Former Sewage Farms 2010

Introduction

Waste waters from Berlin households and streets were drained by a primitive gutter drainage system up into the 1870’s. There were years of dispute over which process should be chosen for urban dewatering and sewage disposal. Field treatment of sewage and its parallel use for agricultural areas was generally accepted as the most favourable form of sewage disposal. A total of 12,500 ha were adapted for 20 official sewage farms and two smaller sites used for field treatment of sewage. The city of Berlin had bought the land for the fields and still owns most of them today.

The establishment of sewage treatment plants in Berlin led to the closure of the great majority of sewage farms by the middle of the 1980’s. Large areas of sewage farms were built-up within Berlin, in Marzahn, Hellersdorf, and Hohenschönhausen; or reforested, as around Buch forest at the end of the 1980s. By 1998, the final sewage farms had been shut down in their original use. Until 2010, the Berlin Water Works (BWB) carried out elution studies (an investigation in the field of environmental chemistry to extract adsorbed substances) to discharge clear water on the fields of the Karolinenhöhe sewage farm, in the district of Gatow. Many of the former sewage farm areas are now used for purposes of agriculture and forestry. Both the nutrients and pollutants in waste water are accumulated in sewage farm soils. This accumulation in closed sewage farms has disadvantages for current uses and, because of the size of the areas, far-reaching consequences on the economy of nature.

Former sewage farms remain important spaces for urban development in the future. A diversity of concepts, some of them competing, have already been discussed for use of the remaining surfaces as residential areas, industrial parks, recreation spaces or for ground water accumulation. Being aware of the specific pollution connected to sewage farms, information about the location and extent of former sewage farms forms an important planning basis for assessing the soils need for protection and for avoiding future conflict regarding use.

Functioning of Sewage Farms

The sewage farms followed a dewatering concept by James Hobrecht. In 1869, the Berlin administration made him director of the Berlin Latrine System. Hobrecht divided the city into 12 districts, called radial systems. Each radial system had a pumping station. Pumping stations received domestic, commercial and industrial waste waters as well as precipitation water through gravity flow pipelines. Sewage effluents were conducted from the pumping station through pressure pipelines to sewage farms located outside the city. Some sewage farms were additionally supplied by direct pipelines.

Pressure pipelines discharged waste water at the sewage farms. Waste water was first collected in sedimentation basins made of concrete or earth. Water flowed through the tank and most sediments settled to the bottom. Immersion panels held back floating matter. Sediments settling in the sedimentation basin were regularly evacuated and dewatered at special sludge drying areas. Dewatered sludge was used as a soil conditioner for agriculture and horticulture in early years. The sewage farm trench system was also regularly cleaned, whereby removed sediments were usually deposited directly alongside the trench. After sewage water had passed through the sedimentation basin, e.g. had been mechanically cleaned, it flowed through gravity feeders to the terraces.

The natural soil surface was not automatically suited for processing sewage waters. Terraces were constructed horizontally or on a slope, depending on the surface. They were about 0.25 ha large, and surrounded by embankments. There were three methods of sewage farm treatment. Horizontal terraces were flooded by surrounding distribution ditches. For sloped terraces, sewage water overflowed the upper bank and irrigated the sloped terrace. Bed terraces with ditch irrigation were also initially used. Waste water flowed through bed terraces in connected parallel furrows, about a metre apart. Only plant roots received water (cf. Fig. 1).

Fig. 1: Diagram of sewage farm types

Wild sewage areas were often found near treatment terraces. The overloading of prepared surfaces could be met by directly diverting unpurified water through sluices onto natural land.

Sewage water contents were retained during the passage through the soil, adsorbed in the humic topsoil, and handled chemically and biologically. This process supplied agriculturally useful nutrients. Initial yields were high and the majority of fields were used agriculturally and served their own sewage treatment plots. There was a mixed use of grasslands and field cultivation.

Most sewage farms were provided during construction with drainage pipelines at regular intervals for a faster discharge of filtered and purified water, and to provide for aeration and regeneration of soils as well. Drainage water passed through collecting drains and dewatering trenches into the preclarification outlet trenches. Some water from soil passage percolated into ground water.

Fields were flooded in normal operation, and then left until water seeped away and the soil was re-aerated. The next flooding was begun only after re-aeration was completed. These sewage farm rhythms were also oriented to the growth periods of agricultural crops. Four to eight field treatment cycles a year were possible on grasslands, with a volume of 2,000-4,000 mm of sewage water. Areas used for cultivation of winter wheat could only be used once a year, with 100-500 mm of waste water.

Sewage farms were overtaxed with increasing amounts of waste water, an intensification of agricultural production, and the closure of other sewage farms. This stimulated some sewage farm operators to establish “intensive filtration areas”. These were permanently flooded and surrounded by high embankments. An inadequate degree of purification was performed here because aerobic processes could not take place. These areas were not used agriculturally.

Sewage farm structures were often levelled to a large extent after sewage treatment use was discontinued. Trenches and terraces were filled with material from the embankments, themselves land-fill material.

Contamination of Sewage Farm Soils

Waste water nutrients and pollutants were retained in the soil during water passage. All such soils were contaminated with heavy metals, some in considerable measure. This impaired the uses of these soils, as crops cultivated in this soil may accumulate heavy metals. Determined loads may be so high in some locations that health risks resulting from direct contact with soils cannot be ruled out. This is relevant, for example, where former sewage farms are planned to be used for sensitive purposes, such as children’s playgrounds.

It may be assumed that pollutant loads of the infiltrating waste water increased during the operational span of sewage farms, because there was an increased use of household chemicals and detergents, and increased amounts of industrial waste water as well. In addition, there was an increasing load of street waste water that was introduced by the combined waste water collection system. Due to the composition of the waste water, soils that were part of a sewage farm are expected to be considerably contaminated not only with heavy metals but also with organic pollutants.

There is considerable variation in the degree of pollution in these soils of former sewage farms, depending upon the amounts of waste water treated. The duration of operation, the type of use and annual sewage water amounts are decisive factors for pollution loads. Particularly high loads are mainly to be expected at former intensive filtration areas. Additional variations are caused by technical processes of operations. Treatment terraces in the vicinity of sedimentation tanks are usually more heavily contaminated than areas somewhat more distant. Particularly high loads are to be expected around sedimentation tanks and sludge drying areas which have no sealing.

Fig. 2: Schematic illustration of sewage farm divisions

After sewage farm operations were stopped, areas no longer used were usually levelled to a large extent, filled, and ploughed up. This resulted in a mixing of soils with different levels of contamination. Contaminated soil material was brought into deeper soil layers in addition.

Not all contents of sewage water were retained in the soil passage. Considerable concentrations of nitrogen and phosphate compounds in sewage farm discharges polluted the receiving preclarification outlet trenches. Waters particularly affected in the urban area were Panke/ Nordgraben, Tegeler Fließ, Wuhle, Unterhavel and Rudower Fließ. The decommissioning of the sewage farms already led to an improvement of water quality in the past. Beyond the contamination of surface waters, a transfer of nitrogen compounds and organic pollutants into ground water has been detected (cf. e.g. Liese et al. 2004). Heavy metals, however, are largely retained in the surface soil.

Ending the intensive use of sewage farms has diverse effects on the eco-system:

Nutrients and pollutants accumulated during the operation of sewage farms are primarily bound in the soil’s organic substances. The changed water economy and chemical condition of soils at abandoned sewage farms results in a decomposition of organic substances, and a reduction of binding capacity can be expected. As the pH value decreases, bound nutrients or pollutants can then be mobilised and washed out into the groundwater, or the bordering preclarification outlet trenches.

Discontinuing sewage farm use also had considerable consequences for the area’s water balance. A significant drop in the groundwater level was registered at gauges in the area of the southern sewage farms. This had direct consequences for the local vegetation and for the yield potential of agricultural areas. Discontinuing sewage farm treatment also led to a reduction in the groundwater supply in the Berlin metropolitan area. After the abandonment of the northern sewage farms, problems arose with the water flow of the Panke and the Tegeler Fließ. They had previously received some water from sewage farm outflows.

In order to mitigate the negative consequences resulting from the closure of the sewage farms, various concepts were discussed and tested. Possible measures include:

• maintaining binding strength of soils by introducing organic substances or lime to stabilise pH values
• removal of pollutants by plants with high biomass production, and
• the renewed wetting or further flooding with purified sewage treatment plant outflows to achieve groundwater accumulation (recharge) and the prevention of organic substance degradation.