An experimental research has been carried out in a small WWTP in order to study the impact of storm events, with a global approach including all CSOs. Special attention has been devoted to the ammonia removal and discharges into the receiving water during storm events. More than 100 rainfalls have been measured between September 1993 and December 1994. The main results are:

- the mass of ammonia discharged by overflows without any treatment ranges from 50 to 90% of the total mass transferred in the sewer system during rainfall periods.

- the mass of ammonia treated in the WWTP ranges from 70 to 140% of the dry weather mass during the same duration, but with a mean concentration ranging from 24 to 76% of the dry weather concentration due to the dilution effect.

- the ammonia removal efficiency remains above 90% even for strong or long rainfall periods, thanks to the overdesign of the WWTP.

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