Sewage pond – rehabilitation and repair

A sewage pond is a popular method of treating wastewater in rural, communal areas. The requirements for a sewage pond are the same as for a modern wastewater treatment plant, as the treated wastewater flows into the environment. Many sewage ponds were built in the 1960s, 1970s and 1980s. The requirements for treatment performance have risen sharply since then, which is why many sewage ponds in Germany are in need of rehabilitation. When updating the water law permit, it is often necessary to increase the area of the sewage pond or to install a technically modern intermediate stage. PPU Umwelttechnik GmbH offers various methods for rehabilitating a sewage pond and complying with the required discharge values.

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What is a sewage pond?

A sewage pond is an artificial, standing body of water that mechanically and biologically purifies municipal wastewater. Since a sewage pond is usually very shallow, a controlled decomposition of impurities takes place there in anaerobic and aerobic spaces. An essential difference lies in the aeration, as there are aerated and non-aerated sewage ponds. In unaerated sewage ponds, oxygen is introduced naturally through the water surface, which is why sewage ponds are usually constructed over a large area.

As a result, wastewater treatment is very slow and the growth of sludge is low. In aerated sewage ponds, an artificial oxygen supply is installed. This ensures a stronger growth of microorganisms, but also increases the concentration of suspended matter in the water, which is why a downstream suspended matter separator is often necessary.

What requirements must a sewage pond fulfil?

A sewage pond must treat the wastewater of a municipality or community. In the process, a sewage pond receives mixed water, which is composed of household sanitary wastewater on the one hand and rainwater on the other. The treated wastewater then flows into a public body of water such as a stream, river or pond and must therefore be harmless to the environment. A sewage pond must reliably purify the wastewater. The location should be chosen by the municipality so that it does not cause any odour nuisance.

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Why do many municipalities have to rehabilitate their sewage ponds?

For some years now, many municipalities have been focusing on environmental protection, which is why the requirements for a municipality’s treated wastewater have also increased. The water management authorities tightened the discharge limits into public waters. Since water law does not provide for automatic renewal of water permits, a new application must be submitted in good time. The resulting measurement programmes give the engineering and planning companies a basis for deciding which treatment method is the best choice. According to the planners, the municipality invites tenders for the rehabilitation of the sewage pond.

What impurities must a sewage pond remove?

Foreign bodies and solids

Almost every sewage pond receives foreign bodies and solids that it cannot biodegrade. These include solids that enter the sewage system through toilet flushing, but also coarse impurities such as small stones that rain washes into the sewage ponds. It is therefore absolutely necessary to install a rake or screen in the inlet of the sewage pond. This efficiently filters out coarse impurities and thus protects the sewage pond. In addition, no electricity is necessary, but regular cleaning by the sewage attendant is required.

Carbon compounds

Carbon forms the main part of the dissolved substances in wastewater. They are formed by carbohydrates, proteins and fats, which enter the sewage ponds mainly through kitchen wastewater. The microorganisms convert these first in the nitrification basin. The greater the pollution, the more oxygen and pond surface the microorganisms need. The degree of pollution of the biodegradable substances can thus be determined as biological oxygen demand (BOD5).

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The microorganisms convert ammonium into nitrite and nitrate, which cannot remain in the wastewater. The addition of carbon activates the microorganisms to break it down. This can be done by discharging sanitary wastewater. The microorganisms take the oxygen they need for carbon decomposition from the nitrite and nitrate. This produces nitrogen, which is a natural component of the atmosphere and thus harmless to humans.


Phosphates usually enter the wastewater through detergents or soaps. These compounds act as a fertiliser and promote plant growth in the sewage pond. This can also lead to strong algae growth, which is usually undesirable, as algae take oxygen from the water, which the microorganisms need to break down substances. By alternately reducing and adding oxygen, the microorganisms absorb phosphates in their cells and thus reduce the phosphate content in the wastewater.

The processes of PPU Umwelttechnik GmbH

PPU Umwelttechnik GmbH offers different processes to efficiently treat wastewater in a sewage pond for direct discharge into the environment. The customised solutions adapt to any local conditions. All systems are robust, reliable and have a particularly long service life.

Sewage pond with intermediate, submerged and aerated biological fixed bed

In municipal wastewater treatment, the biological fixed bed reactor is a system consisting of submerged fixed bed blocks that are always surrounded by wastewater. ClearFox® fixed bed blocks are mesh tubes made of a special growth material for microorganisms. Underneath the blocks is an aeration system.

On the one hand, this supplies the microorganisms with sufficient oxygen, on the other hand, it removes excess biofilm and thus prevents the reactors from clogging. Since the biofilm remains in the reactor, it is not necessary to recirculate the sludge. The discharged sludge can be separated in a secondary clarifier. This process causes carbon decomposition, nitrification and phosphorus precipitation. The ClearFox® biological fixed bed process is one of the most reliable and efficient municipal wastewater treatment processes on the market. It is suitable for wastewater volumes from 50 to 10,000 population equivalents.

Sewage pond with SBR technology

The ClearFox® SBR technology is a cycle consisting of filling, mixing, aeration, settling, discharge and standstill phases. The duration of an individual cycle depends on the cleaning objective and can be set as desired via a control system. As a result, the individual operating steps accumulate one after the other in the same reactor. In the aeration phase, a system supplies the microorganisms with oxygen. The sludge produced during wastewater treatment settles, so that the clear water can flow into the next tank.

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A downstream equalisation tank is advantageous here in order to be able to cope with the intermittent clear water quantities and any sludge discharge. The advantage of a ClearFox® SBR system is the simple conversion of an existing sewage pond. All the operator has to do is add aeration, discharge equipment and controls. Carbon decomposition, nitrification, denitrification and phosphorus precipitation are possible with this process. The ClearFox® SBR process can be expanded as required so that it can treat large volumes of wastewater.

The advantages of the PPU processes

PPU Umwelttechnik GmbH has almost 15 years of experience in municipal wastewater treatment and the rehabilitation of sewage ponds. The processes can also be combined to achieve an ideal cleaning result. For rehabilitation measures on municipal wastewater treatment plants such as pond treatment plants, there is also the possibility of leasing a plant and a complete takeover later. PPU Umwelttechnik GmbH offers an all-round service for its customers. From an on-site inspection to transparent order processing to aftercare, the customer experiences a complete service.

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