Nitrates Removing And N₂O Producing In Sulfur/Limestone Autotrophic Denitrification System

As a major source of water pollution,nitrate has had more and more affection on environmental quality and human health.Therfore,it is imperative to remove nitrate from water.At present,the methods of nitrate removal mainly include physical chemistry(i.e.,ion exchange,reverse osmosis,electrodialysis),chemical method(i.e.,live wave denitrification,catalytic denitrification)and biological method(i.e.,heterotrophic and autotrophic deniitrification).The sulfur autotrophic denitrification system,which has less mud production,no additional carbon source,simple operation,low cost of operation and management,has become more and more popular among scholars all over the world.In the process of nitrate removal,the gas N2 O,which has high temperature increasing potential,has gradually attracted people's attention.Scholars from various countries have made a great deal of research on the origin and the mechanism of N2 O,and the research mainly focused on the aspects of the complete nitrification and denitrification,simultaneous nitrification and denitrification,shortcut nitrification and denitrification,anaerobic ammonium oxidation and aerobic denitrification,but the study of N2 O in sulfur autotrophic denitrification was rarely reported.In this experiment,a uniformly filled sulfur/ limestone autotrophic denitrification reactor was used to study the influence of influent NO3--N concentration and HRT on nitrate removal and N2 O production in the system.The variations of water quality indexes and N2 O production along the reactor height under different influent NO3--N concentration and HRT were analyzed.The community structure in the reactor was further studied from the microscopic point of view by SEM(scanning electron microscopy)and high-throughput sequencing.The results are as follows :(1)When HRT was 5h,the concentration of NO3--N of the system can effectively remove was from 61.7mg/L to 99.48mg/L,the average removal rate of NO3--N was 99.03%.And only when the NO3--N concentration of influent was not greater than 70.1mg/L,there was no N2 O accumulation of effluent.When HRT was 2.5h,the maximum concentration of NO3--N of the system can effectively remove was 68.4mg/L,the average removal rate of NO3--N was 93.74%,and only when the influent concentration of NO3--N was not exceed 34.52mg/L,there was no N2 O accumulate.When HRT was 1.67 h and 1.25 h,the maximum concentration of NO3--N of the system can effectively remove was 33.62mg/L.The removal rate of NO3--N was about 98% with no accumulation of N2 O in effluent.When HRT was 1h,the maximum concentration of NO3--N of the system can effectively remove was 33.62mg/L,the removal rate of NO3--N was 98%,and only when the influent concentration of NO3--N was 16.23±1mg/L,there was no N2 O accumulation in the effluent.(2)When the concentration of NO3--N was 16.23±1mg/L,the decrease of HRT did not affect the removal efficiency of NO3--N,The removal rate of NO3--N maintains about 97%,HRT had no effect on the accumulation of N2 O in effluent,and there was no accumulation of N2 O in effluent.When the influent concentration of NO3--N was 32.67±1.85mg/L,the removal effect of NO3--N was not affected by the decrease of HRT,but N2 O accumulation appeared in the reactor effluent when HRT was reduced to 1.25 h.When the influent concentration of NO3--N was 64.05±6.05mg/L or 103.63±11.55mg/L,the removal rate of NO3--N will decrease greatly with the decrease of HRT,and the accumulation of N2 O will occur.Therefore,we must guarantee a longer HRT for removing high concentration of NO3--N.In addition,the reactor can withstand the minimum HRT was 1h.(3)The optimum influent load of NO3--N was 0.146~0.626kg/(m3·d),the removal load was 0.141~0.626kg/(m3·d),and no N2 O accumulated in effluent.When the NO3--N load of influent was 2.60kg/(m3·d),the maximum removal load was 1.211kg/(m3·d),but the effluent concentration of NO3--N would reach 57.9mg/L.(4)The enrichment of microorganism in the reactor mainly occurred in the biofilm covered by the surface of sulfur and limestone particles.The biomass of the lower part of the reactor was larger than that of the upper part,and the biomass in the biofilm on the surface of the limestone was larger than that in the sulfur surface biofilm at the same height.(5)The diversity and abundance of bacteria in the lower part of the reactor were larger.The microbes at the bottom of the system have a lower similarity to the upper microbes in the reactor.While the central microbial similarity was higher.At the Phylum classification level,there were 41 groups in this system.Proteobacteria was the dominant flora,there were 350 bacterial groups at the genes classification level,the main role in the sulfur autotrophic denitrification was Thiobacillus,Sulfurimonas enhanced the denitrification capacity of the system,and a number of microbial species capable of denitrification was found.The presence of Ideonella further validated that the DNRA reaction occurred in the system.