Sand filter media: when it is done and how to change it

How does a sand filter actually catch anything?

Water is pushed down through a bed of sharp edged silica sand and the particles get caught in the tight, irregular gaps between grains. It is not a sieve so much as a maze, and the sharpness of the grain edges is what makes the maze effective.

A sand filter in good condition catches particles down to roughly twenty to forty microns. That is coarser than a cartridge and considerably coarser than DE, which is why sand pools can look slightly less polished even when everything is working properly.

A well known trick closes some of that gap. A small dose of DE or a cellulose fiber additive through the skimmer coats the top of the bed and tightens what it catches significantly. It washes out with the next backwash, so it gets redone, and it is genuinely effective during pollen season.

How do you know the media is finished?

Because the grains have rounded off. Years of water flow and backwashing wear the sharp edges away, and smooth spheres pack together in a way that lets particles slip through. The filter still passes water beautifully. It just stops catching much.

The symptoms are indirect. Water that never fully clears despite good chemistry. A clean baseline pressure that has crept upward season over season and does not come back after a long backwash. Backwash cycles that get shorter and shorter.

Then there are two visible conditions. Channeling, where water has carved preferred paths straight down through the bed and most of the sand is being bypassed. And mudballs, where oils and fine debris have bound sand into clumps that neither filter nor backwash properly. Pull a sample from the top of the bed and look at it. Clumps mean the bed is done.

Five to seven years is the common working life. Under heavy tree load, on a pool that gets a lot of use, or in water that scales, it can be less.

What is involved in a sand change?

Pump off and depressurized, valve and plumbing disconnected at the multiport, and the tank drained. Then the old sand comes out, which is the actual work. A shop vacuum designed for wet material or a small scoop and a lot of patience. A typical residential filter holds a few hundred pounds of it.

The reason it matters, and the reason people who have never done it underestimate the job, is that the laterals at the bottom have to be inspected. Those are the slotted fingers that collect filtered water, and a cracked or broken lateral is what sends sand back into the pool. While the tank is empty is the only chance you get to see them.

Then the standpipe has to be covered before new sand goes in, or sand pours straight down into the collection assembly. Fill the tank part way with water first to cushion the sand as it lands, then add the correct weight of the correct grade.

Grade matters. Pool filter sand is a specific silica grade, commonly sold as number twenty. Play sand and construction sand are the wrong shape and the wrong size and will pack, channel and pass straight through the laterals.

What about glass media and the alternatives?

Recycled glass media is a common substitute. It is lighter, so a filter takes less of it by weight, it holds its edges longer than sand, and it filters somewhat finer. It costs more up front and many owners find the longer life makes up for that.

Zeolite based media is the other option, and it filters finer again and has some ability to take up ammonia. It has its own maintenance quirks, including periodic regeneration with a salt solution.

Whichever you use, the filter body itself does not care much. What matters is using the quantity specified for the media you chose rather than the weight printed for sand, because the densities are different.

And none of them fixes a broken lateral, a worn spider gasket, or a filter that is too small for the pool. Media is not the answer to every sand filter complaint.

Why is sand suddenly in the pool?

A cracked lateral or a cracked standpipe, most likely. Both let media past the collection assembly and out through the returns, and it shows up as a fine gritty pile on the pool floor near the return eyes.

A worn spider gasket in the multiport valve, which lets water take the wrong path through the valve. This one usually shows as water going to waste during filtering or vice versa rather than as sand in the pool, but it belongs on the list.

Or the media was overfilled, so the bed sits above the top of the standpipe collection area and gets carried over. Filters have a fill line for a reason.

If sand is coming back, stop running the filter in that mode and get the tank opened. Sand circulating through the plumbing gets into pump seals and valve seats and turns a lateral repair into a bigger bill.