EFFECTS OF REACTION CONDITIONS ON SLUDGE DEGRADATION AND ORGANIC ACIDS FORMATION IN LOW-CRITIAL WET AIR OXIDATION

摘要

Wet air oxidation processes are to treat highly concentrated organic compounds including refractory materials, sludge, and night soil, and usually operated at supercritical water conditions of high temperature and high pressure (>374°C, 218 atm). Recent studies have attempted to enhance the formation of organic acids at low critical conditions (<250°C, 40 atm) while oxidizing organic compounds. In this study, the effects of reaction parameters including temperature, time, pressure, and oxidant dose for sludge degradation and conversion into subsequent intermediates such as organic acids were investigated at low critical wet oxidation conditions. Experiments were performed in lab-scale and pilot plant using thickened sludge from a municipal wastewater treatment facility. A 1-liter reactor made of SUS 316 was used for the lab-scale experiments and pilot plant experiments were conducted with a 50-liter reactor at 30 min of hydraulic retention time (HRT), with sludge injected using a high-pressure pump. Results from the lab experiments showed that the reaction temperature directly affected the thermal hydrolysis reaction rather than oxidation reaction, and the efficiencies of sludge degradation and organic acid formation increased as the reaction temperature and time increased. The removal efficiencies of total chemical oxygen demand (TCOD) at reaction temperatures of 180, 200, 220, and 240°C were 29.4, 43.2, 57.7, and 66.9, respectively, for the reaction time of 80 min. Suspended solids (SS) removals were 52.6, 68.3, 72.6, and 74.4%, respectively, for the same reaction temperature conditions at 10 min reaction time, indicating that most organic SS were liquefied at the early reaction stage. At 200 and 240°C of reaction temperatures and 10 min reaction time, the ratios of biochemical oxygen demand (BOD5) to soluble chemical oxygen demand (SCODCr) were 0.67 and 0.50, respectively, showing that the fraction of biodegradable compounds was higher at relatively low reaction temperatures. Pilot-scale experiments showed better results compared to the lab-scale, showing the removal efficiencies of SS and TCOD at 200°C, 30 atm, and 30 min HRT over 80 and 60%, respectively, and yields of volatile fatty acids (VFAs) and SCOD 115 g VFAs / kg SS and 730 g SCOD / kg SS, respectively.