Improving Nitrogen And Phosphorus Removal Efficiency Of Oxidation Ditch Process For Domestic Sewage Treatment Through Adding Anaerobic/Facultative Units

In recent years,with the improvement of the effluent quality of municipal wastewater treatment plants from national and local requirements perspectives?especially the nitrogen?N?and phosphorus?P?indicators?,many wastewater treatment plants established in earlier period did not consider the concentration of N and P in effluent.Although some wastewater treatment plants considered the concentration of N and P,the concentration of N and P in the effluent still did not meet the water quality standards.So these specific sewage treatment plants will upgrading through adding or improving treatment process units of N and P.In view of the fact that the oxidation ditch process of sewage treatment plant of Southwest University of Science and Technology has poor treatment effect on N and P,the existing problems were found out in this study.Combined with the existed processes and pre-expected outcomes of water quality,the anaerobic phosphorus release unit and facultative denitrification unit were added at the front and end of the oxidation ditch,respectively.Then this study further discussed the effect of adding anaerobic/facultative treatment unit on N and P removal in the original oxidation ditch process.The main results of this study were as follows:With the addition of pre-anaerobic unit,the removal efficiency of total phosphorous?TP?in the system obviously improved.Under the conditions that the anaerobic unit hydraulic residence time?HRT?with 1 hour and the sludge reflux ratio with 75%,the released amount of phosphorus in the anaerobic unit was 1.27?1.95 mg TP/g MLSS,and the removal rate of TP in the whole treatment process was 88.5?94.7%.With the addition of post-facultative unit,the removal efficiency of total nitrogen?TN?obviously improved and the removal efficiency of chemical oxygen demand(COD_Cr)also improved,but the removal efficiency of TP and ammonia nitrogen?NH₃-N?almost kept steady.When the HRT of the facultative unit with 3 hour,the concentration and removal rate of TN in the effluent discharged from whole treatment process were 10.4?15.9 mg/L and 44.6%?74.1%,respectively.The temperature had significant influence on anaerobic-oxidation ditch-facultative process.When the temperature was 12??low temperature?,the improved treatment process had a poor effect on the removal of pollutants;when the temperatures were 22 and 30??high temperature?,the improved treatment process had a better effect on the removal of pollutants.When the temperature was 12?,only the concentration of TP of the effluent in the improved treatment process could meet the first-class A standard of the Discharge standard of pollutants for municipal wastewater treatment plant?GB18918-2002?.When the temperatures were 22 and 30?,the concentration of COD_Cr,TP,NH₃-N and TN of the effluent in the improved treatment process could reach the first-class A standard of the Discharge standard of pollutants for municipal wastewater treatment plant?GB18918-2002?.Under low temperature?12??in winter,the nitrification effect could be improved by increasing the concentration of dissolved oxygen?DO?in aerobic stage of oxidation ditch,and the denitrification effect of wastewater treatment system could be improved by adding external carbon source.Although the removal effect of the system on TN could be improved through adding the carbon source to the post-facultative unit,the concentration of COD_Crand TP of the effluent in the improved treatment process would be increased.Especially the COD_Cr/TN ratios of the influent were about 7 and 8,respectively,the concentration of TN of the effluent in the improved treatment process could meet the first-class A standard of the Discharge standard of pollutants for municipal wastewater treatment plant?GB18918-2002?.In low temperature environment,when the COD_Cr/TN ratios of the influent was low,the COD_Cr/TN ratios of the influent could be adjusted to about 7 by adding extra carbon source in the actual wastewater treatment project,which can improve the denitrification performance of the sewage treatment system.The removal efficiency of the improved treatment process in this study can obviously improve removal efficiency of TP and TN by adding anaerobic/facultative treatment unit to the domestic wastewater treatment system which adopts oxidation ditch process.The obtained research results of this study can provide an improved idea for the upgrading of wastewater treatment plants using oxidation ditch process.