Why MFI is edging SDI as a fouling index

Jan C Schippers, Sergio G Salinas-Rodriguez and Maria D Kennedy (TU Delft), UNESCO-IHE, and Siobhan Boerlage, Boerlage Consulting

Figure 1: Fabric in hollow fine fibre permeator of Du Pont Permasep

Figure 1: Fabric in hollow fine fibre permeator of Du Pont Permasep

PARTICULATE FOULING has plagued reverse osmosis (RO) systems since their first use in desalination and remains a persistent issue today for RO and other pressure-driven systems such as microfiltration (MF), ultrafiltration (UF) and nanofiltration (NF). In the early 1960s, the Du Pont company/Permasep Product successfully launched the hollow-fine-fibre (HFF) permeator onto the desalination market, where it dominated for several decades.
A well-known weakness of this permeator was its vulnerability to fouling. Initially, this vulnerability was attributed to suspended and colloidal matter in the feed water, ie, particulate fouling. Therefore, Du Pont developed, the Silt Density Index (SDI), initially named the Fouling Index, as a parameter to characterize the fouling potential of the feedwater for permeators.
The fouling mechanism turned out to be more complicated than just fouling of the membrane surface as initially assumed. Gradually it became clear that the fouling was initiated by local clogging of the woven or non-woven fabric between the fibres, which is needed to ensure equal flow distribution of the feed water (see Figure 1).

This article first appeared in the issue of Desalination & Water Reuse magazine.

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