A Eulerian-Eulerian multiphase CFD model is employed for the air/water flow. A 3D structure grid is used to incorporate the air nozzle and tank geometry. The fixed frictionless wall boundary approximating the free surface acts as a sink to allow the air bubbles to escape. The air/water volume fraction in the flotation tank is evaluated to determine the effective air/water fluid density. The floc particle is then introduced and is tracked in the air/water fluid using a disperse Lagrangean model. Fate of these flocs depends on their sizes and density. Flocs therefore can either escape through the top water surface, settles in the main tank or breakthrough under the outlet weir. The CFD model is developed for a full scale DAF tank to predict the flow dynamic, particle removal and settled solid profile. The general flow pattern is compared with flow visualisation using the underwater camera. Comparison of average fluid velocities is carried out using acoustic Doppler velocimetry ADV measurement.

Thames Water Utilities has developed and patented a Counter-current dissolved air flotation/filtration (COCO-DAFF) process as a compact water treatment system designed to remove particulate material from traditional water sources. In particular it has been developed to overcome operational problems with primary filters caused by seasonal blooms of filter blocking algae such as Melosira sp., Aphanizomenon sp. andAnabaena sp. The process can be run without flotation during periods when algae are not a problem, giving operational cost savings. This process differs from co-current dissolved air flotation in that the recycle water is introduced after the inlet structure, but above the filter media. This generates an even depth bubble blanket in the flotation tank through which all the flocculated water must pass. The advantages are that in moving the recycle inlet away from the flocculated water inlet the potential for floc damage by the recycle is eliminated. Also since the entire sludge blanket is supported by a deep, even, bubble blanket, on de-sludging any fall-out of sludge that occurs near the de-sludging weirs will have to go back down through the process, leading to subsequent re-floating, and a reduced potential for spiking of the floated turbidity. Process validation experiments have been carried out on a 1.4 Ml/d pilot plant based at the Kempton Advanced Water Treatment Centre, London. These tests have identified a required flocculation time of 15 minutes prior to counter-current flotation, and insensitivity to the depth of the air injection below top water level. Dissolved air distribution is achieved using a special high volume flow rate DAF nozzle designed to lower the number of nozzles required per unit area, and to maximise the spread of the bubble cloud for optimum bubble/particle contact. Computational fluid dynamics (CFD) has been used in the scale-up of the pilot plant experience into the first full scale of this design plant to be built, by PWT Projects, at the 200 Ml/d Walton AWTW, for Thames Water Utilities.

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.

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There is increasing interest in treating recovered spent filter backwash water in the drinking water industry. In the USA the Filter Backwash Recycling Rule will come into effect in the near future. The purpose of the Rule is to prevent the concentrated pathogenic agents, potentially in the filter backwash water, from being returned to the head of the water treatment works without some form of treatment or dilution. By treating this flow both public health and financial liability can be better managed by the operating utility. Dissolved Air Flotation (DAF) was investigated as a possible technology alternative to simple or advanced sedimentation techniques. This application is not widespread but sits somewhere in between the two normal applications of DAF as a high solids sludge thickener and a low turbidity clarification system. Given this a pilot plant program, supported by jar testing, was undertaken to determine the process capability and the design parameters for this application. DAF proved to be very suitable for backwash water recovery. DAF effluent turbidities of < 1.0 NTU could be easily obtained, when raw water turbidities were in excess of 50 NTU. Chemical requirements were low with only a single low dose of polymer required to bind the floc particles to form a solids matrix suitable for flotation. Flocculation contact times ranged from 0-10 minutes depending on the nature of the raw water. Recycle rates as low as 5% performed satisfactorily with no significant improvement when increased to 20%. Sludge solids of 3.5-9.6% dry solids were found and very low volumes of sludge, < 0.1% of the incoming flow make the DAF solids handling system very compact.