Nitrous oxide emissions from near-zero water exchange brackish recirculating aquaculture systems

The development of intensive recirculating aquaculture systems (RAS) with low water exchange has accelerated in recent years as a result of environmental, economic and other concerns. In these systems, fish are commonly grown at high density, 50 to 150 kg/m³, using high-protein (30%–60%) feeds. Typically, the RAS consists of a solid treatment and a nitrification unit; in more advanced RAS, there is an additional denitrification step. Nitrous oxide (N₂O), a byproduct during nitrification and denitrification processes, is a potent greenhouse gas that destroys the ozone layer. The aim of this study was to measure and assess N₂O emissions from a near-zero discharge land-based saline RAS. N₂O flux was monitored from the RAS's fish tank, and moving-bed nitrification and activated-sludge (with intrinsic C source) denitrification reactors. N₂O emission potential was also analyzed in the laboratory. N₂O flux from the denitrification reactors ranged between 6.5 and 48 mg/day, equivalent to 1.27 ± 1.01% of the removed nitrate-N. Direct analysis from the fish tank and nitrification reactors could not be performed due to high aeration, which diluted the N₂O concentration to below detection limits. Thus, its potential emission was estimated in the laboratory: from the fishponds, it was negligible; from the nitrification reactor, it ranged between 0.4 and 2.8% of the total ammonia-N oxidized. The potential N₂O emission from the denitrification reactor was 3.72 ± 2.75% of the reduced nitrate-N, within the range found in the direct measurement. Overall, N₂O emission during N transformation in a RAS was evaluated to be 885 mg/kg feed or 1.36 g/kg fish production, accounting for 1.23% of total N application. Consequently, it is estimated that N₂O emission from aquaculture currently accounts for 2.4% of the total agricultural N₂O emission, but will decrease to 1.7% by 2030.