Effects of return mixed liquor recycling ratio on A2/O process

Discharging wastewater effluent to surface water or groundwater is so dangerous for environment, while it includes nutrient. As the phosphorus and nitrogen combinations in the aquatic environments have harmful impacts (mainly the poisonousness of ammonia, overgrowth of aquatic plants, groundwater pollute to nitrate and diseases caused by drinking the polluted water, and also eutrophication, resulting in frequent outbreaks of algal blooms and threatening the reliable supply of drinking water resources), some limitations were imposed on the consistency of these combinations in the entry waste. So nowadays removal of these combinations must be considered in designing of the treatment plants and also systems designed for treating the municipal wastewater must be able to remove nitrogen and phosphorus combinations to reach the standard limit. Therefore, in order to good performance of the aerobic-anaerobic A2/O method, it is proposed in this research and a study in advanced treatment of municipal wastewater using the A2/O method to remove nitrogen and phosphorus in the pilot scale in Ekbatan WWTP has been done.
In this research, firstly the principals of biological removal of nitrogen and phosphorus, and secondly the basis of designing biological treatment plants have been investigated. Then for laboratorial studies, an A2/O pilot has been made. This pilot consists of anaerobic, anoxic and aeration tanks and also sedimentation tank. The volume of these 4 tanks are 40, 60, 170 and 120 L, respectively. In order to simulate the real condition, this pilot has been set up in Ekbatan plant and the experiments were done to observe the effect of hydraulic residence time on nitrate, ammonia and phosphorus removal and also the effect of oxic mixed liquor recycling ratio on nitrate removal has been conducted. In order to observe the nitrate, ammonia and phosphorus removal process efficiency, the experiments were done in a period of three months and in 5 aeration hydraulic residence times, 4, 6, 8, 10 and 12 hours. In these experiments, the returned sludge was 25% and the oxic mixed liquor recycling ratio was 75%. After determining the best hydraulic residence time, experiments continued in 5 different oxic mixed liquor recycling ratios, 75%,150%, 225%, 300% and 375%.
It was concluded that at aeration hydraulic residence time of 8 hours, 96% COD , 95% ammonia and 79% phosphorus (effluent: 9 mg/L COD, 0.87 mg/L ammonia, 2.1 mg/L phosphorus, 18.7 mg/L nitrate) removal were achieved and that was the best HRT. Furthermore, according to the mixed liquor recycling ratio experiments, when the oxic mixed liquor recycling ratio was about 180 - 200%, optimum removing nitrate has been occurred. Although the mixed liquor recycling ratio of 225 - 275% resulted better efficiency for nitrate removal, it is not proposed, because effluent limitations in Iran for the nitrate is up to 10 mg/l which it is resulted in the mixed liquor recycling ratio of 180 -200%, and the other reason is that, with increasing the oxic mixed liquor recycling ratio, energy costs will increase, too.