How to Backwash a Sand Filter and Change the Sand

Why this matters

A sand filter is the most forgiving filter on the pad until it is not. Backwash it wrong and you tear the multiport's spider gasket, send dirty water straight back to the pool, or drop the water level below the skimmer and run the pump dry. Change the sand wrong and you crack a lateral, which shows up a week later as a line of sand on the pool floor in front of every return. Both jobs are short. Both have one or two steps that, skipped, turn them into a callback. Discharge rules and the general cleaning thresholds live in 'Filter Cleaning and Media Replacement'; this card owns the sand-specific work.

Safety before either job

Never move the multiport handle with the pump running. Turning it under pressure tears the spider gasket inside the valve, which then leaks water to waste in every position, and it can drive a pressure surge through the filter. Pump off, move the handle, pump on, every time.

The tank is a pressure vessel. For a sand change, turn the pump off at its disconnect or breaker and lock and tag it, so an automation schedule or the customer cannot start it while the tank is open, under 29 CFR 1910.147. Open the air relief and confirm the gauge reads zero before loosening the valve clamp or unthreading anything. Switch the heater off and let the pump run a few minutes before shutting down, so the heat exchanger is not left hot with no flow.

Filter sand is silica. Pool filter sand is coarse, but every bag carries fines, and pouring it dry lifts respirable crystalline silica into the air at face height over the tank opening. Pour slowly, keep your face out of the plume, and wear a respirator under your shop's written respiratory program per 29 CFR 1910.134. 29 CFR 1910.1053 covers this exposure unless the employer has objective data showing it stays below the action level under any foreseeable conditions. Wetting the tank first, as the procedure does, also cuts the dust. Old sand comes out wet; keep it wet.

Lifting. A sand change moves several hundred pounds in 50 lb bags and wet sand by the bucket. Lift from the legs, keep bags close, and get the truck as near the pad as the site allows.

How the filter moves water

   TOP-MOUNT SAND FILTER, SECTION

        multiport valve on top
   +----[ valve ]-------+   handle positions:
   |    diffuser        |   FILTER  BACKWASH  RINSE
   |        |           |   WASTE   RECIRCULATE
   |  ~~~~~~|~~~~~~~~~  |   CLOSED  WINTER
   |  ......|.........  |  <- freeboard above sand
   |  .. #20 sand bed   |  <- sand to the fill line
   |  ......|.........  |
   |  ......|.........  |
   |  ==\\==|==//====   |  <- laterals at the base
   +--------+-----------+
            standpipe, up the center to the valve
                         drain plug at the base

   FILTER:   valve -> diffuser -> down through sand
             -> laterals -> standpipe -> valve -> pool
   BACKWASH: valve -> standpipe -> laterals -> up
             through sand -> valve -> waste line
   RINSE:    same path as FILTER, out to waste

Backwash reverses flow so the bed lifts and the trapped dirt goes to waste. Rinse runs the filter direction but to waste, which settles the bed back down and flushes the loosened dirt from under the valve before you go back to filter. Skip the rinse and the first minute of filtered water carries that dirt to the pool as a cloud at the returns.

Backwashing

When: at 8 to 10 psi above the recorded clean baseline, per 'Filter Cleaning and Media Replacement', not on a calendar. A sand bed filters finer when it is slightly dirty; backwashing a clean filter wastes water and makes it filter worse for a day.

1. Check the water level first. Backwash takes water out. If the level is already near the bottom of the skimmer mouth, add water before or during, or the skimmer will suck air and lose prime partway through.

2. Check where the waste line goes. The backwash water goes to the point your local wastewater authority permits: sanitary cleanout, a designated area of the yard, never a storm drain or a neighbor's property. A roll-out hose that has been moved since last time is the usual way this goes wrong.

3. Pump off. Handle to BACKWASH. Pump on. Watch the sight glass (or the waste line discharge). It runs dark, then clears.

4. Run until the sight glass runs clear, then about 15 seconds more. Typically two to three minutes on a route pool. Stopping on a color still clouding the glass leaves dirt in the bed; running long after it is clear only wastes water.

5. Pump off. Handle to RINSE. Pump on for 30 seconds to a minute. Until the sight glass is clear again.

6. Pump off. Handle to FILTER. Pump on. Bleed air at the relief if the filter has one. Read the pressure: it should land within about 2 psi of the recorded clean baseline. Note the new reading and recheck the water level.

Changing the sand

When: a backwashed filter that will not return within about 2 psi of its baseline, or a scoop sample that clumps or comes up in pea-sized balls rather than pouring loose and gritty, per the condition test in 'Filter Cleaning and Media Replacement'. Sand also wears smooth over years and stops trapping fine particles, which shows as water that never quite polishes even at good pressure.

Start state: read the label on the tank for sand grade and weight. The common grade is #20 silica, but some tanks call for a different grade or allow glass or zeolite media, and the label governs. Bring the full amount, a cup or cap to cover the standpipe opening, a shop vacuum or a scoop, a spare lateral set if the tank is older, a new valve o-ring, and silicone lubricant.

1. Heater off, run on, pump off and locked out, relief open, gauge at zero. As in the safety section.

2. Drain the tank. Pull the drain plug. On many tanks the plug sits behind a screen that holds sand in; keep the plug, its o-ring and the screen together.

3. Remove the multiport valve. Disconnect the plumbing unions or cut and re-plumb as the installation requires, release the valve clamp or unthread the valve, and lift it straight up off the standpipe. Do not lever it against the standpipe.

4. Cover the standpipe opening. A cup, a cap or tape over the top. Sand that falls into the standpipe goes straight to the pool on the first restart.

5. Remove the old sand. Scoop or vacuum it out, keeping the standpipe centered and upright. Work carefully at the bottom: the laterals are brittle plastic and old ones crack under a scoop.

6. Inspect the laterals. Lift the standpipe assembly if the design allows. A cracked or broken lateral is how sand reaches the pool. Replace any that is cracked, and on a filter with years on it, replace the set.

7. Half fill the tank with water. The water cushions the laterals from the falling weight of the first bags and keeps dust down. Pouring dry sand straight onto bare laterals is the commonest way they crack during a sand change.

8. Add the grade and amount on the label. Pour slowly, keeping the standpipe centered by hand as the sand builds around it, respirator on. Stop at the fill line marked on the tank or at the depth the label states; the freeboard above the sand is what lets the bed lift during backwash.

9. Uncover the standpipe, clean the tank threads or neck, and reinstall the valve. New or cleaned o-ring, lightly lubricated with silicone. Seat the valve squarely on the standpipe and tighten the clamp evenly.

10. Restore: drain plug in, plumbing reconnected, unlock, start in BACKWASH. This is the step where pressure goes back in. Drain plug and screen first. Remove your lock, restore power, and with the handle already set to BACKWASH, start the pump with nobody standing over the valve. Leave the air relief open until it spits a steady stream of water, then close it. New sand carries fines; backwash until the glass clears, rinse, then go to filter. Check the valve, clamp and drain plug for leaks under pressure.

11. Log the new clean pressure. That is the new baseline, and new sand often reads lower than the old baseline did. Restore the heater and any automation schedule, and confirm the heater fires with the pump running.

Worked example: a backwash that did not recover

A 24 in top-mount sand filter on an 18,000 gal pool. Equipment tag: clean baseline 13 psi. Arrival: 22 psi, 9 psi over, inside the 8 to 10 psi window.

Backwash. Water level checked mid-skimmer. Waste hose confirmed to the sanitary cleanout. Pump off, BACKWASH, pump on: the glass cleared at about 2 1/2 minutes, run 15 seconds more. Pump off, RINSE for 45 seconds, pump off, FILTER, pump on. Pressure: 18 psi.

Judgment. 18 psi is 5 psi over the 13 psi baseline, outside the 2 psi return rule. A scoop sample from under the valve came up in pea-sized clumps that held their shape when squeezed. Both tests point to the same answer: the sand is finished. Quoted and scheduled.

The change, next visit. The tank label calls for 300 lb of #20 silica sand, six 50 lb bags. Heater off, pump run three minutes, pump locked out, relief open, gauge zero, drained. Valve off, standpipe capped. Old sand came out with calcium clumps throughout the bottom third. One lateral had a crack at its root; the full set was replaced. Tank half filled with water, six bags poured with the standpipe held centered, sand stopped at the marked fill line. Cap off, valve seated with a new o-ring.

Restart. Lock off, pump started in BACKWASH from beside the tank: dark water for about three minutes as the fines cleared, then rinse and filter. New clean pressure 11 psi, written on the tag as the new baseline. Next backwash is due at 19 to 21 psi. Heater fired and held with no flow trip.

What changes the job

• Sand on the pool floor after a change. A cracked lateral you missed, a standpipe not seated in the valve, or sand that went down the standpipe. Drain and open the tank again.
• A side-mount valve. The valve does not lift off the top, so the tank is emptied through a larger drain and the standpipe arrangement differs; follow the maker's manual.
• Glass or zeolite media. Fill weights differ from sand for the same tank, and some makers state different backwash rates; the tank and media labels govern.

References

• See related: Filter Cleaning and Media Replacement (cleaning thresholds, condition test, discharge)
• 29 CFR 1910.147 (control of hazardous energy, for locking out the pump)
• 29 CFR 1910.134 (respiratory protection program) and 29 CFR 1910.1053 (respirable crystalline silica, general industry)
• Filter and multiport valve manufacturer's manual and tank label (media grade, weight, fill line)