For many years, the percolating filter system of sewage treatment was the most commonly used (although now it is steadily being replaced by the activated sludge process). The incoming sewage, whether it be from a combined or separate sewerage system, is first passed through a 'gate' of parallel iron bars (coarse screen) which removes the large objects that find their way into the sewers, such as pieces of wood, bottles or cardboard boxes. The grit and stones from roads etc. are allowed to settle out in a small tank and the sewage then flows into very large tanks known as primary settlement tanks. Here, the heavier particles of organic matter settle to the base of the tank as a thick muddy layer which is known as primary sludge. This sludge is pushed into a central chamber by scrapers attached to a large beam which is slowly rotated round the tank by a motor attached to its end, and then it is siphoned off. About 50 per cent of organic matter in sewage is removed in the primary tanks.
Whilst the sludge accumulates at the base of the tanks, the cleaner surface layers overflow from the primary tanks on to the next stage of treatment where the sewage is purified by micro-organisms. The settled sewage is spread over the surface of further tanks, which are either circular or rectangular, at least two metres deep and full of stones. The stones are about five centimetres in diameter near the surface and slightly larger further down. The sewage is spread by slowly rotating radial arms in circular tanks, or by a suspended beam driven backwards and forwards across rectangular tanks. The settled sewage trickles down through the tank and over the stones. The stones are covered with a slime of many different micro-organisms, mainly bacteria but also protozoa, worms and the larvae of insects. The bacteria feed on the sewage and purify it, whilst the worms and insect larvae live on the bacterial slime, thereby controlling its growth and preventing it from clogging the spaces between the stones.
The water that collects at the base of the percolating filter tank contains the breakdown products of the purified organic matter, dead microbial cells and insect larvae, and the waste products of the larger organisms in the tank. This purified sewage is piped to more settling tanks, so-called final settling tanks, where the debris from the filters is allowed to settle and the overflow of relatively clean water is allowed to pass to the river. The whole process is shown in Figure 1. Thus the final products of the sewage treatment process are the clean effluent that goes to the river, and the sludge which accumulates in the primary and final settling tanks.
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