Counter-current dissolved air flotation/filtration

Abstract

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.

Effect of Ozone on the Formation and Breakup of Flocs in Raw Waters with High Algae Content

“…The autumn bloom is predominantly composed of filter-blocking algae, such as Melosira sp. [17,18] The basic raw water quality characteristics such as turbidity, pH, temperature, particulate organic carbon (POC) and Melosira sp count are summarized in Table 1.…”

Eutrophic lake water treatment using a diatomite porous ceramic membrane

“…Color, odor, algal toxins, and other harmful substances are produced during the metabolic process of algae [1]. Several processes, including enhanced coagulation, ozone oxidation, flotation, filtration, and other counter-current flotation are often used to remove algae in production [2,3]. But the conventional processes, particularly the chemical methods, will facilitate the release of organic matter from algal cells into water, thus reducing water security, and increasing the difficulty of subsequent processing [4].…”

Improvement of Operational Mode of Counter-Current Dissolved Air Flotation Process

“…Being a new type of flotation technology, countercurrent dissolved air flotation (CCDAF) can overcome some limitations of the traditional co-current DAF [5,6]. The main difference between the two technologies is that in conventional DAF the air and water rise together in the inlet channel and from then on they are free to find their own concentration levels and bubble/floc associations [5].…”

“…The main difference between the two technologies is that in conventional DAF the air and water rise together in the inlet channel and from then on they are free to find their own concentration levels and bubble/floc associations [5]. In the CCDAF the flocculated water enters the flotation tank from the top, and the dissolved-air water is introduced from the releaser installed at a fixed depth below the top of tank.…”