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 which is introduced into the recycle stream the ensuing effluent/air mix is referred to as "White Water". The vast number of very fine bubbles created during the DAF process 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. DAF process has been shown to 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.

The process is also common in the Oil and Petrochemical industry, and  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 the Sand Filtration process is that the sand can easily be backwashed and scoured to allow effective and efficient cleaning of the filtration medium.

The sand used for this process 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 and clarification are settlement based processes which makes 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. The optimal design of the clarifier is generally of a circular shape.

The treated effluent passes over the circumferential wall while the sludge is directed to the base of the clarifier, typically conical in shape to aid sludge collection and removal. The sludge itself is 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 depends on the nature of the effluent to be treated. A simple rule is that the process should aid 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, therefor sedimentation clarifiers should be considered in this instance.