Behaviour of a small wastewater treatment plant during rain events

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