As though the industry didn’t have enough to quibble about when comparing various filter media, we now have a relative newcomer to consider.
Introduced in the late 1990s, glass filtration is catching on as a direct alternative to sand. While it claims to filter finer than silica and require less backwashing, is it all that it’s cracked up to be? PSN spoke with service technicians and pool operators to see if the material meets, or shatters, expectations.
Reduce, reuse, recycle
Glass filtration is the product not only of the industry’s never-ending search for the best media, but also the generous supply of recycled glass.
Municipal recycling programs do a decent job of spurring the public into organizing household waste into paper, plastic, aluminum and glass. It’s a well-intentioned effort to divert stuff from the landfill; however, not all of it finds a second life.
Glass containers are especially problematic.
The majority of recycling programs do not separate bottles by color, so when the glass is crushed, the result is a mix of brown, green and clear shards called cullet. Most beverage companies won’t use cullet for new bottles because, frankly, they want pretty bottles.
That leaves a fairly limited market for scrap glass. By the same token, pool operators were seeking an alternative to sand.
The pool industry can take credit for finding a use for the material beyond cement and insulation applications — its predominant post-consumer uses.
“The problem, I think, that we found in the industry was that sand itself — to get the right co-efficiency throughout the country — was really difficult for us,” says Wayne Smith, president of WMS Aquatics, an equipment supplier and commercial pool contractor in Ellensburg, Wash.
So certain manufacturers obliged by exploring a new material. Glass grains were more consistent in size, and the media has since earned a strong regional preference among commercial pool operators.
Glass vs. sand
By most accounts, crushed glass functions perfectly well. But what advantages does it have over conventional quartz sand?
Let’s do a side-by-side comparison.
High-rate sand filtration: As the saying goes, why fix what isn’t broken? That’s a sentiment shared by some service techs when they consider changing out sand media with crushed glass.
Sand is the old standby. It’s dependable and delivers predictable results.
The concept is simple: Solid turbid particles become lodged into the sand bed. As more dirt is captured in the sand bed, it becomes capable of filtering finer and finer particulate, up to a point.
In order for the filter to be effective, each grain of sand must be roughly the same size, generally 0.4 to 0.6 millimeters. The most common sand filter media is 20-grade silica, which creates a mesh so tight it can sieve particles as small as 20 microns. (To put that in perspective, the eye of a needle is 1,230 microns.)
While silica is an effective filter media, it does have its setbacks. Over time, sand can clump, calcify and create channels where water slips through unfiltered. And in commercial settings, it needs to be backwashed frequently — at least once a week for large-volume public pools.
It’s generally recommended that the sand be replaced every five to eight years, as the grains eventually breakdown.
Glass: All the principals of high-rate sand filtration apply to glass, with a few exceptions. Unlike sand, which traps debris with its jagged edges, glass has a slight negative charge on the surface to attract fine particles, which cling to the media like static-charged socks to a sweater. This weak charge releases the particles upon backwashing for a more efficient cleansing.
Manufacturers say glass has another unique characteristic: filtration takes place through the entire bed depth, unlike sand, in which gunk commonly builds up within the top 6 or so inches. This allows for a larger load of contaminant. This is because glass, which is less dense than sand, doesn’t pack as tight.
Proponents also say glass has a tighter turbid catch, with an ability to filter down to 9 microns. Some brand claims go as low as 3 microns. This DE-quality filtration can be achieved without chloramine-producing flocculants and shock treatments, says one manufacturer, which is one reason glass is most commonly found at indoor pools where air quality is a top concern.
In fact, this new media is gaining the most traction in indoor facilities, says Rich Young, general manager of Aquatic Commercial Consulting in Saratoga, Calif.
This trend also stems partially from glass’ ability to “ripen” more quickly than sand — a concern because pools sheltered from the elements do not deal with as much debris, such as leaves and dirt, that can accumulate to aid filter efficiency.
A water saver?
But glass media’s biggest selling point is its water-savings potential.
Because this media is more efficient, it’s said to require less backwashing.
A Washington state nonprofit put this to the test. In a field study performed by the Clean Washington Center, researchers replaced 1,950 pounds of sand from three high-rate filters with recycled glass and analyzed the results for approximately nine months. Here’s what they found:
• Less water: The average duration of backwashing (in minutes) was 2:34 compared to 3:21 for sand. That’s equates to a 23-percent reduction in water. The improved performance can be attributed to glass’ lower density, with approximately 20 percent less volume by weight than sand. The lighter material floats more easily, the study noted, allowing it to fluidize quicker, reducing backwash runtimes.
• Clearer water: The study showed a 25-percent reduction in National Turbidity Unit Readings.The obvious advantage here is a more polished body of water; however it may also extend to energy savings. Recirculation systems could be operated fewer hours.
In the field, not every professional experiences the same results. While Drew Essig can attest to glass media’s purification performance, he’s not convinced that it consumes any less water than sand.
When the service manager at Capitol Pool Service, serving the Washington D.C. metro area, swapped out sand for glass at a hotel pool, he immediately solved the pool’s most vexing issue: cloudy water.
“It’s much clearer than it was before,” Essig says. “One way that you really noticed was when you looked at the lights. Instead of seeing a slight haze in front of the light, it was crystal clear.”
However, he found that he had to backwash just as often as he did when the water was treated with sand.
“We were backwashing it three times a week whether it needed it or not [when using sand],” he says. “When we moved to glass, it needed it three times a week.” That could be because this was popular party pool with an above-average amount of impurities. But the increased need to backwash was a sign of the glass’ effectiveness. “It was capturing a lot more particles than it had been before,” Essig says.
So Essig is sold. “I can’t tell you the last time we put sand in,” he says. “It used to be that we gave [customers] the option, but now glass is where you’ve got to go.”
Glass media also gets a ringing endorsement from Tonya Kiser. The senior assistant director of aquatics and safety for campus recreation at the University of Montana switched to the media last year.
When the filter’s laterals crushed under the weight of sand, sending grains throughout the 25-yard competition pool, she needed a solution, and quick, since the Grizzly Pool is programmed to the hilt. To rapidly clear up the water, she took a cue from UM’s home town, the City of Missoula, which recently upgraded its municipal pool to glass media.
“We’re not letting the backwash times run as long,” Kiser says. “So we’re not dumping as much water down the drain.”
Plus the water is more polished and the laterals, she believes, will hold up better under the lighter-weight material.
Kiser also likes another earth-friendly advantage of glass: “The fact that we’re using recycled glass was a big draw,” she says.
Another touted glass advantage is longevity. It is said to last 10 years or longer. So how do you dispose of the stuff when it reaches its expiration date? Can recycled glass be … recycled?
Manufacturers are just now beginning to field this question, and the answer is fairly straightforward. First, make sure the glass needs to be replaced. In many cases techs find that it only needs to be cleaned with chlorine or acid, and topped off with a fresh layer. (A little glass may be lost with each backwash, especially with a 10 horsepower commercial pool pump.) Otherwise, the material can be disposed of just like sand. (Unless you’re in the habit of dumping used sand on a beach, in which case, don’t.)
As for installing the stuff in a sand-type filter, that too is pretty simple. Use a pea gravel base to protect the laterals. (Tip: Fill the filter half way with water, and then pour in the gravel to so as to not crush the laterals.)
Some operators find they don’t even need to replace all of the sand with glass.
“The glass sand has been used as a filter bed ‘topping’ of 6- to 12 inches to aid in troublesome clarity situations with excellent success,” says Young, the commercial aquatics consultant.