Forward osmosis coupled with lime-soda ash softening for volume minimization of reverse osmosis concentrate and CaCO_(3) recovery: A case study on the coal chemical industry

Reverse osmosis(RO)is frequently used for water reclamation from treated wastewater or desalination plants.The RO concentrate(ROC)produced from the coal chemical industry(CCI)generally contains refractory organic pollutants and extremely high-concentration inorganic salts with a dissolved solids content of more than 20 g/L contributed by inorganic ions,such as Na^(+),Ca^(2+),Mg^(2+),Cl^(−),and SO_(4)^(2−).To address this issue,in this study,we focused on coupling forward osmosis(FO)with chemical softening(FO-CS)for the volume minimization of CCI ROC and the recovery of valuable resources in the form of CaCO_(3).In the case of the real raw CCI ROC,softening treatment by lime-soda ash was shown to effectively remove Ca^(2+)/Ba^(2+)(>98.5%)and Mg^(2+)/Sr^(2+)/Si(>80%),as well as significantly mitigate membrane scaling during FO.The softened ROC and raw ROC corresponded to a maximum water recovery of 86%and 54%,respectively.During cyclic FO tests(4×10 h),a 27%decline in the water flux was observed for raw ROC,whereas only 4%was observed for softened ROC.The cleaning efficiency using EDTA was also found to be considerably higher for softened ROC(88.5%)than that for raw ROC(49.0%).In addition,CaCO3(92.2%purity)was recovered from the softening sludge with an average yield of 5.6 kg/m^(3) treated ROC.This study provides a proof-of-concept demonstration of the FO-CS coupling process for ROC volume minimization and valuable resources recovery,which makes the treatment of CCI ROC more efficient and more economical.