For optimal water quality management in RAS, a? primary design concern is to provide the biofilter capacity required to control the total ammonia-nitrogen (TAN) concentration in the culture tanks. An optimal biocarrier will have a short start-up time- the time between when the biocarrier is added to the system and when the bacterial population can support the welfare of the needed biomass of aquatic species to be produced- and a high total ammonia nitrogen (TAN) removal rate during operation. The most challenging of these start-ups are in cold water systems in a new system, where there is no existing ‘mature’ biomedia for inoculation of the biological community and lower metabolic activity at low temerature. Experiments assessed the start-up time of five different designs of biocarriers in 14? fresh water and, once nitrifying activity was established, their TAN removal capacity.
Five biocarriers of different designs, specific surface area (SSA) and protected surface area (PSA), from 5 different suppliers were compared. SSA varied between 700 and 5500 and materials included virgin and recycled polyethylene, HDPE, and polypropylene. Experiments were carried out in 300L round tanks with an operational volume of 200L and maintained at 14?. Each biocarrier type was stocked at 50% of the operational volume (0.1m3 per tank) in triplicate and hydrated for 48 hours before the trial start. Aeration was set at 20L/min/tank using blowers in the tanks. Following initial dosing to start biological activity in the tanks (NH4CL (3.8g); NaHCO3 (43g); NaNO2 (0.4g); Na2HPO4 (0.08g) per tank) tanks were “fed” with calculated quantities of NH4Cl and NaHCO3 according to observed conversion rates, to promote ongoing nitrifying activity. Over 10 weeks, DO; temperature; salinity; pH; KH; NH4; NO2; NO3? data were collected at regular intervals using either probes (DO, temperature, salinity) or standard drop test kits, easily available to aquaculture producers, and with spectrophotometric titration.
There was variation between hydration time required for the different biocarriers. Faster hydration times were related to faster start-up times. Start-up of bacterial development differed between biocarriers (2 weeks to 4 weeks) as did cumulative TAN conversion performance after start-up (week 4-10). Maximum daily conversion rates reflected the cumulative trend. Consumption of bicarbonate was found to be proportional to conversion of TAN and was higher than the typical values considered for a mature system, likely due to increasing conversion demand during the start-up phase. Start-up time and TAN conversion performance were not found to be correlated with SSA, contrary to common industry assumptions. Additionally, PSA was not well related to rates of TAN conversion. Degassing affecting CO2 levels; SSA vs. PSA; mixing rates; and the shape of holding tanks may have a greater influence than either SSA or PSA alone. Selection of biocarrier for optimal cold start-up of RAS systems appears to be more complex than selecting that with the greatest SSA. Users should consider their production method and harvest strategy (e.g., continuous, batch all in/all out) ?when considering optimal biocarrier selection.