Simultaneous sewage sludge digestion and metal leaching using an internal loop reactor: effect of suspended solids concentration

Abstract

The objective of this work was to determine the effects of various concentrations of solid sludge on the bioleaching of metals and the stabilisation of sludge using elemental sulphur as a substrate to remove metals by an acid bioleaching process. Two experimental series were carried out in batch cultures, in column-type bioreactors, using sulphur in the form of 25 mm diameter blocks, in a 12-litre mixture which contained 80% of primary and 20% of secondary sludge (on a dry weight basis). Both sludge types came from the waste water treatment centre of the Valcartier military base (Quebec, Canada). The suspended solids (SS) concentrations (g/l) of the first and second series (B and B’): ranged from 8 to 27 g/l, respectively, with eight concentrations per series. The results obtained during the bioleaching–stabilisation process show that the acidification rate was reduced, that the quantity of sulphate produced increased with the sludge solid concentration. But the reduction in SS and volatile suspended solids (VSS) of the leached sludge was greater than that obtained by stabilising the sludge aerobically (control). The value of the degradation constant ( k d) decreased in response to the increase in VSS. The solubilisation rates of different metals during the bioleaching stabilisation decreased as the sludge solid concentrations increased and were as follows: Cd 43–79%; Cr 22–38%; Cu 75–97%; Fe 40–54%; Mn 88–98%; Ni 25–40%; Pb 37–89%; Zn 76–100%; P 30–34%; K 47–60%. The use of sulphur in the form of blocks leads to the formation of anoxic zones in certain regions of the bioreactor, favouring bacterial denitrification. Soluble concentrations of potassium were reduced by the end of the bioleaching process, whereas soluble concentrations of phosphorus and ammonium nitrogen increased. Despite these changes, the fertilizer values of the solid phase of these sludge types were effectively preserved.