Addition of an external carbon source is usually necessary to guarantee a sufficiently high C/N ratio and enable denitrification in wastewater treatment plants (WWTPs). Alternatively, denitrification processes using autotrophic microorganisms have been proposed i.e., with the use of H as electron donor or with the use of cathodic denitrification in bioelectrochemical systems (BES), in which electrons are transferred directly to a denitrifying biofilm. The aim of this work was to investigate and demonstrate the feasibility of applying an easy-to-operate BES as a polishing mechanism for treated secondary clarified effluent from a municipal WWTP, containing low levels of organic matter, buffer capacity and low concentrations of remaining nitrate. In the proposed system, nitrogen removal rates (0.018-0.121 Kg N m d) increased with the nitrogen loading rates, suggesting that biofilm kinetics were not rate limiting. The lowest energy consumption for denitrification was 12.7 kWh Kg N, equivalent to 0.021 kWh m and could be further reduced by 14% by adding recirculation circuits within both the anode and cathode.

Dissimilatory nitrate reduction to ammonium (DNRA) is an undesired pathway occurring simultaneously to denitrification in natural environments as well as engineered systems aimed at biological nitrate reduction/removal. Ammonium formation has previously been detected in cathodic compartments of bioelectrochemical systems performing denitrification, although reported concentrations are generally very low. In order to demonstrate and quantify the occurrence of DNRA from nitrate in a mixed culture denitrifying cathodic biofilm, a carbon cloth working electrode was inoculated with a denitrifying microbial community and poised at -0.9 V vs. standard hydrogen electrode, while nitrate (20 mg/L NO 3 --N) was continuously fed at an HRT of 10 hours. Results showed that more than 40% of nitrogen added as nitrate was converted via DNRA when the biofilm was at initial stages of development. However, ammonium generation decreased to approximately 5% at later stages of development (7 months of operation), indicating that biofilm age plays a key role on biological pathways occurring during cathodic nitrate reduction. A closer insight revealed that the occurrence of DNRA is linked to cathodic Coulombic efficiency: at low efficiency, a large fraction of 2 the incoming electrons are converted to hydrogen or other reduced compounds within the biofilm, increasing the driving force for DNRA; at high Coulombic efficiency, lower reducing power availability leads to nitrogen gas as preferred reduction product.

Maintaining low concentrations of nitrogen compounds (ammonium, nitrate and nitrite) in recirculating aquaculture waters is extremely important for a larger and healthier fish production, as well as for water discharge purposes. Although ammonium removal from aquaculture streams is usually done within a nitrifying step, nitrate removal via denitrification is still partially limited by the low organic matter availability. Therefore, an easy-to-operate autotrophic denitrifying bioelectrochemical system is herein proposed for the treatment of seawater aquaculture streams. The nitrate-containing synthetic stream flows sequentially through a biological denitrifying cathode (placed at the lower portion of a tubular reactor) and an abiotic anode (generating electrons and oxygen from water splitting, at the upper portion). Experimental results with synthetic seawater showed that the system reached denitrification rates of 0.13 ± 0.01 kg N m–3 day–1, operating with minimum ammonium and nitrite accumulation, as well as minimum chlorine formation in the abiotic anode, despite the high chloride concentration. There results support the technical potential for simultaneous bioelectrochemical denitrification and partial re-oxygenation of aquaculture waters either for recirculation or discharge purposes.

No 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...