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The conventional biological treatment needs to set the anaerobic, anoxic and aerobic tank to achieve the purpose of nitrogen and pHospHorus removal. But the long sludge age of oxidation ditch provides a convenient for the removal, especially with the advantage of simultaneous nitrification and denitrification.This technology has been widely used in many wastewater treatment plants,both at home and abroad,and has become a hot-spot.This research is based on Changshou wastewater treatment plant.It has studied the effect of nitrogen removal by simultaneous nitrification and denitrification in Carrousel oxidation ditch,using the Changshou District urban sewage and a similar Carrousel oxidation ditch as pilot model.This paper discussed the SND of this process with ORP as control parameters.In order to monitor and control the running process of sewage and sludge treatment much better,a series of researches on the changes of endogenous denitrification and nitrification process parameters were carried out,including DO,pH and MLSS.Through the experimental study and theoretical analysis of the efficiency in the sewage treatment process,we found the efficiency of ORP,DO,pH,MLSS,HRT and reflux ratio on denitrification,and thus get the critical ORP when nitrification and denitrification occurred simultaneously.Analysis on the key factors of critical ORP ,could provide basis for improvement of the efficiency in simultaneous nitrification and denitrification and support for enhancement of the oxidation ditch denitrification process.Through the experiments and analysis, the following conclusions are obtained:①MLSS,pH,DO,HRT and reflux ratio are all factors influencing ORP.DO has less impact,while pH is the most sensitive.The other three are also relatively key factors. Because nitrification and denitrification occur simultaneously, it’s indicated that oxidation and reduction reaction in the process of denitrification is occurring at the same time, and so is the acidity and alkalinity, thus in this process,the relationship between ORP and HRT,MLSS,pH,DO ,reflux ratio etc. is different from that of traditional biological nitrogen removal.The relationship between ORP and HRT,MLSS,pH,DO ,reflux ratio can be expressed in a formula:y=-231.7+0.38*ln(DO)-4.03*Cos(-2.24*MLSS)+101.43*ln(pH)+5.72*HRT +3.87*(reflux ratio)2-4.46*sin(4.89* MLSS -5.41* pH)This model has a certain significance for the process of the sewage treatment.1)First of all,Do,MLSS,pH,HRT and reflux ratio all have relatively important impacts on ORP.The ORP value can be controlled under a proper control of the five factors,also facilitates further control of nitrification process on activated sludge.2)As can be seen from the equation above,pH has the most impact on ORP,so with the remaining four determined,the proper ORP value can be achieved by adjusting pH.The effect of denitrification in engineering application can be improved through the dual effects of overall adjustment and fine-tuning.3)The value of MLSS is usually relatively fixed with the reflux ratio and the remaining sludge determined,and it also has a obviously influence on ORP. It’s appropriate to adjust the value of ORP in different seasons. The level of ORP values can be maintained by increasing MLSS concentration in the fall and winter because of low temperature and microbial activity, which means slow metabolism and relatively weak nitrogen, while it’s opposite during the spring and summer.4)There is a linear relationship between HRT and ORP,and it’s proportional with a coefficient 5.72,under certain circumstances of other factors. The long time of HRT results in entering into the sludge flocculation body of dissolved oxygen,breaking the balance of anoxic - aerobic,leading ammonification bacteria, nitrifying bacteria, denitrifying bacteria to be unbalanced,and thus impacting on the nitrate,nitrite and ammonia,causing changes of sewage redox potentials.So it’s essential and relatively simple to control the time of HRT in a reasonable range at any time.5)There is a parabolic relationship between reflux ratio and ORP with other factors determined.The growth rate of ORP values increases with the growth of reflux ratio.So ORP can be fine-tuned with a low reflux ratio, while a wide range of adjustments can be made with a high one,depending on different cases.6)DO has less impact on ORP compared with MLSS and pH.In the case of keeping the appropriate dissolved oxygen needed by microorganisms,the values of ORP can be fine-tuned to achieve the best nitrogen effect.②This experiment shows that the best SND activity in the oxidation ditch is:MLSS=3000mg/L,DO=1.2~1.4mg/L,pH=7.5,HRT=10h,reflux ratio=100%.Nitrification and denitrification processes are in balance with MLSS being 3000mg/L,which show a low concentrations of ammonia, nitrate and nitrite in effluent,and a high removal rate of total nitrogen.According to Denitrifying dynamics,the causes that affect denitrification include nitrate, nitrite nitrogen and carbon source.microbial biomass increases along with the concentrations of MLSS,which will lead to an increment of endogenous metabolites and anaerobic denitrifying bacteria.At the same time,due to the penetration resistance increment of DO and reduce of permeability, the denitrification rate increases.Finally, nitrification and denitrification reach a balance.③In this experiment,the value of ORP is 30 with the parameters above determined.When the ORP was about 30, simultaneous nitrification and denitrification reactions have better performance,reaching a removal rate of TN up to 80%.This process is an intermediate state.If the aeration was insufficient or excessive, or influent loads changed,it would Change to the other two states.However,the oxidation ditch has a strong ability to loading shock,therefore this state can be stably controlled. When ORP is greater than 40,95% of effluent is nitrate, which means a good performance for nitrification and a poor one for denitrification. While with ORP under 30,70% of effluent is ammonia,indicating a insufficient nitrification.④The pHospHorus removal in this test achieved a good performance. The pHospHorus removal rate basically reached 90%,and the pHospHorus concentration of effluent is basically under 0.2mg/L,lower than the 1mg / L requirements of level B emission standard. There is no negative impact on the pHospHorus removal,and nitrate value is moderate, in the appropriate range of pHospHorus release and uptake. As electron acceptor, nitrate nitrogen will be fully utilized,which greatly enhances anoxic pHospHorus uptake rate,as long as reducing the nitrate nitrogen by a large amount.
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