Evaluating Glycerol as an Alternative Carbon Source for Denitrification in Land-Based Salmon Recirculating Aquaculture Systems

Heterotrophic denitrification is increasingly used in land-based recirculating aquaculture systems (RASs) to remove nitrate and minimise new-water demand, but the carbon sources traditionally used—methanol and acetic acid—raise safety, corrosion, and cost concerns. This study evaluated glycerol as an alternative external carbon source in a commercial land-based Atlantic salmon RAS in Norway. Trials in two identical post-smolt systems with integrated denitrification bioreactors compared glycerol (80%) against acetic acid for denitrification performance, nitrite accumulation, pH stability, carbon-source consumption, cost, and hazard profile. Glycerol achieved similar or higher apparent total dissolved nitrogen (TDN) removal than acetic acid and, once the biofilm had acclimated, was completely consumed. It maintained a stable outlet pH (≥6.5), avoiding the enzyme inhibition seen when acetic acid depressed pH, and proved about 1.5 times more cost-effective (≈NOK 66 vs. 97 per kg TDN removed). A standardised hazard assessment ranked glycerol as the safest option for flammability, toxicity, and corrosiveness. TDN removal correlated strongly with dissolved nitrogen gas saturation at the bioreactor outlet (r2 = 0.79–0.82), indicating that total gas pressure monitoring is a promising, low-cost proxy for performance. Nitrite accumulation per unit of nitrogen removed was, however, about three times higher under glycerol (≈1 mg NO2-N per mg TDN) than under acetic acid (≈0.3 mg NO2-N per mg TDN), and the nine-day glycerol phase was too short to establish whether this resolves or persists as the biofilm matures. Subject to routine nitrite monitoring in the fish tanks, glycerol is a cost-effective, low-hazard, and pH-stable alternative to acetic acid for nitrate removal in salmon RASs.