Thermal pretreatment in the UK – alternative to advanced digestion
By Patrick Coleman,PhD
CAMBI Thermal Hydrolysis - Dublin Bay Wastewater Treatment Plant.
The preferred method in the
UK to upgrade mesophilic
anaerobic digestion to meet
new stringent sludge quality
regulations is to bolt onto the existing
system a thermal pretreatment stage.
Currently, in the UK more than 500
digesters process over 700,000 tonnes
of dry sludge per year. The average
loading rate in 1969 for these digesters
was 1.5 kg VS/m3/d (kg of volatile
solids per m3 of digester volume per
day). With the increased use of
mechanical thickening using drums
and belts, the average loading is now
between 2.0 and 3.2 kg/m3/d. The average
sludge feed is now between 5% and
7% dried solids.
The last few years have seen the
acceptability of recycling sludge to
agriculture diminish severely. Most
notably the actions of the British Retail
Consortium (supermarkets), in insisting
on sludge pasteurization, have
prompted a detailed look at how
sludge should be treated. In the past a
table of approved processes formed the
basis for design without any quantitative
assessment of the quality or consistency
of the product. New sludge
regulations were introduced in early
2002 and were a combination of introduction
of HACCPs (Hazard Analysis
Critical Control Points), sampling of
product for kill of E. coli and quality
control.
These new regulations introduced
two new standards: treated and
enhanced treated. These standards are
similar but not identical to the US EPA
503 Class B and A standards. The
treated standard can be achieved by 14
to 21 days batch liquid storage post
digestion. However, a more significant
process change is required to achieve
the enhanced standard.
The most common solution chosen
to achieve an enhanced treated or a
secure treated standard was to bolt on
a process to pretreat the raw sludge
prior to digestion. This meant that the
existing system did not need to be modified and could continue to operate
while the new process was being
built.
There are three types of pretreatment
systems now installed in the UK
upstream of mesophilic digestion:
enzymic hydrolysis, pasteurization and
thermal hydrolysis. The Combined
Heat and Power (CHP) engines provide
part of the heat required for these
processes.
United Utilities along with their
technology partner Monsal (www.monsal.com) trialed enzymic hydrolysis at
the Macclesfield WwTP. This demonstration project has been operating for
over 4 years. The first commercial
installation of this new process is at the
Bromborough WwTP. This plant has
been in operation for a year.
The process consists of a plug flow
reactor where the sewage undergoes
hydrolysis at 42 °C. One of the tanks in
this reactor is bumped up to 55 °C to
ensure enhanced treated quality. At
least three more plants are planned,
with the largest at the Blackburn
WwTP at 21,600 tds/yr (tonnes of
dried solids per year).
To achieve the kill using smaller
vessels (lower retention time), the temperature
must be increased. Standard
practice is to lock the raw sludge in at
70 °C for 30 minutes. Typically, the
raw sludge is preheated using the heat
in the pasteurized sludge. The warm
raw sludge is then boosted to 70 °C
using either hot water or steam.
Once the sludge temperature rises
above 50 °C, there is a danger that the
performance of the water to sludge
heat exchanger deteriorates due to
scale forming on the sludge side of the
exchanger. This is a concern for plants
that dose iron for phosphorus removal.
The design is very sensitive to the
rheology of the sludge being pumped.
Many of these types of systems have
failed because they could not pump the
sludge at the design thickness through
the heat exchangers.
Alpha Biotherm Plant - Ellesmere
Port Wastewater treatmemt plant
The Alpha Biotherm system is
more robust because it does not rely on
conventional heat exchangers to heat
the sludge. The system uses a batch
sludge to sludge heat exchanger to preheat
the sludge and a thermophilic aerobic
digester (TAD) to raise the sludge
temperature to 70°C. The TAD reactor
contains a hyperbolic mixer and air
diffusers. The TAD is heated by a
water jacket. Typically 15% of the
reactor volume is exchanged with each
cycle. This system has been installed at
the new Reading WwTP (Thames
Water) and the Ellesmere Port WwTP
(United Utilities).
To achieve kill and increase the
digestibility of sludge, three sites have
installed the CAMBI (www.cambi.com)
thermal hydrolysis plant. The first
CAMBI thermal hydrolysis plant in
Hias, Norway, has been operational for
over 10 years.
The CAMBI plant consists of two stage pulper tanks, batch reactor tanks
and a flash tank. The raw sludge feed is
dewatered to >18%. The sludge is
diluted with hot water to 12% to 14%
and heated to about 100 °C using
recovered process steam. The sludge is
then fed to one of the batch reactors
where it is heated to 165 °C with steam
and put under at least 6-bar pressure.
The time from a reactor filling to it
being empty again is between 90 and
110 minutes.
The largest of these sites is at the
new Dublin WwTP (serves 1.6 million
population equivalents). The plant
processes about 100 te/day (tonnes per
day) of raw sludge. The digesters are
fed a hydrolyzed and pasteurized feed
at 11% to 12% dried solids and operate
at 15 day hydraulic retention time. The
digesters are loaded on average at 6 kg
VS/m3/day and achieve 62% volatile
solids destruction. A hydrolyzed
sludge produces 1.5 times the energy
of that of a purely pasteurized sludge.
These three processes are alternatives
to thermophilic or temperature
phased digestion as well as post thermal
treatment processes such as drying.
There is a direct correlation
between the complexity of these systems
(e.g. operating temperature and
pressure) and operational/maintenance
effort they require. However, with
proper equipment selection, piping
design and an understanding of sludge
rheology, this effort can be minimized.
Dr Coleman is with Associated
Engineering as a Senior Wastewater
Specialist.
E-mail: colemanp@ae.ca
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