Environmental Science & Engineering - www.esemag.com - March 2005
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Fine bubble aeration evaluated for energy savings
By Hans-Christian von Consbruch and George S.Pastoric
The Hoechst Industrial Park
Sewage Treatment Plant in
Frankfurt is the second largest
industrial wastewater treatment
plant in Germany. Wastewaters from a
broad variety of chemical production
processes are treated in a biological
treatment facility.
The treatment process includes a
biological high-load stage, a low-load
stage and an intermediate clarifier.
Bottom mounted jet aerators used for
aeration had caused process problems.
Acceleration of the water inside of the
jets resulted in excessive shear rates,
causing a significant loss of biomass
in the intermediate clarifiers. As well,
high air flow rates were needed to
achieve the required oxygen transfer.
In addition to high power costs, these
high air flow rates made the off-gas
cleaning system (compulsory in
Germany) one of the most expensive
process steps. After examining various
options, plant staff concluded that
retrofitting the plant with an aeration
system which treated the sludge gently,
while reducing the air flow rate,
was necessary.
Fine bubble aeration technology
offers low (if any) shear rates and low
air flow rates. However, due to chemicals
contained in the waste water, the
lifetime of the diffuser, the change in
oxygen transfer efficiency, increased
pressure loss over time, and the change
of the mechanical characteristics have
to be considered.
Pilot Testing
Plant operators conducted extensive
field tests. Diffusers from six manufacturers
were installed on test racks in both
the high load and low load stages. The
test installation provided the same conditions
for each diffuser type. Changes
in pressure loss of each diffuser were
monitored over a six-month period.
After the test run was finished,
additional tests were conducted on the
diffusers to measure the changes in
efficiency and the change of the
mechanical properties of the membranes.
Oxygen transfer tests and
material tests were conducted with
new diffusers and those which had
been in operation for six months.
Results
The evaluation of the Standard
Oxygen Transfer Efficiency (SOTE)
test data and the physical property test
data showed that the OTT System
GmbH & Co. diffuser equipped with
FlexSil silicone-based membranes
achieved the best results. These membranes
showed the lowest increase in
pressure loss, the highest efficiency
and the lowest change in efficiency
over the six-month period of operation.
As a result of the tests, the plant
owners decided to use the OTT diffuser
system. In the summer of 2004
the OTT aeration system passed the
oxygen transfer test with an SOTE of
48%. The diffusers were set at 7.1
metres (23.3 ft) submergence with a
50% density. Start up of the aeration
system was done in September 2004.
Data generated during the first four
months of operation was used to
design the aerations system for a plant
expansion scheduled for later in 2005.
OTT System GmbH of Langenhagen,
Germany, has been awarded the design
and installation contract.
OTT System technology, design
and engineering services are available
in North America from Hydro-Logic
Environmental.
Hans-Christian von Consbruch is with
OTT System GmbH & Co.
George S. Pastoric is President of Hydro-Logic
Environmental.
Contact e-mail: info@hydrologic.ca.
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