Application of Mixer Technology in Water and Wastewater Treatment

The application of mixers in water and wastewater treatment is an important field in fluid mixing. The following diagram illustrates the different applications of mixers in

these two processes, involving operations such as rapid mixing, flocculation, chemical preparation and storage, neutralization, homogenization,

anaerobic mixing, and sludge blending.

I. Rapid Mixing

  The function of a rapid mixer is to uniformly blend the added chemicals with the raw water within the retention time. From a mixing perspective,

the rapid mixing process is a flow-controlled process. Therefore, selecting medium to high-flow axial-flow impellers is most appropriate.

  For a long time, a traditional design method for rapid mixers has been based on the G-value or GT-value. This concept was proposed by T.R. Camp in the forties and fifties,

with the G-value correlation expressed as:

  The GT-value is the product of the G-value and the average retention time (T). Both the G-value and GT-value only consider the volume of the rapid mixing tank, water

treatment capacity, mixer shaft power, and water viscosity, without accounting for the efficiency of the impeller itself.

  The G-value originated from inefficient (low-flow) radial-flow impellers. If efficient axial-flow impellers are selected while still using the traditional G-value or GT-value for design,

the selected mixer's power and the discharge flow generated by the impeller will far exceed the mixing requirements of the rapid mixing process itself.

Therefore, with the advancement of mixing technology, the G-value and GT-value are now only used as reference values for mixer selection and design.

  The design method for efficient axial-flow impeller rapid mixers is based on the mixer's discharge flow, using the ratio (N) of the discharge flow generated by the impeller during the average retention time to the effective volume of the rapid mixing tank as the basis for selecting and designing rapid mixers. Due to factors such as chemical dosage, mixing tank geometry, and other influences, this ratio is not fixed but generally requires n>1.5. For the same impeller achieving the same mixing effect (i.e., the same ratio n), the mixing power will decrease as the D/T (where T is the tank diameter or equivalent diameter) value increases. In other words, there is not just one suitable mixer for a mixing process; multiple combinations of mixing power, rotational speed, and impeller diameter are possible.

II. Flocculation

  In the flocculation process, the role of mixing has two aspects: first, mixing to promote collisions between fine particles, allowing them to gradually grow; second, solid suspension to ensure that the aggregated particles are uniformly suspended in the liquid, preventing sedimentation in the flocculation tank. Additionally, an important constraint in the flocculation mixing process is the shear stress generated by the shear rate, which can break the flocculated particles. This is why the limitation of tip speed is often considered in flocculation mixer design. The FRF series of mixers are specifically designed for the flocculation process, featuring very low speed, high discharge flow, and efficient axial-flow impellers. Therefore, the traditional G-value or GT-value is also only used as a reference.