Residual free chlorine can be reduced by activated carbon or chemical reducing agents such as sodium metabisulfite (SMBS) commonly used for removal of free chlorine and as a biostatic. Residual free chlorine can be detected with oxidant-reduction potential electrodes (ORP). Other oxidizing agents such as chlorine dioxide, hydrogen peroxide, ozone, and permanganate are capable of damaging RO membranes also if not used properly.