| The traditional biological denitrification process of NO3-→ NO2-→NO→N2O→N2,often consumed a large amount of external carbon sources,leading to the high cost of wastewater treatment plants.However,the partial denitrification（PD）reaction controlled the denitrification process at the stage of NO2--N accumulation phase,which can not only save carbon source,but also reduce the reaction time and sludge production.The anaerobic ammonia oxidation（ANAMMOX）used the NO2--N generated by the PD process as the reaction substrate,which in turn provided NO3--N as electron acceptors for the PD process.Therefore,the prosperity of ANAMMOX has led to more research and attention to PD technology.In this study,the influences of C/N ratio,NO3--N concentration,carbon source,temperature,reaction time and other factors on the NO2--N accumulation characteristics of the PD process were primarily investigated.The composition and abundance of bacteria community in the reactor under different conditions were compared by high-throughput sequencing,and the NO2--N accumulation mechanism in the PD process was further explained.On the basis of the above research,the coupling technology of partial denitrification and denitrifying phosphorus removal process（PD-DPR）was carried out to achieve deep nitrogen and phosphorus removal,which provided operational strategies for the practical engineering application by combining with the introduction of coupled process related to PD.The main research contents and relevant conclusions were as follows:（1）The NO2--N accumulation amount in each reactor reached the peak value at anoxic 30 min under different C/N（2.0,2.5,3.3,5.0）ratios（Temperature:20℃）.There was a phenomenon that external carbon source was converted to internal carbon source,because COD was rapidly consumed in the first 30 min.Besides,the higher NO3--N content inhibited the activity of NO2--N reductase and promoted the NO2--N accumulation.The NO2--N accumulation reached to 31.2 mg/L at anoxic 30 min at C/N ratio of 2.5,and the NO2--N and NTR values in the effluent reached to 26.2 mg/L and 72.8%,respectively.Importantly,the kinetic analysis revealed that the reduction of NO2--N was closely related to SN2AR（nitrite accumulation rate）,SN3RR（nitrate removal rate）and SCCR（COD consumption rate）,and the NOB（nitrite oxidizing bacteria）was effectively restrained in aerobic stage,which promoted the stable NO2--N accumulation.（2）It can be seen from the OTU rank level,PCoA analysis and Venn diagram that the microbial characters in the four reactors（C/N=2.0,2.5,3.3,5.0）had changed greatly compared with seed sludge（SS）（Temperature:20℃）.The abundance of microbial composition was similar at C/N ratio of 3.3,2.5 and 2,and the microbial diversity was the lowest at 2.5.In addition,a species of streptomyces bacteria was observed via SEM and microscopic analysis,which was speculated to be the unique bacteria of partial denitrifying sludge.Nitrospira accounted for 1.8%of the SS,and the proportion of the four C/N ratio conditions after domestication was less than 0.1%.Thauera was the dominant species in PD process,and the enrichment of Thauera reached to 2.6%,24.1%,9.1%and 7.2%compared with SS（<0.01%）at C/N ratio of 5,3.3,2.5,2,respectively.Flavobacterium identified as the functional bacteria of partial denitrification accounted for 28.2%at C/N ratio of 2.5,achieving the maximum enrichment of partial denitrifying bacteria.（3）When the initial NO3--N concentration was guaranteed to be 40 and 80 mg/L,the organic carbon source was added to keep C/N at 0.8,1.5,2.5,3.5,4.5,respectively.It was observed that NO2--N and NTR values of effluent with NO3--N concentration of 80 mg/L were significantly higher than those of 40 mg/L,and the higher NO3--N concentration corresponded to higher NO2--N accumulation rate and COD degradation rate through kinetic analysis.The results of high-throughput sequencing showed that the proportion of Thauera was 19.3%,19.4%,31.8%,23.5%,24.3%and Flavobacterium accounted for 0.4%,1.25%,20.3%,13.9%,20.8%with NO3--N concentration of 80 mg/L at C/N ratio of 0.8,1.5,2.5,3.5,4.5.When NO3--N concentration was 40 mg/L,Thauera accounted for 6.6%,13.0%,13.0%,36.6%,25.8%,while Flavobacterium accounted for 2.8%,3.4%,3.9%,2.5%,2.8%,respectively.（4）Temperature,reaction time and the type of carbon source all affected the PD process.Firstly,lower temperatures negatively affected NO2--N accumulation during the PD process.A significant decrease in NO2--N content and NTR values occurred when the temperature decreased from 20℃ to 12℃ at C/N ratios of 5,3.3,2.5 and 2,and a positive correlation between temperature and NO2--N accumulation characteristics was observed.Lower temperature also reduced the NO3--N reduction rate and prolonged the time for NO2--N accumulation to reach its peak,causing the increased optimal C/N ratio for PD process.Secondly,regarding to the reaction time,the effluent of NO2--N was 17.0,24.2,5.3 and 0 mg/L at 3 h for C/N ratio of 2,2.5,3.3 and 5,respectively,while it reached to 17.2,31.2,16.5 and 3.9 mg/L at 30 min.Reasonable control of the reaction time could maintain the NO2--N content in the effluent at a high level.The type of carbon source also showed a considerable impact on the effectiveness of the PD process,mainly due to the differences in molecular structure.The molecular structures of sodium acetate and sodium propionate were simple compared to glucose,and the best NO2--N accumulation was accomplished with NTR of 74.4%using sodium acetate,followed by sodium propionate（50.9%）and glucose（29.3%）,demonstrating that glucose was detrimental to PD process.（5）In the PD-DPR coupling process,the NTR of PD process reached to 70%,and the COD degradation rate and PO43--P removal rate in the denitrifying phosphorus removal reactor were 78%-85.4%and 95%,respectively,and there was almost no NO2--N and NO3--N in the effluent.By analyzing the coupled process related to PD process and summarizing advantages,disadvantages and operational characteristics.It was found that lower C/N ratio（2.5-3）and shorter reaction time（1 h）were more favorable for nitrogen removal,which provided an operational control strategy for the practical application of deep denitrification by PD process. |
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