Seawater denitrification in a closed mesocosm by a submerged moving bed biofilm reactor

The performance of a submerged moving bed biofilm reactor (MBBR) for the denitrification of seawater in a 3.25 million l closed circuit mesocosm was investigated at pilot scale, using methanol as a carbon source at various C/N ratios. Nitrate accumulation in closed systems where water changes are expensive and problematic may cause toxicity problems to marine life. Seawater was pretreated in a recirculated fixed bed to remove oxygen prior to the denitrification step. The 110 l MBBR was partly filled (25%) with spherical positively buoyant polyethylene carriers with an effective surface area of approximately 100 m2 m−3, which represents 35% of the total surface area. Carriers were maintained submerged by a conical grid and circulated by the downflow jet of an eductor. The MBBR mixing system was designed to prevent dead mixing zones and carrier fouling to avoid sulfate reduction while treating seawater containing as high as 2150 mg SO4-S l−1. NO3-N reduction from 53 to as low as 1.7±0.7 mg l−1 and a maximum denitrification rate of 17.7±1.4 g N m−2 d−1 were achieved at 4.2–4.3 applied COD/N (w/w) ratio. Methanol consumption corresponded to denitrification stoichiometric values, indicating the absence of sulfate reduction. Denitrification rates and effluent residual dissolved organic carbon were proportional to the C/N ratio. Such reactors could be scaled up in closed systems where water changes must be minimized.