Raw water (tap water, surface water and groundwater) under pressure delivered to pre-treating system for coarse filtering,then be filtered in precise filter, then delivered to first grade RO system by first grade high pressure pump, water from that then delivered to second RO by second grade high pressure pump to be water product is called double RO.

Double RO, applies for water with conductance more than 500us/cm, water conductance of 1-10us/cm.