All water treatment technologies involve a physical change to the water that they are processing. The nature of the change truly defines the technology.

Subtractive treatment

The most commonly understood and frequently used type of water treatment technology is subtractive; something is removed from water for health or aesthetic reasons. Contaminants are physically removed from the water using a physical separation or adsorptive process. Within the subtractive realm, filters generally comprise depth, barrier or membrane types:

• Depth filters use physical media in granular form of varying sizes and densities to physically filter contaminants.
• Barrier filters use an organic, synthetic or metallic screen that provides a barrier to passage of contaminants, based on their physical size.
• Membrane separators typically use a semipermeable membrane wrapped around a central core, allowing selective passage of water and contaminants, depending on the size, molecular weight and even electrical charge of the contaminant(s). Ultrafiltration, nanofiltration and reverse osmosis purifiers are types of membrane separators.

Reverse osmosis separation is an extremely effective form of water purification. Contaminants are removed from a typical residential RO purifier through the combined technologies of physical filtration, absorption, adsorption and membrane separation. Multiple technologies are used to ensure maximum removal of contaminants from the water. Prefilters in a typical RO purifier are designed to protect the membrane from oxidants and physically large contaminants that would prematurely foul the membrane pores. While protected by the prefilters, the semi-permeable RO membrane works to reject as many contaminants as possible from the water. The membrane operates under pressure from the municipal supply or with the assistance of a pressure-boosting pump. As a crossflow filter, the membrane allows passage of purified water through its pores, while a waste stream flows to drain. Backpressure from the drain restrictor creates sufficient pressure against the membrane to allow for rapid processing while continuously cleaning the membrane. This purification process is quite slow in residential systems, with a typical purifier being able to purify approximately 0.05 gpm. Purified water is typically stored in a pneumatically charged tank that allows for repressurization and accumulation in a sanitary environment.

Manufacturers of residential RO purifiers make substantial claims to remove contaminants. These claims are required by most states to be verified by an independent certifying/testing body, such as the National Sanitation Foundation (NSF), which has established standards of performance. NSF/ANSI Standard 58 establishes the minimum requirements for the certification of POU reverse osmosis systems designed to reduce contaminants that may be present in public or private drinking water. The scope of NSF/ANSI 58 includes material safety, structural integrity, TDS reduction and other optional contaminant reduction claims. The most common optional claims addressed by the standard include cyst reduction, hexavalent and trivalent chromium reduction, arsenic reduction, nitrate/nitrite reduction and cadmium and lead reduction. Consumers seeking RO purifiers are looking for water that is as functionally close to pure as possible. This water is often described as empty water to describe its high level of purity (see Table 1 for typical performance).

Exchange technology

Exchange technology removes one contaminant from the water while replacing it with an alternate. Both natural and synthetic materials can act as an ion exchanger and depending on the functional matrix of the ion exchange media, various positively and negatively charged contaminants can be removed from water. A water softener is an example of ion-exchange technology.

Additive technology

Additive technologies change the nature of water through addition of chemicals to the water. Various things are typically added to water, such as pH neutralizers, ions, phosphate sequesterants, colorants and flavorings. While the types and quantities of additives will vary from application to application, the effect is to add something to the water that will change its character.

Water ionizers are a type of additive water treatment technology, whereby they add ions to water through the process of electrolysis to create both acidic and alkaline streams of water. They do not filter or purify the water to any appreciable degree. The residential water ionizer will accept municipal water and then pass it through an ionizing chamber, which comprises a number of metallic electrodes (usually titanium or platinum). When energized, the electrodes will initiate an electrolysis reaction where the water yields hydrogen (H+) and hydroxyl (OH-) to the remaining water. The unstable hydroxyl radicals then quickly become hydroxides in the water. Sodium chloride is sometimes added to the feedwater to potentiate the reaction and develop hypochlorous acid and caustic soda as byproducts. The electrodes are separated within the chamber by semipermeable membranes that isolate the streams of acidic and alkaline water as they are created. Electronic controls allow the user to vary the electric energy input to the chamber, thereby changing the relative strength of the electrolysis reaction. These devices typically process water at a rate of one gpm and require no additional water storage or repressurization apparatus (see Table 2 for typical performance).

The acidic stream is generally used for cleaning and even disinfection of surfaces. In most residential systems though, this acidic stream is discharged as waste. The alkaline stream is generally used for drinking, skin care and cleaning. Many claims are made about the health benefits of drinking this type of water, particularly the elevated pH and negative ORP antioxidant characteristic.

Glossary

pH

pH is a measurement of the acidity or alkalinity of a solution. It provides a value on a scale from 0 to 14 where 7 is neutral, less than 7 is acidic and greater than 7 is alkaline (or basic). In water, the pH value is related directly to the ratio of positively charged hydrogen ions [H+] and negatively charged hydroxide ions [OH-]. When water has an equal concentration of H+ ions and OH- ions, it is said to be neutral (pH=7). When water has a greater concentration of H+ ions, it is said to be acidic (pH<7). When a solution has a greater concentration of OH-, it is said to be alkaline (pH>7).

ORP

Oxidation reduction potential, or ORP, is an indication of the degree to which a substance is capable of oxidizing or reducing another substance. ORP is measured in millivolts (mv) using an ORP meter. A positive ORP reading indicates that a substance is an oxidizing agent. The higher the reading, the more oxidizing it is. As such, a substance with an ORP reading of +400 mv is four times more oxidizing than a substance with an ORP reading of +100 mv. A negative ORP reading indicates that a substance is a reducing agent. The lower the reading, the more anti-oxidizing it is. As such, a substance with an ORP reading of -400 mv is four times more anti-oxidizing than a substance with an ORP reading of -100 mv.

This article was written by Greg Reyneke and originally appeared on Water Conditioning and Purification Magazine [link].