Monitoring of storm sewer overflows

The poor quality of many receiving waters has been attributed to the frequent operation of combined sewer overflows (C. S. O. s). As the existing need for overflow structures is unlikely to change in the foreseeable future it is imperative that these deleterious effects are minimised. The present study investigates the pollution performance of three common overflow structure designs (the stilling pond, the double high side weir and the double low side weir) and aims to institute a novel pollution monitoring methodology for combined sewer overflows. Four sites in the central Sheffield area were monitored for periods from 9 to 13 months (two high-side weirs, one low-side weir and a stilling pond). Each overflow was monitored with continuous flow measurement equipment and bottle samplers to obtain samples of sewage from the storm and dry weather flows. The bottle samples were analysed for suspended solids (SS), ash, BOD, COD, pH, conductivity and ammonia. Mesh bags and frames were also installed to trap the gross solids (solids with a median size>6mm) from the inflow and the spill flow.

The stilling pond and the high side weirs were found to perform well hydraulically, limiting the flow to treatment to a steady maximum. The low side weir performed unsatisfactorily, hydraulically, as the flow to treatment rose as the incoming flow increased and, for some storm conditions, a hydraulic jump formed towards the downstream end of the chamber.

The first foul flush was regularly observed at the stilling pond and low side weir sites. Peak concentrations for SS were found to be 600 times greater than the dry weather flow for the same time of day. The first foul flush was rarely observed at the other sites. For the majority of storms at each site the spill concentrations were of a similar magnitude to the inflow sample concentrations. However for a large minority of SS, BOD and COD samples, the concentrations of the spill samples were significantly less than the inflow samples. t-Tests suggested that at the stilling pond and high side weir sites there is a significant reduction in the spill sample concentrations for the water quality (bottle) samples.

Although the load of material spilled during an overflow event was found to be small in comparison to the inflow load, large amounts of material were spilled to the watercourse during storm events at each of the sites investigated. The storm load entering the CSO was found to be considerably influenced by peak intensity of the storm at the stilling pond site and antecedent dry weather period at one of the high side weir sites. At the other sites a number of hydrological factors were found to be influential e. g. duration. It is thought that time of year may also be important factor as this influences the type of rainfall (its duration and intensity).

The types of gross solid collected at each site were similar with leaf material and sanitary towels consistently being the major items in terms of total mass. The efficiency of the stilling pond and one of the high side weirs in retaining gross solids in the flow to treatment appeared to be explained by the flow split although for 5 of 14 storms at the stilling pond and 3 of 7 at the high side weir a treatment effect was observed. The treatment factors at the low side weir were noticeably less than those for the other three sites with all being less than unity (average 0.5). This suggests that the low side weir preferentially discharges gross solid material over the weir. The treatment factors at the other high side weir were low due to inadequate sampling of the spill flow.