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<International Source Book On Environmentally Sound Technologies
for Wastewater and Stormwater Management>

4.3 Stormwater treatment

Stormwater can be polluted as discussed in Section 2 (2.1). When collected in a combined sewerage system it is treated with the wastewater, though treatment is not effective during peak heavy stormwater run-off periods resulting in combined sewer overflow (CSO) that is not treated. Storage basins or tanks can be used to accommodate moderate peak flows of combined stormwater and wastewater, and treating the stored water at night when wastewater flow is a minimum. The Regional Overview for Western Europe devotes a full sub-section on CSO.

Separately collected stormwater is generally treated by passing it through a settling basin to remove solids (Figure 2.34). The retention time in the settling basin is designed so that solids can settle in say 20 minutes for a one in five year storm-event. For storm-events less than the design value removal efficiency is greater, while for storm-events greater than the design value removal efficiency is lower. Mechanical devices have been developed that can trap gross solids (see North American RO). Both settling basins and mechanical traps need to be cleaned regularly to maintain solids removal efficiency.

Naturally landscaped stormwater drains can help filter out fine sediments through the action of vegetation slowing down the flow and trapping solids. Permeable surfaces allow rainwater to percolate into the soil, thus treating the water in much the same manner as land based treatment of wastewater (4.2.4.) and at the same time reduce the amount of run-off. Pavements have been designed and manufactured for this purpose. Directing run-off to vegetated area (rainwater harvesting) can reduce down-stream flow and reuse the water for maintaining plant growth. This is especially beneficial in arid climates. Four techniques for stormwater treatment are described below. Used judiciously these can treat stormwater locally (at source, Figure 2.35). Applying these on a sub-catchment scale (site), or whole catchment scale (region) can reduce flooding and the undesirable impacts of stormwater described in Section 2 (2.1), while at the same time improve the amenity value of the landscape through creation of, for example, passive recreation water bodies.

Figure 2.35: Management train for stormwater at the local, sub-catchment and catchment levels (CIRIA, 2000)


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