Monitoring of storm sewer overflows

The poor quality of many receiving waters has been attributed to the frequent operation \nof combined sewer overflows (C. S. O. s). As the existing need for overflow structures is \nunlikely to change in the foreseeable future it is imperative that these deleterious effects \nare minimised. The present study investigates the pollution performance of three \ncommon overflow structure designs (the stilling pond, the double high side weir and the \ndouble low side weir) and aims to institute a novel pollution monitoring methodology for \ncombined sewer overflows. Four sites in the central Sheffield area were monitored for \nperiods from 9 to 13 months (two high-side weirs, one low-side weir and a stilling pond). \nEach overflow was monitored with continuous flow measurement equipment and bottle \nsamplers to obtain samples of sewage from the storm and dry weather flows. The bottle \nsamples were analysed for suspended solids (SS), ash, BOD, COD, pH, conductivity and \nammonia. Mesh bags and frames were also installed to trap the gross solids (solids with \na median size>6mm) from the inflow and the spill flow. \n The stilling pond and the high side weirs were found to perform well hydraulically, \nlimiting the flow to treatment to a steady maximum. The low side weir performed \nunsatisfactorily, hydraulically, as the flow to treatment rose as the incoming flow \nincreased and, for some storm conditions, a hydraulic jump formed towards the \ndownstream end of the chamber. \n The first foul flush was regularly observed at the stilling pond and low side weir sites. \nPeak concentrations for SS were found to be 600 times greater than the dry weather \nflow for the same time of day. The first foul flush was rarely observed at the other sites. \nFor the majority of storms at each site the spill concentrations were of a similar \nmagnitude to the inflow sample concentrations. However for a large minority of SS, \nBOD and COD samples, the concentrations of the spill samples were significantly less \nthan the inflow samples. t-Tests suggested that at the stilling pond and high side weir \nsites there is a significant reduction in the spill sample concentrations for the water \nquality (bottle) samples. \n Although the load of material spilled during an overflow event was found to be small in \ncomparison to the inflow load, large amounts of material were spilled to the watercourse \nduring storm events at each of the sites investigated. The storm load entering the CSO \nwas found to be considerably influenced by peak intensity of the storm at the stilling \npond site and antecedent dry weather period at one of the high side weir sites. At the \nother sites a number of hydrological factors were found to be influential e. g. duration. It \nis thought that time of year may also be important factor as this influences the type of \nrainfall (its duration and intensity). \n The types of gross solid collected at each site were similar with leaf material and sanitary \ntowels consistently being the major items in terms of total mass. The efficiency of the \nstilling pond and one of the high side weirs in retaining gross solids in the flow to \ntreatment appeared to be explained by the flow split although for 5 of 14 storms at the \nstilling pond and 3 of 7 at the high side weir a treatment effect was observed. The \ntreatment factors at the low side weir were noticeably less than those for the other three \nsites with all being less than unity (average 0.5). This suggests that the low side weir \npreferentially discharges gross solid material over the weir. The treatment factors at the \nother high side weir were low due to inadequate sampling of the spill flow.