Municipal and Industrial Water Treatment Plant process: Flash Mixing and Flocculation

What is Wastewater Treatment?

Municipal and industrial wastewater treatment is a complex process that makes wastewater suitable to be released into the public sewers or the environment without causing damage. It goes through several phases of mechanical, physical and chemical nature. The physical-chemical processes of flash mixing and flocculation are two steps of this process.

In order for the treatment process to lead to a high level of water quality, high-quality mixing solutions must be used. This is why PRO-DO-MIX have developed a complete range of vertical agitators covering all water treatment requirements, including flash mixing and flocculation. One of our latest installation has been engineered and manufactured for Sulaibiya Wastewater Treatment and Reclamation Plant in Kuwait City, for which we have supplied flash mixing and flocculation agitators. Installed in 2018, they have successfully worked since February 2019.

Flash Mixing and Flocculation

Flash mixing and flocculation are essential in the treatment of industrial and municipal wastewater, which contains colloidal-type suspension materials, i.e. particles that are too small or light to be able to settle and be eliminated. To allow their sedimentation and agglomeration in larger particles, flash mixing treatments and flocculation are fundamental. But what is it about? Let's see both in detail.

Flash Mixing in Water Treatment

Flash or rapid mixing stage consists in adding chemicals to raw water to foster coagulation, attracting particles which do not settle or are not filtered. Attracted by coagulants, such as ferric chloride (FeCl3), PAC (poly aluminium chloride Al2Cl(OH)5), aluminium sulphate (Al2(SO4)3), these particles are then easy to agglomerate during the flocculation stage.

The duration of the flash mixing stage is a key factor and determines the quality of the whole process: too short retention time will not ensure a homogeneous distribution of the chemicals within the basin volume, resulting in an unsuccessful reaction. On the other hand, too long retention time will lead to coagulant overdosing, preventing the process efficiency. Therefore, it is essential to carefully calculate the residence time taking into consideration several parameters including the reactor volume and the water flow rate.

Vertical agitators are the only possible solution at this stage (as well as during the flocculation stage), which cannot be fulfilled with other mixing solutions e.g. submersible mixers. Our vertical agitators, in particular, are conceived to have an efficient flash mixing phase, thanks to a quick and thorough agitation of the chemicals in raw water before being released into the flocculation chamber. Our high-efficiency impellers are perfect to accomplish flash mixing due to their design: the models 3PM-0134 and 3PM-0030 are the optimal solution to provide best rapid mixing performance, due to their highly hydrodynamical profile that creates a high flow rate while ensuring maximum efficiency with low installed power.

What is Flocculation in Water Treatment?

Flocculation is the phase that occurs after flash mixing. During this phase, the particles to be removed from water are brought to larger agglomerations, called flocs, in order to settle down in the sedimentation chamber and be easily filtered. This happens thanks to the reaction with a polymer solution, which acts as flocculant agent.

Flocculation agitators need to be engineered very carefully to ensure an effective flocculation process, which results in minimum time, energy and chemical consumption and, of course, high quality water as well as increased dry content during dewatering. Same as in flash mixing, the flocculation agitator design is critical: flocculation needs an effective and homogeneous mixing to encourage the flocs contact within the whole basin volume, while ensuring that the agitation is not too vigorous, since that would prevent the flocs agglomeration and break the newly formed flocs. That is why it is crucial to maximise the impeller flow rate while minimizing the shear.

PRO-DO-MIX 2PM-0650 impeller is specifically engineered for flocculation mixing, where flocs are highly shear-sensitive and smooth flow is needed. The high flow rate design makes sure that the mixing energy reaches the complete water volume with no dead spots, including the bottom and the corners of the tank, keeping all flocs in suspension; the wide blade surface and curved design allow a gentle yet effective blending, enhancing the flocs agglomeration.

Combined with our flocculation agitator, variable speed drive is recommended to control the mixing speed during the process, so that the operator can check the process and change the output speed according to the process needs, optimising the mixing performance.

High Efficiency Flocculation Impeller VS Gate Type Agitator

Gate type agitator
Gate type agitator
High efficiency flocculation impeller
High efficiency flocculation impeller

Our high efficiency impellers are the perfect solution to carry out a correct and efficient flocculation process and have several advantages especially if compared to gate design impeller, which have typically been installed in old Municipal WWTPs and used to be considered suitable to flocculation due to their slow speed and low shear. However, we would like to explain why flocculation agitator with PRO-DO-MIX 2PM-0650 Premium impeller should be preferred in order to ensure a correct and efficient flocculation process.

First of all, gate impeller's obsolete and inefficient design results in a high installed power, due to its high energy absorption. Furthermore, its heavy construction with multiple, robust paddles makes installation operations difficult. Also, that typically causes severe mechanical concern due to an excessive load on the agitator drive unit, resulting in frequent drive unit maintenance or replacement. Moreover, gate type agitator does not allow flow rate or peripheral speed control, nor proper contact between flocs and sludge, being the mixing a simple, massive circular motion.

On the contrary, 2PM-0650 impeller high efficiency design, combined with its light mechanical construction, enables to considerably reduce the installed power due to its low power absorption, as well as to ease installation operations and minimise mechanical stress, resulting in no maintenance actions or plant breakdown. From a process point of view, our Premium impeller ensures high flow rate at low peripheral speed, thanks to its highly hydrodynamical profile with wide blade surface, which ensures low shear, meaning no floc breakup. Finally, our high efficiency flocculation impeller creates a smooth bottom-up flow, fostering the floc agglomeration and suspension, resulting in a substantial improvement of the flocculation process.

Vertical Agitators for all Water Treatment Needs

PRO-DO-MIX can support you in planning or optimising your Municipal or Industrial Wastewater Treatment Plant. Our state of the art high-efficiency impellers range and mechanical and process expertise are integrated into our proprietary selection software A.P.P.L.E., available online at, that includes flash mixing and flocculation agitators and can be used whenever designing a WWTP.

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