Biomethanation of alkaline waste sludge in haloalkaline conditions: combined proof of concept experiments and technical economic evaluation

A highly pure biomethane stream (≈97% CH₄) was produced continuously under halo-alkaline conditions (pH > 9, 0.6 M Na⁺) from complex alkaline organic waste residue originating from biopolymer extraction from sewage sludge. During the proof-of-concept operation, the substrate was degraded with similar efficiency (40% of the volatile solids, VS) compared to neutral conditions (36% of the VS). Operational data was utilised in a technical evaluation to identify bottlenecks for full-scale implementation at an early stage of process development and for comparison to conventional biogas upgrading using pressure swing and membranes. Initially identified bottlenecks for alkaline fermentation were related to overcautious assumptions, while others could be technically solved. Alkaline fermentation offers an attractive method for supplying increasingly needed high-purity biomethane using various recalcitrant substrates that have undergone alkaline pre-treatment. This is more feasible than the conventional ex-situ biogas upgrading. Next, upscaling steps for alkaline fermentation should be pursued. Strategies for integrated CO₂ sequestration and nutrient recovery are outlined, which will offer additional benefits in the future. First successful semi-continuous biomethane production under alkaline conditions from complex substrate Techno economic evaluation: Scaling-up and scaling-down analyses identified bottlenecks that are relatively easy to resolve Next research steps and operational modifications to overcome bottlenecks were identified Alkaline fermentation is an attractive in-situ method for biomethane production with eventual CO₂ sequestration and nutrient recovery, instead of ex-situ biogas upgrading technologies.