Introduction

Microfiltration is particularly effective for the removal of particulate contaminants, such as bacteria, algae and protozoa from drinking water. However microfiltration is ineffective for the removal of dissolved contaminants such as natural organic matter (NOM). Residual NOM in drinking water is a cause of colour, disinfection by-products, and microbial regrowth1,2. Residual NOM in microfiltration processes is a cause of membrane fouling3. The conventional approach to removing NOM from drinking water is chemical coagulation and flocculation. This approach is now routinely used in Australia as a microfiltration pre-treatment to enhance colour removal and to reduce the rate of membrane fouling. However, there is considerable scope for further improvement by combining chemical coagulation with adsorption of NOM onto a solid particle. The adsorbent could be selected on the basis of low hydraulic resistance to filtration, and could be used under conditions in which it acts as a site for flocculation. The hybrid adsorbent-coagulant particle formed in this way would then be removed by microfiltration in the conventional way, depositing a more permeable filter cake on the membrane and producing a higher quality drinking water than coagulation alone.

Magnetite is a commonly mined mineral in Australia with potential as an adsorbent in a hybrid microfiltration pre-treatment. Magnetite is a magn├ętisable iron oxide used alone or in conjunction with alum or polyelectrolyte in an existing treatment process to remove particulate and some dissolved contaminants from drinking water4. It has a low surface area (2-3m2/g) and therefore requires low contact times (< 60s) but relatively high doses (> lg/L). In addition, particles are heavy (s.g. = 5.1), and can be magnetically flocculated to settle rapidly (> 30m/hr). The adsorbent is easily regenerated by a caustic wash. These properties would facilitate in-line adsorbent dosing in an existing microfiltration plant. Removal of the flocculated adsorbent is simple and rapid, reduces any additional particle loading to the membrane, and aids recovery for reuse. The potential of magnetite as an adsorbent particle for use in combination with alum and polyelectrolyte as a microfiltration pre-treatment for Australian surface water is investigated in this paper.

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