High Purity Water Technical Overview
What is High Purity Water?
The fist order of business is to determine what is meant by high purity for the proposed application. We must consider suspended solids, dissolved solids, dissolved gasses, organics, and biology. Zero is often stated but theoretically unobtainable because of minimum detection levels by instrumentation and background laboratory interference. Taking the contamination families one by one let’s examine:
1.SUSPENDED SOLIDS.
Materials in the water may consist of inert items such as clay or bits of micro-biology. Backwashing filters can take this to a 20 micron level. Follow with a series of cartridge filters to reduce in steps to a practical level of 0.2 micron absolute. Further reduction to 0.02 micron by ultra filtration. Polish with nano filtration to 0.002 micron and finally to 0.0002 micron with reverse osmosis. Knowing the terminal process in advance may eliminate many of the prior steps.
2. DISSOLVED SOLIDS
The level of removal will be dictated by the application. Measurement is often in parts per million, parts per billion or parts per trillion. As an alternate measurement can be in resistance as ohms/cm with 18.2 million ohms/cm as a theoretical maximum and as another alternate in micro Siemens/cm which is 1 million divided by the resistance in millions. Single pass commercial reverse osmosis will typically reduce the level to 10 PPM and 2 pass commercial reverse osmosis systems to 1/2 PPM with carbon dioxide being a major contributor.
Either stand alone or as post treatment to a commercial reverse osmosis unit, a two column deionizer (regenerate in situ or rental tanks) can be used. Typical results are in the range of 1.5 million ohm water. If a mixed bed deionizer replaces the 2 column system, then the resistance range is from 10 million to 18 million ohms. Final results are based on regeneration chemical rates, chemical quality , flow rates and types of resin employed.
To get to further refinement polishing with special ion selective resins is employed.
3. DISSOLVED GASSES
A de-gas tower can be employed or a membrane system with a vacuum applied is very effective. Care must be taken after gas removal to prevent re-balance of gasses in the air due to the gas laws.
4. ORGANICS
They should be identified by testing before removal if they are critical. A gross process using activated carbon will help with final polishing if required by special organic trap resins. Destruction by ozone and possibly membranes with a design Dalton cut-off MAY BE NEEDED
5. BIOLOGY
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