Bio-hydrogel formulation for co-immobilization of microalgae and bacteria in living biofilters for nutrient recovery from secondary industrial effluents

Abstract

The increasing discharge of nutrient-rich industrial effluents poses a significant environmental challenge, necessitating efficient and sustainable wastewater treatment strategies. This study developed a living hydrogel-based biofilter incorporating co-immobilized Chlorella sp. and Bacillus subtilis TISTR 1415 to enhance nutrient recovery from secondary industrial effluent from vegetable oil factories. Hydrogels were formulated using guar gum and carrageenan, crosslinked with potassium chloride (KCl), and evaluated for their stability and microbial immobilization efficiency. Among the tested formulations, the hydrogel with 0.3 M KCl exhibited optimal properties, including moderate swelling capacity (∼1,005 % or ∼10 g_water/g_{dry hydrogel}), reduced solubility (∼40 %), and enhanced mechanical stability and crosslinking density, leading to improved porosity and microbial retention. These physicochemical properties facilitated efficient nutrient diffusion and sustained cell viability within the hydrogel matrix. The synthetic co-culture biofilter with a 3:1 ratio of Chlorella sp. to B. subtilis significantly enhanced nutrient removal efficiencies compared to monocultures, achieving 98.68 % ammonium (NH₄⁺), 53.45 % phosphate (PO₄³⁻), and 68.60 % COD removal over 7-day trials. The synergistic interaction between microalgae and bacteria facilitated improved nutrient uptake, organic matter degradation, and enhanced effluent treatment performance. Furthermore, pH and dissolved oxygen levels were significantly influenced by microbial activity, with microalgae contributing to oxygen production and pH elevation, while bacteria aided organic matter breakdown. The living hydrogel-based biofilter presents a promising alternative to conventional wastewater treatment methods by harnessing the synergistic interactions between biological processes and hydrogel immobilization technology. This approach enhances effluent quality and contributes to innovative solutions for environmental protection and nutrient recovery.