Katsuya OKANO

Application of ozone pretreatment in anaerobic sewage sludge digestion and energy evaluation of the system

Toshiya KOMATSU Shuji HIMENO

Currently, the amount of sewage sludge generated in Japan accounts for about 20 % of industrial waste, and technologies such as effective utilization and volume reduction of sewage sludge are required. Especially, anaerobic digestion (methane fermentation) can recover methane as energy from waste biomass. However, in anaerobic digestion of sewage sludge, waste activated sludge is difficult to decompose and has low methane conversion rate compared to primary sludge. In this study, by applying ozone-pretreatment to the concentrated waste activated sludge, the solubilization of waste activated sludge and the increase of biogas generation in anaerobic digestion were attempted.
Continuous experiments were performed using ozone-pretreated sludge. In the continuous experiment, the waste activated sludge (untreated, ozone 30, 60 mg-O3 / g-TS) was used as the substrate and the reactors were operated at HRT 30 days and 20 days. Stable operation was confirmed on both HRT 30 days and HRT 20 days, and the increase in methane gas generation and the TS and VS decomposition rates were confirmed as in batch experiments. The increase rate was higher at 48.6 to 61.7 % compared to the untreated sludge at HRT 20 days, indicating that the effect of ozone treatment could be expected. In the series where primary sludge and waste activated sludge were mixed, methane gas generation increased by 5.0 to 20.6 %, though lower than that of waste activated sludge alone.
Based on the results of the continuous experiments, the energy balance was estimated when the ozone pretreatment system was installed in a sewage treatment plant (sewage treatment amount: 121,800 m3 /d) and the ozone pretreatment system was evaluated. As a result, the energy balance showed a positive value in the series of ozone 15, 30 mg-O3 / g-TS. Especially, in ozone 30 the energy balance became maximum, and an increase of 5.2 GJ/d (kWh conversion: 1,444 kWh) was expected. This is equivalent to the power consumption of 118 ordinary households (three-person households).