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3. Wastewater and Stormwater Collection A sewerage system collects wastewater and can be in the form of blackwater separated from greywater, or mixed with it (sewage). Gravity is used wherever possible to convey the wastewater. It is not surprising therefore that natural stormwater drainage is usually used, because this is how rainwater run-off is conveyed in nature by gravity. The principle of using gravity as the driving force for conveying wastewater in a sewerage system should be applied wherever possible, because this will minimise the cost of pumping. Natural stormwater drainage occurs in what is usually termed a catchment basin. In a catchment basin, rainwater run-off flows to a common point of discharge, and in so doing, forms streams and rivers. Crossing a catchment boundary may mean that the water has to be unnecessarily pumped, requiring an energy source. A wastewater sewerage system should therefore be within a stormwater catchment basin. Sewerage systems can be classified into combined sewerage and separate sewerage. Combined sewerage carries both stormwater and wastewater, while separate sewerage carries stormwater or wastewater separately. Recent trends have been for the development of separate sewerage systems. The main reason for this is that stormwater is generally less polluted than wastewater, and that treatment of combined wastewater and stormwater is difficult during heavy rainfalls, resulting in untreated overflows (commonly termed combined sewer overflow, CSO). In practice there is usually ingress of stormwater into wastewater sewerage pipes, because of unsealed pipe joints, and unintentional or illegal connections of rainwater run-off. Conversely there may be unintentional or illegal wastewater connections to stormwater sewerage. Wastewater sewerage systems can be classified into three major types:
3.1 Conventional sewerage Conventional sewerage is also termed deep sewerage because the sewerage pipes are laid deep beneath the ground. Pumping is generally required at various stages of the sewer pipe network, especially if the landscape is fairly flat. The larger the population served by the sewerage system, and the longer the planning horizon is to cope with future population increases, the larger the diameter of the final pipes becomes. The costs of the pipes, inspection manholes, pumps and pumping stations and their construction/installation are therefore high. The costs of operation and maintenance are correspondingly high because of very conservative design assumptions.
3.2 Simplified sewerage Simplified sewerage is also known as shallow sewerage, reflecting the shallower placement of the pipes in contrast to the conventional or deep sewerage. The purpose of simplified sewerage is to reduce the cost of construction and the corresponding cost of operation and maintenance. Simplified sewerage design is based on hydraulic theory in the same manner as for conventional sewerage but has less conservative design assumptions. Smaller diameter pipes are used when water use per person is known to be less and the minimum depth of cover of pipes can be as low as 0.2 m when there is only light traffic. Manholes can be replaced by inspection cleanouts because of the shallow pipes. The design planning horizon can be shorter because the population projection may be uncertain. In a variation of the simplified sewerage, the pipe layout passes through property lots (condominial) rather than on both sides of a street (conventional). Figures 4 and 5 show the sewerage layouts in conventional sewerage and in condominial sewerage. The cost of construction of simplified sewerage can be 30 to 50 % less than conventional sewerage depending on local conditions.
Shallow sewerage is also conducive to local community participation because sewer pipes have to cross property boundaries. The community has to agree to this arrangement which extends after construction for maintenance (e.g. unblocking of sewer pipes). The shallow pipe, and hence the shallow trenches, also allow members of the community to participate by, for example, providing labour for digging the trenches. This is in contrast to conventional sewerage where specialised machinery is required for the deep trenches. Simplified sewerage was originally developed in Brazil and is increasingly being used in other parts of the world. The ‘International Source Book on Environmentally Sound Technologies for Wastewater and Stormwater Management’ (hereafter referred to as the Source Book), published by IWA and IETC, provides useful case studies. 3.3 Settled sewerage Settled sewerage refers to sewerage for conveying wastewater that has been settled, for example, in a septic tank. Settled sewerage originated to convey the overflow from septic tanks where the soil cannot cope or absorb the overflow. This usually occurs when the groundwater table is high, or where the soil permeability is low, or where there are rock outcrops. It can also be used when effluent from septic tanks pollutes groundwater and it is necessary to convey the effluent off-site and treat it. Because there are no solids that can potentially sediment in the sewerage pipes, there is no requirement for the self-cleansing velocity. Smaller pipes and lower gradients can be used. The cost of settled sewerage is between a third and a half of conventional sewerage. Originally developed in South Australia to overcome problems with failing septic tanks, it has been used quite widely worldwide to upgrade septic tank systems.
Where there is no existing septic tank, an interceptor box or tank can be used. It functions like a septic tank and designed in the same way (Figure 6). To reduce cost, the wastewater from a group of houses can be connected to one interceptor tank. Just like in a septic tank, the accumulation of sludge has to be removed regularly from an interceptor tank. 3.4 Stormwater collection Stormwater flows through the landscape’s natural drainage system. Piped
stormwater collection was a development in European cities to overcome odour and
improve aesthetic appearance of wastewater disposed with stormwater. The
covering of ditches used for combined sewerage was an intermediate step in using
natural drainage to construct sewerage for combined wastewater and stormwater.
Piped sewerage also allows more land area for road and footpaths. With the
separate collection of wastewater there is an opportunity to return some
stormwater flow path to its more natural state to improve urban amenity value.
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