Water for laboratory use has been divided into different grades for different laboratory applications. It is quite obvious that you will useType-I water for ICP – MS determinations for trace metals and not for washing of laboratory glass ware. You may be tempted to use highest grade purity water for all your laboratory usage so as to give unquestioned results but imagine the high expenditure involved for taking such a decision. Purification of available water to the required levels of purity is achieved by different technologies. It becomes necessary to understand the advantages and disadvantages of some common techniques.
Distilled water is a common source of purified water in college and university laboratories. Raw water is boiled and the steam generated is condensed as distillate in a clean receptacle.
- Effective removal of inorganic salts and other high boiling point impurities
- The process does not require expensive equipment
- Distilled water is sufficiently pure for majority of laboratory applications
- Slow collection of distillate
- Consumes electrical energy for heating
- Low boiling organic impurities get simultaneously distilled over
- Requires large amount of water for re- circulation in the condensing unit
- Prolonged storage can decontaminate the collected water through leaching from container or subsequent exposure to atmosphere.
Filtration is effective in removing suspended particles, bacteria and other suspended impurities which are above the pore size of the filter. Coarse filtration can remove particles to 25 µm size and final stage filtration can remove particles in the range of 0.45 µm to 0.2 µm. Two popular filtration techniques are ultrafiltration which is effective in removal of bacterial endotoxins and reverse osmosis which is effective in removal of bacteria, pyrogens, inorganic and organic suspended impurities. Reverse osmosis in combination with ion exchange, sorption and UV oxidation provides a viable solution for most high purity water applications.
- Filters remove particles and microorganisms greater in size than pore size of the filters
- Cost-effective and easy replacement
- Ultrafilters are effective for removal of colloids, endotoxins and micro- organisms
- Dissolved organics and inorganics are not removed
- Filters become blocked over prolonged use with large size impurities
- Regeneration is not possible for blocked or damaged filters and replacement is the only available option
Deionization or demineralization is carried out using ion exchange resins. A mix of cation and anion resins is adequate to achieve the desired degree of deionization
- Ion-exchange process is relatively inexpensive
- Regeneration of ion exchange resin bed is possible by using acids and bases
- Possible to achieve Type-I water resistivity of 18.2 mΩ – cm at 25°C
- Not effective in removing bacteria, organics, suspended particles and micro- organisms
- Ion exchange beds over a period of time accumulate microbial contaminants and particulates which can get released into the purified water stream
- Regeneration or replacement becomes necessary once all ion exchange sites get saturated
Adsorption by activated carbon packed columns helps remove organics, chlorine and chloramines and also helps reduce the total organic carbon load
- Significant reduction of the TOC level
- Removal of chlorine minimizes damage to reverse osmosis filter membranes if water is subsequently passed through the reverse osmosis system
- Not able to remove all dissolved organics
- Fines and other soluble components can get released into the purified water stream
UV light irradiation helps photochemical oxidation by breaking down larger organic molecules into smaller ionic species at shorter wavelengths around 185nm which can be subsequently removed by deionization. Irradiation at 254 nm destroys the bacteria
- Effective removal of bacteria through oxidation at 185 nm or destruction at 254 nm
- TOC levels can be lowered simultaneously
- Ions, colloids or particulates are not removed
- Limited reduction of TOC only
It can be seen that different water purification techniques have their own positive features. A combination of two or more techniques can provide the water of required purity specifications that meet your applications.
Several manufacturers provide commercially viable laboratory water purification units. Some of the more popular ones are: