By Srikanth Pathapati, Ph.D.
Mixing is something that is often taken for granted when designing systems for water and wastewater treatment. Perhaps “taken for granted” is too harsh a term. Let’s instead say that while designing a treatment unit operation or process, mixing as a phenomenon is automatically assumed to occur – an assumption that forms the basis for process controls, performance guarantees and measurement methods and locations.
In this article, we will examine where this assumption is generally true, where it might not be true, and what the ramifications are when mixing is not achieved as expected.
Water and wastewater treatment processes are typically designed from a macro, civil engineering (hydraulics, construction) and aesthetic point of view while the actual physical/chemical processes depend on local and often micro-scale flow patterns. While millions of dollars are spent on a treatment system, the inherent errors associated with grab sampling can be easily overlooked. Now, consider that even the grab sample that we are looking at is often an aliquot of an aliquot. Add to that the issue of where the sample is obtained.