The various filter media can be compared in terms of performance and cost, but which of them is nature's favorite?
It takes a moment of reflection to realize that these three selection considerations - price, performance and environmental impact - are becoming virtual coequals in the minds of consumers.
This is driven in part by a rise in the prices of energy and resources, but also due to the increasing sense of planet stewardship among buyers of pool products. Extrapolating this general shift in priorities through the decades to come, a filter's green credential is likely to become its chief selling point.
Thus far, the green movement in filters has focused on energy and water conservation - energy being perhaps the chief concern as it involves not only the direct cost to the consumer, but a host of environmental issues, as well. For every kilowatt-hour of energy saved, air pollution is reduced along with the need to build new power plants and import fuel from foreign countries.
Filters "consume" energy by restricting flow and raising total head for the system. The greater the total head, the larger the pump needed - and the pool pump is by far the biggest energy hog in the backyard, second only to the air conditioner for the entire residence.
As a general rule, cartridge filters have been winning the energy battle so far. They're less restrictive because they are plumbed directly into the circulation line, and do not require the energy-sapping backwash or multiport valves that sand and DE filters must have.
These valves require extra turns and convolutions in the flowpath, and every time flowing water is forced to change directions, especially at sharp, 90-degree angles, system head rises and more pump pressure is required.
However, manufacturers are focused on making more-efficient backwash valves to close that energy gap. Zodiac Pool Systems has a backwash valve that it claims only loses 1 pound of pressure across the unit, and other manufacturers are said to be poised to follow suit.
Valve losses notwithstanding, there is some concern that sand and DE do not filter as well at low flow rates, according to Scott Clay, senior program manager at Pacific Gas & Electric in San Francisco.
Good performance at low flow is crucial, as slowing flow rates in pool circulation is another important factor in reducing head in the system.
"The efficiency of the cartridge filter goes up quite a bit when you slow the water down," Clay says. "There's a minimum flow that you need to maintain, but anything above that, you are wasting more and more energy. We like the cartridge filters best because of the large filter surface area they have, and they work better at low flow.
"In addition, if you operate at low speed, a cartridge filter is very easy to clean because the debris isn't packed into the pores. You just wash it off and put it right back in."
The cleaning process is central to another argument for cartridge filtration. Sand and DE filters must be backwashed periodically in order to flush the debris, and this action uses a tremendous amount of water. With cartridge filters, the user simply removes the filter elements and hoses them off.
"By our estimates," says Bruce Aubrey, product manager, heating and filtration products, Hayward Pool Products, Elizabeth, N.J., "you can save up to 92 percent of the water used for backwashing when you are just rinsing off a cartridge filter.
"That's thousands of gallons of water saved per season when you are rinsing off cartridge filter elements as opposed to backwashing either sand or DE."
Water conservation is a huge issue in some areas of the country and less important in others, but in either case, wherever water is lost, expensive heat (if the pool is heated) and chemicals are lost right along with it.
That makes two green demerits for sand and DE - valves and backwashing. You can add a third for DE, the fact that the material itself is hazardous to health if not handled properly. And it is banned on new pools in some municipalities for its effect on the water system.
The problem is that the DE material is often flushed down a drain or into a sewer in the backwashing process, and "if DE gets into a local sewage treatment plant, it causes havoc with their processes," says Clay.
The DE cleaning procedure can be done using a separation tank, where the DE material settles to the bottom, the dirty water is poured off and the DE is reclaimed, but, as Aubrey says, "you don't see too many separation tanks on pool pads. A service company might have one, but most residents don't."
It's not all bad press for sand and DE, however.
DE and sand are naturally occurring substances. DE is the fossilized remains of diatoms, a type of hard-shelled algae, and can be collected from the earth; and sand is just sand. No manufacturing process is employed in the production of these basic materials, unlike the elements in a cartridge filter, which must be manufactured for the original equipment and periodic replacement.
As for frequency of replacement, Aubrey says, it depends on the size of the pool and usage. "It can vary, but a lot of the cartridge filters are sized to go through a full season without cleaning the elements, and then you should get two or three seasons before replacing them. Conservatively, you should get at least two years."
Over the life of a pool, that adds up to at least half a dozen manufactured and discarded filter elements, as opposed to sand, which can be scooped up on a beach and safely sprinkled in the backyard.
Further improving the green prospects for sand-type filters is the development of alternative media to replace the sand in filters.
In a sand filter, water from the pool runs through the line and is sprayed into the top of the filter, where it sinks down through the sand and then goes back to the pool. Various granular media substitutes, such as perlite, zeolite and pulverized glass, can be used in the bottom of the filter to strain debris from the water, and according to their proponents, these improve a sand filter's performance and environmental report card.
Josh Peterson, technical sales and logistics, ECOsmarte, Minneapolis, ex-plains the green attraction of glass.
"It's a post-consumer, recycled product that is machined down so it's safe to handle. It's the same kind of glass most people have in their recycling bin - green and brown glass from empty beer bottles and things like that."
As one of the few products in the pool circulation system made entirely from waste materials, recycled glass as a filter medium qualifies as fully and legitimately green. Peterson says it is even easier to maintain than sand due to a virtually indefinite product life.
"After about three years you have to replace your sand because of the clumping and the bacteria that grows in it. As for glass, we put a nine-year life on our glass media, but that's just because that's how long we've had the product. It could be 15 years, we don't know. But if you do need to replace it, you can recycle this glass all over again."
Volcanic In Nature
Another medium that can improve a sand filter's green image is zeolite, which sounds like a trade name but is in fact a type of extremely porous volcanic ash.
Not only does zeolite effectively strain debris mechanically like a normal filter, but it works chemically to remove impurities from pool and spa water, according to Brandon Friesen, vice president of sales, MicroPure, Nisku, Alberta, Canada.
"It filters everything, all metals, all minerals, ammonia, nitrates, phosphates, through a natural process of cation exchange. When an ammonia particle is going by, it will capture it and it will be locked into its core."
The process is very similar to that which occurs in a water softener, where minerals in drinking water are exchanged for sodium ions in a brine solution. By reversing that process, used-up zeolite can be recharged. "You can soak the zeolite in a salt solution," Friesen says, "and the sodium ions will replace the ions you've filtered out, and you just dispose of the water and you're ready to go."
Like sand and recycled glass, and unlike cartridge filter elements, disposal of zeolite is easy and earth friendly. "When you're done with it," Friesen says, "you can dispose of it like sand. Just throw it in your garden or in your backyard. It helps your grass grow."
Perhaps the biggest payoff for all the sand alternatives is the lessening of the need to waste water in a backwash.
In the case of zeolite, Friesen says, the material contains an extraordinary amount of carrying space because of the intricate shape of the particle. "One cubic foot of zeolite has the same surface area as a football field. There's so much filtration area that it can absorb more and you can go longer between backwashes."
Glass media works a little bit differently, according to Peterson.
"One of the biggest problems we have with sand filters is that over time, and especially during the winterization process, junk will ball up inside the filter and clump together and it screws up the way that it filters," he says.
Because the glass media does not clump, he adds, it uses more of the entire volume of the media to filter, and can therefore go longer without backwashing.
"The glass media allows the water to spread evenly throughout the filter and filter particles down to the 6- to 8-micron level, which is in the same range as DE, on a platform that is significantly less expensive and requires half the backwash. The glass media requires that you backwash, at the most, once every two weeks. And you use less water during the backwashing process because the media is less sticky, and it releases the dirt and debris to the backwash more easily."
And The Winner Is...
With such gains in the fight against backwash waste, and improvements in the design of backwash valves, which promise to cut head loss, the green credentials for sand-type and, to a lesser extent, DE filters, have improved.
And while the green gap between sand and DE on one side, and cartridge filtration on the other, still exists, it is narrowing. In making a filter medium selection, this comparative advantage is weighed along with other criteria, such as performance, maintenance and cost.
As in all other aspects of the industry, the consumer has the deciding vote, but a strong knowledge of the green selling points of each media not only allows consumers to make an informed choice, but reflects well on the professionalism of the builder.
Opening The Core
One key to reducing the enormous amount of energy pools consume is reducing head in the circulation system. If total system head can be reduced, lower-horsepower pumps, which consume much less energy, can be employed without any loss of effectiveness.
A fair amount of flow restriction takes place at the filter, and for that reason, Pleatco, a filter cartridge manufacturer based in Glen Cove, N.Y., has attacked the problem by redesigning the filter core to give it a much greater flow area. The new Free Flow core offers less restriction for water passing through the filter, according to John Antretter, COO of Pleatco and an expert in manufacturing and product development with a background in a number of high-tech industries.
Antretter notes that traditional cartridge cores are made by extruding PVC pipe to a certain length and drilling holes in it. The amount of area available for flow in such cores can be easily calculated by counting the holes and subtracting their area, r2, from the total area of the pipe.
"It's usually around 20 percent," he says. "And we thought, 'That can't be good for flow.' So we decided to invent an injection-molded core designed to maximize flow. Meaning it has as much open space as possible while retaining enough structure to keep the core solid. Our goal was to make the core 50 percent open for flow.
"What we found was that by opening up the core, it allows for more complete flow throughout the filter. That also means more of the filter is used, which means that you get better overall life from the media."
Antretter believes that the Free Flow cores will improve the already strong green credentials of cartridge filters by reducing their energy usage.
"There are a lot of things that affect psi drop across a cartridge," he says, "and we don't have data yet on exactly what the improvement is, but we had a customer recently that changed the filter from a traditional core to a Pleatco core and went from a 12-psi pressure drop to an 8-psi pressure drop."