Layman report - Currenta

Layman report - Currenta








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


2 Layman´s Report

Biogas from sewage sludge

Industrial sewage sludge is a very special


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


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



Alkaline solution



activated sludge











Alkaline hydrolysis system

Anaerobic treatment system

4 Layman´s Report

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


Washed surplus

activated sludge

Balance of the solids after

hydrolysis and anaerobic



13 %

100 % 98 % 30 %

Solids in



2 %

55 %

Solubilised solids to

wastewater treatment plant

Solubilised solids to

wastewater treatment plant

Layman´s Report 5

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.

6 Layman´s Report

Biogas from sewage sludge

Cost-effectiveness of the combination


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,


Layman´s Report 7

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