Comparative study of bacterial leaching of metals from sewage sludge in continuous stirred tank and air-lift reactors

Abstract A comparative study between continuous stirred tank reactor (CSTR) with cell recycle and air-lift reactor was carried out in 30 l capacity reactors for metal solubilization from anaerobically digested sludge. In CSTR at a residence time of 0·75 day, supplementing the feed sludge with 4 g/l of FeSO 4 ·7H 2 O, using 20% sludge recycle and adjusting the pH of the feed sludge to 4·5 resulted in the following metal solubilization: 91% Cu, 94% Zn, 93% Mn, 67% Cd, 67% Ni, 8% Cr, and 7% Pb. At a retention time of 0.75 day, metals were solubilized to the recommended level in the CSTR as well as in the airlift reactor. The treated sludge was easy to filter in a plate filter press, by centrifugation, or by vacuum filter. The sludge nutrients did not seem to be diminished as a result of treatment and separation of the solid from the liquid. The ammonia-nitrogen fraction seems to be transformed to organic nitrogen. The decontaminated sludge produced has reduced odour.