(Inline) Coagulation or Precoating for Seawater UF/RO Systems?

Ultrafiltration (UF) is gradually being established as the technology of choice for seawater reverse osmosis (SWRO) pretreatment due to the production of consistent high quality RO feed and small footprint, among others. This study focused on the effect of different coagulation conditions and modes of application on UF performance, both in terms of hydraulic operation and product water quality. Coagulation was performed in two different modes – inline coagulation and precoating. Filtration was performed in Amicon unstirred cells, through 300kDa PES membranes in dead-end constant pressure (?P = 1bar). UF hydraulic performance was evaluated using the Modified Fouling Index (MFI). UF permeate quality was characterized in terms of residual phosphate, a nutrient that can trigger biofouling in subsequent RO membranes at relatively small concentrations (~ 0.25?g/L). Results showed that while precoating improved hydraulic performance, permeate quality was better when inline coagulation was applied. precoating may be an effective alternative to direct filtration or inline coagulation (absence of flocculation) in terms of filtration resistance or permeability restoration. However, inline coagulation shows more promising results in terms of permeate quality. Low pH (~6) favours higher phosphate removal and less permeability loss. At 5mgFe3+/L and in the absence of flocculation, coagulant dose seemed to be dominating floc formation as pH did not have a considerable influence on floc filterability and permeability restoration.

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