From the Sewer to the Spree River

Six wastewater treatment plants clean sewage from Berlin and the surrounding regions

In such densely populated regions as Berlin, particularly high requirements are set for cleaning wastewater. During dry weather conditions, Berliner Wasserbetriebe's six wastewater treatment plants clean approximately 665,000 cubic metres of wastewater per day. Treated wastewater flows into the Spree and Havel Rivers directly or via the Erpe, the Panke, the Nordgraben or the Teltow Canal. These bodies of water are slow moving and have a low volume. The wastewater that is pumped by the pumping stations through pressure mains to the wastewater treatment plants initially passes through the mechanical treatment stage. Coarse contaminants such as paper, textiles, wood and plastic are removed in the screening plants. Automatic rakes remove any waste stuck on the screen. Then it is collected, dewatered in containers, and disposed of in an ecologically responsible manner.

The wastewater then flows through the grit chamber. It consists of long channels in which coarse mineral solids such as sand, gravel and stones settle on the floor at a flow velocity of around 30 centimetres per second. These solid materials are pushed by scrapers into hoppers and pumped into grit washing tanks. There, the grit is freed of organic substances, dewatered and later diposed of. In the primary settlement tanks, the flow velocity of the wastewater is reduced to around 1.5 centimetres per second so that lighter, undissolved substances can settle on the floor of the tank. The buoyant particles collect on the water's surface. The sludge is pushed by scrapers into sludge hoppers. The sludge is stored temporarily and then pumped to the sludge treatment plant. Floating matter, which moves on the water's surface and mainly consists of grease, is removed.

The mechanically pre-treated wastewater then flows into the aeration tanks of the biological treatment stage. In this treatment stage, dissolved organic substances, as well as phosphorus and nitrogen compounds, are degraded. The degradation is carried out by bacteria and other microorganisms which form the aerated sludge. The first part of the aeration tanks is kept free of oxygen. This stimulates bacteria to consume phosphorus compounds in the wastewater in the subsequent oxygen-rich zone of the aeration tanks. The nitrogen compounds are reduced by other bacteria, which are also exposed to changing oxygen concentrations. In addition to biological phosphorus removal, simultaneous chemical precipitation can be used if needed. In this case, the precipitant iron (I) sulphate or iron(II) chloride is added to the aerationtanks in a dissolved form. Iron (III) phosphate produced, which then mixes with the biological sludge. The wastewater then flows through the final clarification tanks. Here the activated sludge has several hours to settle. Afterwards, it is pushed into hoppers and then most of it is pumped back into the aeration tanks in order to maintain a constant level of microorganisms. Finally, any excess sludge is passed on to the sludge treatment plant for further processing. The treated wastewater – we call it clear water – is transported into a body of water, so that it returns to the natural water cycle.