Microfiltration/ultrafiltration as pretreatment for reclamation of rinsing waters of indigo dyeing

This paper focuses on the applicability of microfiltration (MF)/ultrafiltration (UF) as a pretreatment to nanofiltration (NF) for the reclamation of rinsing waters of indigo dyeing process. Two different MF alternatives, single 5 μm MF and sequential 5 μm+0.45 μm MF, were evaluated as prefiltration to improve the performance of the proceeding UF stage. UF tests performed after 5 μm MF using 100 kDa, 50 kDa, 20 kDa, 2 kDa and 1 kDa membranes have indicated that all the UF membranes provided the same cumulative color retention of 98–99%. However, in terms of COD retention, UF membranes presented different performances and the highest retentions were by both 50 kDa (56%) and 100 kDa (59%) membranes. As regards permeation rate, 100 kDa and 50 kDa UF membranes appeared to have the highest flux decline due to fouling. When sequential MF (5 μm+0.45 μm) was adopted to decrease this fouling, 0.45 μm MF before UF was found not to provide any significant improvement. Thus, single stage 5 μm MF followed by 100 kDa UF was selected as the best prefiltration train to NF that permits the textile producer to reclaim indigo dyeing wastewater.