Are your water softeners messing with RO permeate quality of your two pass system?

Published on by in Technology

We were experiencing an issue in the last couple of years where the output quality of our two-pass (also staged feed each pass) was deteriorating badly on a temporary basis, only during the first steps of softener regeneration on the exhausted vessel.  I suspected something odd about the softener vessel contents that had been offline on standby.  The change in RO product was so profound that the 2nd-pass feed pH would at times reach almost 10.0 pH, and the permeate quality would go south, so the RO (once above 10 micro-Siemens/cm) would be forced to route to drain, instead of to the RO storage tank.  

Checking the soft water output from the "new" softener bed revealed a pH range of anywhere from 8.3 to 8.6 when "normal" softener output pH was around 7.8 pH.  It seemed evident to me that ammonia arising from decomposition of chloramine could be the culprit, but no ammonia above 1.5 ppm was found.  Odd that.

At any rate, the solution to this issue was introducing one more chemical pump feeding dilute (2.5-3%) hydrochloric acid just upstream of the cartridge filter housing of the pretreatment skid.  This appropriately shifts the pH back to "normal" or perhaps sub-normal.  A slight positive shift on the ORP that is used for control of sulfite injection for total chlorine neutralization is also noted.

The output of the RO not only goes back to a good low "normal" conductivity, under some conditions even lower.  No more wasting water unnecessarily.