Hybrid UF/NF process treating secondary effluent of wastewater treatment plants for potable water reuse: Adsorption vs. coagulation for removal improvements and membrane fouling alleviation

Coagulation and adsorption are gradually adopted as pre-treatments to produce reclaimed potable water. However, previous researches on membrane fouling mechanisms were currently insufficient to minimize dual membrane fouling. This study aimed at investigating the effects of pre-coagulation and pre-adsorption on the removal performance and membrane fouling alleviation of dual membrane UF/NF process in treating secondary effluent from a wastewater treatment plant. The results indicated that both types of pretreatments conferred positive effects on organic membrane fouling removal of the UF process whereas diverse effects on NF process. Pre-coagulation could enhance the removal of nitrogen and phosphorus to contribute towards producing microbiologically-stable water. On the other hand, introduction of Al³⁺ reduced the removal efficiency of UF/NF systems on heavy metals. From the perspective of UF membrane fouling, two pretreatments employed could increase the flux of UF, but simultaneously aggravating irreversible membrane fouling. Hermia and Tansel models revealed an unstable cake filtration was caused by pre-coagulation and pre-adsorption. Both the models consistently demonstrated the rapid formation of cake filtration onto UF membrane surface. Interestingly, the powdered activated carbon (PAC) adsorption could significantly reduce cake layer fouling onto the surface of NF membrane, while pre-coagulation aggravated the NF fouling. These results are essential to developing robust, cost-effective and energy-efficient strategies based on membranes to produce reclaimed potable water.