Desalination

Reverse osmosis concentrate (ROC) produced in water reclamation and desalination plants can endanger the environment if it is not treated before discharge. Volume minimisation of ROC can help in its easy disposal. The study examined the use of forward osmosis (FO) with and without granular activated carbon (GAC) fixedbed adsorption pretreatment for volume minimisation of ROC and removal of organic micropollutants. Five repeated FO steps using 2 or 3 M NaCl as the draw solution reduced the volume of ROC to 8%. With each successive step the flux decreased due to membrane fouling and scaling caused by increased concentrations of organics and inorganics resulting from volume reduction of ROC. However, flux decline was arrested in the second or third step by reducing the pH of the feed solution from 7.0 to 5.0. FO treatment rejected 9 of the 18 organic micropollutants at N82% and GAC treatment removed 15 of them at N 82%. GAC pre-treatment followed by FO treatment removed almost all the organic micropollutants from the ROC. GAC pretreatment also reduced total organic carbon concentration in ROC by adsorption, thus controlling membrane fouling. © 2015 Elsevier B.V. All rights reserved.

The study showed that FO is a promising technology for reducing the volume of ROC leading to zero liquid discharge. Five repeated FO steps using 2 or 3 M NaCl as the DS reduced the volume of 6 or 8 L of ROC to 8%. With each successive step the flux decreased due to increased concentrations of organics and inorganics caused by the volume reduction of ROC which led to membrane fouling and scaling. Humic acids, building blocks, and acids + low molecular weight organics and carbonates of calcium and magnesium were found to have accumulated in the membrane. However, the flux decline was controlled by reducing the ROC pH from 7.0 to 5.0. Forward osmosis treatment of ROC as FS was successful in rejecting most of the organic micropollutants from ROC due to the membrane operation restricting them going to the DS. However, GAC fixed-bed adsorption pretreatment removed most of the micropollutants from the ROC. GAC pretreatment followed by FO treatment restricted almost all the organic micropollutants from the ROC going to the DS. The removal of individual organic micropollutants varied widely and depended on many factors, such as their molecular weight, charge, hydrophobicity, and H-bonding. Used DS can be reused in subsequent FO treatment processes. Alternatively, after salts have been partially recovered from the used DS by RO treatment, the resultant RO permeate can be blended with the main RO permeate.