Investigation Of The Removal Of Nitrate From Wastewater By Biocathode Under Long-term Successive Enrichment

Biocathode denitrification represents a promising high-efficiency and cost-effective technology for nitrogen removal.The metabolism ability that microorganisms can respire anaerobically with an electrode serving as the electron donor allows microbes to grow without traditional chemical electron donors such as acetate and lactate.This feature makes the biocathode denitrification technology offer the advantages of easy operation,because of the controlled electron delivery and the elimination of organic chemicals that induce secondary pollution.However,this technology is still in its infancy stage where the cultivation of cathodic microorganisms is usually considered to be difficult,and most previous studies in the literature were conducted with the artificial wastewater or synthesizing groundwater.Therefore,our first effort was made to explore the feasibility of biocathode denitrification targeting the biological effluent of coke wastewater,a real and complex wastewater.It was found that biochemical reducation of nitrate in this effluent was unable to occur in the initial stage.Nevertheless,the long-term successive enrichment promoted denitrifiers adapt to biorefractory compounds available in the effluent,resulting in apparent cathodic currents and signifcant reduction of nitrate.The confocal laser scanning microscope?CLSM?images revealed that the long-term operation caused the rise in the relative activity of biocathode from 48.8%?graphite felt?and 42%?graphite granules?to 66.1%?graphite felt?and 82%?graphite granules?,respectively.Analysis of bacterial structures suggested that cathode-driven autotrophic denitrifiers?CDAD?represented by Thiobacillus played an important role in nitrate reduction.Moreover,little information on the effect of heavy metals is available in the literature.Our second effort was thus made to investigate the effects of heavy metals on the rate and efficiency of biocathode denitrification.A series of experiments were conducted in a bioelectrochemical reactor in which NO3--N?40 mg L-1?and Cd²⁺,Cu²⁺,Ni²⁺ or Zn²⁺ were amended.The concentration of dosed metal ions and the corresponding free ion concentration that caused 80% inhibition of the denitrification activity were determined for each case,described as IC80.Further studies with IC80-Cd²⁺ showed that three-cycle feeding of Cd²⁺ resulted in the recovery of the denitrification performance.This was evident from the decline in the electron-transfer resistance and the rise on the relative viability of biocathode from 43.4%?graphite felt?and 38.7%?graphite granules?to 79.1%?graphite felt?and 74.3%?graphite granules?,respectively.Furthermore,analysis of bacterial structures revealed a distinct increase in the relative abundance of cathode-driven autotrophic denitrifiers represented by Thiobacillus and Sulfurimonas.It can be concluded that the long-term successive enrichment enabled the biocathode denitrifying film adaptable to heavy metal exposure.