Enhanced nitrogen removal in recirculating aquaculture systems using a downflow hanging sponge reactor and Poly(butylene succinate-co-adipate) as a biodegradable electron donor for nitrate removal.

Nitrate accumulation remains a major constraint to the sustainable intensification of recirculating aquaculture systems (RAS). This study provides the first long-term, in situ evaluation of a Poly(butylene succinate-co-adipate) (PBSA) -integrated Downflow Hanging Sponge (DHS) reactor operating in a live, brackish-water RAS, enabling simultaneous nitrification-denitrification without soluble carbon or alkalinity supplementation. Supplementing PBSA in the RAS stabilized water quality, maintaining total ammonia nitrogen and nitrite below 0.3 mg-N L-1 and nitrate below 50 mg-N L-1 when PBSA concentrations ranged from approximately 3.3 to 1.2 g L-1. Scanning electron microscopy and FTIR analyses confirmed progressive PBSA degradation, including surface erosion, bond scission, and a shift in the glycosidic-to-ester ratio from 1.3 at day 0 to 0.8 at day 104 and 1.0 at day 316. Lipase assays revealed strong biofilm-associated enzymatic activity in DHS sponges, and 16S rRNA sequencing showed functional partitioning: nitrifying microorganisms (Nitrosomonas and Candidatus Nitrosotalea) and dominated the DHS sponges, whereas PBSA selectively enriched denitrifying taxa (Pseudomonas, Azoarcus, and Acidovorax). Cost analysis estimated a removal cost of $80-180 per kg NO3--N under current operating conditions, with optimization projected to reduce costs to approximately $50-120, consistent with reported ranges for other biopolymer donors. Unlike conventional soluble donors, PBSA functions simultaneously as a structural support and a sustained electron reservoir, bridging material biodegradation with microbial nitrogen cycling. Collectively, these findings position PBSA-integrated DHS systems as a multifunctional strategy for nitrate control and provide a practical blueprint for low-input, biopolymer-integrated RAS design that supports circular bioeconomy principles.