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Due to extensive usage of nitrogenous fertilizers and discharge of industrial and domestic wastewater, nitrate contamination of water is becoming a main environmental concern. High levels of nitrate in drinking water causes serious health problems such as methemoglobinemia in infants and gastric cancer. Because of such health problems, nitrate removal from water is urgent and has been a hot topic over the recent years. Various technologies such as the ion exchange, reverse osmosis, electrocatalytic, adsorption, electrodialysis and biological process, have been used to eliminate nitrate ion from water and wastewater. Nevertheless, these methods have several drawbacks such as high installation and maintenance costs, difficult operation, brine production, membrane fouling, further treatment, slow process and carbon source requirement. A large number of investigators thus have focused on the reduction of nitrate by the electrochemical process usually because of its efficiency, very low sludge production, small area occupation and facile operation. Integration of electrochemical and biological processes as bioelectrochemical systems has been recommended to overcome the potential problems. In bioelectrochemical denitrification, denitrifying microorganisms make use of hydrogen generated at the cathode by the electrolysis of water as an electron donor to reduce nitrate into nitrogen gas. Autohyrogenotrophic denitrifying bacteria commonly adhere to the cathode surface and make a biocathode. Therefore, Cathode electrode material is one of the major factors that affecting in the bioelectrochemistry efficiency. Cathode material can directly affect to denitrifying bacteria attachment, hydrogen production, electron transfer and electrical conductivity. Bioelectrochemical process can be used to eliminate nitrate through a catholic reduction process. Carbon material has high mechanical strength and a rough surface which is ideal for the formation of biofilm as compared with metal materials. However, carbon materials are difficult to apply in large scale processes due to high electrical resistivity that tend to increase electrode ohmic losses. Hence, carbon electrodes are supported by a conductive material current collector such as carbon nanotubes and metallic materials. Carbon nanotubes have a good biocompatibility with bacteria and have not shown negative effect on biofilm formation. It had been reported that carbon nanotubes can facilitate transfer of electrons between bacteria and electrode in bioelectrochemistry. The aim of this study is bio-electrochemical removal of nitrate from wastewater using carbon nanotubes immobilized in cathode. This study has been done in a bathe scale bioelectrochemical rector with a two chambers. Considering that nitrate reduction done in biocathode, carbon nanotube used in cathode for increasing nitrate removal. The effects of pH, current density and retention time were evaluated for nitrate removal in a bio-electrochemical reactor.The highest nitrate reduction rates were occurs in neutral pH and current density of 15 mA/cm2. Furthermore, at current density of 15 mA/cm2 and retention time of 8 hours, the bioelecterochmical system can reduce the nitrate levels to below the environmental standard.The results showed that multi-wall carbon nanotube as cathode modifier increase the nitrate reduction efficiency about 14 persent. The use of multi-wall carbon nanotube can increased biofilm formation and therfor the reduction time for achieving to nitrate standard was reduced.

Keywords: Bioelectrochemical, Nitrate, Denitrification, Carbon nanotubes

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