Early Warning Signs and Dynamics of Hydrogen Sulfide Production in Recirculating Aquaculture System (RAS): with Nitrate and Iron (III) Addition

Aquaculture sector is adopting more Recirculating Aquaculture System (RAS) for fish production. In Norway, RAS was earlier used only for smolt production in freshwater, but now industry is slowly shifting post-smolt production and even full production cycle to marine RAS. Such a production system is claimed to be a better sustainable alternative method for fish farming. The use of seawater in RAS increases the risk of production of hydrogen sulfide (H2S), due to high sulfate concentration. H2S is extremely toxic for the fish and events of H2S induced mortality is being reported from several RAS facilities across Norway. Previous studies reported that H2S is produced through sulfate reduction by action of Sulfate Reducing Bacteria (SRB) and the redox reaction takes place in the order O2, NO3-, Mn (IV), Fe (III) oxides and oxy-hydroxides, sulfate and CO2. It is evident from earlier studies that addition of Nitrate (NO3-) delays the H2S production. Here in this thesis we are checking the ability of Fe (III) to delay H2S production and how effectively we can use Fe (II) as an early warning detection for H2S production.

An experiment was designed where sludge from RAS was mixed with seawater and incubated in 33 screw cap bottles for a period of 19 days. The bottles were equally divided among control, nitrate-added treatment (NAT) and iron-added treatment (FAT). The control was with sludge and seawater. In addition to sludge and seawater to NAT, NO3- was added at a concentration of 6 mM. In FAT bottles, Fe (III) was added at a concentration of 0.4 mM. Samples were drawn from each of the groups following a fixed schedule and analyzed for H2S, Fe (II) and nutrients. Results shows NO3- delayed the H2S production by 8 days while Fe (III) additions suppressed the H2S production for about 5 days. In FAT, there was a delay of 5 days between increase in concentrations of Fe (II) and H2S. Here lies the possibility of using Fe (II) as an early warning sign for H2S production in RAS.