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3. Wastewater and stormwater collection (Topic b)

Collection of wastewater is by use of a sewerage system. Depending on whether blackwater is generated separately from greywater, or mixed with it, we need to collect greywater or the mixture of blackwater and greywater (sewage). Gravity is used wherever possible to convey the wastewater. It is not surprising therefore that natural stormwater drainage has been used, because this is how rainwater run-off is conveyed in nature by gravity. The stages of development of the use of a natural drainage system for conveying both wastewater and stormwaster have been described in Section 1, outlining its evolution from lining and covering of the drains, to the trend of separately collecting wastewater and returning the stormwater drainage to its more natural state.

Figure 2.10. Plan of London's sewerage, showing the main sewers and drainage areas of Beckton and Crossness wastewater treatment plants.

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 form 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. Figure 2.10 shows an example of wastewater collection in a 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: 1. Conventional sewerage, 2. Simplified sewerage and 3. Settled sewerage

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