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Understanding the physics of flocculation

By Dr. Jeff Williams and Nathan Kenney

Figure 3

If the physics of particle removal by filters seem unexpectedly complicated, the factors involved in flocculation of suspended particles and their aggregation into stable clumps, firm enough to withstand filtration pressures, are even more complex.

Molecular weight, shape, charge, and absolute and relative concentrations of the biopolymers, plus timing of exposure, are all critical factors affecting the ability of flocculant treatments to bring about particle removal by filters.

By using the proper polymer treatment, laboratory experiments showed the population size profile of inert particles (e.g. bentonite, shown in the Figure above) can be shifted to the right, enabling many of the resulting aggregates to be removed from the pool water—via the filter—in a single turnover. Further experiments involving biological as well as other inert particle types showed similar success, even to the extent that suspensions of live Cryptosporidium (Crypto) oocysts had a 99.9 per cent removal rate in a single pass through sand.

Accomplishing this required sequential treatment of pool water with two differently acting biopolymers—one negatively charged and the other positively charged—but in the right proportions, and at the right concentrations, these results can be achieved reliably and repeatedly.

Dr. Jeff Williams is the chief technology officer and senior vice-president of R & D for Halosource Inc., a clean water technology company based in Bothell, Wash. He is an emeritus professor of microbiology and molecular genetics at Michigan State University, where he was a teacher and biomedical researcher for almost 30 years before founding Halosource in 1998. Williams can be reached via e-mail at

Nathan Kenney is a water microbiology specialist for Halosource’s product development division. He joined the company in 2012 and is involved in development and testing. He received his bachelor of science (B.Sc.) in microbiology from the University of Washington. He can be reached via e-mail at

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