9. It is unusual that raw water is pumped directly from its source to the treatment plant. The use of intermediate processes, which can collectively be called pre-treatment, increases the effectivity and life-span of the treatment plant. Raw water storage, pre-chlorination, aeration, algal control, straining, preliminary settling, coagulation, mixing and flocculation are all pre-treatment processes. Each performs a particular function and unless the quality problem they are intended to resolve is part of the raw water’s characteristics, they should be omitted.
10. In general terms, the quality of water that is left undisturbed in containers, tanks or reservoirs improves since some pathogenic micro-organisms die and heavy matter in suspension settles (sedimentation). Efforts to provide maximum storage capacity at refugee sites at the onset of emergency assistance operations is a logical step. This task may not always be accomplished, however, especially when water demand is large (large refugee populations) or when water is limited in quantity. To bring about a substantial improvement in water quality, storage should be possible for at least 12 to 24 hours; the longer the period of storage and the higher the temperature, the greater the improvement. Storage periods of up to two weeks are recommended as necessary to achieve maximum improvement in raw water by storage. The fact that other organisms are encouraged to develop in stagnant water should, however, not be forgotten. Storage of raw water may create a silt problem; reservoirs tend to silt up very quickly in the absence of some sort of a silt trap. The cost of building reservoirs large enough to be effective for water quality improvement is fairly high and, on this basis, they should normally be omitted from the treatment processes.
11. The practice of injecting chlorine into the raw water soon after it is abstracted from its source (normally a surface stream) is called pre-chlorination. This step is usually omitted for reasonable quality water and is normally more effective in low turbidity water having a high bacteria content. The amount of chlorine used is fairly high (2-5 mg./l). Chlorine oxydizes organic matter, iron or manganese during the time water spends in settling basins; it will also reduce colour and slime formation. As much greater quantities of chlorine are used than in post-chlorination and complete water disinfection may very seldom be accomplished with it, pre-chlorination should not be regarded as a substitute for post-chlorination (See 8.21) but as a safeguard to be adopted only when extremely polluted (but fairly clear) raw water has to be used in emergency situations.
12. Aeration is practiced to add oxygen from the atmosphere to water and to liberate undesirable gases such as carbon dioxide or hydrogen sulphide. It is commonly done by splashing the water over trays or by blowing air bubbles through the water. It is a viable and cheap means of controlling tastes, odours and corrosion but its results may not be considered complete in all cases. Among the equipment normally used for aeration, the most common are some special nozzles which direct thin jets of water into metallic plates to produce fine sprays exposing water to the atmosphere; cascade-type aerators which create turbulence in thin streams of water flowing down; tray-type aerators consisting of some five perforated trays, increasing in size from top to bottom, where water (falling from tray to tray) is exposed to air; and diffused air aerators, which are tanks where air is bubbled upwards from diffuser pipes laid on their floor. The latter method is the most efficient; the amount of air needed may be regulated; the tanks are normally about 4 metres deep and have a retention time of about 15 minutes. Among all the methods, however, trays are the most commonly used because of their low cost, simple operation and reasonably high efficiency.
13. Algal control is necessary to eliminate outbreaks of these organisms which are usually classified as plants and which proliferate in rivers and reservoirs. These outbreaks tend to be sporadic or seasonal but normally severe and can cause trouble to waterworks’ operators. Fairly alkaline waters, with an appreciable concentration of nitrates or phosphates, are likely to develop important algal colonies. Although heavy pollution may impede the growth of algae, water treatment, by itself, may encourage it (once pollution has been eliminated). Chlorine doses of up to 1 mg./l may kill the algae (See 9.8.23). Algal growth is inhibited by Copper sulphate in concentrations of 0.3 mg./l; these doses are, however, toxic to some fish species and may therefore not be acceptable in some circumstances. Strainers are widely used to remove algae, some of them functioning as rapid sand filters (See 8.18) which, if their filtration medium is coarse, are known as “roughing filters” (See 8.14). Other devices, called “microstrainers”, which are mainly of proprietary make, are excellent, provided that the water is relatively free of silt.
14. Where sediment loads in raw water can reach concentrations of more than 1000 mg./l, it is helpful to put in small, non-chemically assisted, horizontal flow basins immediately upstream of other treatment works, such as normal sedimentation basins, to increase the effectiveness of the treatment process, minimize plant maintenance and save on the use of chemicals. These facilities are called “pre-settlement basins”. Alternatively, horizontal roughing filters may be used to improve the quality of raw water that will undergo further treatment through slow sand filtration devices; they are rectangular boxes similar to the basins used in plain sedimentation (See 8.16); their raw water inlet is situated on one side of the box, their outlet at the opposite side (Fig 29). In the main direction of flow, water passes through various layers of graded coarse material (in the sequence coarse-fine-coarse). Vertical depths of filtration are in the range of 0.8 to 1.5 m.; suitable filtration rates are in the range of 0.4 to 1.0 m/h); the total length of the filter would vary between 4 and 10 metres. Pre-settlement basins and horizontal roughing filters are sometimes built as a remedy, where changing raw water characteristics have put in jeopardy existing waterworks facilities (a common occurrence in developing countries).
Fig. 29 Features of a Horizontal Flow Roughing Filter