Calcium carbonate scaling in forward osmosis for textile reverse osmosis concentrate treatment. (June 2020) - Wang, Shu-Yue
Graphical abstract: Schematic of possible membrane scaling mechanism for the FO dewatering of textile reverse osmosis concentrate with NaCl and NH4 HCO3 draw solutions. Highlights: The real textile ROC could be concentrated by 10 times using FO process. CaCO3 scaling is the dominant fouling while the organic fouling is not significant. The CaCO3 scaling could not cause the decline of water flux. The CaCO3 precipitation occurred much earlier while Ca 2+ in the ROC concentrate was significantly lower for NH3 -CO2 as DS. Abstract: The reverse osmosis concentrate (ROC) of textile wastewater produced in the process of the reverse osmosis usually has a high concentration of salts and organic matters, and is hard to be biodegraded. In this study, forward osmosis process was used for reusing ROC of real textile wastewater, and the effect of sodium chloride (NaCl) and ammonium bicarbonate (NH4 HCO3 ) on the water flux and the recovery of ROC were then investigated. The results showed that the water recovery could reach 90 % when 2 M NaCl was used as DS. The calcium carbonate (CaCO3 ) scaling could be both observed using NaCl and NH4 HCO3 as DS. Compared to the DS of NaCl, the CaCO3 precipitation occurred much earlier when NH4 HCO3 was used as DS. Due to the reverse diffusion of HCO3 −, the concentration of Ca 2+ in the concentrated ROC was significantly lower than 3 mg/L. Although the FO membrane was covered by CaCO3 scaling, the water flux was slightly affected by it. In addition, the water flux showed no difference through four cycles of FO tests during which the FO membrane was not cleaned after each cycle of test. This finding provided the possibility of applying FO process for the near-zero liquid discharge of textile wastewater.