Autoheated thermophilic aerobic sludge digestion and metal bioleaching in a two-stage reactor system

A two-stage process has been developed for stabilization of sludge and removal of heavy metals from the secondary activated sludge with high rate of energy and time conservation. The first stage of the process involves autoheated thermophilic aerobic digestion at 55–60°C inoculated with less-acidophilic thermophilic sulfur-oxidizing microorganisms (ATAD). The results show that it is possible to maintain the autoheated conditions (55–60°C) in the ATAD reactor up to 24 hr, leading to reduction of 21% total solids (TS), 27% volatile solids (VS), 27% suspended solids (SS) and 33% volatile suspended solids (VSS) from the sludge. The sludge pH also decreased from 7 to 4.6 due to the activity of less-acidophilic thermophilic microorganisms. In the second stage operation, the digested sludge (pH 4.6, TS 31.6 g/L) from stage one was subjected to bioleaching in a continuous stirred tank reactor, operated at mean hydraulic retention times (HRTs) of 12, 24 and 36 hr at 30°C. An HRT of 24 hr was found to be sufficient for removal of 70% Cu, 70% Mn, 75% Ni, and 80% Zn from the sludge. In all, 39% VSS, 76% Cu, 78.2% Mn, 79.5% Ni and 84.2% Zn were removed from the sludge in both the stages.