Stirring things up: Do surface mixers in drinking water reservoirs improve water quality?

Clean water is an important resource and a crucial worldwide challenge for both society and industry. Drinking water reservoirs in the UK and around the world become depleted in oxygen in the summer, when layers of different temperature form due to increased solar heating. This strong temperature stratification in the reservoir acts as a barrier to vertical transport of dissolved substances including oxygen and nutrients. All major water utilities in Great Britain use engineering interventions to attempt to mitigate the cost and water quality impacts of low oxygen in their reservoirs.

Besides being lethal for fish and other aquatic organisms, low oxygen leads to the growth of nuisance algae, leading to blooms. In addition to the malodourous "pea soup" state that these blooms create, making the reservoir unappealing for recreation and as a landscape feature, the algal matter that enters drinking water treatment plants interacts with the chlorination process to create carcinogenic compounds. The stratification also creates ideal conditions for the growth of cyanobacteria, who have evolved to be able to regulate their buoyancy and location in the water column; cyanobacterial toxins are one of the key contributors to taste and odour problems in drinking water. Lack of oxygen also causes manganese in the bottom sediments to be released into the water. Like iron, manganese turns colour when oxidised (black in this case), which then makes drinking water supplies unsuitable for consumption until expensive treatment processes are utilised to remove the manganese.

To deal with this problem, water utilities employ reservoir destratification systems that mix the water so that oxygen can mix downwards from the surface. There are several methods of destratification; bubble plumes have most commonly been used by water utilities in the UK, but recently utilities have been making new capital investments in surface mixers, which act by pushing water from the surface down to the bottom, creating circulation in the reservoir. Yet these destratification systems are imperfect, and utilities still suffer from water quality problems that must be dealt with through the water treatment process. Thus better understanding and optimization of destratification systems can lead to improved source water quality, lighter treatment load at drinking water plants, and a decrease in energy use by water utilities if less mixing is required.

The overarching goal of this project is to improve the management of water supply reservoirs that depend upon destratification for maintaining water quality. To this end, a field campaign is proposed that has two aims:

1. Assess in the field the spatial distribution of mixing and water quality in a reservoir with a surface mixer system and other interventions.
2. Determine through a modelling study if changes in water quality are due to the mixer or to natural variation in climate controls.

These aims will be achieved through combined field measurements and modelling of Durleigh Reservoir. In 2015, Durleigh's bubble destratification system was replaced with a surface destratification system. Through long-term measurements in Durleigh, the performance of the mixer will be assessed throughout the summer. By modelling the reservoir without the mixer, changes in water quality can be attributed to the intervention or to natural variability in climate forcing. Through these aims, we will generate knowledge that can be used to better understand the hydrodynamics and water quality of reservoirs with artificial mixing, with a future aim of improving management of our water resources.