Flotation processes
Content Table
Section A - Introduction
Flotation is a process that involves producing small air bubbles in the water being treated. The bubbles then attach to floc particles produced by coagulation (Link?) and flocculation (Link?). The resulting bubble-floc agglomerates are buoyant and so float to the surface of the water where they accumulate as a floating layer of sludge, known as float, before being removed.
Section B - Dissolved Air Flotation
The normal method in municipal water treatment of producing the small air bubbles is to recycle a small proportion of the water after treatment by flotation (Image?), or preferably after subsequent filtration, and dissolving air in this recycle stream under pressure (Image?). When the pressurised recycle stream, which is saturated with dissolved air, is released into the water entering the flotation tank, the dissolved air comes out of solution and forms the small air bubbles. As a result of this, the method of flotation is referred to as dissolved air flotation (DAF). There are other methods of achieving flotation but most are not applicable to potable water treatment.
DAF is a method of clarification, and is an alternative to sedimentation (Link?), for removing particles in water prior to filtration (Link?). It has several advantages compared to sedimentation, including:
a. Notably it is more effective for treating waters containing algae.
b. It is more cost effective for the treatment of waters when coagulation results in flocs that settle particularly slowly, including when water is cold.
c. The cost structure of flotation is different from most methods of sedimentation, in that it has lower capital cost but higher operating cost. This means that DAF tends to be cheaper to use when the water treatment plant has low utilization.
d. DAF can have a much smaller footprint than most other methods of clarification. Therefore, it is likely to be the process of choice when weather conditions require the treatment plant to be enclosed by a building.
e. DAF can be placed directly over rapid gravity filters (Link?) without a floor to separate them, so reducing the water treatment plant footprint more. This arrangement of DAF over filtration is generally known as DAFF or DAF/F.
Sub-section B.1 - History
DAF is a relatively modern process, so much so that many water treatment plant operators have yet to realise how appropriate it might be for them. DAF has two basic origins. Firstly, it was a process that was initially developed in Scandinavia in the 1950’s to treat paper manufacturing effluent, but it was then realised in the early 1960’s that DAF would be appropriate for potable water treatment (Ref?). In South Africa DAF was encountered by chance, because of pumps drawing air in through their glands, as a process for removal of algae from recycled treated sewage. Dr Packham from the Water Research Association (WRA), UK, (subsequently known as the Water Research Centre, WRc) visited South Africa in 1969 and realised that such an algal removal process was needed for treating stored eutrophic waters that posed treatment problems because of algae. This led to WRA demonstrating during the early 1970’s the viability of DAF as a more effective process than sedimentation especially in the treatment of algal-rich waters (Ref?). A satisfactory theory for DAF wasn’t developed until the 1990s by Edzwald and others (Ref?). This has helped, during the past 10 years or so, in developing high-rate DAF, which can operate at about twice the surface loading rates of the first generation of DAF plants (i.e. normal DAF).
Sub-section B.2 - Theory
Standard texts should be referred to (for the time being) for the theory of flotation (Refs?). It is relevant to note that development of the theory is based on the initial assumption the particles to be removed from suspension are spherical and solid, whereas in practice they are irregular in shape, exist as a wide range in size and can be permeable to flow of water. The challenge for researchers is to develop the theory to take account of these realities.
Sub-section B.3 - Saturated recycle production
Air can be dissolved in the stream of recycled water by a number of methods. The most efficient method is to pass the water downwards through a vessel, the saturator, containing a proprietary packing whilst releasing air at a slightly greater pressure into the base of the vessel. The air dissolves into the water as it cascades down through the packing and the water saturated with air is taken from the base of the saturator. The pressurised water is fed as required to the flotation tanks. The pressure of the recycle must be sustained until it is released through orifices into the flocculated water entering the flotation tank. Any reduction in pressure between the outlet of the saturator and the release valve or orifice results in premature release of air from solution and a corresponding reduction in efficiency of the recycle stream. Maintaining pressure to the point of release helps ensure the ideal bubble size of about 40-80 µm is achieved.
Typically, the rate of recycle is 8-10% of the flow through the flotation tank, and the pressure in the saturator tank is 400-600 kPa. The amount of air that is needed to maximise flotation is independent (for potable water treatment) of particle concentration. The air dose needs to be about 8-10 g/m3.
A relatively recent development is a special recycle pump that is designed to specifically draw air into it. Depending on circumstances, the saturated recycle can be achieved cost effectively without using a saturator vessel when using this kind of pump.
Sub-section B.4 - Float removal
Float is normally removed by one of three methods. The simplest is to periodically flood the float up over the beach into the sludge channel, but this can result in producing a more dilute sludge than necessary. Sludge is generally removed by a mechanical method. One method is a rotating set of blades that sweeps the float up the beach, and this action draws float from the rest of the tank towards the beach end. Alternatively, a chain-linked set of scraper blades is arranged to sweep along the surface of at least half the length of the flotation tank pushing the float to and up the beach.
The choice of float removal method depends much on the expected quality of the float as governed by raw water quality and the choice of chemical treatment. When fragile flocs are formed then removal of float must be relatively frequent. In these cases flight scrapers are more appropriate. When relatively strong flocs are formed then float removal does not need to be frequent because it is less likely to fall back into the main body of water. In these cases rotating beach brushes can be appropriate.
Sub-section B.5 - DAF tank design
When DAF first became acceptable for water treatment in the 1960’s, it was designed for treatment rates of 5-12 m/h (i.e. surface loadings of 5-12 m3/m2.h). Unlike sedimentation, the treatment rate is relatively unaffected by water temperature. Developments have lead to designs that can reliably operate at 15-25 m/h, and greater. This has been achieved mainly by improving the water flow pattern through the flotation tank by use of baffles and pipes.
Even if a DAF plant is not wholly installed within a building, it is usual to have the flotation tank itself covered, yet fully accessible, so that the float itself is protected from frost, wind and rain or snow.
DAF tanks are usually rectangular, with flow in at one end and out at the other end. DAF can be constructed as circular units but generally are not adopted for municipal water treatment. Particularly suited for DAF over filtration is CoCoDAF® (i.e. CoCoDAFF®). In this design, the flocculated water is introduced over the whole area of the tank (not just at one end) with the saturated recycle introduced below it (Image?). The water then flows downwards through the rising cloud of bubbles. Thus, CoCoDAF® resembles floc blanket sedimentation whilst normal DAF resembles horizontal-flow sedimentation.
Section C - Electrolytic Flotation
Section D - Dispersed Air Flotation
Section E - ??
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References
Gregory R. and Edzwald J. (2010) Sedimentation and Flotation, Chapt.9 in Water Quality & Treatment, 6th Edtn., AWWA & McGrawHill.
Haarhoff, J. (2008) “Dissolved air flotation: progress and prospects for drinking water treatment”, Jour. Water Supply: Research and Technology.- Aqua, vol.57, no.8, pp.555-567.
Edzwald J.K. and Haarhof J. (2012) Dissolved Air Flotatin for Water Clarification, AWWA & McGrawHill.
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