When you backwash a pool filter, the dirty water flows out through the waste port on your filter valve and exits through a connected pipe or discharge hose. Where it goes from there depends on your local plumbing setup and regulations, but the most common destinations are your home’s sanitary sewer line, a dedicated drainage area in your yard, or a dry well designed to absorb the water underground. A typical backwash cycle on a 25,000 to 30,000 gallon pool lasts one to two minutes and sends roughly 150 to 300 gallons of water down the waste line.
How Water Leaves the Filter
During normal filtration, water flows into the top of your sand or DE filter, passes down through the filter media, and returns clean to the pool. When you flip the multiport valve to the “Backwash” setting, that flow reverses. Water pushes up through the filter media from the bottom, lifting out the trapped dirt, algae, and debris that had accumulated.
That dirty water exits through the waste port on the valve. Most setups connect a rigid PVC pipe or a flexible backwash hose to this port. Before you start, you need to make sure the hose is rolled out or the valve on the waste pipe is open, because the water comes out with real force. The water that leaves is gone for good. It does not return to the pool, which is why your water level drops noticeably after each backwash.
Sanitary Sewer Connections
In many cities and suburbs, the backwash line is plumbed directly into the home’s sanitary sewer system, the same drain system your toilets and sinks use. This is the preferred disposal method in most jurisdictions because sanitary sewers carry wastewater to a treatment plant where chlorine and other chemicals are processed before the water is released into the environment.
Plumbing codes require a specific setup for this connection. The backwash pipe can’t plug straight into the sewer line. Instead, it must discharge through an indirect waste pipe with an air gap, a physical space between the end of the pipe and the drain receptor below it. The air gap prevents sewage from ever backing up into your pool system. The gap must be at least twice the diameter of the waste pipe’s opening. The receptor it drains into needs its own trap and vent, just like a sink drain, before connecting to the building’s main drainage.
If you’re not sure whether your pool waste line connects to the sanitary sewer, look for a capped cleanout or a floor drain near your pool equipment pad. In homes where the pool was professionally installed, this connection is usually already in place.
Why Storm Drains Are Off Limits
Storm drains and sanitary sewers are two completely separate systems. Storm drains carry rainwater directly to local rivers, lakes, and streams with no treatment at all. Sending chlorinated pool water into a storm drain puts chemicals directly into the environment.
The EPA warns that water treated with chlorine, bromine, or salt is toxic to fish and wildlife when it reaches surface waters. Many communities have passed ordinances that specifically ban discharging chlorinated pool water into storm systems or local waterways. Oklahoma City directly prohibits chlorinated water from entering its stormwater system. Virginia Beach bans pool water from entering the public sewer system entirely. Washington, D.C., imposes fines that increase with each repeat offense. The specifics vary by location, but the direction is clear: chlorinated backwash water should never flow into a storm drain, a roadside ditch, or a nearby creek.
Draining Backwash Into Your Yard
Some pool owners route the backwash hose onto their lawn or a landscaped area, and in many areas this is perfectly acceptable as long as the water doesn’t flow off your property or into a storm drain. The chlorine question is less alarming than it might seem. Pool water typically contains 1 to 3 parts per million of free chlorine. Research from Colorado State University found that chlorinated water at 5 ppm only killed soil organisms in the top half inch of soil. It took 65 ppm, far beyond any residential pool level, to affect organisms six inches deep. So occasional backwash water at normal pool chemistry levels won’t sterilize your lawn.
That said, repeatedly saturating the same patch of grass can cause problems from sheer water volume, salt content (especially in saltwater pools), or slightly elevated pH. Rotating where you point the hose helps. Avoid directing backwash water toward vegetable gardens or sensitive ornamental plants, which are less tolerant of the chemical mix than turf grass.
Dry Wells and Gravel Pits
A dry well is a buried pit filled with gravel or stone that collects water and lets it slowly percolate into the surrounding soil. Some homeowners install a dedicated dry well near their pool equipment specifically for backwash water. The backwash pipe runs underground to the pit, and the water filters through the stone and disperses naturally.
A basic dry well involves digging a hole several feet deep, filling it with three-quarter-inch crushed stone, wrapping it in landscape fabric to keep soil from clogging the gaps between stones, and connecting a buried PVC pipe from the filter’s waste port. This works well on properties with sandy or loamy soil that drains freely. If your soil is heavy clay, water may pool instead of absorbing, making a dry well a poor choice.
Dechlorinating Before Discharge
If your local rules require dechlorination before you can discharge pool water (even to your yard), the process is straightforward. The most common method uses a chemical called sodium thiosulfate, which the EPA recommends for neutralizing residual chlorine. Pool supply stores sell dechlorinating products based on this compound. You add it to the water before or during discharge, and it breaks down the chlorine almost immediately.
The simpler approach, if you have time, is to stop adding chlorine to the pool a few days before a planned backwash. Sunlight naturally breaks down chlorine, and levels can drop below 0.1 ppm within 24 to 48 hours of sun exposure without any chemical addition. Once chlorine is effectively zero, the backwash water is far less of an environmental concern, though other chemicals like stabilizer and salt remain.
Choosing the Right Option for Your Setup
Your best disposal path depends on three things: what your local code requires, what plumbing already exists, and how much water you’re dealing with. A sanitary sewer connection is the most universally accepted option and requires the least ongoing thought. You backwash, the water goes to the treatment plant, and you’re done. If that connection doesn’t exist and installing one is expensive, a dry well or yard discharge can work as long as you stay within local rules.
Keep in mind that 150 to 300 gallons per backwash adds up over a season. If you backwash every week or two, you could be sending 3,000 to 7,000 gallons down the waste line over a summer. That water needs to be replaced, which affects your water bill and means you’ll need to rebalance your pool chemistry each time. Backwashing only when your filter pressure gauge rises 8 to 10 psi above its clean baseline, rather than on a fixed schedule, saves water and keeps your filter running efficiently.