If you are using ...

If you are using chlorine/hypochlorite prior to RO membranes, differential ORP is acceptable but I find marginal. It's ok for basic control. Most importantly, after you kill the bleach, measuring that it's all gone. ORP won't reliably give you a that reading. Several manufacturers make a chlorine meter that is much safer to use. I do not recommend bleach ahead of RO membranes. If you are, a GAC bed with sulfite feed is very important to kill the oxidant.  Chloramines or chlorine dioxide only for oxidizing biocides. Are your RO's on continuously, then there are several other treatments that are less dangerous to the membranes. slug fed non-oxidizers and IDE Technologies system are better. If they are on intermittently, a good low conductivity water flush and layup with a non-ox biocide is most effective.