Elevated nitrate levels can adversely affect the health and growth of cultured species, thereby constraining water reuse in recirculating aquaculture systems (RAS). Solid-phase denitrification (SPD) using polyhydroxybutyrate (PHB) as a carbon source has been demonstrated as an effective method for nitrate removal in RAS. However, the high cost of PHB limits its commercial application in SPD biofilters. Soy wax (SW) and stearic acid (SA) are inexpensive, non-toxic, and biodegradable long-chain fatty acids with strong potential for this purpose. Despite these favorable properties, their use in SPD remains largely unexplored. Therefore, this study evaluated the denitrification performance of SW and SA bio-pellets in comparison with that of PHB.
SW and SA bio-pellets were prepared in batch mode using a mold-and-cast method with a hexagonal silicone trivet (7.2 in L × 7.2 in W × 0.24 in) serving as the mold. PHB pellets were remolded to the same hexagonal geometry for direct comparison. In a laboratory-scale setup, SW and SA bio-pellets were compared with PHB in terms of denitrification rates, carbon consumption, and cost. A 13-day experiment was conducted in triplicate in a temperature-controlled dark room. Nine identical 29-cm clear PVC biofilter columns were used. Three of the biofilters were operated in an upflow configuration for PHB, while six were operated in a downflow configuration for SW and SA, to account for buoyancy. Each biofilter was packed with 300 mL of its respective carbon substrate and connected in a flow-through manner to a 985-L source water reservoir enriched with nitrate. The biofilters were backwashed every 48 h using pure nitrogen gas.
The study found that under stable operating conditions, the maximum denitrification rates for SW, SA, and PHB were 1.13 ± 0.10, 1.05 ± 0.10, and 0.80 ± 0.03 kg NO₃–N m⁻³ d⁻¹, respectively, and these differences were statistically significant (p < 0.05). SW was consumed at a rate of 2.95 ± 0.30 kg SW per kg NO₃–N, SA at 2.63 ± 0.50 kg SA per kg NO₃–N, and PHB at 3.95 ± 0.25 kg PHB per kg NO₃–N. The differences in consumption rates were statistically significant (p < 0.05), with PHB exhibiting a higher consumption rate than SW and SA. SW and SA biofilters exhibited nitrite accumulation, which was seldom observed in the PHB biofilter. Both SW and SA were more cost-effective than PHB, demonstrating their potential as effective carbon sources for SPD applications.