Pretreatment for seawater desalination by flocculation: Performance of modified poly ferric silicate (PFSi-δ) and ferric chloride as flocculants

Submerged microfiltration system coupled with in-line flocculation as pretreatment for seawater reverse osmosis has the potential to remove organic compounds and mitigate fouling. The effect of flocculation on the performance of microfiltration (MF) was investigated using two flocculants, namely, modified poly ferric silicate (PFSi-δ) and ferric chloride (FeCl3). The experimental results were analysed in terms of organic removal efficiency and critical flux. Both PFSi-δ and FeCl3 were found to be suitable as pretreatment because of their capacity in removing organic compounds. The results show that PFSi-δ was better than FeCl3 in terms of removing turbidity and dissolved organic carbon (DOC), particularly in removing hydrophilic compounds. The highest DOC removals of 68% and 57% were obtained when PFSi-δ at a dose of 1.2 mg Fe3+/L and FeCl3 at 3 mg Fe3+/L were used, respectively. It was observed that PFSi-δ (1.2 mg Fe3+/L) and FeCl3 (3 mg Fe3+/L) led to an increase of critical flux from 20 L/m2 h to 35 L/m2 h and 55 L/m2 h, respectively.

Research Highlights
► The aim is to assess a new flocculent of modified poly ferric silicate (PFSi-δ) performance for seawater desalination.
► The PFSi-δ performance was compared with FeCl3 performance.
► PFSi-δ was better than FeCl3 in turbidity, dissolved organic carbon (DOC), particularly for hydrophilic organic carbon removal.
► DOC removal by PFSi-δ at 1.2 mg Fe3+/L was 68%, and higher than 57% by FeCl3 at a higher dose of 3 mg Fe3+/L.
► Critical flux increased from 20 to 35 L/m2 h when PFSi-δ was used in-line flocculation.