Effect of feed pH on an integrated membrane process for the reclamation of a combined rinse water from electroless nickel plating

This study explored the effect of feed pH on an integrated membrane process of ultrafiltration/reverse osmosis (UF/RO) for the reclamation of spent rinse water from an electroless nickel-plating operation. Feed pH in the range of 2.54–6.64 was investigated. For UF pre-treatment, the experimental results showed that iron was not rejected by UF membrane when feed pH was below 3.68. However, when feed pH was raised to 3.68, iron rejection increased significantly and reached 98.7% at pH 6.64. For the RO process, it was found that for a particular wastewater containing weak acids and weak bases in this study, permeate pH was higher than the feed pH when the feed pH was low, but lower than the feed pH when the feed pH had reached 6.64. The critical point at which permeate pH would be equal to feed pH was about 6. This observation has not been reported previously. An attempt at explaining the observation was offered. The results showed that concentration of all measured contaminants in RO permeate decreased with an increase in feed pH. As a consequence, the permeate conductivity decreased with increasing feed pH. The membrane flux decreased with increasing feed pH. Total organic carbon (TOC) rejection increased with increasing feed pH due to the enhanced electrostatic repulsion between the negatively charged membrane surface and the negatively charged HOCH2CO2−. Moreover, the caustic soda consumption used for neutralization could be reduced by 39% in the integrated membrane process if the feed pH was set at 3.68 instead of 6.64. It was concluded that pH of the feed had a significant effect on the integrated membrane UF/RO process and the optimum pH of feed water for the treatment was in the range of 3.7–5.6.