Reverse osmosis concentrate treatment by a PAC countercurrent four-stage adsorption/MF hybrid process

• A PAC countercurrent four-stage adsorption/MF hybrid process was developed.
• PAC consumption was further reduced compared with that in two-stage adsorption.
• Calculation method correlating PAC dose and organic removal was established.
• Membrane fouling was mitigated compared with that in two-stage adsorption.

In this study, in order to reduce the impact of reverse osmosis concentrate (ROC) on the receiving body of water and/or improve the recovery rate of reverse osmosis (RO) system by reusing the treated ROC as a feeding, a powdered activated carbon (PAC) countercurrent four-stage adsorption/MF hybrid process was developed for organic removal from ROC. The process could achieve good organic removal at lower PAC consumption. For dissolved organic carbon (DOC) removal efficiency of 70.0%, the PAC dose was 21.6% less than that in countercurrent two-stage adsorption and 50.9% less than that in single-stage adsorption. The calculation method for correlating removal efficiency and PAC dose was deduced and validated. The validation result showed that the relative error between the average experimental DOC removal efficiency and the calculated one was less than 5% throughout the experiment, exhibiting good accuracy for the calculation method. Compared with countercurrent two-stage adsorption, the membrane fouling in the new process could be mitigated to a certain extent. The total number of sub-cycles in a cycle (n) influenced the hydraulic retention time (HRT) and the total volume of the reactors. Results showed that the HRT increased as n increased.