Introduction
In recent years, hydrogen sulfide (H2S) incidents leading to acute and severe fish mortality has been identified as one of the most important challenges for marine, land-based recirculating aquaculture systems (RAS). It has been demonstrated that the highest potential for H2S production in RAS is in saline environments with high sulfate (SO42- ) content (Letelier-Gordo et al., 2020) and in RAS biofilters with the higher sulfate-reducing microbial activity (Rojas-Tirado et al., 2021). Other factors can also play an important role in H2S incidents, such as organic matter and nitrate availability. Therefore, it is important to find solutions that reduce the risk of H2S -related incidents in RAS. The presence, level and availability of sulfate is still one of the key drivers of H2 S-related incidents. One of the options to prevent H2S formation is to remove sulfate from the seawater intake via nanomembrane filtration to use in saline-environment RAS.
Material & Methods
A nanofiltration membrane setup (Hydronautics , Nitto-Group Company, USA) w as installed at a commercial salmon smolt farm in Vestland county (Norway) to remove the SO42- ions of the seawater intake. The farm was operated at 15-17 ppt and with a feed loading of 1.3-2.2 kg feed/m3 of make-up water. At continuous and constant operation, the membrane maintained the SO42- levels in the farm water at 102 mg SO42- /L. The sulfate content of the local seawater varies between 2181-2830 mg SO42-/L. This means that, before mixing with freshwater to a salinity of 17 ppt, the constant operation of the membrane had the capacity to decreas e sulfate by 10 to 14 times.
Mature RAS nanofiltered water, freshwater and seawater, and mature m oving bed biofilter elements (BTW 15, Biowater Technologies, USA) w ere obtained from the farm only after two consecutive weeks of membrane operation without significant downtime. 400 mL of biofilter elements (ca. 278 biomedia elements) were incubated in closed glass bottles with 2 L of nanomembrane filtered water (n =3) or 2 L of unfiltered water (n = 3) to a salinity of 15 ppt. Initial sulfate levels in the filtered water (102 mg SO42- /L) were 10-fold lower than in the unfiltered water (1060 mg SO42-/L).
Water samples were collected from each reactor every second day and analyzed for O2, pH, temperature, ORP (oxy-redox potential), total sulfide and H2 S, sCOD (dissolved chemical oxygen demand), NH4+, NO2-, NO3-, PO43-, and SO42- . All reactors were spiked with 180 mg acetate/L on day 27 to maximize H2 S production. The trial lasted 42 days. TCOD (total chemical oxygen demand) from water and biomedia, and microbial samples from biomedia and water of each reactor were collected at the start (day 0) and end of the trial (day 42).
Results
Data analysis is ongoing, and we expect to present the results during the conference.
References
Letelier-Gordo, C.O., Aalto, S.L., Suurnäkki, S., Pedersen, P.B. (2020) Increased sulfate availability in saline water promotes hydrogen sulfide production in fish organic waste. Aquacultural Engineering, 89: 102062.
Rojas-Tirado, P., Aalto, S.L., Åtland, Å., Letelier-Gordo, C. (2021). Biofilters are potential hotspots for H2S production in brackish and marine water RAS. Aquaculture, 536: 736490.