Continuous two-stage anaerobic digestion of potato processing wastewater enhanced by graphite rods coated with nano-nickel ferrite toward a zero-waste approach for biohydrogen and biomethane production

Abstract Potato processing wastewater (PPW) is known for its high organic load and suspended solids, which pose considerable environmental and operational challenges for the food industry. This research presents an integrated, near zero-waste treatment strategy that not only enhances wastewater quality but also facilitates the recovery of renewable energy. A continuous two-stage anaerobic digestion system was employed for the sequential production of Biohydrogen and Biomethane, utilizing graphite rods coated with Nano-nickel ferrite as conductive catalytic media to boost microbial activity and electron transfer. The initial stage was optimized for Biohydrogen production under mesophilic conditions with controlled pH, while the subsequent stage transformed the effluents from the hydrogen stage into Biomethane. The treated effluents underwent further polishing through reverse osmosis (RO) and UV disinfection to allow for water reuse. The incorporation of Nano-nickel ferrite–coated graphite markedly improved system performance in comparison to control reactors. The yield of Biohydrogen rose from 148 to 273 mL H₂ g⁻¹ VS, with peak daily production nearing 3000 mL. Similarly, the Biomethane yield increased from 143 to 274 mL CH₄ g⁻¹ VS, reflecting the effective conversion of volatile fatty acids during the second stage. The integrated system achieved high removal efficiencies for total and soluble chemical oxygen demand, total solids, volatile solids, and nitrogen compounds, showcasing effective stabilization of organic matter. An energy recovery analysis indicated a significant enhancement in net energy output attributable to the combined production of hydrogen and methane. After RO treatment, the final effluent satisfied quality standards for reuse in potato processing activities. In summary, the findings illustrate that the integration of Nano-enabled conductive materials with two-stage anaerobic digestion and membrane filtration constitutes an effective and sustainable approach for the treatment of PPW. This methodology aligns with circular economy principles by merging wastewater reclamation, renewable energy recovery, and resource recovery.