NF process for the recovery of caustic soda and concentration of disodium terephthalate from alkaline wastewater from polyester fabrics

The recovery of caustic soda and concentration of disodium terephthalate (DST) from alkali waste of polyester fabrics were investigated using SelRo (MPT-34) nanofiltration membranes with membrane area of 0.024 m2 and 5.2 m2, respectively. Flux behaviors were observed with respect to filtration time, volumetric concentration factor (VCF), operating pressure, temperature, and cleaning. The mass balance of caustic soda, DST, and ethylene glycol (EG) were estimated according to different values of VCFs. As the VCF increased, more amounts of disodium terephthalate and ethylene glycol permeated while almost the same concentration of sodium hydroxide was permeable. The NF operation end with a concentrated disodium terephthalate solution that needed a reduced amount of acid for precipitation and a permeate of caustic soda solution containing EG and DST. Concentrations of EG and DST that govern reusability of permeate in the caustic treatment of polyester fabrics were measured to be dependent of VCF. The most suitable VCF in terms of maintaining relatively good membrane productivity and high rejection of DST was approximately 4.5. At a VCF of 4.5, the rejections of DST and EG were 86% and 42%, while NaOH recovery was 84% with fluxes between 12 to 36 LMH.