Organic compounds and silica are the compounds weakly held by the ion exchange resins. After a period of use, the ion exchange capacity of a purification pack starts to be used up and these compounds are released first, even before the resistivity has fallen much below 18.2 Mohm-cm. This is shown in Figure 1 where the TOC and silica content of the water rises rapidly as the product water resistivity starts to fall. These problems are avoided in the Cascada Lab Water System as any organics and silica released from the first pack are retained on the second polishing pack which is still highly regenerated.

Figure 1
Release of Organics and Silica From Resin Pack on Exhaustion

The Cascada BIO- and AN-water Systems’ product water quality are not dependent on accurate resistivity and temperature measurements. Even the best meters are only accurate to ± 0.2 Mohm-cm and those used in the laboratory water purifiers are more typically ± 0.4 Mohm-cm. A conventional water purifier relies on immediately changing the purification packs as soon as the output resistivity drops below 18.2 Mohm-cm in order to maintain the highest water quality. Any error in the resistivity or temperature will seriously threaten the water quality from the unit. This could result in organic or silica release as shown in Figure 1 above, or the presence of other ions as in Table 1. In the Cascada BIO- and AN-water Systems, minor errors in the measurements will only affect the change-over point of the cartridges and have no effect on water purity of the product water.

Table 1
Impurity Levels in Purified Water of Different Resistivities

A further advantage lies in the security provided by the Cascada BIO- and AN-water Systems. If the “Intermediate Quality” alarm warning is ignored, the Cascada Lab Water Systems will continue to produce 18.2 Mohm-cm water for another 1,000 liters or more with a 35 µS/cm feed. This is due to the polishing pack that is virtually unused when the first pack needs to be changed.

Where regular logging of water quality is required, the security provided by the Cascada BIO- and AN-water Systems offer the advantage that, even with very heavy usage, daily recording is sufficient to guarantee that the product water will be maintained at ultrapure quality, 18.2 Mohm-cm, through the whole day. This would be the case even if the intermediate quality fell below 1 Mohm-cm early in the day.

In a conventional water purifier, if the resistivity drop is not detected or acted upon, serious contamination can result as shown in Table 1 and frequent logging and validation of resistivity monitors would be needed.