Treatment and Energy Recovery of Sewage Sludge by High-pressure Superheated Steam Oxidation

High-pressure superheated steam oxidation, which uses hot and high-pressure steam above the critical temperature (374°C) and below the critical pressure (22.1 MPa) of water, is an environmentally friendly treatment of waste biomass. This technique was applied to the treatment of sewage sludge and the recovery of the thermal energy from it. Sewage sludge has high moisture content of around 75 wt% and relatively large nitrogen content of about 7 wt%. It is one of refractory waste biomass. In this study, sewage sludge was decomposed by a flow-type two-stage high-pressure superheated steam oxidation bench plant, in which the first reaction was high-pressure superheated steam oxidation without catalyst and the second was that with solid catalyst. This method was proposed to decrease the reaction temperature and pressure of conventional supercritical water oxidation process largely, keeping high decomposition efficiency. The optimal condition of the first reactor was 400 °C, 8-10 MPa, 15-20 min of reaction time, 1.2 of oxygen supply ratio and that of the second reactor was 385 °C, 8-10 MPa and 0.5-1.0 min of space time. The sewage sludge was mainly decomposed to safe CO2, water and nitrogen. The product yields of TOC, solid carbon, toxic ammonia and nitrogen oxides were negligible. The discharged water from the second rector was clean and satisfied the emission standard established by environment agency in Japan. The thermal energy of the effluent from the second reactor was recovered by a double-piped high pressure heat exchanger. The recovery ratio was 86 % on average. The energy profit ratio of the proposed process as energy production from the sewage sludge was 1.47.