You are here

Station 8: Water Clarification

Why does wastewater have to be clarified?

Every person contributes to wastewater, whether through daily showering, doing laundry or washing dishes. So-called domestic wastewater contains many different types of pollutants. If these dirty waters were to be dumped into rivers and lakes, grave environmental damage would result. Above all, these bodies of water would sustain serious damage. Thus wastewater treatment is necessary in order to ensure water pollution prevention, as well as for hygienic and aesthetic reasons.

How does a water clarification plant work? 

Figure: [Prof.Blumes BILDUNGSSERVER 2004]

Incoming wastewater is initially directed through the sewage system to the respective clarification plant. Here, coarse solids such as branches, paper and bottles are removed by a bar screen. A grit chamber separates mineral sediments, such as sand and gravel, from the wastewater. These sediments sink to the bottom and can be easily removed. In the settling basin, water is held back so that granular and flakey substances can settle. These substances are known as mixed primary sludge and sent on to sludge treatment. Sludge is treated in two basins that form one unit; first of all it undergoes anaerobic digestion in the activated sludge tank, where microorganisms are intentionally brought into contact with the waste in order to biologically purify it. The microorganisms absorb the dissolved organic waste components and transform them into inorganic substances (water, salts, carbon dioxide). After a certain period of time, the sludge water is delivered to the secondary clarifier, where the activated sludge sinks to the bottom and is thus separated from the purified water. This activated sludge will be returned in part to the activated sludge tank. The remaining secondary sludge is sent to sludge treatment. The remaining water in the secondary clarifier is filtered and sent along into a natural body of water.

Sludge treatment takes primary and secondary sludge, and makes it more concentrated, reducing the water content. The sludge is subsequently led into a hermetically sealed digester, where putrefactive bacteria are introduced, and sludge gas is produced through a process of fermentation. The resulting gas can be stored and used to supply energy for the clarification plant. After four weeks, the rest of the sludge is completely digested and odourless. This substance can be used as agricultural fertilizer, or sent on to further treatment stages, such as draining, incineration or dumping.