Potential and limitations of ozone for the removal of ammonia, nitrite, and yellow substances in marine recirculating aquaculture systems

The high levels of water-reuse in intensive recirculating aquaculture systems (RAS) require an effective water treatment in order to maintain good water quality. In order to reveal the potential and limitations of ozonation for water quality improvement in marine RAS, we tested ozone's ability to remove nitrite, ammonia, yellow substances and total bacterial biomass in seawater, considering aspects such as efficiency, pH-dependency as well as the formation of toxic ozone-produced oxidants (OPO). Our results demonstrate that ozone can be efficiently utilized to simultaneously remove nitrite and yellow substances from process water in RAS without risking the formation of toxic OPO concentrations. Contemporaneously, an effective reduction of bacterial biomass was achieved by ozonation in combination with foam fractionation. In contrast, ammonia is not oxidized by ozone so long as nitrite and yellow substances are present in the water, as the dominant reaction of the ozone-based ammonia-oxidation in seawater requires the previous formation of OPO as intermediates. The oxidation of ammonia in seawater by ozone is basically a bromide-catalyzed reaction with nitrogen gas as end product, enabling an almost complete removal of ammonia-nitrogen from the aquaculture system. Results further show that pH has no effect on the ozone-based ammonia oxidation in seawater. Unlike in freshwater, an effective removal of ammonia even at pH-values as low as 6.5 has been shown to be feasible in seawater. However, as the predominant reaction pathway involves an initial accumulation of OPO to toxic amounts, we consider the ozone-based removal of ammonia in marine RAS as risky for animal health and economically unviable.