| Because of the advantages of low construction and operation costs,high organic matter removal efficiency,and both landscape and ecological value,constructed wetlands have been widely used in several wastewater treatment fields in recent years.However,the low efficiency of nitrogen removal and the ease of clogging are the main factors limiting the further application of artificial wetlands.Traditional nitrogen removal method(the coupling of nitrification and denitrification)requires sufficient oxygen and carbon sources,otherwise it cannot achieve efficient nitrogen removal.Clogging occurs with increasing operational time in constructed wetlands,mainly due to the abundantly secreted biofilms by bacteria within the system and extracellular polymers(EPS)accumulating at the substrate surface and adsorbing inert organic matter(OM)and inorganic matter(IM).Previous studies have shown that free nitrous acid(FNA)can specifically inhibit nitrous acid oxidizing bacteria,rapidly initiate and maintain short-cut nitrification reactions,and significantly enhance the nitrogen efficiency of constructed wetlands.At the same time,FNA can reduce the extracellular polymeric substances(EPS)adhesion ability and cleave proteins and polysaccharides in EPS,which in turn can effectively alleviate constructed wetland clogging.Therefore,the addition of FNA is expected to simultaneously reduce the clogging of constructed wetland and significantly enhance its nitrogen removal efficiency.In this study,the effects of FNA addition on the clogging and nitrogen removal efficiency of constructed wetlands were systematically investigated using a vertical flow wetland as the research target.The main findings are as follows:(1)The effect of FNA on nitrification reactions was determined by setting up six sets of vertical flow constructed wetlands.Six sets of FNA concentrations(0.0,0.4,0.8,1.2,1.6,2.0mg N / L)and four sets of treatment times(6,12,18,24 h)were set to reflect the rate of ammonia oxidation and nitrite accumulation reactions in the in the constructed wetland system in terms of ammonia nitrogen removal rate and nitrite accumulation rate.It was found that increasing concentrations of FNA led to a general decrease in ammonia nitrogen removal rate,while nitrite accumulation rates increased with increasing FNA concentrations.When the treatment time was found to be18 h and FNA = 2.0 mg N / L,the nitrite accumulation rate increased by 42.40% before and after treatment.In this context,no damaging side effects of functional microorganisms were found,except for a significant decrease in nitrite oxidising bacteria(NOB).And under the treatment time of 18 h,it was found that FNA mainly acted on TB-EPS,and the protein(PN)and polysaccharide(PS)in TB-EPS changed with the increase of FNA concentration,the concentration of PN decreased from 50.74mg/L to 22.20mg/L,and the concentration of PS decreased from 16.52mg/L to 3.22mg/L,where the There was no significant trend in humic acid(HA)concentration,but the concentration of HA in the group with FNA added was lower than that in the group with FNA = 0 mg N/L.The concentrations of PS and PN in S-EPS and LB-EPS were lower and mainly dominated by humic acid.(2)To investigate the effect of FNA on the extent of clogging in constructed wetland,initial porosity was 35.00% in both groups through the establishment of long-term operated constructed wetlands(CW1,CW2),and it was found that porosity gradually decreased with increasing operation time,and the porosity decreased sharply from26.84% to 16.00% when the constructed wetlands entered the stable stage(50-139days).When the FNA treatment was followed,the porosity of the treated wetland recovered to 23.12%,while the porosity of the control group remained unchanged.The factors responsible for this porosity change were as follows: the concentrations of PN and PS were all decreased in TB-EPS,and the treatment with FNA rendered the PN inactive and PS hydrolyzed;The particle size distribution width and average size of LB-EPS,TB-EPS all decreased,indicating that FNA had a dispersive effect on EPS,making the structure of EPS with adhesiveness loose and alleviating the clogging degree of constructed wetlands;The absolute value of zeta potential of LB-EPS and TB-EPS increased,and EPS after FNA treatment exerted an inhibitory effect on microbial aggregation via electrostatic repulsion.(3)In order to elucidate the impact of FNA on the nitrogen removal effect of constructed wetlands,through continuous monitoring of the hydration index of two groups of intermittent aeration artificial wetlands(CW1,CW2)in long-term operation,it was found that the low dissolved oxygen strategy through intermittent aeration in the early stage of operation(15-90 days)led to higher nitrogen removal efficiency of the constructed wetlands was increased by a low dissolved oxygen strategy with intermittent aeration.but as the operation time of the artificial wetlands increased,the total nitrogen(TN)removal rate decreased from 60.64% to 37.94%,and ammonia nitrogen removal rate decreased from 87.65% to 74.12%.After the FNA treatment,it was found that the TN removal rate of CW2 in the treatment group gradually increased to a stable level of about 70% while the TN removal rate of CW1 in the control group remained stable at 50.99%.By high-throughput sequencing,it was found that FNA treatment did not affect the metabolic function of other nitrifying denitrifying microorganisms besides nob.Microbial community richness and diversity are on the rise in constructed wetlands.The relative abundance of the major microbes Proteobacteria,bacteroidota,Chloroflexi increased,and that of Firmicutes decreased,so that TN removal from CW2 increased. |
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