How Do Water Filters Work?

Posted by
John Woodard on August 11, 2023

Ever paused to consider what makes your drinking water so pure and refreshing? It's not magic; it's science. Water filters work tirelessly behind the scenes, turning murky, contaminated water into the crisp, clean drink we often take for granted. But how exactly do they achieve this transformation? Dive in as we explore the mechanics of various filtration systems.

What is a water filter?

A water filter is a cartridge, system, or device that removes impurities, such as sediment, heavy metals, chlorine, VOCs, and TDS from water. A water filtration system can service an entire home or connect to a single faucet or fixture. A system that filters water for an entire home may be used to protect against bacteria that can be harmful to ingest, sediment or minerals that can damage pipes, or contaminants that give water an unpleasant odor or taste. Water filters that connect to a single faucet are most often used in the kitchen, where clean water is needed for drinking, cooking, and washing dishes.

How does a water filter work?

A water filter runs water through a porous material to remove impurities. Some water purification systems, like distillation, do not use a filter medium to remove impurities. While technically not filters, these systems are in the same discussion as water filters because they achieve similar goals. While some water filter types work very similarly to each other, each one removes contaminants in its own way.

Types of water filters

A variety of water filtration systems are available for household use, and they each remove different contaminants from the others. When purchasing a water filter, it is important to understand the makeup of the water at your home so you can make the most informed decision.

Click to read about the following filter types:

Mechanical water filters

Mechanical water filters contain some type of mesh, strainer, or other porous material that removes suspended particles from water as it flows through. Two common types of mechanical filters are sediment and ceramic filters.

Sediment water filters

Sediment water filters are designed to remove sediment, dirt, and other particulate matter from water. They are one of the most widely used filtration types because of the benefits they provide other filtration systems. For example, reverse osmosis systems and UV water purification systems require sediment filtration to protect the systems from damage and ensure the quality of the final product.

On top of protecting other filtration systems, whole-house sediment filters can also protect a home’s plumbing by removing particulates before they can clog pipes and appliances. Once this sediment builds up in your plumbing, it is expensive and difficult to remove. Consequently, sediment filters are an excellent addition to any water filtration system.

Learn more: What is a sediment filter?

sediment filter cartridges

Ceramic water filters

Ceramic filters remove suspended particles from water as it flows through tiny pores in the ceramic surface. The holes in ceramic filters are 0.5 microns in diameter, meaning objects larger than 0.5 micrometers (0.0005 millimeters) cannot pass through. These filters remove sediment, turbidity, about 99% of pathogenic bacteria (such as E. coli and salmonella), and microbial cysts from water.

Ceramic water filters are often configured in multistage designs for more comprehensive filtration. These filters can include a ceramic shell impregnated by silver ions, a carbon core, and ion exchange resin. In this configuration, the ceramic shell removes particulate matter and repels bacterial growth, followed by an activated carbon filter that removes chlorine and VOCs, and finished with ion exchange resin that removes heavy metals.

Learn more: What is a ceramic filter?

Reverse osmosis systems

Reverse osmosis (RO) systems produce some of the purest water possible from a home water filtration system. RO water is so pure because of the multi-stage filtration process it passes through. The basic stages of a reverse osmosis system include a sediment filter, carbon filter, and reverse osmosis membrane. Additional stages, such as a remineralization filter and an extra carbon filter, are included in some RO configurations. In regions where hard water is prevalent, a water softener is recommended as a pretreatment for an RO system to protect the membrane from becoming clogged with minerals.

RO systems are commonly used in what are called point-of-use applications. This means that they are connected to only one outlet, such as a faucet. These systems provide extremely pure water used for drinking, cooking, and washing dishes. Because of their effectiveness, they are one of the most common filtration systems available today. However, some homeowners are turned away from reverse osmosis systems because of their inefficiency. A standard home reverse osmosis system generates about 5 gallons of wastewater for every 1 gallon of purified water. Some RO systems are more efficient than others, and there are steps that can be taken to increase the efficiency of an already installed RO system. For many homeowners, the cleanliness of RO water is worth the drawback of the amount of wastewater produced.

Learn more: What is a reverse osmosis system?

Activated carbon water filters

Activated carbon filters can be found in almost every home, whether they be in the refrigerator or a water pitcher filter. They work through a process called adsorption, adhering particles to the carbon as it passes through the filter. Carbon filters are often used to reduce levels of chlorine and chloramine in water that is treated by a municipal water plant. This gives the water a cleaner, crisper taste and odor. In addition to chlorine and chloramine, activated carbon filters remove trihalomethanes, mercury, pesticides, and herbicides. Certain carbon filters may also be certified to remove lead, iron, heavy metals, and coliform.

Carbon filters are used in many types of filtration systems. For example, reverse osmosis systems always utilize at least one carbon filter in the filtration process. Refrigerator filters, as well as water pitcher filters, often use carbon to remove foul tastes and odors from water. When using a water softener on city-treated water, you should run a carbon filter before the softener to lengthen the life of the water softening resin.

Learn more: What is an activated carbon filter?

carbon filter cartridge

Water softeners

Water softeners, also called ion exchange systems, remove water-hardening minerals from water by exchanging sodium ions with calcium and magnesium ions. Water softeners use plastic resin beads charged with negative sodium ions to attach to positively charged magnesium and calcium ions. As water passes through the beads, ion exchange occurs, removing hardness from the water.

In regions with hard water, water softeners prevent the damage hard water can cause to a home’s plumbing and appliances. Limescale buildup in pipes can result from the long-term flow of hard water, resulting in low water pressure and shortened appliance lifespan. Hard water can also shorten the lifespan of water filtration systems. While a reverse osmosis system can remove water hardness by itself, these minerals can clog the RO membrane and cause the system to become less efficient. Incorporating a water softener before a reverse osmosis system ensures the RO system will not become clogged with water-hardening minerals.

Learn more: What is a water softener?

Ultrafiltration systems

Ultrafiltration systems produce extremely clean water by passing it through a .02-micron hollow fiber membrane. This removes practically all contaminants from water except for dissolved minerals. Because it does not remove minerals from water, ultrafiltration is often used instead of a reverse osmosis system by those who wish to maintain the mineral content of water. Unlike reverse osmosis systems, ultrafiltration systems do not produce any wastewater, so homeowners in states with water restrictions may prefer ultrafiltration over RO. Like reverse osmosis systems, ultrafiltration systems are most often installed under the kitchen sink for drinking and cooking water.

Learn more: What is ultrafiltration?

Water purification systems

By definition, water purification systems are not water filters because they do not possess a medium that water flows through. Rather, they use various methods to clean water without passing it through a physical filter. However, because they output pure water from a contaminated source, water purification systems are colloquially referred to as filters. The most common types of water purification systems are water distillers and UV water purifiers.

Water distillers

Water distillers deliver water that is highest in purity among other home water systems. Because of their effectiveness, they are the only acceptable water treatment method for specific applications in labs, hospitals, and automotive cooling systems. Water distillers work like the hydrologic cycle, converting water into vapor and leaving contaminants behind. Contaminated water is loaded into a water distiller in a boiling chamber, where the water is heated into vapor. This vapor rises to the ceiling of the distiller, where it condenses back into a liquid state. After enough water collects on the distiller’s ceiling, it drips into a fresh container. Because the boiling point of water differs from that of contaminants, all impurities remain in the boiling chamber.

Renters who want high-quality water but are not allowed to install a reverse osmosis or ultrafiltration system under their sink can greatly benefit from a countertop water distiller. While the distillation process is slow, taking about 4 hours to produce a single gallon of pure water, it grants peace of mind knowing that the water it outputs is as pure as possible.

Learn more: What is a water distiller?

UV water purifiers

UV water purifiers deactivate microorganisms in water by exposing them to a specific wavelength of UV light. They are used on water that contains microbiological contaminants, such as bacteria, viruses, and parasites, so they are not often used for treating municipal water. Because of this, they are most often used in home’s that get their water from a well. However, during the event of a boil water advisory, a UV system can protect homeowners using a municipal water supply from waterborne illness.

UV water purifiers rely on sediment filtration to maintain their efficiency and effectiveness. If sediment enters a UV system, it blocks the UV light from reaching some microorganisms. UV systems are primarily used as the last step of a point-of-entry household filtration system, disinfecting all water used in the home. They can also be sized and used in conjunction with other filter types, like reverse osmosis, to complete a robust point-of-use drinking water system.

Learn more: What is a UV water purifier?

Whole-house vs point-of-use water filters

A whole-house water filtration system connects at the main water line as it enters your home, while a point-of-use system connects to only one fixture. A whole-house system, also known as a point-of-entry system, is used in cases where water can damage plumbing or contains contaminants that make it unsafe to ingest. Common whole-house water systems include sediment filters, carbon filters, water softeners, and ultraviolet purification systems.

A point-of-use (POU) water filtration system is often used to make potable water a much higher quality for drinking and cooking. The two most common locations for POU systems are kitchen sinks and refrigerators. Carbon filters are often used on tap water to remove foul tastes and odors caused by chlorine and chloramine. Reverse osmosis systems are often installed under the kitchen sink to provide extremely clean water for the entire kitchen.

reverse osmosis system

How much do water filters cost?

The cost of a water filtration system varies greatly on a few factors, including the type of system and where the system is being installed. Point-of-entry water filtration systems must be able to process enough water for your home, so they are much more expensive than point-of-use filters.

With installation costs included, the average cost of a whole-house (point-of-entry) water filtration system typically ranges from:

  • Sediment and carbon filters - $800 - $1400
  • UV filtration system (prefiltration included) - $1800 - $3000
  • Water softener - $1500 - $2300

The average cost of a standard point-of-use system, including installation, can be found in the range of:

  • UV system - $300 - $1000
  • Water distiller - $250 - $650
  • Reverse osmosis system - $200 - $500
  • Ultrafiltration - $200 - $500
  • Ceramic filter - $70 - $150
  • Refrigerator filter - $30 - $60
  • Water pitcher filter - $10 - $80



If you have any additional questions, please do not hesitate to contact us.

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