removing
21-33
21-34
FIGURE 21.7
Stage arrangements and flow patterns for rotating biological contactors.
a baffle, as shown in plan C of Figure 21.7. The baffles may be constructed of either perforated concrete or slotted boards. To reduce the organic loading of a stage, baffles are
often removable. This allows two or more shafts to operate in a single stage, as illustrated in plan D of Figure 21.7. Staging is often used to improve effluent quality. Four
or more stages, combined with lower organic loading, are typically used to obtain a nitrified or well-treated effluent.
Staging is sometimes used to overcome oxygen-transfer problems in the first shafts
(first stages) of an RBC facility. Often, baffles are removed on the first several stages of
multistage facilities to distribute loading among several shafts. This method, among
others, has been used to reduce the organic loading and overcome oxygen-transfer difficulties in heavily loaded RBC reactors.
A train (several parallel series of stages) is also typically used to reduce loadings
on the first RBC stages. Baffles may also be removed between shafts to increase the surface area available in the first stage. Figure 21.7 illustrates the use of both trains and
stages in either parallel or series flow modes. Good designs will include a number of
gates and baffles to provide for system flexibility. This allows operators to vary the
flow pattern to accommodate facility-specific load and effluent quality criteria.
Designers should consider including one or more of the following to ensure that
the process will have adequate operating flexibility:
• Supplemental aeration to increase dissolved oxygen levels in the first and second stages;
• A means for removing excess biofilm growth (e.g., supplemental aeration, rotational speed control, and reversal);
• Multiple treatment trains;
• Removable baffles between all stages;
• Variable rotational speeds in the first and second stages;
• Load cells for first- and second-stage shafts;
• Alternate flow distribution systems (e.g., step feeding); and
• Recirculation of secondary clarifier effluent.
A significant number of RBCs have encountered oxygen limitations or overloads in the
first stages. In other cases, the RBCs have simply reached their design loads. Regardless of the reason, RBC upgrades are typically accomplished by adding more RBCs or
by constructing the following:
• New processes in parallel (i.e., side-by-side) with existing RBCs (e.g., activated
sludge processes, trickling filters, or aerated lagoons); and
• New processes in series (i.e., preceding or following existing RBCs), such as activated sludge processes, trickling filters, aerated lagoons, preaeration processes,
or solids contact processes.
DESCRIPTION OF PROCESS. Rotating biological contactor systems consist of
plastic media, typically a series of vertical discs, mounted on a horizontal shaft that
21-35
21-36
slowly rotates, turning the media into and out of a tank of wastewater. Rotating biological contactor shafts are rotated by either a mechanical or a compressed air drive so
the media on up to 40% of its diameter are immersed in the wastewater. The wastewater being treated flows through the contactor by simple displacement and gravity.
Bacteria and other microorganisms that are naturally present in the wastewater adhere
and grow on the surface of the rotating media. The biological film sloughs off whenever the biomass growth becomes too thick and heavy for the media to support. The
sloughed biofilm and other suspended solids are carried away in the wastewater and
removed in the secondary clarifier.
The biological slime on the first stages is typically 0.15 to 0.33 cm (0.06 to 0.13 in.)
thick. A healthy biomass on the first stage tends to be light brown, while the biomass
on later stages tends to have a gold or reddish sheen. Lightly loaded units may be
nearly devoid of visible biomass. A white or gray biomass indicates domination by filamentous (Beggiatoa, Thiothrix, or Lepothrix) bacteria—an unhealthy sign.
Like the trickling filter process, many of the process choices are fixed during design, and RBC operators have limited opportunities to make changes. The design choices
discussed in the following section will help both operators and designers better understand the RBC process.
Loadings for RBCs are typically based on BOD5 loading to the RBC units divided
by the media’s surface area. Organic loading is typically calculated for all units online
or simply for the first stages, where an oxygen limitation may exist. The organic load
may be based on either the soluble or total BOD.
Organic (BOD 5 ) load Where
BOD5 applied
BOD 5 applied, lb/d
Area of media, 1000 sq ft
(21.4)
kg of primary effluent BOD5/d;
(primary effluent BOD5, mg/L)(flow, ML/d); and
2
Media surface area, 100 m3 Surface per shaft, m number of shafts
100
Or (in U.S. customary units)
BOD5 applied
lb of primary effluent BOD5/d;
(primary effluent BOD5, mg/L)(flow, mgd)
(8.34 lb/gal); and
Surface per shaft, sq ft number of shafts
Media surface area,
1000
1000 sq ft
Although RBCs are typically not classified by hydraulic loading, the hydraulic load is
a useful operating parameter and is calculated as follows:
Hydraulic load, gpd/sq ft Example 21.2.
Total flow into plant, gpd
Surface area per shaft, sq ft number
(21.5)
Calculate the organic and hydraulic loads of an RBC system.
Given:
Q
Primary effluent BOD5 (TOL)
Primary effluent soluble
BOD5 (SOL)
Total number of RBC shafts
Solution:
Total system
3 mgd,
120 mg/L,
80 mg/L, and
8 at 100 000 sq ft/shaft. Four of the eight
shafts are in the first stage (four trains of two
shafts each).
100 000 sq ft 8 shafts
Surface area
800 000 sq ft
First stage
100 000 sq ft 4 shafts
Surface area
400 000 sq ft
TOL
(120 mg/L)(3 mgd)(8.34)
3002 lb BOD5/d
SOL
(80 mg/L)(3 mgd)(8.34)
2002 lb SBOD5/d
System TOL
3002 lb BOD 5 /d
800 unitsa
3.75 lb BOD5/d/1000 sq ft
2002 lb SBOD 5 /d
800 units
2.5 lb SBOD5/d/1000 sq ft
System SOLb
First-stage TOL
3002 lb SBOD 5 /d
400 units
21-37
21-38
First-stage SOLb
2002 lb BOD 5 /d
400 units
System hydraulic load
(3 mgd)(1 10 6 gpd/mgd)
800 000 sq ft
3.75 gpd/sq ft
(Notes: aEach unit is equal to 1000 sq ft. bFor system SOL and first-stage SOL, organic
loading exceeds the acceptable limits often used to gauge the ability to operate without
problems. System upgrading may be required to improve performance or prevent adverse effects.
DESCRIPTION OF EQUIPMENT. Rotating biological contactor systems typically include the following six equipment items (many also include instrumentation):
•
•
•
•
•
•
Tankage,
Baffles,
Filter media,
Cover,
Drive assembly, and
Inlet and outlet piping.
Figures 21.8 and 21.9 illustrate various equipment components that are typically used in
the RBC process. The names for individual equipment components may differ slightly,
depending on the manufacturer. The purpose of each part is described in Table 21.6
and discussed further in the following sections.
Tankage. Containment structures or tanks for RBC equipment may consist of metal
tanks for small pilot plants or single-shaft units. However, multishaft units almost always include tankage made of concrete basins (Figure 21.8). The tank volume typically
provides approximately 1 hour of hydraulic contact time; this typically corresponds to
4.9 L tank volume/m2 (0.12 gal/sq ft) of standard-density filter media.
Baffles. Internal baffling or weir structures separate the stages of the RBC reactors.
Baffling along one shaft is accomplished by removing a section of discs and replacing
it with a stationary bulkhead (Figure 21.9). Baffling used to separate multiple shafts
may be made of either concrete or wood. Removable baffles are often used to allow
process changes after the facility is constructed.
FIGURE 21.8
Air-driven rotating biological contactor.
Filter Media. The media used for RBCs is composed of high-density polyethylene.
Manufacturers vary the thickness and shape of the material to conform to their own
standards. Similar variations occur in the shapes and sizes of the shafts and structural
frames used to support individual discs.
Two broad categories of RBC media exist: standard-density media (Figure 21.10) and
high-density media (closer disc spacing). Standard-density media have approximately
21-39
21-40
FIGURE 21.9
Mechanically driven rotating biological contactor.
9300 m2 (100 000 sq ft) of surface area/shaft. High-density media are typically used in
the later or nitrification stages of RBCs and may have between 11 200 and 16 700 m2
(120 000 and 180 000 sq ft) of surface area/shaft (Gross et al., 1984).
Covers. Covers or enclosures are used with RBCs to:
•
•
•
•
•
Protect biological slimes from freezing;
Prevent rain from washing off slime growth;
Prevent media exposure to sunlight, which results in algae growth;
Protect the media from UV rays, which can weaken them; and
Provide protection from the elements.
Covers are often made of fiberglass or other reinforced resin plastics. Another approach involves housing a number of shafts in a building. In either case, the RBC enclosure must have ventilation, humidity and condensation control, and heat loss
provisions.
Rotating Biological Contactor Drives. The discs can be rotated by either mechanical
or air drive units. Both types of drives have bearings to support the RBC shafts. Every
TABLE 21.6
Parts of a rotating biological contactor.
Part
Purpose
Concrete or steel tank divided
into bays (sections) by baffles
(bulkheads)
Tank: holds the wastewater and allows it to come in
contact with the organisms on the discs
Bays and baffles: prevent short-circuiting of wastewater
Orifice or weir in baffle
Controls flow from one stage to the next or from one bay
to the next
Rotating media
Provides support for organisms; rotation provides food
(from wastewater being treated) and air for organisms
Cover over contactor
Protects organisms from severe fluctuations in the
weather, especially freezing; also helps contain odors
Drive assembly
Rotates the media
Influent lines with valves
Influent lines: transport wastewater to the RBC*
Influent valves: regulate influent to RBC and isolate
RBC for maintenance
Effluent lines with valves
Effluent lines: convey treated wastewater from the RBC
to the secondary clarifier
Effluent valves: regulate effluent from the RBC and isolate
the RBC for maintenance
Underdrains
Allow for removal of solids that may settle out in the tank
*Rotating biological contactor.
RBC shaft has at least one bearing designed to accommodate thermal expansion as the
shaft heats and cools; most shafts have one expansion and one non-expansion bearing.
Mechanical-drive RBCs use a chain and sprocket assembly (Figure 21.9) to rotate
the shaft. The motors, typically rated at 3730 to 5590 W (5 to 7.5 hp)/shaft, may be
equipped to allow changing shims or sprocket sizes and installation of an electronic
speed controller to vary rotational speed.
Air-driven RBC units have a blower and air diffuser at the bottom of each RBC
shaft (Figure 21.8; California State University, 1988). Air cups pinned to the edge of the
plastic disc trap air bubbles released from the air header. As the air bubbles rise, they
cause the RBC shaft to rotate.
The advantages of air-driven units are that less torque is applied to the shaft,
the biomass tends to be thinner (sheared by the air), and the wastewater may contain slightly more dissolved oxygen. In some cases, diffused air has been used in
mechanical-drive RBCs to reduce solids accumulation at the tank bottom and miniCopyright © 2007 Water Environment Federation.
21-41
21-42
FIGURE 21.10
Standard-density rotating biological contactor media.
mize localized anaerobic conditions. The air for RBCs is typically not captured for
reuse because of its low pressure [17.2 to 20.7 kPa (2.5 to 3 psi)].
Positive-displacement or centrifugal blowers may be used; however, centrifugal
blowers are easier to control by adjusting the throttle on the blower suction valve.
Positive-displacement blowers must be equipped with a variable-speed drive or conCopyright © 2007 Water Environment Federation.
tinuously vented to the atmosphere. A major problem with air-drive units has been
loping or unbalanced rotation. In extreme cases, rotation has stopped, and mechanical
rotation, cleaning, or the use of high air rates has been necessary to re-establish rotation. Other disadvantages of air-drive units are higher power use and the need to balance and adjust the air flow to the diffusers.
Influent and Effluent Lines and Valves. The piping and valves associated with the
RBC process typically do not affect performance significantly. If channels are used to
transfer water, the hydraulic velocities should be adequate to maintain solids in suspension. Another approach is to aerate the channels to prevent solids deposition.
Because the flow velocities are typically low, the flow can typically be distributed via
weirs, slide gates, or other conventional structures. A flow-splitting structure is best for
distributing wastewater to a number of trains.
Instrumentation. Electronic or hydraulic load cells are useful to periodically measure
total shaft weight. Some shafts have a load cell device installed under the shaft support
bearing on the idle end of the shaft. Such a cell has a hand-operated hydraulic pump to
lift the bearing from its base and generate a hydraulic pressure that can be converted
to shaft weight. Load cell information is used to judge the condition of biological growth
and the weight of the biomass. Electronic speed indicators and remotely activated air
valves have been provided at air-drive installations to facilitate maintenance and
adjustment of rotational speed.
PROCESS CONTROL. The most important element of process control is daily
shaft inspection by a trained operator. Principal observations typically include the biomass condition in each stage and the dissolved oxygen levels exiting the individual
stages.
Observations about the first-stage biomass are typically the most critical. A healthy
first-stage biomass is uniformly brown and distributed in a thin, even layer. A heavy,
shaggy biomass in the first stage indicates an organic overload, which can be caused by
an insufficient number of RBCs, industrial waste, or the effect of sidestreams (e.g., digester supernatant).
A U.S. Environmental Protection Agency (U.S. EPA) study (Chesner and Iannone,
1968) reported difficulties with the initial stages of RBC systems indicated by heavy
biofilm growth, the presence of nuisance organisms (e.g., Beggiatoa), and a reduction in
BOD5 removal rates. These problems have been attributed to excessive organic loading
rates that result in low dissolved oxygen levels, which subsequently lead to Beggiatoa
growth and deteriorating process efficiency. Beggiatoa—whitish autotrophic sulfur
bacteria—use hydrogen sulfide and sulfur as energy sources in the presence of oxygen.
21-43
21-44
Beggiatoa organisms compete with heterotrophic organisms for oxygen and space
on RBC media surfaces. Their predominance can increase the biomass concentration on
the media, while substantially reducing BOD5 removal per unit area. The study suggested that, whenever the first stage load exceeded 20 to 40 g/m2d (6 to 8 lb BOD5/d/
1000 sq ft), the media surface was associated with Beggiatoa or sulfide-oxidizing organisms. This loading should correspond approximately to an SBOD5 loading in the range
of 12 to 20 g/m2d (2.5 to 4.0 lb/d/1000 sq ft).
If white or gray splotches develop on the disc surfaces, then Beggiatoa or Thiothrix
bacteria are developing in large numbers and are a nuisance. Both Thiothrix and Beggiatoa are filamentous-type organisms that reduce the RBC’s removal rate capability
and cause poor settling sludge in the secondary clarifier. These organisms typically develop in the presence of high concentrations of hydrogen sulfide (H2S). Sulfides may
result from the following:
•
•
•
•
•
Low oxygen levels caused by an extreme overload of the first stage,
Septic wastes,
Industrial discharges,
Anaerobic deposits on the bottom of the RBC tank, or
Reduced dissolved oxygen levels during warm-weather operations.
A second indicator closely observed in RBC operation is the dissolved oxygen concentration throughout various stages. High organic loading may result in low dissolved
oxygen levels. For carbonaceous BOD5 removal, a minimum dissolved oxygen level
ranging from 0.5 to 1.0 mg/L is needed at the end of the first stage, and at least 2 to
3 mg/L is needed at the end of the last stage of the RBC unit.
Depending on the biomass and dissolved oxygen observations, operators may
need to make process-control changes to the following:
•
•
•
•
•
•
•
Other processes,
Stages and trains,
Supplemental aeration,
Step feeding or an enlarged first stage,
Recirculation,
Rotational speed, or
Secondary clarifier.
Other Processes. Rotating biological contactors depend, to a great extent, on the preceding treatment steps to effectively reduce the solids or BOD levels in high-strength
influent streams that could otherwise result in interference or overload. For example,
efficient grit removal and screening are essential to prevent solids buildup under the
discs. Good primary treatment is required to reduce the organic and hydrogen sulfide loadings to the RBC units. Although all RBCs need periodic cleanout or flushing,
the need for frequent cleaning indicates that either preliminary or primary treatment
units need improvement.
Sidestreams (e.g., supernatants from anaerobic or aerobic digesters and filtrate or
concentrate from dewatering processes, drying beds, or lagoons) may significantly affect both the BOD and ammonia levels entering the RBCs. Therefore, these recycle
streams demand careful monitoring and control to avoid adverse effects on RBC operations (U.S. EPA, 1984).
Staging and Trains. The number of trains and stages is a principal process-control
variable affecting RBC performance. The number of stages is typically increased to improve RBC performance (this assumes no overloading of the first stages). A minimum
of two to three stages is typically necessary to reliably achieve secondary effluent quality when treating domestic wastewater. Three to four stages are needed to achieve effluent BOD5 concentrations of less than 20 mg/L.
Any changes in the number of stages should follow consideration of organic loading. Placing additional trains online may be necessary to reduce the organic load. Typically acceptable loading limits are 1.5 to 2.0 kg BOD5/100 m2d (3 to 4 lb BOD5/d/
1000 sq ft) for the overall shaft loading and 2.0 to 2.9 kg BOD/100 m2d (4 to 6 lb
BOD5/d/1000 sq ft) for the first stage. The RBC system is typically designed and analyzed on an SBOD5 basis. On this basis, acceptable loads are approximately 50% of
those for total BOD5. Pre-aeration before the first stage merits consideration to reduce
the possibility of oxygen deficiencies.
The number of stages and trains may be adjusted to provide proper hydraulic
retention time, typically considered less significant than organic loading. Hydraulic retention time depends directly on the available liquid volume in the RBC tank and the
wastewater flow. Research has indicated that retention as low as 4.9 L of tank volume/m2 of media (0.12 gal/sq ft) does not reduce RBC efficiency (however, lower values lack such tests). Hydraulic loadings for carbonaceous BOD removal are typically
maintained in the range 0.4 to 0.12 m3/m2d (1.0 to 3.0 gpd/sq ft), while nitrification
loading is typically 0.04 to 0.10 m3/m2d (1.0 to 2.5 gpd/sq ft) or even less than 0.04
m3/m2d (1.0 gpd/sq ft).
Supplemental Aeration. The importance of dissolved oxygen in aerobic wastewater
treatment is well-known. An inadequate dissolved oxygen concentration may be a major cause of process failure. In the RBC system, supplemental aeration should be used
21-45
21-46
whenever the BOD5 loadings to the first stage are high or when dissolved oxygen concentrations in the incoming wastewater are low (Surrampalli and Baumann, 1989). The
reported benefits of supplemental aeration include the following:
•
•
•
•
•
•
Elimination of Beggiatoa growth,
Thinner biofilms on the media,
Increased dissolved oxygen levels,
Higher SBOD5 removal rates,
Higher ammonia-nitrogen removal rates, and
Enhanced shaft and media life.
The increased removal rates with supplemental aeration are attributed to higher dissolved oxygen concentrations and thinner, Beggiatoa-free biomass growth that enhances
mass diffusion of substrate and oxygen into the inner layers of the active biomass.
Step Feeding or Enlarged First Stage. It is desirable to operate RBC plants with an enlarged first stage, particularly when the first stage is organically overloaded (Surampalli and Baumann, 1993). Step feeding, or step aeration, as it is called when applied to
activated sludge processes, is used extensively in activated sludge plants to improve
the oxygen demand situation at the head end of treatment systems that would otherwise be organically overloaded. To avoid a high oxygen demand at the beginning of an
activated sludge aeration tank, the incoming wastewater is distributed along the aeration tank at several locations to result in a more even oxygen demand throughout the
tank. Similarly, an enlarged first stage can be used effectively to avoid overload and
to attenuate variations in wastewater characteristics, thereby eliminating oxygen-limiting conditions and the development of nuisance organisms. Fortunately, an enlarged
first stage can be created simply by removing the baffle between the first and second
stages of RBC systems.
Recirculation. Recirculating RBC-treated effluent (either before or after the secondary
clarifier) does not significantly improve the treatment efficiency. Nonetheless, under
certain conditions (e.g., during startup or when high industrial wastes are present), recirculation may avoid overloading and thus lead to more reliable RBC performance.
Recirculation may also be advantageous where large hydraulic fluctuations occur (e.g.,
flow changes from industrial parks or schools). Recirculation via holding or thickening
tanks should be avoided, however, because the sludge could produce high sulfide concentrations in the recycle flow that would stimulate the growth of nuisance microorganisms.
Rotational Speed. The equipment manufacturer’s recommendations should govern
the selection of rotational speed for the RBC discs. Typical rotational speeds range from
1.0 to 1.6 rpm.
While less energy is required to rotate RBCs at slower speeds, the slower speeds
will also reduce the oxygen-transfer capability. Some facilities have observed increased
efficiency by rotating countercurrent to influent flow (when perpendicular to the shaft),
while others have found no change, regardless of the direction of rotation.
Secondary Clarifier. Clarifier operation with the RBC process resembles that already
described for trickling filters. Use of the secondary clarifier as a principal means of
thickening should be avoided to minimize denitrification or solids carryover with the
treated effluent.
Sloughing of the discs may result from toxic loads, temperature changes, or normal
hydraulic shear action. Daily monitoring the clarifier sludge blanket and accounting
for the amount of solids pumped [in kilograms per day (pounds per day)] will help operators identify potential solids buildup or other problems.
Nitrification. Rotating biological contactors are also used to nitrify secondary effluent.
Secondary treatment systems used ahead of separate-stage RBC nitrification include a
variety of activated sludge and attached-growth systems. Separate-stage RBC nitrification is typically used as an add-on step to existing secondary biological systems that are
required to meet ammonia-nitrogen (NH3-N) effluent limits.
Major variables influencing ammonia nitrification in an RBC system include influent organic nitrogen and ammonia-nitrogen concentrations, dissolved oxygen concentrations, wastewater temperature, pH, alkalinity, and influent flow and load variability.
Staged or plug-flow configurations promote development of nitrifying organisms. Thus,
appropriate staging is necessary for nitrification to take place in RBC systems. The
growth of nitrifiers depends on the SBOD5 concentration present in the stage’s wastewater. Typically, nitrification is observed when the SBOD5 concentration in the stage’s
wastewaters is reduced to 15 mg/L, and maximum nitrification is observed when
SBOD5 declines to 10 mg/L or less.
Wastewater temperature is the major variable controlling full-scale RBC nitrification below 13°C (55°F), becoming increasingly documented when temperatures of 4°C
(40°F) are approached. Wastewater temperatures higher than 13°C (55°F) do not result
in higher nitrification rates in full-scale units, because the oxygen-transfer rate, rather
than the biological growth rate, controls the reaction at these temperatures.
Nitrification is more sensitive to dissolved oxygen concentration than heterotrophic
carbonaceous removal systems. A minimum desired dissolved oxygen level of 2 mg/L
21-47
21-48
is often quoted. High effluent dissolved oxygen concentrations combined with low levels
of SBOD5 can lead to deterioration of nitrification rates via the proliferation of higher
life forms that ingest nitrifying microorganisms. To discourage selective predation of nitrifying bacteria, it is suggested that dissolved oxygen concentrations of no more than
3.5 mg/L and SBOD5 concentrations of 6 to 8 mg/L be maintained in the polishing
stages of RBC nitrification trains.
Nitrification is an acid-producing biochemical reaction. Approximately 7.1 mg
of calcium carbonate alkalinity is theoretically consumed per milligram of ammonianitrogen oxidized. Depending on initial alkalinity and unoxidized nitrogen concentrations, the nitrification process could reduce wastewater alkalinity until the pH drops to
6.5 and even to 6.0 or less. Increased nitrification is observed when the pH is between
7.0 and 8.5, with nitrification efficiency falling off dramatically as the pH decreases
from 7.0 to 6.0.
TROUBLESHOOTING. When properly designed and operated, RBCs can provide trouble-free secondary treatment. However, some RBC plants constructed during
the late 1970s and early 1980s required significant troubleshooting or plant modifications to achieve the desired treatment level without operating problems.
Troubleshooting operational problems begins with obtaining good records and
data associated with the RBC process. Tracking total and soluble BOD5, suspended
solids, organic nitrogen, ammonia-nitrogen, pH, alkalinity, dissolved oxygen, and
other parameters is necessary to recognize trends that may have an adverse effect on
the RBC system. The sampling frequency may have to be increased to ensure representative data.
Equipment failures (e.g., broken shafts or failed filter media) were common occurrences on many of the early RBC installations. Most of these problems were resolved
through equipment warranty or performance specifications. Many of the problems associated with the early designs have been mitigated by using more conservative design
practices, improving equipment design and manufacturing practices, shaft weighing
devices, and using supplemental aeration to improve biomass uniformity. Table 21.7 is
a troubleshooting guide for other problems associated with the design, operation, and
maintenance of RBCs.
PLANNED MAINTENANCE. Like any treatment process, the RBC system demands routine attention, or operations and maintenance problems will occur. Chain
drives, belts, sprockets, rotating shafts, and other moving parts need inspection and
maintenance according to the manufacturers’ instructions or with guidance from the
TABLE 21.7
21-49
Troubleshooting guide for rotating biological contactors (RBCs) (continued on next page).
Indicators/observations
Decreased treatment
efficiency
Probable cause
Check or monitor
Organic overload
Check peak organic loads; Improve pretreatment or expand
if less than twice the daily plant
average, should not be the
cause
Hydraulic
equalization
Check peak hydraulic
Flow equalization; eliminate
loads; if less than twice the source of excessive flow; balance
daily average, should not flows between reactors
be the cause
pH too high or too
low
Desired range is 6.5 to 8.5
for secondary treatment,
8 to 8.5 for nitrification
Low wastewater
temperatures
Temperature less than 15 °C If available, place more
(59 °F) will reduce efficiency treatment units in service
Snails in nitrification Evaluate nitrification rate
units
Excessive sloughing of
biomass from discs
Development of white
biomass over most of
disc area
Solids accumulating in
reactors
Solutions
Eliminate source of undesirable
pH or add an acid or base to
adjust pH; when nitrifying,
maintain alkalinity at seven
times the influent ammonia
concentrations
Periodically remove unit from
service and add caustic to clean
media
Toxic materials in
influent
Determine material and its Eliminate toxic material if
source
possible; if not, use flow
equalization to reduce variations
in concentration so biomass can
acclimate
Excessive pH
variations
pH below 5 or above 10
can cause sloughing
Eliminate source of pH
variations or maintain control
of influent pH
Septic influent or
high hydrogen
sulfide
concentrations
Influent odor
Pre-aerate wastewater or add
sodium nitrate, hydrogen
peroxide, or ferrous sulfate;
supplemental aeration may also
help, especially in the first
stages
First stage is
organically
overloaded
Organic loading on first
stage
Adjust baffles between first and
second stages to increase
fraction of total surface area in
first stage
Inadequate
pretreatment
Determine if solids are grit Remove solids from reactors and
or organic
provide better grit removal or
primary settling
21-50
TABLE 21.7
Troubleshooting guide for rotating biological contactors (RBCs).
Indicators/observations
Probable cause
Check or monitor
Solutions
Shaft bearings running
hot or failing
Inadequate
maintenance
Maintenance schedules
and practices
Lubricate bearings as per
manufacturer’s instructions
Motors running hot
Inadequate
maintenance
Oil level in speed reducer
and chain drive
Lubricate as per manufacturer’s
instructions
Improper chain
drive alignment
Alignment
Align properly
Wastewater
environmental
conditions prone to
snail growth
Biomass growth or snail
accumulation in tanks
Periodic chemical cleaning
Low organic
loading
Low organic load
Rearrange loading to RBC units
to increase organic load
Snail growth
Periodically increase RBC speed
design engineer. Although these requirements will vary among plants, the RBC maintenance guide given in Table 21.8 provides many of the typically required maintenance
procedures for RBCs.
Routine maintenance should include the inspection of shafts and replacement of
broken air cups or media that might otherwise jam or interfere with shaft rotation.
Housekeeping should include the removal of grease balls via a net device. The manufacturer may provide advice on making field repairs to media that become separated.
Unbonded surfaces may sometimes be repaired by melting the plastic with a heated
metal rod or other manufacturer-recommended product.
Mechanical Drive Systems. Shaft bearings should be inaudible above the splashing.
A screwdriver or metal rod can be used to transmit bearing noise to the operator’s ear.
Vibration meters can also sense noise (vibrations). Drive motors, which need daily
inspection, typically should run cool enough to touch with a bare hand [less than 60 °C
(140 °F)]. If motor amperage readings are recorded, they should be taken and logged at
least semiannually. During daily observations of the belt drive, a squealing noise is the
first indication that a problem has occurred. Because belts are often sold as a set, the
whole set should be replaced with identical belts from the same manufacturer.
Air-Drive Systems. Air-drive systems require more careful monitoring and attention
than mechanical drive units, because shaft speed and balance must be maintained via
TABLE 21.8
Rotating biological contactor maintenance guide.
Interval
Procedure
Daily
Check for hot shaft and bearings. Replace bearings if temperature exceeds
93 °C (200 °F).
Daily
Listen for unusual noises in shaft and bearing. Identify cause of noise and
correct if necessary.
Weekly
Grease the mainshaft bearings and drive bearings. Use manufacturer’s
recommended lubricants. Add grease slowly while shaft rotates. When grease
begins to ooze from the housing, the bearings contain the correct amount of
grease. Add six full strokes where bearings cannot be seen.
4 weeks
Inspect all chain drives.
4 weeks
Inspect main shaft bearings and drive bearings.
4 weeks
Apply a generous coating of general-purpose grease to main shaft sub ends,
main shaft bearings, and end collars.
3 months
Change oil in chain casing. Use manufacturer’s recommended lubricants. Be
sure oil level is at or above the mark on the dipstick.
3 months
Inspect belt drive.
6 months
Change oil in speed reducer. Use manufacturer’s recommended lubricants.
6 months
Clean magnetic drain plug in speed reducer.
6 months
Purge the grease in the double-sealed shaft seals of the speed reducer by
removing the plug located 180° from the grease fitting on both the input and
output seals’ cages. Pump grease into the seal cages and then replace the plug.
Use manufacturer’s recommended grease.
12 months
Grease motor bearings. Use manufacturer’s recommended grease and
recommendations for lubrication. To grease motor bearings, stop motor and
remove drain plugs. Inject new grease with pressure gun until all old grease
has been forced out of the bearing through the grease drain. Run motor until
all excess grease has been expelled. This may require up to several hours’
running time for some motors. Replace drain plugs.
indirect air lift. Shaft speed should be checked daily and compared with manufacturer
recommendations. Shaft balance also requires periodic checks to ensure that excess
biomass has not built up on one side of the discs. This check involves timing quarter
turns of the RBC shaft. If the shaft becomes badly out of balance, correcting the problem requires stopping or isolating the shaft, draining the tank, or otherwise chemically
stripping the biomass.
Periodically increasing the air capacity to 150% of the normal volume may help
control biomass growth. Purging often strips excess biomass that would otherwise
21-51
21-52
cause imbalance. Daily monitoring of blower oil pressure will help indicate the possible presence of clogged diffusers or other interferences.
COMBINED PROCESSES
Treatment is typically thought of as occurring in sequential major steps—preliminary,
primary, secondary, and tertiary. However, the major secondary treatment step sometimes combines use of fixed-film reactors (e.g., trickling filters and RBCs) in series or in
conjunction with other forms of biological treatment (e.g., activated sludge). These
combined processes are often referred to by a number of terms, such as step systems,
two-stage, series, dual, coupled, or combined processes. This section covers the operation
and maintenance of what, for lack of a better term, will be called combined processes—the
coupling of fixed-film and suspended growth processes.
In the mid- to late 1970s, filter media improvements included the development
of high-rate media (Figure 21.2), as described earlier in this chapter. The first applications of high-rate media were in roughing filters used primarily by industry to accommodate high loadings (Harrison and Daigger, 1987). The new media allowed
trickling filters to be organically loaded 10 to 15 times higher than rock media loadings without odor or plugging problems. It soon became evident that biological treatment could often be accomplished with a combination of highly loaded trickling filters followed by activated sludge. Advantages and disadvantages of the parent
trickling filter and activated sludge processes are given in Table 21.9. Combining the
processes has often coupled the simplicity, shock resistance, and low maintenance of
the trickling filter with the improved effluent quality or increased nitrification of the
second-stage activated sludge or contact basin. Figure 21.11 compares the reliability
of combined, activated sludge, and trickling filter processes in achieving good effluent quality.
ALTERNATIVES. Numerous combinations of processes are possible, depending on
the trickling filter and activated sludge processes used, the loading of individual units,
and the point at which sludge or other recycled streams are reintroduced to the main
flow stream. The most common combined processes (and their typical acronym) are
listed in Table 21.10.
Process schematics for each combined process are illustrated in Figure 21.12. Table
21.11 presents loading criteria typically considered appropriate for both the component processes and the combined process. These criteria are not absolute values, because site-specific conditions may cause the loading balance to vary. A brief description of each combined process follows.
TABLE 21.9 Advantages and disadvantages of trickling filter and activated
sludge processes.
Advantages
Disadvantages
Trickling filter systems
Simplicity
Thick secondary sludge
Low operating costs
Shock resistance
Low maintenance
Little power required
Activated sludge systems
Increased operational flexibility
Lowest initial cost
Less land area required
Reduced odor
Nitrification control
Higher initial cost
More land area required
Odor problems
Temperature sensitivity
Poor response to operational changes
Complexity
Greater sludge volume
Sensitive to shock loads
High power requirements
Greater operating costs
Activated Biofilter. The activated biofilter process (Figure 21.12) uses a lightly
loaded trickling filter with high-rate media. Biological or activated solids are recycled
from the bottom of the secondary clarifier and returned to the trickling filter (hence,
the term activated biofilter or biological filter). Many consider that recycled solids improve settleability, similar to that in a selector-activated-sludge process. An initial
high food-to-microorganism (F⬊M) ratio and high dissolved oxygen concentration
are sometimes attributed to the ABF process’ ability to improve the settling characteristics of secondary solids.
While performing well at low organic loads, the ABF process proved to be unable
to consistently achieve good effluent quality as organic loads approach 1.6 kg BOD5/
m3d (100 lb BOD5/d/1000 cu ft). The ABF process without short-term aeration also
proved to be susceptible to poor performance in cold climates. To overcome these problems, the ABF tower was later modified to include a relatively small aeration basin,
described in the Biofilter Activated Sludge section of this chapter (Harrison, 1980).
Trickling Filter–Solids Contact. The trickling filter–solids contact process (Figure
21.12) typically uses a moderately to highly loaded trickling filter, followed by a small
contact channel only 8 to 17% of the size typically required by a traditional activated
sludge process. By combining the trickling filter with the contact channel, the filter size
is typically reduced to 50% or less of that required by a traditional trickling filter process (Harrison and Timpany, 1988).
21-53
21-54
FIGURE 21.11 Comparison of the effluent quality provided by rock trickling filters,
activated sludge, and dual treatment systems.
The trickling filter–solids contact process results in low power requirements because the trickling filter removes most of the SBOD. Other benefits include the ability
to easily upgrade existing rock trickling filters by polishing their effluent when return
activated sludge (RAS) is used as a bioflocculating agent (Timpany and Harrison, 1989).
TABLE 21.10
Names and acronyms for combined processes (Harrison et al., 1984).
Name
Acronym
Activated biofilter
Trickling filter–solids contact
Roughing filter–activated sludge
Biofilter–activated sludge
Trickling filter–activated sludge
ABF
TF/SC
RF/AS
BF/AS
TF/AS
FIGURE 21.12 Combined system process schematics: (a) activated biofilter (ABF),
(b) trickling filter–solids contact (TF/SC) and roughing filter–activated sludge (RF/AS),
(c) biofilter–activated sludge (BF/AS), and (d) trickling filter–activated sludge (TF/AS).
21-55
21-56
General design criteria for combined processes (continued on next page).
TABLE 21.11
Appropriate design criteria
Item
Range
Typically used
Component processes
Activated sludge
F⬊M ratioa
MLSSb
Retention time
Mean cell residence timec
RAS
0.2–0.4
0.3
1500–4000
2500
4.0–8.0
6.0
5–15
10
6200–12000
8000
0.8–1.8
0.6–1.4
1.6
1.0
Rock or high rate
High rate
Basin oxygend
Totally available
Typically on
Trickling filter
Media type
e
BOD loading
Hydraulic loadingf
5–40
20
0.02–1.0
0.8
High rate
High rate
10–75
30
Combined processes
ABF
Media type
BOD loading
Hydraulic loading
Filter MLSS
0.8–5.0
2.0
1500–3000
2000
Rock or high rate
High rate
20–75
40
TF/SC
Media type
BOD loading
Hydraulic loading
0.1–2.0
1.0
1500–3000
2000
Hydraulic residence timeg
0.5–2.0
1.0
Mean cell residence time
0.5–2.0
1.0
6000–12000
8000
2000–4000
3000
60–130
100
Channel MLSS
RAS
Minimum channel mixing
Diffused air (sq ft/mil. gal)
Mechanical (hp/mil. gal)
TABLE 21.11
General design criteria for combined processes.
Appropriate design criteria
Item
Range
Typically used
High rate
High rate
BOD loading
100–300
150
Hydraulic loading
0.8–5.0
2.0
1500–4000
2500
0.5–4.0
2.0
RF/AS, TF/AS, and BF/AS
Media type
Basin MLSS
Basin retention time
Mean cell residence time
1.0–7.0
3.0
F⬊M ratio
0.5–1.2
0.9
0.6–1.2
0.3–0.9
0.9
0.6
Basin oxygen
Total available
Typically on
F⬊M ratio lb primary effluent BOD/d/lb MLVSS (lb/lb 1 000 g/kg).
MLSS mg/L.
c
Mean cell residence time (MCRT) days (aeration basin only).
d
Basin oxygen lb O2/lb primary effluent BOD.
e
Biochemical oxygen demand (BOD) loading lb BOD/d/1000 cu ft for trickling filter (lb/d/1000 cu ft 1.602 102 kg/m3d).
f
Hydraulic loading gpm/sq ft for trickling filter [gpm/sq ft (6.791 104) m/ms].
g
Hydraulic residence time hours based on influent Q only (no RAS).
a
b
Roughing Filter–Activated Sludge. A common method of upgrading existing activated sludge plants is to install a roughing filter ahead of the activated sludge process
(Figure 21.12). This process is also used in situations where a plant receives a waste high
in SBOD. The roughing filter is typically one-fifth to one-eighth the size required if treatment were accomplished with the trickling filter process alone. Hydraulic retention
time in the aeration basin is typically 30 to 50% of that required with activated sludge
alone.
Both the TF/SC and roughing filter–activated sludge (RF/AS) process have the
same process schematic (Figure 21.12), but the RF/AS uses a much smaller trickling filter than the TF/SC, so the former depends more on aeration to provide oxygen, remove BOD, and digest solids. This differs from the TF/SC process, where the trickling
filter provides almost all wastewater treatment, and the contact channel only enhances
solids flocculation and effluent clarity. Differences in capital costs, often influenced by
the availability of existing units, often determine the choice between the TF/SC or
RF/AS process.
21-57
21-58
Biofilter–Activated Sludge. The biofilter–activated sludge (BF/AS) process (Figure
21.12) resembles the RF/AS process, except that its return activated sludge is recycled
over the trickling filter in a way similar to the ABF process. Incorporating RAS recycle
over the trickling filter sometimes reduces sludge bulking from filamentous bacteria,
especially when treating difficult food processing wastes. However, there is no evidence that sludge recycle improves the trickling filter’s oxygen-transfer capability
(Harrison, 1980).
Recent evidence indicates that adding aerated RAS can help reduce odors from
trickling filters. It is believed that, by providing a bacteria population to the filter influent, sulfides are metabolized and therefore not released as odorous compounds (Joyce
et al., 1995).
Trickling Filter–Activated Sludge. The trickling filter–activated sludge (TF/AS)
process (Figure 21.12) is loaded in a manner similar to that of either the RF/AS or
BF/AS. However, the TF/AS process includes an intermediate clarifier between the
trickling filter and the aeration basin. The intermediate clarifier removes sloughed
solids from the trickling filter underflow before it enters the aeration basin.
A major benefit of using the TF/AS mode is that solids generated from carbonaceous BOD removal can be separated from the second-stage treatment. This two-stage
approach is often best where nitrogen oxidation (nitrification) is required, and the second stage of the process is designed to be dominated by nitrifying microorganisms.
Another advantage of intermediate clarification is the reduced effects of trickling filter
sloughing on the activated sludge portion of the process. However, researchers have
not shown clear evidence of reduced oxygen requirements or improved settleability in
intermediate clarification. Therefore, most high-rate or roughing filters in combined
processes are designed as RF/AS or BF/AS to eliminate the cost of intermediate clarification, unless nitrification is required.
DESCRIPTION OF PROCESSES. Combined processes consist of a two-stage or
two-step method of removing pollutants. The first-stage filter (fixed-film) supplies 30
to 50% of the oxygen requirements for total biological treatment with RF/AS, BF/AS,
or TF/AS. Essentially all of the oxygen for biological reactions is supplied in the filter
with either the TF/SC or ABF processes.
Biological solids are produced from the use of soluble organic material as food matter. These biological solids attach to the fixed-film filter media until hydraulic shear, excessive growth, or other conditions induce sloughing of the biomass. The type of filter media
used, hydraulic loading rate to the filter, and the organic loading all result in variations
in sloughing frequencies and in the characteristics and mass of the sloughed solids.
Biological activity in the aeration basin after the fixed-film reactor allows additional contact time for suspended-growth bacteria to synthesize any remaining SBOD.
In addition, the bacteria undergo endogenous respiration (aerobic digestion of cell
matter). Basins with longer contact times (1 to 3 hours) provide greater opportunities
for stabilization of biological solids and removal of additional SBOD than do smaller
basins (e.g., contact channels). A reaeration basin (Figure 21.12) provides another
method of allowing time for solids digestion. Because solids from the clarifier underflow to be reaerated are much thicker than the aeration basin mixed-liquor solids, less
volume is required to digest solids in a reaeration basin than in an aeration basin.
The mixture of digested biological solids and treated effluent undergoes separation in the final clarifier. A mixed liquor from combined processes, similar to that of activated sludge (although perhaps less susceptible to upset), requires good separation of
treated effluent and solids to achieve high effluent quality.
DESCRIPTION OF EQUIPMENT. The basic combined processes (Figure 21.12)
use several common reactors and equipment items, regardless of the type of process
mode. Common equipment items are listed below (Figure 21.13).
•
•
•
•
Trickling or biotower (fixed-film reactor),
Filter pumping station,
Contact channel or aeration basin (suspended-growth reactor), and
Clarifier (intermediate or final).
Trickling Filter or Biotower. The trickling filter structure for rock media can be incorporated into any combined process. However, many combined processes are loaded at
rates that typically favor the use of high-rate synthetic filter media, which are less susceptible to plugging, odors, or other problems. This is particularly true if treatment of
high-strength wastes is the reason for combining processes. Accordingly, most combined systems use biotowers rather than the trickling filter structures associated with
rock media.
Filter Pumping Station. The filter pumping stations used with combined systems are
similar to those described in the Trickling Filters and Biotowers section of this chapter.
Contact Channel or Aeration Basin. The suspended-growth reactor may consist of
a relatively large aeration basin, contact channel, or re-aeration structure. If an aeration basin is used, it is smaller than the aeration basin in an activated sludge
21-59
21-60
FIGURE 21.13 Combined system components: biotower (top), contact channel (bottom),
and clarifier (right).
process, but its features are similar (see Chapter 20). Contact channels resemble aeration basins, except that most contact channels have a length-to-width ratio of approximately 5⬊1 for plug flow. Water depth is not as critical in a solids contact unit,
because oxygen transfer is typically not limiting. Re-aeration basins are typically designed with sizes similar to those of the contact channel to provide a redundant basin
and allow interchange of the contact and reaeration units, depending on actual operating needs.
Aeration Equipment. The description of aeration equipment in Chapter 20 adequately
represents the variety of types used in combined processes. Current practices tend to
favor the use of fine-bubble diffusion or low-energy aeration to minimize floc shear, esCopyright © 2007 Water Environment Federation.
pecially preceding the secondary clarifier. A wide variety of aeration equipment is
used, depending on the specific situation.
Clarifier. The final clarifier for combined processes is similar to that of the activated
sludge process. Although some proponents of combined processes claim more stable
or better settling sludge results, the consensus indicates no basis for departing from the
types of clarification equipment used for a corresponding activated sludge plant (see
Chapter 20).
PROCESS CONTROL. Combining fixed-film and suspended-growth processes
greatly increases the operator’s ability to make process-control changes. By varying the
arrangement or number of biological reactors online, combined systems can often be
broadly modified to operate similar to either an activated sludge process or a trickling
filter. If the combined process has an unequal balance in treatment unit sizes (i.e., constructed with large trickling filters and small basins or vice versa), the possibility of
making broad process changes is reduced.
Process Changes. Combined processes can often be designed or modified to operate in
several modes (Figure 21.12). Often, only minor piping and valve position changes are
required to provide the opportunity to make complete process mode changes. For example, combined plants are typically designed to operate in either the RF/AS or BF/AS
modes. Likewise, an existing activated sludge plant upgraded with a roughing filter
ahead of the aeration basin is often designed to operate as a BF/AS, RF/AS, TF/SC, or
as only an activated sludge process.
Biotower. Process control for the biotower is similar to that already described in the
Trickling Filters and Biotowers section of this chapter. However, operating a combined
system allows operators to bypass or flow split, as necessary, a certain portion of the
primary effluent direct to the downstream activated sludge or contact channel. Combined process modes also provide a mixed liquor that can be recycled over many types
of filter media (except rock or high-density media), should the biofilter mode be
needed to minimize filamentous growth or other problems.
Contact Channel or Aeration Basin. The three methods typically used to control the
amount of mixed liquor in the suspended-growth basin or channel are constant mixed
liquor, F⬊M ratio, and solids retention time (SRT). Calculations for these three parameters are presented in Chapter 20, and the Appendix contains sample calculations. However, the biomass associated with a trickling filter is not included in either the F⬊M ratio or SRT calculations (i.e., only the aeration basin solids are used to calculate the
biomass). Also, the food (F) is based on the primary effluent (ignoring removal via the
21-61
21-62
trickling filter), because filter underflow entering the suspended-growth system contains sloughed solids similar to those of RAS returning to an aeration basin. The filter
underflow food would impart an internal BOD load if included in the calculations.
Some operators also find it helpful to determine the SBOD loading to the aeration basin
as a control parameter.
Operators of activated sludge processes directly following trickling filters may experience swings in mixed liquor concentrations from sloughing. This may make tight
control via the constant mixed-liquor method difficult. Some plants experience higher
RAS concentrations from the final clarifiers following sloughing periods. In plants that
waste sludge from the clarifier underflow (as opposed to the aeration basin), this will
tend to return the mixed-liquor concentration to acceptable levels.
Typical SRT values for plants operating with activated sludge alone are typically
not applicable to combined processes. As mentioned above, the portion of the residence time in the trickling filter is not accounted for in the SRT calculation. The SRTs
for activated sludge can often be significantly lower than textbook values, when the
process is proceeded by an attached growth process.
Clarification. Process control of the secondary clarifier for combined processes resembles that presented in Chapter 17 for activated sludge). Field experiences have
shown that nitrification can be more difficult to control with combined processes
than with either component process. To minimize the effects of denitrification, many
operators maintain less than 0.2 m (0.5 ft) of sludge in the secondary clarifier and
operate at relatively high RAS rates to prevent rising solids or other problems from
denitrification.
The clarity or turbidity of combined process effluent is often less than that from
either component process. This is especially true when the combined process is operated at relatively high loads. Changing process modes or increasing the SRT may be
necessary to improve effluent clarity.
TROUBLESHOOTING. Combined systems historically have had fewer operating problems than either activated sludge or trickling filter processes. However, trouble-free operation demands good operator control and initial plant design. Table 21.12
presents a troubleshooting guide that, with Tables 21.3 and 21.7, will enable the operator to identify common problems and develop solutions. Steps must be taken to correct these problems, or plant efficiency and performance will be adversely affected.
PLANNED MAINTENANCE. In general, combined processes offer greater flexibility for maintenance of individual treatment units. These coupled processes tend to
TABLE 21.12
General troubleshooting guide for combined processes.
Effect/observationa
Problem
Uncontrolled sloughing
Large variations in MLSS concentration
Poor effluent quality
Too many units on line
High energy use
Inability to obtain good bacterial reduction without
excessive chlorine use
Rising sludge
Poor primary clarification
Plugging
Standing water
Odors
Reduced efficiency
Filter flies
Hydraulic overload
High effluent TSS
Nitrification
High effluent TSS
High chlorine demand
Low pH
Nutrient shortage
Filamentous bacteria
Rising sludge
Pass through of soluble BOD
Organic overload
Pass through of soluble BOD
Odors
Low DO
Poor effluent quality
Snails
Snail cases settle in basins and transfer structures
Heavy industrial load
Shock organic loads
Nutrient deficiency
Low pH
Sludge contamination
Odor problems
Corrosion problems
Cold weather
Loss in removal efficiency
Icing problems
Loss of nitrification
Organic underload
High energy use
Nitrification
MLSS mixed liquor suspended solids; TSS total suspended solids; BOD biochemical oxygen demand;
and DO dissolved oxygen.
a
21-63
21-64
be more forgiving when a single treatment unit is removed from service, particularly
during warm weather, when biological activity is higher. The maintenance of combined systems is similar to that of the component processes discussed in the Trickling
Filters and Biotowers section of this chapter and in Chapter 20. These portions of the
manual and Chapter 5 discuss safety considerations.
REFERENCES
Albertson, O. E. (1989) Slow Down That Trickling Filter! Oper. Forum, 6 (1), 15.
Albertson, O. E.; Eckenfelder, W. Jr. (1984) Analysis of Process Factors Affecting Plastic
Media Trickling Filter Performance. Proceedings of the 2nd International Conference on
Fixed-Film Biological Processes, Arlington, Virginia, 1155.
California State University (1988) Operation of Wastewater Treatment Plants, Vol. I; Report prepared for U.S. Environmental Protection Agency, Office Water Programs:
Washington, D.C.
Chesner, W. H.; Iannone, J. J. (1968) Review of Current RBC Performance and Design Procedures. Report prepared for U.S. EPA under contract no. 68-02-2775; U.S. Environmental Protection Agency: Washington, D.C.
Harrison, J. R. (1980) Surveys of Plants Operating Activated Biofilter/Activated Sludge. Paper
presented at the Calif. Water Pollut. Control Assoc.
Harrison, J. R.; Daigger, G. T. (1987) A Comparison of Trickling Filter Media. J. Water
Pollut. Control Fed., 59, 679–685.
Harrison, J. R.; Timpany, P. L. (1988) Design Considerations with the Trickling Filter
Solids Contact Process. Paper presented at Joint Can. Am. Soc. Civ. Eng. Conf. Environ.
Eng., Vancouver, British Columbia, Canada.
Harrison, J. R.; Daigger, G.; Filbert, J. (1984) A Survey of Combined Trickling Filter and
Activated Sludge Processes. J. Water Pollut. Control Fed., 56 (10), 1073–1079.
Joyce, J. J.; Battenfield, T.; Whitney, R. (1995) Biological Oxidation of Hydrogen Sulfide.
Water Environ. Technol., 7 (3), 40–43.
Metcalf and Eddy (1979) Wastewater Engineering: Treatment, Disposal, Reuse; McGrawHill: New York.
Gross, C.; Gilbert, W.; Wheeler, J. (1984) RBCs Reach Maturity. Special Report: Rotating
Biological Contactors. Water Eng. Manage., 131 (6), 28–37.
Surampalli, R. Y.; Baumann, E. R. (1989) Supplemental Aeration Enhances Nitrification
in a Secondary RBC Plant. J. Water Pollut. Control Fed., 61, 200.
Surampalli, R. Y.; Baumann, E. R. (1993) Effectiveness of Supplemental Aeration and
Enlarged First Stage in Improving RBC Performance. Environ. Prog., 12 (1), 24–29.
Tekippe, R. J.; Bender, J. H. (1987) Activated Sludge Clarifiers: Design Requirements
and Research Priorities. J. Water Pollut. Control Fed., 59, 865.
Timpany, P. L.; Harrison, J. R. (1989) Trickling Filter Solids Contact from the Operator’s
Perspective. Proceedings of the 62nd Annual Conference of the Water Pollution Control
Federation, San Francisco, California, Oct. 15–19; Water Pollution Control Federation:
Alexandria, Virginia.
U.S. Environmental Protection Agency (1984) Design Information on Rotating Biological
Contactors, EPA-600/2-84-106; U.S. Environmental Protection Agency: Washington,
D.C.
Water Pollution Control Federation (1988) O&M of Trickling Filters, RBCs, and Related
Processes, Manual of Practice No. OM-10; Water Pollution Control Federation:
Alexandria, Virginia.
Zickefoose, C. S. (1984) Rotating Biological Contactors; Linn-Benton Community College:
Albany, Oregon.
21-65

Trickling Filters, Rotating Biological Contactors

Transcription

Similar documents

Poly-Trace™ Capabilities

MADE IN THE USA - FreshWaterSystems.com

FleetValue ™ Oil Filters

Hydro-Filter™ - Phoenix Truckmounts

PREPERATION AREAS

ecopure® dual stage filter

VALUE Ad Sheet

Greater Cincinnati Water Works Filter Distribution

Airscreen 1000 14 x 24 Brochure

Extend Lube Filter life and reduce operating cost