Of course, all those retained particles alter the total amount of path available within the filter bed for the next influx of water. On the other hand, the filters should be periodically backwashed, which will restore the pathway for future entrapment. In this way, properly maintained depth filters do a fine job of keeping pool water free of much of the undesirable bits and pieces that become suspended in the media during normal use.
Giving pool filters the ability to trap more is the idea behind enhanced filtration products. Using characteristics of unique biopolymers has been the solution—already widely used for clarification of pools—because they showed a superior capacity to flocculate (or form larger clumps) and sediment suspensions of particles in water. The pore paths in sand beds are typically 50 microns and up, so the enhanced filtration products must generate clumps this large to be successful.
Enabling filter media to remove particles that were in the submicron range allows for the removal of microbial organisms. The most troublesome of those measure only a few microns or even less, and therefore routinely pass through filters with the flow of water.
Molecular weight, shape, charge, as well as absolute and relative concentrations of the biopolymers, plus timing of exposure, all turned out to be critical factors. Data from laboratory experiments conducted at Auburn University showed with the proper polymer treatment, inert particles (in this case, bentonite clay) (see figure 2) 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 suspensions of live Crypto cysts could be removed 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. Using the right proportions and concentrations, this could be achieved reliably and repeatedly.
Here is how it works
Polymer molecules from stage one alter surface charges on small particles in the 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 will not occur. Stage two polymers (negatively charged) then entangle the complexes, firming them up so they can withstand being trapped in the filter bed, until the filter is backwashed, removing them to waste.
The net effect is Crypto cysts, normally able to pass through sand bed and other particulate filter media, become trapped as cyst-polymer complexes and are removed. Crypto is not the only biological agent that can be trapped in this manner; other waterborne microbes (e.g. Giardia, E. coli) are similarly affected.
Stage one and two polymers can be used while bathers are present. With particle removal possible at the submicron level, overall water clarity also improves. This gives sand filters the ability to trap minute particles, and offers aquatic facility managers a new way to fight RWIs while improving water clarity. (see Figure 3)
Polymer additions for large pool applications can be accomplished by controlled metering, but a properly timed manual process is also entirely practical for smaller scale operations.
