Maintaining a clean and healthy swimming pool requires more than just skimming the surface and checking chemical levels. A crucial part of this maintenance process is backwashing, a technique used to clean the pool filter. While it’s a common practice among pool owners, many wonder what happens to the water during and after a backwash cycle. This article will explore every detail of the backwashing process, focusing particularly on the question: When you backwash a pool, where does the water go?
We’ll walk through the mechanics of backwashing, what happens to the water, where it drains to, and the environmental considerations involved. Whether you’re a new pool owner or just curious about sustainable pool maintenance, this article will ensure you walk away with a solid understanding of this essential process.
What Is Backwashing in Pool Maintenance?
Before we answer the question of where the water goes, it’s helpful to first understand what backwashing is and why it’s an integral part of managing a swimming pool.
Backwashing is the process of cleaning a pool filter by reversing the flow of water through it. Most pools use sand or diatomaceous earth (DE) filters, both of which trap debris and contaminants as water passes through them during regular filtration cycles. Over time, these particles accumulate, reducing the filter’s efficiency and increasing pressure in the system.
By initiating a backwash cycle, pool owners or automated systems reverse the water flow, which lifts the filter media (such as sand or DE grids) and flushes out the trapped dirt. This not only keeps the filter working correctly but also maintains the overall water quality of the pool.
The Mechanics of Backwashing
Understanding how backwashing works can help illuminate where the water ultimately flows:
- Water Flow Reversal: When the backwash mode is engaged, usually through a multiport valve, the water flow through the filter is reversed.
- Debris Expulsion: As water flows backward through the filter medium, it dislodges trapped particles, turning the water in the filter dirty and heavily laden with contaminants.
- Drainage Through Waste Line: This dirty water, along with the debris, exits the filter through a designated waste line, rather than returning to the pool.
It’s important to note that backwashing doesn’t simply recycle dirty water; it expels it to keep your pool’s filtration unit clean and operational.
Where Does the Water Go After Backwashing?
The short answer to the question is: The backwashed water goes through the waste line and is expelled from the system. But how exactly that happens — and where the water ends up — depends on your setup and local regulations.
This is where the multiport valve comes into play, and understanding its function will help clarify what happens during and after a backwash cycle.
The Role of the Multiport Valve
The multiport valve is typically found on sand and DE filters and serves several functions. It allows the user to direct the water flow in various directions, such as filter, rinse, recirculate, closed, and waste (used for backwashing).
When switched to the backwash setting:
- Water flows in reverse through the filter.
- Cleaned debris is flushed into the waste line.
- The backwashed water travels to a drainage point instead of returning to the pool.
This waste discharge can go to several places, depending on how your system is set up, as we’ll discuss below.
Common Drainage Options for Backwashed Water
- Storm Drain or Municipal Sewer Line: If your pool is connected to a municipal system, backwashed water often goes into either a storm drain or a sewer line. However, this depends on local regulations and whether the water is considered hazardous due to chemical treatments.
- On-Site Retention Drain Field: Many homeowners have a dedicated underground drainage system to accept pool filter waste water. This field disperses the water safely into the soil, often through gravel-filled trenches.
- Surface Disposal (Outdoor Drainage): Some systems simply discharge backwashed water onto a lawn or landscaped area. While convenient, this method must be managed carefully because chlorinated water can harm plants and soil life.
- SwimClub or Commercial Pool Septic System: Commercial pools may have a dedicated treatment system to manage waste water safely before releasing it into the environment.
The correct setup is often dictated by local water regulations, so ensure you understand municipal codes or state laws before installing or modifying your backwash drainage system.
Environmental Impact and Considerations
The environmental consequences of backwashing depend largely on the chemical content of your pool water and how it’s disposed of. Chlorine, algicides, and flocculants are commonly present in pool water, and if not managed carefully, these substances can affect local waterways, plants, and animal life.
What to Watch Out For With Backwashed Pool Water
Here are some elements of environmental concern when it comes to where pool backwash water goes:
- Chlorine and Chloramines: These common pool sanitizers can be toxic to aquatic life and vegetation. Before disposal, ensure the chlorine level is at or close to 0 ppm by allowing the water to dechlorinate using a chemical neutralizer or by running it through charcoal filters.
- High Phosphate and Algae Content: If your pool is green (algal bloom), backwashing such water can introduce harmful nutrients into the environment, potentially feeding more algae in nearby bodies of water.
- Contaminants and Metals: Metals like copper from algaecides or iron from source water can accumulate and cause staining or long-term soil impact — particularly important when draining to grass or landscaping.
Understanding and mitigating these risks is essential, especially if you’re draining to municipal lines or into the natural environment.
Dealing with Regulations and Local Ordinances
It’s vitally important to check your local municipality’s or state’s regulations before discharging pool water from backwashing. In some cases, chemical content limits apply — especially for businesses and commercial pools. Penalties may be imposed if chemical levels exceed thresholds or if homeowners dispose of water improperly.
Some cities may require the installation of a neutralization system or even encourage water conservation methods like filtering and reusing pool water rather than draining it entirely.
How Much Water Is Used During Backwashing?
Along with understanding where the water goes, it’s also useful to know how much water a backwash typically uses. This can help in planning for both water efficiency and disposal.
On average, a backwash cycle for a medium-sized residential pool can use anywhere from 150 to 300 gallons per session. This amount can vary based on pool size, filter media, frequency of backwashing, and duration of the cycle.
This is one reason why understanding how often — and what triggers — a backwash is important in managing your water use responsibly.
Factors Affecting Backwash Water Volume
| Factor | Description |
|---|---|
| Filter Type | Sand filters generally require longer backwash cycles than DE filters, potentially using more water. |
| Pool Usage | Heavily used pools or those in dusty climates typically need more frequent backwashing, increasing water usage. |
| Water Pressure | Higher pump pressure can mean faster flushing but may consume more water if the system is not efficient. |
Opting for an efficient filtration unit, monitoring your filter pressure regularly, and backwashing only as needed are key practices to conserve water in your pool maintenance.
Backwashing Frequency and How to Know When to Do It
To keep your pool’s filtration system running efficiently, knowing when to backwash is just as crucial as knowing where the water goes afterward.
As a general guideline:
Backwashing should be done when the filter’s pressure gauge rises 8–10 psi (pounds per square inch) above the starting pressure.
This pressure increase typically indicates that enough debris has built up in the filter to hinder proper flow and decrease filtration performance.
However, frequency will vary:
- A private backyard pool may need backwashing every few weeks
- Commercial pools might need backwashing every few days
- Seasonal or frequent swimmers may find the pressure increases more quickly in summer months
Don’t backwash just because it’s been a certain number of days — always rely on pressure readings and visual water clarity to make the decision.
Alternatives to Backwashing
If you’re looking to reduce water usage but still maintain a clean filter, there are a few modern alternatives to traditional backwashing.
Cartridge Filters
Unlike sand or DE filters, cartridge filters don’t require backwashing. Instead, they are manually cleaned by removing the filter and hosing it down. This option significantly cuts back on water use and avoids the question “where does the water go?” altogether.
High-Efficiency Filters
Some advanced sand and DE filters are fitted with systems that optimize flow and reduce the need to reverse the system as often. These filters can help maintain lower pressure spikes and delay the need for backwashing.
Reclaimed Backwash Systems
Innovative residential and commercial settings are beginning to install systems that capture and reuse a portion of backwash water. These systems filter out debris and sanitize the water before reusing it for landscape irrigation or toilet flushing, improving overall water sustainability.
Best Practices for Responsible Backwashing
To promote safe, environmentally sound pool maintenance and water conservation, consider adopting these best practices:
- Monitor your filter pressure regularly — never backwash unnecessarily.
- Use a test kit to ensure chlorine levels are at 0 ppm before disposal.
- Direct waste lines to gravel-filled drain fields rather than directly onto grass.
- Consider switching to a cartridge filter to avoid backwash cycles entirely.
- Explore greywater systems to reuse some backwash water for non-potable needs.
By approaching your pool maintenance with sustainability in mind, you’ll gain peace of mind over both the mechanics and the impacts of your pool habits.
Conclusion
So, when you backwash a pool, where does the water go? Now you know the answer is multifaceted: the water goes through a complex system governed by your filter type, your plumbing setup, and local environmental regulations.
Backwashing is a vital part of keeping your pool clean and your filter efficient, but it doesn’t have to come at the expense of water conservation or environmental safety. By managing your backwash properly, using responsible filtration alternatives when available, and following disposal guidelines, you can maintain your backyard oasis sustainably.
Whether you have a small above-ground pool or an elaborate in-ground design, understanding backwashing — and where the water ends up — is an essential skill for any conscientious pool owner.
About the Author
This article was written by experienced pool maintenance professionals committed to delivering clear, actionable information to help pool owners manage their systems efficiently and sustainably. We combine decades of experience in pool systems engineering with an ongoing commitment to green practices and water conservation.
If you found this article useful, share it with fellow pool owners — and take the extra step to schedule your next filter backwash wisely.
What is pool backwashing, and why is it important?
Pool backwashing is a cleaning process used for swimming pool filters to remove accumulated debris and contaminants. During normal operation, pool water passes through the filter media—such as sand, diatomaceous earth (DE), or cartridges—trapping dirt and other particles. Over time, this buildup can reduce the filter’s efficiency and increase pressure within the system, which is why backwashing is essential to maintain optimal filtration and water clarity.
Backwashing reverses the flow of water through the filter, lifting and expanding the media to flush out trapped debris. This helps restore the filter’s performance and prolongs its lifespan. Regular backwashing not only keeps the water clean and safe for swimmers but also supports the proper function of the pool’s circulation system. Without it, filters become clogged, leading to poor water quality, increased chemical demand, and potential damage to pool equipment.
Where does the water go during backwashing?
During backwashing, the water is redirected through the pool filter’s waste line, which is connected to a dedicated plumbing line that leads to a drainage system. This line generally flows to a sanitary sewer, a septic tank, or directly to the ground, depending on local regulations and the pool’s plumbing setup. The backwash water carries away dirt, oils, and other contaminants that have been filtered out of the pool.
It is important to know local codes about backwash discharge because the water contains chemicals like chlorine, which can harm plants, aquatic life, and public water systems. Some areas require that backwash water be dechlorinated before disposal or directed to appropriate wastewater treatment systems. Improper disposal can result in environmental harm or legal consequences, so understanding where the backwash water goes and how it should be handled is a key part of pool maintenance.
How much water is used during a backwashing cycle?
The amount of water used during a backwashing cycle depends on the size and type of filter, as well as the duration of the backwash. For a typical residential pool with a sand filter, a single backwashing session can use anywhere from 150 to 300 gallons of water. This process generally lasts between 2 to 5 minutes, with water flowing out of the waste line until it runs clear.
Because this volume adds up with regular maintenance, backwashing contributes significantly to water usage in pool ownership. Pool owners in drought-prone or water-restricted areas should consider efficient backwashing practices, such as timing and frequency optimization, or explore alternative systems like cartridge filters that do not require backwashing. Conserving water during pool maintenance can reduce environmental impact and lower utility bills.
How often should I backwash my pool filter?
The frequency of backwashing depends on several factors including the filter type, pool usage, and the amount of debris in the surrounding environment. As a general rule, most pool owners should backwash every 4 to 6 weeks. Signs that it’s time to backwash include increased pressure on the filter’s gauge, reduced water flow, or visible dirt accumulation in the pool despite regular cleaning.
Over-backwashing can waste water and may disturb the filter media, reducing its effectiveness. Conversely, waiting too long can strain the filtration system and compromise water quality. Monitoring the pressure gauge and being attentive to changes in water clarity will help determine the ideal backwashing schedule. It’s also recommended to consult your filter’s manual or a pool professional for model-specific guidance.
Can backwashing water be reused or recycled?
Reusing or recycling backwash water is challenging due to its high contaminant and chemical content, particularly chlorine and fine particulates. Most backwash water is not suitable for direct irrigation or household reuse without proper treatment. However, advances in pool water recovery systems and environmentally friendly water recycling methods are emerging in some commercial and residential applications.
Filtration and treatment units can remove solids and neutralize chemicals, making the water safer for non-potable uses like landscape irrigation or dust control. Some pool owners also explore onsite retention systems where backwash water can settle before being used cautiously for cleaning or toilet flushing. The feasibility of recycling depends on local water regulations and environmental conditions, so it’s essential to seek guidance before setting up any greywater reclamation system for backwash water.
What are the environmental effects of pool backwashing?
Backwashing releases water containing chlorine, algaecides, and other chemicals into the drainage system, which can affect local waterways and ecosystems if not properly managed. Chlorine, in particular, is toxic to aquatic life and can impact water treatment plants if large volumes are introduced without regulation. Additionally, the sediment and organic matter flushed out during backwashing can contribute to pollution in stormwater systems.
To minimize environmental effects, pool owners should follow local disposal guidelines and consider dechlorinating backwash water before discharge. Reducing the frequency of backwashing through proper maintenance and using eco-friendly pool chemicals can also help. For environmentally conscious pool owners, alternative filter systems and water recapture technologies provide sustainable options to lessen the ecological footprint of pool maintenance.
Is it possible to stop backwashing entirely with a different filtration system?
Yes, some filtration systems eliminate the need for backwashing. Cartridge filters, for example, do not require backwashing—instead, they use a physical cartridge that can be removed and rinsed clean. These systems are more water-efficient since they only need periodic manual cleaning and may be preferable in areas where water conservation is a priority or in locations with restrictions on backwashing.
While cartridge filters reduce water waste and eliminate the need for a backwash line, they may require more hands-on maintenance and have a limited capacity to filter fine particles compared to sand or DE filters. Sand and DE filters remain popular for their filtration efficiency but do require occasional backwashing. Choosing the right filtration system depends on your maintenance preferences, environmental concerns, and the size and usage of your pool.