[APD] RO/pH question

[Russell Edward Vance <revance at indiana_edu>]

I recently purchased a RO/DI filter and just hooked it up. It is a 6 stage
filter with 3 prefilters a 75gpd RO membrane, DI filter and a final carbon
post-filter. It has not run long enough to get all the sediment out of the
post carbon filter and DI unit, so the product water still has a TDS of
10ppm. This must be partly due to the sediment in those last two
filters because the water coming out of the RO membrane is lower
than 10ppm. Somehow the water still has a pH of 9! I don't understand how
almost pure water can still have such a high pH. The KH is 0 (or at least
my test indicates 0... I am sure it isnt exactly 0) and the general
hardness is also very close to 0. Does anybody know why this is and how I
can correct it? Thanks.

Russ

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