Biofilters are potential hotspots for H2S production in brackish and marine water RAS

The production of hydrogen sulphide (H₂S) is a major concern and challenge in marine land-based recirculating aquaculture systems (RAS). In order to gain a thorough understanding on H₂S production dynamics in RAS, an anaerobic batch reactors experiment was performed, evaluating the production potential of total dissolved sulphide (TDS) and H₂S and characterizing the microbial communities in different RAS environments (solid waste, biomedia+RAS-water, biomedia+seawater [SW], and RAS-water) during 33 days. The highest H₂S concentrations were achieved in the reactors with biomedia+RAS-water, where biofilms growing on biomedia hosted a high abundance of sulphate reducing bacteria (SRB), which activated once anoxic conditions and carbon sources were introduced. The biofilms in the biomedia+SW reactors produced lower amounts of H₂S than the biomedia+RAS-water reactors, due to the lower abundance of SRB. The solid waste produced H₂S at low rate, the production being limited by the substrate competition between SRB and other microbial groups. No production of H₂S was observed in the reactors with only RAS-water, due to the absence of SRB and suitable carbon sources. Based on measurements of production of H₂S in this study, a moving bed biofilter submerged in brackish water (17 ppt) can potentially produce 139 g of H₂S per biofilter m³, when the biomedia is not adequately mixed and oxygenated. Altogether, the results demonstrate that although SRB are found both in biofilters and solid waste, these two environments exhibit different H₂S production dynamics. Biofilters produce large quantities of H2S under anoxic conditions, while solid waste deposited to the rearing tanks exhibits a lower but constant production of H₂S. However, both of these H₂S sources can be suppressed through fast removal of solids from rearing tanks and optimal operational conditions for biofilters.