What is Backwashing in Water Filtration?

By Rehan Iqbal

There are a ton of choices for filtering municipal and well water, including UV, sediment, activated carbon, catalytic carbon, ion exchange, air injection and more. Where do backwashing filters fit into all this? They aren’t a different way to treat water. Instead, they have a cleaning system that extends the life of the filter media. How does backwashing work, and why does it only work for some filters? Is recharging a filter the same as backwashing it? How do you deal with drain water, and where can you dump it? Here’s everything you need to know about these systems, so you can make the right choices when you upgrade your water treatment system.

US Water Matrixx Catalytic Carbon Backwashing Filter

What is a Backwashing Filter?

A backwashing filter flushes water through the filter media to remove contaminants. There are several types of filter media that benefit from backwashing, so it’s not used exclusively on one type of water treatment system. If something is marketed as a “backwashing filter,” it’s usually a system that combines several types of media in a single tank, or a simple sediment filter. However, there are plenty of single media systems that use backwashing to keep the filter clean.

How Does Backwashsing Work?

The media tank has a valve head that controls the flow of water through the system. When the system is in service, water enters the head, passes through the media for treatment, and exits to the rest of the plumbing system. In backwashing mode, the positions of the valves switch, so inlet water is forced through the media and out of the system to a drain line. backwashing uses a higher flow rate than service use. The force of the water shifts the media around inside the tank, which helps shake contaminates loose. This also resettles the media in the tank, removing blockages and channels. Here’s how backwashing restores the function of different types of filters:

Sediment filters - The filter media acts like a sieve, trapping particles in its fine pores. backwashing pushes these trapped particles out of the filter median and into water to be drained.
Carbon filters - Activated and catalytic carbon filters use their massive surface area to attract particles in the water. In effect, it’s less like a sieve and more like a piece of Velcro. To be effective, the carbon granules must be tightly packed, which makes them easy to clog. backwashing pushes this debris out of the filter medium, restoring its flow rate. While some companies mistakenly call this “recharging,” it doesn’t remove the particles stuck to the surface of the media. Once activated carbon is saturated with contaminates, it must be replaced.
Ion exchange (salt-based) water softeners and nitrate filter recharging - Salt-based water softeners remove hard water minerals from water using ion exchange. As water passes through the filter, an electrical charge makes mineral ions swap places with salt ions on the filter media. Eventually, the media becomes saturated with these minerals, and it stops working. Nitrate filters use a different type of media, but the ion exchange process is the same as water softeners.

Regenerating the filter replaces the hardness minerals with salt using backwashing. Salt is mixed with water to form brine, which is pushed through the filter. The salt swaps places with the minerals on the filter media, creating hard, salty flush water.

Water conditioners, which are sometimes called salt free softeners, don’t use ion exchange. Instead, the media acts as a nucleation point. Hardness minerals latch onto the media, forming microscopic crystals that break off when they get too big. These crystals don’t react with appliances and plumbing fixtures, so they don’t need to be removed from the water before it travels through the rest of the plumbing system. Since there’s no salt to deplete or buildup of debris, these softeners don’t need to backwash.

Do Reverse Osmosis (RO) Filters Backwash?

No. Reverse osmosis filters have a drain line, but this isn’t for backwashing. Instead, reverse osmosis uses pressure to concentrate contaminants like salt on one side of a membrane. This dirty water goes down the drain line, while the clean water on the other side goes to the tap.

Due to the submicron size of the pores in an RO filter’s membrane, backwashing isn’t effective. Fortunately, this filtration stage is at the end of the water treatment system. By this point, anything that could cause clogging has already been removed from the water. Most clogging problems in RO filters are caused by hard water. Adding an ion exchange filter or water conditioner fixes this problem.

What are the Benefits of a Backwashing Filter over a Cartridge Filter?

A cartridge filter has a housing with replaceable filter media. When the media is spent, you just pop in a new cartridge. Since the media is constantly replaced, these systems don’t need a huge tank. This makes them ideal for installations in confined spaces. Cartridges are used for sediment, carbon and reverse osmosis filtration.

Due to their small size, cartridge-based systems usually have more restriction and lower flow rates than backwashing filters. While they’re fine for small homes, you won’t find systems built for homes and building with more than 3-5 bathrooms. Cartridge systems cost less to buy and set up than backwashing filters, but the price advantages are quickly negated by the cost of replacement cartridges. Point of Entry (POE) water softeners, oxidizers and sediment filters are almost always large backwashing units.

When Do I Need to Backwash My Treatment System’s Filters?

This depends on a number of factors, including your water use rate, the level of contaminates in water, and the manufacturer’s recommendations.

Modern electronic control heads measure water flow through the system, running a backwashing cycle after the system processes a certain amount of water. These systems also monitor time, running the backwashing cycle when you aren’t likely to need water, typically late at night. Older systems only have a timer. It’s up to you to choose how often the system cycles, and when.

Naturally, the more contaminants the filter removes, the sooner it needs to be cleaned. The type of contaminants being removed also affects cleaning cycle frequency. For example, while carbon filters can remove small amounts of iron, the presence of this metal greatly increases clogging. When filtering water that contains iron, manufacturers may recommend flushing two to three times as often.

It’s typical for an ion exchange filter to recharge anywhere from twice per week to once every two weeks, depending on water hardness. Other filters typically cycle every week or two.

How Long Does Backwashing Take?

A typical cycle takes 10 to 20 minutes, depending on the flow rate and the size of the tank. After recharging, ion exchange systems refill the brine tank. This uses regular water pressure, so it takes a while. However, the rest of the system can be in service while the tank is refilling.

How Do I Get Rid of the Flush Water?

The water from backwashing has to go somewhere. How it exits your home and where it goes depends on how your system is installed, and where you live.

Regulations governing waste water from flushing vary from area to area. The chief concern with backwashing is salt contamination from brine. In some areas, the release of brine isn’t allowed, effectively eliminating the use of backwashing ion exchange filters. Instead, you need to use a water conditioner to treat hard water, or subscribe to a service that collects and processes the ion exchange tank when it’s saturated with hardness minerals.

In most cases, you can dump flush water into the ground. However, spent brine from ion exchange filter recharging contaminates ground water. Many areas don’t allow dumping of brine, and even if it’s legal, it can harm the plants on your land. The salt from the brine can reach your well, contaminating your drinking water.

If you’re connected to a city sewer system, you can usually empty the water into the sewer. If you have your own sewage system, you can dump the water there, as long as it doesn’t lead to groundwater contamination.

There are several ways to connect your drain lines to your disposal system. A floor drain is the simplest, most common option. Usually, the backwashing cycle provides enough pressure to push water into sewage lines, even if the tank is in the basement. If it doesn’t, you need to add a sump pump or a sewage ejector pump to your system.

Waste water can be sent to a dry well, which is just a hole in the ground for dumping water. A dry well can be used in tandem with a sump pump. If there’s too much water for the well to handle, the excess is pumped outside or to your sewage system.

A septic drain field sends water through lines that cover a wide section of your lawn. If you have a septic tank, you also have one of these fields. You can connect the drain lines to this field directly, sending the water out across your lawn.

A French drain is a simplified septic field. Instead of a series of pipes, it’s just one long pipe in a ditch. This pipe is covered in holes, letting drain water escape. The longer the pipe is, the more the water spreads out across the lawn. The trench is filled with fine gravel, keeping the pipe holes from being blocked by soil. French drains are also used in landscaping to correct drainage problems.

What is an Air Gap, and Why Does My Drain Line Need One?

Plumbing code requires an air gap on all drain lines used for backwashing. This gap separates dirty and clean water systems. Without one, dirty water can flow up the drain line into the filtration system, contaminating your water.

An air gap is exactly what it sounds like. It’s an open air space between two parts of a plumbing system. Flush water can go out to the drain, but water backing out of the drain spills over the gap instead of entering the flush water outlet. If you’re using a floor drain or a dry well, the drain outlet just needs to stay above the water line. Pipe adapters that can be added to closed systems. These adapters have a small pipe sleeved inside a large pipe connected to the drain. This large pipe has a hole in the side to let air and water exit before it reaches the flush water pipe.

Air gaps are also required for multiple drainage systems. In other words, you can’t merge two drain lines to form a single line before the drain.