Toxic Effects Of Several Poison Substances Onto A Combined Sulfur Autotrophic And Electrochemical Hydrogen Autotrophic Denitrification System

Because excessive amounts of nitrate in surface and ground waters pose serious threats to water environment and human health,exploring high-efficiency,low-consumption and non-secondary pollution methods for nitrate removal is becoming an important subject for water treatment.The biological treatment is regarded as the most dominant nitrate removal technology because of its high-efficiency and low-consumption features.Although biological nitrate removal method have been widely studied and demonstrated,the nitrate polluted waters may face threats from heavy metal ions and toxic organics.Nowadays,even though some researchers have focused on simultaneous removal of nitrate,heavy metals and organics,the effects caused by heavy metals and organics concerning denitrification process and the simultaneous degradation mechanisms are not systematically studied.This work aims to explore the effects of denitrification efficiency,dominant microbial community,key enzyme activity,denitrifying functional gene,and extracellular polymeric substance caused by heavy metals and organics concerning denitrification process,and reveal the toxic effects of heavy metals and organics onto denitrification from the macroscopic and microscopic aspects.This study will provide important supports for the practical application of denitrification process.This work develops a combined sulfur autotrophic and electrochemical hydrogen autotrophic denitrification system in order to investigate the effects of heavy metal ions Cr(?)and V(?),toxic organics phenol,p-nitrophenol and chloramphenicol onto denitrification process.Results demonstrate that nitrate removal efficiency decreases with initial Cr(?)concentration increases from zero to 150 mg/L in the combined denitrification system.Under the stress condition of 100 mg/L Cr(?),the combined system can achieve completed denitrification when the nitrate concentration is lower than 100 mg/L.However,nitrate removal efficiency decreases to 81.69%under 200 mg/L nitrate condition.Under the stress condition of 100 mg/L Cr(?),the combined system achieves optimal denitrification efficiency at pH 7 and the highest nitrate removal efficiency reaches 93.01%.When the initial Cr(?)concentration is 150 mg/L,the nitrate reductase activity and nitrite reductase activity both decrease obviously.The effects of biofilm formation are not obvious when the initial Cr(?)concentration is lower than 100 mg/L,however,when the initial Cr(?)concentration is further increased to 150 mg/L,the microorganisms are apparently influenced and then secrete fewer amounts of EPS.Under the stress condition of Cr(VI),some denitrifying microorganisms may decrease or disappear because of the toxic effects.The combined system can achieve satisfied denitrification efficiency,attributing to the denitrification abilities of phyla Proteobacteria and Firmicutes and classes Betaproteobacteria and Clostridia.When the initial Cr(VI)concentration increases from zero to 150 mg/L,the copy numbers of denitrifying functional genes nirS,nirK,narG,and napA show declining trends.Cr(VI)is reduced to Cr(III)through biological reduction process and then combined with biofilms,or dropped off with biofilms,then removed from the reactor.When the initial V(V)concentration increased from 0 to 100 mg/L,the denitrification efficiency of the combined system presents decreased tendency.Under the stress condition of 60 mg/LV(V),the denitrification efficiency increases with current increases from 50 to 200 mA,However,the denitrification efficiency decreases when current increases to 400 mA.Under the stress condition of 60 mg/L V(V),the combined system achieve optimal nitrate removal efficiency under neutral condition,acidic or alkaline conditions cause decreased denitrification efficiencies.When the initial V(V)concentration is higher than 60 mg/L,the nitrate reductase activity and nitrite reductase activity both decrease obviously,which is in accordance with the decreased nitrate removal efficiency.The results of LDH release rate and ROS production rate demonstrate that the combined denitrifying microorganisms can be adversely affected by V(V)which concentration is higher than 60 mg/L.The stress of V(V)decreases the bacterial abundance,the total number of species,and microbial diversity.The dominant phyla and classes are Proteobacteria,Firmicutes and Betaproteobacteria,Clostridia,Gammaproteobacteria,Alphaproteobacteria,respectively.The main denitrifying functional genes are inhibited by the increasing V(V)concentration,so that the denitrification efficiency constantly decreases.V(V)is ultimately reduced to V(IV)through biological reduction process in the system.When initial phenol concentration increases from zero to 100 mg/L,denitrification efficiency shows decreased trend and the lowest denitrification efficiency achieves at 100 mg/L phenol.Under the stress condition of 50 mg/L phenol,nitrate removal efficiency gradually decreases with increasing initial nitrate concentration.Under the stress condition of 50 mg/L phenol,the combined system can achieve optimal denitrification efficiency at pH 7.With increasing initial phenol concentration,nitrate reductase activity and nitrite reductase activity both present decreased tendencies,which is in accordance with decreasing denitrification efficiency.With increasing initial phenol concentration,the combined system secretes declining EPS,and thus affecting the biofilm formation and decreasing nitrate removal efficiency.When initial phenol concentration is lower than 50 mg/L,the bacterial abundance shows increased tendency under the phenol stress condition.However,when initial phenol concentration increases to 100 mg/L,the community diversity shows decreased trend,demonstrating that 100 mg/L phenol causes negative effects onto microorganisms in the combined system.Phenol is removed in the denitrification system due to participating heterotrophic denitrification process as carbon source.The denitrification process in this system contains autotrophic and heterotrophic process.When initial p-nitrophenol concentration increases from 0 to 100 mg/L,nitrate removal efficiency presents slightly decreased tendency.Under the stress condition of 60 mg/L p-nitrophenol,nitrate removal efficiency decreases with increasing initial nitrate concentration,and the combined system achieves optimal denitrification efficiency at pH 7.Under acidic or alkaline condition,the microbial activity is inhibited,so that denitrification is not completed and nitrite concentration is increased.The nitrate reductase activity and nitrite reductase activity are not obviously influenced when initial p-nitrophenol concentration is lower than 100 mg/L.LDH release rate and ROS production rate present slight increased trends,demonstrating that p-nitrophenol which concentration is lower than 100 mg/L brings relatively small damage to microbial cell membrane.Under the stress condition of p-nitrophenol,the percentage of the dominant phylum Proteobacteria presents relatively small variation,which demonstrates that the dominant phylum in the denitrification process is less affected by p-nitrophenol,which is also in accordance with the decreasing denitrification efficiency in the combined system under p-nitrophenol condition.p-Nitrophenol is mainly reduced to p-aminophenol in the system.When initial chloramphenicol concentration increases from 0 to 150 mg/L,nitrate removal efficiency presents decreased tendency.Under the stress condition of 100 mg/L chloramphenicol,the limited electron donor cannot support nitrate reduction so that nitrate removal efficiency decreases with increasing initial nitrate concentration.Under the stress condition of 100 mg/L chloramphenicol,nitrate removal efficiency shows increased trend with current increases from 50 to 200 mA.However,denitrification efficiency decreases to 85.11%when current increases to 400 mA.The nitrate reductase activity and nitrite reductase activity both decrease with increasing initial chloramphenicol concentration.The increased LDH release rate and ROS production rate demonstrates that the microbial community suffers more and more serious damage with increasing initial chloramphenicol concentration,so that denitrification efficiency is getting worse and worse.The chloramphenicol condition causes relatively great influence onto microorganisms in the combined system.When initial chloramphenicol concentration increases from 0 to 150 mg/L,the copy numbers of the main functional denitrification genes nirS,nirK,narG,and napA show decreased trends,demonstrating that the abundances of main functional denitrification genes decrease under chloramphenicol stress condition in the combined system.Chloramphenicol is removed in the denitrification system probably due to participating heterotrophic denitrification process as carbon source.