Study On Nitrogen Removal Performance And Microbial Diversity Of Low C/N Ratio Wastewater With Added Iron Source

Although the traditional biological denitrification technology has the advantages of high denitrification efficiency and easy operation,it has high operating costs,increases carbon emissions,and heavy burdens of subsequent sludge disposal.In view of the low carbon to nitrogen ratio(C/N)wastewater,urban wastewater treatment plants need to add additional organic compounds(sodium acetate,methanol).It is urgent to seek environmentally friendly,economic and efficient,product multi effect technology to effectively solve the problem of low carbon to nitrogen ratio(C/N)wastewater treatment in China.As a ubiquitous metal element in the environment,iron is easy to obtain;At the same time,it is also an important trace element required in microorganisms,which widely involved in various biochemical reactions and metabolic processes in microorganisms.It is of great practical significance to study the denitrification performance of activated sludge by adding trace element Fe²⁺to enhence microbial activity under the condition of low carbon source.Aiming at the problems of the traditional low carbon to nitrogen ratio(C/N)urban wastewater treatment plant,it investigates the effect of the addition of iron sources under different C/N conditions(C/N:6,3,2)organic matter removal,denitrification performance,electron transfer system(ETS),sludge concentration(MLSS),EPS and its components(PN,PS),activated sludge dynamics analysis,and microbial community structure,diversity and functional gene of nitrogen removal research on the impact;Therefore two SBR reactors operating in parallel were set up,of which 2mg/LFe²⁺was added when the reactors A enters water,the reactors B is the control group without iron source,the results show that:(1)When C/N=6 and 3,the utilization rate of COD in the effluent of A and B reactors is basically maintained at about 90%;while the COD in the effluent of the reactor B fluctuated at low C/N conditions(C/N=2),and the overall removal rate dropped by 4.26%,indicating that the addition of iron source is more conducive to the SBR reactor facing the impact load and maintaining the stability of the system under the conditions of low-carbon source.In addition,the removal rate of ammonia nitrogen is increased from 73.27%and45.50%to 79.23%and 47.24%under the conditions of C/N=3 and 2,respectively,which was 5.96%and 1.74%higher than the without adding Fe²⁺,indicating that iron ions at low concentrations can promote the nitrification reaction;In the later stage of system operation,by adjusting p H,the average NH₄⁺removal rates of A and B reactors were 81.61%and75.47%,respectively.Compared with the blank group,the ammonia nitrogen removal rate increased by 6.47%.At the same time,under the condition of low C/N(C/N=3),The average removal rate of TN in the reactor A and B was 59.89%and 43.85%;The TN removal rate of the A reactor was higher than that of the reactor B 16.04%;Therefore,the addition of iron source is more conducive to denitrification,and the efficiency of microbial nitrogen removal is higher.The overall TTC-ETS activity of the reactor A is 2 orders of magnitude higher than that of the reactor B;the INT-ETS activity of the reactor A gradually increases with the decrease of C/N,and eventually reaching 80.39mg INTF/(g TSS?h);while the reactor B is basically maintained at 57.89mg INTF/(g TSS?h).With the continuous extension of the operating cycle,the contents of EPS,PN and PS showed a decreasing trend;the EPS and PS of the reactor A were higher than the control group as a whole.With the extension of operation time,the increase of Fe²⁺in the reactor increased would stimulate the cells to secrete more EPS.(2)With the decrease of C/N(6?3?2),the number of OTUs in different stages of A,B,C,D(experimental group)and E,F,G(control group)have the same decreasing trend;However,the total number of OTUs in the sludge samples with iron ions was higher than in the control group(E,F,G),and the number of OTUs unique to groups A,B,C,and D was also higher than that in the control group.Among,the changes of Chao1 index and ACE index indicate that the microbial community gradually decreases with the decrease of carbon-nitrogen ratio,but the index value of the sludge sample with iron ions is higher than that of the sludge sample without iron ions;Which indicated that Fe²⁺addition could promote the diversity of the microbial community,help the system to withstand a greater impact load,and then maintain the stability of the system microbial community,and screen out the most suitable dominant species in the population.The carbon-nitrogen ratio(C/N)at different stages provides different growth environments for the microbial community,and the microbial community structure is different,and the most suitable population for this environment is selected;In addition,the microbial populations in the two reactors were similar,but the relative abundance content of some microbial species is still different;Among them,the proportion of Proteobacteria in the reactor with Fe²⁺was higher than that of the control sludge samples,reaching the highest of 52.09%;The proportion of Betaproteobacteria,which is essential for nitrogen removal,was 32.17%at low carbon nitrogen ratio(C/N=2),which was 9.25%higher than the control group;In addition,the proportion of partial aerobic denitrifying bacteria containing Anaerolineae is reduced with the decrease of carbon-nitrogen ratio,but it was higher than that of the reactors without Fe²⁺.At the same time,the relative abundance of Rhodocyclales,which plays an important role in the denitrification process,increased under the condition of lack of carbon source,and the maximum relative abundance is 2.24 times that of the control group.(3)In the case of C/N=3,2,the copy number of nar G,nir K,and nos Z genes in the activated sludge of the reactor with Fe²⁺was higher than that of the control reactor(no iron source was added).With the decrease of carbon nitrogen ratio,the nar G,nir K,and nos Z genes reached the maximum when C/N=2;This indicated that the addition of Fe²⁺increased the activity of denitrifying bacteria.When C/N was 6,3,and 2,nar G was less than(nir K+nir S)gene copies number,and there was no accumulation of nitrite in reactors A and B.With the continuous addition of Fe²⁺in the system,the activated sludge exhibits a good settling performance,and the activated sludge flocs become denser,which is conducive to the impact load of the influent water and maintains the stability of the system.The selection of dominant species in the reactor where Fe²⁺is added has increased the number of protozoa;its stalks and filamentous bacteria are used as the“skeleton”of the floc to strengthen the sludge structure;the small molecular weight secreted by microorganisms Iron chelate improves the utilization of iron.Figure[33]table[22]reference[128]...