Contribution of Combined Sewer Overflow-Derived Sediments to Annual Participate Load and Oxygen Demand in the River Roch, England

Abstract

The nature and behaviour of sediments released from a Combined Sewer Overflow (CSO) and their impact on the steep, gravel-bedded River Roch in northwest England were monitored by sampling suspended and bedload sediments, and by analysis of bed material. CSO-derived suspended sediments characteristically had 103 478 mg kg−1 wet weight COD, and 17 536 mg kg−1 wet weight BOD, respectively 1.1 and 3.5 times those of river suspended sediments. However, over a two-year monitoring period, overflows accounted for only 0.3% of the river water discharge, and 2.0% of the river sediment load. The CSO sediment-related BOD was only 3.5 to 7% of the river's natural sediment-related BOD in the two study years, while CSO derived COD and ammonia accounted for only 3% and 0.5% of the river pollutant loads. Under certain hydraulic conditions during storm events, sediment-BOD from the CSO could account for 60% of the river's instantaneous BOD load. However, the only high sewer-derived sediment oxygen demands occurred during storm events. No lagged impacts on dissolved oxygen were observed. In this naturally turbulent, welloxygenated river, sediments suspended in overflow liquor have a short-lived impact on DO levels, but they are too fine to settle out in the first kilometre downstream. This welldesigned, new, CSO has a minimal impact on the stream.