The sewage treatment processes described above can purify sewage so that at least 90 per cent of the organic matter is biodegraded and the concentration of nutrients is substantially reduced. The effluent can be discharged into most rivers without any adverse effect on the receiving water quality. There are some rivers though that drain into lakes, and others may be shallow and slow flowing. In these cases, the nutrients present in the purified sewage effluent may encourage the growth of water weeds or algae to excessive amounts (see Chapter 3). The excess weeds or algae can be harmful to other aquatic life because they cover
the river bed or the water surface and prevent light from penetrating to the deeper water. The nutrient of particular concern is phosphate as the concentration of this is usually the deciding factor in the growth of aquatic plants and algae. These nuisance growths can be prevented by giving the purified sewage an extra treatment stage for nutrient removal. This can be either a physico-chemical process or a biological one.
In the physico-chemical process, a metal salt is added to the sewage. This forms an insoluble compound of phosphorus which settles out in a sedimentation tank. The metal salts added are those of either iron or aluminium. The reaction between aluminium and phosphate can be represented simply by the following equation:
The most common form of aluminium used is alum which has the formula
Al2(SO4)3.14H2O, and the reaction is as follows:
Iron salts are commonly used to precipitate phosphate at sewage treatment works and both ferrous (Fe2+) and ferric ions (Fe3+) are used in the form of sulphate or chloride. Their reactions are:
3FeCl2 + 2PO3- ^ Fe3(PO4)21 + 6Cl-3FeSO4 + 2PO|- ^ Fe3(PO4)21 + 3SO42-
There are various biological methods of removing phosphorus from sewage before it is discharged into a river but a particularly attractive one is to construct an artificial 'wetland' or a reed bed. These not only remove the nutrients but also further reduce the amount of organic matter in effluent.
In its simplest form, a wetland comprises a large shallow tank with a very gradual slope extending from a wide distribution system at the inlet end, to a collecting channel at its lowest end. The tank is filled with a bed of gravel into which reeds are planted. The roots of the reeds filter out organic matter and suspended solids whilst the reeds themselves extract nutrients from the effluent before the drainage water overflows into the river.
In some locations the reed beds can be extensive and are constructed by creating large flat areas of ground adjacent to the river or lake. They become havens for wildlife which take advantage of the cover provided by the reeds and exploit the food available in the marshy ground. (In Chapter 7, the use of a reed bed to clean up mine-water drainage is described.)
In some countries, the nutrients in sewage effluent are used to grow other crops. In Scandinavia and Poland, the effluent from sewage works is diverted into large areas planted with willow trees. The trees grow rapidly because of the extra supply of nutrients in the water and they are regularly pruned to provide wood for burning in domestic fires, for power generation or for paper manufacture.
Nowadays, we are much more aware of the loss of nutrients from sewage and how we have been throwing away this valuable resource. The next section describes how sewage sludge is increasingly being recovered and utilized to improve the quality of soil by increasing its nutrient status.
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