Anaerobic fermentation mixture from waste activated sludge under alkaline condition improving treatment effect of wastewater with low C/N

Abstract: Direct discharging of high nitrogen and phosphorus concentration wastewater can bring serious eutrophication to the environment. The activated sludge process has been widely used for solving this problem. However, the process exhibits poor removal of nitrogen and phosphorus due to the shortage of carbon source, and also produces a lot of waste activated sludge (WAS) which needed to be treated and disposed. On the basis of these considerations above, anaerobic fermentation of WAS and fermentation liquid utilization processes were developed. However, fermentation liquid separation from fermented sludge was difficult and had high energy consumption. To avoid the problem of liquid-solid separation difficulty involved in traditional WAS anaerobic fermentation liquid utilization process, the feasibility of using fermentation mixture directly as external carbon source for nitrogen and phosphorus removal of wastewater treatment was discussed. Firstly, the characteristics of anaerobic fermentation mixture under the acidic (pH value of 4), neutral (pH value uncontrolled) and alkaline (pH value of 10) conditions over 90 days were compared. Secondly, biological denitrification and biological phosphorus removal processes by adding alkaline fermentation mixture of different volumes were investigated. A set of sequencing batch reactors (SBR) were respectively fed with 0.5 L seed sludge and 0.5 L wastewater, and then alkaline fermentation mixture of 0, 10, 20, 30, 50, 100 and 200 mL were added into the 7 reactors. The feasibility of using anaerobic fermentation mixture from WAS under alkaline condition as external carbon source was indicated by nitrate nitrogen (NO3--N) removal efficiency and pure phosphorus release amount. Meanwhile, the influence of ammonia (NH4+-N) concentration on biological nitrification process was analyzed. The results indicated that: 1) Alkaline fermentation mixture showed the best merit of reusable carbon source, with soluble chemical oxygen demand (SCOD) reaching 3428.5 mg/L corresponded to short chain fatty acids (SCFAs) accumulation of 1521.4 mg COD/L. Meanwhile, a small amount of nitrogen (C/N=18.9) and phosphorus (C/P=57.0) was released during alkaline fermentation. 2) The NO3--N removal efficiency was 69.3% when the initial NO3--N concentration was 15.0±0.5 mg/L and the maximum pure phosphorus release amount was 18.5 mg/L in blank test. Compared with the blank test, the highest NO3--N removal efficiency (100%) occurred with the optimal additive volume of more than 30 mL in biological denitrification process. The maximum pure phosphorus release amount (22.8 mg/L) occurred with the optimal additive volume of 20 mL, 4.3 mg/L higher than that of blank test. However, excessive fermentation mixture could significantly reduce the nitrification rate. The highest NH4+-N removal rate was 0.293 mg/(L·min) with the optimal additive volume of 30 mL in biological nitrification process. When the additive volume was more than 30 mL (50, 100 and 200 mL), the NH4+-N removal rates were 0.244, 0.109 and 0.004 mg/(L?min), respectively. Therefore, the strategy using anaerobic fermentation mixture from WAS under alkaline condition as external carbon source is feasible. It can solve the problems of carbon source lack and WAS disposal, and also simplify the process of traditional WAS anaerobic fermentation liquid utilization. This strategy is suitable for the treatment of rural domestic wastewater with low C/N ratio.