Optimized Operation And Microbial Community Change Characteristics Of DPR-SPNA Process For Low Carbon Wastewater Treatment

Activated sludge process is the main treatment method of urban sewage at present,However,urban sewage in China is characterized by a low-carbon nitrogen（C/N）ratio,and it is difficult for traditional treatment methods to meet current treatment discharge standards without additional investment.In the denitrification phosphorus removal（DPR for short）process,the same carbon source can be used twice,reducing 30%of aeration energy consumption,50%of carbon source and sludge production.Single-stage of Partial Nitrification and Anammox（SPNA）is a cost-effective denitrification process for wastewater treatment.The combined process of DPR-SPNA enables wastewater treatment plants to achieve energy self-sufficiency and reduce greenhouse gas emissions.The subject studied the influence of the influent water quality and operation mode on the startup and stable operation of the SPNA system,and analyzed the changes in the bacterial community structure in flocs and biofilms through high-throughput sequencing,and discussed the mainstream application of DPR-SPNA system in sewage treatment plants.The main research contents and conclusions are as follows:（1）Adopting SBR process,inoculate the denitrifying sludge,and the EBPR system was started under the condition of COD of 250 mg/L by anaerobic/aerobic operation,and then the start-up of the DPR system was realized by the anaerobic/anoxic/aerobic（A/A/O）operating mode at a COD of 250 mg/L and NO₃^--N of 15 mg/L.Then,by changing the operating conditions such as COD concentration and NO₃^--N concentration in the influent water,the changes in the denitrification and phosphorus removal performance of the DPR system were studied.The results showed that the effluent COD,PO₄^3--P,NO₃^--N,NH₄⁺-N concentrations were 29.84 mg/L,respectively,during the stable operation of the DPR system,which could be combined with the new autotrophic denitrification process.The COD removal rate,PO₄^3--P removal rate and NO₃^--N removal rate increased from 79.52%,74.27%,75.84%to 85.32%,92.58%,90.28%,respectively,within 220 days of system operation.The typical phosphorus accumulating bacteria unclassified＿f＿Rhodobacteraceae（relative abundance:12.04%）and Dechloromonas（relative abundance:15.06%）,which dominated at high COD concentration,lost their dominance after COD concentration decreased,while Defluviimona（relative abundance increased from 0.01%to 3.49%）and hypomicrobium（the relative abundance increased from 0.95%to 1.65%）,which were more adaptable to the low COD environment.（2）Through batch experiments,the effects of different influent NO₃^--N concentrations on denitrification,phosphorus removal and N₂O release in DPR system were studied.The results show that in the DPR system,when the concentration of NO₃^--N in the initial anoxic range is 10-20 mg/L,NO₃^--N does not significantly inhibit the denitrification and phosphorus removal process,while at 30-40 mg/L,heterotrophic denitrification occurred in the system,ΔPO₄/ΔNO₃ decreased,and the phosphorus removal effect was inhibited.When the nitrate concentration is too high（>20 mg/L）,nitrite accumulation will occur in the denitrification and phosphorus removal process,showing the characteristics of partial denitrification,and the phosphorus removal rate in the partial denitrification and phosphorus removal process is significantly higher than denitrification.In the DPR system,the amount of N₂O generated during the phosphorus absorption process is positively correlated with the initial NO₃^--N concentration,and the N₂O concentration in the reaction process is positively correlated with the concentration of NO₂^-;under closed conditions,with the reduction of NO₂^-concentration in the denitrification phosphorus removal system,the activity of some functional bacteria recovered,and the accumulated N2O in the system could be completely removed within 30 min after the NO2-concentration dropped to 0.（3）Adopting SBR process,the SPNA system was started and combined with DPR system to explore the feasibility of DPR-SPNA system operation.The results show that the SPNA system can be successfully started under the condition of low NH₄⁺-N concentration influent（26-30 mg/L）through hypoxic intermittent aeration（23min/7min）.During the stable operation,the removal rate of NH₄⁺-N in the system was 93.22%,and the ratio of NO3--N produced by the consumption unit NH₄⁺-N was 35.12%.The SPNA system can be combined with the DPR system to form a DPR-SPNA combined process.After the combination,the simulated urban sewage is used as the influent.The effluent COD<20mg/L,NO₃^--N<1mg/L,TN<12 mg/L,TP<0.5 mg/L,in line with the national first-class A emission standard.In the SPNA system,under the conditions of low NH₄⁺-N,low DO concentration,and no sludge discharge,although the activity of NOB is inhibited,it cannot be effectively screened out.The main nitrobacteria in the system was Nitrosomonus,with relative abundances of 0.82%and 2.03%in biofilms and flocs,respectively;the main nitrifying bacteria was Nitrospira,with relative abundances of 4.61%and 3.73%in biofilms and flocs,respectively.Two anammox bacteria mainly exist in the biofilm,Candidatus＿Brocadia（relative abundance is 3.49%）and Candidatus＿Kuenenia（relative abundance is 1.65%）.AOB,NOB and Anammox have long-term competition and coexistence,which promoted the removal of TN from SPNA system together.