Calcium carbonate scaling in forward osmosis for textile reverse osmosis concentrate treatment

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 (NH4HCO3) 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 NH4HCO3 as DS. Compared to the DS of NaCl, the CaCO3 precipitation occurred much earlier when NH4HCO3 was used as DS. Due to the reverse diffusion of HCO3, the concentration of Ca2+ 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.