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Thames Water treats approximately 2800Ml/d of water originating mainly from the lowland rivers Thames and Lee for supply to over 7.3million customers, principally in the cities of London and Oxford. This paper reviews aspects of Thames Water's research, design and operating experiences of treating algal rich reservoir stored lowland water. Areas covered include experiences of optimising reservoir management, uprating and upgrading of rapid gravity filtration (RGF), standard co-current dissolved air flotation (DAF) and counter-current dissolved air flotation/filtration (COCO-DAFF®) to counter operational problems caused by seasonal blooms of filter blocking algae such as Melosira spp., Aphanizomenon spp. and Anabaena spp. A major programme of uprating and modernisation (inclusion of Advanced Water Treatment: GAC and ozone) of the major works is in progress which, together with the Thames Tunnel Ring Main, will meet London's water supply needs into the 21st Century.

A novel media combination comprising of two layers of commercially available expanded clay (EC) media was tested for use in existing roughing filters (RF). Results have indicated a potentially suitable alternative to anthracite/sand, a dual media combination of a fine grade of high density EC media and a coarse grade of low density EC media. This novel dual media consistently achieved longer run lengths, whilst producing filtrate of adequate quality for slow sand filter influent – even when challenged with algal laden water.

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.

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