In order to investigate the performance of thermophilic UASB reactor and the behavior of methanogenic activities of the retained sludge, a laboratory-scale UASB reactor was operated at 55°C over 8 months by feeding with an alcohol distillery wastewater. The reactor was inoculated with granular sludge grown on a carbohydrate (fruit juice) wastewater under a mesophilic condition (35°C).
The maximum COD loading achieved was 30kgCOD/m3/d, corresponding to a HRT of 7.2h for influent strength of 9000mgCOD/l, with a COD removal efficiency of around 85%. During a period of COD loading of 30kgCOD/m3/d, propionate and acetate accumulated in the effluent up to about 300-600mgCOD/l and 100mgCOD/l, respectively. Although the settleability of the retained sludge deteriorated from the initial SVI (based on VSS) of 25ml/gVSS to 52 during the first 80 days, its value gradually improved finally to 21 due to a significant proliferation of thermophilic methanogens.
Thermophilic cultivation caused a drastic shift in temperature dependence of methanogenic activities of the retained sludge. On the day 202, acetate-and hydrogen-utilizing methanogenic activities exhibited their optimum values at 65°C, while that of propionate-fed methanogenic activity occurred at 55°C. Comparing in methanogenic activities at 55°C between the retained sludge and the seed sludge, acetate-fed activity increased finally by 4.4 times, propionate-fed activity by 4.6 times, and hydrogen-fed activity by 3.5 times. Fairly lower value of propionate-fed methanogenic activity, i. e., only 1/5 of acetate-fed activity and furthermore only 1/23 of hydrogen-fed activity, suggested that the propionate degradation is subject to be a rate-limiting step in thermophilic condition.
