A growing trend in swimming pool filtration uses recycled glass in lieu of quartz silica sand in conventional pool sand filters. In fact, glass media was first used in slow sand filters by the English in the late 1700s, and was the basis for early patents.
Some builders and service technicians say they’re achieving greater water clarity with this media than with diatomaceous earth (D.E.) filters — sometimes with even less maintenance and cost. Here are some important points to remember when considering a switch to this type of media.
Glass filter media is compatible with ozone, salt chlorination, UV, ionization, active oxygen and stabilized conventional chlorine or bromine. The media works with biguanides and aqua catalyst mineral platforms as well.
Unlike some kinds of sand and D.E., glass filter media is very unlikely to short-cycle under the influence of biguanides and related sanitizers. It requires no backwash to put in service, and is designed to be backwashed, on average, every fifth week — as opposed to an average once-a-week backwash for many sand or D.E. filters.
Many service technicians who use glass filter media say that it’s reduced the need for monthly backwash frequency — some report that it’s saved as much as 100,000 gallons of backwash water per pool per swim season, while others report savings in the 10,000-gallon range on an average-sized pool in northern climates.
It’s also worth noting that a glass filter will permit techs to vacuum through the filter and get results — a very helpful feature in desert pools.
Glass particles have a slight negative charge on their surface, which tends to hold onto fine particles during the filtration cycle. Upon backwashing, this weak charge releases these fine particles to the effluent, enhancing filtration action.
In addition, where a sand filter uses the top six inches of media bed only, regardless of filter diameter, the glass media in the same model filter uses the entire bed depth — as much as five feet of total depth on a 900-pound filter. In other words, glass filter media also allows a wider, larger mesh size than the top six inches traditionally provided by conventional sand, permitting the entire filter bed to load contaminant.
Because bottle glass — the type used by many manufacturers of glass filter media — is much harder than the plate or window glass some glass filters used in the past, it provides finer particle separation and longer media life than those older glass types. As the grains are nearly all angular in shape and have a fairly high degree of sphericity, the filter bed tends to have more opened packing than that of a traditional sand filter, resulting in a greater permeability than a filter of spherical silica grains.
Also, because glass is amorphous and has no internal crystal structure, the particles are more homogeneous and have no grain boundaries. This gives glass more resistance to breakdown through filtration backwashing cycles. Furthermore, the lack of grain boundaries minimizes cracks where bacteria can lodge and resist flushing in backwashing.
This benefit has not gone unnoticed in the commercial pool sector, especially in light of the recent NSF 61 certifications that have been given to glass filter media. This drinking water standard goes beyond the conventional commercial pool standard (NSF 50) to ensure that the media is free from bacteria, mold and chemical contaminant. As with both standards, the processing facilities are subject to continued inspections to keep the standard. Many of the larger northern commercial pool sites budget the media changeout on an eight-year cycle, with over $1 per pound already budgeted and available to the dealer.
One recent 50-meter pool (Olympic competition size) was discovered with a $46,000 budget for the three filters totaling 39,000 pounds. (In every case, all filter laterals should be replaced in the dealer’s cost budget.)
Another cost-saver in the installation plan is to enlist the sewer department, street maintenance department or fire department for extraction of the old media. Nearly every city, park district, school district or county is under a “green initiative,” allowing for cross-departmental benefits from the savings that media like glass can bring.
The dealer can save even more by ordering the product in 3,000-pound “super sacks” at a lower price point — though it’s important to specify only one per shipping pallet, in order to avoid allowing workers to cut open and handle the bags. Rail freight with as little as two weeks’ delivery notice can usually be accomplished, further increasing profitability.
Sites using this media do require a multiport valve as opposed to a slide valve; purging the entire bed depth needs both a rinse and backwash cycle with two complete cycles of rinse, followed by a backwash, then a repetition of the cycle.
In residential pools, the media life of this media is approximately five years, but many sites require a bag to restore the bed depth after two or three seasons’ use.
The material is machined like sugar, and specific state statutes govern the millimeter size minimum of the glass, the minimum bed depth for the gallonage of the pool, and the four-hour pool turnover rate needed in most states.
Glass filters require a pea gravel base (approximately $6 per 50 pounds) above the laterals, consistent with several filter manufacturers’ preferred silica sand methods of installation. This gravel helps prevent glass or silica sand from plugging up the laterals.