Pure Water Systems for Lab Grade Water

ASTM (American Society of Testing and Materials) is the most commonly used standard for laboratory water. ASTM lab water is divided into four categories, with the upper tiers being Type 1 (ultrapure) and Type 2 (pure).  Pure water can be produced by distillation, or other alternative technologies, to achieve a conductivity of < 1 µS/cm (or resistivity which is > 1 MΩ*cm) at 25 °C. Other quality parameters are TOC (Total Organic Carbon, 50 µg/L), sodium (5 µg/L), chloride (5 µg/L) and silica (3 µg/L). Deionization technology and reverse osmosis are examples of alternative technologies that can be used to produce ASTM Type 2 water.

You can usually find the feed water specifications in the water purification system’s technical information such as the datasheet. A pure water purification system can generally be connected directly to tap water, as water that is good for drinking is typically good as feed water. However, if it does not meet specifications and has a high concentration of e.g. calcium carbonate (CaCO3), iron or silica particles, organics or bacteria - an extra  pretreatment step is recommended to avoid calcification and clogging. Reverse osmosis, activated carbon and 5 µm depth filters are pretreatment examples that are generally used either individually or in combinations. CO2 value is also important when using an EDI module, as it impacts regeneration capabilities of the EDI module and can pass through the semipermeable membrane in the RO module. Therefore, it is important that the feed water CO2 value is below 40 ppm.

A deionization system provides consistent removal of charged molecules, which would otherwise cause interference with reagents, protein-protein interactions, inhibition of polymerase activities in samples, buffers etc. However, pure deionization systems without additional pretreatment techniques (e.g. Membrane filtration) do not remove organic material, particles or bacteria. Further processing could therefore have a positive impact  on downstream process or application. 

It is important to choose the correct quality of water for your applications. While too low quality may affect your experiments or analyses, too high quality may be unnecessarily expensive. Applications such as media and buffer preparation, AAS and photometry analysis, which are less critical or sensitive, are suitable for pure water. Pure water can also be used for general laboratory tasks such as rinsing, or as feed water for water-consuming systems (e.g. washing machines, autoclaves or Type 1 water purification systems).

With water purification, the terms deionization and demineralization are often used interchangeably to mean the use of ion exchange technology. Most minerals in water have a charge and are therefore ions. However, deionization is the process of removing ions, while demineralization removes minerals. The deionization process is typically based on artificially produced ion exchange resin, which consists of both negatively and positively charged particles. As the ions in the water come into contact with the resin, they are bound to the resin with the help of ion exchange and removed from the feed water. The deionization process does not, however, remove neutral molecules or particles. For this, an internal or external pretreatment step with activated carbon, depth filters or RO modules - or a combination of the technologies - is required.

DI (deionization) and EDI (electrodeionization) are both ion exchange technologies used in water purification systems. The term “deionization” also describes the process of deionizing (removing ions from) the water in general. The biggest difference between DI and EDI is that EDI additionally uses electricity to remove ions  supported by semipermeable membranes, while DI cartridges use only resin. The electrical current in the EDI also regenerates the resin automatically , while resin used in a DI cartridge is either exchanged or regenerated manually with chemicals. 

The main difference between ASTM Type 2 (pure) and ASTM Type 1 (ultrapure) water is conductivity. For ultrapure water, the conductivity is a maximum of 0,055 µS/cm. For pure water, it is maximum 1 µS/cm. Other quality parameters that differ are sodium and chloride concentrations, which have a maximum level of 1 µg/L and 5 µg/L, respectively. To produce ultrapure water, you need a DI cartridge, designed for producing ultrapure water, for polishing. In comparison, to produce pure water you can use either a DI cartridge or an EDI module suitable for producing pure water. Depending on water purification system and feed water, a pretreatment step may be needed.

The main difference between ASTM Type 2 and RO (Reverse Osmosis) water is conductivity. ASTM Type 2 water has a maximum conductivity of 1 µS/cm, while for RO water it is usually higher. The quality parameters, which except for conductivity include TOC, sodium, chloride and silica, must be met to be categorized as ASTM Type 2, while RO water does not have any set values. The quality of RO water depends completely on feed water, as the RO module only removes a certain percentage based on the rejection rate of the module. Therefore, RO water is not consistent but may vary depending on factors like region and season.  Reverse osmosis is commonly used as a pretreatment step because of its excellent and broad retention capabilities, as it filters out impurities based on size. Deionization technology, which is commonly used for production of ASTM Type 2 water, only binds ions using ion exchange. Therefore, a pretreatment step is highly recommended to remove other impurities, such as organics and bacteria.

For producing pure water, a system with deionization technology - either DI or EDI - is generally used. It is very common to have a pretreatment step of a reverse osmosis module prior to the deionization cartridge. This is especially recommended when the pure water is used as feed water for producing ultrapure water. Additionally, there can be steps consisting of activated carbon and/or depth filters, depending on quality of the feed water. As membrane filtration reduces the flow rate - RO and EDI technology both use membrane filtration - a storage tank, either internal or external, is generally used to compensate the flow rate. From the storage tank, the water can then be directed as feed water to instruments or to a dispensing unit. 

Purification methods based on membrane filtration limit the flow rate to such an extent that they usually require a storage tank in laboratory environments. As membrane filtration (EDI and reverse osmosis) is commonly used to produce pure water a storage tank is then needed.    The use of a storage tank allows water to be dispensed at a higher flow rate and pressure, which is needed when used as feed water for example. If the pure water is produced with a DI cartridge only, then a storage tank is not necessarily needed as the DI cartridge does not use membrane filtration. However, to only use a DI cartridge removes only ions and e.g. no particles, organics or bacteria and the cartridge is at risk for clogging and calcification. Therefore, a pretreatment with RO module is highly recommended and therefore also the use of a storage tank.