RO water pH, what is typical? OK for sparging?

[Silver_Is_Money]

My homes RO is measuring pH 6.04. Does that sound about right for RO water? Would this water be good for sparging, without any mineral additions?

My well water has alkalinity at a whopping 436 ppm (as CaCo3). The well water goes through a softener, and then the RO unit. Since the rejection ratio is roughly 9 or 10 to 1 for our RO unit, can I ballpark assume that our RO water likely has alkalinity at about one tenth of the well, or about 43.6 ppm (as CaC03)?

 

[ajdelange]

A new RO unit ought to have a rejection of 98 - 99%. Thus we would hope that your alkalinity would be less than 10 ppm as CaCO3. If the RO water pH is really 6.04 that is indicative of low alkalinity but to be certain you should really use an alkalinity test kit. Short of doing that you could let a tumblerfull of the water stand out overnight (to bring it to equilibrium with atmospheric CO2) and recheck the pH. If it is still in the 6's then you can be assured that the alkalinity is low.

In any case yes, this water would be fine for sparging w/o adding anything to it.

 

[mabrungard]

Any RO unit owner or RO water user should have a TDS meter for checking what the total dissolved solids (TDS) of their RO water really is. While those are not precision instruments, they will provide you with a relative assessment of the mineral content of your water. Observe the TDS readings during the first few weeks of operation and remember what the TDS typically is. When the TDS rises significantly, it could be a signal that the membrane is failing and needs replacement.

A properly operating RO machine will produce water that is probably less than 50 ppm TDS. Since the TDS of the treated water is proportional to the TDS of the raw water, you can't assume that all RO water is virtually mineral-free or has very low TDS. Therefore, a TDS reading between 0 and 50 ppm can be expected.

pH of RO water can be driven by more than alkalinity. Dissolved gases will preferentially pass through the membrane into the product water. If the water source has a lot of dissolved CO2, the RO water pH is likely to be driven low due to the carbonic acid content (the water is essentially carbonated).

 

[ajdelange]

Dissolved gases will preferentially pass through the membrane into the product water. If the water source has a lot of dissolved CO2, the RO water pH is likely to be driven low due to the carbonic acid content (the water is essentially carbonated).

True, and that's why I advised him to let it equilibrate with atmospheric CO2 IOW give any CO2 that came through the membrane a chance to dissipate. If the water is at pH appreciably lower than 7 when in equilibrium with air its alkalinity is low.