Bio-cathode Denitrification Technology And Its Mechanism

Nitrogen-containing pollutants have gradually become prominent water pollutants in recent years,thus it is imperative to remove nitrate and nitrite(oxidative pollutants).After the implementation of "source control and emission reduction" across the country,the organic pollution has been effectively controlled and the C/N ratio of wastewaters has been greatly reduced.The conventional nitrification and denitrification processes have become powerless.The bio-cathode can directly provide electrons to make up for the lack of carbon source for denitrification and to avoid secondary pollution.This work aims to develop a new bio-cathode denitrification technology to solve the problem of low C/N ratio;to discover new resources of electrotrophic micro-organisms;to reveal the mechanism of bio-cathode denitrification;finally to promote theoretical innovation of pollutant removal and power generation.The main conclusions are as follows:1)Electrotrophic denitrifier was successfully enriched and its respiration properties were determined.With the heterotrophic denitrifying culture as inoculum,the electrotrophic culture could be successfully enriched by utilizing electricity as electron donor when exogenous organic electron donor gradually decreased to zero.The electrotrophic culture showed a red colour.During the enrichment of electrotrophic bacteria,both the diversity and homogeneity of electrotrophic community increased.The proportion of Thauera significantly decreased while the proportion of Acholeplasma significantly increased.Acholeplasma was the final dominant genus in the electrotrophic community.Nitrate respiratory activity of electrotrophic culture was proportional to electricity intensity when electricity served as the sole electron donor.Electrotrophic bacteria could utilize electricity to synthesize ATP for growth.2)The impact of substrate concentration on the performance of electrotrophic denitrification was studied.The nutritional conditions were optimized and the process potential was determined.With the prepared electrotrophic denitrifier cathode,the bio-cathode denitrification technology was investigated.The fitted equation between nitrate reduction rate and nitrate concentration is r =74.17×([S]-500)/(1894.03+[S]);the fitted equation between nitrate reduction rate and current intensity is r =47.02/(1+1.99/[S]+[S]/158.71).The optimal nitrate-N concentration and electricity intensity for electrotrophic denitrification are 1500 mg/Land 20?A,respectively.The functional genes of electrotrophic denitrifier responded to the changes of substrate concentration.The relative content of functional genes increased significantly at the optimal substrate concentration while it decreased at self-inhibitory concentration.3)The effect of inocula on performance of bio-cathode denitrification was studied.The inocula selection strategy was optimized and its microbial mechanism was explored.The bio-cathode denitrification technology demonstrated a good and stable nitrogen removal and electricity generation with denitrifying,denitratating or anammox inocula.The NRR was 101.79 mg/L·d,47.31 mg/L·d and 100.04 mg/L·d,and the output power was 24.01 mW/m2,14.11 mW/m2 and 45.93 mW/m2,respectively.The denitrifying,denitratating or anammox inocula endowed the bio-cathodes with different performances.Denitrifying sludge was the best inoculum when wastewater treatment was the main purpose,while Anammox sludge turned to be the best inoculum when wastewater treatment and energy recovery were combined in practical engineerings.The different performances of three bio-cathodes were ascribed to the different microbial community from inocula and the electricity selective enrichment of microorganism during cultivation.Pseudomonas stutzeri was deduced to be the key functional microorganism that used electricity as the sole electron donor for both the ATP synthesis and the denitrification.4)The extracellular electron transfer of electrotrophic denitrifier was explored.The impact of exogenous electron mediator on the performance of bio-cathode denitrification was studied and the response of function genes in electrotrophic denitrifier was determined.Electrotrophic denitrifier could secret electron mediator that contained nitrogen heterocyclic ring or benzene ring to transfer electron.Nedifloramide was the potential secreta.Mediated electron transfer played a significant role in the extracellular electron transfer for electrotrophic denitrifier.The addition of exogenous electron mediator pyocyanin could enhance nitrate removal efficiency of bio-cathode denitrification reactor,and the relative amount of denitrification functional genes as well as cytochrome genes of electrotrophic denitrifier had a positive response.5)The biological characteristics of electrotrophic denitrifier were studied;the functional characteristics of electrotrophic denitrifier were determined,the electron transfer pathways were deduced,and the bio-cathode denitrification mechanism was ascertained.Long-term operation experiment showed that bio-cathode denitrification technology could tolerate high nitrate concentrations of wastewater.The volume loading rate was 0.1 kgN/m3·d,nitrate removal rate was 100 mg/L·d,nitrite accumulation rate was 8.33 mg/L·d.Nitrite accumulation accounted for 7%of nitrate removal.Stappia indica was the potential electrotrophic denitrification microorganism.NarG was the characteristic nitrate reductase for electrotrophic denitrifier,and the relative amount of inorganic pyrophosphorylase and phosphate kinase was significantly larger than that for chemotrophic denitrifier.Based on the genomic and proteomic analysis,two possible electron transfer pathways were suggested for the electrophilic denitrifier.One is that the secreted electron transfer mediator was delivered to periplasm by the TonB-dependent receptor protein,and then it was delivered to cytoplasm by the LIV-I system.The electron was finally transfered to coenzyme Q.The other is that the secreted electron transfer mediator was delivered to periplasm by the TonB-dependent receptor protein,and the electron was then transferred to periplasmic cytochrome c by formate dehydrogenase,or transfered to coenzyme Q by multiple cytochromes.