Newsletter and Technical Publications
<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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