Performance And Start-up Strategy Of Dual Biofilm DPR-SPNA Process For Deep Nitrogen And Phosphorus Removal

In recent years,phosphorus removal mostly depends on chemical phosphorus removal,which requires the use of a large number of chemical reagents,greatly increasing the cost of treatment.Research on denitrification and phosphorus removal(DPR)process had broken the barrier of simultaneous nitrogen and phosphorus removal,phosphorus absorption and denitrification were carried out simultaneously,and carbon stored in the previous anaerobic stage was used at the same time.However,if the ratio of ammonia nitrogen(NH4+-N)to nitrate nitrogen(NO3--N)is 1:1,DPR cannot achieve complete denitrification,while anaerobic ammonia oxidation can reduce NO3--N produced to a lower level.In this study,a sequencing batch biofilm reactor(with a filling ratio of 22%)was used to investigate the feasibility of DPR on biofilms based on two processes of biofilm formation on fixed carrier and domestication of denitrifying phosphorus accumulating bacteria(DPAOs).After the deterioration of the fixed biofilm denitrification(DPR-SBBR)system,the causes of the deterioration were analyzed and a strategy to delay the deterioration of DPR-SBBR was proposed.In addition,the performance of the system after adjusting DPR-SBBR to mobile biofilm denitrification(DPR-MBBR)was studied,and a strategy to quickly restore DPR on biofilm was proposed.The differences and changes in microbial communities of activated sludge(AS)and biofilm samples were revealed through batch experiments and highthroughput analysis.By metagenomic analysis,functional pathways,nutrient removal dominant genes and functional genes related to biofilm formation were identified.Finally,the rapid start-up strategy of the integrated short-cut nitrification and anaerobic ammoxidation(SPNA)system was discussed,and coupled with DPR-MBBR to obtain a dual biofilm coupling system,and the feasibility of the coupling system was discussed.The main conclusions are as follows:(1)Starting stage of DPR system treatment of low C/N municipal sewageAfter the operation mode was changed from anaerobic/aerobic(A/O)to anaerobic/anoxic/aerobic(A/A/O),the phosphorus removal rate(PRE)remained at 55.65%.Although sludge concentration(MLSS)dropped below 60 mg/L,PRE was still above 54.01%on day 33.However,in the absence of AS drainage,PRE gradually decreases,leading to deterioration of DPR performance.In situ batch test showed that phosphorus absorption mainly occurred at the anoxic stage.However,the presence of glycan bacteria(GAOs)may result in a low PO43-/NO3-ratio.Later aerobic phosphorus absorption ensured lower effluent PO43--P concentration,but was inevitably accompanied by NH4+-N oxidation.In the process of biofilm formation,GAOs(Candidatus_Competibacter)had a more active secretion of extracellular polymer(EPS),accounting for the dominant position in biofilm(22.35%).After 100 days of A/A/O operation,the dominant genus of PAOs was Pseudomonas(1.27%)and Thiothrix(2.24%).Biofilm-based DPR was expected to combine with anammox to achieve efficient nutrient removal and low carbon requirements.Therefore,the longterm stable operation of DPR-SBBR,especially in the absence of sludge discharge,still needs further study.(2)Degradation and recovery of low C/N municipal wastewater treated by DPR systemThe DPR-SBBR system was operated under alternating A/A/O conditions,and the effluent PO43--P concentration decreased to 2.90 mg/L and PRE to 41.38%.In addition,the PRA was lower than 10 mg/L and the PUA was also lower than 10 mg/L on day 329.The deterioration of the system could be delayed or stopped by backflushing.DPRMBBR system was operated under A/O condition,and the average concentration of PO43--P in effluent was 0.73 mg/L,while PRE increased to 89.37%.When the NH4+-N concentration of effluent was 0.18 mg/L,the ammonia nitrogen removal rate(ARE)increased to 99.63%.In addition,it was observed that the concentration of aerobic nitrous nitrogen(NO2--N)reached 14.09 mg/L on day 401,but the nitration products eventually changed to NO3--N over time.When DPR-MBBR system was operated under A/A/O conditions,the effluent PO43--P concentration could be reduced to less than 0.83 mg/L,while PRE was maintained above 84.65%.The system was maintained above 93.11%,but the products of nitrification process ARE mainly NO3--N,whose average concentration was 16.64 mg/L.After shortening the aerobic time,the PO43/NO3-ratio(average 4.34 mgP/mgN)also showed a stable trend.Proteobacteria(76.7%)occupied the highest proportion in AS-227 of DPR-SBBR system,and the genera with relative abundance greater than 20%were Candidatus_Competibacter(42.91%)and Thiothrix(20.2%).Proteobacteria and Bacteroidota were still the dominant groups in the biofilm samples.In the sample of DPR-MBBR system Bf-518,the phylum with the highest proportion was Bacteroidota,while in other samples,Proteobacteria was the phylum with the highest proportion.The genus with the highest proportion in AS-403 was Thiothrix,the genus with the highest proportion in AS-518 was OLB 8(11.39%),and the genus with the highest proportion in other samples was Candidatus_Competibacter.(3)Dual biofilm DPR-SPNA system start-up and optimized operationSPNA system was inhibited at high NH4+-N concentration.After influent concentration was reduced to 20 mg/L,effluent NH4+-N concentration was about 12.85 mg/L,and ARE was gradually increased to 39.85%.When the aerobic and anaerobic time(tO:tA)was adjusted to 20:20 min,the NH4+-N concentration of the effluent was gradually reduced to 9.67 mg/L,and the ARE was increased to 55.21%.The dissolved oxygen(DO)of the system was controlled at 0.25-0.35 mg/L,and the ARE could be stabilized above 91.94%.The system had stable nitrogen removal performance.After coupling DPR-MBBR with SPNA,PRE gradually increased to 97.53%,NH4+-N concentration in effluent remained at 1 mg/L,and ARE increased to more than 95%.The average concentration of PO43--P was 0.13 mg/L,the PRE increased to 97.68%,and the ARE increased to more than 98.06%.Therefore,the coupling of DPR-MBBR and SPNA could achieve efficient nitrogen and phosphorus removal.For the SPNA system,Chloroflexi(44.42%-60.97%)was the gate with the highest proportion before and after anammox film mounting and after SPNA system operation.norank_f_norank_o_SBR1031(37.14%;45.38%)and Calorithrix(22.19%;19.25%).At the gate level of the DPR-SPNA system,Proteobacteria(57.50%-67.27%)dominated in DPR samples,while Chloroflexi(48.09%)was the highest in SPNA-78 samples.Species level,the function AS samples unclassified_f_Rhodobacteraceae dominant in phosphorus removal(4.78%).The abundance of Candidatus_Competibacter and Defluviicoccus will gradually increase to 47.07%and 2.54%,accounting for 25.37%and 1.73%respectively in Bf samples.