Physico-Chemical Treatment Processes

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Physico – chemical separation is a general term covering a number of solid liquid separation processes, including the following:

Dissolved Air Flotation (DAF)

DAF is an effluent treatment process which utilises air bubbles to enhance the natural buoyancy of certain solids to allow separation by flotation. A proportion of the treated effluent is recycled with compressed air also introduced into the recycle stream – the ensuing effluent/air mix being referred to as ‘white water’. The vast number of very fine bubbles creates a large and buoyant surface area to enhance the flotation process.

The DAF process is useful as a separation process in its own right but is also very effective in reducing the organic loading (measured as COD or BOD) onto any downstream biological process – aerobic or AD. It can also be seen that DAF will be most effective in the removal of insoluble (ie particulate) COD. Chemically enhanced DAF (see below) may also assist in the removal of a certain quantity of dissolved or soluble COD.

The DAF process is common in food and drink processing due to the possible inclusion within the effluent of high levels of Fats, Oils and Greases (FOG) which may be prevalent.

For the same reason it is also common in the Oil and Petrochemical industry, and also in the Paper industry whereby the voluminous and fibrous solids within the effluent are amenable to flotation due to entrapment of the air bubbles within the particulate material.

Sand Filtration

This process has historically been the main method for the removal of suspended solids from an effluent stream, and is still in common usage today. It is also frequently used in removal of solids from drinking water supplies.

The advantage of this process is that the sand can easily be backwashed and scoured to allow effective and efficient cleaning of the filtration medium. 

The sand is generally of different grades depending upon the nature of the material requiring removal. It can also be enhanced with larger particulate material, such as anthracite, which allows stratification of the overall filtration media. The stratification is maintained after back-washing and scouring due to the variable buoyancy of the different materials being utilised as media.

Sedimentation/Clarification

Sedimentation and clarification are settlement based processes which again make use of the higher density of material to be removed from the water or effluent stream. The influent is usually introduced via a central ‘stilling’ chamber which allows dissipation of the hydraulic energy before directing the effluent to the perimeter of the clarification vessel. For this reason, optimal design of the clarifier is generally of a circular shape.

The treated effluent then passes over the circumferential wall while the sludge is directed to the base of the clarifier – again typically conical in shape to aid sludge collection and removal. The sludge itself is generally ‘helped’ to the base of the clarifier by bottom based sludge scrapers, attached to and driven by a bridge arrangement on the surface of the clarifier.


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Physico-Chemical Treatment Processes FAQ's

This very much depends on the nature of the effluent to be treated, but a simple rule is that it should help the effluent do what it is trying to achieve naturally. For example, the Dissolved Air Flotation (DAF) Cell is very common in the food industry, particularly the dairy industry, where the fatty effluents tend to float anyway. The same situation occurs in the oil and petrochemical industry. Conversely, metal plating and other heavy industries produce an effluent that is generally trying to settle, and hence sedimentation clarifiers should be considered in this instance.