Pool and Spa Filtration
A pool or a spa filter acts like a human kidney, cleaning water that circulates through it. Without a filter, the water would be murky, cloudy, difficult to sanitize and generally unsuitable for swimming or bathing. The earliest swimming pools didn’t have the luxury of filtration, and the only way to clarify the water was to replace all or part of it with fresh water. Today, refilling the pool or spa every time you wanted to use it would be not only inconvenient but also costly and unecological. By filtering pool and spa water, we’re able to conserve water, reduce chemical use and maintain a healthy swimming environment.

A properly filtered and chemically treated spa may still need to be drained and refilled once every two to three months. A well-maintained pool, on the other hand, will need fresh water added only to replace water lost to evaporation, filter cleaning, and splashing from normal use.

There are three types of filters commonly available to pool and spa owners: sand, diatomaceous earth (DE), and cartridge. Most portable spas and above-ground pools use cartridge filters; all three types of filters are popular for in-ground pools. Each type of filter has its advantages and disadvantages; you should be familiar with all of them to make sure you’re using the best filter for your particular pool and spa setup. In general, DE cost the most but filters out the smallest particles. Sand cost the least but does the least effective job. Cartridges fall somewhere in the middle.

The effectiveness of a filter is gauged by the smallest particles it can screen out. For this purpose, particles are measured in microns. A micron is a unit of linear measurement equivalent to one millionth of a meter. Though manufacturers will argue about the effectiveness of their filter medium over others, as a general guide you can assume the sand will filter particles down to 20 to 40 microns, cartridges will filter down to 10 to 20 microns, and DE will filter down to 2 to 5 microns. As a means of comparison, a grain of salt is 90 to 110 microns wide, and particles under 35 microns can’t be seen with the naked eye. Even so, particles smaller than 1 micron can make water appear cloudy if there are enough of them.

That said, here’s a detailed look at each filter type.

Sand Filters
Granules of sand are irregularly shaped and make an efficient filtering medium, as evidenced by the many freshwater creek that are naturally filtered by sand. Sand filters contain a deep layer of sand that traps dirt and debris as water flows through it. The trapped particles fill the spaces between granules of sand and actually help the filter collect even smaller particles.

Over time, the accumulation of trapped particles will restrict water flow, the pressure inside the filter will increase and it will be time to backwash or clean, the filter. Backwashing simply calls for reversing the flow of water through the filter tank to loosen the trapped dirt so that it can be washed away.

The amount of dirt a filter can hold depends on how large the sand bed within the filter is. You want to put the maximum amount of sand in the filter that’s recommended, but make sure to leave a space between the top of the sand bed and the lowest opening in the distributor (where the water enters the filter). This space, called the freeboard, should be at least half the depth of the sand bed. Because every sand filter is unique, check the manufacturer’s recommendations for proper operation.

Sand filters require a particular type of sand. Check your owners manual for the correct sand specifications. If the grain of sand is too large or too small, the filter will not run efficiently and may even become damaged.

Many sand filters are fitted with two pressure gauges: the inlet gauge and the outlet gauge. When the sand is clean, the difference between the two gauges is usually about 3 psi (pounds per square inch). As the sand becomes dirty, however, the pressure on the inlet gauge will increase while the pressure on the outlet gauge will decrease. Most manufacturers recommend that you backwash the filter when the difference between the two gauges is 18 to 20 psi.

If your filter has just one gauge (it will be on the inlet), you’ll need to take note of the starting pressure the first time the system is turned on with clean sand in it. The filter may have projected starting pressure stamped on it, but don’t rely on this estimate to determine when it’s time to backwash. Instead, record the actual starting pressure. When the inlet gauge shows and increase of 8 to 10 psi over the starting pressure, it’s time to backwash the filter.

To backwash a sand filter, turn the control valve to “backwash” and divert the outlet water to the “waste” line. This will reverse the flow of water through the sand bed, thereby loosening dirt particles so that water can carry them away to the waste line. The process usually takes 2 to 4 minutes. Some waste lines are plumbed with a sight glass so that you can observe the water as it’s flowing through to make sure the water is clean before you end the backwashing cycle.

When backwashing is complete, turn off the pump so that the redistributed sand can settle in the tank. If you have a filter-to-waste option on your filter, use it now. This allows you to divert the first 20 seconds or so of filtered water to the waste line to make sure that no settled dirt that was missed during the backwash cycle is able to enter the pool. Once this is done, you can switch to regular filtration. If you don’t have the filter-to-waste option, you might see a puff of dirt or debris enter the pool when you first start up the pool following a backwash cycle.

Improving Performance
Flocculants are often used to improve the performance of sand filters. As an alternative, diatomaceous earth can be used so long as long your municipality doesn’t prohibit the discharge of DE during the backwash process. If DE is acceptable, pour 0.5 cup (0.12L) of DE for every 3 square feet (0.28 m2) of filter area into the filter after the unit has been filled with sand.

Eventually the rough sand in your filter will become smooth and you’ll need to replace it. Sand is usually good for a few years, but you’ll know its time for a change when the filter’s effectiveness becomes greatly diminished. To change the sand, simply turn off the power, open the filter, and scoop out the old sand by hand. Then fill the bottom third of the tank with water to cushion the impact of new sand on the laterals ( the pipes that carry filtered water out of the filter). Fill the tank about two-thirds full with sand. Reassemble the filter and backwash for several seconds to remove any impurities from the sand, then filter as normal.

Diatomaceous Earth Filters
Diatomaceous earth filters produce sparkling clear pool and spa water because they’re capable of filtering out even the smallest particles. This level of quality explains why DE filters cost the most to operate.

As the name implies, DE filters use diatomaceous earth as a filtering medium. DE is a fine, chalky powder made from the fossilized remains of microscopic sea organisms (diatoms). DE is mined from large deposits found where ancient inland seas have dried up. These deposits take the form of soft rock, which is pulverized to produce a fine, talcum-like powder. Though DE may feel soft, it actually has an incredibly large number of microscopic jagged edges that help it trap extremely small particles when used as a filtering medium. The dusty nature of DE, however, means that users must take special care not to inhale the dry material. In fact, DE is a known carcinogen, and many municipalities don’t allow residents to discharge used DE into the municipal wastewater system. (To check out the regulations in your area, contact your local health department.) If used properly, however, DE is as safe as it is effective.

The inside of a DE filter is very different from that of a sand filter. Inside the filter tank are grids covered with a polypropylene cloth. A typical filter has eight grids that total 24 to 72 square feet (2.2 to 6.7 m2). There are two basic types of grids: vertical and spin.

Vertical grid filters have grids assembled vertically on a manifold. A hole wheel secures the grids to the manifold and a retaining rod screws into the base of the tank to secure the assembly. Water enters the tank at the bottom. It flows up and around the outside of the grids and then down the stem of each grid, into the hollow manifold, and out of the filter.

Spin grid filters are now obsolete but may still be found in older pools. The grids are shaped like wheels and line up horizontally like a box of doughnuts. They operate in a manner similar to that of vertical grid filter, but they’re not as effective. To clean the grids, the user must turn a crank to spin them.

Once the filter is assembled and the pool’s circulation system is turned on, you’ll pour a thin slurry of DE and water through the skimmer basket. The slurry ends up in the filter, where the DE coats the grid cloth, which is woven tight enough ti keep the DE from passing through. This layer of DE – ideally 1/16 to 1/8 inch thick – does the actual filtering, not the grid cloth.

To avoid damage to the grids- which are costly to repair – make sure you add the right amount of DE for the type and size of filter you have. The owner’s manual for the filter should specify the proper amount of DE to use and how to prepare the slurry mixture.

As the DE traps dirt and debris from the pool water passing through it, the pressure inside the filter increases. When the pressure gauge (located on the top or side of the filter) reaches to level specified by the manufacturer, it’s time to backwash the filter, following the instructions in the owner’s manual.

During the backwash cycle, water flows up through the filter and dislodges the cake of DE and dirt from the filter grids. The DE and debris are flushed into the waste line. After the backwash cycle is through, a fresh slurry of DE and water is added through the skimmer basket to recoat the filter grids. It’s important that all DE be backwashed to waste during the cleaning process. If DE is allowed to remain in the filter, adding new DE will result in too much DE, which could cause bridging, a situation wherein DE and dirt close the necessary gap between the filter grids, reducing the flow rate as well as the square footage of the filter area.

DE Disposal

If you municipality doesn’t permit DE to be flushed into the waste-water system, you’ll need to fit your filter with a separation tank. This tank is smaller than the filter and is equipped with a mesh bag or other screening device that filters the DE out of wastwater. The DE can then be easily removed and disposed of properly, either with the regular trash or through a hazardous waste disposal site.

Regenerative DE filters
Many DE filters available today are regenerative. This means you can redistribute the DE when the filter becomes clogged so that clean particles of DE are exposed, thereby extending the life of the DE.

How do you redistribute the DE in the filter? Most regenerative DE filters have a handle on the top. Every few days, you should turn off the pump and move the handle up and down according to the manufacturer’s directions. Other filter designs call for rotating the grid assembly. Whatever the design, the objective is the same: to dislodge the DE cake and allow the DE to redistribute itself when the pump is turned back on.

If after regenerating the DE filter pressure doesn’t drop back down to the starting pressure, it may be time to replace the DE.

Washing DE grids
Sometimes it may be necessary to remove and wash the grid elements to clean away body oils, embedded dirt, and calcium deposits. You’ll know it’s time to clean the grids when the filtration cycle is shorter than usual or the pressure in the filter doesn’t return to normal after backwashing. One common method is to acid-wash each of the grids in a 25 percent solution of muriatic acid (one part acid to four parts water). After soaking the grids in a solution, rinse them thoroughly with fresh water. Overexposing the grids to acid can damage the elements, so make sure you rinse them immediately after cleaning.

Before reassembling a DE filter, inspect the inside and remove any debris. Carefully look at each grid for damage. If there’s a small hole in the filter element , you might be able to repair it with fingernail polish or silicone sealer. If you don’t, DE will wash through the grid and enter the pool. If the hole is too big to repair this way, you’ll need to replace the entire grid.

Cartridge Filters
Because of their ease of maintenance, cartridge filters are growing in popularity for all types of pools and spas. Unlike sand and DE filters, cartridge filters don’t need to be backwashed. The cartridge filtering medium – a cylinder pf pleated, polyester fabric – can be removed and hosed off when it becomes clogged. Not only does this conserve water, it also saves on pool chemicals because you’re not backwashing treated pool water down the drain.

The cartridge filter is housed inside a tank or canister. Water is drawn into the tank, passes through the filter, which traps dirt and debris and then travels back into the pool.

The effectiveness of a cartridge filter depends greatly on the quality and size of the cartridge. Cartridges are differentiated by the weight of the polyester fabric, the overall square footage of the fabric, the depth of the pleats in the fabric, and the type of end cap construction.

Most manufacturers use a 4-ounce fabric, although a 6-ounce material is more effective for filtering small particles. However, the lighter-weight material cost less and a cartridge can hold more square footage of 4-ounce material than of 6-ounce material, increasing its effectiveness. You might find some cartridges using 3-ounce material, but their effectiveness will be greatly reduced, even though they contain more square footage of material. A 3-ounce filter might suffice for a spa application, but it won’t be adequate for a swimming pool.

Though greater square footage of filtering medium can mean better filtration, it’s not necessarily the case if the pleats of the fabric are crammed so tightly together that water has difficulty flowing through them. In essence, of you put too much fabric in a filter, you restrict the flow of water and hinder the filter’s performance. To increase the filtering square footage without squeezing the fabric too tightly, some filter canisters are designed to hold two or three cartridges.

Pleats that are too deep – more than 2.5 inches (6cm) – also reduces the effectiveness of a cartridge filter. Water always seeks the path of least resistance. As a pleat in a cartridge becomes clogged with dirt, the water works its way up the pleat and into adjacent pleats. If the pleats are too deep, the water is trapped in them and the flow is restricted.

Cartridge end caps are designed to fit specific filter applications. Some need to pull double duty by supporting the pleated filter medium and creating a tight seal when placed in the canister. Be sure to check the cartridge’s end cap design, not just the overall dimensions, when purchasing replacement cartridges.

Cleaning the Cartridge’s
As with other types of filters, a cartridge filter should be cleaned when the pressure gauge on top of the unit reads 8 to 10 psi about the filter’s starting pressure. Unlike other filter types, however, cartridge filters don’t require backwashing. The basic steps for cleaning a cartridge filter are as followed:

Remove the filter from the canister or filter tank according to the manufacturer’s instructions

Using a garden hose with a straight-flow nozzle, spray water through the fabric to remove dirt. Work from the top down, making sure to spray between the pleats where dirt and debris build up. Rinse the cartridge until all the dirt and debris appears to be gone.

Replace the cartridge in the canister or tank
If the filter has been exposed to heavy bather loads or high levels of perspiration and body oils (which is especially common with spas), you may need to soak it in a cleaning solution overnight after hosing it down. You can purchase a filter-cleaning product from a pool and spa supply store, or you can use 1 cup (0.24 L) of trisodium phosphate or dishwasher detergent mixed with 5 gallons (18.9 L) of water. After soaking, rinse the cartridge thoroughly.

If the filter is coated with algae or calcium deposits, you may need to acid wash it. Do not attempt to acid wash a cartridge filter without first removing all oils and subsequent cleaning solution. Otherwise the acid will destroy the filter element. When you are ready to acid wash, prepare a solution of 1 part muriatic acid mixed with 20 parts water. Soak the cartridge in the acid solution just until the solution stops bubbling. Then rinse the cartridge thoroughly and immediately place it back in the canister or tank.

Replacing Cartridges
In terms of life expectancy, cartridges can be expected to last one to six years, depending on the quality of the cartridge and the care it’s been given. Replace the cartridge when they’re no longer cleanable, when the webbing of the fabric appears shiny and closed or when the fabric has begun to deteriorate or tear.

To extend the life of your cartridge filter, consider having your vacuum line run directly to waste rather than through the filter. Sometimes a heavy and sudden dirt load can clog and even damage the cartridge.

Sizing a Filter Correctly

No matter what kind of filter system you choose – sand, DE, or cartridge – the filter must be sized to the volume of water in your pool or spa. The basic rule is to size the pump and motor to the pool or spa capacity, then size the filter (its flow rate and capacity) to the pump.

For a pool, the recommended turnover time (the time it takes for the pump to circulate the entire body of water) is about eight hours. An eight-hour turnover time does not necessarily mean the filtration system must run eight hours daily to ensure clean water. In fact, if your pool water is correctly balanced and has the recommended level of free or available sanitizer, you may find that you can reduce filtration cycles to six, four and sometimes as little as two hours per day. But short filtration cycles must conform to individual pool conditions and requirements, which may vary from season to season. A professional pool equipment dealer or pool builder can help you figure out the right filter size and filtration cycle for your pool or spa.

In many cities and counties throughout the United States, filter rates on pools (especially commercial pools) are regulated. The faster water passes through the filter medium, the less effectively it’s cleaned. Thus, to ensure effective filtration, the National Sanitation Foundation offers recommendations for maximum filter rates, which may or may not be adopted by local building codes.

Just remember, if the filter is too small, it won’t keep the pool clean. If the filter is too large, it will cost more to buy but not necessarily more to operate. So don’t opt for the minimum-size filter to fit your requirements. Such a filter may work fine under normal conditions, but any sudden influx of dirt and debris from heavy rain and wind could quickly overload it. You’re better off slightly over sizing the filter than using a filter that is too small for the pool. If you oversize the filter, make sure the pump is sized accordingly, because a pump that is too small for the filter may not backwash adequately.

When something goes wrong with your pool or spa filtration system, water quality can deteriorate quickly. Poor filtration also leads to increase chemical cost and greater need for sanitizer. Following are some basic filter problems and possible solutions. For the most part, problems affect all filter types – sand, DE, and cartridge. As always, these guidelines are for general information only. Be sure to consult the owner’s manual for your filter for specific recommendations and operating guidelines.