DIFFUSE SOURCES OF HEAVY METALS TO SEWERS

Nov 1994

As large industrial sources of heavy metal to sewers are being reduced, diffuse sources are becoming more significant and may have to be controlled, if there are to be continuing improvements in the quality of sewage sludge recycled to agricultural land. This final report considers closely the significance of domestic, runoff, light industrial and commercial diffuse sources as contributors of heavy metal loads entering sewage works.

Studies carried out in two non-industrial catchments (Bracknell and Shrewsbury), revealed domestic inputs to be the dominant sources of copper and zinc, and in some areas lead also. For a hard water catchment such as Bracknell 55%1 of the copper load to the sewage treatment works was shown to be derived from household plumbing interactions. Light industry and commercial premises were the other major contribution accounting for some 20%1 of the total copper load at Bracknell STW. For catchments of lower hardness, however, domestic concentrations of copper were substantially lower. This was reflected by a good correlation being obtained for hardness against copper concentration in domestic sewage sludge for a range of UK sewage works.

For the domestic catchments in this study domestic sources of zinc and nickel were also the major inputs to sewers, being derived mainly from activities such as washing, bathing and from faeces. The significance of domestic sources of lead varied greatly, depending on the extent of lead plumbing present in the catchment. In Bracknell only 4%1 of the lead in sewage was derived from plumbing interactions, where as in Shrewsbury it accounted for ca. 25%1. Town centre/commercial heavy metal concentrations were not significantly higher (excluding mercury) than those measured from domestic sources. Higher concentrations of mercury were probably derived from dental practices. However, overall metal loads to the sewer from the town centre were smaller than from domestic households, for example, the town centre contributed some 15%1 of the total copper and zinc load at the STW. This reflected the smaller flow from the town centre in comparison with that from domestic households.

Raw sewage from light industry displayed elevated concentrations of lead, cadmium, chromium and nickel compared with domestic levels, this reflected the predominance of metal and car-related workshops found in the sub-catchment. It was estimated that runoff contributed 16%1 of the flow to Bracknell STW, even though it has a predominantly separated sewer system. In Shrewsbury, a combined catchment, greater quantities of runoff reached the sewer (22%1 of total flow) and therefore the contribution of runoff to total metal loads was of greater significance.

This study has identified copper and zinc from domestic diffuse sources as important contributors of total copper and zinc loads to sewers and to sludge metal concentrations. When considering the scope for future reductions in heavy metals in sludge, contributions from domestic sources of copper and zinc therefore need to be measured. A spreadsheet model was developed to predict the heavy metal loads going to sewers from different sources, and therefore help to identify any areas where significant reductions could be made.

1 Figures derived from spreadsheet model data presented in Appendix A.

Copies of the report are available from FWR, price £35.00, less 20% to FWR Members.