Performance And Optimization Control Of Modified Step Feed Process In Treating Low COD/TN Domestic Sewage

Nitrogen and phosphorus pollut ion is one of the most important water environment problems. In order to protect the natural water from pollut ion and improve the water environment quality, urban sewage treatment plants are required to enhance the ability of simultaneous nitrogen and phosphorus removal. But nitrogen and phosphorus removal rate is low and difficult to be improved in the urban sewage treatment plants because of the ubiquity of low C /N domestic sewage. How to removal nutrients such as nitrogen and phosphorus stably in the condition of low C/N has been an immediate problem for the urban sewage treatment plants.The modified step feed process was used to treat low C/N domestic sewage because it can make use of the carbon source extremely in the raw water. Through the method of material balance calculation, 4 influencing factors of the process: volume distribution、influent flow distribution、inner cycle ratio and the aeration rate in aerobic zones were analyzed in order to obtain the best operation condition in treating low C/N domestic sewage using this process.Research shows: effective carbon source utilization rate of the modified step feed process is much higher than that of the ordinary UC T process, COD effluent concentration can meet the national discharge requirements stably; Ammonia nitrogen effluent concentration cannot meet the discharge requirements stably when the actual HRT of aerobic zones downs to 2.143 h or the DO concentration of aerobic zone 3 downs to 1.5mg/L;The removal of TN not only depends on the nitrification and denitrification effect of the system,but the SND and microbial assimilation intensity. Because of the special form of the process, sedimentation performance of sludge is good at the condition of suitable DO concentration.With the increase of Vt ot al anoxic zones/ Vt ot al aerobic zones, the removal rate of phosphate increases gradually, but the remo val rate of TN only increases in a certain range; With the improvement of the influent proportion in the first zone, the removal rate of phosphate is improved partly, but the removal rate of TN will deteriorate if the proportion too high; The removal rate of phosphate decreases gradually with the increase of the inner cycle ratio, but the removal of TN is less affected. Whether the aeration rate in aerobic zones is higher or lower, the removal rate of phosphate and sedimentation performance of sludge will b e both affected, but it has little effect on the removal of TN.After systematic study,the best treatment effect is obtained under the following condit ions:volume distribution ratio for 4:3:3:3:3:3:3, influent flow distribution ratio for 6:3:1, inner cycle ratio for 75% and the aeration rate in aerobic zones for 0.10m3/h, the effluent concentration of COD, ammonia nitrogen, TN and phosphate is respectively 46.07mg/L, 1.49mg/L, 15.83mg/L and 1.48mg/L,the corresponding removal rate is respectively 77.82%, 97.4%, 75.3% and 72.8%.