BEHAVIOR OF A SMALL WASTE-WATER TREATMENT-PLANT DURING RAIN EVENTS

New legislation in France specifies that all effluents should be treat ed before being discharged into receiving waters and that effluent cri teria should be met 95% of the time. Therefore, all flows routed to wa stewater treatment during rain events will have to be treated. To eval uate the impacts of these flows on the behaviour of a treatment plant, a small works has been instrumented with samplers, flowmeters, and pr obes (dissolved oxygen, turbidity, and oxidation-reduction potential). A total of 10 rain events were monitored along with one dry-weather e vent. Sludge blanket overflows have been observed for half of the even ts. Results clearly show for the influent an increase in all particula te pollutants at the start of the event followed by some dilution, and a dilution for dissolved constituents. Also observed is the mixed liq uor movement from the aeration tank to the settler.