High water recovery of RO brine using multi-stage air gap membrane distillation

• A multi-stage AGMD process for further concentrating RO brine is investigated.
• The maximum value of JD and GOR can reach 6.8 kg/m2 h and 7.1.
• 24.7% of water recovery for the 4-stage AGMD process by experiment is obtained.
• 88.2% of water recovery for the 14-stage AGMD process is theoretically analyzed.

A multi-stage AGMD process for further concentrating RO brine and obtaining a high water recovery is investigated. One-stage AGMD, using artificial RO brine as feed and an improved AGMD module with energy recovery, is implemented in this study. The maximum value of JD and GOR could reach 6.8 kg/m2 h and 7.1 respectively. The relationship between water recovery (R), gained output ratio (GOR), permeate water flux (JD) and temperature difference at the top of the AGMD module (ΔTtop) was studied. To get a high water recovery, a 4-stage AGMD process was experimentally studied. JD decreased by 5.5% in the case of equal Fb,i. The multi-stage AGMD process was designed in a series of AGMD stages for large scale plants. A theoretical analysis of such stages has been carried out and the relationship between overall water recovery (Rall) and the number of stages was generated. 88.2% of water recovery for the 14-stage AGMD process was obtained. This could have important implications for the use of AGMD in treating high salinity solutions.