Investigation of the immobilisation / mobilisation of nickel, copper, chromium and zinc following co-disposal of activated sewage sludge with synthetic refuse

Abstract

Laboratory microcosms were packed with synthetic refuse and waste activated sewage sludge in two ratios (4.1:1 and 4.1:2 v/v). The sludge was spiked (100 mg·l-1 or 200 mg·l-1) with the salts of each of four heavy metals (Ni2+, Cu2+, Cr3+ and Zn2+). Subsequently, the metal concentrations were increased progressively. For all the microcosms, including an unperturbated control, unbalanced fermentations resulted as evidenced by low pH values. Thus, heavy metal presence was not the sole cause. The leached metal concentrations were in a consistent order with high Zn and Ni concentrations recorded compared with immobilised Cr and Cu. After 15 weeks of operation, despite extensive retention, increases in Cr, Ni and Zn were detected in the leachates following elevated loadings. Due to the high redox potentials (+2 to +170 mV), precipitation of the metals as insoluble sulphides was not attainable.


WaterSA Vol.27(4) 2001: 455-460