Severn Trent plc is a water company based in Coventry, England. It is traded on the London Stock Exchange and a constituent of the FTSE 100 Index. Severn Trent, the trading name owned by the company, applies to a group of companies operating across the United Kingdom, United States and mainland Europe, with some involvement in the Middle East. It took its name from the two predecessor River Authorities, which managed the catchment of the Severn and the Trent.
The current understanding of activated sludge conditions and configurations that favour the removal of micropollutants (MPs) is equivocal. This study conducted a series of activated sludge process trials using pilot-scale reactors and a synthetic feedstock, to achieve controlled and representative conditions. The fate of ten perfluoroalkyl substances (PFAS), four brominated flame retardants, two triazoles, three pharmaceuticals, three bio-/pesticides, one phthalate, one nonylphenol and four metals was studied with three levels of mixed liquor suspended solids (1-3 g/L), three redox configurations and three levels of dissolved organic carbon in the feed (starch vs. acetate). The tested range of operational conditions had a limited effect on MP removals (standard deviation < 20%, n = 6), demonstrating the resilience of the activated sludge process. At the same time, this indicated that such process interventions may not achieve improved removal efficiency. A new approach based on three confidence levels was proposed to support the prediction of the likely fate of MPs in biological wastewater treatment (sludge, sludge/effluent, effluent, effluent/degraded, degraded). High confidence was assigned to substances with consistent behaviour in both the control and the test reactor, while low confidence was linked to concentrations near or below the reporting limit and inconsistent or partial removal. Finally, the MPs were analysed in the particulate, the colloidal and the truly dissolved fraction of the activated sludge effluent to inform the selection of suitable post-treatment processes, which are deemed necessary for a high-quality final effluent.
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.
Reactive media present an alternative to gravel in constructed wetlands and have the potential to sustainably and efficiently remove phosphorus from wastewater. In this study, a full-scale steel slag wetland has been operated for its whole lifecycle at which 1.39 mg P/g media were retained. During its lifecycle, this wetland met strict consents below 0.5 mg P/L for the first 6 months and was operated for 266 and 353 days before the effluent phosphorus concentration rose above the typical consents of 1 and 2 mg P/L, respectively. A detailed analysis of the system demonstrated that the performance was directly associated with the release of materials from the media into the water which in turn affected other critical parameters such as pH. Further analysis of the media suggested that greater understanding was needed concerning the role of carbonates and in particular calcite if steel slag is to be effectively managed for use on constructed wetlands. Importantly, controlled release of calcium oxide from the media surface is required by managing the concerns of pH and vanadium release.
In this manuscript, CH4 emissions from sludge treatment centres are quantified using an unmanned aerial vehicle (UAV) framework, with particular focus on anaerobic digesters and digestate storage tanks. The outcomes are compared to those obtained using the carbon accounting workbook (CAW), which is the most commonly used industry tool by UK and Irish water companies to estimate the annual greenhouse gas emissions from their process operations. Path integrated concentrations are monitored with the use of an open-path tuneable diode laser absorption spectroscopy sensor embedded on a UAV. Measurements are interpolated using geostatistics (Kriging) and coupled with the mass balance approach to estimate emissions. The findings show that the CAW seems to underestimate emissions from digestate storage tanks by up to an order of magnitude. The results also show that CH4 emissions are linked with the residence time in the tank and temperature of the digestate. This study highlights the limitations of assumptions made using current reporting methods based on the carbon accounting workbook. This study proves that the UAV framework, together with the mass balance approach, provides high spatial resolution data; it captures the dynamic nature of emissions compared to the CAW and can be a cost-effective solution to estimate CH4 fluxes compared to other sensor-based systems.
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.
