Rex Barry Thorpe

In a net-zero emissions scenario, a secure supply of electricity involves renewable generators that can flexibly increase their production when needed. Currently, electricity generation from biogas in the water industry is most commonly at a steady level, given Anaerobic Digestion (AD) is traditionally operated in steady state. This research demonstrated at different scales that demand-driven biogas production from AD of sewage sludge is feasible. Performance parameters are not negatively affected by a flexible feeding schedule and stability parameters show transitional imbalances that do not threaten the overall process. This paper presents the trial implementation in digesters of volume 3800 m3, which became permanent. Economic and environmental benefits exist; however, in order to unlock the full potential of flexible electricity generation from sewage sludge, synergies between technical, operational and political factors in the water and energy sectors need to be developed.

The earlier in the development of a process a design change is made, the lower the cost and the higher the impact on the final performance. This applies equally to environmental and technical performance, but in practice the environmental aspects often receive less attention. To maximise sustainability, it is important to review all of these aspects through each stage, not just after the design. Tools that integrate environmental goals into the design process would enable the design of more environmentally friendly processes at a lower cost. This paper brings together approaches based on Life Cycle Assessment (LCA) including comparisons of design changes, hotspot analysis, identification of key impact categories, environmental break-even analysis, and decision analysis using ternary diagrams that give detailed guidance for design while not requiring high quality data. The tools include hotspot analysis to reveal which unit operations dominate the impacts and therefore should be the focus of further detailed process development. This approach enables the best variants to be identified so that the basic design can be improved to reduce all significant environmental impacts. The tools are illustrated by a case study on the development of a novel process with several variants: thermal cracking of mixed plastic waste to produce a heavy hydrocarbon product that can displace crude oil, naphtha, or refinery wax or be used as a fuel. The results justified continuing with the development by confirming that the novel process is likely to be a better environmental option than landfill or incineration. The general approach embodied in the toolkit should be applicable in the development of any new process, particularly one producing multiple products.

Dissolved air flotation (DAF) is often used after a primary gravity separator to enhance the quality of wastewater, so it can be released to streams, rivers or the sea. The main aim of the DAF experiments reported here was to measure the oil droplet removal efficiency, (η) mostly in the range 15-80 μm from oil-in-water mixtures. The DAF tank used in this investigation was a scale model of real DAF unit. Two kinds of oil, vegetable and mineral and two types of water, fresh and salty were used, and four other operating parameters were varied. A droplet counting and oil-in-water measuring methods were used to estimate the η. Dimensional analysis concluded that the η in this experiment is a function of eight other dimensionless groups and the experimental data has been subjected to multivariable linear regression. The resulting correlation was found to have a root mean square error of 6.0%, but predict η outside the range zero and one. An alternative mathematical formulation was devised that cannot predict η outside the range. Regression of the data by this formulation, which had the same number of adjustable parameters as the linear regression, was successful with a lower root mean square error of 5.5%.