The most effective pre-treatment to nanofiltration for the recovery of print dyeing wastewaters

This paper presents the determination of the best pre-treatment method to nanofiltration (NF) for the recovery of print dyeing wastewaters (PDW) of the carpet manufacturing industry. Chemical precipitation (CP) with optimum alum (Al2SO4.18H2O) dose was chosen among five alternatives; 1) Dead-end microfiltration (MF), 2) CP with optimum coagulant dose, 3) CP with low coagulant dose and polyelectrolyte aid, 4) CP with low coagulant dose followed by MF, 5) CP with high coagulant dose followed by ultrafiltration (UF). In the selected process, an optimum alum dose of 150 mg/L provided color and turbidity removal efficiencies of >90%. The effectiveness of the selected pre-treatment method for flux control in post NF was also tested. Perfect rejection of COD, color and turbidity was achieved in NF, accompanied by a stable flux decline of around 22%. Moreover, the flux decline was mainly due to concentration polarization rather than fouling, indicating reversible flux decline.