INVESTIGATION ON THE POLLUTION POTENTIAL OF WATERWORKS SLUDGES

Several contaminated sludges from water treatment plants with known or estimated concentrations of trace elements were investigated for thei r leaching characteristics and long-term stability using standard and advanced test procedures. Potentially hazardous elements in the sludge are zinc, nickel, and arsenic with concentrations of up to 1.2 g/kg d ry matter (mass). Preliminary sorption tests with synthetic sludge com ponents like iron hydroxide, manganese oxide, silicate clay minerals, and chitine powder as a model organic component showed that Cu is asso ciated with the organic phase whereas arsenic is predominantly bound t o the iron oxide minerals. The recently suggested pH(stat) test proced ure was used to assess the leaching characteristics of metals at typic al pH values. This procedure was compared with the DEV-S4 test, the cu rrent standard test in Germany, consisting of a simple lixiviation of the solids with water, without pH control. The pH(stat) test yields re sults which are much better to interprete than those obtained by the D EV-S4 procedure. The iron and manganese sludges are well buffered agai nst changes in pH and redox potential so that low pH values and/or red ucing conditions can hardly occur. Thus, in deposited material a sudde n leaching of heavy metals is unlikely and due to the presence of iron and manganese oxides the pentavalent arsenic is protected against con version into the highly mobile trivalent form at neutral to low pH. Co -deposition with reducing organic matter and alkaline stabilisation ma terial or waste (like fly ash) could influence the binding properties and should be strictly avoided.