Quantification of anammox activity in a denitrification reactor for a recirculating aquaculture system

Abstract

The activity of anammox bacteria in a denitrification reactor in a recirculating aquaculture system (RAS) for gilthead seabream production was investigated. Organic matter, extracted from the pond's solid filter, was used as the electron donor and carbon source for the denitrification reaction. The reactor was operated at four solid retention times (SRT). At steady state, anammox activity showed similar activity at SRTs of 12.5, 8 and 6 days (approximately 35 mg N l reactor − 1 day − 1 ), and a much lower activity of 12 mg N l reactor − 1 day − 1 at a SRT of 4 days, indicating that anammox bacteria were washed out of the reactor at SRT< 6 days. These results were corroborated by fluorescence in situ hybridization (FISH) that showed that at SRT of 12.5, 8 and 6 days the anammox bacteria population in the denitrification reactor was on the order of 10 8 cells ml − 1 as compared with 10 6 cells ml − 1 at SRT of 4 days. The reportedly long cell division time of anammox bacteria together with the relatively short SRT for anammox washout in the denitrification reactor suggested that a substantial quantity of anammox bacteria were being supplied on a daily basis to the denitrification reactor with the solids captured in the pond's filter system. Since close-to-saturation conditions for oxygen prevailing in both the pond and the solids filter do not favor anammox growth, it was further conjectured that anammox bacteria propagate in fish intestines. This paper shows qualitative evidence that anammox bacteria are indeed present in significant quantities in both the feces of seabream fish and in the solids backwashed from the pond's filter.