Sludge Management on Sewage Treatment Plants

Systematic analysis – basis for the optimization of sewage treatment plants

Sewage sludge “only consists of 1 % of the flow of wastewater, but produces about 30 % of the treatment costs and at the end about 90 % of the headaches.” Imhoff, 1993. Fraunhofer IGB offers a systematic assessment of sewage treatment plants through the specific evaluation of operational data books, design documents and installed technology. This translates into operational transparency, on the basis of which measures can be taken to optimize the sewage treatment plant.

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High-load digestion of sewage sludge

Fraunhofer Institute for Interfacial Engineering and Biotechnology IGB has developed an energy-effcient high-load process for the fermentation of sewage sludge. It was put into operation for the first time in 1994 at the sewage treatment plant in Leonberg. In the meantime, this process is being successfully applied by four other municipal sewage treatment plants. The outcome: The high-load digestion converts the sludge into biogas in a considerably smaller space and more cost-effectively than the conventional digestion towers.


Fundamental advantages of high-load digestion:

  • shorter retention time
  • smaller digestion space
  • enhanced degradation rate
  • higher biogas yield
  • no operational problems (foaming)
  • easier to dewater
  • lower operational and disposal costs


Limited disposal possibilities

The possibilities for disposal of sewage sludge from municipal wastewater treatment are being increasingly restricted. In future there will no longer be any demand for use in landscaping, utilization in agriculture is controversial, dumping of most sludges no longer possible. Incineration of sewage sludge will gain in importance, disposal is thus becoming more expensive. Aerobic sludge stabilization is expensive, frequently insuffcient and no alternative for sewage plants > 10,000 PE.


Intelligent utilization of sewage sludge as an energy carrier

The high-load digestion process developed at Fraunhofer IGB makes sewage sludge digestion a process that can, as a result of the effcient conversion of the sewage sludge contents into biogas, contribute substantially to the cost-effectiveness and energy effciency of sewage treatment plants. The process is therefore also suited for smaller treatment plants (10,000 PE) that so far stabilize the sludge aerobically with a high power consumption.

The sewage sludge is stabilized with net energy production by means of high-load digestion, can be dewatered to a higher TS level and the residual sludge disposed of by incineration at the lowest possible cost. The regenerative energy carrier biogas is derived as a product. The thermal energy requirements of the sewage treatment plant can be covered by the biogas obtained and further expenses can be saved by means of combined heat and power generation. The high-load digestion process therefore also represents an economically intelligent alternative and considerably improves the energy effciency of municipal sewage plants.

Reference projects

Analysis of the biogas production potential in sewage sludge digestion plants

Duration: July 2014 – June 2017

In an extensive study, Fraunhofer IGB investigated the potential of high-load digestion of sewage sludge for producing biogas, which can be used to generate electricity and heat. The experts at the Institute evaluated the overall potential of all wastewater treatment plants in Baden-Württemberg in size class 4.

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Brazilian vehicle fleet drives on bio-methane from the sewage plant

Duration: February 2009 – December 2017

In the project, which is funded by the International Climate Initiative (IKI) of the German Federal Ministry for the Environment, Fraunhofer IGB has developed a plant to treat the biogas produced at the wastewater treatment plant in Franca, Brazil, to natural gas quality. The purified biomethane will be used to power the water supplier's own fleet of vehicles.

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ESiTI – Wastewater treatment plant of the future: energy storage in interaction with technical infrastructure is as a trade-off between energy production and consumption

Duration: May 2014 – April 2017

Biogas – such as wind and solar energy – is one decentralized source of renewable bioelectricity. Among other sources, it is produced during high-load digestion of sewage sludge in wastewater treatment. Fraunhofer IGB is investigating the extent to which high-load digestion can be operated efficiently without affecting the actual functions.

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High load digestion on the sewage treatment plant Leipheim

Duration: October 2015 – June 2019

The city of Leipheim has decided to replace its 40-year-old digester with a modern high-load digester (HLD). The HLD process was developed by the Fraunhofer IGB for anaerobic decomposition and sludge stabilization.

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