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Pool filters catch debris, including soil, leaves, dead skin, hair  and sometimes bacteria, viruses and protozoan cysts (more commonly treated by the disinfection system). Sounds simple enough but keep in mind, filters can only catch particles from the water that travels through them. According to Gage and Bidwell’s Law of Dilution, after a single turnover of a pool, less than one-half of the water really sees the filter.

Most health agencies require a National Sanitation Foundation listing for filters installed within their jurisdictions, and some states require that American Society of Mechanical Engineers standards be met. Either way, always follow manufacturer’s recommendations for installation and operation.

In commercial pool environments, sand filters — which use sand as the medium to trap debris — and diatomaceous earth (DE), a disposable media-type filter, are the most common; however, there are circumstances where cartridge filters can and should be used.

There are four types of sand filters: pressure high-rate sand (most common), pressure sand and gravel (rapid sand), vacuum sand, and gravity sand. All are considered permanent media filters. More recently recycled, crushed glass has become popular in several areas, and zeolite sand media, which originates from volcanic rock, is another option.

Here’s a look at each type of sand filter.

  • Pressure high-rate sand filters work like this: As a new filter cycle begins the filter is not capable of catching turbid particles much smaller than 15-18 microns. The first few inches do all the filtering work and as the sand bed traps dirt, the dirt becomes part of the filter medium and begins to trap smaller and smaller particles, polishing the water while creating more resistance to flow. Pressure high-rate sand filter cycle rates range from 12 to 20 gallons per square foot; however, many health departments and engineers limit the maximum filter rate to 15 gallons per minute/square foot.

Today many high-rate sand filters are automated. Filter controllers monitor the pressure differential and upon reaching a preset value, usually between 12 and 15 pounds, automated backwash control valves are initiated to accomplish an automatic backwash. These should be fitted with fail-safe features to ensure heaters and chemical control systems are turned off during the backwash process and that stuck valves don’t end up draining a pool.

  • Pressure sand and gravel filters have been around for more than a century. Sand and gravel filters are usually found in groups of three or four, for backwash purposes, and filter water at between 3 and 5 gpm/sq. ft. Filter tanks were always typically welded steel and usually coated with a water-impervious lining and fitted with an incoming water distribution system to prevent incoming water from directly hitting the sand bed. The actual sand bed consists of various layers of sand and gravel.
  • Though few in number now, gravity sand filters are older and slower than the sand and gravel units with a filter rate of only around 1 gpm/sq. ft. The name itself implies how the filter works. Gravity “pulls” water through a sand bed. Filter tanks are constructed below grade to collect the pool water that drains down and the pump delivers the collected water back to the pool. Backwashing is accomplished in the same manner as the S&G units by pushing all the water through one tank, which then overflows into a pit that drains to sewer.
  • Vacuum high-rate sand filters are a newer option, but remain very similar in design to the gravity sand filters. Pool water flows into open filter tanks, which also act as surge pits, and circulation pumps draw the water through the filter at rates similar to conventional high-rate sand. Backwashing also is similar to gravity sand, but often is aided by the injection of air into the under-drain laterals.

Both vacuum sand and gravity sand filters require a vacuum gauge installed upstream of the circulation pump to determine the condition of the filter and the need for backwashing. Some are fitted with vacuum-limit switches, which shut down the circulation pump if the vacuum increases to a point causing damage to the pump. Diatomaceous earth filters are the alternative to sand filters. In general, DE systems begin catching debris particles as small as 6 micron immediately and do not require “ripening” of the filter as do sand systems. Manufacturers and health agencies accept 2 gpm/sq. ft.

DE particles are actually small porous skeletons of crestations that when airborne become a hazardous dust. There's also a pulp fiber replacement.

As with sand filters, DE filtration systems come in several varieties using either pressure — with a pump pushing the water through the filter — or a vacuum, drawing water through the filter. Filter rates for DE vary only slightly. Pressure DE filters are enclosed systems and vacuum DE filters are configured in the same way as the vacuum sand filters, with the pump actually drawing pool water through the filter elements. Most vacuum DE filters are found on larger pools.

Unlike sand, DE is not a permanent filter media, so it must be continually purchased, stored and periodically handled. Even so, DE systems provide excellent water quality and often are less expensive to install. Cartridge filters use “accordion-like” filter elements that trap dirt directly onto the filter surface and must be manually cleaned rather than backwashed.  We most often see them in areas where no sewer connection is available.  Many health agencies require not only a second set of filters be available, but also an area with a sewer drain to adequately wash them. The filter rates are listed as high as 1 gpm/sq. ft.; however, common practice uses something closer to .2 gpm/sq. ft.

Maintenance

Some keys to maintaining quality high-rate sand filtration include:

  • Use quality sand. Filter sand should be hard, irregularly shaped, sharp-edged and of the same sieve size.
  • Monitor pressure and or vacuum gauges daily. Remember that the best indication of the filter condition is the pressure differential between the influent and effluent pressure gauge readings.
  • Make sure the filter rate is appropriate. A higher filter rate results in shorter filter runs (time between backwashes); however, avoid filter-rates above 15 gpm/sq. ft. if possible. In addition, avoid filter rates below10 gpm/sq. ft. Dirt particles will end up being caught on the sand bed surface only, decreasing the amount of dirt the filter can hold.
  • Make sure automatic and manual air relief features remain in operating order. A filter filled with air functions poorly.
  • Once a year, after a backwash, shut down the system, release the pressure, remove the access cover and examine the filter bed surface. It should be flat. Valleys and mounds of sand are an indication there is a water distribution issue. Also drag the sand bed surface with a stiff rake. This will help remove stubborn hairballs and loosen up any sand “clods.”
  • Sand really doesn’t “wear out,” and if your system is backwashing properly, and you are maintaining a healthy disinfection and oxidation level, you should never have to “clean” or replace your sand. Ultimately maintaining proper water balance and water chemistry will prevent calcification and sand bed contamination.

Following are some keys for successful DE operation:

  • Maintain the appropriate filter rate, between 1.5 and 2.5 gpm/sq. ft.
  • Never use hydrogen peroxide with a DE system, it dissolves the DE.
  • Record pressure differentials and vacuum readings to know when element cover cleaning is necessary.
  • For optimum backwashing understand how your system functions and follow manufacturer guidelines. Some pressure DE filters, termed regenerated-DE use flexible elements that can be agitated or bumped to knock the DE from the element so the DE can be redistributed.  This significantly extends the filter run and lessens the frequency of backwashing and re-coating the elements. A crude variation of this also can be accomplished by turning off the recirculation pump and then turning it back on 10 or 15 seconds later. Bumping, however, also redistributes dirt particles and cleaning of the actual elements may be required more frequently.

With vacuum DE systems there is no real ”backwash” procedure. The vacuum DE filter tank must be isolated from the pool, pumped down and the soiled DE washed down with a stiff stream of water. The old DE and wash-down water then must be pumped out usually with a submersible trash pump located in a low corner or sump within the filter pit to waste or in some circumstances to an evaporation pit, where the DE is collected and shoveled into bags for disposition to a landfill waste site.

  • Periodically the DE filter elements themselves must be cleaned. Some operators do this annually. To establish a cleaning schedule, observe the pressure differential or vacuum gauge reading on the new filter elements after pre-coating. Record those values and compare them with the values after subsequent backwashes or wash-downs. If later values show a measurable difference — that is, a large pressure differential or higher vacuum reading — chances are, it is time to clean the element covers. Tri-sodium-phosphate (TSP) is the best option for this job. Give the elements a good soaking and rinse thoughroughly. A very mild acid wash also may be necessary if the covers are scaled with calcium carbonate.
  • Troubleshooting

    Sand Filters:

    • Sand in the pool.  First, make sure it is filter sand. Keep a sample of the original sand for comparison. If the sand in the pool is from your filter, check the air relief tube inside.  Lots (shovels full) of sand would indicate a broken lateral.
    • Remember that the need to use clarifiers to obtain clear water could indicate something else is wrong. It could be the wrong sand, incorrect filter rate or poor backwashes. Continued use of some clarifiers is not only expensive, but will eventually foul the sand making it “sticky.”
    • Improper backwashing will result in the sand media failing and can significantly reduce the filter run and affect the clarity of your water. Only backwash when necessary and remember that the pressure differential is the best way to determine how dirty the sand bed is, and whether backwashing is needed. Don’t go by day of the week. Backwashing before the filter has had a chance to ripen will prevent the filter from catching the smaller dirt particles and really polish the water. When backwashing, make sure the backwash flow rate reaches 15 gallons per minute and that the rate is maintained for at least three minutes. Failure to achieve a proper fluidization of the sand bed will result in turbid particles and hair remaining in the bed, eventually resulting in “break through” (small particles passing all the way through the bed) or hairballs and even channeling.

    DE Filters:

    • DE in the pool. This usually means a filter element cover has been damaged allowing the filter media to travel back out to the pool. Small tears will end up being sealed by the DE.
    • Higher than normal pressure or vacuum readings after a filter backwash or cleaning indicates the element covers themselves have become soiled and are clogged (see maintenance procedures).

    Replacement

    The decision as to what type of filter to install is normally based on the original budget, availability of labor and most importantly, if a sewer connection is available. Most DE filters are less expensive than their high-rate sand counterparts; however, consideration of the required maintenance procedures should be included.


    About the Instructor

    Rich Young has more than 30 years’ experience with municipal, public and commercial pools and waterparks. He has operated and maintained many public and commercial swimming pools as well as several waterparks. Besides being an AFO instructor for the National Recreation and Park Association and technical adviser/board member at the Professional Pool Operators of America, Young has been published numerous times in industry journals.