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Aggressiveness: When Water Attacks

I had a water quality case some years ago in the southern Midwest. The issue was that the sinks, instruments, sterilizer chambers, and pretty much everything else was turning a deep green color. The water supply for the boiler, sinks, and washers was the same supply, generated in an anteroom for use in these devices.

Water was “generated”? Yes. The water was taken from the municipal supply to the building and softened, then run through copper plumbing to its points of use.

So, aside from algae, what is green or could become green in this system? Copper.

"But we use copper plumbing because it is easy to work with, bactericidal, reasonably inexpensive, and lasts a long time." Yes, but…

There is a “figure of merit” from the HVAC industry called “aggressiveness index”. The aggressiveness index measures how aggressive water is toward copper plumbing. Copper plumbing is used for commercial cooling applications by circulating cooled water through it and blowing air over the copper plumbing. It is also used for baseboard heating applications. So, what is the aggressiveness index?

Aggressiveness index = pH + log10(total hardness in ppm) + log10(alkalinity in ppm)

If the aggressiveness index is > 12, the copper will likely last forever. In this case, the parameters in the aggressiveness index were:

·pH: 7.7

·hardness: 0.33

·alkalinity: 80

Plugging these value into the calculation, the result was:

Aggressiveness index = pH + log10(0.33) + log10(80)

= 7.7 + (-0.48) + 1.9

= 9.1

This is fairly aggressive water. The cure was plastic piping. The only issue there is to make sure it meets code.

Enough chemistry for this month. See you next month!


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