Ammonia recovery and utilisation for biogas upgrading in membrane contactors.

This thesis has developed an innovative system for biogas upgrading using hollow fibre membrane contactors (HFMC) whilst recovering ammonia from wastewater as a reactive solvent to intensify CO₂ absorption. An expanded two- phase region for ammonia-water separation was identified and exploited to foster selective, low energy recovery of concentrated gas-phase ammonia from wastewater by vacuum thermal stripping. Selective stripping was translated to a gas-liquid contacting column which demonstrated mass transfer rates analogous to commercially established stripping processes. Investment in selective ammonia recovery from anaerobic digester centrate represents a cost saving over a 20-year economic lifetime relative to biological nitrogen removal. During physical CO₂ absorption in HFMC, solvent chilling and gas pressurisation were observed to increase flux and selectivity, thereby reducing membrane area and path length for biogas upgrading. Chilled conditions will promote wetting resilience to favour the application of microporous membranes, which are low- cost and technologically mature. Translation to recovered ammonia solvents will further intensify CO₂ absorption, but can result in gas-side reactions within the ternary CO₂-NH₃-H₂O system which reduce process stability. In a positive synergy, chilled, pressurised conditions could limit ammonia ‘slip’ and maintain the system below a critical threshold to prevent gas-side reactions and improve process resilience. Pressurised, reactive crystallisation in HFMC during CO₂ absorption by ammonia solvents was demonstrated for the first time, and observed to occur at a consistent supersaturation level. Consequently, ammonium bicarbonate crystals exhibited consistent characteristics independent of pressure which supports simplified online control and solids recovery for scale- up. The integrated system proposed in this thesis presents a cost effective, circular economy solution for ammonia recovery and biogas upgrading which is closely aligned to net zero ambitions within the water sector and wider society.