Microbial electrochemical systems for nitrate removal from diluted streams

Abstract

Increased concentrations of nitrogen compounds comprise one important factor contributing to deterioration of aquatic systems, both engineered and natural environments. In engineered Recirculating Aquaculture Systems (RAS), fish excretion and the addition of excess feed lead to build up of ammonium, which is further converted to less toxic nitrate within a (biological) nitrification unit. Although nitrate can accumulate to concentrations higher than 500 mg L -1 NO3 --N, it is suggested that its concentration should not overcome 50 mg L -1 NO3 --N in freshwater cultures and 100 mg L -1 NO3 --N in seawater cultures. Furthermore, the increased environmental regulations and licencing requirements have further imposed considerably stricter limits for discharge of aquaculture effluents to natural environments (i.e. <3 mg L -1 as Total Nitrogen in Queensland prawn farms). Similarly, other industrial activities and commissioned domestic wastewater treatment plants are also required to treat their wastewater streams to very low (diluted) concentrations of nitrogen before discharging them into natural environments. Therefore, either for recirculation of aquaculture water streams or for discharge purposes of various types of wastewaters, it is recommended that the concentration of nitrogen compounds should be monitored and controlled at low levels.Since the organic matter present in wastewaters is generally not sufficient to enable complete biological heterotrophic denitrification, typical denitrification processes require addition of an external carbon source (e.g. methanol). Recently, bioelectrochemical cathodic denitrification have been proposed as an alternative to organic matter addition, providing electrons through an inert conductive surface (cathode) directly to a biofilm performing (autotrophic) denitrification. Furthermore, electrons can be generated abiotically by splitting water, which simultaneously oxygenates the water, providing additional benefits for water reuse in aquaculture systems.Thus, the overall aim of this research project is to develop a Bioelectrochemical system as an alternative technology for denitrification and simultaneous oxygen generation in recirculating streams, which will enable the maintenance of diluted streams with low levels of nitrogen either for recirculation or discharge purposes. Furthermore, the technology should also be applicable as a polishing mechanism for denitrification from other diluted ii streams containing low levels of nitrate, such as secondary (treated) effluents from activated sludge systems.Since Dissimilatory Nitrate Reduction to Ammonium (DNRA) can be considered an undesired competitive pathway during cathodic denitrification, it is important to demonstrate and quantify the occurrence of this pathway. To assess this phenomenon, a carbon cloth cathodic electrode was inoculated with a mix culture denitrifying microbial community and poised at -0.9 V vs. standard hydrogen electrode (SHE). Results showed that more than 40% of nitrogen added as nitrate was co...