Optimization of ultrafiltration as pre-treatment for seawater RO desalination

Abstract

Ultrafiltration (UF) is increasingly used as pretreatment for reverse osmosis (RO) desalination, as an alternative to granular filtration. To ensure a sustainable and cost-effective operation of UF, key operational parameters must be optimized. In the present study we used a pilot UF system to optimize filtration flux, length of filtration cycle, coagulant dosing, and membrane cleaning procedure. Filtration flux of 82 LMH resulted in reversible fouling, which could be completely removed during chemically enhanced backwash, and did not affect filtration performance. Increasing the flux to 85 LMH on the other hand, resulted in the formation of irreversible fouling, which rapidly deteriorated UF operation. Filtration time of 35 min between backwashes did not affect filtration, whereas, at longer filtration time of 45 min, (hydraulically) irreversible fouling was formed. Addition of Ferric chloride coagulation prior to UF (up to 0.6 mg/L-Fe) improved the removal of reversible fouling during backwash (BW), through the formation of pinflocs. In parallel, high ferric chloride concentrations generated ferric-based irreversible fouling, which could only be removed by chemical cleaning. Finally, the study proposes a unique analysis of the trans membrane pressure (TMP) curve shape, for the optimization of coagulation and hydraulic cleaning during UF.