Nitrifying biofilters in recirculating aquaculture systems (RAS) effectively remove ammonia and nitrite but require long acclimation periods before reaching full performance, often leading to ammonia accumulation during start-up. Heterotrophic assimilation driven by fast-growing heterotrophs that incorporate ammonia into their biomass offers a practical solution, especially when supported by organic carbon supplementation. Although soluble carbon sources like methanol enhance this process, they require precise dosing to prevent water quality deterioration. Solid-phase carbon substrates such as poly(3-hydroxybutyrate) (PHB) provide a more stable alternative by supporting biofilm growth while minimizing excessive biomass accumulation and oxygen depletion. However, PHB’s high cost limits its large-scale use. Natural wax-based substrates such as soy and coconut wax are inexpensive, biodegradable, and can serve as solid carbon and electron donors for heterotrophic ammonia assimilation. This study evaluated the volumetric total ammonia removal rates (VTR) of different soy and coconut waxes as solid-phase carbon substrates, with relevance to RAS start-up.
A 7-day experiment was conducted using three soy wax types with distinct melting points MWSW, FSW, and MSW, alongside a control with no carbon source in a recirculating fixed-film setup. A follow-up trial under similar conditions tested two coconut wax variants pure coconut wax (PCW) and FCW, a commercial soy–coconut blend to assess potential improvements in TAN conversion. Water quality parameters, including total ammonia nitrogen (TAN), chemical oxygen demand (COD), temperature, dissolved oxygen (DO), and pH, were monitored every 24 hours.
After the 7-day trials (Figure 1), PCW achieved the highest TAN reduction with a mean VTR of 0.23 ± 0.13 kg TAN m⁻³ day⁻¹ but also exhibited elevated COD release, affecting water quality. The soy wax treatments showed more stable but lower TAN reduction rates with good carbon release profile. Overall, natural waxes demonstrated potential as cost-effective solid carbon sources for heterotrophic ammonia assimilation during RAS start-up, though further optimization is needed to balance TAN removal efficiency and organic loading.