Avoiding recreational water illnesses

How does enhanced filtration work?

First, it is important to understand the complicated inner workings of a depth filter (e.g. sand bed), as it traps particulates suspended in the incoming water. For example, there are lots of influences on surface charge, particle size, conformation, rigidity, density, etc., on the nature of the path taken by the particles, and the frequency of their collisions and interactions with the filter medium.

The outcome is simple to understand; smaller particles tend to make it through the filter bed and come out in the effluent, whereas larger particles stand a greater chance of getting trapped en route. All of the retained particles alter the total amount of tortuous path available within the filter bed for the next influx of water. The pathway is restored for future entrapment after periodical backwashing of the filter medium. Properly maintained depth filters do a fine job of keeping swimming pool water free of debris, which become suspended in the water during normal use of the pool.

In terms of enhanced filtration, molecular weight, shape, charge, and absolute and relative concentrations of the biopolymers, plus timing of exposure, are all critical factors. In fact, data from laboratory experiments showed with the proper polymer treatment, inert particles (bentonite clay was used for the purpose of the study) could be flocculated into larger clumps, enabling many of the resulting aggregates to be removed in a single filter bed pass. Later experiments involving biological as well as other inert particle types showed similar success, even to the extent that suspensions of live Crypto cysts could be taken out at a rate of 99.9 per cent in a single pass through sand.

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

Positively charged polymer molecules (stage one) alter surface charges on small particles in water, destabilizing the normal tendency they have to repel one another (and therefore, keep separate and fully suspended, indefinitely). The particles aggregate and become enmeshed in the lattice of long, cross-linked polymer molecules to form much bigger clumps. If the concentration of the cationic (positively charged) polymer is too low or high, this does not happen.

Negatively charged polymers (stage two) then entangle the complexes, firming them up so they can withstand being trapped in the filter bed until the filter is backwashed, taking them into the waste stream. The net effect is Crypto cysts, which are normally capable of passing through sand bed and other particulate filter media, become trapped as cyst-polymer complexes and get removed from the circulating water.

Crypto is not the only biological agent that can be trapped in this manner; other waterborne microbes (e.g. Giardia and E. coli), as well as algae, are similarly affected. Polymer additions for large pools can be accomplished by controlled metering, but a manual process, properly timed, also is entirely practical for smaller scale operations.

Stage one and two polymers are completely biodegradable and safe for bather exposure. The concentrations required are also extremely low (in the parts per billion [ppb] range). With particle removal possible at the submicron level, overall water clarity also improves. Thus, giving sand filters the ability to trap such small particles offers operators a new way to fight RWIs and improve water quality.

Dave Callahan is the commericial aquatics manager for Halosource Inc., the parent company of the SeaKlear® brand of water treatment products based in Bothell, Wash. He has several years’ experience in the commercial sector, and previously managed the company’s sales in the mid-west U.S. He can be reached via e-mail at dcallahan@seaklear.com.