Study On The Effect Of Fe(Ⅱ) On Biological Nitrogen Removal Performance And Microbial Population Of Low-C/N-Ratio Sewage

High concentration of nitrogen not only causes eutrophication of surface water,but also threatens the quality of groundwater.Low-C/N-ratio sewage becomes increasingly common in China nowadays,which make it hard to meet the criteria of denitrification in traditional biological denitrification process.As an essential mineral nutrient for microbial growth,Iron is widely involved in microbial biochemical reactions and energy metabolism.Iron cycle and nitrogen cycle interacting also contribute to nitrogen transformation.Besides,ferrous salt has good economic benefits because it is cheap and accessible.In sewage treatment plants,ferrous salt is usually added after the fine screens to remove phosphorus and part of COD,but will cause iron ions residue.To study biological denitrification mechanism in low C/N ratio sewage with Fe（II）will provides a way for the utilization of remaining iron ions,so as to decrease additional organic materials and promote the reasonable material circulation.Therefore,it conforms to the new concepts of sewage treatment plants and has major practical significance.This study combined heterologous iron with low-C/N-ratio wastewater treatment.It was under the SBR lab-scale apparatus operating in A/O mode,simulated with low-C/N-ratio（COD/TN）sewage influent.To investigate the effect of Fe（II）on biological short-term and long-term nitrogen removal performance under low-C/N-ratio conditions,and the anti-load capability of activated sludge under low-C/N-ratio load.The community structure and diversity of activated sludge microbes were firstly detected by high-throughput sequencing technology based on 16S rRNA gene sequence.Then it is used to explore the mechanism of Fe（II）in promoting biological nitrogen removal performance from the perspective of molecular biology.The conclusion can be summarized as follows:（1）The biological nitrogen removal process of activated sludge was investigated under four different C/N ratios（C/N=1,2,3,5）.The results showed that the low-C/N ratio could lead to the incomplete denitrification;moreover,it could decrease specific nitrification rate which results in poor nitrification effect.However,it had less impact on removal of organics.（2）The activated sludge acclimated under the condition of C/N=5 had high capability of bearing shock load.Its treatment performance soon went into stable again after it is shocked by low-C/N-ratio（C/N=1,2,3）loads.However,the low-C/N-ratio load could decline both ammonia nitrogen and total nitrogen removal rate,while increase nitrate nitrogen concentration in effluent because of incomplete denitrification.（3）When adding Fe（II）to each of four C/N-ratio（C/N=1,2,3,5）systems to explore the effect of Fe（II）on short-term biological nitrogen removal performance,it showed that Fe（II）could promote the nitrification effect of low-C/N-ratio system rapidly and significantly,and enhance the stability of system treatment performance.Specifically,under the condition of C/N=1 and C/N=2,the average removal rate of ammonia nitrogen were 83.68%and 96.98%,which improved by 21.62%and 7.11%respectively compared to those without adding Fe（II）.And for system with relatively abundant carbon sources in influent（C/N=5）,the average total nitrogen removal rate was 68.23%,which increased by6.15%,showing that Fe（II）could promote the total nitrogen removal efficiency soon.But for systems with insufficient carbon source in influent（C/N=1,2,3）,Fe（II）had less impact on total nitrogen removal efficiency in same time period.（4）To investigate the long-term biological nitrogen removal performance in different Fe（II）-adding stages,the low-C/N-ratio group（NH₄⁺-N=100 mg/L,C/N=2）was compared with the control group（NH₄⁺-N=40 mg/L,C/N=5）.The results showed that the specific nitrification rate of the groups in the first 30 minutes of the aerobic aeration was accelerated with the addition of Fe（II）.The average specific nitrification rate of the control group and the low-C/N-ratio group in stage IV were 4.56 mgNH₄⁺-N/（gMLVSS·h）and11.59 mgNH₄⁺-N/（gMLVSS·h）,which improved by 29.8%and 53.5%compared to stage I that without adding Fe（II）.The average ammonia nitrogen removal rate of the low-C/N-ratio group in stage IV was 94.87%,which increased by 6.66%compared to stage I.Additionally,Fe（II）facilitated the denitrification of the system.The annamox reaction might occur in the system because of the iron-nitrogen coupling transformation,thus reduced the accumulation of nitrite nitrogen in the anaerobic stir of the low-C/N-ratio group.The concentration of Fe（II）was an important factor for promoting total nitrogen removal performance:the effect is more obvious when the total iron concentration in the mixed liquo was greater than 20 mg/L.The average removal rate of total nitrogen of the control group and the low-C/N-ratio group in stage IV were 67.65%and 44.58%,which improved by 10.37%and 14.32%respectively compared to stage I.（5）To investigate the community structure and diversity of activated sludge microbes in different Fe（II）-adding stages,the low-C/N-ratio group（NH₄⁺-N=100 mg/L,C/N=2）was compared with the control group（NH₄⁺-N=40 mg/L,C/N=5）.When Fe（II）was not added,the abundance and diversity of activated sludge microbes in the low-C/N-ratio group were lower than control group.After adding Fe（II）,the Chao1 index,the ACE index and the Simpson index increased,and the Shannon index decreased,indicating that Fe（II）increased the microbial population abundance and made the dominant microbial population more significant.When analyzing the microbial community structure in different adding Fe（II）stages,it showed that before Fe（II）was added,the dominant microbial population of the two groups were similar,and the abundance of some microbial population were different:there were more Nitrospirae in the low-C/N-ratio group.After adding Fe（II）,Proteobacteria,which is essential to the nitrogen removal,was still the most dominant microbial population and its relative abundance increased.The relative abundance of Planctomycetes（containing ANNAMOX bacteria）,Nitrospirae（containing Nitrospira）,and some aerobic denitrifiers were also increased.These demonstrated that Fe（II）promoted the abundance of the microbial population.As these microbial population have nitrification and denitrification capability,they improved nitrogen removal performance.The results of cluster analysis further evidenced that Fe（II）had a more significant impact on the microbial community structure than the C/N ratio.