Evaluating the Ecological Performance of Recirculating Aquaculture Systems Under Variable Nutrient Loading

Though RAS is considered a less impacting technology compared to open systems, there is little information on the ecological performance of RAS with changing feeding loads. This paper reports on experiments conducted on three replicate pilot-scale freshwater RAS with low, medium, high and shock feeding loads. An increase in the feeding load from low to high increased the total ammonia nitrogen from 0.18 to 0.71 mg L⁻¹ and nitrate-nitrogen from 18 to 61 mg L⁻¹ and decreased bottom-line dissolved oxygen from 6.8 to 5.6 mg L⁻¹. The efficiency of total nitrogen removal decreased from 68% to 54%, while effluent nitrogen discharge increased from 11.2 to 18.9 g N per kg of feed. Following a tripling of shock loading, ammonia reached its maximum value of 1.42 mg L⁻¹ in 14 h before returning to baseline within 36 h, whereas nitrite took 60 h. There were strong correlations between feeding loading and ammonia, nitrate, and effluent nitrogen loading (r > 0.94). It is evident from these findings that RAS can handle moderate loadings quite well, although the discharge per unit of feeding loading increases with increased loading.