Effects of shear on microfiltration and ultrafiltration fouling by marine bloom-forming algae

Pretreatment of seawater for desalination can be accomplished with microfiltration (MF) or ultrafiltration (UF) membranes. One problem with MF and UF pretreatment is fouling by marine algae, which is most prevalent during algal blooms. Algal cells quickly block MF and UF pores and decrease permeability. This paper investigates an important consideration in algal fouling: shear. A strain of bloom-forming dinoflagellate algae, Heterocapsa pygmaea, was grown in the laboratory for experiments. Algae were sheared by pumping through a highly restrictive valve. Sheared and non-sheared algal samples were filtered with four MF and UF membranes to determine the effects of shear on flux. Feed and permeate samples were analyzed to determine how shear affected organic-matter rejection. Sheared samples caused more drastic flux decline than non-sheared samples and rejection of algogenic organic matter (AOM) was diminished after shear. To determine the size of the foulants most responsible for flux decline, sheared and non-sheared samples were size fractionated before filtration on 0.1-μm PVDF membranes. The highly fouling fraction was cell-derived material larger than 0.22. μm. The algal cells themselves played only a small role in flux decline. High-molecular-weight organic material was released during shear, but flux decline did not correlate with its release; thus, adsorption of dissolved algogenic organic matter was not a significant fouling mechanism in these short-term experiments.