Everyone in the biotech industry has inevitably heard the term RO/DI water, but aside from lab end users, not all of us may understand the importance or process involved in creating RO/DI water. From cleaning glassware, feeding water to equipment, or in reagent water, pure water is crucial in providing accurate/repeatable results and ensuring longevity of critical and costly equipment. So, let’s dive in to understand how water goes from city provided source to lab quality.

Although typically provided in a single package system, RO/DI is created in two separate steps. The reverse osmosis (or RO) process and the deionized water (or DI) processes. From City water, the first step is RO. City water is fed to a pre-filter that captures any large contaminants, generally filtering down to 5 microns. Enough to capture contaminants about the size of a blood cell but not enough to capture bacterial cells. From the pre-filter, water is then passed through either a single or series of tanks containing osmotic membranes. Like cells in the human body, water naturally wants to pass through cell walls to create balanced water concentrations across the cells membranes but by applying pressure to one side of the membrane, we can “unnaturally” force water across the membrane from low concentration to high concentration and remove contaminants. In this stage, filtration to .2 microns is typical. This results in removal of all bacteria and contaminants.

To achieve DI water, the RO product water is then fed into another series of tanks containing a mixture of ion-exchange resins. These resins, which look like small plastic beads are composed of polymer chains that are positively or negatively charged. The filter medias positive or negative charge attract positive and negative ions in the feed water and output neutralH2O. The filter media does have a limit on the quantity of negative and positive ions it can take on, so it does need to be periodically replaced. Ions in water are good electrical conductors and typical tap water has 1,000-5,000Ohms of resistivity while quality DI water has 15-18 megaohms of resistivity.

The output of the RO/DI water system has been removed of all large contaminants, organic matter, and is neutrally charged. Perfect for a lab.

by Eric Rodan

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