Water softening involves the removal of cations, namely calcium, manganese, and iron, in hard water. This is often achieved using various resins in order to decrease lime build up, iron staining, soap scum, dry skin, and can extend the life of various plumbing equipment and laundry fabric. The water softener replaces the incoming magnesium and calcium ions with sodium ions or potassium ions, depending on the type of regenerant used, salt (NaCl) or potassium Chloride (KCl). Regeneration is normally achieved with an automatic control head. The common steps are backwash, brine draw, slow rinse, refill brine tank and fast rinse and return to service. The method of initiating a resin bed regeneration and clean up can be based on a time estimate, gallons usage of water or hardness sensors.

The hardware comes in various sizes depending on daily usage, peak gallon per minute, incoming hardness load and the desired water hardness out of the system. The number of tanks can be one, two, three, four or more. The method of initiating regeneration can be based on time, volume of water previously passed through the system, or the measured water hardness coming out of the system.

Ultra-filtration (UF) in water treatment typically uses hollow fibers of membrane material The feed water flows either inside the fibers, or around the outside of the fibers. Suspended solids and solutes of high molecular weight are retained, while water and low molecular weight solutes pass through the membrane. This separation process is used in industry and research for purifying and concentrating macromolecular compounds. When combined with other purification technologies in a complete water system, UF is ideal for the removal of colloids, bacteria, pyrogens and macromolecules larger than the membrane pore size from water. The primary removal mechanism is size exclusion. UF can be used as pretreatment for reverse osmosis systems or as a final filtration stage for purified water. Periodic flushing and cleaning are required.

Reverse osmosis (RO) membrane systems force purified water through a semi-permeable membrane that rejects inorganic, sediment, and high molecular weight contaminants. The whole house system is meant to filter all incoming water to a home or business and may include pre filtration devices. These devices may be carbon filters, water softeners and sediment filters. Typically a whole house RO system will treat drinking water and work water. These systems require a holding tank with a water re-pressure pump and tank. In a home, about a 8×3 foot print and 20 amp outlet are required.

This system consists of both cation and anion ion exchange resin. The water passes through the cation ion exchange resin, where the positive ions are exchanged for hydrogen. Next, the acidic water is passed through the anion ion exchange resin where the negative ions are replaced with hydroxyl. Once through the cation and anion phases, all impurities are now either hydrogen or hydroxyl, which forms water. This process yields what is called deionized water. An acid such as HCl is used to regenerate the cation resin. NaOH is used to regenerate the anion. The waste streams need to be monitored and treated to meet regulated discharge requirements. Water quality is less than 1ppm, so instead of measuring conductivity (expressed as parts per million), water purity is measured in ohms of resistance, and the higher the resistance, the higher the purity. The above serial process from a cation tank to anion tank can yield about 1 mega ohm water. To reach higher levels of about 16 mega ohm water, a mixed resin bed system is needed with multiple stages. Higher levels of purity require recirculation. Materials of construction must be compatible with water purity.

Carbon tanks work to remove taste, smells, odors, chemicals, chlorine, and other impurities in water. Carbon tanks loaded with carbon media come in various sizes and number; they can be plumbed in series or parallel. They can be backwashing type or non-backwashing. The backwashing can be initiated on pressure drop, time or volume of water filtered. The carbon can be a fine particle size mesh or a standard particle size. The type of carbon can be coal, coconut or wood based. It can be standard activation or catalytic with enhanced oxidation. Specifying a carbon tank system is application based.

Ultraviolet (U.V.) disinfection is a chemical-free process used to directly attack microbes and viruses in water. Even difficult to kill parasites like Cryptosporidia or Giardia are greatly reduced in this process. Essentially a specific 254 nanometer wavelength will scramble the organisms DNA such that they will not reproduce. Systems are rated at so many gallons per minute which is a function of contact time and bulb energy. The fluid needs to have sufficient UV transmission, and clarity is a must for transmission. However, low UV transmission can be present with a clear fluid. A 5 micron pre-filter is a standard design component. Equipment efficiency is expressed as log reduction or % removal such as 99.99%, which is 4 log reduction. Usually bulbs have a useful life of one year. Even though the light is on, it does not mean it is as effective as claimed. Check with manufacturer.

Water aeration can be defined as the process of adding a gas such as oxygen to water or the process of removing a gas from raw water. This is often used to oxidize troublesome components in water such as iron for subsequent filtration. Aeration is also used to strip undesirables such as volatile organic carbons and radon gas. Depending how easily the problem gas releases from the water, simply spraying the gasified water into an open tank will adequately de-gasify.

Cartridge filters key parameter is surface area, enabling them to operate for a period of time. As water flows through the filter, the material is able to catch unwanted contaminants in the water. Properly sized cartridges require minimal maintenance, and may only need to be rinsed off or replaced a few times per year. Cartridge filters can be of many different forms. There are carbon block, carbon granular activated, sediment, ion exchange, ultra filtration, submicron, PFAS filters and absolute filter cartridges. The second step in the application is sizing the cartridge for efficacy and reasonable filter change periods. Lastly the designer needs to consider how many filters are needed and if they are plumbed in parallel or in a series.

This system is designed for the removal of iron and manganese from water. These filters oxidize ferrous iron to turn in into an insoluble Ferric particle (rust), and then trap it in the filter media. Sometimes the oxidizer is injected before the back washing filter tank. The oxidizer could be bleach, ozone or the oxygen in air. This is a 3 step process: oxidizer injection, retention time (for particle growth) and finally filtration. Sometimes the oxidation and filtration can take place in one step. With a two tank design, the tanks clean each other with clean water and the media is kept clean longer. The media may require a regenerant and may be fairly heavy, requiring high backwash rates.

Reverse osmosis systems push purified water through a semi-permeable membrane. The membrane itself removes inorganic chemicals, sediment, and high molecular weight organics from the water. These systems also include sediment and carbon filters to catch contaminants before and after the membrane itself. As a system, the remove many common inorganic and organic contaminants, sediment, and odor. The point of use system is smaller and is intended to purify small volumes of water. Typical applications are drinking water and humidification. About 50-300 gallon per day systems are used. A surge tank is often supplied for peak flows, membranes are under-rated for cold water less than 77 F and over-rated above 77 F. Even though the RO membrane will reject microbes, these systems do not meet disinfection standards.

Ozone is O3 instead of the O2 found in air, and it can give up an atom of oxygen. This results in a very powerful oxidizing agent, toxic to most waterborne microorganisms. After about an hour, it decays back to ordinary oxygen, making the water safe for human consumption and greatly reducing the amount of harmful contaminants in the water. Additional benefits of this type of system are the minimal byproducts, and absence of odor or taste changes. This type of oxidizer can be used to oxidize iron and manganese before filtration. Ozone is created by two methods: corona discharge or ultraviolet light at 185 nanometers. The corona method takes dehumidified air to make O3, or nitric acid will form. Pre-concentrating the O2 in air increases O3 concentration. Systems are sized for total oxidation load. The UV method is commonly used for spa applications.