Aim
The performance and welfare of Atlantic salmon (Salmo salar) in land-based recirculating aquaculture systems (RAS) depends on a well matured microbiota of the nitrification biofilter (Dahle et al., 2023). To achieve cash flow in land-based operations as early as possible after start-up, the time needed for nitrification biofilter maturation should be short. The “Core-RAS” (AKVA group, Norway) includes the nitrification bioreactor(s) for ammonia removal by autotrophic microbial nitrification. While performance of mature biofilters and maturation of lab-scale nitrification bioreactors is well documented, data on the maturation of a juvenile filter of commercial scale is lacking. To assist project managers and RAS operators with these challenges, we tested a protocol that allows achieving complete nitrification within 6 weeks of biofilter maturation. We will share our recommendations to minimize maturation time.
Materials and Methods
The 2 RAS tested are identical and part of a smolt production facility in Finnmark, Norway. The removal of solids is achieved by 2 mechanical drum filters, the nitrification by a combination of 1 moving bed bioreactor (30 m³ bio-media × 800²/m3 ~25,000 m²), 6 fixed bed bioreactors (FBBR) (~16 m³ bio-media × 800²/m3 × 6 ~157,000 m²). Ozone is used to control turbidity and degassing of CO2 is achieved in a degassing tank, located above the FBBR. The system is illustrated in Figure 1. To allow for inoculation of biofilters with high numbers of nitrifying microorganisms adapted to target salinity once commissioning was completed, 2 pre-cultivation systems were set-up. Each pre-cultivation system consisted of 1 Intermediate Bulk Containers (IBC), 1 submersible heater + regulator (500W, Aqua Medic), and 1 shared aeration system (Aqua Forte V60). To supply nutrients, a premix was added (adapted from Navada et al., 2020): to supply ammonia oxidizing bacteria (AOB) in with ammonia, ammonium chloride (40 g/IBC) was added initially. To kick start nitrite-oxidizing bacteria (NOB), sodium nitrite (30 g/IBC) was added initially. To provide for phosphate, trisodium phosphate (10 g/IBC) was added initially. To control pH (target 8.5), alkalinity was added in form of sodium bicarbonate (250 g/IBC). Salinity was set to 3 ppt using sodium chloride. To accelerate microbial growth, the water temperature was set to 27°C. To inoculate nitrifying microorganisms, organic substrate (100g/IBC) was added.
Maturation of the biofilters: the bio-media (PP, 800 m2/m3) were first soaked in water for 4 weeks and then introduced in the FBBRs. Both RASs where then filled with fresh water and saltwater was added to set salinity to 11ppt. Circulation over the biofilters was started using the main RAS-pumps. To feed AOB and NOB, ammonium chloride (14 kg/RAS) and sodium nitrite (10kg/RAS) were added to set levels of 5 mg/l NH4+-N and 3 mg/l NO2--N. To supply phosphate, 5 kg Na3PO4 were added to set a ratio of 0.2 g P per g NH4+-N. The operating temperature was set to 24°C. Per day, 50% (500L) of the starter cultivate from each IBC was added to each RAS into moving bed. The IBC was then refilled with water, ammonium chloride, sodium nitrite, trisodium phosphate and sodium bicarbonate to create conditions as during pre-cultivation. Levels of NH4+-N, NO2--N, ortho-phosphate were measured daily using spectrophotometry (Hach Lange DR3900), and alkalinity using drop count test kits (Hach Lange dct). pH, temperature, salinity, and oxygen levels were measured daily (Hach Lange HQ2200).
Results
Pre-maturation in IBC: on day 1, NH4+-N was set to 10 mg/l. On day 5, NH4+-N levels were 0.1 mg/l in both tanks, and it was decided to reset NH4+-N to 30mg/l daily. The daily removal rate averaged ~2 g/m3/d. Maturation of the biofilters: the removal of NH4+-N was observed in RAS 1 on day 11 and in RAS 2 on day 16. Removal of NO2--N observed in RAS 1 on day 31 and in RAS 2 on day 37. Levels of NH4+-N were reset to 3 mg/l daily. A nitrification rate of 0.09 g/m2/d was documented on day 42 (Figure 2).
Discussion and Conclusions
This study demonstrates that the maturation protocol applied, allows to achieve full nitrification within 6 weeks. Levels of NH4+-N (Kolarevic et al., 2013) and NO2--N (Gutiérrez et al., 2019) were well below the tolerance levels for Atlantic salmon, allowing for stocking of juveniles 6 weeks after starting the maturation of the biofilters. The results enable project managers and farm operators to integrate the maturation period within a clear timeframe of a project. Because adequate housing conditions for the fish are established within 6 weeks, fish welfare is accounted for prior to stocking. This sets the ground for better growth performance and fewer losses, so economic returns can be achieved earlier.
References
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NAVADA, Sharada, et al. A salty start: Brackish water start-up as a microbial management strategy for nitrifying bioreactors with variable salinity. Science of the Total Environment, 2020, 739. Jg., S. 139934.
KOLAREVIC, Jelena, et al. Influence of long term ammonia exposure on Atlantic salmon (Salmo salar L.) parr growth and welfare. Aquaculture Research, 2013, 44. Jg., Nr. 11, S. 1649-1664.
GUTIÉRREZ, Xavier A., et al. Effects of chronic sub‐lethal nitrite exposure at high water chloride concentration on Atlantic salmon (Salmo salar, Linnaeus 1758) parr. Aquaculture Research, 2019, 50. Jg., Nr. 9, S. 2687-2697.