Biofilter and degasser performance at different alkalinity levels in a brackish water pilot scale recirculating aquaculture system (RAS) for post-smolt Atlantic salmon

Abstract

Maintaining pH stability in Recirculating Aquaculture Systems (RAS) is essential, as it has a direct impact on the toxicity of ammonia, CO₂, and other metabolite compounds, as well as the efficiency of critical processes like nitrification and CO₂ removal. Alkalinity is necessary for pH stability and for the inorganic carbon supply to nitrifying bacteria. The relationship between alkalinity, nitrification rate, pH, and CO₂ concentration emphasize the need to determine the optimal alkalinity levels in RAS. However, the consequences of operating RAS under non-optimal alkalinity levels, especially in marine systems, are not well understood. This study aims to investigate the influence of alkalinity on nitrification rates and CO₂ removal efficiency in RAS with brackish water, with a specific focus on ammonia, total inorganic carbon (TIC), and CO₂ levels, as well as their removal rates. To accomplish this, the study was conducted in a pilot-scale Atlantic salmon post-smolt RAS. The RAS was operated with a moving bed biofilter. Alkalinity treatments of nominal 70, 100, and 200 mg/L as CaCO₃ were maintained by supplying sodium bicarbonate and NaOH. Each of the three treatments were operated for 2 weeks and replicated three times. During every two weeks of treatment, five water samples on days 3, 5, 8, 10, and 15 were collected. The system received 12-hour feeding of 1.5 kg/day, with continuous lighting and a water temperature and salinity of 14.2 ± 0.16^◦C and 15.4 ± 0.53‰, respectively. The results indicated that high alkalinity levels (100 and 200 mg/L as CaCO₃) resulted in significantly lower CO₂ concentrations after the water treatment, due to higher pH. The highest alkalinity treatment (200 mg/L) showed lower CO₂ removal efficiency. Furthermore, high alkalinity levels (above 100 mg/L as CaCO₃) related to reduced total ammonia nitrogen concentrations and increased volumetric nitrite removal rate. Elevated alkalinity levels can also help prevent rapid pH fluctuations, benefiting fish health and production. To summarize, for Atlantic salmon post-smolt RAS, operated in brackish water, maintaining alkalinity slightly above 100 mg/L as CaCO₃ appears to be the optimal choice in terms of TAN and CO₂ concentration and rate of removal or consumption.