The provision of large volumes of in-sewer storage to control Combined Sewer Overflow (CSO) discharges has various effects on downstream sewage treatment. The discharge of longer periods of high flow, caused by the need to empty the storage tanks, has been recognised in existing procedures. Less is known about changes in the quality of discharged water caused by storage, and its subsequent effects on sewage treatment.
This project for UK Water Industry Research (UKWIR), was undertaken by WRc and looked at two possible effects:
- The conversion of less degradable forms of chemical oxygen demand (COD) to volatile fatty acids (VFAa), which are readily degradable and may overload the treatment capacity of the initial part of an aeration lane.
- Creation of large quantities of hydrogen sulphide (H2S) and other odourous compounds.
The project assessed the effect on sewage treatment performance of VFAs, and the likely odour increase caused by the H2S.