In the first stage, the still completely untreated wastewater is mechanically treated; this removes about 20 - 30% of the contained solids. To realize this, the wastewater is guided into a screening plant, where a screen or sieve drum filters out coarse impurities like leaves, paper or textiles. Various screens, from coarse screens with several centimetres gap width, to fine screens with a niche width of a couple of millimetres, through which the water flows at different speeds, filter the coarse materials step-by-step. The mechanically-recovered screen debris is dewatered and disposed of in an incineration plant.

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The pre-purified water then passes into what's called a sand collector. In wastewater treatment technology, a sedimentation tank is employed to get rid of coarse particles, like stones, glass splinters or sand, also as coarse organic material that has not been separated out by the screens. This happens with a comparatively high flow velocity of about 0.3 m/s. A distinction is formed between the non-aerated long sand collector, the aerated long sand collector – also called a cylindrical sand collector –, and therefore the round sand collector.

The aerated sand collector removes additional fats and oils from the wastewater, and therefore the following occurs: the introduced process air produces a rolling motion within the water, which carries lighter substances, like oils and fats, to the surface. They will be easily faraway from the water here.

A round sand collector separates substances from the waste water with force and sucks them away. After cleaning within the sand collector, the sand collector debris is washed and free of organic substances. This improves the dewatering of the collected inorganic material, which can, for instance, be reused in construction . If further recycling isn't possible, the sand collector debris must be disposed of properly; it's landfilled or destroyed in waste incineration plants.

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The primary wastewater treatment tank is that the next stage of wastewater treatment. The speed of the wastewater is approx. 1.5 cm/s, significantly slower than within the sand collector. The reduction of the flow velocity is achieved by widening the basin. A coffee flow velocity is important in order that the finer dirt particles can, counting on their nature, choose rock bottom or on the water surface. The sludge produced by sedimentation (settling to the bottom) is named primary sludge. it always consists of organic material. The first sludge is pushed from rock bottom into a fresh sludge hopper by a scraper. The floating substances are transferred to a floating sludge duct. A pump transports the fresh sludge to what's referred to as a digestion tower.

In the digestion tower, methane gas is produced in four phases (hydrolysis, acidification, acetone gene and methanogene phase); it's converted into electricity during a block heating system and may be wont to supply the plant with energy. The digestion process within the digestion tower is completed after approximately four weeks. What remains is an odourless sludge, which is usually utilized in agriculture after dewatering by centrifuge or filter.

 The mechanical cleaning stage ends here. On the average, 30% to 40% of the pollution is faraway from the wastewater during this phase. On its way through the wastewater treatment plant, the wastewater now reaches subsequent stage of wastewater treatment.  

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