Effects Of Nanoscale Zero-Valent Iron On The Performance Of Denitrifying Granular Sludge

Nanoscale zero-valent iron,as a new type of nano materials,can be used for the degradation of organic pollutants,heavy metals,nitrate,and so on because of its large surface area,small particle size,and the high strength reduction activity.However,nanoscale zero-valent iron will discharge into the sewage treatment system and may be adsorbed on the surface of microorganisms or accumulated in microbial body,which may influence on the microbial activity and the stability of the system.At present,there were few reports about the effects of nZVI on the biochemical treatment system,and the influences on denitrifying granular sludge were never mentioned.In this study,the effects of nanoscale zero-valent iron on the nitrogen removal performance and the activity of the denitrification granular sludge were investigated to explore the potential threatening and improve the stability of denitrification system,main contents and results were as follows:(1)Using anaerobic sludge granule as seed sludge,sodium acetate and potassium nitrate as carbon source and nitrogen source,the denitrifying granular sludge was cultivated in USB.The granular sludge was pale yellow or white,the mean diameter was above 1.93 mm,and the denitrifying rate was higher than 22.5 mg N /(g·h).(2)The effects of n ZVI dosage from 0 to 100 mg/L on the performance of denitrifying granular sludge(DGS)at different C/N ratios were investigated in this study.During the period,the transformation of nitrogen forms,the specific denitrifying rates(? values),the composition of extracellular polymeric substance(EPS)and the distribution of element iron were analyzed in detail.Results showed that the removal efficiencies of nitrate nitrogen and COD would decrease with the n ZVI dosage higher than 5 mg/L,while the residual concentration of nitrite nitrogen increased significantly.Moreover,the inhibition of nZVI dosages on the specific denitrifying rates was well fitted by Haldane model,since the value R2 was in the range of 0.944 to 0.995.And both the higher adsorbing capacity and the lower inhibition coefficient(KIH)were observed,with the increase of C/N ratios.It indicated the resistance of DGS to nZVI toxicity getting weakened gradually.When the dosage of nZVI was higher than 25 mg/L,the concentrations of protein and polysaccharides in EPS reduced obviously,which would be against to maintain the structural stability of granular sludge.(3)To explore the potential stressing effect of nanoscale zero-valent iron(nZVI)on denitrifying granular sludge(DGS),the evolution of DGS denitrifying performance under different C/N ratios was investigated in this study,by carrying out batch tests of eight successive periods with the nZVI shock-loading.Results showed that the specific denitrification rate of ? value would be decrease with the n ZVI dosage higher than 5 mg/L.Meanwhile,a positive correlation between the inhibition ratio(IR)of ? value and substance C/N ratios or n ZVI dosage was observed.When the nZVI dosage reached 100 mg/L,both extracellular protein and polysaccharides concentrations decreased obviously.It would be beneficial to promote the recovery of DGS denitrifying activity and reduce the COD demanding to remove unit mass of nitrate,by increasing external carbon source with C/N ratios higher than 4.On the basis of Freundlich and Langmuir adsorption isotherms,the higher C/N ratio was provided,the stronger bioadsorption of nZVI would be achieved.During the recovery period,a significant improvement of DGS denitrifying performance under the high C/N ratio was expected,due to the continuous washout of total iron in sludge phase(Qe),while the ? value would reach or approach the one of control group with Qe lower than 0.4 mg/g.(4)The effects of nZVI concentration on nitrogen removal performance and activity of denitrifying granular sludge were studied in long term,and then explored whether the inhibition was reversible.The long-term test showed that there was no impact on denitrifying granular sludge to organic pollutants and nitrogen removal when nZVI concentration was less than 2 mg/L,meanwhile,it had a process of domestication for microorganism in the system,NO3--N and COD removal were falled and then stabled when the influent concentration of n ZVI at 5 mg/L and 10 mg/L.when the concentration of nZVI was 30 mg/L,the activity of sludge was inhibited significantly,NO3--N and COD removal rate was only about 20%,and the colour of the granular sludge changed from yellowish-brown to grey black.Denitrifying granular sludge activity can be recovery without nZVI in the influent.