The presence of chlorine and chloramine in the water supplied to a reverse osmosis system has been a continuous problem and threat. Free chlorine damages the membrane and combined chlorine (chloramine) is toxic for applications such as hemodialysis and aquaculture.
Filtration with activated carbon has and continues to be the primary technology for dechlorinating RO feedwater. However it has several shortcomings such as space requirements, breakthrough, difficulty in monitoring, bacteria generation, cost, and poor operation in water with a high load of competing organics. Carbon is much less effective in removing chloramine versus free chlorine.
A recognized alternative is neutralizing the chlorine by chemical addition of a reducing agent. In practice, this has most often been done with some form of sodium sulfite.
An alternative chemical is ascorbic acid – Vitamin C. Compared to a sulfite, ascorbic acid is physiologically benign or even beneficial. Learning that the municipal water industry was using Vitamin C to dechlorinate water from mains flushing, etc. spurred me to investigate its use for reverse osmosis.
I developed a reliable capillary feed apparatus that could add the vitamin C to the water in a steady, continuous manner without any moving parts. Monitoring the reaction with an ORP probe and alarm proved to be reliable. The reaction with chloramine was slower than that with free chlorine, but a delay volume prior to electrode monitoring solved the kinetic problem.
The process worked so well, we imagined applications for both portable and stationary RO systems of small to moderate size adopting this method of dechlorination.
Unfortunately further testing exposed a fundamental problem with using ascorbic acid to dechloriminate RO feed water. The ascorbic acid does efficiently remove the chloramine and this can be well monitored by the ORP and confirmed by the DPD reagent total chlorine test. However, a mysterious thing happens when this reduced water is fed to a reverse osmosis system. The chloramine is regenerated in the RO product water! Only witnessing this happening removes the obvious doubt as to its possibility.
Water that has contained 2.5 ppm of chloramine with an ORP of 600 mV is treated with ascorbic acid and results in water with 0.0 ppm chloramine and an ORP of less than 200 mV. However, when this water is supplied to an RO membrane, the ORP of the product water rises to more than 500 mV and the DPD test shows more than 1.0 ppm total chlorine. How can this happen???
This membrane reconstitution does not occur with free chlorine or with chloramine when using sulfite. Can anyone explain why it happens with ascorbic acid and chloramine?
