As the aquaculture sector development intensified, concerns regarding the impact caused by the high discharge of nutrients to the environment are rising. Recirculating aquaculture systems (RAS) allow high water reuse alongside a good control of the farming conditions. In RAS, ammonium (NH4+) is oxidized into nitrate (NO3-) via nitrite (NO2-) by nitrifying bacteria in aerobic biofilters. NO3- can later be removed from the system through anaerobic denitrification or by regular water exchanges. As an alternative, the anammox (anaerobic ammonia oxidation) process is a cost-effective and environment-friendly way to remove nitrogen compounds from RAS wastewater (WW), where NH4+and NO2- are directly transformed into nitrogen gas (N2).
We evaluated the potential of the marine anammox Candidatus Scalindua to clean RAS WW through a series of experiments in laboratory conditions. A sudden exposure to RAS WW, enriched in NH4+ (28 mg.L-1) and NO2- (34 mg.L-1), reduced its removal activity for these nitrogenous compounds, without impairing its relative abundance in the granule. To gradually expose Ca. Scalindua to RAS WW on the other hand resulted in a successful acclimation of the bacteria (Figure 1), even in the absence of trace element supplementation, while a slight decrease in relative abundance was observed. High NO3-concentrations typically encountered in RAS did not affect the removal rate of Ca. Scalindua for neither NH4+ nor NO2- but did reduce its relative abundance. Preliminary results suggest that Ca. Scalindua can maintain high removal (> 70%) for both NH4+ and NO2- rates when exposed to concentrations of NH4+ and NO2- commonly encountered in RAS (< 1 mg.L-1). We conclude that Ca. Scalindua can be successfully used to treat marine RAS WW under laboratory conditions. Future studies need to validate this findings in a pilot RAS.