The process of assimilation utilizes carbon-enriched beads to promote the growth of heterotrophic bacteria, which have a higher specific growth rate than nitrifiers. This makes them ideal for faster biofilter start-ups and for absorbing ammonia during sudden increases in organic loading rates in RAS systems, mitigating the negative impacts of ammonia on cultured animals. Recently, carbon beads, such as poly(3-hydroxybutyrate) (PHB), have been introduced as passive assimilation beads, serving as both biofilm carriers and electron donors for heterotrophic bacteria, which allows farmers the opportunity to separate the cultured animals from the filtration unit. While PHB eliminates the need for external control systems, its application as a carbon substrate is limited by high cost. To reduce the cost of PHB while maintaining biodegradability, it has been blended with cheaper materials like cellulose and starch. This present study extruded PHB/cellulose and PHB/starch blends with compositional ratios (100:0, 80:20, 70:30, and 60:40), and tested their effectiveness in ammonia removal in an aerobic assimilation process to reduce toxic ammonia buildup in RAS facilities. The COD release of these materials was evaluated to assess their solubility.
A single-screw extruder was used to produce PHB/cellulose and PHB/starch beads. A 5-day experiment was conducted using these blends as carbon substrates in an up-flow fixed-bed bioreactor to promote bacterial growth and remove ammonia (~20 mg/L ammonia-N) from synthetic aquacultural wastewater. Water quality parameters, including ammonia-N, nitrite-N, COD, and nitrate-N, were measured every 24 hours, with oxygen, pH, and temperature in the reservoir maintained at 7.85 mg/L, 8.1, and 25°C, respectively.
After 5 days of experimentation PHB/starch beads had lower cost, and higher ammonia-N removal rates but with high COD accumulation, which eliminates their applications in RAS facilities. PHB/cellulose had good assimilation rates and reduced the cost of PHB while maintaining low solubility. PHB showed the lowest solubility, but its application is hampered by high cost.