Afin Sans Frais


Sand Filters and Backwashing

by Dion Rodrigues

The sand in a sand filter is specially graded to trap particles in the 20 - 100 micron range. As a sand filter collects dirt, its efficiency increases, trapping more dirt. When your pressure gauge shows a reading 8 - 10 lbs. over its clean, start-up reading, it is time to backwash the captured dirt out of the filter.

"They say" that a sand bed should be replaced after seven years. Gradual loss of efficiency may be hard to notice if you don’t know what to look for. Here are some signs that it may be time to change the sand bed. If your filter requires frequent backwashing, every week or two, the sand bed may be "mudballed", or it may be "channeled". It may also "calcify" with calcium deposits. Other water balance problems may also contribute to sand deterioration, but a properly sized filter could go over 10 years between sand changes.

Use of Biguanide chemicals, i.e., Soft Swim or Baquacil require annual cleaning of the sand to prevent it from "gumming-up". High amounts of bather oils can gum-up a sand bed. And just the years of a pump forcing water over the grains wears away the sharp edges of the sand. Such sand becomes more circular, and traps dirt less efficiently.

Remember that for sparkling water, we need the trio of sanitation, filtration and circulation. If one of these areas is lacking, the water won't look so great.

Adding a small amount of aluminum sulfate or "alum", through the skimmer will form a gelatinous layer on top of the sand bed, which is useful in cleaning up an undesirable water condition. You can also add a small amount of D.E. powder or other filter media.

How do I Backwash my Sand Filter?

When the pressure gauge is reading 8 - 10 lbs above the clean, starting pressure (after backwashing), it is time to backwash the filter. This process involves making the water flow through the filter backwards which flushes out the dirt. All this requires is turning a valve. Sand filters can have either a push-pull valve (also known as a slide valve) or a multiport valve. The multiport valve has multi-ports on the valve, usually 6 positions:

1. FILTER: Keep it here 99%, except when backwashing, rinsing or wasting
2. RINSE: Use this setting for 20 seconds after backwashing to rinse tank
3. RECIRCULATE: Use this if the filter's broken; at least you're circulating.
4. BACKWASH: Use this setting to reverse the flow in the filter and send water out of the waste line. Make sure valves are open or hoses rolled out
5. CLOSED: Put here to close off flow from the pool, usually to work on the equipment. Do not operate pump with valve in closed position
6. WASTE/DRAIN: Another filter bypass setting, but this setting sends the water out of the waste pipe (hose), instead of returning it to the pool. This setting is used to lower pool water level or to vacuum to waste.
To backwash a Sand filter with a multiport valve

· Shut off pump motor
· Press down on valve handle, set the valve from FILTER to BACKWASH position
· Roll out any backwash hose or open any waste line valves
· Open the air bleeder assembly on filter, and turn pump on.
· Watch pressure gauge for backpressure and hose for kinks. Be prepared to shut off pump quickly if necessary
· After hose fills with water, let it run for 2 - 3 minutes or until water runs clear
· Shut off pump motor and move multiport valve handle to RINSE position Run on rinse for 15 - 25 seconds.
· Shut off pump motor and move multiport valve handle to FILTER position
· Turn pump back on and note lower pressure. Roll up backwash hose
  To Backwash a Sand filter with a slide valve:

· Shut off pump motor, roll out backwash hose if there is one
· Twist to unlock plunger T-handle, pull or twist plunger upwards 2 - 3"
· Open air bleeder assembly on filter, and turn pump on
· Watch pressure gauge for backpressure (somewhere around+ 40 PSI) and hose for kinks. Be prepared to shut off pump quickly if problems arise
· After hose fills with water, run for 2 - 3 minutes or until water runs clear
· Shut off pump motor and push T-handle back down into locked position
· Turn pump back on and note lower pressure. Roll up backwash hose


A properly sized sand filter should be able to operate continuously for 4 weeks between backwashings. If it is less than 4 weeks, this may indicate sand problems (or sizing problems).

Sand in the pool?

This is never a good sign. If it hasn't made its way into the pool by some other means, it's likely coming from the filter. This could be because of a broken lateral or standpipe. You'll need to empty the tank, locate and make the repair, refill with fresh sand and test.

Sand bed replacement:

To replace filter sand, you'll first need to get rid of the existing sand. One method is to spread a tarp out beneath the filter drain assembly. Then remove the entire assembly and turn on the pump. The water pumping through the filter should remove most of the sand by pushing it out through the drain hole. Another method, which will take much longer is to remove the drain plug only and allow the filter to drain for several hours or days. Then, remove the top dome or multiport valve.

If you have the Triton style dome on the top of the filter, you'll need the special octagonal dome wrench to remove the dome from the filter. Once the dome is removed, gently twist the baffle/pipe out of the way so you have enough room to scoop out the sand.

If you have a Top Mount Multiport, you may need to cut some pipes to remove the valve, however, you can reconnect them later with unions or couplings. Once these pipes are cut, remove the clamp band connecting the valve to the filter. The valve should then pull straight up and off. Plug, tape, or cover the standpipe so you don't accidentally spill sand in it. Then you can use a shop vacuum to suck out the sand, or a small cup to scoop out the sand.

Be very careful as you scoop or suck, not to hit or break the laterals at the bottom of the tank. They can be brittle when they get older, and it’s probably a good idea to replace laterals at the time you replace the filter sand. Use a hose to wash out the sand beneath the laterals. When the tank is empty of sand, replace the drain assembly, using silicone sealant on the threads. Then add enough water to cover the laterals, this is so the new sand pouring in won't crack them.

Pour the sand in. Use only specially graded pool filter sand; #20 silica sand, 45 - 55 mm. If you have a top mounted multiport filters, use care to keep the lateral/ hub assembly centered, and on the bottom of the tank. After each bag of sand is added, make sure it is still centered. It may be useful to have a helper hold the standpipe steady while the sand is added.

Add the recommended amount of sand only; do not add more than necessary. This can lead to problems. If you don't know the right amount, contact your dealer or manufacturer. Most tanks are filled only about 2/3 of the way full, to leave enough free space on top. When full, lubricate the o-rings and reassemble filter top. Make sure lid is very secure, otherwise the lid could blow of and create a hazardous situation. It's a good idea to replace the o-ring on the filter domes.

When the filter is started up, begin on "RINSE" setting first (if you have a multiport valve). Then backwash and rinse again. If you have a push-pull valve, backwash first. This will prevent putting a lot of "sand dust" into the pool after a sand change.

Leaking filter?

Sand filter tanks rarely leak themselves, however leaks often occur in and around the multiport interface. One of the most common complaints is that water is leaking out of the backwash port of the multiport valve. Slight adjustments of the handle may solve this problem, but only temporarily. A more permanent repair may require replacement of the spider gasket inside of the multiport. You may also have leakage up around the middle of the handle on the valve. This usually requires replacement of a Teflon washer and sometimes the spring as well.

You may have a push-pull valve, or slide valve, instead of a multiport valve. Leaks can occur through the top of the index plate, or out of the backwash line. This may help to determine what o-rings need to be replaced. Leaks can also occur at the bulkhead unions where the valve attaches to the side of the filter, or around the threads on a top-mounted multiport. The drain plug can leak if not tightly secured or properly sealed.

Filter replacement?
A filter will last you for a long time, but not forever. A new filter may be in order if your current filter is between 15 and 20 years old, or difficult to use or get parts for. If the filter tank has cracked, usually from freeze damage or possibly from closing off return valves while the pump is running, a new filter is in order. Replacement is usually fairly simple, with just a few plumbing fittings needed.

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