How filters and chemicals work together to keep the pool water clean

Scott Webb Headshot

Clump0909The vision that forms in the head of every pool owner when swimming season is mentioned is that of sparkling water - beams of sunlight reflecting off its surface and passing unhindered to the bottom and back as if through fine crystal.

They yearn for water that not only reflects and transmits light like a pane of clean glass, but that is free of bacteria and germs that cause illness.

And that means getting the stuff swirling around in suspension out of the pool. It's a more involved process than one might imagine.

Straining is the most familiar means - where particles stick in the filter like berries in a colander. But there's also hydrostatic pressure, where the force of flowing water presses particles against filter media, and electrostatic forces hold particles against the filter through the attraction of opposite charge.

For that matter, particles can merely drop out of suspension in the filter, or they may just stick to the slime that tends to coat filter surfaces.

These are just some of the ways that a filter can gather up unwanted debris from a pool and hold it until it can be cleaned and discarded. But the effectiveness depends on size.

Regardless of media type, all filters are great at straining out large particles. Sand filters are capable of grabbing anything down to about 25 microns in diameter, while cartridge filters can catch particles that are 15 microns across.

DE filters, known for the clarity they bring to turbid pool water, can filter out 4-micron particles. (By comparison, a relatively fine human hair is about 17 microns in diameter.

Unfortunately, many of the particles that annoy or endanger swimmers are too small to be snatched out by an unaided sand filter, and some of them can even slip past DE.

These include the fine dust that makes water dull and unattractive, says John Puetz, vice president of research and development for Advantis Technologies, Alpharetta, Ga., a subsidiary of Arch Chemicals. These tiny particles might not seem to be a big deal, but as time goes on they will build in number because they continue to avoid filtration.

If this situation is left unchecked, over time, more and more particles will populate the pool, and the water will take on a hazy or cloudy appearance.

The proliferation of underwater lighting in recent decades has brought the problem into focus, so to speak, because this haze and turbidity is most apparent in a pool when the light is switched on at night. And customers, ever more finicky about the appearance of their pools, are quick to notice.

"You can have what appears to be a beautiful clear pool in the day," Puetz points out, "but when you turn on that bright pool light at night, it can reveal a kind of hazy look to the water. And you think, 'What's going on?'

"It's a similar effect to turning a headlight on in a light fog. The fog is hardly noticeable at all until you turn the headlight on and you see these refracted beams of light, which tells you, 'Hey, that light is bouncing off of something.'"

If indeed light is bouncing off of something dirty before it hits the bottom of the pool, it's always a good idea to check to be sure the filter isn't the problem before money is spent on chemicals.

"Neither a clarifier nor a flocculant should ever be used to overcome a damaged filter," Puetz says. "If the filter is not functioning properly and that's giving the pool a haze, that needs to be taken care of first. Could a clarifier improve it a little bit? Yes. But will it solve it? No."

Attractive Cats

Indeed, solving the problem - assuming the filter checks out - means overcoming the dirt particle's solitary nature and enticing it into a coagulated mass for filtration. Dirt particles dislike and avoid each other because they all have the same, negative charge, and of course, like charges repel.

Instead, they are attracted to the positively charged, cationic polymer molecules in clarifiers and flocculants. ("Cationic" means positively charged, and a "polymer" is a molecule composed of a repeating series of identical, smaller segments.)

The majority of clarifiers and flocculants are just that - long chains of repeating, positively charged segments, just like a ladder is made up of repeating, identical rungs.

Some of these "ladders" are longer and some are shorter, but they all contain positively charged rungs or niches where negatively charged particles of dirt and debris will nestle in and be held, forming an ever larger, more massive structure.

"Now you have this large molecule with all these small particles of trash clinging to each positively charged rung," says Puetz, "which forms a bigger particle. And that bigger particle is easily trapped by the filter.

"Another nice thing about this big particle besides its size is the fact that it is inert. All the positive sites along the molecule have negatively charged particles filling them, so there are no charges associated with it - the next time the pool owner gets out his garden hose and flushes out the cartridge or backwashes the DE or sand filter, these particles just wash right out instead of clinging to the filter media."

Or, in the case of a flocculant, instead of being removed from the filter, the heavy coagulated masses will be vacuumed to waste at the bottom of the pool. That process should be done with care, however, cautions Leanne Levy, technical service manager, BioGuard Pool and Spa Products, Lawrenceville, Ga.

"The key there is to vacuum slowly," she says, "because otherwise you're just going to stir the pot again, so to speak, and all that material will swirl back up. Then you'll have to wait until it settles back down and try again.

"And remember that some of the flocs must be predissolved before adding them. If you don't do that, it's just going to float on top."

Going After Germs

While most clarifiers and flocculants focus on debris, one company has recently devoted development effort toward a product that helps rid the pool of germs like Crypto, Giardia and E. Coli.

Crypto, the best known of these, is one of the most common waterborne diseases in the world, passed through the feces of infected swimmers to other bathers. It is a particular problem, notes Terry Arko, SeaKlear, Bothell, Wash., because it's chlorine resistant and can pass through some filtration systems.

"Even though it's large for a germ, 4 to 6 microns, it can still get through sand filters, which make up a significant percentage of the market," he says.

The desire to deal with Crypto was the genesis of the development effort at SeaKlear to come up with a way of using a two-stage chemical approach to clearing a pool of germs. "That's what we looked at in developing this product," Arko says, "a way to make something like Crypto larger and filterable in a sand filter."

The system, based on technology in use in the water-treatment industry, uses two polymers of opposite charge. "When you add one," Arko says, "you collect all the positively charged material, and when you add the other, you pick up all the [negatively charged material], and then both of those materials are attracted to each other, and they end up attaching and becoming an even bigger particle."

It doesn't create heavy flocs, but ones that are neutrally buoyant - they neither sink nor float. Instead, they are sucked up by the filter. The system requires two doses and an interval between - either a dose in the morning and in the afternoon, or a return to the pool the next day, according to Jack Hulse, president of Jack's Pool Service, El Dorado Hills, Calif., who has used the product with good success.

Hulse originally used it on some of his commercial pool clients due to its effectiveness against Crypto, but has since transitioned many of his residential customers to it due to the clarifying effect on the water. Clarifying being the point of the exercise. Unless, of course, one is flocculating.

What's the difference? "The only difference between a clarifier and a flocculant," says Puetz, "is the flocculant is capable of picking up bigger masses."

The chemistry is the same, he adds, but the flocculant will create heavy masses that settle to the bottom of the pool, where a clarifier's coagulated masses are aimed at the filter.

"You would use a flocculant to clean up an extremely cloudy pool in relatively short order without putting stress on the filter. A clarifier on the other hand, is the ideal routine-maintenance chemical for adding sparkle and clarity to the water. It's one of those chemicals that can give your customer the best-looking water possible."

Comments or thoughts on this article? Please e-mail [email protected].

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