Experimental Study On Advanced Removal Of Nitrogen And Phosphorus In A Novel Continuous Flow Process Of A/O+PD/A

Due to its benefit of not requiring extra carbon sources and oxygen supply,the anaerobic ammonia oxidation（Anammox）process acquired widespread concern in the treatment of municipal wastewater with limited carbon sources.However,there are several problems when using the Anammox process to treat municipal wastewater:First,as an essential electron acceptor for Anammox,the content of nitrite（NO₂^--N）in urban wastewater is usually low,and pretreatment is required to obtain sufficient NO₂^--N;Secondly,the nitrate（NO₃^--N）contained in the Anammox process effluent needs to be further removed;Finally,Anammox is an efficient nitrogen removal process for wastewater,but it needs to be combined with the phosphorus removal process to complete the nitrogen and phosphorus removal treatment of municipal wastewater.Therefore,based on the partial denitrification coupled Anammox（PD/A）process and anaerobic/aerobic（A/O）biological removal of phosphorus technology,a continuous flow A/O+PD/A process was constructed in this paper.In the study of this subject,the formation and NO₂^--N accumulation characteristics of PD granular sludge were analyzed,and the nitrogen and carbon removal performance of the integrated PD/A coupling process was investigated.Through controlling the aeration method,A/O biological phosphorus removal was achieved,and appropriate influent nitrate/Ammonia nitrogen（NO₃^--N/NH₄⁺-N）was provided for the PD/A process.Finally,the combined process achieved the goal of efficient nitrogen and phosphorus removal.The main findings are as follows:（1）Analysis of the formation and NO₂^--N accumulation characteristics of PD granular sludge.（1）Inoculated with undomesticated activated sludge from ordinary municipal wastewater treatment plants in the Expanded Granular Sludge Bed（EGSB）reactor,controlled the p H and water temperature at 8.0 and 20～24℃,respectively,and used synthetic wastewater to maintain the influent NO₃^--N concentration at 30 mg/L.The influent COD/NO₃^--N（C/N）was controlled to gradually increase from 2.0 to 3.5,and the HRT was gradually reduced from4 h to 2 h.The results showed that the highest nitrate to nitrite conversion ratio（NTR）of 91.99%was obtained when C/N was 3.0 and HRT was 2 h,while the average NTR reached 83.23%and NO₂^--N accumulation reached 23.44 mg/L.（2）The particle size of sludge in EGSB gradually increased from 0.107 mm of inoculated sludge to 1.72 mm of granular sludge on day 90.The MLVSS/MLSS ratio gradually decreased from a maximum of 81.44%to 66.97%,indicating a gradual accumulation of nonvolatile substances in the granular sludge.（3）The changes in the loosely bound and tightly bound layers of proteins（L-PN,T-PN）in the extracellular polymeric substances（EPS）correlated well with the changes in the particle size of the granular sludge（r=0.99,P<0.001）,and the main reason for the change in EPS was the growth of PN.（4）Bacteroidetes and Chloroflexi,microbial populations favoring granular sludge formation,were detected in PD-EGSB,with relative abundances increasing from 18.33%and 13.70%of inoculated sludge to 27.57%and 36.48%of granular sludge,respectively;The relative abundance of the PD functional genus Thauera increased from 0.55%to 1.35%,and denitrifying glycogen accumulating organisms（DGAOs）such as Candidatus Competibacter and Defluviicoccus were also the main genera performing PD reactions in the system.（2）A preliminary study on the performance of PD/A coupled process for wastewater treatment.（1）Inoculated PD granular sludge and Anammox sludge（MLSS and MLVSS were13,893 and 6,164 mg/L,respectively）in PD/A-EGSB to treat synthetic wastewater,maintained the influent NO₃^--N and NH₄⁺-N concentrations at 30.0 mg/L,temperature at 32±2℃,controlled the influent C/N from 3.0 to 2.5,and HRT from 5 h gradually reduced to 3 h,which can quickly start the PD/A coupling process.The nitrogen removal performance of the PD/A system with and without filler was compared,and it was found that the nitrogen removal efficiency of R2 with the addition of porous sponge filler was higher than that of R1.When the influent C/N was 2.5,the HRT was 3 hours,and the influent NLR was 0.48 kg N/m³/d,the NRR of R1 and R2 reached 0.40 and 0.46 kg N/m3/d,respectively,and the TN removal efficiency was 83.69%and 91.48%,respectively.（2）The biomass increased significantly,MLSS reached17.41 and 18.93 g/L and MLVSS 7.53 and 9.32 g/L in R1 and R2,respectively,indicating that the coupled granular sludge-biofilm system is more favorable for the enrichment of functional microorganisms.（3）EPS increased from 47.16 and 50.26 mg/g VSS to 61.41 and 65.43mg/g VSS,respectively,and the growth of T-EPS plays an important role in the stabilization of sludge in the PD/A system.（4）Nitrogen removal pathways were analyzed for the PD/A system,and when C/N was 2.5 and HRT was 3 h,the contribution of Anammox to nitrogen removal reached 90.62%and 93.71%,respectively,and the contribution of denitrification to nitrogen removal decreased to 9.38%and 6.29%,respectively,and Anammox became the most dominant nitrogen removal pathway in the PD/A system.（5）The Planctomycetes which contained multiple An AOBs were the dominant phylum in R1 and R2.The major An AOBs in the two reactors were Candidatus Brocadia and Candidatus Kuenenia,and the relative abundance of An AOBs in R1 and R2 was 27.57%and 51.66%,respectively.The retention and attachment of porous sponge filler in R2 were more beneficial for the enrichment of An AOB.（3）Start-up and stabilization of A/O half amount full nitrification and biological phosphorus removal.（1）The A/O continuous flow process was used to treat the actual municipal wastewater with a controlled HRT of 8 h and SRT of 15 d.There was no nitrification liquid return,100%sludge return was set,and the aeration method was adjusted to intermittent aeration and continuous low oxygen aeration.The results showed that when the volume of aeration was 60 liters per hour and the ratio of aeration to stop was 60 min:40 min,the effluent NO₃^--N/NH₄⁺-N of the A/O system was close to 1.0,which was able to provide a suitable influent substrate ratio for the PD/A system,but the longer anoxic time made the average effluent orthophosphate(PO₄^3--P)concentration at this stage greater than 0.5 mg/L.（2）When the continuous aeration was limited to 30 L/h and the corresponding dissolved oxygen was0.5～0.8 mg/L,the effluent NO₃^--N/NH₄⁺-N of the A/O system was also close to 1.0,which satisfied the influent requirements of the PD/A system,Meanwhile,the effluent PO₄^3--P of the A/O system was able to be lower than 0.5 mg/L.（4）A preliminary study on the efficiency of a continuous flow process for deep nitrogen removal and phosphorus removal by a new continuous flow process of A/O+PD/A.（1）The A/O+PD/A continuous flow process was used to treat actual municipal wastewater,the aeration volume in the aerobic zone of continuous aeration A/O was 30 L/h,the dissolved oxygen was0.5～0.8 mg/L,the PD/A influent was the A/O effluent,and the PD/A temperature was maintained at 32±2℃.When the influent COD,TN and PO₄^3--P concentrations were 250.64,55.88 and 5.21 mg/L,the effluent concentrations after the combined process were 21.14,5.16and 0.45 mg/L,respectively,and the mean percentage of removal reached 91.01%,91.36%and90.54%,respectively.The contribution of the A/O system and PD/A system to COD removal was 90.39%and 9.61%,and the contribution to TN removal was 67.08%and 32.92%,respectively,while PO₄^3--P was mainly removed in the A/O system.（2）TN conversion analysis along A/O+PD/A showed that after the effluent and return sludge enter the anaerobic zone of the A/O reactor together,the TN is removed by about 44.56%because the NO₃^--N carried in the diluted and return sludge is reduced by denitrifying bacteria.TN removal of 14.41%due to simultaneous nitrification-denitrification reactions that occurred in the low DO environment in the aerobic zone,followed by a continued decrease in TN concentration of 2.88%in the settling tank.Finally,28.93%of TN was removed by PD/A-EGSB.The combined A/O+PD/A process combines biological phosphorus removal,heterotrophic denitrification and autotrophic denitrification technologies to achieve efficient nitrogen and phosphorus removal from municipal wastewater.