SIMULTANEOUS SEWAGE-SLUDGE DIGESTION AND METAL LEACHING AT CONTROLLEDPH

The presence of toxic metals and pathogenic microorganisms are the two principal obstacles in using sewage sludge on agricultural land. This problem resulted in the development of a biological process which sim ultaneously digests the sludge and removes toxic metals (SSDML). In or der to verify whether the SSDML process could be operated in a continu ous mode at a constant pH range, the effect of pH on the sludge digest ion and metal leaching was studied. The experiments were conducted at different constant pH. The acid production and acidophilic thiobacilli growth rates were increased in the following order of pH: 3.5 > 2.0 > 5.0 > 6.5. The less-acidophilic thiobacilli growth was not detected a t pH 2.0 with an optimum at pH 5.0. Oxygen utilisation rate, sludge so lids degradation and first order rate constant for solids degradation were found to be almost similar in pH ranges of 2.0-2.5 and 6.5-7.0. T his suggested that the SSDML process could be operated in continuous m ode at a pH range of 2.0-2.5. The sludge treatment in strong acidic me dium resulted in more efficient destruction of indicator bacteria than conventional aerobic sludge digestion. At low pH, the neutrophilic he terotrophic population decreased appreciably whereas the acidophilic h eterotrophic population remained almost unchanged indicating that soli ds degradation in this pH range could be in part due to the chemical p rocess. The solubilization of nutrients was found to be moderate if fi nal sludge pH was not allowed to decrease below 2.0.