Layman report - Currenta

Disposal,
RECYCLING,
Engineering
Waste
Incineration
Wastewater
Treatment
EU-LIFE-Project Sludge Redox
Biogas from Sludge
Layman´s Report
A company of
Bayer and LANXESS

Biogas from sewage sludge
Final report on a demonstration project - with financial support from the EU LIFE program
This report presents the results of a CURRENTA
project to investigate a combination process to reduce
the volume of sewage sludge from industrial sources
on a pilot scale. A total of 20,000 metric tons per
annum of dewatered surplus activated sludge are
available for the generation of biogas at the
CHEMPARK Leverkusen This is the outcome of a
two-year demonstration project that received financial
support from the European Union within the framework
of the LIFE program.
Sewage sludge is an ongoing subject
Germany produces approximately 2 million metric tons
(solids) of sewage sludge per annum. Around one third
of this is currently used to fertilize areas in agricultural
Wastewater treatment plant in the Leverkusen Waste Management Center
The project “Sludge Redox” presented here is the last
link to date in a long chain of research projects that
have been carried out by Bayer AG, Bayer Industry
Services and CURRENTA for over 20 years. The Porteous
process – a thermal process for treating sewage
sludge with a high organic content – was applied at the
Leverkusen site in the 1980s to improve the dewatering
of the sludge. The sludge was heated to 170 °C in
that process. In the 1990s a process was developed
for the alkaline hydrolysis of sewage sludge. Sludge
digestion, a common process in municipal wastewater
treatment plants, was also tested. Unfortunately, it was
not possible to convert industrial sewage sludge
efficiently into biogas with this method.
use. Sewage sludge from industrial sources – which
often contains residues e. g. heavy metals – is incinerated.
This process is expensive. The search for new
sewage sludge disposal processes that are less
expensive and have a lower environmental impact is
important because of the large quantities of industrial
sewage sludge generated. The EU therefore supports
research activities that are aimed at developing new
processes to reduce the quantity of sewage sludge and
save energy.
Membrane filter press for dewatering sludge at the wastewater treatment plant
Leverkusen
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Biogas from sewage sludge
Industrial sewage sludge is a very special
substance
The challenges facing scientists in wastewater treatment
plants of the chemical industry were fundamentally
different to those in the communal sector. Wastewater
from a Chemical Park has high concentrations of
many different chemicals. Frequently it is necessary to
pretreat plant effluents to make it biodegradable.
Numerous chemicals are harmful to anaerobic bacteria
that generate biogas from organic substances in the
absence of air. For decades it proved impossible to
generate biogas from chemical wastewater. But the
wastewater loads have changed considerably in many
Chemical Parks since the end of the 1990s as a result
of new, more efficient manufacturing processes.
Bacteria that could not withstand the wastewater
conditions a few years ago can survive in wastewater
today.
Günter Müller shows the participants of the LIFE symposium lab-scale plant for sludge reduction
Combined treatment process for industrial
sewage sludge
However, despite all of the process improvements the
industrial sewage sludge generated today can still not
be digested that readily. Chemists, biologists, engineers
and technologists of CURRENTA have now
shown how this is possible: The key to the process is
the judicious combination of treatment steps that are
already known. In the first step all water-soluble
substances are removed using large volumes of water.
Sulfate is also removed in this step as this would
otherwise be converted by bacteria to foul-smelling
hydrogen sulfide which is toxic to bacteria. The
washing step is necessary to avoid subsequent
adverse effects on digestion. The sludge floccules
(which form surplus activated sludge) are then
destroyed with sodium hydroxide at temperatures
above 90 °C.
Organic components, especially proteins, lipids and
carbohydrates are very largely converted into watersoluble
substances. By this step the solid concentration
can be reduced significantly. Before anaerobic
treatment neutralization of the hydrolysate is necessary.
The neutralized hydrolysate is treated in an anaerobic
bioreactor at 35 °C. In anaerobic treatment first longchain
carboxylic acids are formed; these are further
converted to short-chain carboxylic acids. In the final
step anaerobic bacteria use this compounds to generate
biogas (approx. 60 % methane and 40 % carbon
dioxide). Biogas can be used for heating, electricity
generation or after purification in compressed natural
gas vehicles.
Layman´s Report 3

Biogas from sewage sludge
SLUDGE REDOX Method
Water
Alkaline solution
Biogas
Surplus
activated sludge
Washing
step
Alkaline
hydrolysis
Anaerobic
treatment
Residual
sludge
Watersoluble
components
Alkaline hydrolysis system
Anaerobic treatment system
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Biogas from sewage sludge
Activated sludge, untreated
Activated sludge after alkaline hydrolysis
Sludge after anaerobic treatment
The photomicrographs of sewage sludge bacteria
below show that the sludge floccules are very largely
dissolved after hydrolysis with sodium hydroxide. The
subsequent anaerobic treatment converts the remaining
floccules partially into biogas. All that remains are
fragments of the previously numerous sludge
floccules. The remaining sludge will be dewatered and
incinerated. The sulphate containing washing water
and the effluent of the anaerobic bioreactor are treated
in the wastewater treatment plant.
Surplus activated
sludge
Washed surplus
activated sludge
Balance of the solids after
hydrolysis and anaerobic
treatment
Biogas
13 %
100 % 98 % 30 %
Solids in
residual
sludge
2 %
55 %
Solubilised solids to
wastewater treatment plant
Solubilised solids to
wastewater treatment plant
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Biogas from sewage sludge
Use of sewage sludge for energy production
At the industrial wastewater treatment plant (wwtp) in
Leverkusen sewage sludge is first thickened before
adding lime and iron salts for conditioning. Afterwards
the sludge is dewatered in a membrane filter press.
the calorific value is low (3,000 to 4,000 kJ/kg).
Therefore it is necessary to add substances with high
calorific value for sludge combustion.
The Sludge Redox process generates biogas with a
calorific value of 22,000 kJ/Nm 3 from. Each day 2,000
to 3,000 Nm 3 biogas can be generated at the wwtp
Leverkusen. More than 50 % of the energy in the
surplus activated sludge can be recovered in the
biogas. In the same way the amount of waste from
surplus activated sludge is reduced by 70 %. The
remaining sludge is reduced in organic substances and
therefore the calorific value is also reduced.
Karl-Heinz Stürznickel shows a component of the pilot plant for sludge reduction.
After this treatment the filter cake still has a water
content of about 60 %. Because of the water content
and high content of inorganic substances in the sludge
LIFE symposion
The results of the LIFE Project “SLUDGE REDOX”
were presented at 17 th September 2007 in the
Leverkusen Waste Management Center. Operators of
industrial and municipal wastewater treatment plants
and scientists from universities and research institutes
participated the symposion.
Presentation of Hartmut Mayer (Emschergenossenschaft, Essen) at the LIFE symposium.
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Biogas from sewage sludge
Cost-effectiveness of the combination
process
Although pilot-scale studies were successful, further
studies will be necessary to make the new process
economically viable. The large amount of sodium
hydroxide solution needed to dissolve the sludge
floccules means that the entire process costs €120 –
140 per metric ton of sewage sludge and is therefore
markedly more expensive than conventional sewage
sludge incineration which costs €50 – 100 per metric
ton of sludge. Even if the savings made through the
use of biogas for electricity generation are included in
the calculation, the process still does not reach the
point of being viable economically. Further work should
focus on improving the economic viability of the
process in a larger pilot plant.
The process can be further improved by:
- application of low-cost alkaline waste
- reduction of the energy consumption in
the process
- increase of the biogas output
Sludge digestion in the future
Future work will focus on the utilization of the energy
present in the sewage sludge. The use of sewage
sludges to yield renewable energy is an important step
to improve the energy efficiency of wastewater treatment
plants and to reduce carbon dioxide emissions.
Dr. Fritz Bremer explained the participants of the symposion the control room of the wastewater treatment plant and the process engineering unit at the Waste Management Center.
For further information on the project please
contact: Dr. Guenter Mueller, Currenta, Geb. 4242,
51368 Leverkusen , phone +49-214-3066227,
e-mail: guenter.mueller@currenta.de
Layman´s Report 7