Backwashing a pool reverses the flow of water through your filter to flush out trapped dirt, debris, and other contaminants that have accumulated during normal filtration. Over time, all that captured material builds up inside the filter, restricting water flow and reducing the filter’s ability to keep your pool clean. Backwashing pushes water backward through the filter media, dislodging the trapped gunk and sending it out through a waste line.
How Filters Get Clogged
Your pool filter works by forcing water through a filtering medium, whether that’s sand, diatomaceous earth (DE), or a cartridge element. As the filter catches leaves, algae, body oils, sunscreen, and fine particles, that material builds up on and within the filter media. Think of it like a coffee filter slowly filling with grounds. The more debris it collects, the harder your pump has to work to push water through.
This is where your pressure gauge becomes important. Every filter has a “clean operating pressure,” the PSI reading you see right after a fresh cleaning or new install. When that pressure climbs 7 to 10 PSI above your clean baseline, it’s time to backwash. That pressure increase tells you the filter is struggling to move water through the accumulated debris. Ignoring it means poor circulation, cloudy water, and unnecessary strain on your pump motor.
What Happens During a Backwash
On a multiport valve (the dial on top or side of your filter), switching to the “backwash” setting reverses the direction water flows through the filter tank. Instead of entering from the top and filtering down through the media, water pushes up from the bottom, lifting and agitating the sand or DE powder. This loosens all the trapped debris, which then exits through the waste port and out a discharge hose or line.
The process typically takes 2 to 3 minutes. You run the backwash until the water visible in the sight glass (a small clear window on your valve) runs clear. After that, you switch to the “rinse” setting for 1 to 2 minutes. The rinse cycle resettles the filter media and flushes any remaining loose debris so it doesn’t get pushed back into your pool when you return to normal filtration.
Sand vs. DE vs. Cartridge Filters
Not every pool filter uses backwashing. Sand filters and DE filters both rely on the backwash process, but they handle it differently. Sand filters are the most straightforward: reverse the water flow, flush the sand, and you’re done. DE filters are a bit more involved because backwashing strips away the diatomaceous earth powder that actually does the filtering. You need to add fresh DE powder after every backwash to restore the filter’s ability to catch fine particles.
The standard ratio is roughly 1 pound of DE per 10 square feet of filter area. So a 60-square-foot DE filter needs about 6 pounds after a full cleaning. After a routine backwash (as opposed to a complete teardown), you typically only need about 80% of the full amount, since some powder remains on the grids. For that same 60-square-foot filter, roughly 4.5 to 5 pounds will do the job.
Cartridge filters skip backwashing entirely. Instead, you remove the cartridge element and hose it off or soak it in a cleaning solution. This eliminates the water loss that comes with backwashing, which is one reason cartridge systems have become popular in drought-prone areas.
Water and Chemical Loss
Every backwash sends several hundred gallons of pool water down the drain. That water carries with it not just dirt, but also the chemicals you’ve carefully balanced: chlorine, stabilizer (cyanuric acid), algaecides, and dissolved minerals. After backwashing, you’ll need to top off the pool with fresh water and retest your chemistry. Expect to adjust chlorine and possibly stabilizer levels, since both leave with the discharged water.
This chemical loss is the main reason you shouldn’t backwash more often than necessary. Some pool owners backwash on a weekly schedule regardless of pressure, which wastes water and chemicals without providing any benefit. Let the pressure gauge guide you instead. For most residential pools, that means backwashing every few weeks during swimming season, though heavy use, storms, or algae blooms can shorten the interval.
Where Backwash Water Goes
The water you discharge during backwashing contains chlorine, bromine, algaecides, and other pool chemicals that can harm plants, soil, and waterways. Many municipalities have specific rules about where this water can go. The EPA notes that numerous communities either directly prohibit discharging chlorinated pool water into storm drains and local waterways, or regulate what types of chemically treated water can enter sewer systems. Cities including Oklahoma City, Los Angeles, Phoenix, and Virginia Beach have ordinances addressing this, with some imposing fines that increase for repeat violations.
In practice, most areas allow you to discharge backwash water into a sanitary sewer cleanout (not a storm drain) or onto a landscaped area where it can percolate into the ground, provided chlorine levels are low. Directing it toward storm drains, creeks, or ditches is where you’re most likely to run into trouble. Check your local codes before your first backwash to avoid a surprise fine.
Signs You Need to Backwash
The pressure gauge is your primary indicator, but there are other clues. Reduced return flow from your jets means the filter is restricting water movement. Cloudy water that doesn’t respond to chemical treatment can signal a filter that’s too clogged to do its job. And if you notice the pump making more noise than usual or the skimmer not pulling with its typical suction, a dirty filter could be the cause.
On the flip side, pressure that stays elevated even after backwashing suggests the filter media itself needs deeper cleaning or replacement. Sand typically lasts 5 to 7 years before it breaks down and loses its filtering ability. DE grids can crack or develop holes. If backwashing no longer brings your pressure back down to its clean baseline, the issue is the media, not the debris.