Cu²⁺ and Ni²⁺ ions were successfully removed by the RO process and the rejection efficiency of the two ions increased up to 99.5% by using Na₂EDTA (Mohsen-Nia et al., 2007). Dialynas and Diamadopoulos (2009) applied a pilot-scale membrane bioreactor system in combination with RO and they found heavy metal removal efficiencies were very high. The major drawback of RO is the high power consumption due to the pumping pressures, and the restoration of the membranes.