How Does a Well Water Filtration System Work?

Building a Complete System

What does a well water system look like? That depends on the types of contaminates in your water:

At each stage, your equipment choices are dependent on the level of contamination in your water, whether that means using larger filters or different filtering technologies. The cleanest well water may only need sediment and carbon filtering, while the dirtiest water may need advanced filtration and chemical treatment to remove minerals, metals and toxins.

Testing Your Water

How do you know what your well filtration system needs? It starts with a water test. Your well water should be tested when it’s first dug, so you can plan your initial treatment system. After that, the water should be retested if there are changes to your well or treatment system. This includes flooding, land disturbances, and the replacement of any plumbing or filtration components. You also need to get a new test if you notice changes in the flavor, odor or color of your water.

Water tests can be comprehensive, or just focus on one factor of water quality. Here’s what these tests look for, and how they relate to water quality.

Coliform bacteria don’t make you sick, but they’re easy to test. By looking at the amount of these bacteria present in the water, you can get an idea of how many harmful bacteria, viruses and parasites may be present.

E. Coli is a form of coliform bacteria. There are harmful and harmless forms of this bacteria. Like the coliform test, this doesn’t look for harmful bacteria directly, but gives you an overview of water possible contamination.

pH is a measurement of how acidic or basic the water is. This changes how water tastes and looks. Water with an extreme pH can leach heavy metals from pipes.

While nitrates occur naturally in foods, high nitrate levels in water can lead to illness. Nitrates come from a variety of sources, including agricultural runoff and flooded sewers. However, the amount of nitrates that make it into your well water can vary greatly, depending on your local geology. While your well might have high levels of nitrates, your neighbor’s water can be almost nitrate free, depending on the types of rocks and soil around the well.

Volatile Organic Compounds (VOCs) are solvents and other chemicals that may be present in your water. The most common pollutants are benzene, toluene, MTBE and carbon tetrachloride. This pollution is regional, so you’ll get recommendations on what to test for when you order your test. VOCs have been detected in about a third of the wells tested by the sampled by the National Water-Quality Assessment (NAWQA) Program.

Iron leaves difficult-to-remove stains around faucets. These stains are particularly hard to remove from grout. It also builds up inside pipes, decreasing water flow and leading to clogs in plumbing and appliances.

Once you know the issues you need to address, you can plan out your treatment system. Most well water treatment systems treat water in this order:

Using this order protects water treatment equipment. For example, by removing sediment first, larger debris can’t clog filters and media used to remove metals and adjust water hardness.

For the most part, each step uses the same methods as municipal water systems, just scaled down for home use. The sole exception is the use of chlorine and chloramine. These chemicals are used to disinfect water in municipal systems because they remain active as they pass through pipes to your home. That means the water is still safe to drink, even if it passes through a contaminated area. Since well water has to travel a few feet instead of a few miles to your faucets, this is much less of an issue. These chemicals can be used, but they’re much less common.

One term you’ll see repeatedly when looking at filter equipment is microns. A micron is one millionth of a meter. There are 25,400 microns in an inch, while a human hair is about 45 microns wide. For comparison, most single cell organisms are between 1 and 5 microns wide, while viruses are less than one third of a micron in size. The best straining filters can only remove particles as small as half a micron. That means many contaminates, like minerals, have to be removed using chemicals or filters that use methods other than straining. Here are the most common treatment methods, in order by where they’re used in a well water treatment system.

Water Softeners

Water dissolves minerals as it passes through certain types of rock. If it picks up calcium and magnesium, it becomes hard water. Calcium and magnesium ions leave mineral deposits on plumbing, water heaters, sinks and bath tubs. They also leave skin and hair dry. Water softeners deactivate these ions. This keeps water flowing through your plumbing, makes it easier to clean around faucets, and decreases the amount of soap you need for cleaning.

Water hardness is measured in Grains Per Gallon (GPG.) One grain is equal to 17.1 Parts Per Million (PPM.) Soft water contains one grain of calcium or less per gallon. Moderately hard water is between 1 and 7 GPG, while very hard water is over 10 GPG. The hardness of your water is the most important factor when choosing a water softener.

Salt-based systems replace mineral ions with sodium ions. They use a media tank full of negatively-charged resin saturated with salt, and a brine tank. As hard water passes through the main tank, the positively-charged calcium and magnesium ions bind to the resin, while the salt binds to the water. The amount of salt added to the water is minimal. On average, an 8 ounce glass of salt-treated water only has 12.5 mg of sodium. For comparison, a ¼ teaspoon of salt contains 590 mg of sodium.

Eventually, the salt is stripped away from the resin, while the spaces on the resin’s surface are filled with hard water ions. The resin is recharged by flushing it with brine from the brine tank. The salt switches places with the calcium and magnesium ions. This leaves behind extremely hard, salty water that is sent through a drain. The resin tank is rinsed with clean water to remove any remaining hard water minerals and loose salt. A correctly-sized system needs to recharge once or twice per week. Recharging is handled automatically by a controller, so all you need to do is make sure the brine tank has plenty of salt.

Salt-free systems use media with nucleation points to crystallize mineral ions. As the ions pass through the media, they latch on, drawing in more molecules that bind together to form crystals. Eventually, these crystals flake off into the water. These crystals won’t stick to surfaces or react with your skin and hair. The crystallization process also releases microscopic carbon dioxide bubbles. These bubbles scrub deposits off of pipes. Technically, this system is a descaler or conditioner, not a softener, since it isn’t removing hard water molecules. There’s no salt used in this system, so it doesn’t change the water’s pH.

Most water conditioners can only handle moderately hard water. If your water has a hardness over 10 GPG, you need a salt-based water softener.

Sediment Filters

A sediment filter uses mechanical filtration to purify water. These filters use a variety of materials, including ceramics, polypropylene, polyester, cellulose, cotton, glass fiber and even string. The pores in this media ranges in size between 1 to 80 microns. These filters remove large particles, including sand, silt, plant material, dirt and rust flakes. Sediment filters are the first step in a well filtration system, because they get rid of large particles that can clog other filtration equipment.

Sediment filters can be divided into three main types, based on how water flows through them. Spindown filters use a screen and centrifugal force to filter water. This forces the largest contaminates out of suspension, leaving them in a container below the water line. Every week or so, this container must be emptied. Spindown filters only remove heavy dirt, so they have to be used with other sediment filters.

In axial flow filters, water goes in one end of the filter and out the other end. The filter media has to be loosely packed to let water flow through. Over time, channels can form, letting water pass without being filtered. However, restriction is minimal, so pumping requirements are low for this type of filter.

In radial flow filters, water enters the outside of the filter. From there, it has to pass through the filter media to the inside. Since the media is fixed in place, it can use multiple layers. When water first enters the filter, it goes through media with large pores, trapping the largest contaminates. Each successive layer has smaller and smaller pores, filtering out more and more sediment. Since all the filter media is used equally, radial flow filters have up to 15 times as much usable surface area as axial flow filters. However, it takes a lot of pressure and time to push water through all those layers.

Sediment filters come in different diameters and lengths. The larger the filter, the more water it can handle. A 10 inch diameter filter may be enough for a two or three bedroom home, while you’ll need a 20 inch filter to keep up with the demands of a 4-5 bedroom home.

Radial flow filters are made of a pleated fabric or solid polypropylene. If a pleated filter clogs, it can be removed and rinsed to remove the clog. Once a solid polypropylene filter clogs, it has to be replaced. If you’re treating exceptionally dirty water, you can use a system with multiple filters. After a spindown filter removes the largest particles, a coarse filter handles mid-sized particles, and then a fine filter removes anything that’s left. By splitting up filtering duties, clogs are less common, and filters last longer.

The majority of dirt particles are larger than 20 microns. Pores in sediment filters are between 50 and 5 microns in size.

Iron Filter

Iron leaves rust deposits on pipes, plumbing fixtures, sinks and bath tubs. It also gives water a metallic taste and smell, and encourages the growth of bacteria. High iron concentrations encourage the growth of iron bacteria. Water high in iron also tends to have high levels of arsenic, manganese and sulfur. Not all well water has enough iron in it to require a filter. A water softener can remove up to 0.5 PPM of iron. However, this requires more frequent backflushing. The media also needs to be treated with special iron removing chemicals.

Iron can be soluble (ferrous) or insoluble (ferric.) Soluble iron is held in suspension, while insoluble iron falls out of suspension. Soluble iron can be turned into insoluble water using oxygen. However, the resulting ferric iron leaves deposits on anything it touches. Iron bacteria also ferrous iron into ferric iron, resulting in a slimy metal sludge that floats on top of water.

Iron filters are designed specifically to remove iron, but they have some major limitations. Most cartridges only work with water that has a pH of 7.0. They also won’t work with hydrogen sulfide present in the water. These filters are mostly used in small scale systems, like cabins that see occasional use. Instead, most well water treatment systems use chemical or air injection to remove iron from water.

Chemical injection systems use a strong oxidizer, typically chlorine or hydrogen peroxide, to react with iron, sulfur and manganese. This makes these metals insoluble, so they fall out of suspension. Off-the-shelf systems can usually handle up to 20 ppm of iron. This process takes time, so these systems use a large storage tank to hold water during treatment. Once the chemicals have had time to react, the treated water goes to a backwash tank to separate out the metals. The treated water passes through a carbon filter to remove the oxidizer. This system is effective at removing both iron and iron bacteria.

Air injection systems spray water through an air bubble inside a tank. This aerates the water, exposing iron directly to oxygen. To increase effectiveness, some systems add an ozone generator. Ozone is more effective than the air’s O2 molecules at oxidizing iron. These systems require less maintenance than hydrogen peroxide-based filters, but they’re usually limited to treating water with up to 10 PPM of iron. Ozone is also hard on plumbing, which shortens the life of the backflush valve and other components that have direct contact with this gas.

Like hydrogen peroxide systems, air injection systems use a catalytic carbon filter to remove metals before the water enters the rest of the plumbing system. Like water softeners, these systems have to backwash occasionally to remove built-up iron and sulfur. However, there’s no salt or brine tank involved. They just use water to push contaminates out of the tank and into your drainage system.

Carbon Filters

While this is the smallest part of a well water filtration system, it’s also the most complicated looking. Most carbon filter systems have multiple filter chambers, plus valves, hoses and an external tank.

While some people still call these charcoal filters, modern filters use activated carbon. The carbon can come from many sources, including coal and peat, but most filters use coconut shells. The material is heated to extreme temperatures in an oxygen-free environment, then bombarded by superheated steam. This breaks up the carbon, creating tremendous amounts of surface area in a small space. One gram of activated carbon has 500-1,000 square feet of surface area.

Carbon filters work by using adsorbtion. As contaminates pass through, they’re attracted to the surface of the carbon. This type of filtration is effective on chlorine, organic chemicals, VOCs and THMs. That means they’re effective at removing most industrial chemicals, including solvents and pesticides. These aren’t just bad for you, they also make the water taste terrible. There are also specially treated carbon filters that remove lead, but these filters won’t remove arsenic or dissolved inorganic compounds. Catalytic carbon filters have an extra coating that is effective at removing chloramine. However, it’s unlikely that this chemical is present in your well water.

Carbon filters can be solid blocks or loose granular activated carbon (GAC.) GAC has a higher flow rate, but carbon block filters are more effective at filtration. On average, a carbon filter will have a 6-12 month lifespan, depending on the level of contaminates in the water. Once the carbon is saturated with contaminates, it stops filtering and can start to break down. This means metals aren’t being removed, and some of the collected contaminates may be mixing with the water.

Passing water through all these filters takes time, so most systems have a reserve tank. When you open a tap, a valve on the filters switches the direction of flow, so water is drawn from the tank. When the tap closes, the valve switches back to directing water from the filters to the tank. Since capacity is limited, carbon filters are only connected to faucets used for drinking water. They’re also connected to ice makers, as the filtered water makes clear, odor-free ice.

Reverse Osmosis

Originally created for submarines in WWII to convert sea water into drinkable water, this filtration method is commonly used to produce bottled water. Reverse osmosis filters are often combined with carbon filters, creating a single unit that filters water for drinking.

Osmosis uses a membrane that blocks large molecules. If you use this membrane to separate clean water and salty water, the clean water will flow through the membrane to the salty side until the salt concentration is equal on both sides. Reverse osmosis uses pressure to reverse this flow, so water molecules in the salt water side pass through the filter. The result is clean water on the filtered side and increasingly salty water on the unfiltered side.

Most reverse osmosis systems use multiple filters to purify the water as much as possible. First, water passes through a sediment filter to remove any fine particles that may be left in the water. Next, the water flows through two carbon filters, typically one GAC and one solid block.These are a must have if you use chlorine at any point in your water treatment system, since this chemical destroys osmosis filters.

The filtered water moves through the membrane. This membrane uses a radial design, like some sediment filters. Reverse osmosis filters are made from several layers of material. A support layer provides space for water to flow through the filter. By restricting flow, the water is forced through the filtration membrane. The filtered water goes to the rest of the plumbing system, while the runoff is sent to a drain.

Due to the restrictions in the membrane chamber, flow rate through these systems is slow. To compensate, the water is held in a small reserve tank. This tank only has one hose. When you open the faucet, a valve switches flow. Instead of water going from the membrane to the tank, it goes from the tank to the faucet. This valve operates by using the pressure differences between the membrane side and the faucet side. Once the faucet valve is open, the drop in pressure opens the valve. Likewise, it shuts off flow from the filter to the tank once the tank is full. Most reverse osmosis systems have a final GAC carbon filter. This “polishing” filter removes any remaining contaminates before it reaches the faucet.

The better the osmosis filter is at filtering, the less water goes down the drain. A standard efficiency filter typically makes 3-3.5 gallons of runoff water for every gallon of clean water. High efficiency filters make as little as one gallon of runoff water for every gallon of clean water. The carbon and sediment filters are the same on standard and high efficiency systems.

Since this level of filtering isn’t needed for washing or cooking, reverse osmosis systems have a dedicated faucet just for drinking water. You can also use this system as a water source for your ice maker. This creates clear, odor-free ice.