Drinking Water Filter Buying Guide

Mar 21st 2017

How to Select a Drinking Water Treatment System

Step One - identify the contaminants in your water that need to be reduced or the water conditions that need to be corrected. This is accomplished through a comprehensive water quality test from a certified water testing laboratory. Our mail in laboratory Water Test Kits are certified in all states as an out of state testing lab. Click here for the EPA list of State Offices that provide a directory of State certified Laboratories for water quality testing. Community water treatment systems must produce an annual Consumer Confidence Report (CCR) and provide it to their customers. These reports indicate the condition of the drinking water and any contaminants found. These reports are based on water leaving the water treatment facility and will not determine the water condition at your faucet. Distribution piping and home plumbing systems can alter the water quality leaving the treatment facility. Click here for EPA Local Drinking Water Information to read local water quality reports and other local water quality information.

Step Two - select an appropriate Drinking Water Treatment System based on the results of the water quality testing. Not all treatment systems will remove all contaminants or improve all water conditions. Some water quality problems may require professional evaluation, design and installation of treatment systems. Evaluate Countertop, Under-Counter and Whole House water treatment system options to determine if these products meet your requirements for contaminant removal. Each Drinking Water Treatment System has specific operating parameters that are necessary for effective and safe operation. Make sure your water quality and the household plumbing system meet these requirements when selecting your water treatment system and that the daily output and flow rates meet your needs.

Point of Use (POU) - water treatment applied at the point of water use. Usually this is a single location application such as at the kitchen or bathroom sink for water to be used for drinking and cooking.

Countertop Systems - these units sit on the counter and attach to the kitchen faucet through hose connections leading to and from the treatment system. Treated water may be delivered back at the faucet or through a spout at the treatment unit. Another type of countertop unit uses a container that is filled with tap water, which is then treated and pumped through the treatment unit and treated water is delivered into a pitcher for use. This type of unit requires no direct faucet connection.

Under Counter Systems - treatment systems under the counter require that they be plumbed into the water supply line. If using a Reverse Osmosis (R.O.) system, a drain connection is necessary and some R.O. units require a storage tank. Under counter systems require a faucet be installed at the sink or countertop for delivery of treated water.

Point of Entry (POE) - water treatment at the point where untreated water enters the home. Locate the system after the water meter or pressure tank. Also called "Whole House" water treatment. The purpose of this treatment option is to reduce contaminants in water that will be distributed throughout the house. This can have benefits for water using appliances such as washing machines and water heaters and address contaminants that can not be removed from Point of Use systems and Shower Filters. Household plumbing can be configured so that certain water supply lines are treated and not others. For example, you may not want to treat water going to outside faucets or used for lawn watering.

When evaluating any Water Treatment System, consider the following:

Certified Systems - Is the entire system or components tested and certified by an independent third party to remove or reduce the specified water contaminants and are safe for contact with drinking water? Click here to viewCertification Standards for Water Treatment Systems.

Performance Data - Will the system remove the contaminants you are most concerned about and are there any performance tests to demonstrate the effectiveness of the treatment system at removing those specific contaminants? If the system is reverse osmosis, will it produce enough water for your family?

Initial Cost - What is the cost of the installed system?

Operating Costs - What are the costs to replace the filter cartridges and how often should they be replaced? Divide the cost of the replacement filter by the recommended service life (in gallons of water filtered). This is the operating cost per gallon for filtered water, excluding any electrical costs if applicable.

Step Three - install and maintain the Water Treatment System as required by the equipment manufacturer. This will assure your system will consistently perform at the level needed to meet your contaminant reduction goals. Regularly changing the system filters and membrane cartridges and sanitizing the units and storage tanks are necessary maintenance procedures to ensure safe, high quality water.

Drinking Water Treatment Technologies

Topics Covered

Mechanical Filtration



Ion Exchange and Deionization

Reverse Osmosis

Water Softening



Ultraviolet Light

pH Modification & Acid Neutralization

Mechanical Filtration

This is the process of physically separating and removing suspended solids from a liquid by a physical means, such as with filter media, rather than through a chemical process. Mechanical filtration can be obtained through large whole house tank-type systems that hold loose media in layers or with replaceable filter cartridges.

Tank-Type Mechanical Filtration - A large tank is used to hold various layers of different media inside. The systems are designed according to the filtration requirements and contaminant removal needs. Usually the filter medium includes substances such as loose Activated Carbon, Copper-Zinc alloy, Activated Alumina and various sands and gravels. This media is layered and graded in particle size so the least dense and coarsest material is at the inlet water side and the more dense and finer material is at the outlet side. These systems may be designed for single use and disposed of when their service life is complete or they may be serviceable and the interior filter media can be replaced when the capability and/or capacity is exhausted. Sometimes these tank-type systems require backwashing, whereby water is pumped in the opposite direction as would normally occur through the treatment system and is sent down a drain. This backwashing process regenerates the media and removes captured particles to allow for continued and effective operation.

Some of the advantages of Tank-Type systems over Cartridge-Type systems include:

  • Lower operating cost over time.
  • Less clogging with high turbidity and heavy particulate water supplies.
  • Less servicing visits/requirements for automatic/backwashing systems.

Cartridge-Type Mechanical Filtration - this option utilizes fixed media and has most of the capabilities of tank-type filtration. Due to the nature of the fixed media used in Cartridge filtration, these systems can go beyond the limits of tank-type systems that utilize loose granular media. Mechanical filtration cartridges are available for Surface Filtration or Depth Filtration. Surface Filtration is more of a straining process in which the filter medium is designed to allow a certain size particle to pass through and anything larger will be blocked. As the larger particles are blocked, a layer develops on top of the filter media to help with further filtration. These type of filters clog easily and need more frequent maintenance. Depth filtration media allows particles of differing sizes to enter the filter and travel in irregular pathways. A depth filter will have layers of media to effectively screen out and capture smaller and smaller particles as water travels through it. Depth filters will have larger capacities and longer service life. Cartridge filters are rated for particle size based on two Industry standards.
  1. Nominal Rating - the filter will remove 85% of the particles at the rated size.
  2. Absolute Rating - the filter will remove 99.9% of the particles at the rated size.

Some of the advantages of Cartridge-Type systems over Tank-Type systems include:

  • Simple in-line plumbing installation or connection to plumbing fixtures. No backwash or wastewater connection issues.
  • Quick and easy servicing that can be done without professional assistance.
  • Lower initial system and installation costs.
  • Greater versatility to handle a broad range of particle sizes and flow rates.
  • Less space requirements. Most systems are mounted on a wall or are installed under a sink or placed on a countertop.

There is a variety of media used for mechanical filtration cartridges. Many cartridges are dedicated to particle capture only. Other cartridges are multifunctional and will integrate activated carbon to assist with additional contaminant removal along with particle removal. The following list describes several of the mechanical filtration cartridge types:

Pleated Cellulose - the most inexpensive option made with a resin-coated paper. It has a high surface area provided by the pleats. It works as a surface type filter and is good for dirt, rust, scale and fine silt. The disadvantage of this filter type is it's vulnerability to cellulose eating bacteria which will decompose the filter and make it ineffective. Requires frequent changes and higher maintenance than depth filter types.

Pleated Synthetic Fabric - pleated media provides a high surface area. It is a surface type filter and is good for clay, scale and fine silt. The synthetic material defends it from cellulose eating bacteria. Requires frequent changes and higher maintenance than depth filter types.

String Wound Fiber - natural or synthetic yarns are tightly wound around a perforated core creating built-up layers to form a depth type filter cartridge. Can meet the requirements for very fine and course filtration.

Resin Bonded Filter - synthetic or cellulose fabric is rolled or cast into a preformed shape and then treated with a polymer resin and cured. The cartridges are then machined to shape with grooves added to increase surface area. Provides a broad range of particle filtration capability.

Spun Bonded Polypropylene - filters made primarily from synthetic fiber that are formed by spun bonding the matted fibers onto a center core.

Ceramic - constructed from fired ceramic material and designed for use with very fine dirt, sediment and other small contaminants including cysts. One advantage of this type of cartridge is that they are cleanable with a cloth or brush. Wiping the exterior removes the outer layer of media and exposes a new layer. The cartridge can then be put back in service until the filter thickness is reduced to a specific value.

Multi-functional - these cartridge types incorporate activated carbon along with synthetic fabric to act as both a mechanical filter and adsorptive filter to remove chlorine and other contaminants such as organic chemicals. The configuration of these filters can be Activated Carbon impregnated fabric or alternating layers of mechanical filtration media and Activated Carbon to create a depth type filter. Activated Carbon does have mechanical filtration capabilities because of its structure, but is a more expensive alternative as a mechanical filter compared to non Activated Carbon options.

Adsorption Filtration

Adsorption is the process when liquid, gaseous and solid matter adheres to the surface or in the pores of an adsorbent media. This is a process that holds particles to the adsorbent media through physical and chemical forces. It is often confused with "Absorption", which is the process of one substance penetrating into another substance. When the adsorbent media forces are greater than the forces that keep the material in solution, then the materials will adhere to the Adsorbent media surface. "Physical Adsorption" takes place when the surface energy of a solid adsorbent material attracts a substance from a liquid. "Chemisorption" is the process of attracting substances though a chemical ionic process and is discussed further below under Ion Exchange. There are three key factors that impact the effectiveness of Adsorption filtration:

Temperature - generally, the lower the temperature, the better the adsorption process. High temperatures can cause "desorption" or the release of removed contaminants. This is why it is recommended to connect Water Treatment Systems to cold water supply lines. The most efficient temperature range for adsorption is 40° to 55°F.

pH - most nonpathogenic (not disease causing) organic contaminants in water are more soluble in a more alkaline (higher pH) solution than in an acidic (low pH) solution allowing better Adsorption. Substances such as Chlorine and Chloramine are more effectively removed when the pH is below 7 (neutral pH).

Contact Time and Flow Rate - the amount of time water spends passing through an adsorbent media has an impact on Adsorption. The longer the contact time, the greater the effectiveness of Adsorption filtration. A Water Treatment System must take the flow rate of the water into consideration to determine system effectiveness.

There are several Adsorbent medias used in Water Treatment Systems and each media type has specific characteristics and contaminant removal properties. The most common Adsorbent medias follow:

Activated Carbon - this a product that has been "Activated" through a process in which temperatures around 1300°F are applied to a carbonaceous substance in the absence of air to produce a carbonized char. The next step is activating the carbonized char at 1500°F to 1800°F with steam, Carbon Dioxide or acid to create a highly porous, clean and adsorbent material. Each teaspoon of Activated Carbon has the equivalent surface area as a football field and one pound equals about 125 acres of surface area. The most common carbon based substances used for Activated Carbon are Bituminous Coal and cellulose based substances such as Wood or Coconut Shells. Depending on the base material and activation methods used, the Activated Carbons will differ in their capabilities and can be selected for their performance characteristics and contaminant removal effectiveness. Activated Carbon media is found in primarily two forms:

  1. Granular Activated Carbon (GAC) - this is Activated Carbon that has been broken into small pieces and granules. It is common in tank-type Water Treatment systems as part of the multi-media bed and is also used in cartridge form in a sealed container.

  2. Carbon Block - most commonly used for cartridge type systems, this is a pressed block media produced from a blend of finely crushed and powdered Activated Carbon and a binder that is then molded and hardened or extruded to form the desired shape and size. Often, specialized media will be added with the Activated Carbon to provide customized contaminant removal performance. In general, carbon block exhibits faster adsorption rates and 2- 4 times the adsorption capacity compared to GAC.

Activated Alumina - a media that is produced by treating Aluminum oxide to become highly porous and adsorptive. The mineral is first regenerated with a caustic solution of Sodium Hydroxide and then followed by sulfuric acid neutralization. It has a surface area of over 22 acres per pound weight. It is a selective type Ion adsorbent (see below) and used to target specific contaminants such as Fluoride, Selenium and Arsenic.

Reverse Osmosis

To understand how Reverse Osmosis (R.O.) works, you must first understand Osmosis and its natural process. Water with dissolved solids in a less concentrated solution will naturally move toward a more concentrated solution in an effort to dilute the more concentrated solution. This is called Osmosis. In "Reverse Osmosis", the opposite happens, water from a more concentrated solution of dissolved solids are forced to move toward the less concentrated solution. This water movement of the different concentrations of dissolved solids occurs across a "Semipermeable Membrane". This membrane is called semipermeable because it is selective and allows some materials to pass through it (permeable) and prevents other materials from moving across it. The force that causes the water to move in Reverse Osmosis across the semipermeable membrane against the natural forces of Osmosis is provided by the water pressure supplied to a Reverse Osmosis system. The solution with the higher concentration of dissolved solids and contaminants is the feed water to the system supplied by the home. The solution of less concentration of dissolved solids and removed contaminants is the product water that has been treated for drinking.

In a typical Reverse Osmosis System, the path of treated water in the home begins with a connection to a cold water supply. Water passes through the Pre-Filters to remove any contaminants that can effect the R.O. membrane. The water stream travels through the membrane and is split into two paths, one path to become the Permeate (product) water and the other water path is flushing the membrane of captured contaminants so it can continue to perform effectively. The product water is sent to a storage tank to be used later or in the case of a Tankless system, directly out the faucet. The water used to flush the membrane is called the concentrate and is sent down the drain. The whole system is controlled by pressure limiting and flow controls and automatic shutoff valves to ensure safe and effective operation. To fully understand how a Reverse Osmosis system works, you will need to know the components of a system and some terminology.

Total Dissolved Solids (TDS) - defined as the total weight of solid matter that is dissolved in water. Dissolved solids can not be seen by the naked eye in water. The measurement for TDS is Parts per Million (ppm). A Reverse Osmosis system efficiency is measured by the amount of TDS removed from the water. TDS can be easily monitored to determine when replacement of the R.O. membrane is necessary or if a system is not functioning properly.

Pre-Filters - Reverse Osmosis membranes are sensitive to certain contaminants, such as Chlorine and particulates. The cartridge filters located before the water passes through the R.O. membrane are called "Pre-Filters" and treat the feed water so the membrane will not be damaged and will function effectively. These Pre-Filters also provide additional contaminant removal capability to a R.O. system as well as reduce Chlorine and particulate matter that can harm and clog a membrane.

Membrane - the membrane functions to remove Total Dissolved Solids (TDS) down to a size of chemical molecules and Ions (charged particles). It is usually made of layers of polymer films that are spiral wound around a core with spacer screens between each layer. As the feed water passes through the membrane, the layers have smaller and smaller pores, so the resulting product water has been removed of most of the dissolved solids. Membranes act like super mechanical filters to screen out particles larger than the pores of the membrane. Membranes are designed to meet certain TDS rejection levels and operating conditions. In general, a R.O. membrane can remove over 90% of TDS which can include salts, minerals, metals, micro-organisms and some organic substances. Low molecular weight organics such as Volatile Organic Chemicals (VOC's) require additional treatment for removal. It is important to note that R.O. membranes reject different contaminants at different rejection levels. The two most common R.O. membrane types are:

  1. Cellulose Triacetate (CTA) - the material of this membrane type is subject to some bacterial attack, it operates over a lower pH range, has a TDS rejection rate that decreases as TDS increases, has excellent Chlorine resistance, has a lower operating temperature and the Nitrate rejection is low.
  2. Thin Film Composite (TFC) - this membrane has good bacteria resistance, operates over a large pH range, has consistent high TDS rejection performance, higher operating temperature and high nitrate rejection. It has poor resistance to chlorine and water must have pre-treatment for Chlorine prior to reaching the membrane. It is the most common membrane used in high quality R.O. systems.

Permeate - this is the supply water that has passed through the Reverse Osmosis membrane to become treated water, also called product water.

Concentrate - the water that is diverted to flush the membrane of contaminants becomes the concentrate water. This is the water that flows to the drain and is also called the "Reject Water".

Post-Filter - a post filter is used after the product water leaves the R.O. membrane and before the water travels to the storage tank or directly to the faucet. R.O. systems will use a Post-Filter to remove additional contaminants not removed by previous stages in the system. Most often this stage would be for organic chemical removal and contaminants that require specialty media such as Lead or Arsenic.

Storage Tank - most water treatment systems need to produce a reasonable flow rate, about 0.5 to 0.75 Gallons per Minute (64 - 96 ounces per minute) at the faucet, to allow acceptable filling of containers. Standard tank-type R.O. systems make drinking quality water at about 1-3 ounces per minute, therefore treated water is directed to a storage tank as it is produced until the storage tank is full and the R.O. unit shuts off. When the faucet is opened, water flows from the storage tank out the faucet. When the faucet is shut off, the R.O. system will replenish the water level in the storage tank until full. Storage tanks have an air charged bladder that exerts pressure on the water in the storage tank as the water fills the tank. This tank pressure is what propels the water to the faucet when needed. Typical storage tank capacity is 2 - 4 gallons and is affected by line pressure. The greater the water pressure, the more water storage capacity a tank has because the water pressure must work against the storage tank bladder pressure. Countertop R.O. Systems use a non-pressurized reservoir to hold the water.

Tankless R.O. System - the pressure in the storage tank used in most R.O. systems also works against the R.O. membrane to some degree. As tank pressure increases the backpressure on the system, the differential pressure across the membrane decreases and reduces effectiveness and the treated water output rate of the system. Tankless R.O. systems do not require a storage tank and therefore overcome some of the drawbacks of a traditional R.O. system. Specialty high output membranes are used to produce treated water at a rate that can be directly delivered to the faucet on demand.

Automatic Shut Off Control Valve - this device shuts off the the feed water to the membrane when the R.O. system senses the storage tank is full. Usually this is done through pressure monitoring. When the tank reaches about 2/3 of the incoming feed water pressure, the control valve will shut off. When the tank is drained down to about 1/3 of the feed water pressure, the control valve will open. This Automatic Shut Off Control Valve functions to conserve water by preventing the continuing draining of concentrate "Reject Water" when the R.O. system storage tank is full.

TDS Creep - when an R.O. system has not been in use for sometime, or if there is a low differential pressure across the membrane, TDS will continue to permeate through the membrane due to the natural process of Osmosis and the differential solution concentrations. This may cause undesired contaminants to "Creep" across the membrane. Once the system is back in operation, the membrane will be flushed and return to design performance levels.

Final "Polishing" Filter - this is the last stage in the water treatment process before the treated water exits the faucet. Usually, water leaving a storage tank in a tank-type R.O system or water leaving the final membrane in a tankless system will travel through a final filter to remove any contaminants not removed from previous stages and to provide the highest quality drinking water. Final taste and odor improvements are made at this stage.

Drain Connection - the concentrate water that was derived from the flushing of the R.O. membrane is directed down a drain line. The drain line is connected to the household waste drain and in a typical undersink installation, this is usually the waste pipe from the kitchen sink. The drain connector clamps around the waste drain pipe with a connection for the reject water to flow from the R.O. System.

Air Gap Faucet - an Air Gap provides a clear vertical space in the R.O. system between the drain line and the flood level rim of the sink, preventing the potential back-up of waste water in the drain from contaminating the drinking water supply. The Uniform Plumbing Code requires that there is no direct connection between the drinking water and the sewer wastes. Some local plumbing codes across the country may not require an Air Gap as part of the R.O. system and not all systems are sold with one. Having an Air Gap as part of a R.O. system is important and recommended. The R.O. system is connected to a drain that can potentially back-up if a plumbing problem materializes. The Air Gap component can take several forms, but the most common is the integration with the faucet dispenser.

ooster Pump - a critical requirement for a R.O. system to operate effectively is water supply pressure. In situations where the water supply pressure is below operating requirements, a "Booster Pump" can be installed to increase the water supply pressure and improve the performance of the Reverse Osmosis system.

System Output - Reverse Osmosis systems are rated on water production output capability and indicated as "Gallons per Day" (gpd). This can be confusing to a consumer as two types of output are advertised for R.O. Systems.

  1. R.O. Membrane Output - manufacturers provide output ratings for their R.O. membrane cartridges. This is the tested performance of a membrane component outside of a system installation in optimal conditions. Membrane "Gallons per Day" output ratings are not achieved as a system output rating due to the fact the system is working against differing water pressures from the water supply and storage tank as well as restricted flow of the pre and post filter cartridges. This membrane rating should only be used as a reference to determine what component is installed and to re-order the correct membrane for a system. Since R.O. membranes are part of a R.O. system, the membrane rating should not be used to compare R.O. systems. Only the tested system performance will provide an indication of daily output of the R.O. system.
  2. R.O. System Output - the average family uses 1-2 gallons of water per day per person for cooking and drinking. Tested R.O. systems provide an indication of the average daily output achieved by the system design. Consider the needs of your family when determining the output requirements of your R.O. system. Many factors impact the performance of a R.O. system such as water pressure, water temperature and amount of TDS. Generally, the higher the rated output for a system, the faster the R.O. system will produce treated product water.

Recovery Rate - this is a measure of a R.O. system's efficiency and is the percentage of feed water that has traveled through the membrane to become permeate (product) water. The higher the recovery rate, the more efficient the system. The formula for Recovery Rate is as follows:

(Permeate in Gallons per Day ÷ Feed Water in Gallons per Day) x 100 = % Recovery Rate

Rejection Rate - is the percentage of TDS that is rejected by a membrane in a R.O. System and is also a measure of the system efficiency and performance. The higher the rejection rate, the more efficient and effective the system. Rejection Rates can vary with different contaminants that make up the TDS feed water. R.O. system performance data will include the Rejection Rate of specific contaminants. The formula for Rejection Rate is as follows:

((Feed Water TDS in ppm - Permeate TDS in ppm) ÷ Feed Water TDS in ppm) x 100 = % Rejection Rate

There are several factors to consider when purchasing and using a Reverse Osmosis system. No water treatment system can remove all contaminants in water and some treatment systems have more critical operating requirements to be effective at contaminant removal. A Reverse Osmosis system is more complicated that a mechanical type filtration system and incorporates more specialized components. These systems should be installed where the supply water to the system meets certain operating requirements as indicated by the R.O. system manufacturer. Always consult the R.O. system manufacturer's operating requirements before purchasing a R.O. system. The following are general guidelines to R.O. system requirements,

Quality of Supply Water - Reverse Osmosis systems require that the feed water meet certain water quality requirements for the systems to operate effectively. If the supply water does not meet these requirements, some type of pretreatment of the supply may be needed before the water enters the R.O. system. Any pre-treatment of the supply water will depend on the water quality problem. Some options can include a whole house "Point of Entry" system or filter cartridges installed before the R.O. system. Unless a R.O. system is specifically designed to be a microbiological filter, systems should be installed only on microbiologically safe water. The following list describes several of the water quality requirements of a R.O. system.

pH - is a measure of the Acidity or Alkalinity of the water supply with 7.0 being neutral. Higher numbers mean the water is Alkaline and lower numbers than 7.0 means the water is Acidic. The recommended range for R.O. systems utilizing TFC membranes is pH 3 to 11.

Iron - as soluble iron in water is exposed to air, it will oxidize and precipitate out of solution to become a solid in the form of Iron Hydroxide and/or Iron Oxide. These gelatinous substances will "Foul" the R.O. membrane and reduce or prevent it from functioning effectively. The recommended level for Iron in feed water should not exceed 0.2 ppm or mg/L.

Total Dissolved Solids (TDS) - the amount of suspended solids in the feed water will impact the effectiveness of the R.O. system. The higher the concentration in the feed water the lower the quality of the treated product water. A R.O. system recommendation is for TDS to not exceed 1800 ppm.

Turbidity - is a measure of the amount of suspended matter in water. This suspended matter blocks light rays and makes the water look cloudy. It is measured in Nephelometric Turbidity Units (NTU) through the use of an instrument called a "Nephelometer", which uses a photometric analysis to measure the light scattered by the suspended matter and generate a value. Recommendation for R.O. systems is a NTU value below 5.

Hardness - primarily the amount of dissolved compounds of calcium and magnesium. Excessive hardness will not necessarily prevent the R.O. system from operating, but it will shorten the life of the R.O. membrane. General recommendation for feed water hardness is that it should not exceed 10 grains per gallon or 170ppm. There are five Hardness classifications ranging from soft to very hard. The level of 170ppm falls into the fourth classification of "Hard".

Temperature of Feed Water - as water gets colder the viscosity increases making it thicker, which slows down the production rate. Each R.O. system will have a recommended temperature operating range and should never be allowed to freeze. In general, this range is 40°F to 100°F. Increasing the temperature of the supply water will not improve the quality of the permeate (product water) or contaminant removal capability of the R.O. system, but it will improve the rate of producing treated product water.

Water Pressure - critical to the operation of a Reverse Osmosis system is water pressure. In general, the recommended operating pressure range is 40psi to 85psi. The concept of Reverse Osmosis is based on the differential pressure across the membrane to overcome the natural forces of Osmosis and drive high concentrations of dissolved solids from the feed water, across the membrane, to lower concentrations of dissolved solids in the product water. Without a high pressure difference across the membrane, the effectiveness and contaminant removal capability will drop in a R.O. system. Another factor in storage tank-type R.O. systems is the backpressure exerted by the air charged bladder in the storage tank. The higher the water line pressure, the more it will overcome the storage tank pressure and the more water will be stored in the tank. Operating at higher feed water pressures improves the quality of the permeate (product) water. In situations where there is not enough water line pressure to operate the R.O. system effectively, a booster pump can be installed to increase the water line pressure to a level needed by the system.

The following is an example of how Temperature and Water Pressure have an impact on Reverse Osmosis system performance. R.O systems may be rated at different Water Pressures and Water Temperatures, so consult with the specific system specifications. Water Temperatures and Water Pressures other than the rated manufacturer's values will impact treated water output (either increasing or decreasing).

Rated R.O. System Performance

TFC Membrane 60psi, 77°F

Water Supply Pressure 40psi

Water Supply Temperature 50°F

Water Supply Pressure 60psi

Water Supply Temperature 65°F

Rated Output

15 gallons per day (gpd)

Pressure and Temperature Adjusted Output

6.1 gallons per day (gpd)

Pressure and Temperature Adjusted Output

12.4 gallons per day (gpd)


Ozone is a gas compound composed of three Oxygen atoms to make the molecule O3. The naturally occurring element Oxygen exists as two atoms, O2. When energy is used to break O2 bonds, single O1 Oxygen atoms form . These O1 atoms combine with O2 molecules to form O3 Ozone. Ozonation is the process of feeding Ozone into a water source for disinfection and to improve several water quality problems such as color and odor. The Ozone process leaves no residual taste and odor as would occur in other disinfection options and adds no chemicals to the water. Ozone has been used in community water treatment applications since 1906 and there are hundreds of water treatment plants in the US using Ozone today.

zone is unstable and will change back to Oxygen over time. Temperature, pH and water quality all affect the time Ozone takes to revert back to Oxygen. The production and introduction of Ozone into water must occur consecutively. Ozone's instability is due to the weak bond from the third Oxygen atom and causes an oxidizing reaction with any oxidizable substance. Oxidation is a chemical reaction to cause one substance to gain electrons and another substance to release their electrons with the end result being a change in the structure of the substances. This change in structure allows the substance to become deactivated or be effectively treated or removed. Ozone is considered the most powerful Oxidizer available for water treatment that can be used safely and a strong Disinfecting agent. Cyst organisms are considered the most resistant to all disinfectants because of their protective shells and is effectively treated, along with Bacteria and Viruses, with Ozone.

Ultraviolet Light

Ultraviolet Light (UV) is Radiation with wavelengths of energy shorter than the visible light spectrum and longer than the X-ray wavelengths, about 265 nanometers. It is the invisible violet end of the light spectrum. UV is used as a disinfectant to destroy Pathogens, which are organisms such as bacteria, viruses or parasites that can cause disease. Water passes through a chamber in which an Ultraviolet light source emits UV Radiation on the water. The UV source is similar to a fluorescent lamp, but it does not have a phosphor coating on the inside of the tube that normally converts UV energy into visible light. The UV radiation causes changes in the genetic material of the organism and inactivates it. However, some bacteria are capable of repairing themselves and UV light is not effective at killing Giardia and Cryptosporidium cysts because of the thick protective coats of these organisms.

UV treatment of water is automatic and adds no taste, odor or chemicals to the water. For UV treatment to be effective, it must have a specific Radiation intensity to provide the necessary penetration power to kill microorganisms. The design of a UV treatment stage considers the UV source strength, water flow rate (contact time) and surface area, There are several factors that can reduce the effectiveness of UV treatment. Turbidity (suspended matter in water) and impurities such as Iron can block UV rays from reaching microorganisms. Hardness mineral deposits from the water can coat the chamber to reduce UV effectiveness. Water should be pre-filtered before reaching the UV stage to remove any potential contaminants that could decrease disinfection effectiveness. UV light treatment should be monitored regularly and maintenance can include removing the UV bulb four times a year to clean the lamp and UV chamber to maintain UV intensity. The UV source will gradually lose strength over time and most residential systems require annual replacement. The power needed to operate the Ultraviolet light source can range from 10 to 30 watts.


The process of separating the water from organic and inorganic matter using evaporation and then condensation to capture the treated water is called Distillation. The basis for the distillation process is the differences in volatilities of chemical substances. Volatility is a measure of how fast an element or compound evaporates. Low volatility means it will evaporate slowly, high volatility chemical compounds will evaporate quickly. In the Distillation process, water is heated until boiling to produce a water vapor. Dissolved solids and other contaminants with boiling points higher than water (lower volatility) do not change to vapor. The water vapor is captured in a chamber where it cools and condenses to form liquid water and is stored in a reservoir for use. The impurities with the higher boiling points than water are left behind in the boiling chamber, so the condensed water has lower mineral content and is of higher quality.

Distiller Water Treatment System DistillationDistillation is effective at removing biological contaminants such as bacteria and viruses. The prolonged boiling action at high temperature will kill microbes and microorganisms. These dead organisms are not evaporated along with the water and are left behind. Inorganic contaminants such as minerals are also left in the boiling chamber and are not carried over during evaporation and condensation. Volatile Organic Chemicals (VOC's) have lower boiling points than water and can be carried over to the condensate side to contaminate the product water. Some home distillers use an Activated Carbon stage to help with VOC removal although water Distillers are not certified for removal of these contaminants. While countertop water Distillers may remove several categories of contaminants, they are certified for Total Dissolved Solids (TDS) removal only. These are the suspended solids in water such as inorganic minerals.

Countertop home water distillers produce drinking water slowly with output at about 1 gallon per 4.5 hours or about 5 gallons per day. With the average person using 1 to 2 gallons of water per day for cooking and drinking, and a water reservoir storage capacity of about 0.5 to 1 gallon, quality drinking water may be in short supply. Using several plastic storage containers requires space and is bulky. Transferring treated water to larger containers is inconvenient and may require hand pumps for dispensing. Water Distillers require electricity and will not operate during a power failure such as in an emergency situation when drinking water is needed.

Distillers also require significant maintenance because of the residue left behind when the water evaporates. Scale build-up in the boiling chamber can reduce the efficiency of heat transfer to the water requiring more energy and longer processing time. Maintenance recommendations include disassembly and cleaning the boiler before each use and removing scale deposit build-up in the boiling chamber with solutions and abrasion pads. The amount of maintenance is dependent on source water quality and amount of dissolved solids. Distillers are electrical appliances and the heating element may eventually need replacement or the electrical controls may fail. The cost of using a water Distiller can be high. Power consumption for typical home Distillers is about 750 watts and uses about 3 to 4 kilowatt hours of electricity per gallon. With electricity rates of $0.05 to $0.15 per kWh, this translates to $0.15 to $0.60 per gallon for distilled water. Additionally, water distillers use small carbon post-filters that must be replaced about every few months at a cost of about $6 to $7 a piece.

Distillation units also get very hot and extra care needs to be taken in handling and using these devices. The use of a home water Distiller will release hot steam into a room and may make the room uncomfortable or increase cooling requirements and costs. The lower boiling point Volatile Organic Compounds (VOC's) in water vaporize first before water and are allowed to be released into the surrounding room air through the gas vent. These VOC gases may create a concern for Indoor Air Quality. VOC's can pose health risks and should not be inhaled as a gas or ingested in drinking water. Consideration should also be given to the fact that unnecessary electrical use contributes to greenhouse gas emissions and environmental pollution. Countertop home water distillers are certified to remove only Total Dissolved Solids (TDS). There are other water treatment options available that are certified to treat TDS and additional water contaminants that will require less maintenance and will operate at a lower cost per gallon of treated drinking water.

Ion Exchange and Deionization

Ion Exchange occurs when an insoluble permanent solid medium, called the Ion Exchanger, exchanges Ions with the solution surrounding the insoluble medium. In a neutral atom that is neither negatively or positively charged, the negative charges of the electrons revolving around a nucleus of an atom are balanced by the positive charge of the protons in the nucleus. An Ion is an atom or group of atoms that carry an electrical charge because they have lost or gained one or more negatively charged electrons. "Cations" are positively charged Ions because they have lost a negatively charged electron. "Anions" are negatively charged Ions because they have gained a negatively charged electron. Substances that become Ions often display different properties than the original element from which it was formed.

Ion Exchange is used primarily to remove hardness minerals from water (water softening), but has applications for a variety of water quality issues. As Ion Exchange resin is depleted by giving off its Ions in exchange for obtaining the desired Ions from substances in water, the resin must be "regenerated" to bring it back to its fully Ionized form so the Ionic Exchange process can continue. Regeneration is performed with a combination of backwashing the resin, which cleans the resin and resets the resin configuration, and flooding the resin with a regeneration solution that will bring the resin back to the correct chemical state. The selection of the Ion Exchange resins and the "regenerant" used determine the substances that will be exchanged in the water and the application.

How It Works - a molecule of Hydrochloric Acid (HCl) contains one Hydrogen atom (H) and one Chloride (Cl) atom. When this molecule is forced to Ionize in water, the two atoms split apart into a positively charged Hydrogen Cation (H+) and a negatively charged Chloride Anion (Cl-). Once a substance separates into its Ions, these Ions are now available to combine with other Ions with an opposite charge, even if the other Ions are from a different type of molecule. For example, if Sodium Hydroxide (NaOH) is added to water containing Ionized Hydrochloric Acid, the result would be Sodium Chloride (table salt) and water as shown in this example:

(H+ + CL-)


(Na+ + OH-)





Hydrochloric Acid Ions


Sodium Hydroxide


Table Salt



"Deionization" is a two phase Ion Exchange process that removes all ionized minerals and salts, both organic and inorganic, from water. Positively charged ions in a solution are removed first by a Cation exchange resin which exchanges them for a chemically equivalent amount of Hydrogen Ions (H+). In the second stage, negatively charged Ions are removed from the solution by an Anion exchange resin and an equivalent amount of Hydroxide Ions (HO-) are released. The Hydroxide and Hydrogen Ions produced by this process join to form water molecules (H+ + HO- = H2O) while the mineral Ions are removed. This process is also called "Demineralization" by Ion Exchange. It is not a common treatment technique used in residential applications and does not remove microorganisms or non-electrically charged substances such as synthetic organic chemicals (pesticides, solvents, herbicides).

There are primarily two configurations of Ion Exchange water treatment equipment:

Replaceable Cartridge Systems - for small applications such as Point of Use (POU) drinking water at a single faucet, an in-line cartridge type unit can be used. Larger cartridge type Point of Entry (POE) systems are available for "Whole House" applications where the contaminant demand can be met by the system. The cartridges can contain a mixed media bed with both Anion and Cation Ion Exchange resin. When the media becomes exhausted and is no longer effective, the cartridges are replaced. These units provide an economical and compact alternative to water softeners for low sodium mineral free water.

Tank-Type Systems - for larger water flow rates and greater contaminant removal, tank-type systems may be required. These configurations have one or more 6' to 10" diameter portable tanks that use meters and electronic controls for their effective operation. When the tank media becomes exhausted in about 6-10 years, the tanks are returned to a regeneration plant to reprocess the resin mix. Generally, these systems are for industrial and commercial use and are expensive to purchase and maintain.

The principles of Ion Exchange can be applied to water quality issues beyond mineral removal with specific Ion Exchange resins. The following water quality treatment examples demonstrate the versatility of the Ion Exchange process:

Dealkalization - the reduction of Alkalinity (high pH) in water.

Decolorization - the treatment of water discoloration referred to as "Tannins", usually a yellowish tint, caused by microscopic, unsettleable particles.

Fluoride Removal - use of Activated Alumina as a selective Ion Exchange resin to adsorb Fluoride.

Nitrate/Nitrite Removal - Anion Ion Exchange resin is used and is regenerated with Sodium Chloride (NaCl) similar to a water softener.

Manganese & Iron Removal and pH Modification - a special Cation Exchange resin is used to soften water, remove dissolved Iron and Manganese and modify low-pH simultaneously.

Uranium Removal - found in water as Ion complexes, Uranium can be effectively removed with Anion Exchange resins.

Water Softening

The treatment of water through Water Softening is the most common type of Ion Exchange process. Water with hardness minerals, such as Calcium and Magnesium, are passed through a bed of Cation Exchange media to exchange the Calcium and Magnesium Ions in the water with Sodium or Potassium Ions from the Ion Exchange resin. This effectively removes these hardness minerals from the water, but increases the Sodium and Potassium levels of the water leaving the treatment stage. The term "hardness" was originally applied to waters that were hard to wash in because hardness minerals prevent soap from lathering and produces an insoluble "curdy" precipitate in water. Dissolved Calcium and Magnesium are also responsible for most scale (coating) build-up in plumbing pipes and water heaters. Hard water in the home makes bathing and laundry cleaning more difficult, leaves visible white chalky residue on plumbing fixtures and can affect the life and efficiency of electrical appliances and water heaters.

There are several Water Softener system configurations and some designs may include more than one resin tank. Water Softener systems are primarily composed of three components:

  1. Pressure Vessel - this holds the bed of Cation Exchange resin and where the actual water softening takes place.
  2. Additional "Brine" Tank - to hold the regeneration material used to regenerate the Ion Exchange resin and get it back to the necessary chemical state to continue to perform the water softening function.
  3. Controls - these are valves and water meters or timers usually located on top of the pressure vessel that direct the flow of water and Sodium or Potassium regeneration solution, called "Brine", during the regeneration cycle.

Most Water Softening systems are automated to provide continuous and efficient operation. Two classes of Automated Water Softeners are available

Demand Control Systems - these systems operate based on the demand for regeneration and can also alter the amount of Brine solution needed to regenerate the Ion Exchange resin. The system will either meter the amount of water passing through it or have a sensor that registers when the softener is nearing the end of its capacity and needs to be regenerated. These type of systems are more efficient than Timer Control systems because the regeneration cycle is based on the amount of use and the amount of Brine needed to replenish the system. This can save money on regenerants, Sodium Chloride or Potassium Chloride, as well as water and pumping costs.

Timer Control Systems - a time clock built into the system control is set for a specific time period to regenerate the Ion Exchange resin. This is based on a predetermined estimate of softened water usage and a predetermined estimate of brine regenerant solution, whether it is needed or not.

Water hardness forming salts are measured in grains per gallon (gpg) or parts per million (ppm) and is expressed in terms of equivalent quantities of Calcium Carbonate. This allows a common basis for comparison of different hardness salts and compounds. 1 gpg = 17.1 ppm. In a water test report, total hardness is the sum of Calcium and Magnesium Ions. The following is the industry standard for classifying the levels of hardness:

Hardness Designation

Grains per Gallon

Parts per Million (mg/L)


less than 1.0

less than 17.1

Slightly Hard

1.0 to 3.5

17.1 to 60

Moderately Hard

3.5 to 7.0

60 to 120


7.0 to 10.5

120 to 180

Very Hard

10.5 and over

180 and over

How It Works - water being treated for Hardness flows into a tank of Ion Exchange Water Softening resin. The most common resin used for this application is insoluble beads of polystyrene bonded with divinylbenzene that are about 1/64" to 1/32" in size. This Ion Exchange resin is permanently negatively charged and attract positively charged Ions (Cations). The Ion Exchange resin holds positively charged monovalent (one+ charge) Sodium (Na+) or Potassium (K+) Ions. When positively charged divalent (two++ charge) Calcium (Ca++) and Magnesium (Mg++) Cations approach the Ion Exchange resin, a chemical reaction occurs in which the Sodium or Potassium Ions on the Ion Exchange resin are replaced by an equivalent quantity of Calcium and Magnesium Ions that were in the water. There is a stronger attraction for divalent ions over monovalent ions because of the greater positive charge, therefore the less attracted Sodium and Potassium ions are released from the Ion Exchange resin and the more attracted Calcium and Magnesium Ions are adsorbed in the Ion Exchange resin. The less harmful Sodium or Potassium Ions have replaced the troublesome hardness Ions in the water flowing through the home plumbing system. The chemical reaction looks like this:








Sodium Ion Exchange Resin


Calcium Bicarbonate in Water


Calcium Ion Exchange Resin


Sodium Bicarbonate in Water

R = Cation Exchange Resin

When the Water Softener system determines the Ion Exchange resin is close to "exhaustion" and will no longer be capable of capturing more Calcium and Magnesium Ions from the water, a regeneration cycle is performed on the system. The most common regenerate used is Sodium Chloride (table salt), but Potassium Chloride can also be used. The advantage to Potassium Chloride is that it does not increase the sodium level in the drinking water, which may be important to individuals on a low sodium diet. Potassium Chloride is more costly and usually a larger quantity is needed for regeneration compared to Sodium Chloride. These regenerants are put into a storage tank of water to create a highly concentrated regenerant solution called "Brine". Newer, more efficient Water Softeners use about 3-7 lbs. of Sodium Chloride per cubic foot of resin for regeneration, while older units can use 10-15 lbs. of salt per cubic foot of resin.

Water Softening systems are rated by removal capacity in grains. Average Ion Exchange resin used for this purpose has a removal capacity of 30,000 grains per cubic foot of resin. To determine when the Water Softener system will become depleted and need regeneration, you would divide the capacity of the system by the amount of grains per gallon of hardness that exists in the water. For example, if your water had 6 gpg of hardness and your Water Softener system capacity is 30,000 grains, you would have to regenerate the resin after 5,000 gallons (30,000 ÷ 6 = 5,000).

Systems can be designed as a "concurrent flow" or a "countercurrent flow" regeneration process. In the concurrent flow, the Brine solution flows in the same direction as the water flow through the resin bed. When the brine first enters the resin tank, it flows through a fresh water zone at the top. This mixing with fresh water initially reduces the concentration of the Brine solution. This may cause the lower levels of the resin bed to not fully regenerate and create a situation where hardness leakage can occur when the system becomes operational. In the countercurrent flow, the Brine solution flows in the opposite direction as the water flow through the resin bed. The advantage to the countercurrent flow design is that the concentrated Brine solution immediately contacts the last portion of the resin bed first for maximum regeneration. This ensures that when the unit is back in service, the water leaving the resin bed passes through the most highly regenerated resin region last, ensuring maximum softening effectiveness.

Ion Exchange Water Softener Process Ion Exchange Water Softener Process Ion Exchange Water Softener Process Ion Exchange Water Softener Process Ion Exchange Water Softener Process

Source: Water Quality Assoc.

There are several steps in Water Softener regeneration:

Backflushing - water flows into the resin tank at a high flow rate to clean and flush out particulates and suspended dirt that the resin may have filtered out. Backflushing also creates a stirring and scrubbing action on the resin beads and expands the resin bed back to the design state. The water used to backflush the resin bed is sent down the drain. This stage prepares the resin for the regeneration step.

Regeneration - the brine solution is pumped into the resin bed at a very high concentration of Sodium or Potassium Ions. This very high concentration forces the adsorbed Calcium and Magnesium Ions to be released from the resin and be carried away to the drain. The Sodium or Potassium Ions are then received by the Ion Exchange resin and the resin has been regenerated and ready to begin the Water Softening process again.

Other Steps - depending on system design, there could be a quick rinse at the end of the regeneration cycle to rid the system of any remaining high concentrations of brine solution. Other steps can include a settling rinse to get the resin bed in a state to function optimally.

Brine Refill - this may be an automatic or manual process that is done when needed to get the Brine solution back to a full level and at the necessary concentration in the Brine tank.


The process of bringing water and air into contact with each other is called Aeration. This can be accomplished by spraying or cascading the water into air or by injecting air into the water. The primary purpose of Aeration for Water Treatment is for "Degasification" and "Oxygenation". Aeration of water helps in the reduction of dissolved residual gases like Radon, volatile organic chemicals (VOC's) and removal of some undesirable odors. It can also be helpful with the chemical reduction of Ferrous Iron and Manganous Manganese.

There are two configurations of Aeration systems:

  1. Open Gravity Aerators - used primarily for degasification such as for the removal of dissolved gases such as VOC's and Carbon Dioxide, Hydrogen Sulfide (rotten egg smell) and Radon. Iron and Manganese must be controlled in these systems or fouling of the system can occur. For treating Radon and Methane gas, a fan must be incorporated to expel the gas to the outside and away from the home and system. These systems require that water be re-pumped to re-pressurize the water supply after leaving the Aeration system.
  2. Closed Pressure Aerators - oxygenation of the water is the primary purpose of these systems. These treatment systems are under constant pressure and the line pressure generated serves the distribution system. No re-pumping is needed. Due to the system being under constant pressure, they are not well suited for the release of dissolved gasses and VOC's. They are used to transform dissolved metals into precipitates that can drop out of solution and be filtered.

pH Modification and Acid Neutralization

The pH scale determines how Acidic or Alkaline a solution is. The scale ranges from 1 to 14 with 7.0 being "Neutral". A pH below 7.0 is considered "Acidic" and a pH above 7.0 is "Alkaline". Each single numerical increase or decrease represents a tenfold increase in Acidity or Alkalinity. Acid is a substance that releases Hydrogen Ions (H+) when dissolved in solution and the solution develops a higher concentration of Hydrogen Ions (H+) compared to Hydroxyl Ions (OH-). An "Alkali" substance will cause a solution to become Alkaline and produce a concentration of Hydroxyl Ions (OH-) that is greater than the Hydrogen Ions (H+).

One of the main reasons for the control of pH is to reduce corrosion. Low pH water has a corrosive effect on metal surfaces such as Brass, Copper, Cadmium, Lead and Zinc and can leave stains on plumbing fixtures. Water may become Acidic from the presence of Carbon Dioxide (CO2) and the lack of alkalinity to offset the Acid. Carbon Dioxide may get into well water from absorbing the CO2 released from decaying vegetation. The CO2 can combine with water to form Carbonic Acid (H2CO3). Rainwater can also be Acidic (acid rain). High pH Alkaline water, pH above 9.0, is also corrosive to metals such as Brass, Zinc, Aluminum and Copper. Highly Alkaline water can cause drying of the skin when bathing, give water a "soda-like" taste and cause scale to form on metal surfaces.

Treatment of low pH water is primarily done by passing the water through chemically reactive media or feeding a liquid chemical solution into the stream of water. Calcite and Magnesia are used as medias to reduce Acidity in water. Ion Exchange can be used with a weak Cation Exchange resin to absorb the Carbonic Acid and also helps soften the water and remove Iron. Soda Ash can be used in a chemical feeder application to reduce pH. High pH Alkaline water is unusual in a residential environment and considered less critical. Treatment for high pH water includes the use of chemical feed pumps to add acid-type solutions to the water. Ion Exchange can also be used in a process called "Dealkalization".