Effects of power tariffs and aeration dynamics on the expansion of water resource recovery facilities

Abstract

Water resource recovery facilities are major points of electricity demand and exhibit dynamic electricity demand due to their influent load as well as their diurnal and seasonal cycles. This dynamic demand along with the diurnal and seasonal nature of grid electricity cost as well as greenhouse gas emission intensity (i.e., emission produced to generate unit grid electricity) together demonstrate the importance of modelling these facilities dynamically to accurately assess their energy demand, operating cost, extent of removal, and greenhouse gas emissions. This article reports the results of a dynamic modelling effort that examines the effect of an expansion project on the extent of treatment, electricity demand, carbon footprint, and the operating cost of a large water resource recovery facility in California. This expansion consists of the addition of a differently configured activated sludge unit and associated secondary clarifiers to this plant's existing unit. The presence of these two parallel activated sludge configurations in the expanded plant was expected to provide an opportunity to achieve electricity demand and cost savings especially during the facility's coinciding peak of electricity demand with the grid's demand by maximizing the use of more energy efficient configuration. This electricity saving approach is an example of a grid demand-side flexibility project. However, the observed inverse correlation of the facility electricity demand and its cost with the overall operating cost and extent of treatment after the addition of the second activated configuration, which was determined to be the less energy efficient of these two configurations, demonstrated the infeasibility of such a grid demand-side flexibility project. This inverse correlation is changed to a positive correlation if the electricity efficiency of the added activated sludge configuration as part of this expansion project exceeds the existing electricity efficiency. As a result, recommendations were made accordingly for future studies to improve this configuration's electricity efficiency and to make the proposed grid demand-side flexibility project feasible. Such electricity efficiency improvement is also expected to lower the plant's overall carbon footprint which was not significantly impacted as part of this expansion project.