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 c ake. 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 th e methane production rate and the lag-phase time under various conditi ons, based on the cumulative methane production curves. The relative m ethanogenic activity decreased with the decrease of moisture content a nd dropped from 100 to 53% when the moisture content decreased from 96 to 90%. The rate of the methane production of the high-solids digesti on 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 sp ecific methane production rate, K-m, and the saturation rate constants for hydrogen ion, K-H, and hydroxyl ion, K-OH, were 14.5-7.5 ml CH.g( -1) dry weight.d(-1), 5.0 x 10(-1)-8.0 x 10(-1) [M] and 2.0 x 10(-9)-8 .0 x 10(-9) [M], respectively. (C) 1997 Elsevier Science Ltd.