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2.1.08 Semi-decentralised concept for a new building development – the Knittlingen “water house”

When it comes to connecting new building developments to the water supply and disposal system, local authorities are faced with a choice: should the existing sewer system be expanded, or should a decentralised solution be implemented? The Fraunhofer Institute for Interfacial Engineering and Biotechnology used a model project in Knittlingen near Pforzheim to demonstrate the benefits of a semi-decentralised concept: as this method uses rainwater, it reduces the level of fresh water consumption. It also produces fertiliser for farmers and its operation is energy-neutral.

Industrial nations generally apply a combined-system principle to wastewater disposal: rainwater is used to dilute the wastewater before it enters the central sewage plant. This process is counter-productive as the sewage plant has the laborious task of extracting the contents from the water. A practical alternative from both an economic and ecological perspective could be to use cycle-oriented, semi-decentralised systems for water supply and wastewater disposal – in emerging and developing countries too.

The Fraunhofer Institute for Interfacial Engineering and Biotechnology (IGB) implemented such a concept as part of the “Decentralised Urban infrastructure System 21” (DEUS 21) project in 2005, selecting a new business development with 100 buildings in Knittlingen near Pforzheim. The Fraunhofer Institute for Systems and Innovation Research (ISI) is working alongside the project to compare the ecological and economic aspects of the system with those of conventional processes.

Treated rainwater

At the heart of the system is the “water house”, located at the edge of the DEUS 21 residential area. It serves as both the technical operations building and an information centre for residents and visitors. The rainwater flowing off the roofs and streets is stored in underground cisterns and treated in the water house. The aim is to treat the rainwater to render it potable and then use a separate network to channel it into homes so it can be used for flushing toilets, watering gardens, operating washing machines and dishwashers and also for washing and showering. The first step is to examine the treated rainwater over an extended period; the residents receive a second source of drinking water from the Knittlingen authorities during this test phase.

The water house in Knittlingen

The water house in Knittlingen
The water house in Knittlingen
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A vacuum sewage system sucks the domestic wastewater from transfer chutes in front of the houses; it is then treated in the water house in an anaerobic cleaning reactor with built-in membrane technology. The central vacuum station, commissioned in 2005, produces the vacuum required for this process. The builders can also lay a vacuum pipe in the home, enabling installation of a water-saving vacuum toilet and a shredder for kitchen waste.

Solids separated

Preliminary tests have shown that wastewater cleaning is more effective if the solids are separated beforehand. The output solids are therefore treated separately at 37°C using the high-load digestion process developed by the Fraunhofer IGB with integrated microfiltration. Fermentation of the solids produces up to 5,000 litres of biogas every day. The hydraulic retention time in the reactor is approx. ten days; the solids retention time is freely adjustable to a certain extent but is considerably higher. An unheated, fully mixed bioreactor with a volume of ten cubic metres is used to treat the overflow from the sedimentation tank (approx. 99% of the inflow); the outflow is handled by four parallel rotating disc filters (pore diameter of 0.2 μm).

The anaerobic sewage plant offers reliable functionality even at low temperatures: at reactor temperatures as low as 13°C the outflow values register less than 150 milligrams of the chemical oxygen demand (COD) per litre (limit value for sewage plants serving less than 1,000 residents).

Reactor

Reactor
Reactor
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The inflow concentrations are between 400 and 1,100 milligrams COD per litre, with the average level of degradation at 85%. The maximum biogas production in the reactor for cleaning the primary treatment overflow is around 3,000 litres a day. The increase in biomass produces around 10% of the expected amount of excess sludge from the activated sludge procedure . Since its commissioning in March 2009, the membrane filtration has been cleaned through automatic filtrate backwashing, with the first chemical clean taking place in April 2010.

Agricultural benefits

A vacuum station (left) and bioreactors in the water house

A vacuum station (left) and bioreactors in the water house
A vacuum station (left) and bioreactors in the water house
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The water flowing out of the plant was suitable for irrigating and fertilising farmland. The bioreactor breaks down virtually no ammonium or phosphate, both of which occur in relatively high concentrations in the wastewater. The membrane filtration removes the bulk of germs from the water, so it is therefore safe to use as fertiliser. Spot checks in the outflow from the rotating disc filters used in the membrane filtration revealed no traces of Escherichia coli bacteria, although it was present in the reactor sludge at a level of one million per millilitre.

If this cannot be used as fertiliser, one alternative is to recover the ammonium and phosphate: an electrochemical process precipitates the nutrients as struvite (MAP, magnesium ammonium phosphate). Excess ammonium is bound to zeolite by means of an ion exchange process and recovered as ammonium sulphate through air stripping.

Energy-neutral operation

The fully anaerobic process technology is able to convert most of the organic materials from the wastewater into biogas: a daily yield of 40 to 60 litres per resident – as opposed to just 25 litres from conventional wastewater cleaning through sludge digestion. The energy in the biogas produced through anaerobic wastewater cleaning is over 100 kilowatt hours per resident each year. Large sewage plants consume around 30 kilowatt hours of electrical energy per resident per year (and around 30 kWh thermal energy): in comparison, anaerobic conditions enable energy-neutral wastewater cleaning at the very least.

Fraunhofer Institute for Interfacial Engineering and Biotechnology (IGB)
Prof. Dr. Walter Trösch
Dipl.-Ing. Marius Mohr
Nobelstraße 12
70569 Stuttgart, Germany
Tel.: +49(0)7 11/9 70-42 20
Fax: +49(0)7 11/9 70-42 00
E-mail: walter.troesch@igb.fraunhofer.de
marius.mohr@igb.fraunhofer.dee
Internet: www.igb.fraunhofer.de/en.html
Funding reference: 02WD0457
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