Document 6503786

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How to Evaluate a
W/Oxidation System
Ultraviolet oxidation is a proven technology for ground and process water treatment.
Learn to identify when to consider it and how much it will cost.
................................. by Robert Notarfonzo and Wayne McPhee .................................
No single technology offers the solution for
every water treatment problem. Although
the conventional technologies of air stripping and activated carbon have proved
robust and usually cost effective, continued
advances in ultraviolet (UV)/oxidation have made it the
U.S. Environmental Protection Agency (EPA) proven technology of choice in an ever increasing number of groundwater and process water applications. The number of fullscale UV/oxidation installations has increased in the last
five years from a handful to more than 150.
Principles of UV/oxidation
In the UV/oxidation process, a high-powered lamp emits
W radiation through a quartz sleeve into the contaminated
water. An oxidizing agent, typically hydrogen peroxide, is
added, which is activated by the UV light to form oxidizing hydroxyl radicals:
H,O, + UV + 2.OH
These radicals indiscriminately destroy the toxic organic
compounds in the water. Depending on the nature of the
organic species, two types of initial attack are possible: it
can abstract a hydrogen atom to form water, as with alkanes
or alcohols, or it can add to the contaminant, as is the case
for olefins or aromatic compounds. The following equation
represents the simplified general oxidation process:
Chlorinated 0,
0,
organic ++ Oxygenated ++ CO,+H,O+Clmolecule .OH intermediates .OH
The attack by hydroxyl radicals, in the presence of oxygen, initiates a cascade of reactions leading to mineralization, such as CO, and H,O. In certain applications,
catalysts, which are photo active and non-toxic, are added
to significantly enhance the system’s performance. A
UV/oxidation system can be designed to treat to any discharge requirement.
W/oxidation’s key advantage is its inherent destructive
nature; contaminated water is detoxified with no requirement for secondary disposal. There is no transfer of contaminants from one medium to another. Furthermore, UV
systems in combination with hydrogen peroxide have no
vapor emissions, hence no air permit is required. The
equipment is quiet, compact and unobtrusive, and preventive maintenance and operating requirements are low in a
carefully designed system.
A typical system
In a typical UV/oxidation system, reagents are injected and
mixed using metering pumps and an in-line static mixer.
The contaminated water then flows sequentially through
one or more UV reactors, where treatment occurs.
Pretreatment, such as solids removal, pH adjustment and
oil and grease removal, sometimes is required. In practice, if
the W system is designedcarefully with provisions for automated cleaning of the quartz sleeve which surrounds the UV
lamp, pretreatment often can be avoided, reducing both the
capital investment and the ongoing maintenance costs.
The UV lamp inside the reactor is operated at high voltage, typically between 1000 and 3000 volts. Safety interlocks are fitted to protect personnel from both the UV
radiation and the high voltage supply. These interlocks
usually are linked to a programmable logic controller
(PLC), which can be used to control the whole installation, including feed pumps, the UV lamps and the reagent
delivery systems. A PLC can be accessed via a modem to
facilitate diagnostics for easier servicing, and can be
reprogrammed to accommodate changes in operation
throughout the remediation cycle.
In most groundwater applications, the material specified
for the UV reactor is 316L stainless steel, which protects
against the oxidants and the UV light while providing
excellent resistance to corrosion.
UV/oxidation checklist
The competitive universe for UV/oxidation systems is not
between the various vendors who manufacture these systems, but with the conventional media transfer technologies
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1994 POLLUTION
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9
of activated carbon and air stripping. Before conducting a
preliminary cost estimate, potential applications can be
quickly screened with yes or no answers to the following
checklist questions:
Is a destruction technology preferred?
Are there any restrictions on air discharge?
Does the principle contaminant air strip poorly Henry’s Law Constant 100 atm/mole fraction?
Do any of the principle contaminants load poorly on activated carbon in the liquid phase -50 mg/g carbon at 1 ppm?
Does the background water chemistry consume large
amounts of liquid phase activated carbon or interfere with
carbon operation, such as high iron or high chemical oxygen demand (COD)?
Are there handling or disposal concerns associated with
loading contaminants on to activated carbon, such as loading and concentrating explosive or carcinogenic compounds on to the carbon bed?
If the result is three or more yes answers, UV/oxidation
should be considered and a preliminary cost estimate is in
order using the design parameters provided as shown in
Calculation Example 1. If the preliminary cost estimate
looks favorable, arrange a design test to confirm and guarantee costs.
Electrical energy per order and UV dose
The key design variables are the exposure to UV radiation
and the number of orders of contaminant concentration
removed. These two variables are combined into a single
function, the Electrical Energy per Order (EE/O). The
EE/O is a powerful scale-up parameter and is a measure
of the treatment obtained in a fixed volume of water as a
function of exposure to UV light. It is defined as the kilowatt hours of electricity required to reduce the concentration of a compound in 1000 gallons by one order of
magnitude, or 90 percent. The units for EE/O are
kWh/lOOO gallodorder.
For example, if it takes 10 kWh of electricity to reduce
the concentration of a target compound from 10 ppm to 1
ppm - 1 order of magnitude or 90 percent - in 1000 gallons of groundwater, then the EE/O is 10 kWh/1000
gaVorder for this compound. It will then take another 10
kWh to reduce the compound from 1 ppm to 0.1 ppm, and
so on.
The EWO measured in a design test is specific to the
water tested and to the compound of interest, and it will
Figure 1. Esfimafedcapifal costs are given in ranges.
vary for different applications. Typical EE/Os for a range of
organic contaminants are provided in Table 1.
With the EE/O determined, either through design tests
or by using Table 1, the UV dose, or the amount of electrical energy required to treat 1000 gallons, needed to
treat a specific case is simply calculated using the following equation:
(1) UV dose (kWW1000 gallons) = EE/O X log (initial/
final), where initial is the starting concentration (any
units), and final is the anticipated or required discharge
standard (same units as initial).
For streams with several contaminants the required
energy is not additive but determined by the contaminant requiring the greatest UV dosage. See Calculation
Example 2.
Operating costs
Once the required UV dose is known, the electrical operating cost associated with supplying the UV energy can be
calculated using the following equation:
( 2 ) Electrical cost ($/lo00 gal) =
UV dose (kWh/1000gal) X Power cost ($/kWh)
The second key parameter from the design test is the concentration (ppm) of chemical reagents used, specifically
hydrogen peroxide and any catalyst added to improve performance. The peroxide dose is based on the UV
absorbance and COD of the water, and is typically in the
range of 50 to 200 ppm (mg/L). For the purpose of a preliminary cost estimate, the simplest rule of thumb for estimating the amount of peroxide necessary is the greater of
25 ppm or twice the COD concentration. Hydrogen peroxide cost varies from $0.005 to $0.008 per ppm concentratiordl000 gal. If a catalyst is required, its selection and concentration will vary with the target compound and must be
based on design test results. Lamp replacement costs typically range between 40 percent to 50 percent of the electrical cost. Therefore,
( 3 ) Total operating cost ($/lo00 gal) = 1.45 X Electrical
cost + peroxide cost, where peroxide cost = (H202
concentration in ppm) X ($O.O05/ppm/1000 gal).
OCTOBER
1994 POLLUTION
ENGINEERING
15
r,
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UVpower is influenced more by a change infow rate
than a change in concentration.
for a single contaminant can vary significantly from those
listed depending on the water matrix and concentration.
It is important to note that the total UV power, and hence
the capital and operating cost, is influenced much more by
a change in flow rate than by a similar change in concentration because of its logarithmic dependence on concentration through the log (initialhnal) term.
These are the most accurate cost figures possible without
performing actual design tests. The design tests consist of
batch treatment runs of sampled water while varying UV
dosage and reagent concentrations. Capital and operating
costs are optimized over the lifetime of the remediation
project by selecting the combination of total UV power and
reagent concentration where the most economical treatment is obtained.
Cost comparisons
Capital costs
Capital cost is a function of system size, which is a function of the UV power required to destroy the selected contaminants. Using the EE/Os provided in Table 1, the following equation is used to determine the total UV power
(kW) required:
(4) UV power (kW)
= EE/O X 60 X flow (gpm) X log (initial/final)
1000
= UV dose X 60 X flow (gpm)
1000
Once the required UV power is known, Figure 1 can be
used to look up the associated capital cost in U.S. dollars.
The capital costs are given as ranges to allow for the actual number of discrete reactors which will be required along
with any additional system options required.
The total UV power varies proportionally with flow rate
and orders of concentration of contaminant removed. For
example, doubling the flow rate, or treating from 10ppm to
0.1 ppm instead of down to 1 ppm, will double the UV
power required. The equation theoretically can be used to
obtain total UV power for any combination of flow ratc or
concentration, but its accuracy depends on the EE/O, which
76 POLLUTION
ENGINEERING
OCTOBER
1994
Compared with other treatment technologies, such as activated carbon, unit operating costs for UV/oxidation increase
much more slowly with increasing influent concentrations.
Typical operating costs as a function of influent concentration are shown in Table 2. UV/oxidation treatment costs are
almost always less for those contaminants which load poorly on carbon, regardless of Concentration. UV/oxidation
treatment costs are competitive for most of the average loading compounds but a definitive answer can only come from
the results of 2 design t m .
Since UV/oxidation capital costs typically are higher
than that of carbon for the same size flow, longer term projects may favor UV/oxidation where the cumulative savings
in operating costs offset the higher capital expense.
Robert Notarfonzo is a market analyst and Wayne McPhee is
a process engineer with Solarchem Environmental Systems,
Toronto, Ontario, 905-477-9242.
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