Integrated management of urban water systems to prioritise river quality

Water quality is an increasingly recommended goal for the management of water systems. A range of studies have addressed how water resources and river quality interact and can be managed in a joint manner. However, considerations of water resources and river quality often omit a critical factor - that wastewater systems exist on the same rivers as water supply systems. Actions taken for water supply, for example to abstract a given amount of water, will interact with the wastewater system, for example by changing the flow and dilution in effluent receiving waters. We have identified that around half of the river catchments in England and Wales have both significant abstractions (>2 million litres/day) on and significant wastewater treatment plant (>100,000 people) discharges into the same rivers. In this work we develop a systems urban water management model, CityWat, which enabled us to propose decisions that can be taken on the water resources side to manage river quality. We term the novel operational rule `abstraction-effluent dilution', that is, to reduce abstractions when precipitation is such that combined sewer overflows are expected in order to dilute the untreated effluent discharged into the river. We test abstraction effluent-dilution on a stylised model of London's water system and find that it can reduce phosphorus levels by an amount that would require an equivalent £100 millions of wastewater infrastructure, despite only requiring a change in decision making. We also examine whether the technique would have any adverse impacts on the reliability of water supply or flooding and find these negligible.