5.2 Preliminary treatment
The abstracted water is first screened to remove suspended and floating debris, such as leaves or branches, which could interfere with the operation of machinery in the treatment works. The water may then enter a preliminary settlement tank or storage reservoir. It then passes through screens again and goes to the treatment works. Screens may be classified by the size of their openings as coarse or fine, and may be in the forms of bars or continuous belts. Coarse screens are used primarily to protect the treatment works from physical damage, while the fine screens serve to remove material which might eventually block pipework in the system. Coarse screens usually consist of a series of metal bars spaced 5–15 cm apart. Fine screens, which follow the coarse screens, have a bar spacing of 5–20 mm. Screens are positioned in the inlet channel of the treatment plant at an angle of 60° to facilitate removal of the collected material or screenings by raking. The cleaning of the screens is important to prevent them choking. Bar screens can be raked by hand but are more usually cleaned by a mechanical raking operation, either on a time basis or by pressure-sensing probes which are activated by an excessive head loss (pressure drop) across the screen. A continuous chain scraper can also be used to clean bar screens (Figure 20). There are also fine mesh screens which are cleaned by water jets. These meshes can be on frames or, more commonly, in the form of a drum.
A variation of the fine screen is the microstrainer (Figure 21). This consists of a rotating drum with a stainless steel micromesh fabric. The mesh size can range from 15 μm to 64 μm so that very fine suspended matter such as algae and plankton can be trapped. The trapped solids are dislodged from the fabric by high-pressure water jets using clean water, and carried away for disposal.
Storage of the screened water in a preliminary settlement tank or reservoir smoothes out fluctuations in the water quality and helps to reduce the suspended solids content. It also reduces the number of pathogenic bacteria present, and the oxidation which can occur will allow the degradation of organic matter and the precipitation of soluble iron and manganese as oxides and hydroxides. It is generally recommended that storage should be for at least seven days in the case of river-derived supplies. The storage of water is particularly valuable when abstraction is not possible, e.g. during droughts, or when the water source is badly contaminated or in flood condition.
After preliminary settlement, it may be necessary to aerate the water in the case of poor quality water with a low dissolved oxygen content. There are several ways in which this can be done but the simplest is to allow the water to fall over a series of steps so that it is able to entrain oxygen from the air. This is known as cascade aeration. In addition to increasing the oxygen content, aeration also helps to liberate soluble gases, such as carbon dioxide and hydrogen sulphide, and volatile organic compounds which could give an undesirable taste to the water.
Aeration can reduce the corrosiveness of raw waters which are acidic due to their carbonic acid content. When the water is aerated, some of the dissolved carbon dioxide is displaced by the oxygen dissolving in the water. This causes some of the carbonic acid that has been formed in the water by the carbon dioxide to be converted back to carbon dioxide and water in order to maintain chemical equilibrium, as we discussed in Section 2.4.
Aeration is also used to remove iron and manganese from solution. Iron and manganese can cause peculiar tastes and can stain clothing. Iron is soluble in water only in the absence of dissolved oxygen and at pH values below 6.5, when it is in the ferrous (Fe2+) state. Aeration converts soluble iron into its insoluble hydroxide [Fe(OH)3] which can then be removed by filtration. Manganese can be removed in the same way.
After aeration, the water may be passed through a further fine screen before entering the treatment works proper.