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MINORITY REPORT
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the chemical engineer|issue 869|november 2013
Game changers
Planning a cleaner future for fossil fuels
tce
WASTE MANAGEMENT
The value of sludge
Rob Lei, Steve Sopora and Daniel Gapes report on how researchers have
found hidden value in the disposal of biosolids at a popular tourist destination
M
ANAGING waste is a growing
headache for municipal
authorities, especially for a
city like Rotorua. While this New Zealand
tourist destination has just 60,000
permanent residents, it regularly plays
host to around 3.2m visitors a year, who
come mainly for the geothermal activity,
pristine forests, and lakes.
As is often the case though, a thriving
tourist industry also comes with a risk of
an associated negative environmental
impact. In 2006 the Rotorua District Council
found itself with a major municipal waste
disposal problem – landfilling biosolids
(from sewage sludge) was costing it a
hefty NZ$1m/y (US$0.8m/y). Protecting
Rotorua’s waterways was, and still is, an
environmental and business necessity.
In response to that challenge, scientists
38
and engineers at the New Zealand
government research institute Scion have
developed a hydrothermal process to
transform wastewater biosolids into reusable
chemical products. Seven years after the
research began the technology has now
progressed beyond pilot-scale validation,
with the first commercial-scale plant due to
be completed in late 2014.
Biosolids are generated as
a byproduct of wastewater
treatment processes and
represent a considerable
fraction of overall municipal
solid waste production.
www.tcetoday.com november 2013
the biosolids problem
Population growth, urbanisation, and higher
living standards are driving waste generation
throughout the world. The World Bank
estimates1 that about 3bn urban residents
generate 1.2 kg of municipal solid waste
per person per day (1.3bn t/y). By 2025 this
will likely increase to 4.3bn urban residents
generating about 1.4 kg per person per day, or
around 2.2bn t/y.
Biosolids are generated as a byproduct of
wastewater treatment processes and represent
a considerable fraction of overall municipal
solid waste production (estimated at 10%
in New Zealand). They have around 80%
moisture content with large proportions of
carbon, nitrogen, and phosphorus presenting
good opportunities to generate value through
energy and chemical recovery. But despite
WASTE CAREERS
MANAGEMENT
New Zealand has a low population density compared with
many countries and a seeming abundance of land available
for recycling biosolids back into agriculture. But land-based
application of biosolids waste is fraught with difficulties, chief
among them being the potential to degrade waterways.
Technology
Capital
costs
Volume
reduction
Value recovery
Hazard removal
Compliance
costs
High
High
Low (moisture
content limits
energy recovery)
High
High (air)
Med/low
(common
practice in
US)
Incineration and
gasification
Autothermal
aerobic digestion
(land application)
Moderate
Moderate
Low
Moderate (metals &
nutrients remain in
product)
Thermal drying
Moderate
Moderate (water
removal only)
Low
Moderate (metals
& nutrients remain
diluted in product)
Med/low
Wet oxidation-full
destruction
High
High
Low
High
Med/low
Composting (inc
vermicomposting)
Low
Low (negative if
bulking agents
used)
Low (limited
product markets
Low (pathogen
removal only)
Med/high
Moderate
Moderate
(further
treatment
needed)
Moderate
(biogas)
Moderate (hazards
transferred to liquid
and residue phases
Low
High
High
High
High
Med/low
Anaerobic
digestion
TERAX: combined
biologicalhydrothermal
Table 1: Comparison of biosolids management technologies
tce
their value opportunity, only a small number
of advanced material recovery technologies
(such as struvite precipitation) are used
commercially, typically in recovering
phosphorus.
Regulatory constraints, land availability
and ultimately economics mean that
solutions for biosolids disposal vary greatly
by region and country. Technology to
manage biosolids include dewatering,
anaerobic digestion, or incineration (either
alone or in combination) – see Table 1. The
residuals commonly end up in landfills or
agricultural applications, where fugitive
emissions such as greenhouse gases and
nutrient-rich leachates can have unwanted
environmental impacts.
New Zealand has a low population density
compared with many countries and a
seeming abundance of land available for
recycling biosolids back into agriculture. But
land-based application of biosolids waste is
fraught with difficulties, chief among them
being the potential to degrade waterways
due to nutrient inflows and negative export
market perceptions associated with food
safety.
Incineration is not an option either. It is
very difficult under New Zealand legislation,
is energy-intensive and an expensive option
requiring a significant scale for viable
biosolids processing. Like many developed
countries, New Zealand has shifted towards
fewer, larger and more modern landfills.
Fewer landfills mean waste is transported
longer distances. This, coupled with tighter
environmental regulation, has led to steadily
increasing landfill costs.
a hybrid approach
Figure 1: Simplified process diagram of the hybrid
TERAX technology
Feed preparation
Anaerobic
fermentation
Hydrothermal
oxidation
Product recovery
Nitrogen
Heat
Carbon
Oxygen
Phosphate
Core process
• 6–8% dry solids
• Screening
• 4–6 days retention
• 35–55ºC
• 1–2 hours retention
• 180–260ºC
• 30–60 bar
• Inorganic solids separation
• Ammonia stripping
• Dissolved carbon
reuse
Scion’s technology (TERAX) was spurred
by the desire to extract value from organic
wastes, which are typically disposed of on
a lowest-cost approach rather than adding
value via energy and products. It combines a
biological stage and a thermo-chemical stage
(see Figure 1).
The first stage is a short retention anaerobic
fermentation targeting initial solids
reduction and organic acid production. This
is followed by the hydrothermal oxidation
stage which completes the organic solids
degradation, generating additional shortchain organic acids (specifically acetic acid),
ammonia and a high phosphorus ash.
The biological process – anaerobic
fermentation – is very efficient in reducing
organic solids concentrations with relatively
low unit cost energy inputs required. It
achieves 40–50% solids reduction, requiring
just 4–6 days to complete.
The second stage is a hydrothermal
process known as wet oxidation that has
been in commercial use since it was first
developed in the 1930s. It is ideally suited
november 2013 www.tcetoday.com
39
tce
WASTE MANAGEMENT
Commercial-scale implementation of the process in Rotorua
will eliminate approximately 10,000 t/y of biosolids currently
exported from the site.
Figure 2: transforming biosolids
Simplified kinetic diagram showing the destruction of biosolids (brown) and
associated conversion to breakdown products.
CO2
Carbon mass
CO2
Dissolved organic carbon
Organic acid fraction
Hydro
thermal
oxidation
Anaerobic fermentation
Time
Solid
Liquid
Gas
to the treatment of wet materials, and water
is an integral part of the process. Unlike the
fermentation stage, wet oxidation can achieve
greater than 90% reduction in organic solids,
albeit with much higher unit operating costs.
The wet oxidation process is operated in
a pressurised hydrothermal reactor at subcritical water temperatures and pressures
resulting in rapid reaction rates. This stage
typically takes 1–2 h, or around a hundred
times faster than the fermentation stage.
The surplus heat from the hydrothermal
oxidation reactions is sufficient to supply
heat for the overall process without requiring
external energy inputs.
This combination of processes has been
optimised to degrade the complex solid
organic carbon into simple compounds with
potential for recovery and re-use (see Figure
2). The degradation proceeds via an initial
solubilisation to dissolved organic carbon
prior to formation of short-chain organic
acids (also known as volatile fatty acids, or
carboxylic acids). The organic acids produced
in the fermentation survive the hydrothermal
oxidation and are increased.
This hybrid approach of combining the
processes couples the efficiency of the
biological process with the effectiveness of the
chemical process to enable lower cost inputs
and greater product recovery yields than wet
oxidation alone.
The hybrid combination of the anaerobic
fermentation step and the hydrothermal
deconstruction step is novel in its own right.
The greater novelty though, is that these two
Rotorua lies within a sensitive lake catchment
40
www.tcetoday.com november 2013
WASTE CAREERS
MANAGEMENT
Biological nutrient removal processes are
used to achieve a high quality effluent
tce
third of variable operating costs for the
site. The dissolved organic carbon from
TERAX is highly biodegradable, substituting
an estimated 40% of these ethanol
requirements.
the payoff
The process recovers more than 60% of
nitrogen and 95% of phosphorus – resulting
in a reduced load returned to the wastewater
treatment plant.
Nutrients are recovered through stripping
ammonia and separating the ash. At
about 30% phosphate content, this ash is
comparable to rock phosphate used for
fertiliser production. The relatively small
scale (50 t/y nitrogen, 40 t/y phosphorus)
is expected to provide useful quantities to
supply niche applications in the fertiliser
market.
future applications
steps are optimised to target value recovery
in the form of carbon, nitrogen, phosphorous
and energy.
progress
The Scion team started investigating several
existing technologies for improved waste
management back in 2006. Of the available
technologies, hydrothermal processing
was found to be the most promising to
meet the objective of recovering value from
solid organic waste. A life cycle analysis
identified potential environmental benefits
of a 96% drop in landfill volume; 76%
drop in greenhouse gases; 40% drop in
eutrophication potential; and 90% drop in
Expanding primary industries
such as pulp and paper, dairy,
meat and fruit processing
represent a further potential
resource within New Zealand.
Applying this technology
to managing these organic
wastes is the subject of
ongoing research at Scion.
photochemical ozone creation potential.
Four years later, in 2010, the technology
was piloted at Rotorua Council’s wastewater
treatment plant (WWTP). Through the
laboratory research and piloting stages,
techno-economic modelling provided targets
for optimising the capital and operating costs
of the process.
The results from the pilot stage provided
sufficient justification to proceed to initial
engineering and business case phases of
a commercial-scale demonstration at the
Rotorua WWTP.
full-scale implementation
Commercial-scale implementation of
the process in Rotorua will eliminate
approximately 10,000 t/y of biosolids
currently exported from the site.
Process modelling the full scale
implementation of TERAX within Rotorua’s
WWTP ensured that the downstream effects
were well understood and the performance
parameters could be met.
To achieve strict discharge limits for
nitrogen, the existing WWTP uses a biological
nutrient removal process where a carbon
supplement is required as a food source for
de-nitrification bacteria.
In the case of Rotorua, ethanol is the
carbon source and represents about one-
The carbon-rich product stream (if not
required as a supplement in the wastewater
treatment process) can become a feedstock
for conventional biogas production. This
material also provides a potential feedstock
for biopolymer production and many other
industrial biotechnology applications.
Expanding primary industries such as pulp
and paper, dairy, meat and fruit processing
represent a further potential resource within
New Zealand. Applying this technology to
managing these organic wastes is the subject
of ongoing research at Scion. tce
Rob Lei ([email protected])
is business development manager at
Terax 2013; Steve Sopora is general
manager at Terax 2013; Daniel Gapes
is environmental technologies leader
at Scion.
further reading
1. Hoornweg, D, Bhada-Tata, P, What
a waste: a global review of solid waste
management, Urban development series,
The Worldbank, Washington DC, 2012.
bit.ly/10tnIGK
Chemical Engineering Matters
The topics discussed in this article refer to the
following lines on the vistas of IChemE’s technical
strategy document Chemical Engineering Matters:
Water
Lines 3, 4, 17, 18
Health and wellbeing
Lines 4, 5, 7, 27
Visit www.icheme.org/vistas1 to discover where
this article and your own activities fit into the myriad
of grand challenges facing chemical engineers
november 2013 www.tcetoday.com
41