The Basics of Lab Water Purification
Ultra-Pure Water, containing nothing but hydrogen and oxygen, has a specific resistance of 18.2 megohm-cm at 25 °C. Since conductance or conductivity is the reciprocal of resistance or resistivity, a cubic centimeter of pure water has a specific conductance of about 0.055 micromhos (i.e. microsiemens) per cm at 25 °C. The conductance arises from the partial dissociation of pure water into hydrogen (H+) and hydroxyl (OH-) ions. Since conductivity is an increasing function of increasing water temperature, the temperature at which conductivity is measured MUST be taken into account. Most conductivity measuring devices simultaneously measure the water temperature, and compensate the conductivity reading as if it were taken at 25 °C.
But, type I Ultra-Pure Water does not like to stay pure! In its "ultra-pure state", water is defined as the world's strongest solvent! It aggressively attacks its surroundings, and begins to dissolve them! And, it doesn't care what they are made of! It’s just as happy dissolving solids as it is dissolving gases or other liquids. It likes to dissolve both metallic and nonmetallic solids. It likes to dissolve both organic and inorganic compounds. In the process, it also likes to acquire suspended solids, colloids, microorganisms, pyrogens, endotoxins, viruses and enzymes, such as DNase and RNase!
At the beginning of a lab water purification train is a source of tap water to be purified. But what is tap water? Can it be defined? Not in general. The local tap water quality is the result of many variables having to do with where it came from, where and how long and under what conditions it was stored, and what has the human race done to contaminate it?
Tap water comes with a lot of particles, free of charge. Particles are described as having a diameter of so many microns. A micron is 1/1000 of a meter. For comparison, a human hair is about 70-100 microns in diameter, a pinpoint is about 60 microns, and a red blood cell is about 7 microns. Your local water treatment plant operates on the theory that if you can't see it, you won't mind drinking it! Therefore, they typically filter tap water down to about 25 microns, which means that they remove most particles larger than 25 microns in diameter. Just coincidentally, the smallest particles visible to the naked eye are in the range of 30-40 microns in diameter! The particulate matter in tap water comes from its surroundings, and can include pieces of rock, iron, sand, sediment, pieces that fall off the piping and container system (including leaded solder), organic and inorganic colloids, organic matter from decaying plants, remains from dead animals, live microscopic animals, bacteria (dead or alive), pyrogen and endotoxin (fragments of bacterial cell walls), viruses, and enzymes.
Dissolved inorganic solids, such as silicates, sulfates, chlorides, fluorides, bicarbonates, phosphates, nitrates and ferrous (metallic) compounds, are present in the tap water as cations (positively charged ions) and anions (negatively charged ions). Dissolved inorganic solids (and gases) are what gives water its conductivity. Each one contributes to conductivity at its own rate, that is standardized to a CaCO3 or NaCl equivalent. These contaminants dissolve into the water from its surroundings, such as aquifers, lakes, streams, and rivers. In some cases, humans introduce them into the water supply through the use of fertilizers, pesticides, and plumbing. The longer water sits in its surroundings, the more chemicals it dissolves. Calcium and magnesium compounds come from the natural limestone caves that water collects in. They are what causes "hardness" in water. They have limited solubility in water, and tend to chemically react with soaps and detergents by coming out of solution and forming "soap scum". Dissolved ionized gases include CO2, which purified water will actually absorb right from the air! CO2 is also one of the by-products resulting from ultraviolet oxidation. CO2 contributes to the water's conductivity.
Dissolved organic compounds come from animal and plant decay, and from human pollution. They can include natural proteins, alcohol resulting from plant decay, chloramines resulting from chlorination of tap water, and compounds left over from the use of fertilizers, pesticides, detergents, and gas & oil spills. Dissolved non-ionized gasses, of which oxygen is the most common, do not contribute to conductivity, but can corrode metallic surfaces that the water is contained in.
In order to purify water to levels acceptable to laboratories, several technologies must be combined in a logical sequence to achieve the desired results. Note that, although distillation is a water purification technology, we don't include it herein, because deionization has essentially replaced distillation in the modern world. In addition, distillation has very high energy and maintenance costs. While a few researchers still swear by (and at) their stills, most have switched to a DI system.
5 Lab Water Purification Technologies
Activated Carbon Filtration
Carbon is the oldest, and probably the safest, form of liquid purification technology. It dates back to the earliest biblical recordings of beer and wine making, where it was used to improve the flavor. And, activated carbon is so safe, you can actually eat it in small quantities without any harmful effects! An activated carbon pre filter can effectively remove chlorine and many of its compounds, along with many nonionic organic compounds. The sintered carbon block type of activated carbon pre filter offers the highest capacity, since it has the most surface area. In addition, the water cannot create internal "rivers" and bypass the carbon granules found in other types of carbon filters. Thus, the second stage of a properly designed laboratory water purification system, should include an activated carbon prefilter unless, of course, the feed water to the system has already been pretreated via a "house" DI system, or a reverse osmosis pretreatment system, which has its own activated carbon prefilter.
Reverse Osmosis Filtration
Reverse Osmosis (RO) is aptly named. It actually reverses the natural osmotic process by using pressure to force pure water through a porous membrane. The membrane's pores are sized such that they allow pure water to pass through, while rejecting the contaminants in the water at up to 99% efficiency. In actual situations, the rates of rejection can vary from about 85% to 99% for various contaminants, based on the molecular weight or size of the contaminant and the operating conditions, which include pressure and temperature.
On the average, it is very safe to say that the RO pretreatment system removes 90-95% of the contaminants found in the incoming tap water. The net result is, that whatever water purification technology follows the RO pretreatment system (i.e., a Type I, Type II or Type III DI system), its operating cost for DI modules will be reduced by 90-95% when compared to not having an RO pretreatment system.
To justify the added cost of the RO system, calculate the total capital and operating costs for the DI lab water purification system, with and without the RO over 2-3 years. RO pretreatment should also be used when the DI system alone cannot provide the required water quality. RO should be considered whenever the incoming tap water contains more than 170 parts per million of total dissolved solids, and/or usage on a Type I DI system exceeds 20 liters per day, and/or when usage on a Type II DI system exceeds 40 liters per day.
Deionization (DI), a.k.a. ion exchange or demineralization, is a process whereby tap water is passed through charged cationic and anionic resin beds containing sites with available hydrogen (H+) and hydroxyl (OH- ) ions. As the ionized contaminants, such as Na+, Ca++, Mg++, Cl-, SO4--, and HCO3-, pass by the ionized sites, the cationic resin exchanges its H+ ion for the Na+ ion, and anionic resin exchanges its OH- ion for the Cl- ion, etc.
Originally, the cationic and anionic resins were in separate tanks, and the process could not go to full completion, since some impurities were left behind from each process. The maximum resistivity that could be achieved was in the range of 2-4 megohm-cm. This was known as a "two bed" DI system, and some companies still use two bed DI modules in an attempt to convince you that they have higher capacity than mixed bed DI modules, but don't be fooled by this. While they can process more water, they can only process it to a 2-4 megohm-cm endpoint, and it still takes just as much ion exchange capacity to get it up to 18 megohm-cm! By mixing the cationic and anionic resins together in a single tank, the net result is an essentially infinite sequence of ion exchanges that do go to completion, resulting in 18 megohm-cm water.
Ion exchange resins come in a wide variety of types, and most of them are unsuitable for producing 18 megohmcm type I "ultra-pure" reagent grade water for laboratory applications. Those that do, have been specially processed and rinsed, and are usually classified as "Semiconductor Grade", since the semiconductor industry stands to lose the most if their final rinse water contains any impurities. The ion exchange or "DI" portion of the laboratory water purification system does (essentially) all of the work in removing ionic contaminants from the feed water except, of course, for the contaminants removed by the RO system, if any.
A 0.1 micron absolute-rated final filter cartridge or capsule, prevents suspended solids, particulate matter and bacteria from exiting a lab water purification system along with the purified water. The absolute rating means that nothing larger than 0.1 micron in diameter can pass through the filter. This is essentially how beer and other liquids are "cold sterilized". The typical final filter capsule is manufactured per GMP standards from USP Class IV materials with no glues or surfactants, and is autoclavable, bubble point testable, and non-pyrogenic.
When do you require ultra-low TOC Type I ultra-pure water?
Pure water is a commodity in many industries in particular in analytical and biologic laboratories. Laboratory grade water is defined by its resistivity, which is determined by the amount of ionic contaminations and by its Total Organic Carbon content (TOC). Resistivity of the water determines the water quality based on American Society for Testing and Material (ASTM) definitions. Many routine laboratory applications use ASTM Type II water that has a resistivity of >1MΩ/cm2 which corresponds to less than 500ppb total ionic contamination. More specialized applications in analytical chemistry and molecular and cell biology require water of ASTM Type I with a resistivity of >18 MΩ/cm2 which corresponds to about 1ppb total ionic contaminations in the water.
ASTM water quality definitions do not give any information about the amount of organic compounds in the water. Organic compounds, however, can cause serious problems for certain analytical chemistry and biological applications. Liquid phases and buffers prepared for HPLC need to have ultra low TOC and ultra low ionic contamination, as these contaminants can show up as separate peaks in the chromatogram and also increase background and decrease sensitivity. These same is true for gas chromatography (GC), Atomic Absorption Spectroscopy (AAS), Ion Chromatography (IC), Electromagnetic Spectroscopy, and mass spectrometry (MS). Several molecular biology applications require water that is not only ASTM Type I, but also nuclease- and pyrogen-free, which is most reliably achieved with ultra-pure water with ultra-low TOC.
Electrophoresis, polymerase chain reaction, DNA and RNA extraction and other methods that use nucleic acids need water that is nuclease-free to avoid degradation of the DNA. Due to the stability and abundance of RNAses as contaminants, a source of water that is nuclease-free is even more important for applications that use RNA. Enzyme Linked Immuno-Sorbent –Assays (ELISAs) can detect minute amounts of specific contaminants that can increase background and decrease sensitivity of the assay. Many analytical assays that use fluorescence for detection also need low TOC as many organic compound fluoresce to some extent in UV light.
Cell biology is another area where low TOC and purity of water is critical. Water used for the preparation of buffers and media for tissue culture does not only need to be sterile (free of microbes like bacteria and viruses), but also free of organic contaminants like endotoxin. Endotoxin contaminations in tissue culture can interfere with cell growth, cytokine responses and significantly alter the results of experiments. Drug preparations intended for the use in animals or humans also need to be made with high-quality ultra-pure, ultra low TOC, and endotoxin-free water, as endotoxin cannot only alter results of in vivo experiments dramatically, but it can even cause toxicity, though endotoxin contaminations at a level that can cause toxicity www.aquaa.com are rare. FDA requirements for human and veterinary medications restrict the use of water for production of medications to the highest purities.
To purify tap water to ASTM type I and ultra-low TOC standards, purification systems are used that employ a series of different purification methods, each of which has different maintenance requirements. A vital part of any water purification system that is supposed to achieve ASTM Type I grade purity is a deionizing system. A deionizer removes cationic contaminants in exchange for H+ ions and anionic contaminants in exchange for OH- ions. The purity of the water depends on the quality of the ion exchange resin used and the quality of the maintenance of the system. Ion exchange resins have a limited capacity of ions they can exchange and need to be replaced when they have reached their limit. The resin can be regenerated, but the regeneration process can cause damage to the resin which can lead to organic contaminations. Ion exchangers do not reduce microbial contaminations, and bacteria can accumulate within the resin, leading to a build up of endotoxin within the resin. Virgin ion exchange cartridges, i.e. cartridges with new rather than regenerated ion exchange resin are usually used for the highest purity water. UV-sterilized feed water prevents the buildup of bacteria.
Most water purification systems that produce ASTM Type I water use feed water that is already lower in contaminants than tap water. Pretreatment using activated charcoal can remove chlorine and, to a certain extend, organic compounds from tap water and reverse osmosis is a very economical way to reduce about 95% of all contaminants, which makes it a very useful pretreatment for feed water for ASTM Type I purifiers. Reverse osmosis membranes can be damaged by CaCO3 deposits and deposits of organic compounds and colloids.
Water softeners and pretreatment of the water with activated charcoal can significantly increase the live of the reverse osmosis unit. Since ion exchangers cannot remove non-charged organic compounds, these need to be removed in a separate step. Different filtration steps will remove, depending on the cutoff of the filtration membrane, starting from large particles, bacteria and viruses, down to endotoxin and nucleases. However, while microporous filters like e.g. the ones used for the removal of bacteria with a cutoff of 0.2 micron can remove 100% of particles above the cutoff diameter, ultrafiltration filters that are used for smaller particles and organic compounds like endotoxin and nucleases remove most, but not all contaminants above the cutoff diameter.
Both microporous filters and ultrafiltration membranes can clog when too many contaminants are deposited on the surface. If this happens they need to be replaced. Ultrafiltration membranes clog much easier than microporous filters, so that they are usually placed behind other filtration steps in a water purification system. To further reduce TOC a UV-oxidizer needs to be used.
Since UV oxidizers increase the amount of dissolved ions and therefore reduce resistivity in the water, the most logical position of a UV-oxidizer in a water purification system is before the deionizing step that produced water with very high resistivity. The UV-oxidizer also sterilizes the water reducing the risk of bacterial contamination of the subsequent ion-exchanger resin in this configuration. The low pressure mercury lamps in UV-oxidizers/sterilizers have a limited lifespan and need to be replaced from time to time. In many systems another ultrafiltration step is used at the very end to ensure the sterility of the water.
Types of Reagent Grade Water
Common sense tells us that dilution water must contain significantly lower levels of impurities than the sample to be analyzed. Most modern analytical instruments and procedures generally call for Type I water, since they are analyzing at the “parts per billion” or very low parts per million level. Type I water is almost always produced by a “DI polishing system” at the “point-of-use”. Thus, Type I water is not produced and stored for later use, but is produced and used “on the spot” as required. In fact, those who specify reagent grade water quality, specifically state that Type I water should not be stored for later use.
In most modern labs, Type II water is only used for rinsing glassware, and as feed water to a Type I DI polishing system. Type IV reagent grade water (typically generated by reverse osmosis) has become meaningless, in that it has almost no practical use in the modern laboratory other than as feed water to a Type I or Type II DI system.
While “distilled water” is still used in some undergraduate and high school laboratories, the high energy and labor costs of a still, combined with the relatively poor quality of the distilled water, are serving to rapidly phase out distillation as a practical technique for purifying water. “Standard Methods” points out that there are significant problems with distilled water in that the quality is dependent on the incoming tap water quality, and upkeep on the still can be difficult, and distilled water degrades in the storage vessel.
In discussing reagent grade water, Section 9020 of “Standard Methods” advises as follows:
- “To avoid contamination, do not store such water.”
- “Stills produce water that characteristically deteriorates slowly over time as corrosion, leaching and fouling occur.”
- “Stills efficiently remove dissolved substances but not dissolved gases or volatile organic chemicals.”
- “Freshly distilled water may contain chlorine and ammonia.”
- “Type I water should be used immediately after processing.”
- “Type I water cannot be stored because its resistivity will decrease, metals and/or organic compounds will be leached from the storage container, and bacterial contamination will occur."
- "Storing water in large vessels (carboys) for extended period of time is unacceptable because of the inevitable, unpredictable rate of degradation of the water quality."
The Right Water System For Laboratory Glassware Washer
You are ready to buy a new glass washer for your lab, but you are unsure what water feed system to use? You probably already know what grade water you want to feed your glass washer. In most cases, except for special circumstances for very sensitive applications an ASTM Type II reagent grade water source will be sufficient for your lab glassware washer. ASTM Type II is defined as water that has greater than 1 MΩ/cm2 resistivity. Lower grades are not recommended for most application and Type I water rinses (>18 MΩ/cm2 might be necessary for especially sensitive application like HPLC and mass spectrometry. While there are several different ways for water purification like filtration, different kind of filtration, sterilization by UV radiation, adsorption by activated charcoal, to achieve the resistivity required for lab-grade water, the water need either to be distilled or deionized. Distillation requires the heating of water to the boiling point and collection of the condensate from the vapor. This method is the oldest method to produce lab-grade water, but it consumes large amounts of energy, and is unable to remove contaminants that have a lower boiling point than water. The most commonly used method today is deionization. This process uses two ion exchange resins over which the water flows. One resin exchanges cationic contaminants for an H+ and the other exchanges anionic contaminants for a OHion. Both resins can be mixed together in one container. The quality of the water depends on the lengths of the exchange column and the quality of the resin. The resin has to be replaced from time to time. It can be regenerated, and many water purification companies sell resin cartridges with regenerated resin, but new (virgin) resin has a higher cleaning capacity, and does not carry the risk of cross-contamination from other applications.
One main feature you need to be aware of when choosing a lab water purification system for your lab glassware washer is the water use of the washer. If you do not require a large cleaning capacity and you are looking for washer with a small footprint like e.g. the 24” Miele G789, the Labconco undercounter FlaskScrubber, the Lancer LX, or the undercounter washers from Hotpack, a low flow water system might be the right solution for you. Low flow glass washers with DI-water requirements for up to 6 liters/min can use low-flow water systems that might be a much more affordable alternative to a high-throughput-system. Most glasswasher, however, require a larger water flow, and high flow system with flow capacities of 15-40 l/min will be able to fill the need. Some lab water purification system comes with pressurized storage tanks and spare DI-filter cartridges.
Another feature that should be considered when choosing a water purification system is the ease of the maintenance. Filter cartridges need to be exchanged from time to time, and some systems require a maintenance plan from the manufacturer. Other systems can be serviced easily and within a few minutes by the user.
In most cases, except for special circumstances for very sensitive applications an ASTM Type II reagent grade water source will be sufficient for your lab glassware washer. ASTM Type II is defined as water that has greater than 1 MΩ/cm2 resistivity. Lower grades are not recommended for most application and Type I water rinses (>18 MΩ/cm2 might be necessary for especially sensitive application like HPLC and mass spectrometry. While there are several different ways for water purification like filtration, different kind of filtration, sterilization by UV radiation, adsorption by activated charcoal, to achieve the resistivity required for lab-grade water, the water need either to be distilled or deionized.
Distillation requires the heating of water to the boiling point and collection of the condensate from the vapor. This method is the oldest method to produce lab-grade water, but it consumes large amounts of energy, and is unable to remove contaminants that have a lower boiling point than water. The most commonly used method today is deionization. This process uses two ion exchange resins over which the water flows. One resin exchanges cationic contaminants for an H+ and the other exchanges anionic contaminants for a OHion. Both resins can be mixed together in one container.
The quality of the water depends on the lengths of the exchange column and the quality of the resin. The resin has to be replaced from time to time. It can be regenerated, and many water purification companies sell resin cartridges with regenerated resin, but new (virgin) resin has a higher cleaning capacity, and does not carry the risk of cross-contamination from other applications.
The following questions should be answered before choosing a water purification system.
What water quality do I need? Water quality needs are based on the applications forwhich the glassware will be used. Most application will require ASTM Type II water witha resistivity of >1 MΩ/cm2.
What water flow capacity is needed? Some small glassware washers have a low flow requirement and might be able to be fed by a low flow water purification system that can be more affordable alternative.
How much maintenance am I willing to do myself? Service contract for routine maintenance and cartridge exchanges can offer peace of mind, but doing cartridge exchanges yourself might be more cost efficient alternative that has potentially the additional advantage of reducing downtime of the system.
What is ASTM Type I and Type II water? The American Society for Testing and Material (ASTM) sets standard for the amount of ionic contamination in lab-grade water. ASTM type I water has the highest purity and a resistivity of >18 MΩ/cm2, which corresponds to an ionic contamination of less than 1ppb. ASTM Type II water has a resistivity of >1 MΩ/cm2 or less than 500ppb total ionic contaminants.
Is a point-of service DI-water source needed additionally to the glass washer feed? Is the water purification only meant to provide a source of the DI-water feed for the glassware washer, or is the possibility to obtain water for other applications also required?
Do I need a rinse with ASTM Type I water? ASTM Type II water is sufficiently pure for rinsing of laboratory glassware for most common applications. However there are certain applications that are more sensitive to ionic contaminants like e.g. most analytical chemistry, molecular biology and tissue culture applications. In these cases
Do I need a service contract for my DI-water system? All deionizing water purification systems require regular maintenance as the ion exchange resins have a limited capacity and need to be exchanged on a regular basis. This need to be done by trained technicians for many systems. However, there are some systems that allow exchange of the filter cartridges by the user within minutes. This might reduce downtime and costs for maintenance contracts.
Will I have access to the DI-water aside from the feed to my glassware washer? Some water purification systems provide feed water for the rinse cycles of laboratory glassware washers alone. However, there are system available that will have a point of service (POS) access to DI-water.
Is ASTM Type II-water nuclease-free? Working with nucleic acids in particular RNA can be challenging due to the ubiquitous nature of RNAse-contaminants. ASTM-type II water is not rated for its content of organic matter including nucleases. If nuclease-free rinse water is necessary for your applications the water purification system should include appropriate ultrafiltration and, if very low organic contaminant concentrations are necessary, UV-oxidizer-steps.
Tips on Buying a Lab Purification Water System
Here are some buying tips to help you purchase the best lab water purification system for your money:
Silence is golden - but make sure the system runs 24/7. Noise does not purify water! The Type I System you buy should have a pump that continuously and silently recirculates water 24 hours a day, 7 days a week. Don’t be fooled by intermittent recirculation. It could be a ploy to cover up noisy pumps that were never intended to operate continuously, or pumps that heat up the water.
Save time and money - go for user friendliness. Make sure the lab water purification system you buy can easily be installed, operated and maintained, like ours. Before buying, check for hidden panels, and complex construction and service requirements, like weekly chemical sanitization, that can complicate maintenance and your life, while costing you money and downtime.
Insist on a two-year warranty. Don’t settle for a one-year warranty.
Make sure it can run on ordinary tap water without costly pretreatment. That’s right,our Type I DI systems can run on ordinary tap water containing as much as 170-250ppm of total dissolved solids without RO or other types of pretreatment.
Make sure you get a remote dispenser included in the price. Some Type I systems don’t even have or include a remote dispenser. And others have one located up so high on the wall you have to purchase a second, or “remote-remote dispenser”, to dispense water at bench top level!
Think Green. We all need to protect the environment! Make sure you can recycle the exhausted DI modules back to the manufacturer, like ours. Some locations in the US consider the disposal of ion exchange resin to be an ENVIRONMENTAL HAZARD.