A six-year study of CSO problems showed that in most cases the settleable sewer solids are responsible for poor water quality. This is the reason why the settling behaviour of settleable solids should be taken into account at stormwater treatment to a greater degree than it is done up to now. New efficiency measurements at stormwater tanks with clarifier overflows for combined sewage showed two important results:

1) The annual mean surface loading qA of a clarifier-type CSO tank is only about 25 % of the design surface loading of 10 m3/(m·h) recommended by the German guideline A 128.

2) The settling process in stormwater tanks is effective:

- The median settling velocity vs of sewer solids is reduced from about 0,21 cm/s to vs = 0,035 cm/s.

- The concentration of settleable solids is reduced about 80 %, that of total solids about 65% and COD concentration about 50 %.

With this efficiency, well designed stormwater tanks will be a good measure in many cases to reduce significantly CSO impacts on receiving water quality.

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