Nitrate contamination in recirculating aquaculture systems (RAS) can harm aquatic species, affecting reproduction, growth, and survival. Biological denitrification is considered the most cost-effective method for nitrate removal in RAS. Traditional methods rely on liquid carbon sources like methanol or acetate, which require constant monitoring with sophisticated computer systems to avoid overdosing, increasing operational costs. Solid-phase denitrification (SPD) using biodegradable polymers, such as polyhydroxybutyrate (PHB), offers a promising alternative but is not cost-effective. Cellulose, being renewable, cost-effective, and biodegradable, is a suitable candidate for blending with PHB to reduce costs. This study investigates the impact of cellulose on the biodegradability and denitrification performance of PHB.
PHB/microcrystalline cellulose blends (PHBC) containing 20, 30, and 40% cellulose were prepared using a benchtop single screw extruder. In a laboratory-scale setup, pure PHB bio-pellets were compared to PHBC blend bio-pellets in terms of nitrate reduction rates, COD accumulation, consumption rates, and denitrification costs. A 7-day experiment was conducted in triplicate in a temperature-controlled dark room, using four identical 12” up-flow PVC bioreactor columns, each filled with 200 mL of different bio-pellets, along with four 109 L source water reservoirs and four 20” degassing columns for stripping dissolved oxygen with pure nitrogen gas. The bioreactor units were backwashed at 12 h intervals using pure nitrogen.
The study found that PHBC60:40 and PHBC70:30 achieved average apparent peak nitrate reduction rates of 3.9±0.80 kg NO3−N/m3-d and 3.9±0.24 kg NO3−N/m3-d, respectively, on day 2. PHBC80:20 reached 3.7±0.017 kg NO3−N/m3-d on day 3, and PHB100% attained 3.4±0.03 kg NO3−N/m3-d on day 4, with statistically significant differences between treatments. COD accumulation on day 7 increased with cellulose content, ranging from 19±2.9, 29±4.2 in PHB100% to 58±11.5 in PHBC60:40, partly due to solid release during backwashing. The blends were more cost-effective than PHB. The results show that the blend is effective for SPD.