1.Small volume  
2.Low energy consumption  
3. Operation cost   less  
4. A high degree of automation  
5. No acid-base emissions 

Two stage RO,applies for water with conductance le  than 500μs/cm,water conductance of 1-10μs/cm.    

Raw water 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 perssure pump to be water product is called doubile RO.