Fulvic acid: a novel biofilm denitrification enhancer

To address the challenges of prolonged start-up cycles and poor operational stability in biofilm reactors for treating high ammonia nitrogen pollution in aquaculture effluent, this study adopted fulvic acid (FA) as a novel biofilm enhancer to systematically analyze its regulatory mechanisms on denitrification performance and biofilm formation in sequencing batch biofilm reactors (SBBR). Through microplate biofilm formation experiments and SBBR process validation, researchers found that the biofilm formation index reached 1.4 at an FA concentration of 1200 mg/L. FA improved biofilm structural stability by promoting extracellular polymer secretion, yielding a 22-fold increase in biofilm thickness compared with the control group (the biofilm thickness peaked at 2208.3 μm). Additionally, high FA concentrations (>500 mg/L) strengthened denitrification processes by facilitating carbon source supply and electron transfer, enriched denitrifying functional bacteria such as Comamonadaceae, and inhibited Gram-positive bacteria (e.g., Actinobacteriota). The removal efficiencies of NH₄⁺-N, NO₃^--N, and total nitrogen reached 79.67%, 80% and 62.4%, respectively. This study reveals that FA enhances SBBR denitrification through dual mechanisms of carbon source supplementation and physicochemical property regulation, providing an efficient and cost-effective regulatory strategy for aquaculture effluent treatment.