Biological Nitrogen And Phosphorus Removal In A~2/O Process For Municipal Wastewater Treatment

Recent years, as water body eutrophication becomes increasingly serious in China,the state’s emission standards for nitrogen and phosphorus and other nutrients alsobecomes increasingly stringent. Many sewage treatment plants fand it difficult tocompliance in actual operation. A²/O is one of the most widely used simultaneousnitrogen and phosphorus removal process in urban sewage treatment plant in China,wich enjoys many obvious advantages such as simple process,short hydraulic retentiontime,easy to control etc. However, due to the mutual influence of a variety of factors,A²/O process in nitrogen and phosphorus removal function is difficult for furtherenhance. This paper mainly studys A²/O process in below aspects: organics removal,nitrogen and phosphorus removal performance, the influence of C/N and C/P to nitrogenand phosphorus removal, the settings of the primary settling tank about low carbonsource sewage of simultaneous denitrification and phosphorus removal process, sludgebulking causes and countermeasures. The results are as follows:（1）Temperature has a greate influence on the nitrification rate in short SRT （mean8.5d）. When water temperature above21℃, the average effluent ammonia is less than5mg/L, the average removal rate is94.31%, the maximum of AUR and NUR can reach8.40mg NH₄⁺-N/（gVSS h） and6.02mg NO2N/（gVSS h）; Water temperature below21℃（minimum11.7℃）, ammonia removal efficiency decreases with the temperature drop,the minimum of AUR and NUR can reach2.53mgNH₄⁺-N/（gVSS h） and2.34mg NO₂^--N/（gVSS h）.（2）The nitrate concentration in the return sludge has a great impact on phosphorusrelease in anaerobic tank. When the return sludge nitrate concentration is less than 5mg/L, anaerobic phosphorus release achieve maximize, anaerobic tank phosphorusconcentrations can be achieved20.27mg/L, phosphate average removal rate is91.87%.When the verage return sludge concentration of nitrate is15.09mg/L, anaerobicphosphorus release reduced greatly, anaerobic tank phosphorus concentration is reducedto8.42mg/L, the average removal of phosphate rate droppes to less than20%.（3） The ratio of C/N and C/P increase from4.23and39.13to6.41and54.92respectively, the removal efficiency of TN increased to72.97%from63.18%, theeffluent concentration of NH+4N and TN is4.76mg/L and14.66mg/L, reached the thefirst of urban sewage treatment plant effluent discharge standards （GB189182002） Aemissions standard. The anaerobic average phosphorus release concentration reached20.48mg/L, the verage effluent phosphate concentration isonly0.41mg/L, the averageremoval rate is92.51%.（4）For the low carbon source of urban sewage treatment plants of simultaneouslynitrogen and phosphorus removal process, it is suggested to abolish the primarysedimentation tank. This conclusion is on the basis of water quality contrasting,practical application needed to further test demonstration.（5）Low temperature （16℃）, low DO （an average of0.24mg/L）, low COD organicload （0.27kgCOD/（kgMLSS d）） are the main reasons of sludge bulking. Organic loadresume normal, increase DO to34mg/L, sludge bulking at low temperature can not befully recovered.（6） The excessive proliferation of Microthrix parvicella is the main reason ofsludge bulking in the A²/O process in winter as water temperature lower than16℃.（7）The SS of influent increased from83mg/L to324.50mg/L, after20d, the SVIand SV of the system dropped from a high of348mg/L and96%to about140mg/L and30%respectively at normal temperature in2SRT, and the system maintains stability,sludge settling properties return to normal. SS plays the role of inorganic coagulant.