Study On Nitrate-contaminated Groundwater Treatment By Heterotrophic Coupled With Electro-autotrophic Denitrifying Packed Bed Reactor(HEAD-PBR)

Groundwater is the major source of drinking water all the time. Nitrate contaminated groundwater is becoming serious due to the agricultural fertilization. However, drinking the water polluted by nitrate may cause serious damage to human’s health. Due to its high removal efficiency and friendly to environment, biological denitrification is used widely to treat nitrate contaminated groundwater. Heterotrophic denitrification coupled with autotrophic has more advantages. Hydrogenotrophic denitrification is more available than sulfur-based autotrophic. Packed bed reactor form could be used to separate electrochemical and microbial denitrification part to overcome the shortage that microbial activity is easily inhibited by electric current in the biofilm-electrode reactor former.A novel heterotrophic coupled with electro-autotrophic denitrifying packed bed reactor(HEAD-PBR) was developed in this study. Stainless steel mesh and Ti/RuO2 mesh were employed as cathode and anode respectively, which became the electrolysis segment(ES). Denitrification segment(DS) was made up of sawdust as carbon source and haycite as carries. DS and ES were working in series vertically and heterotrophic and autotrophic denitrification(HAD) was established through producing H2 by electrolyzing water. Performance of HEAD-PBR treating varies nitrate concentrations in varies conditions was investigated, and also the mechanism.Results showed that, the best electrolysis conditions have been got when the gap between electrodes was 30 mm and current applied was 0.1 A with 1.78 mmol/h H2 producing rate and less by-product in water. In these conditions, nearly 99% of nitrate was removed by HEAD-PBR with 126.46 g sawdust in 24 h and 18 h as fast when initial nitrate concentration was 50 mg N/L and 25 mg N/L, respectively. None of nitrite and ammonium was detected in effluent at the same time. The fastest nitrate volumereduction rate of the whole reactor was 28.19 ± 0.70 g/(m3·d). Electrolysis, heterotrophic denitrification and autotrophic denitrification were responsible for 23.3 % ± 6.8 %, 61.3 % ± 7.0 % and 15.4 % ± 4.6 % removed nitrate respectively. Thauera and Hydrogenophaga and so on were the major denitrifiers. Hydrogenotrophic denitrifiers were eugonic mainly in the middle part of DS. Oxygen generated during electrolysis could be consumed by microbes rapidly so that it maintained anoxic well in the upper part of DS. The OH- generated during electrolysis could be neutralized by sawdust and microbial metabolism and pH in effluent kept at 7.82 ± 0.09 averagely. Furthermore, thanks to the Chlorobium, carbon cycle in HEAD-PBR was completed and H2 utilization efficiency was also improved. The HAD system was confirmed being established by analyzing the carbon transformation and the bacteria corresponded.