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.