Optimized Strategy Investigation Of Energy Saving In Modified Oxidation Ditch And It's Application

With the secondary treatment of wastewater was popular in our country, organic pollution was improved in rivers and lakes. But the eutrophication, which induced by excessive discharge of nutrient, became the major problem in water system. The new urban sewage treatment plant pollutants standards (GB 18918-2002) strictly controlled the nitrogen and phosphorus concentrations of effluent in China. As the new standard actualized, nitrogen and phosphorus need deeply removal both in the establishing and established WWTPs. In order to achieve the new criterion, and decrease cost price, all WWTPs are upgrading and reconstructing.Oxidation ditch(OD) process is simple, resist the wastewater and pollutants impaction, and can simultaneous remove nitrogen and phosphorus, so it is popular in China, but the removal rates of nitrogen and phosphorus is not high, because the removal is effected by many factors in the real WWTPs, and the cost is high. In order to solve the problems, the researches have been carried out in a pilot-scale anoxic-anaerobic OD, and the influent is the same with the municipal wastewater. Optimal strategy was brought forward, and applied the results to the cooperator Wolongkou WWTP.In order to improve the removal rates of nitrogen and phosphorus and decrease the energy consumption in pilot-scale anoxic-anaerobic OD, the quantum of aeration was reduced in the front of the OD, and anoxic zone was formed in this place. The running mode is anoxic/anaerobic/anoxic/aerobic instead of anoxic/anaerobic/aerobic. The study investigated the effect of anoxic-anaerobic OD on COD, nitrogen and phosphorus removal in different temperatures. Established the optimal parameters in different temperatures, such as DO, sludge age, MLSS, return sludge rate, and the optimal rate of anoxic zone in the front of the OD and aerobic zone in the back of the OD. The results indicate that all the concentrations of COD, TN, NH₄⁺-N, TP in the effluent all can reach the first level A criteria of (GB 18918-2002) in different temperatures, through adjust the parameters. In the temperatures of 25℃, 20℃and 15℃, the average concentrations of COD, TN, NH₄⁺-N, TP in the effluent are 19mg/L, 13.7mg/L, 3.6mg/L and 42mg/L; 26mg/L, 10.6mg/L, 3.1mg/L and 0.32mg/L; 26mg/L, 12.1mg/L, 3.9mg/L and 0.34mg/L respectively。After the survey of Wulongkou WWTP, we found the removal rate of total nitrogen was not considered in the design. The removal of TN depended on nitrate in the return sludge denitrifying to N2 in the anoxic zone and biological assimilation. Because there was no special anoxic zone for denitrification, the removal rate of TN was not high. There were so many nitrate that it could not be used up in the anoxic zone, and some nitrate came in anaerobic zone, which had a negative impacted to phosphorus release, so the quantum of phosphorus uptake was not enough, and the removal rate of phosphorus was low.In allusion to problems of the WWTP, and on the basis of polit-scale experiment results, the quantum of aeration was optimized in the aerobic zone of the OD, according to strategy of low DO, besides with no more structures and equipments. The quantum of aeration in the front of the OD was reduced, at the same time the DO in the back of the OD was controled, and then the anoxic zone was formed in the front of the OD, create the environments for denitrifying phosphorus removal(DPR) and simultaneous nitrification and denitrification(SND). The results indicate when the quantum of aeration in the 1, 2 and 3 galleries, was reduced to little, and the concentration of DO in the effluent was 1¹.5mg/L, the removal rates of TN and TP improved distinctly, the average concentrations of COD, TN, NH₄⁺-N and TP in the effluent were 20mg/L, 9 mg/L, 0.5 mg/L and 0.22 mg/L respectively, and the average removal rate were 94%, 76%, 98% and 96% respectively. all reach the first level A criteria of (GB18918 -2002). In the steady phase, per ton wastewater consumed 0.241 kw.h, electricity consumption decreased more than 20% compared with before debugging.