Cartridge filter acid soak on a daylight equipment pad

How to Acid-Wash a Pool Cartridge Filter

May 21, 2026

An acid-wash for a cartridge filter is the deep-clean step that removes embedded calcium scale and mineral deposits that hose-rinsing and TSP soaks can’t reach. Done once a year on hard-water pools, it restores cartridge performance and adds years of useful life. Done wrong, it destroys the cartridge fabric. Here’s how to do it correctly.

When to acid-wash a cartridge

You see white crusty calcium deposits on the pleat surfaces or end caps.
Filter pressure won’t drop to baseline even after TSP/degreaser soak.
Annual deep-clean at closing or opening.

Acid wash is the second step in a thorough cleaning — always degrease first with TSP (trisodium phosphate) to remove oils, then acid wash to remove minerals. Doing acid first leaves grease in the fibers that prevents the acid from contacting the scale.

What you’ll need

A 5-gallon bucket dedicated to acid work (mark it with permanent marker)
Muriatic acid (typically labeled 31–38% hydrochloric acid)
Water for dilution
Rubber gloves and safety glasses
A garden hose with a spray nozzle for rinsing
A spare cartridge so you can keep the pool running during the soak

Acid safety. Always add ACID to WATER, never water to acid. Work outdoors with the wind at your back. Wear gloves and eye protection. Never mix acid with bleach, chlorine, or TSP.

Step-by-step

1Pre-clean the cartridge.

Hose-rinse to remove loose debris. Soak overnight in a 1-cup-per-5-gallon TSP solution. Rinse thoroughly. Acid wash is for what TSP can’t remove.

2Mix the acid bath.

Fill the bucket with 4 gallons of water. Slowly pour in 1 quart (4 cups) of muriatic acid. Mix gently with a wooden or PVC stir stick (never metal).

3Soak the cartridge for 5–15 minutes.

Submerge the cartridge fully. You’ll see bubbles as the acid reacts with calcium. Soak time depends on how scaled the cartridge is — 5 minutes for light scale, 15 minutes for heavy. Never exceed 15 minutes; acid degrades the fabric over longer exposure.

4Lift the cartridge out and rinse thoroughly.

Hose-rinse for at least 5 minutes. The cartridge needs to be completely acid-free before going back into the pool. Run water through the pleats from top to bottom.

5Inspect.

The cartridge fabric should look noticeably whiter and the pleats should be more flexible. Check for any holes or tears that the acid may have exposed.

6Neutralize and dispose of the used acid bath.

Add 1 cup of baking soda to the used bath in the bucket. Wait until bubbling stops (acid is neutralized). Pour down the drain with running water. Never pour live acid into the pool, the yard, or down the drain.

7Reinstall the cartridge.

Drop the cleaned cartridge back into the filter housing. Replace the body o-ring with a fresh one. Reassemble and prime per normal.