Waste Water Treatment Coagulation and Flocculation – Static Mixers vs Dynamic Mixers

In waste water treatment, the term Dynamic Mixer means any mixer with a moving part, such as an impeller. This is usually a square tank with a center mounted mixer motor, shaft and impeller. Where Static Mixers are usually several sections of pipe with a convoluted insert that causes the water to change direction over and over again to achieve mixing. The mixing energy comes from the flow rate through the pipe.

When adding coagulants and flocculants for waste water treatment, dynamic mixers work well for both chemicals. However static mixers work best for coagulants, but not for flocculants.

Flocculants are polymers with high molecular weight that must be diluted and mixed with clean water prior to use. Flocculants require time to unwind and become useful in pulling the coagulated particles together to form a floc. Most polymers require 15 to 20 minutes of mixing time with clean water before being added to the waste water reactor tank.

Static mixers have a very short period of mixing time. Mixing time is simply the time it takes to flow through the static mixer pipe. This is usually less than a minute. Turning the flow rate up on a static mixer to increase the mixing energy is counterproductive because you shorten the mixing time. This results in unmixed globs of polymer in the reactor tank and over consumption of the polymer. You can often observe the white unmixed globs in the reactor tank. Many companies still use static mixers but In our experience switching to a dynamic mixer can save around 40 % in chemical costs.

Coagulants are inorganic metal salts with high electrical charge such as ferric chloride or Aluminum Sulfate. Coagulants mix easily and quickly with water. In this case a static mixer will work. However, a dynamic mixer is the preferred choice for most mixing.