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 150kg/m, 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 (NO), 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 NO emissions from a near-zero discharge land-based saline RAS. NO flux was monitored from the RAS's fish tank, and moving-bed nitrification and activated-sludge (with intrinsic C source) denitrification reactors. NO emission potential was also analyzed in the laboratory. NO flux from the denitrification reactors ranged between 6.5 and 48mg/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 NO 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 NO emission from the denitrification reactor was 3.72±2.75% of the reduced nitrate-N, within the range found in the direct measurement. Overall, NO emission during N transformation in a RAS was evaluated to be 885mg/kg feed or 1.36g/kg fish production, accounting for 1.23% of total N application. Consequently, it is estimated that NO emission from aquaculture currently accounts for 2.4% of the total agricultural NO emission, but will decrease to 1.7% by 2030.