Study On Volatile Fatty Acids Production From The Hydrolysis And Acidification Of Waste Activated Sludge

Waste activated sludge (WAS) contains large amounts of organic matter. Hydrolysis and acidification of WAS can reduce the sludge concentration and the production of volatile fatty acid (VFAs) can be used as carbon source of biological nutrient removal technology. Domestic and international studies focus on the treatment of primary sludge or the mixture of primary sludge WAS, while there are few studies on hydrolysis and acidification of the WAS. In this study, the effects of sludge inoculum, pH, temperature and sludge retention time (SRT) on the process of hydrolysis and acidification of the WAS are researched so as to reveal the principle of the impact and to conclude of the best operating conditions of the process.In the short-term batch experiments, results of effects of sludge inoculum and reaction time on hydrolysis and acidification of WAS show that: in the SRT of 7 days, WAS produces highest quantitiy of VFAs, while SRT>7 days the reaction results in the methanogenic conditions. Results also show that the appropriate sludge inoculum is 35% and the best reaction time is 7 days.After the esponse time of 7 days, the combined effect of pH and the sludge inoculation (35%) is investigated. Results show that in alkaline condition (pH range from 8 to 11) the sludge hydrolysis, acidificaton and sludge reduction effect can be substantially improved and the best pH is 10 in which causes the highest VFAs, high SCOD and sludge reduction. The main reason is alkaline matter would damage sludge cells and react with the cell wall which leads to the dissolution of materials(such as RNA, DNA, etc.). Another important reason is that the surface of sludge cells carries negative charge and the high pH will cause high electrostatic repulsion which results in dissolution of part of the extracellular polymers (EPS) VFAs mainly consistes of acetic acid and propionic acid, which makes 62.3% of total VFAs production in pH of 10. Acetic acid and propionic acid are the favorable substrate of the nitrogen and phosphorus removal process. In addition, inoculated sludge under different pH can promote hydrolysis, acidification and sludge reduction of the WAS.Under the conditions of sludge inoculation of 35% and pH of 10, the effects of SRT and temperature on hydrolysis and acidification of the WAS are analysed. Results show that SCOD, VFAs and sludge reduction increase with SRT and temperature in the semi-continuous reactor. The best SRT results in 11 days and the best temperature is 30℃. The semi-continuous reaction system significantly increases the hydrolysis, acidification and sludge reduction effect under alkaline conditions. The best operating conditions are sludge inoculum of 35%, pH of 10, SRT of 11 days and temperature of 30℃. Hydrolysis and acidification of WAS udner best operating conditions in the semicontinuous reactor produces an average SCOD of 539.8mg/gVSS and total VFAs of 274.6mgCOD/gVSS. VSS and SS are 39.3% and 28.3% less than those at the beginning.