Properly maintained sand
filters do a fine job of keeping swimming pool water free of much
of the undesirable bits and pieces that become suspended in it
during normal use.
But what if you could make the filter do a better job, improving
its performance in ways that could even contribute to better
sanitation and hygiene in the pool?
In the laboratory, we were looking for ways to link together
tiny, suspended particles into larger, stable aggregates that would
have difficulty squeezing through the pore tunnels in depth
filters. In other words, we wanted to give the filter the ability
to trap more.
If we could enable filter media to remove particles that were in
the submicron size range, we stood a good chance of being able to
remove microbial organisms as well. The most troublesome of those
measure only a few microns, or even less, and therefore routinely
pass right through filters with the flow of water. While proper
pool sanitation can be counted on to deactivate most
disease-causing agents, there are some, such as
cryptosporidium, that show little sensitivity to
Being able to remove these parasites by enhanced filtration
would be useful in reducing the chances of recreational water
illnesses. Thus, we designed experiments to find the best method of
flocculation. (For a closer look at the chart above that shows how
flocculation treatment causes particles to increase in size, click
To enhance filtration, we used characteristics of naturally
occurring biopolymers — already widely used for clarification
of pools — because they showed a superior capacity to
flocculate (or form larger clumps) and sediment out suspensions of
particles in water.
We needed a formulation with the right molecular characteristics
to cause the combination of aggregation and stability necessary to
retain clumped particles in depth filter media such as sand. We set
our sights high, targeting the removal of submicron-sized
particles. The pore paths in sand beds are typically 50 microns and
up, so we had to generate clumps this large to be successful.
It was not easy. Molecular weight, shape, charge, and absolute
and relative concentrations of the biopolymers, plus timing of
exposure, all turned out to be critical factors.
But data from laboratory experiments showed that with the proper
polymer treatment, inert particles (for our purposes, bentonite
clay) 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 and inert particle types
showed similar success, even to the extent that suspensions of live
cryptosporidium cysts could be taken out at a rate of 99.9
percent in a single pass through sand.
Accomplishing this high level of filtration required sequential
treatment of pool water with two differently acting biopolymers
— one charged positively, the other negatively. But in the
right proportions and concentrations, this could be achieved
reliably and repeatedly.
Here’s how it works: Polymer molecules from stage 1 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 doesn’t happen.
Stage 2 polymers (negatively charged) then entangle the
complexes, firming them up so they can withstand being trapped in
the filter bed, until the backwash takes them out into the waste
stream. The net effect is that crypto cysts, normally able
to pass through sand bed and other particulate filter media, become
trapped as cyst-polymer complexes and get removed from the
Cryptosporidium is not the only biological agent that
can be trapped in this way; other waterborne microbes
(giardia, 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.
Stages 1 and 2 polymers are completely biodegradable and safe for
bather exposure, and the concentrations required are extremely low
— in the 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.