Study On Depth Denitrification Efficiency Of Denitrification Filter For Rural Domestic Sewage

With the accelerating process of the agricultural modernization in China as well as the continuous improvement of the rural residents’living standards,various areas have successively formulated stricter local standards（COD≤30 mg/L,TN≤10 mg/L and TP≤0.3 mg/L）for pollutant discharge from domestic sewage treatment plants in order to more properly treat rural domestic sewage and improve the water environment in rural areas.Currently,through the field investigation,it is found that the secondary effluent treatment indicators（such as TN,SS,and COD）of the ordinary township’s domestic sewage treatment plants in Sichuan Basin can no longer reach the standard.Therefore,it is necessary to carry out advanced treatment on the tail water from the sewage treatment plants.This research takes the tail water of the township sewage treatment plant in Sichuan Basin as the research object,and applies the biological ceramsite as a filler to establish a denitrification filter model to carry out the advanced treatment test for the tail water of the sewage treatment plant,select the external carbon source materials,study the influence of different hydraulic retention times and the carbon-nitrogen ratio on the operation effect,explore the back-flushing methods,optimize the relevant operating parameters of the denitrification filter operation,and provide data support for the upgrading of the township sewage treatment plants in the Sichuan Basin.The main conclusions are as follows:（1）The alkaline pretreatment and thermal pretreatment have an effect on improving the static carbon release and carbon release persistence of the three carbon source materials（wheat straw,corncob,and grass clippings）,and the static release of other pollutants also increases correspondingly;the carbon release characteristic curves of the three carbon source materials as well as the materials after the two pretreatment methods in the first 5 days meet the second-order kinetic equation with good fitting degree;the maximum carbon release c_mvalues（the maximum carbon release per unit mass of the carbon source material in the immersion test）of wheat straw,grass clippings,and these two materials after pretreatment are all too high.The5-day c_mvalues of the raw wheat straw materials,alkaline-pretreated wheat straw,and heat-pretreated wheat straw are 405.35 mg/（g·L）,609.54 mg/（g·L）,and 886.24 mg/（g·L）,respectively,and the 5-day c_mvalues of the raw grass clippings materials,alkaline pretreatment clippings,and heat-pretreated clippings are 548.54 mg/（g·L）,1116.74 mg/（g·L）,and 1055.66 mg/（g·L）respectively,which may bring the risk of secondary pollution.In addition,the release of other pollutants will also have a certain impact on the effluent quality.Therefore,it is preferable to apply corncob raw materials,heat-pretreated corncobs,and alkaline-pretreated corncob soaking liquid as the external carbon source for the denitrification filter.（2）When the glucose is taken as the carbon source to culturing the biofilm in the denitrification filter,the successful biofilm culturing time of the artificial inoculation method is shorter than that of the natural continuous culture biofilm culturing method.After the two biofilm culturing methods succeed,the removal effect of TN,NO₃^--N and COD is not much different and relatively stable.Under the same influent conditions,the average concentration of the effluent TN after the completion of the artificial inoculation biofilm culturing is 9.10 mg/L;the removal rate is above 50%stably;the removal rate of NO₃^--N is about 60%stably;the average concentration of COD in the effluent is 32.14 mg/L,and the removal rate is above 65%stably;after the natural biofilm culturing is completed,the average concentration of TN in the effluent is 8.50 mg/L;the removal rate is about 53%stably;the removal rate of NO₃^--N is more than 60%stably;the average concentration of COD in the effluent is35.41 mg/L,and the removal rate is above 66%stably.（3）Through the single factor experiment and the orthogonal experiment,it is found that the carbon-nitrogen ratio and hydraulic retention time have a significant effect on the removal effect of TN,NO₃^--N,and COD,while the removal effect on SS,TP and NH₄⁺-N is not obvious;the optimal operating parameters determined by the orthogonal test are as follows:the external carbon source is the soaking liquid of the raw corncob material;the hydraulic retention time is 60 minutes,and the C/N is 5.5;under the optimal operating parameters,we compare the filters between taking the heat-pretreated corncob soaking liquid as the carbon source and taking glucose as the carbon source.It is found that the average concentrations of the effluent TN,NO₃^--N,SS,COD,NH₄⁺-N,and TP in the filter applying the raw corncob material soaking liquid as the carbon source during operation are 7.77 mg/L,3.99 mg/L,7.43 mg/L,24.23 mg/L,1.13 mg/L,and 0.27 mg/L,respectively,and the removal rates are52.65%,68.46%,53.81%,74.04%,47.95%,38.99%,respectively.Its denitrification effect and the removal effect of other pollutants are better than that of the filter applying heat-pretreated corncob soaking liquid as the carbon source.However,the nitrogen removal effect of the filter adopting glucose as the carbon source is slightly better than that of the filter adopting the soaking liquid of the raw corncob material as the carbon source;the seasonal changes occurred during the period when the optimal operating parameters are applied.When the average water temperature in autumn is24.30℃,the average concentrations of TN in the effluent from each filter are 6.85mg/L,6.27 mg/L,and 4.00 mg/L,and the average concentrations of COD are 21.76mg/L,22.59 mg/L,and 16.43 mg/L,respectively;when the average water temperature in winter is 15.20℃,the average concentrations of TN in the effluent from each filter are 8.42 mg/L,9.79 mg/L,and 6.19 mg/L respectively,and the average concentrations of COD in the effluent are 25.87 mg/L,28.17 mg/L,and 23.38 mg/L,respectively.The concentration of other pollutants such as SS,NH₄⁺-N,and TP in the effluent did not change significantly due to the temperature drop.The average concentration of each pollutant in the effluent from the three filters can meet the Water Pollutant Discharge Standards of Minjiang River and Tuojiang River Basin in Sichuan Province（DB51/2311-2016）and the first level standard of the Water Pollutant Discharge Standard for Rural Domestic Sewage Treatment Facilities（DB51/2626-2019）.（4）When the corncob raw material soaking liquid is taken as the carbon source,the cost of removing the unit TN（Yuan/kg）is only about one sixth when adopting glucose as the carbon source,which saves the cost of the advanced sewage treatment in rural areas,can also promote the resource utilization of the agricultural waste,and is suitable for the advanced treatment of the tail water from the township sewage treatment plants in the Sichuan Basin.