Reclamation of acid dye bath wastewater: Effect of pH on nanofiltration performance

Abstract

This paper describes the effect of pH on the selection of the best treatment scheme for the reclamation of acid dye bath wastewaters (ADBW) of carpet manufacturing industry. ADBW, having acidic pH (5.0–5.9) originally, was subjected to pre-filtration through 1.0 μm microfiltration (MF) media prior to nanofiltration (NF). Three treatment alternatives were adopted: (1) single NF without pH neutralization; (2) sequential NF without pH neutralization; (3) single NF with pH neutralization. Color was removed completely and turbidity was rejected by 79–92% in all alternatives. However, COD, total solids, total hardness and conductivity were partially removed in single NF without pH neutralization. The inadequate rejection of pollutants lead to the implementation of sequential NF consisting of three stages, where COD rejection increased from 55%–77% to 94% at the end of the third stage. Finally, the wastewater pH was neutralized by adjusting to 7.0, which provided COD rejection efficiency as high as 97% in single NF. The flux decline in single NF at acidic pH was 14–23%. In sequential NF, the flux decline decreased from 21% to 9% in three stages. The flux declines increased up to 25% due to pH neutralization, however chemical cleaning was found to be effective in restoring the fluxes. Besides, the main cause of flux decline, which was fouling at acidic pH became concentration polarization at neutral pH. Therefore, the best treatment scheme for the recovery of ADBW came out to be single NF with pH neutralization preceded by 1.0 μm MF. These results revealed that pH is an important operational parameter affecting the selection of the best treatment scheme for ADBW.