In order to produce pure water suitable for use in scientific applications, water (usually mains supplied potable water) must pass through a series of technologies to remove its impurities. Various laboratory applications require the removal of different impurities and therefore, a range of technologies are utilised.
Depth filters are commonly used as a pre-treatment. Raw water passes through a series of winding fibres which attract and trap impurities. This offers protection to the purification technology that follows as less impurities pass through. Carbon is also used to bind chlorine ions which, if not removed, will cause rapid deterioration of RO membranes. Membrane sub micron filters are traditionally used as a final step to remove bacteria and other particles which have not been dealt with by the preceding technologies.
Reverse Osmosis (RO)
This is the most economical method of removing up to 99% of feed water contaminants. During natural osmosis, water flows from a less concentrated solution through a semipermeable membrane to a more concentrated solution until concentration and pressure on both sides of the membrane are equal. In water purification, external pressure is applied to the more concentrated side of the membrane to reverse the natural osmotic flow. This forces the feed water through the semipermeable membrane. The impurities are deposited on the membrane surface and flushed to drain. The pure water passing through the membrane is referred to as the permeate. Most RO systems will need a tank to store the purified water as the production rate is usually less than the peak demand.
Deionisation (DI)/Ion exchange
This process removes ions from water, usually RO water, with the use of synthetic resins. The ions are removed from the water through a series of chemical reactions. These reactions occur as the water passes through the ion exchange resin beads. Gradually, all unwanted ions are replaced by hydrogen and hydroxyl ions which combine to form pure water. Deionisation is the only process which can produce the quality required for Type 1 water.
Electrodeionisation is an active purification technology which combines electrodialysis and ion exchange. Within the EDI cell, water is passed between an anion permeable membrane and a cation permeable membrane. The cell chamber contains loosely packed ion exchange resin. Ions are attracted to the oppositely charged electrode and are flushed away before they reach it, removing them from the final water. Multiple EDI cells can be stacked within a unit. The resin in the EDI cell is continuously regenerated thus removing the requirement for replacement cartridges and making it a long term cost effective technology for large water volumes. By passing an electrical current through water, the H2O molecule is split into H+ and OH-. These ions continuously regenerate the resin.
Ultraviolet (UV) photo oxidation at 254nm and185nm
Photochemical oxidation and ultraviolet light eliminate trace organics and inactivate microorganisms in feed water. The 254nm light reacts with bacterial DNA resulting in denaturation. The 185nm light breaks up long chain organics which can then be removed from the water by ion exchange.
Ultrafiltration is used to remove pyrogens (bacterial endotoxins) and nucleases. This process is critical when producing water for use in tissue or cell culture and media preparation. Ultrafilters
use size exclusion to remove particles and macromolecules. The filter may also be charged to help attract contaminants. Particles are captured on the surface of the membrane and flushed to drain via a reject stream. Ultrafilters are usually employed at the end of the system to ensure near total removal of macromolecular impurities like pyrogens, nucleases and particulates.