Northumbrian Water

Raw water transfer (RWT) schemes (engineered networks of pipelines, tunnels, and supply canals that move large volumes of freshwater between separate waterbodies) represent one of the most understudied invasion pathways in freshwater ecology. This is despite global prevalence and rapid expansion of RWTs driven by population growth, urbanisation, and climate-driven water scarcity. Environmental regulators in Britain have recently introduced requirements for stakeholders to risk assess and manage the RWT invasion pathway, yet critical assessment of the available literature reveals that few RWT studies have been conducted worldwide and pathway understanding remains limited. Field investigations of three structurally distinct RWTs in northeast England revealed that RWTs disperse a taxonomically diverse range of aquatic taxa, under propagule pressure levels potentially exceeding those of any other freshwater pathway of unintentional spread. Sampling yielded viable specimens of 16 distinct species across all three RWT schemes, including fish, molluscs, and arthropods, with a further seven species sampled in non-viable form. Introduction risk was associated with the type of physical infrastructure barrier, particularly pumps, rather than transfer distance. Reconstruction of historic invasive non-native species (INNS) dispersal using a network-distance analysis method confirmed RWTs as key drivers of INNS spread, particularly of small-bodied invertebrates and aquatic and riparian plants. Results also showed that RWT infrastructure has substantially restructured natural hydrological connectivity at a regional scale, consequently facilitating INNS dispersal. Given the significant and growing threat RWTs pose to global freshwater biodiversity, this research calls for integration of RWT pathway management within broader INNS surveillance frameworks and accurate reclassification of RWTs within international pathway classification systems.

Vacuum thermal stripping permits the recovery of ammonia from wastewater in a concentrated form, which is key to its exploitation in the circular economy, but the latent heat demand for thermal separation remains a critical barrier to exploitation. In this study, we investigate the vapor-liquid equilibrium (VLE) for ammonia-water as a mechanism to enhance recovered ammonia quality and minimise the thermal energy required for ammonia separation. Below the dew point (65 degrees C at 0.25 bar) a two-phase region of the VLE exists where 48 %wt gas-phase ammonia could be produced (61 degrees C) compared to only 2 %wt within the stripping region adopted widely in the literature. This was complemented by a 98 % reduction in thermal separation energy, since limited water vaporization can occur when the feed is maintained below the activation energy threshold for bulk evaporation. Operation within this practically unexplored region of the ammonia-water VLE fosters a gas-phase product suitable for energy generation in gas turbines or solid oxide fuel cells. Comparable product quality was achieved using concentrated wastewater, which validated the VLE for design in the presence of a broad range of dissolved gases and volatile inorganic compounds. Rapid desorption of CO2 occurred during vacuum stripping, subsequently increasing pH >9 without the requirement for alkali addition to shift the ammonia-ammonium equilibrium in favor of gaseous ammonia. Consequently, the two-phase region of the VLE defined for vacuum thermal stripping provides a synergistic strategy to mitigate chemical demand, minimise separation energy and recover gas-phase ammonia for zero carbon energy generation, constituting a significant advancement toward the net zero ambitions of the water sector.

Solid and liquid products can form in the gas phase of membrane contactors applied to reactive ternary systems for CO 2 absorption, which poses a critical barrier for carbon capture applications. The mechanism initiating these unwanted phase changes in the gas phase is unclear. This study therefore systematically characterises CO 2 absorption in distinct regions of the vapour -liquid equilibrium (VLE) within an illustrative ternary system (CO 2 - NH 3 -H 2 O), to provide an explanation for the formation and mitigation of these solid and liquid products in the gas -phase. Unstable CO 2 absorption and increased pressure drop indicated product formation within the gasphase, which occurred at high CO 2 capture ratios. Temporal analysis of gas -phase composition enabled gasphase products to be related to the relative ternary composition. This was subsequently correlated to distinct regions of the VLE. Consequently, mitigation strategies can be developed with recognition for where products are least likely to form. Pressurisation was proposed to modify the relative gas -phase ammonia composition to reposition conditions within the VLE. The commensurate increase of CO 2 into the solvent shifts the ammoniaammonium equilibrium towards ammonium to indirectly reduce vapour pressure. This synergistic strategy allows sustained operation of membrane contactors for CO 2 separation within reactive ternary systems which are critical to delivering carbon capture economically at scale.

Biogas upgrading uniquely requires pressurisation of hollow fibre membrane contactors (HFMC) to be competitive with classical water absorption, and when complemented with an ambient industrial temperature range, these conditions will determine CO2 mass transport phenomena that are distinct dependent upon whether microporous or nonporous membranes are used. This study therefore examines the independent and concomitant role of temperature and pressure in determining CO2 mass transport, and selectivity, within microporous and nonporous HFMC. At low solvent temperatures, higher CO2 flux was achieved which indicates that solvent solubility is more critical than CO2 diffusivity to enhancing mass transport. Low temperatures also favoured mass transfer within the microporous membrane, explained by the reduction in solvent vapour pressure which limited pore wetting by condensation. In contrast, the nonporous membrane exhibited poorer mass transfer at low temperatures due to a decline in dense polymer permeability. Crucially in this study, neither wetting of the microporous membrane or plasticisation of the nonporous membrane were observed following pressurisation. Consequently, CO2 flux increased in proportion to the applied pressure for both membrane types, emphasising the critical role of pressurisation in augmenting process intensification for biogas upgrading which is typically facilitated at pressures of 7-10 bar. Resistance-in-series analysis illustrated how pressurisation reduced gas-phase resistance, and subsequently enhanced selectivity. Consequently, an outlet gas quality of 98% methane could be achieved within a single microporous module at 4.5 bar, meeting the industrial standard for biomethane whilst reducing solvent requirements, separation energy and methane losses. Comparable behaviour was observed during pressurisation of the nonporous membrane, but with a less significant benefit to CO2 mass transfer and selectivity, ostensibly due to the resistance imparted by the dense polymer. When considered collectively, low solvent temperature and high gas pressure enhance process intensification subsequently reducing process size (e. g., membrane area) and separation energy, while also advancing selectivity to deliver a gas product at the composition required for biomethane with minimum methane losses, which are critical factors in demonstrating microporous HFMC as an industrially competitive solution for biogas upgrading.

Solids formation can substanitally reduce the energy penalty for ammonia solvent regeneration in carbon capture and storage (CCS), but has been demonstrated in the literature to be difficult to control. This study examines the use of hollow fibre membrane contactors, as this indirect contact mediated between liquid and gas phases in this geometry could improve the regulation of solids formation. Under conditions comparable to existing literature, NH4HCO3 was evidenced to primarily crystallise in the gas-phase (lumen-side of the membrane) due to the high vapour pressure of ammonia, which promotes gaseous transmission from the solvent. Investigation of solvent reactivity demonstrated how equilibria dependent reactions controlled the onset of NH4HCO3 nucleation in the solvent, and limited ‘slip’ through transfomation of ammonia into its protonated form which occurs prior to the phase change. Crystallisation in the solvent was also dependent upon ammonia concentration, where sufficient supersaturation must develop to overcome the activation energy for nucleation. However, this has to be complemented with a reduction in solvent temperature to offset vapour pressure and limit the risk of gas-phase crystallisation. While changes to the solvent chemistry were sufficient to shift from gas-phase to liquid phase crystallisation, wetting was observed immediately after nucleation in the solvent. This was explained by a local region of supersaturation within the coarse membrane pores that promoted a high nucleation rate, altering the material contact angle of the membrane sufficient for solvent to breakthrough into the gas phase. Adoption of a narrower pore size membrane was shown to dissipate wetting after crystallisation in the solvent, illustrating membrane contactors as a stable platform for the sustained separation of CO2 coupled with its simultaneous transformation into a solid. Through resolving previous challenges experienced with solids formation in multiple reactor configurations, the cost benefit of using ammonia as a solvent can be realised, which is critical to enabling economically viable CCS for the transition to net zero, and can be exploited within hollow fibre membrane contactors, eliciting considerable process intensification over existing reactor designs for CCS.