EVALUATION OF THE PERFORMANCE OF MARINE ANAMMOX, Candidatus SCALINDUA, UNDER REAL RECIRCULATING AQUACULTURE SYSTEM NITROGEN CONDITIONS

Abstract

In an effort to find a sustainable and nutritious food source to meet the demands of an increasing population, recirculating aquaculture systems (RAS) have proven to be a promising candidate. RAS are tank-based setups which mechanically and biologically filter water before recirculating it back into the system. A current challenge with RAS is the potential accumulation of nitrogenous wastes, ammonium (NH4 +), nitrite (NO2 −), and nitrate (NO3 −), which could impact the health and welfare of fish. This waste, primarily in the form of NH4 +, is generated as a byproduct of protein synthesis during digestion, and from the microbial decomposition of organic matter such as feces. In RAS, nitrifying bacteria transform NH4 + into NO3 −, via NO2 −. This NO3 − product needs to be removed from the system through frequent water exchange or by denitrification. Recently, an alternative pathway has been explored involving anaerobic ammonium oxidizing (anammox) bacteria for nitrogen removal from marine wastewater. Anammox bypasses nitrification and denitrification, and directly converts NH4 + to harmless nitrogen gas (N2) by using NO2 − as an electron acceptor. The performance of the marine anammox species Candidatus Scalindua in treating enriched synthetic RAS wastewater has previously been established, however the concentrations of NH4 + and NO2 − (ca. 30 mg/L) used in previous experiments are unsuitable for fish. As such, Experiment I of this thesis assesses the performance of the bacteria when exposed to real RAS nitrogen conditions in two phases. In the first phase, the bacteria were fed with a synthetic feed with 1.4 mg/L of NH4 +, 1.7 mg/L of NO2 −, and an additional trace element (TE) mix. Average removal rates of 80% for NH4 + and 85.2% for NO2 − were achieved. During the second phase, NH4 + and NO2 − concentrations remained the same but the TE mix was removed, this phase is still ongoing. Similarly, previous research showed that Ca. Scalindua tolerated NO3 − concentrations up to 1600 mg/L, but at 3200 mg/L the total nitrogen removal rate collapsed. Therefore, Experiment II of this thesis aims to identify the exact NO3 − tolerance threshold of Ca. Scalindua by incrementally increasing the concentration of NO3 − starting at 1600 mg/L and going up by 200-400 mg/L every 20-40 days. This experiment remains ongoing but preliminary results showed that the bacteria were able to tolerate concentrations up to 2800 mg/L and had an average removal efficiency of 92.1% between 2000 mg/L and 2600 mg/L. With these two experiments, it can be concluded that Ca. Scalindua can successfully be used to treat marine wastewater under real RAS conditions and adapt to incrementally increasing NO3 −concentrations.