Sand Filter Use and Care

Sand Filter

Sand filters capture particles down to 20-40 microns.

A slightly dirty sand filter works better at capturing and filtering stuff. Only backwash when your filter pressure raises 25% over your baseline clean pressure.

Adding a small amount of pool grade DE (diatomaceous Earth) can improve the filtering capability of a sand filter.

Pool grade DE MUST be used, food, pet or garden grade will not work. The use of the wrong type of DE will cause it to go directly through the filter and cloud up your pool worse.

Sand

The sand in a pool sand filter is #20 silica sand; 0.45 - 0.55 mm. A discussion about different qualities of pool sand can be found in My findings on how not all sand is created equal. Pool sand that is too fine with small particles will be blown out from the laterals.

Pentair uses US Silica Mystic White II sand for all of their testing. It may not be readily available at Ace/Home Depot though. In general, we recommend getting your pool sand from a pool supply store and not a hardware store as they are far more likely to stand behind what they sell than a big box store.

Be aware that members have had issues with Quiikrete Pool Sand. Members find that Quikrete Pool Sand is too fine and perceptibly heterogenous (i.e., not filtered to #20 specifically). It does not filter well and tends to blow back into the pool. A search in the Forum can provide more information about the issues.

Swimming Pool Sand filters generally use #20 silica sand which is approximately .40 to .55 mm.

If the slits in the laterals are a little bit large and the sand is a little bit small, then you can get some sand passing through the laterals.

MYSTIC WHITE II filter sand has three effective sizes, 0.49 mm, 0.65 mm or 0.80 mm.

MYSTIC WHITE® II 0.49

MYSTIC WHITE® II 0.65

MYSTIC WHITE® II 0.80

TFP recommends using pool sand. There is no need for an alternative media like glass.

Alternative Media

You have to be careful when using the term "sand" because "sand" is not a well-defined material. There are a bazillion types of "sand" and where it comes from determines its composition.

The construction material that we think of as "sand" that is used in cement forming as well as in pool filters, is actually not a sustainable material. It's taken mostly from beaches. Its a aggregate material that is mostly composed of silica (SiO2) at about 60 wt% or higher. The rest of the material is made up of amphiboles, or "rock-forming" minerals such as aluminosilicates and feldspars. There's a smaller percentage of light weight materials like zirconia and other ceramic forming minerals. What you don't want in "sand" are carbonate or alkali minerals like calcium/magneisum carbonates. Those materials are typically of biological origins (shell forming and coral forming organisms). Alkali carbonate minerals have pH dependent solubilities that make them poor sand components.

Glass is nothing more than refined silica (SiO2) that has had alkali elements (sodium, calcium, potassium, boron, etc) added to it to modify its mechanical, optical, and rheological properties. It can have colorants added to it as well, typically from transition metals that produce distinct color centers. Very high purity silica is called quartz and it's a crystalline material that is very chemically inert with very unique optical properties. So, one can use "glass" in their filter but it's just going to be 10X more expensive than "sand" and not really offer any benefit.

Zeolites are a completely different class of minerals that are microporous materials consisting mainly of aluminosilicates. They are extremely light weight materials will huge surface to volume ratios. They are typically used in advanced filtration process for their abilities to absorb various kinds of organic and inorganic compounds on to their surfaces.

"Activated" glasses are just silica glass materials that have had their surfaces chemically modified to present "active" elements (iron, titanium, manganese, zirconium, etc) to their surroundings in the hope of catalyzing certain chemical reactions.

Is any of this relevant to pool water chemistry ... I think it's easy to be fooled into believing that these materials will perform inside a pool filter the same way they do in a lab setting. While the zeolites might offer greater filtration performance, it's easy for them to become overwhelmed with "crud" and then have those huge surface areas fouled up and useless. Zeolites are typically restored to their original performance by baking them at high temperatures to essentially burn off the crud. Activated glasses are similar - pool water likely presents enough "junk" to them to completely foul and INACTIVATE the material. Reactivation is also heat driven.

Pea Gravel

Pea gravel is sometimes recommended in the larger filters to help support the laterals from the tremendous weight on the sand. In some applications it can also help prevent clogging and improve filtration in the filter.

For example, for the Pentair Triton II TR 24" sand filter Pentair says it requires 75 lbs. of pea gravel & 250 lbs. of sand or 325 lbs. of "all sand".

Pump Sizing

The maximum flow rate for a sand filter should be about 15 gpm per square foot.

Backwash should be 15 gpm per square foot.

There is no minimum flow rate.

Add Pool Grade DE to a Sand Filter

Add DE to a Sand Filter describes how to improve filtering by adding pool grade DE to a sand filter.

Pool grade DE MUST be used, food, pet or garden grade will not work. The use of the wrong type of DE will cause it to go directly through the filter and cloud up your pool worse.

Adding Sand to a New Sand Filter

Before filling with sand, there should be water at least covering the laterals in the tank so that sand will flow under the lateral. No problem adding more than that to be sure.

• Remove the MPV valve and add the water with a hose or bucket
• Pour the sand in
• Reinstall the valve re-installed
• Plumb the system
• Backwash the filter before using it to filter the pool.

There is a lot of very fine material in the sand that will get through the laterals and into the pool if the filter is not backwashed first. Watch the sight glass. It will look brown at first as that debris goes through, then it will appear white. That's when the backwash is done. Even then, you may get a little back into the pool, but not enough to worry about.

How to Deep Clean a Sand Filter?

Deep Cleaning a Sand Filter has instructions with pics of the deep cleaning process.

Changing Sand in a Sand Filter

In general sand in a sand filter rarely needs to be replaced.

If you are going to replace the sand then carefully scoop the sand out without breaking any laterals or use a ShopVac to vacuum sand out.

Fill with water to just above the laterals. That way you're sure the sand will flow under them without leaving a void. Or, follow Hayward's instructions from their manual:

• We recommend filling tank approximately 1/2 way with water to provide a cushioning effect when the filter sand is poured in. This helps protect the underdrain laterals from excessive shock. (Be sure the winter drain cap is securely in place on drain pipe).

How to Open A Sand Filter

You may want to open your sand filter to check the sand, deep clean it, or check the level of the sand. After a few years some sand filters need sand to be added to get it back to the proper operating level.

You may find you do not have unions to easily disconnect the pipes from the MPV valve. If the filter doesn't leak at the fittings, don't remove them. Just cut the inlet and outlet pipes in an area that a union can fit. Use unions designed for pool use, don't grab something from Home Depot. You can find CMP Pool Unions at Leslie's and other retailers.

If you cut clean, then remove 1/2" more, CMP unions will fit. That way the valve can be removed and replaced without disturbing the threaded fittings that aren't leaking now. Just remove, and DO NOT lose the screws that hold the clamp together.

The backwash hose can be removed and the whole assembly set aside. Put one half of a union on the pipes attached to the valve, the other to the pipes from the pump and to the return, put the valve back and tighten the unions. Put a small amount of pool lube on the valve O ring before reassembling. The unions are self-aligning.

Repairs

My sand is "channeled" how to fix it?

• Open up the filter so the sand bed is exposed.
• Stick a garden hose into the sand and turn on the water. The sand should loosen up and all the dirt and gunk should float and overflow out of the filter.
• You might need to stir up the sand a bit. Keep this up until the water overflowing out of the filter is clean.
• If any sand is clumped together CAREFULLY break it up with something like a broom handle but BE VERY CAREFUL NOT TO DAMAGE THE LATERALS. You will be surprised at how much stuff this gets out of your filter that backwashing doesn't and I feel this 'deep cleaning' should be done yearly.
• This should take care of the channeling but if the sand has a lot of calcium deposits you might want to follow this up with an acidic sand filter cleaner from the pool store.
• GENTLY tap on the sand tank with a rubber mallot and watch the pressure gauge climb about 2 psi.

.