Heterogeneous catalytic ozonation of thermal hydrolysis-anaerobic digestion leachate for enhanced cultivation of Galdieria sulphuraria

Thermal hydrolysis technology is widely used to pretreat sewage sludge prior to anaerobic digestion. However, the wastewater produced after thermal hydrolysis-anaerobic digestion process, or thermal hydrolysis-anaerobic digestion leachate (TH-ADL), contains higher concentrations potentially inhibitory complex macromolecular organic compounds. To overcome microalgal growth inhibition caused by refractory organics in full-strength TH-ADL, a novel strategy was developed by integrating heterogeneous catalytic ozonation of effluents prior to mixotrophic cultivation of Galdieria sulphuraria (G. sulphuraria). This study investigated the combined use of catalytic ozonation pretreatment and thermophilic and acidophilic G. sulphuraria cultivation to achieve efficient remediation of TH-ADL. The results demonstrated that catalytic ozonation effectively reduced macromolecular organic compounds and chromaticity of TH-ADL, promoting microalgae growth. Under mixotrophic cultivation with glucose supplementation, the microalgal biomass reached 3.46 g/L, with chemical oxygen demand (COD), NH₄⁺ , and PO₄³⁻ removal efficiency of 31.5 %, 48.8 %, and 99.1 %, respectively. Supplementing with phosphate to optimize the N/P mass ratio further increased the microalgal biomass to 4.25 g/L, achieving 66.9 % COD removal and 53.0 % NH₄⁺ recovery. G. sulphuraria biomass accumulation in catalytically ozonated unsterilized TH-ADL effectively exhibited an inhibitory effect on heterotrophic competitors (e.g., Proteobacteria) ensuring effluent biosafety compliance. This study provides a cost-effective solution for full-strength TH-ADL treatment without dilution, with potential for industrial-scale nutrient recycling. Additional research is needed to explore catalytic ozonation mechanisms, optimize microalgal cultivation conditions, and enhance nutrient recovery to further improve wastewater treatment efficiency.