Are you referring to adding ...

Are you referring to adding biocide before the membrane or to the permeate after the membrane? Before the membrane use chloramine (never free chlorine) at probably a couple of mg/l. Peracetic acid is also a possibility. Post RO you can use almost anything, since the  water will be very low in TOC and is should be already free of pathogens assuming the membrane and seals are in tact.