Enhancing fish sludge bioconversion kinetics for nutrient recovery in aquaponics using a modified biological aerated filter with a novel media of polyhedral hollow spheres

Nutrient recovery from aquaculture sludge is vital for promoting hydroponic plant growth and achieving near-zero solid waste discharge in aquaponic systems. Modified biological aerated filters (MBAFs) are promising because of the dual capabilities of aquaculture sludge collection and aerobic mineralization. However, the bioconversion kinetics, which is indirectly related to the packed media, need to be improved. In this study, a novel polyhedral hollow sphere (PHS) medium was used in an MBAF (MBAF-PHS) to overcome the shortcomings of the current medium, facilitating fish sludge retention and enhancing subsequent bioconversion kinetics for nutrient recovery. An average rate of 36.9 g/d for dry weight of fish sludge was achieved during 29 d of filtration and an average reduction rate of 31.30 g/d during 26 d of bioconversion. The total mass of fish sludge was converted by 76.2% via the co-action of the solubilization of organic solids and degradation of dissolved organic matter. MBAF-PHS was competitive for macronutrient recovery compared with the MBAF-sponge previously used. The ratios of the final concentrations of the macronutrients (P, Mg, and S) to the concentrations in Hoagland solution (C_f/C_H, %) were 278.1, 162.8, and 200.9%, respectively, whereas the ratios of N, K, and Ca were 65.9, 37.1, and 51.0%, respectively. High bioconversion kinetics of NO₃^--N and PO₄^3--P were obtained within 7 d with an M_NO3-N/M_TN of 79.9% and M_PO4-P/M_DTP of 80.3%. The nutrient bioconversion of fish sludge was associated with the diversity of the microbial community in the MBAF-PHS, especially the population of nitrogen-removing microbial species that developed after 9 d of mineralization.