Water purification is the process of removing undesirable chemicals, biological contaminants, suspended solids, and gases from water. The goal is to produce water fit for specific purposes. Most water is purified and disinfected for human consumption (drinking water), but water purification may also be carried out for a variety of other purposes, including medical, pharmacological, chemical, and industrial applications. The methods used include physical processes such as filtration, sedimentation, and distillation; biological processes such as slow sand filters or biologically active carbon; chemical processes such as flocculation and chlorination; and the use of electromagnetic radiation such as ultraviolet light.
Below are helpful resources that will make understanding water purification much less confusing and frustrating. WET has written many articles on various aspects of water filtration and compiled a glossary that containes the industry terminology with their definitions. There is also a small section on questions and answers that may be helpful.
Water Filtration Systems Terminology
Below is a useful glossary of terms dealing with reverse osmosis systems and processes.
- Antiscalants - RO antiscalants, sometimes called scale inhibitors, are specialized chemical formulations that are added to increase the solubility of sparingly soluble salts. In reverse osmosis systems CaCO3, CaSO4, SrSO4, BaSO4 and CaF are the most common scales of concern. The use of such antiscalants can form an essential component in the operation of well run reverse osmosis systems.
- Anti-telescoping Device - An anti-telescoping device or ATD is a plastic device located on the ends of a reverse osmosis membrane element. This device helps to provide structural support to the RO membrane envelopes preventing unraveling and extension. Also refer to “Telescoping”.
- Array - In a reverse osmosis system an array describes the physical arrangement of the pressure vessel, e.g., 4:3:1 - 3 stages with 8 total vessels; stage 1 has 4 pressure vessels, stage 2 has 3 pressure vessels and stage 3 has 1 pressure vessel. The reject of each stage is the feed stream for the next successive stage.
- Asymmetric - An asymmetric membrane is one that is constructed of the same material (cellulose acetate or polyamide) which has an increase in porosity from surface to base. The surface has a dense thin barrier skin and a thick porous support layer.
- Brackish Water - Brackish water is technically defined as water having TDS levels up to 6,000 mg/L, but is generally water that is too salty to be considered potable, but not salty enough to be considered seawater. The World Health Organization (WHO) salt limit for potable water is 1,000 mg/L TDS. By comparison, seawater ranges from about 33,000 mg/L to about 40,000 mg/L.
- Brine Seal - Brine seals are plastic or rubber devices that seal the outside of one end of a spiral wound RO membrane element against the wall of the component housing. Brine seals are designed to prevent by-passing of feedwater around the element.
- Cellulose Acetate - Cellulose acetate reverse osmosis membranes comprise an asymmetric polymer commonly used in the construction of RO membrane elements (other membrane materials include polyamide and polysulfone). Originally cellulose acetate membranes were made of cellulose diacetate, cellulose triacetate or a blend of these materials. Cellulose acetate membranes need a residual of chlorine to protect them from biological attack. They also have tight pH operating requirements.
- Membrane Compaction -
- Concentration Factor (CF) - The concentration factor is the degree that the RO feedwater dissolved solids are concentrated in the brine.
- Concentration Polarization - Concentration polarization refers to the concentration gradient of salts on the high pressure side of the reverse osmosis membrane surface created by the less than immediate redilution of salts left behind as water permeates through the membrane itself. The salt concentration in this boundary layer exceeds the concentration of the bulk water. This phenomenon impacts the performance of the RO process by increasing the osmotic pressure at the membrane’s surface leading to the following:
Increasing the velocity (turbulence) of the brine stream helps to reduce the concentration polarization.
- Reduced flux
- Increased salt leakage
- Increased probability of scale development
- Conversion - Conversion, sometimes also referred to as recovery refers to the percentage of the RO feedwater which is converted into permeate.
- Cross Flow Separation - Cross flow separation referes to a filtration process with the feedwater stream running parallel to the filter media and a concentrate stream continuously removing contaminants from the surface media. Cross flow separation differs in that it has three streams associated with it compared to only two streams found in dead head separation.
- Elements - An element or elements are often referred to as a "module". Elements are the physical devices that house the RO membrane. Spiral wound reverse osmosis systems can have up to six elements per pressure vessel. Hollow fiber RO systems have only one element per pressure vessel.
- Feed Channel Spacer - Feed channel spacers are found in spiral wound elements. Feed channel spacers are a netting material placed between the flat sheets of a reverse osmosis membrane to promote turbulence in the feed / concentrate stream. This material is referred to as “Vexar.”
- Feed Stream - Feed stream refers to the flow into the first stage of a reverse osmosis system. The feed stream is separated into a permeate or product stream and a concentrated or brine stream.
- Flux - Flux or water flux is typically expressed as volume per area per unit of time. Flux is used to express the rate at which water permeates a reverse osmosis membrane. Typical units of measurement are gallons per square foot per day (i.e. GFD or GSFD) or litres per square meter per hour (l/m2/hr).
- Hollow Fiber Element - Hollow fiber elements represent one of four possible RO membrane configurations (others configurations are spiralwound, plate and frame, and tubular).
- Hydrolysis - Hydrolysis refers to the chemical breakdown of a reverse osmosis membrane from exposure to low or high pH, bio-activity and temperature. Normally associated with cellulose acetate membranes where the acetyl groups are replaced by hydroxyl groups. Hydrolysis in increased salt leakage (i.e. greater conductivity of the permeate) and a lower feed pressure requirement. Oxidants and temperature can also cause hydrolysis in Thin Film Composite membrane elements.
- Langelier Saturation Index (LSI) - The Langelier Saturation Index, often abbreviated to LSI is an important measurement of CaCO3 potential and is used as a key performance indicator in the management of reverse osmosis systems. A positive Langlier Saturation Index indicates that CaCO3 can precipitate. A negative LSI indicates that the water is corrosive to steel.
- Membrane Compaction - Membrane compaction refers to the physical compression of the RO membrane itself. This compression results in a decrease in flux. The rate of compaction is directly proportional to an increase in temperature and pressure.
- Nanofiltration - Nanofiltration is a process similar to reverse osmosis but it is less effective at removing dissolved solids. Nanofilters are commonly referred to as membrane softeners because they will usually reject the double-positively charged hardness ions (i.e. calcium and magnesium) fairly well but cannot reject the single-positive charged soft ions (e.g.,sodium and potassium).
- Net Driving Pressure - The net driving pressure refers to the difference between the feed pressure and the osmotic pressure. The net driving pressure is the measure of the actual driving pressure available to force the water through the membrane. As net driving pressure increases, the flux increases proportionally (given all other factors are held constant).
- Normalized Permeate Flow (NPF) - Normalized permeate flow (NPF) refers to a calculation that allows the comparison of a measured permeate flow rate to a standard (or start up) condition.
- O-rings - In reverse osmosis systems O-Rings are used to seal the permeate water tube inter-connectors of adjacent elements. O-Rings help prevent the intrusion of high pressure feedwater (poor quality) into the low pressure permeate water (good quality). A damaged O-Ring will result in higher salt concentration of the permeate in that section of the system.
- Osmotic Pressure - Osmotic pressure is the pressure required to prevent the flow of water across a semi-permeable membrane separating two solutions having different ionic strengths. For reverse osmosis systems it is the osmotic pressure that has to be overcome in order to produce permeate. A useful “rule of thumb” is for every 100 mg/L of TDS difference between feed and permeate, 1 psi of osmotic pressure exists.
- Permeate - The term permeate is also referred to as "product" and describes the portion of the reverse osmosis feedwater stream water which passes through the RO membrane.
- Polyamide - Polyamide membranes were first Introduced during the early 1970’s. Asymmetric polymer is used in the construction of thin film composite (TFC) spiral wound membranes.
- Post Treatment - In a reverse osmosis contect the term post-treatment generally refers to the water treatment processes that take place downstream of the reverse osmosis plant system. Post-treatment processes often include disinfection using suitable biocides, pH adjustment, and possibly the addition of suitable corrosion inhibitors.
- Pre Treatment - In a reverse osmosis contect pre-treatment refers to the various physical and chemical water treatment processes that take place upstream of the reverse osmosis plant. Pre-treatment usually includes the use of sand filters and fine pre-filters. It may also include chemical treatment if scaling, corrosion or biological fouling of the RO membranes is anticipated.
- Pressure vessel - A pressure vessel is a tubular device which contains the reverse osmosis membrane elements. For spiral wound elements the pressure vessel is often referred to as the pressure tube and it can contain up to six RO membrane elements. In hollow fiber systems the pressure vessel is often referred to as the "permeator."
- Product Channel Spacer - A product channel; spacer is also known as a "permeate water carrier". In the construction of a membrane element, the product channel spacer is placed between two layers of the flat sheet membrane. This spacer is made using a knit fabric called Tricot, and is used to prevent the RO membrane from closing-off on itself under the high pressure of operation. Permeate water will flow in a spiral path across the product channel spacer into the product collection tube.
- Product Collection Tube - The product collection tube collects the permeate water and directs it to a product water header. The product collection tube is located in the center of a spiral wound membrane element with the "membrane - product channel spacer - membrane - feedwater channel spacer" sandwich wrapped around it.
- Product Stream - The product stream is the same as the permeate. Often referred to as "product", permeate is the portion of the reverse osmosis feedwater stream water which passes through the RO membrane.
- Recovery -
- RO Membranes - A reverse osmosis membrane is a semipermeable material, i.e. a material through which water passes relatively quickly, while other substances cannot (or do so relatively slowly). Membranes provide the barrier layer or interface for cross flow separation. RO membranes are made from a thin, porous material constructed of organic polymer (e.g. cellulose acetate, polyamide and charged polysulfone).
- Salt Passage - The salt passage describes the quantity of salt, as a percentage, which passes through the reverse osmosis membrane into to the permeate stream. Salt passage is a function of temperature, velocity and concentration gradient (i.e. concentration of salt in the brine versus the permeate).
- Salt Rejection - The salt rejection describes the quantity of salt removed from the reverse osmosis feedwater stream as a percentage.
- Silt Density Index (SDI) -
- Spiral Wound Element - A spiral wound element refers to a membrane configuration which is comprised of "flat sheet membrane - permeate channel spacer - flat sheet membrane - feed channel spacer" combinations rolled up around a product collection
- Staging -
- Telescoping - Telescoping refers to the longitudinal unraveling of spiral wound elements which results in the reverse osmosis membrane leaves extending beyond the spacing material between the leaves. This can be caused by hydraulic surges or by temperature extremes. Telescoping is physically damaging to the construction of the RO membrane element. Most manufactures install anti-telescoping devices (ATD’s) on their elements. Depending on the severity, it may effect salt rejection.
- Thin Film Composite (TFC) - A thin film composite or TFC is a reverse osmosis membrane composed of and manufactured as three layers bonded together. The two base layers of the asymmetric design have a thin skin (3rd) layer of either polyamide or charged polysulfone deposited on the surface. The thin film is the salt rejecting layer where the two base layers provide a porous structure whose primary function is strength.
- Total Dissolved Solids (TDS) - The Total Dissolved Solids or TDS of reverse osmosis water is often a measure of the dissolved salt content. For seawater the TDS is generally between 33,000 mg/L and 40,000 mg/L. The World Health Organization's (WHO) limit for drinking water is a TDS of 1,000 mg/L.
- Ultrafiltration - Ultrafiltration is the term used to describe a cross flow separation technique used to remove colloidal, very fine particles and macromolecules from a water stream. Pore sizes in an Ultrafiltration (UF) system range from 0.001 to 0.1 micron.
- Ultrapure Water - Ultrapure water is the term used to characterize electronic grade process water. Essentially ultrapure water is free of particles, colloids, organic and inorganic contaminants.
The helpful articles below covers how-to's and helpful information on water system filtration products. The sectio is constanly updated with new articles, so check back often for updates.
Frequently Asked Questions & Answers
- Q: What is Reverse Osmosis?
- A: Formally, reverse osmosis is the process of forcing a solvent from a region of high solute concentration through a semipermeable membrane to a region of low solute concentration by applying a pressure in excess of the osmotic pressure. The largest and most important application of reverse osmosis is the separation of pure water from seawater and brackish waters; seawater or brackish water is pressurized against one surface of the membrane, causing transport of salt-depleted water across the membrane and emergence of potable drinking water from the low-pressure side. Read the full article.
- Q: What is Dimeneralization?
- A: Demineralization is often a term used interchangeably with deionization. Demineralization is essentially removing all the minerals that can be found in natural water. This process is usually done when the water will be used for chemical processes and the minerals present may interfere with the other chemicals. All chemistic and beauty products have to be made with demineralized water for this reason. With the demineralization process, the water is "softened" replacing the undesired minerals with different salts (NaCl). Demineralized water has a higher conductivity than deionized water.
- Q: What is Nanofiltration Membrane Softening?
- A: Nanofiltration is a membrane filtration-based method that uses nanometer sized through-pores that pass through the membrane. Nanofiltration membranes have pore sizes from 1-10 nanometers, smaller than that used in microfiltration and ultrafiltration, but just larger than that in reverse osmosis. Membranes used are predominantly created from polymer thin films. Materials that are commonly used include polyethylene terephthalate or metals such as aluminum. Pore dimensions are controlled by pH, temperature and time during development with pore densities ranging from 1 to 106 pores per cm2. Membranes made from polyethylene terephthalate and other similar materials, are referred to as "track-etch" membranes, named after the way the pores on the membranes are made. "Tracking" involves bombarding the polymer thin film with high energy particles. This results in making tracks that are chemically developed into the membrane, or "etched" into the membrane, which are the pores. Membranes created from metal such as alumina membranes, are made by electrochemically growing a thin layer of aluminum oxide from aluminum metal in an acidic medium.