Pre-oxidation and coagulation-flocculation as a pretreatment to UF-RO applied for surface water treatment and arsenic removal

Abstract

In this paper, the effects of applying pre-oxidation, coagulation, and flocculation (C/F) as pretreatment of an ultrafiltration (UF) - reverse osmosis (RO) system treating water containing arsenic were assessed. For this, the performance of a pilot scale – with a maximum treatment capacity of 2.25 m3/h - UF-RO system with and without pretreatment was evaluated. The membranes were able to generate high-quality permeates, even when the feed presented a remarkable increase in color (>2100 uH) and turbidity (>300 NTU) due to the beginning of the rainy season. UF was not efficient for arsenic removal, so RO was essential to guarantee safe water, generating permeates with concentrations below 0.5 μg/L of arsenic. The effective removal of iron and manganese, among other ions, was verified using membranes. Given the high quality of the permeates, blend formulation strategies were carried out, mixing filtered water from the conventional water treatment plant and RO permeate (Strategy I) and mixing filtered water, UF permeate, and RO permeate (strategy II). Strategy II was viable for arsenic concentrations in filtered water of 13 and 15 μg/L. Pretreatment was important in maintaining a higher membrane permeability and ensured a more stable performance in the face of variations in feed quality, which may be seasonal in some places, as in the present case, where feed quality is significantly altered by the rainy season. Overall, although UF and RO membranes are effective processes for treating surface water containing arsenic, the results suggested that pretreatment, which is already part of the treatment of several water treatment plants, guarantees more robustness to the treatment concerning a higher and more stable membrane permeability, directly impacting operational expenses with membranes operation.