Cleaning Pool Filter with Muriatic Acid - Cartridge and DE

When Acid Cleaning Is the Right Choice

A garden hose rinse removes debris and loose particles from a pool filter cartridge or DE grid. It does not remove:

• Calcium scale - white or grey crusty deposits from hard water
• Mineral deposits - iron staining (orange/brown), copper (blue-green tints)
• TSP-resistant scale from heavily mineralized water

If your cartridge passes through a degreaser soak and still has white, crusty deposits on the pleats or end caps, acid cleaning is the correct next step.

Signs that scale is the problem:

• White or grey hard deposits visible on pleat surfaces
• Filter fouls faster than it did when new, even after thorough chemical cleaning
• Pool water leaves calcium rings at the waterline
• Your area has hard water (above 300 ppm calcium hardness)

If your cartridge is showing collapsed or cracked pleats, torn end caps, or visible holes, replace it - acid cleaning will not fix structural damage.

Two-Step Cleaning Sequence

Acid alone does not clean a pool filter completely. Oils, body waste, and sunscreen require an alkaline cleaner, while calcium requires acid. Use them in order:

Step 1 - Degrease first

Soak the cartridge in either:

• A commercial cartridge filter cleaner solution (follow label instructions - typically overnight soak)
• A TSP (trisodium phosphate) solution: 1 cup TSP dissolved in 5 gallons of hot water, soak for several hours

Rinse thoroughly with a garden hose after the degreaser soak.

Step 2 - Acid soak

Only after degreasing and rinsing. The acid step removes mineral scale, not organic waste.

If you reverse the order and acid-soak first, the acid reacts with calcium but leaves oils and organic material embedded in the pleats. The cartridge will reload faster than if you had cleaned it correctly.

Muriatic Acid Soak Procedure

What you need:

• Muriatic acid (31% hydrochloric acid, sold at pool supply stores and hardware stores)
• Large plastic bucket (5-gallon minimum, not metal)
• Chemical-resistant rubber gloves
• Safety goggles
• Garden hose with water running nearby
• Old clothing

Safety first:

Work outdoors or in a very well-ventilated area. Muriatic acid reacts with calcium deposits to release chlorine gas - this is harmless at low concentrations outdoors but irritating in enclosed spaces. Keep children and pets away from the work area.

Mixing the solution:

1. Fill the bucket with water first
2. Add muriatic acid slowly to the water - 1 cup of acid per 5 gallons of water (1:20 ratio)
3. Never add water to acid - always acid to water

The soak:

1. Hose down the cartridge first to remove any loose debris
2. Lower the cartridge into the acid solution - it should be fully submerged
3. You will see fizzing where the acid contacts scale deposits - this is normal
4. Soak for 1-4 hours depending on severity (longer for heavy deposits)
5. The fizzing will slow or stop when most scale has dissolved
6. Remove the cartridge and rinse very thoroughly with a garden hose for at least 2-3 minutes
7. Verify no acid smell remains before reinstalling

Neutralizing and disposing of the used solution:

Add baking soda to the spent acid solution (about 1 pound per gallon) until fizzing stops - this neutralizes the acid. The resulting neutral solution can be diluted with plenty of water and poured down a drain. Do not pour concentrated acid solution down a drain or into the yard.

Cleaning DE Grids with Muriatic Acid

The same two-step process applies to DE filter grids:

1. Remove grids and hose off loose DE and debris
2. Soak in degreaser solution (filter cleaner or TSP) for several hours
3. Rinse thoroughly
4. Soak in 1:20 muriatic acid solution for 1-4 hours
5. Rinse very thoroughly, checking that all acid residue is gone
6. Inspect grids for tears before reinstalling - a torn grid will allow DE powder to pass into the pool

After reinstalling, recharge the DE filter with fresh DE powder through the skimmer.

• How to Clean a DE Pool Filter - full grid teardown procedure
• How Much DE for Pool Filter? - recharge amounts

Commercial Alternatives to Muriatic Acid

If you prefer a ready-made product rather than working with raw muriatic acid, several commercial filter cleaners contain acid or strong chelating agents that dissolve scale:

• Natural Chemistry Filter Perfect - enzyme-based, gentler on cartridge material
• HTH Pool Filter Cleaner - alkaline cleaner, effective for oils and scale
• BioGuard Strip Kwik - concentrated degreaser

These products are safer to handle and produce less fumes, but for severe calcium buildup, diluted muriatic acid often does a more thorough job.

You can find cartridge filter cleaning products here - search for “pool cartridge filter cleaner” on Amazon.

How Often Should You Acid Clean?

Acid cleaning is not a routine step every cleaning cycle. It is a periodic deep treatment:

• Once per season if you have moderately hard water (150-300 ppm calcium hardness)
• Twice per season if water is very hard (above 300 ppm) or if the cartridge loads up quickly even after regular cleaning
• Whenever you see white scale deposits that hose-rinsing does not remove

For soft water areas where calcium hardness stays below 150 ppm, a degreaser soak alone is often sufficient and acid cleaning may only be needed every other season.

Related Guides

• How to Clean a Cartridge Pool Filter - full rinse and soak procedure
• How Often to Clean Pool Filter - cleaning frequency by filter type
• Best Pool Cartridge Filter Cleaner - commercial cleaner options
• When to Replace Pool Filter Cartridge - when to replace instead of clean
• Pool Filter Pressure Too High - diagnosing high pressure after cleaning