What We Utilize

Water often times contains particles that are incapable of being dissolved in solution.  The particles may be comprised of fine sand, silt, clay or other materials.  We often refer to these particles as particulate matter, suspended solids and/or turbidity.  The size of the particulate matter varies and is usually measured in microns.  Suspended solids that are greater than 30 micron size are generally visible to the naked eye.  Filtration of particulate matter in water, in many ways, is the simplest of the treatment processes however thought must be given when deciding which type of equipment should be chosen.

 

Cartridge and Bag Filtration:  Cartridge and/or bag filtration usually involves the use of a filter housing designed to hold a single or multiple number of filter elements.  The filter housings, depending upon the application, may be constructed of carbon steel, stainless steel or a form of plastic such as PVC,  polypropylene or polyvinylidene flouride (PVDF).  The filter cartridges or bags that are used in the filter housings are rated for removal of a specified particle size identified in microns. Cartridge and/or bag filters may be used to remove essentially any particle size even down to submicron ranges. Cartridge and bag filters can be extremely effective for removal of particulate matter but can be costly if not applied properly.  Cartridges and bags are generally disposable and ongoing replacement costs may be expensive.

 

Single-Media Filters:  Single media filters have been widely used for decades in municipal water treatment.  The most commonly used single media is sand.  The size of the media particles determines the effectiveness of the filter to remove particulate matter.  If the media is too coarse, fine particles will pass through the filter media and into the filtered water system.  If the media is too fine the particulate matter will build up rapidly on the top of the filter bed thereby creating the need for frequent backwashing of the filter.  Typically single sand media filters will remove suspended matter down to around 25 micron size particles.  Since single media filters only use one grade or size of media throughout the filter, the efficiency of the unit is limited to the top 3 - 4 inches of the bed.  Design engineers usually choose a fine media in a single filter since inefficient filtration cannot be tolerated and does more harm than good.  Increased frequency of backwashing is tolerated as a concession to higher filtering efficiencies.  The inability of sand filters to utilize their entire media bed for filtration quite often renders them inefficient when compared with other forms of filtration.

 

Multi-Media Filtration:  Multi-media filters, also known as depth filters, combine more than one media in a single vessel.  The top layer or level is generally a coarse anthracite media designed to remove particulate matter down to approximately 50-70 micron size particles.  The second layer or intermediate level is generally comprised of sand with a rated removal of approximately 25-30 microns.  The bottom layer is usually made of fine garnet designed to remove down to 10 micron size particles.  As water flows downward through the filter vessel it encounters the different media layers thus increasing the filtering efficiency of the unit.  Because of its multiple layers of media a depth filter traps and holds more impurities than a single media filter because impurities are trapped throughout the entire bed.  As a result of the increased efficiency of media utilization and less frequent clogging of the filter relatively small diameter vessels that require less floor space can be used for multi-media filters.  The multi-media filter is like having three filters in one without the cost of acquiring and maintaining three separate filters.

 

Screen Filtration: Automatic screen filtration systems are quickly becoming the choice for architectural and engineering firms that are faced with the efficient design of new buildings. As the availability for mechanical space is diminished in new construction more efficient boilers, hot water generators and smaller foot prints for equipment are being sourced. Screen filtration systems can be stacked and require much less floor space than large cumbersome sand and multimedia filter vessels. They also require a fraction of the backwash water required by media filters. The increased operating efficiency of screen filtration coupled with its minimal maintenance requirements makes it an economical choice. 

Softening of water by ion exchange process involves the exchange or substitution of sodium minerals/ions for hardness minerals, mainly calcium and magnesium.  The exchange is made possible because the minerals are ionic in nature (often called ionized impurities), which means they have an electrical charge. The ion exchange process is based on the fact that like charges repel one another and unlike charges attract one another.  Calcium and magnesium are positively charged ions known as “Cations”.

 

Calcium and magnesium ions in water are dissolved in solution.  They have been dissolved into the water, as a result of the water trickling down through strata of rock and soil.  The water actually dissolves the calcium and magnesium deposits as it goes. These dissolved minerals, in the water, eventually find their way into underground aquifers.  As water from the aquifers is brought to the earth’s surface either naturally from springs or pumped it contains the dissolved calcium and magnesium.  Calcium and magnesium are considered to be “hard” minerals and are detrimental to water using equipment and appliances such as boilers, steam generators, hot water heaters, commercial dishwashers, etc.

 

An ion exchange water softener uses a man-made media called sodium zeolite resin to remove hard minerals (calcium and magnesium) from water.  The resin, made of polystyrene divinyl benzene, consists of millions of tiny plastic beads all of which contain many negatively charged exchange sites designed to attract positively charged “Cations”.   When the resin is in the regenerated state, the negatively charged exchange sites hold positively charged “sodium” cations.  The sodium cations are weakly bonded onto the resin exchange sites.

 

As water, containing hard minerals such as calcium and magnesium, is passed through the resin the resin beads displace their sodium ions from the exchange sites and attract the calcium and magnesium ions. During the ion exchange process, relatively small amounts of other strongly charged cations, such as iron and manganese, are also removed along with the calcium and magnesium.  The water existing the resin and water softener now contains sodium ions which are less harmful to equipment and appliances.

 

This process of ion exchange is possible for two reasons: (1) All Cations do not have the same strength of positive charge and (2) the resin prefers the more strongly charged Cations calcium and magnesium than it does the weaker charged sodium Cations.  Although resin has an extremely vast number of exchange sites, eventually all of the resin exchange sites become occupied by calcium and magnesium, and no future exchange can take place. The resin is then said to be exhausted and must be regenerated.  The resin of the softener is regenerated with a dilute solution of sodium chloride (common salt) and water -- brine. During regeneration, the flow of service water from the softener is first stopped. Brine is drawn from the brine tank mixing with a separate stream of water. The brine solution flows downward through the resin, contacting the resin beads which are loaded with calcium and magnesium ions. Even though the calcium and magnesium are more strongly charged than the sodium, the concentrated brine solution contains literally billions of the more weakly charged sodium ions which have the power to displace the smaller number of calcium and magnesium ions. When the calcium and magnesium ions are displaced (exchanged), the positive sodium ions are then attracted to the negative exchange sites. Eventually all sites are taken up by sodium ions and the resin is said to be regenerated and ready for the next softening cycle.

Reverse Osmosis is a process for removing dissolved mineral salts, submicron particulate matter, organic molecules and compounds and microorganisms from water by forcing water, under increased pressure, through a semi-permeable membrane.  This process is the “reverse” of the natural osmotic process in which fluids with a low concentration of dissolved solids pass through a membrane into an area of higher concentration.  With reverse osmosis, water is made to pass from a state of high concentration to a state of low concentration.  Since reverse osmosis does not occur naturally it must be created by applying pressure to the high solids water in order to force it through the membrane.  The membrane material must be strong and resistant enough to withstand the high pressures of RO operation.  Most membrane applications such as the processing of potable water utilize pressures that range between 200 and 400 psi.  Other applications such as seawater desalination may require pressures as high as 1000 to 1200 psi.  The pressure applied to the inlet/feed side of the reverse osmosis membrane must be significantly higher than the natural osmotic pressure of the water in order for the osmotic process to be reversed.  As a result high pressure pumps are generally used to create the pressure needed to produce product/permeate flow rates that are economically acceptable.

 

The product flow termed as “permeate” of a reverse osmosis unit is mainly a function of temperature and pressure.  Some membranes require a tempered water (77 ºF) in order to optimize their production while others require higher pressures.  The quality of a reverse osmosis product/permeate water is based on a percentage of the dissolved solids supplied to the membrane.  Typical “rejection” of dissolved solids will range between 96% - 99.5% depending upon the chosen membrane.   Since reverse osmosis membranes are “semi-permeable” they must have a reject/waste stream termed as “concentrate” to carry away the impurities that have been removed/rejected.  System “recovery” (product water divided by feed water) is limited by the characteristics of the feed water.  Single pass reverse osmosis units typically have a recovery of between 50 and 75%.  System recovery can be increased/controlled with a recycle stream. 

 

In order to maintain the effective and efficient operation of a reverse osmosis system an economic balance between product water quality and system recovery needs to be determined.  High recovery rates will appear to make a system more efficient and decrease waste/concentrate water however they will also increase the concentration of dissolved solids in the system which will hinder the quality of the product/permeate water.

 

Pretreatment of water prior to a reverse osmosis unit is almost always required.  The majority of reverse osmosis membranes in service today are of TFC (thin film composite) construction.  TFC membranes have high salt rejection rates, exhibit good performance over a wide range of pH and temperature conditions and are not degradable by microorganisms.  When using thin film composite (TFC) reverse osmosis membranes chlorine removal is a must.  The chlorine may be eliminated by using activated carbon, injection of sodium metabisulfite or ultraviolet light.  In addition, it is generally recommended that hardness minerals such as calcium and magnesium be removed from the feed water so as to prevent scale formation on the reverse osmosis membranes.  Turbidity, iron and other impurities must also be controlled for optimum performance of a reverse osmosis system.

Deionization is the process of removing ionizable solids from water using the principles of ion exchange.  In a water softener, the ion exchange process is relatively simple, and consists essentially of exchanging “softer” sodium minerals/ions for “harder” calcium and magnesium minerals/ions.  Deioniza­tion, as an ion exchange process, is more complicated because it involves the removal of virtually all ionizable solids from water.

 

All dissolved minerals in water are comprised of a metallic part (a positively charged cation) and a non-metallic part (a nega­tively charged anion).  A water softener only requires one resin to accomplish its job because it exchanges only cations.  A deionizer, on the other hand, requires two resins because it exchanges both cations and anions.  It is important to note that no single resin can exchange both cations and anions because ion exchange depends on the tiny electrical charges in which like ions repel one another and unlike ions attract one another.  A single resin cannot be both positive and negative.  A cation exchange resin is chemically formulated to attract positive ions and an anion exchange resin is formulated to attract nega­tive ions.

 

The simplest form of a deionizer system involves the use of two independent columns or vessels.  This form of deionization is known as a Two-Bed Deionizer.  The first column/vessel contains cation exchange resin and the second column/vessel contains anion resin.  The water to be treated must first pass through the cation deionizer and then the anion deionizer.  As water passes down through the cation vessel it encounters millions of resin beads each of which contains a large number of negatively charged exchange sites in the pores and microscopic paths of its structure.  When the resin is in the regenerated state each exchange site is occupied by a positively charged hydrogen ion.  As the positively charged cations in the water, contact the beads, they are attracted to the negatively charged exchange sites.  Since they are stronger in their positive charge than the positive hydrogen ions, they drive off the hydrogen ions and attach to the exchange sites.  By doing so, they maintain a balance between positive and negative charges. The displaced hydrogen ions (H) pass down through the resin bed and exit the vessel in the water stream.  Because the hydrogen ions are acidic the exchange can also be described as a displacement of acidic ions by metallic ions.  As a result the water from the cation vessel is a stream of dilute mineral acid.  Since the cation resin only removes positively charged ions the negatively charged ions or anions pass through the cation resin bed with the acidic water stream.

 

The anion exchange process is similar to the cation exchange process.  A strong base anion resin is made of beads which have positive exchange sites.  When the resin is in the regenerated state the positive exchange sites are occupied by negative hydroxide ions (OH).  As the negatively charged non-metallic anions contact the beads, the same attraction-repulsion process takes place, as with the cations, and the negative hydroxide ions are dislodged and replaced by the stronger negative non-metallic anions.  The hydroxide ions (OH) pass down through the anion resin and are discharged from the vessel.  At  the same time, the hydrogen ions (H) from the cation vessel have passed unchanged through the anion resin and they join the hydroxide ions to form HOH or H2O…….water.

 

Cation and anion exchange resins have limited capacities and have to be periodically regenerated.  Cation exchange resins are regenerated by hydrochloric or sulfuric acid.  When acid is introduced to the cation resin the positively charged hydrogen acid cations, in the chemical, force the positively charged cations (calcium, magnesium, sodium, etc.) off of the resin that were attracted and held during the deionizer service cycle.  The positive hydrogen ions attach to the negative exchange sites on the beads thereby restoring the resin to its regenerated hydrogen form.

 

Anion exchange resins are regenerated using sodium hydroxide (caustic soda).  In a strong base anion resin the alkaline solution passes down through the resin bed and exchanges hydroxide ions for the anion ions (chlorides, sulfates, bicarbonates, silica, etc.) which were attracted and held by the beads during the service cycle.  The negative hydroxide ions attach themselves to the positive exchange sites on the beads thereby restoring the resin to its original basic, hydroxide form.

 

A two-bed deionizer provides a low dissolved solids water which results in a high quality water.  If the two-bed deionization (Cation/Anion) exchange process could be repeated many times, the efficiency of ion exchange and removal would improve remarkably.  Since no exchange process is 100 percent efficient, successive ion exchanges would remove even more ions since, in effect, it would be deionization of water that had already been deionized.  The result would be an improvement of water purity with each successive ion exchange.  This is exactly what happens when cation and anion resins are mixed together, in a single column/vessel to form a mixed-bed deionizer.  With the resins thoroughly mixed the water molecules to be processed have millions of chances to contact a cation resin bead, then an anion, then another cation, another anion, and so on.  The exchange process takes place, of course, only when a positive cation contacts a negative exchange site and a negative anion contacts a positive exchange site.  With each exchange, the purity of the water improves because more ions are removed and held by the resin beads.  The end result is a higher quality of water from the mixed-bed deionizer.

 

The quality of water provided by deionization is measured in a number of ways.  It can be measured quantitatively in milligrams per liter (mg/L) or parts per million (ppm) of total dissolved solids (TDS) or electrically by conductance or resistivity.  Electrical measurements are based on the fact that the electrical conductance or resistance of water is directly related to the amount of ionizable solids/impurities in the water.  Thus, a measure of the conductance or specific resistivity is in effect a measure of the ionic content, or purity/quality of the water.

 

Mixed-bed deionizers are quite superior to two-bed deionizers in terms of the water quality they produce.  A two-bed strong base deionizer yields water of about 2.5 mg/L (2.5 ppm) TDS which equates to a conductivity of 5.0 microsiemens/cm or a specific resistivity of about 200,000 ohms/cm. Mixed-bed deionizers can yield water with less than 0.04 ppm TDS with conductivities as low as 0.06 microsiemens/cm and resistivity values as high as 18,300,000 ohms/cm (18.3 megohms/cm).

 

It should be pointed out that deionizers remove ionizable solids only and have little or no effect on most dissolved gases, particulate matter, colloids, dissolved organic matter or biological impurities.  In addition, although a strong base resin will remove CO2 chemically it may be more economical to remove it with a mechanical degasifier especially when large amounts of CO2 are involved.  Such considerations underscore the need for a systems engineering approach when addressing the problems of water treatment.

 

Ultraviolet irradiation is a powerful technology that has been employed by several diverse industries such as pharmaceutical, semiconductor, power generation, food and beverage, cosmetics, aquaculture, etc. for several decades.  The most common use of ultraviolet radiation is for the disinfection of water.  It is used primarily for the control of water borne microorganisms.

 

The use of ultraviolet technology for water treatment has several advantages.  UV radiation does not add anything to the water.  No chemicals, no undesirable color, no odor, no taste or flavor and it does not generate harmful by-products.  It only imparts energy into the water stream in the form of ultraviolet radiation.  It is a fast, efficient, cost effective and environmentally friendly solution to controlling the growth of bacteria, viruses, molds, and spores in water.

 

Disinfection: This is the most common application of UV radiation in water treatment.  Using a 254 nanometer UV wavelength the radiation penetrates the outer cell wall of the microorganism, passes through the cell body, reaches the DNA of the microorganism and alters the genetic material.  This process inhibits the microorganism's ability to replicate and therefore destroys the microorganism.

 

TOC Reduction: A number of water quality standards include a limitation for "total organic carbon".  USP Purified Water and Water for Injection (WFI) have an upper limit of 500 ppb while ASTM and CLSI standards have varying limitations for TOC.  Total organic carbon levels may be reduced in water by using UV radiation with a 185 nanometer wavelength.  The more powerful 185 nm wavelength, with proper dose, decomposes the organic molecules.  The level of TOC reduction desired dictates the UV dose required.  Typical UV dose is 4X - 10X (120 - 300 mJ/cm2)

 

Ozone Destruction: Ozone is commonly used in the Life Sciences industry for sanitizing storage tanks, vessels, piping, auxiliary equipment, etc.  In some cases ozone is continuously fed on return piping loops to storage tanks and/or occasionally used as a disinfectant.  Ozone can be detrimental to downstream water treatment equipment and can harmful to humans if in a concentrated vapor form.  UV (254 nm), with a proper dose, can be used to destroy residual ozone by breaking down the O3 to O2 (oxygen).  Typical UV dose is 3X or 90 mJ/cm2.

 

Chlorine/Chloramine Destruction: While the addition of chlorine and chloramines to city water may control bacteria levels, they have detrimental effects on the some water treatment equipments such as reverse osmosis membranes.  Popular methods of chlorine/chloramine removal such as activated carbon or sodium metabisulfite injection have sometimes proven to be problematic.  Adding chemicals, such as metabisulfite to a high purity water system is sometimes frowned upon and activated carbon media can be a breeding ground for bacteria.  UV (185 nm) in a proper dose can destroy chlorine and chloramines.  Care must be taken when employing this technology for this application.  Depending upon the level of chlorine and/or chloramines UV doses of 20X - 50X (600 - 1500 mJ/cm2) may be required.