Removal of toxic metals during biological treatment of landfill leachates

Abstract

Progressive implementation of the European Water Framework Directive has resulted in substantial changes in limits for discharges of heavy metals both to watercourses, and to sewer. The objective of this paper is to provide original, real, full-scale data obtained for removal of metals during aerobic biological leachate treatment, and also to report on studies carried out to look at further trace metal removal. Polishing technologies examined and investigated include; the incorporation of ultrafiltration (UF) membranes into biological treatment systems, the use of ion exchange, and of activated carbon polishing processes. Ultrafiltration was able to provide a 60 percent reduction in COD values in treated leachates, compared with COD values found in settled/clarified effluents. Removal rates for COD varied from 30.5 to 79.8 percent. Additionally, ultrafiltration of treated leachates significantly reduced both chromium and nickel concentrations of effluents by 61.6% and 34.3% respectively (median values). Despite mean reductions of chromium (9.7%) and nickel (13.7%) noted during the ion exchange trials, these results would not justify use of this technology for metals removal at full-scale. Further preliminary studies used pulverized activated carbon (PAC) polishing of UF effluents to demonstrate that significant (up to 80 per cent) removal of COD, TOC and heavy metals could readily be achieved by doses of up to 10g/l of suitable activated carbons. Additional evidence is provided that many trace metals are present not in ionic form, but as organic complexes; this is likely to make their removal to low levels more difficult and expensive.