Articles & Case Studies

Flash mixing and flocculation - Meeting today's challenges

Posted: Wednesday 14th June 2006

Flash mixing and flocculation commonly occur in water treatment applications where the main objective of the process is to separate and decant solid particles.

However, from a hydrodynamic standpoint, effective flash mixing and flocculation is relatively complex to achieve. For example, effective neutralization of charged particles and the promotion of ‘floc’ requires the correct combination of velocity gradient, liquid velocity and energy dissipation.

Clearly, the optimum technical and commercial mixer selection requires liquid, vessel and mixer elements to be considered. Also, another complication is that today tanks and vessels tend to be deeper than they would have traditionally been because of increasing demands on space and other site limitations.

This results in the need for mixers to be fitted with a number of impellors up the shaft to ensure adequate movement of liquid throughout the tank, as opposed to a mixer needing only one impellor in a conventional tank layout.

Due to the relatively short contact time in flash mixing applications, both high shear and a pumping action are required, with the shear related to the amount of energy dissipated into the mixture expressed as the ‘G’ factor.

In this case it is necessary to consider the impellor shape, diameter and speed in order to achieve the best compromise of flow and energy dissipation. Clearly, it is critical that during the short contact time complete mixing, as well as sufficient energy input to allow change of particle charge takes place and specific types of impellor have been developed for this purpose.

For example, Milton Roy Mixing offers a new design of impellor on its VRG mixer range that provides a good balance between flow-rate and energy dissipation. Flocculation is another application that requires careful control of the energy put into the process, sufficient to ensure that the whole contents are mixed, whilst not breaking up the flocs that are formed.

Mixing a liquid slowly requires large diameter impellors that displace the whole contents of the tank on each revolution without allowing any ‘dead’ zones to form. Milton Roy Mixing offer a specific range of mixers (FRF) that are fitted with 2 bladed impellors. The turbulence generated by the first blade does not affect the stream of liquid that meets the second blade and this allows better control of the amount of shear.

An added benefit is that it is easier to introduce a large diameter impellor (up to 4 metres) with 2 blades into a tank, than a 3 bladed impellor. To fine tune the control of the process a variable speed mixer can be provided.

Sludge, as a product of flocculation / decantation, requires conditioning. The rheology of the mixture is particularly complex and variable and requires a specific study to establish a customised mixer design. Again, for each sludge application Milton Roy Mixing offers a specific design from their HM range.

Finally, sludge digestion process also requires careful mixer specification taking into account fluid behavior, tank volume and the amount of methane released during anaerobic fermentation. Milton Roy Mixing has also supplied mixers after considering other parameters such as homogeneity time, biphase mass (gas-liquid), whether a surface crust will form and the ATEX zone for hazardous areas.

Contact Details

Milton Roy Mixing UK
Oaklands Business Centre
Oaklands Park
RG41 2FD

Tel - 0118 977 1066
Fax - 0118 977 1198

Email -
Web -

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