Simultaneous sewage sludge digestion and metal leaching at controlled pH

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 simultaneously digests the sludge and removes toxic metals (SSDML). In order to verify whether the SSDML process could be operated in a continuous mode at a constant pH range, the effect of pH on the sludge digestion 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 at pH 2.0 with an optimum at pH 5.0. Oxygen utilisation rate, sludge solids 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. This suggested that the SSDML process could be operated in continuous mode at a pH range of 2.0-2.5. The sludge treatment in strong acidic medium resulted in more efficient destruction of indicator bacteria than conventional aerobic sludge digestion. At low pH, the neutrophilic heterotrophic population decreased appreciably whereas the acidophilic heterotrophic population remained almost unchanged indicating that solids degradation in this pH range could be in part due to the chemical process. The solubilization of nutrients was found to be moderate if final sludge pH was not allowed to decrease below 2.0.