Cartridge, sand, and DE filters: how to identify and care for yours

The fastest way to identify a filter is to look at the tank shape and fittings. Cartridge filters are typically vertical cylinders with a top-mounted or clamp-band access lid and no multiport valve. There is no backwash port. To service them, you open the top and pull out one or more pleated cartridge elements. Sand filters are usually a rounded tank (taller than wide) with a multiport valve on top or side with a selector dial showing positions like Filter, Backwash, Rinse, Waste, and Recirculate. DE filters look similar to sand filters from the outside but are plumbed with a backwash port and have a DE powder charge port. The internal media (grids or fingers versus sand) is the real difference.

If you are still uncertain, look at the label on the filter tank. Manufacturer nameplates on filters typically say Sand Filter, Cartridge Filter, or DE Filter. If the label is gone, look for whether there is a pressure gauge on the tank (all types have this), whether there is a multiport or push-pull backwash valve (sand and DE yes, cartridge no), and whether there is an access lid at the top (cartridge typically yes for element removal).

Cartridge filters are the most common type on newer Austin pools, particularly those built in the last ten to fifteen years. The filter element is a pleated polyester cylinder that captures particles as water passes through it from outside to inside. When pressure rises 8-10 psi above the clean baseline, the cartridge needs cleaning. Turn off the pump, relieve pressure via the air bleeder valve, remove the tank lid (usually a band clamp), pull the cartridge, and rinse it thoroughly with a garden hose. Work top to bottom, rotating through the pleats.

Do not use a pressure washer on cartridges. It damages the pleating media and shortens the life of the element dramatically. Once a year, soak the element in a commercial filter cleaner or a dilute acid solution to remove grease, sunscreen oils, and mineral scale that hose rinsing cannot reach. This annual soak restores flow capacity significantly. Replace cartridges when they no longer return to a clean baseline pressure after rinsing, when pleats are collapsed or torn, or after roughly three to five years of service.

Sand filters use a bed of silica sand (typically 0.45-0.55 mm grade) to trap particles as water flows down through the bed. They are backwashed by reversing the flow through the multiport valve, sending the waste water to the backwash hose or drain. Backwash when pressure rises 8-10 psi above clean baseline: turn the pump off, move the multiport to Backwash, run until the sight glass runs clear (usually 2-3 minutes), then switch to Rinse for 30 seconds to reseat the sand before returning to Filter.

Sand needs replacement every five to seven years in Austin, more often if the filter has seen heavy algae loads or the sand has calcified from hard water. Calcified or channeled sand does not filter effectively; the water finds paths around the resistance rather than through the media. Glass filter media (recycled glass beads) is an alternative to silica sand that claims better filtration and resistance to channeling and is worth discussing with your service tech if your filter is due for a media change.

Diatomaceous earth filters use a coat of DE powder on internal grids or fingers to achieve very fine filtration. After backwashing, the DE powder is flushed to waste and must be replaced through the skimmer with the pump running. The correct DE dose is listed on the filter label and is based on square footage of filter area; use the right amount or the grids will either be undercoated (poor filtration) or overloaded (early pressure rise).

DE filters should be fully disassembled and cleaned at least once per year. This means backwashing, opening the tank, removing the grid assembly, and rinsing each grid by hand before inspection. Torn or collapsed grids pass DE and debris to the pool. If you see a white cloud entering the pool returns after a DE filter is freshly charged, a grid is likely torn. Full disassembly also lets you inspect the tank manifold, o-rings, and hub for cracking.

If you are replacing or upgrading a filter, pool size and debris load are the two primary criteria. Most manufacturers publish flow rate ratings and square footage of filter area; bigger is generally better in Austin because more filter area means longer run times between cleanings. Oversizing the filter for a given pool is rarely a problem; undersizing on a treed lot is a persistent frustration.

Cartridge filters are the most popular choice for new Austin builds because they do not require a backwash waste line, can filter finer particles than sand, and are straightforward to maintain. DE is favored on pools where crystal-clear water quality is the priority or for pools that are particularly prone to fine algae particles. Sand is still a reliable choice and is often the most cost-effective option when a filter needs replacement on an existing pool that is already plumbed for backwash.