OPTIMUM RESIDENCE TIME (IN CSTR AND AIRLIFT REACTOR) FOR BACTERIAL LEACHING OF METALS FROM ANAEROBIC SEWAGE-SLUDGE

The bacterial process of metal solubilization from sewage sludges was studied using two types of reactors: a continuously stirred tank reactor (CSTR) and an airlift reactor, with partial sludge recycling in each case. Both reactors showed similar metal solubilization efficiencies. After parameter optimization and steady state were obtained, a mean hydraulic residence time (tBAR) of 0.75 day allowed 91% Cu, 94% Zn, 93% Mn, 67% Ni and 67% Cd solubilization in the CSTR while 89% Cu, 91% Zn and 82% Cd were solubilized in the airlift reactor. The equation r(v) = 1.23 C + 0.44, expressing the solubilization rate of Cu or Zn according to their respective concentrations, had an explained variance (r2) of 0.93 for tBAR = 0.75 d. This equation applies equally to the CSTR and to the airlift, thus showing the equality of the efficiency of either reactor. A significant first order kinetic equation has been formulated for each tBAR. These results represent a 4:1 reduction of the required tBAR in comparison to tBAR used in earlier work and 0.75 day is therefore an optimum mean hydraulic residence time for bacterial leaching of metals from anaerobic sewage sludge.