Mixing Energy Matters

Your jar testing efforts are centered around finding the right chemistry to solve the wastewater issues in your facility. You may be missing a critical component to this process if you are not considering the mixing energy being applied in the plant and in your testing.

How much do you need?

In general, you really can't over mix coagulant additions and pH adjustment chemistry (foaming challenges aside). These parts of your process are not attempting to build any significant flocculation particle and usually the more the merrier.

Once you have pH dialed in and the coagulation process going (charge neutralization is complete and pin flocc is forming), you typically will be adding some form of polymer to really build the flocc particle into something you can easily sink (clarifiers) or float (DAFs). Here's where energy can really make or break you. Most polymer additions need two energy steps to optimize the process. The first step is a rapid mixing process when the polymer is first introduced. In your plant, this can look like a flash mix tank, a static mixer at the point of injection, a recycle or drive water line or similar. You will have anywhere from a few seconds to a few minutes of time in this rapid mix. After this step you typically have and need a zone of slow gentle mixing to allow the flocc to build. This can be on the order of a few minutes or much longer, depending on flows and equipment size. Once the flocc is formed, reintroducing rapid mixing again can potentially shear the flocc particles apart, and they are often not easily reformed.

So what do you do about it?

Your job in jar testing is to identify the right chemistry and approximately how much time and how aggressively your plant process can mix each step. The good news is, you're not splitting atoms here! You just need to get close and mostly be careful about over mixing the polymer. If you do, you will struggle to find a polymer that "works." You may have tested two or three that would work but beat the flocc particle to death in the jar and it looks too fine when you are done. The other potential to have a miss is assuming you have enough mixing on coagulants and pH adjustments in the plant. Be sure there's adequate time for these chemistries to do their job. As with all things wastewater testing, it's up to you to do the work!!