Effects of alkalinity on ammonia removal,carbon dioxide stripping,and system pH in semi-commercial scale water recirculating aquaculture systems operated with moving bed bioreactors

When operating water recirculating systems (RAS) with high make-up water flushing rates in locations that have low alkalinity in the raw water, such as Norway, knowledge about the required RAS alkalinity concentration is important. Flushing RAS with make-up water containing low alkalinity washes out valuable base added to the RAS (as bicarbonate, hydroxide, or carbonate), which increases farm operating costs when high alkalinity concentrations are maintained; however, alkalinity must not be so low that it interferes with nitrification or pH stability. For these reasons, a study was designed to evaluate the effects of alkalinity on biofilter performance, and CO₂ stripping during cascade aeration, within two replicate semi-commercial scale Atlantic salmon smolt RAS operated with moving bed biological filters. Alkalinity treatments of nominal 10, 70, and 200 mg/L as CaCO₃ were maintained using a pH controller and chemical dosing pumps supplying sodium bicarbonate (NaHCO₃). Each of the three treatments was replicated three times in each RAS. Both RAS were operated at each treatment level for 2 weeks; water quality sampling was conducted at the end of the second week. A constant feeding of 23 kg/day/RAS was provided every 1–2 h, and continuous lighting, which minimized diurnal fluctuations in water quality. RAS hydraulic retention time and water temperature were 4.3 days and 12.5 ± 0.5 °C, respectively, typical of smolt production RAS in Norway.It was found that a low nominal alkalinity (10 mg/L as CaCO₃) led to a significantly higher steady-state TAN concentration, compared to when 70 or 200 mg/L alkalinity was used. The mean areal nitrification rate was higher at the lowest alkalinity; however, the mean TAN removal efficiency across the MBBR was not significantly affected by alkalinity treatment. The CO₂ stripping efficiency showed only a tendency towards higher efficiency at the lowest alkalinity. In contrast, the relative fraction of total inorganic carbon that was removed from the RAS during CO₂ stripping was much higher at a low alkalinity (10 mg/L) compared to the higher alkalinities (70 and 200 mg/L as CaCO₃). Despite this, when calculating the total loss of inorganic carbon from RAS, it was found that the daily loss was about equal at 10, and 70 mg/L, whereas it was highest at 200 mg/L alkalinity. pH recordings demonstrated that the 10 mg/L alkalinity treatment resulted in the lowest system pH, the largest increase in [H⁺] across the fish culture tanks, as well as giving little response time in case of alkalinity dosing malfunction. Rapid pH changes under the relatively acidic conditions at 10 mg/L alkalinity may ultimately create fish health issues due to e.g. CO₂ or if aluminium or other metals are present. In conclusion, Atlantic salmon smolt producers using soft water make-up sources should aim for 70 mg/L alkalinity considering the relatively low loss of inorganic carbon compared to 200 mg/L alkalinity, and the increased pH stability as well as reduced TAN concentration, compared to lower alkalinity concentrations.