Screening, then Coagulation, Flocculation and Clarification or Dissolved Air Flotation, followed by Filtration and Disinfection, is a typical treatment which converts raw water to drinking water. It removes colour, suspended and colloidal matter from reasonably clean water but cannot reduce their salinity without desalination. A range of other processes may be used in water treatment to remove specific ranges of contaminants in the water. Examples include activated carbon filters, ion exchange and membrane processes. The sedimentation that occurs during raw water storage can also be regarded as part of the water treatment.
The first stage in preliminary treatment usually involves a simple screening or straining operation to remove large solids. In the case of water treatment some form of sloping protective boom or coarse screen with openings of about 75 mm is used to prevent large objects reaching the intake. The main screens are usually provided in the form of a mesh with openings of 5-20mm and arranged as a continuous belt, disc or drum through which the flow must pass (Figure Water Treatment). The screening mesh is usually rotated slowly so that the material collected can be removed before an excessive head loss is reached The screenings removed from water are normally returned to the source downstream of the abstraction point.
The use of chemicals for coagulation brings a further level of complexity to them treatment process and should only be adopted if the necessary supplies and skills are available locally. The use of natural coagulants like Moringa oleifera as described in Chapter 12 can make coagulation feasible in situations where conventional coagulants are unaffordable or unavailable. Chemical coagulation will only be successful if the appropriate dose can be determined and then applied to the water in such a manner as to ensure adequate mixing and flocculation. A simple form of chemical feeder is one based on hydraulic control of a solution such as the Marriotte vessel which provides a constant rate of discharge regardless of the level in the storage container. The coagulant must be added at a point of turbulence such as a weir or in a baffled channel and flocculation is best achieved in a baffled basin connected to a settling basin. In practice it is difficult to prevent floc carry-over from the settling basin so that the output water quality may at times have fairly high turbidity levels. A more satisfactory water may therefore arise by omitting the coagulation stage and proceeding directly with filtration.