Study On Impact Factors Of Nitrogen And Phosphorus Removal For Urban Sewage Treatment Plant

The discharge requirements of sewage becomes more and more strict in ourcountry, and it should meet the requirements of Standard A of Rank Ⅰin “Dischargestandard of pollutants for municipal wastewater treatment plant”（GB18918-2002）.Therefore, a lot of sewage plants have to be improved to comply with the Standard. Inthis thesis, based on the actual performance of the secondary treatment in a municipalsewage plant in Shijiazhuang, the pollutant removal efficiency of the sewage plantwas analyzed with the method of material balance and microbial growth dynamics.The effect of temperature, DO, carbon resources and the initial concentration ofpollutant on nitrogen and phosphorus removal was also investgated.A research that according to the statistical analysis carried on the effluent datacollected in the recent13months in the sewage plant showed that there are thefollowing three problems in the process of biological treatment.（1） The concentrationof the inlet organic matter, and the nitrogen and phosphorus in the anaerobic tank arerelatively high, that is, compared with the total water design values, COD, NH₄⁺-N,TN and TP in the anaerobic tank are over39mg/L,21.6mg/L,27.8mg/L and3.32mg/L.（2） The effluent COD and SS are difficult to comply the standard. During the periodof monitoring, there are another152days and138days for the COD and SSrespectively to reach the Rank Ⅱ of National Discharge Standard.（3） The efficiencyof nitrogen and phosphorus removal is not very good. The removal rates are63.0%and64.5%, and the average effluent TN and TP are30.7mg/L and2.03mg/L,respectively.After the problems was analyzed, it is shown that the quality of inlet water in theanaerobic tank is high. Simultaneously, the sludge dewatering filtrate in the mud fielddischarged into the front devices also increases the concentration of inlet water qualityin the anaerobic tank. The low removal rate of COD is due to the slow degradation rateof COD in the sewage, while the low removal rate of SS is due to the higher MLSS andthe bad settling property of sludge in the aerobic tank. The bad nitrogen removal isdue to the low concentration of NO₃^--N in the anoxic pool which could reduce thedenitrogenation lower, only32.2%. The bad phosphorus removal is due to the NO₃^--Nexisted in the anaerobic tank which will bring the adverse affect on the phosphorusaccumulating bacteria anaerobic phosphorus release. Meanwhile, the filtrate of sludgedewatering straight discharged in will increase the phosphorus load in the process. When the inlet temperature of the sewage plant varies from16.3to27.7℃, therates of nitrification and denitrification increase as the temperature rises. The rates ofnitrification and the phosphorus uptake increase as the DO rises. All things considered,keeping DO at3mg/L is much better in the aerobic tank. The concentration of carbonsource has great influence on the nitrogen and phosphorus removal reaction. It isshown that the higher concentration of carbon source is good for the anaerobicphosphate release and anoxic denitrification rate, but the anaerobic denitrifyingphosphorus removal is significantly reduced. As the initial concentration of NH₄⁺-N islower, it has little influence on the nitrification rate, when it reaches more than58.4mg/L, the nitrification rate is significantly reduced. The nitrogen removal wasraised by increasing the internal reflux ratio to200%. The concentration of NO₃^--N inthe anaerobic pool will have negative effect on the release of thephosphorus-accumulating bacteria, and the phosphorus release rate became muchlower as the initial concentration of NO₃^--N increases.