Carbon-negative biomethane fuel production: Integrating anaerobic digestion with algae-assisted biogas purification and hydrothermal carbonisation of digestate

Highlights: • New process integrates algae-based CO₂ capture with HTC of digestate • D. tertiolecta CCAP 19/30 regenerates carbonate at concentrations up to 80 g L⁻¹ • Above 40 g L⁻¹, bicarbonate conversion exceeds algae growth, releasing excess CO₂ • Spent growth medium successfully used for biogas purification • 27% of feed carbon captured in biochar rendering process net carbon negative This paper presents a new integrated process for producing carbon-negative biomethane fuel which combines anaerobic digestion of sewage sludge with biogas purification, algae generation and carbon capture. Biogas CO₂ is recovered through absorption into sodium carbonate solution, producing bicarbonate, subsequently used as substrate for algae growth. Anaerobic digestate is hydrothermally carbonised into biochar, stabilising unused biomass carbon for long-term storage. Proof-of-concept studies are provided, together with a system carbon balance to demonstrate its overall carbon capture potential. D. tertiolecta (CCAP 19/30) was cultivated at different bicarbonate concentrations (2.5–90 g L⁻¹), producing equimolar amounts of carbonate and carbon-derived products (algae, gaseous CO₂). Above 40 g L⁻¹, carbonate regeneration significantly exceeded algae growth, achieving higher than expected CO₂ uptake potential, but lower overall carbon sequestration potential due to the discharge of excess CO₂ to the environment. Single-stage CO₂ absorption from a model biogas mixture (60% methane) into carbonate solution was rate limited by the reaction of dissolved CO₂ with solution hydroxide ions, achieving steady state methane outlet purities of up to 85%. Spent absorbent was successfully used for algae culture, and the regenerated medium showed equivalent CO₂ uptake as the fresh carbonate solution, demonstrating the cyclability of the system. Carbon distribution to biochar and algae by-products was estimated as 35.1%, exceeding the expected emissions associated with the process, to render the process carbon negative.