Innovative Photoelectrocatalytic filter for Recirculating Aquaculture Systems

Ammonia poses a threat to biodiversity in aquatic environments, having toxic effects on fish living in freshwater and seawater. In this work, we have designed a photoelectrochemical reactor and tested in a recirculating aquaculture system (RAS, Fig.1a). This technology exploits the UV-electrochlorine advanced oxidation process to improve water quality and fish welfare, performance and sustainability. The photoanode is prepared by anodization of a metal titanium mesh in a ethylene glycol, water and hydrofluoric acid electrolyte solution at 30 V for 6 h in order to obtain a self assembled titania nanotube arrays film (Fig.1b). The illuminated semiconductor film promotes the oxidation of chloride ions naturally present in fresh- (or sea-) water, to chlorine radical which is highly reactive and selective in ammonia oxidation yielding N2 or nitrate. Preliminary ammonia degradation tests were performed in 5∙mM KCl cotaining 100 ppm NH3 solutions at 4 V potential bias. A stable photocurrent up to 550 mA under UV- irradiation was obtained during the test. An almost full ammonia conversion was reached after 6 h-long test with a selectivity towards N2 and NO3- of 80% and 20%, respectively. The setup has been shown to be biocompatible in a real rainbow trout farming plant showing a lower nitrate accumulation in water with respect to a traditional bio-filter.