Traditional and novel reservoir management techniques to enhance water quality for subsequent potable water treatment

Thames Water treats approximately 2800MI/d of water originating mainly from the lowland rivers Thames and Lee for supply to over 7.3 million customers, principally in the cities of London and Oxford. Most of the river water sources are stored in bank-side, pumped, storage reservoirs prior to treatment for potable use. Storage reservoir sizes vary and typical theoretical retention times lie between a few days to several weeks or months. During storage the riverine biota is largely replaced by lacustrine taxa which can cause problems for subsequent water treatment, particularly filtration. Recent concerns about cyanobacterial toxins has heightened interest in reservoir management. This paper reviews aspects of Thames Water's research, design and operating experiences of managing eutrophic, algal rich, reservoir stored, lowland water. Areas covered include experiences of optimising reservoir water quality to both control algal productivity and to aid subsequent potable water treatment. Traditional reservoir management techniques are reviewed as is research into biomanipulation. Whilst changes in reservoir water quality using these techniques have been marked, actual retention time and quality changes have traditionally been difficult to predict. Computational fluid dynamic (CFD) modelling has been used successfully to substantially increase retention and subsequent changes to water quality. Information from CFD modelling may also be used to reduce risks from protozoan parasites such as Cryptosporidium and Giardia.