Influences of pH and moisture content on the methane production in high-solids sludge digestion

The effects of pH and moisture content on high-solids sludge digestion were investigated using a mesophilic batch digester fed with sludge cake. The experiments were carried out by changing the initial moisture contents from 90 to 96% and the initial pH from 5.0 to 10.0. A simple model developed from the Gompertz equation was applied to estimate the methane production rate and the lag-phase time under various conditions, based on the cumulative methane production curves. The relative methanogenic activity decreased with the decrease of moisture content and dropped from 100 to 53% when the moisture content decreased from 96 to 90%. The rate of the methane production of the high-solids digestion at moisture contents of 90 to 96% functioned over a range of 6.6–7.8 with an optimum of pH 6.8, whereas the process may fail if the pH is lower than 6.1 or higher than 8.3. Moreover, a minimum lag-phase time for methane production was also found at around pH 6.8. In addition, a modified Haldane equation was suitable to represent the effect of pH on methanogenic activity at various moisture contents. The maximum specific methane production rate, Km, and the saturation rate constants for hydrogen ion, KH, and hydroxyl ion, KOH, were 14.5-7.5 ml CH . g−1 dry weight . d−1, 5.0 × 10−1–8.0 × 10−5 [M] and 2.0 × 10−9–8.0 × 10−9 [M], respectively.