Stimulating Denitrification in a Marine Recirculating Aquaculture System Biofilter Using Granular Starch as a Carbon Source

Abstract

Maintaining superior water quality in intensive recirculating aquaculture systems (RAS) by controlling levels of inorganic nitrogenous waste - ammonia, nitrate and nitrite - derived from uneaten food and fecal excretion is often a challenge. In most systems, solids are removed mechanically and ammonia is oxidized to nitrate by nitrifying biological filtration; nitrate is subsequently eliminated through numerous water exchanges. Alternatively, nitrate removal is achieved using a bacterial-mediated denitrification component that reduces nitrate to nitrogen gas under anoxic conditions, a process that depends on the application of external or endogenous electron and carbon donors, e.g. carbohydrates or organic alcohols. In this study, we compared the capacity of acetate, glucose, soluble starch, and granular starches to promote the denitrifying activity of heterotrophic bacteria in biofilm-coated polyethylene beads from a marine RAS moving bed bioreactor (MBB) under anaerobic conditions. Granular starches (corn, wheat, and rice) were as effective as glucose in supporting denitrification, and were 7.6 and 9.8 times more effective in removing nitrate when compared to soluble starch and acetate, respectively. Furthermore, granular starches retained their denitrification potential for longer time periods than soluble starch or acetate. The low cost, ease of use, and non-toxic nature of granular starches make them an ideal exogenous carbon source for promoting denitrification in RAS bioreactors.