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RO+DI vs DI

RO+DI Advantages

Removes up to 98-99% TDS, including Silica which is difficult to remove by the DI process.
Removes colloidal sized particle such as Silica and other submicron sized particles which can easily pass through a DI filter.
When used as a pretreatment to DI filters, an RO system can extend the useful life of the DI resin by up to 50 times. Its use makes it practical and economical to use disposable DI filter cartridges rather than large DI exchanges tanks which must be regenerated using dangerous chemicals. Eliminates the possibility of receiving contaminated DI tanks containing left over strong chemicals.
Eliminates dependency upon outside vendors to regenerate exhausted DI exchange tanks on a timely basis. If the DI filters in a combination RO+DI systems do become exhausted, the water entering the weathering equipment is still good quality RO water. If only DI filters are used and become exhausted, raw city/tap water will enter the equipment which can cause serious damage.
The HWS RO/DI systems are designed to supply RO water only, for equipment such as environmental and corrosion testing equipment that do not require high purity water, and RO + DI to equipment such as Xenon Arc weathering equipment that requires high purity water.

 

RO+DI Disadvantages

Higher initial investment and more mechanically complex than the use of DI filters only.
Depending upon the size of the RO system and water storage tank, its daily production capacity and short term peak flow capacity can be limited. The production of RO water is reduced with the temperature of the feed water. All of these factors must be considered when selecting an RO system.
An RO system does discharge some water to the building drain. If the “recovery rate” is 50% for example, the system will recovery 50% of the feed water as RO product water and 50% will be rejected to the drain. The rejected water does not contain any additives of substances that were not already in the city/tap water going into the RO system.

 

DI only Advantages

Lower initial investment, simple mechanical operation.
Can usually handle a sudden increase or high short term consumption rate, however the DI tanks may exhaust more rapidly.
Not as effective in removing Silica. Both dissolved silica and colloidal sized silica can pass through the DI filters. The presence of silica in the final DI water does not always register on a typical Resistivity-Conductivity Meter misleading the user as to the actual water purity.

 

DI only Disadvantages

Not as effective as RO on removal of Silica. Does not remove colloidal sized particles and other contaminants such as bacteria.
Regenerated resin tanks can be contaminated by past users. Regeneration process involves use of strong acids and strong bases, which if not properly rinsed can result in a low or high pH when first placed into service. Water with a low or high pH can damage a water cooled Xenon lamp and other components. It can also damage the test specimens.
Many Labs must shut down their weathering tests due to exhausted DI tanks if the vendor does not respond on a timely basis. If the user fails to notice that the tanks have become exhausted, raw city/tap water will enter the weathering equipment which can cause serious damage to the equipment and test results.
The cost of regeneration can be very high for applications using specimen sprays, especially if the feed water has a high level of TDS.

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