Abstract
Microgranular adsorptive filtration (μGAF) is a water treatment technique in which adsorption and granular media filtration operate simultaneously as water passes through a thin layer of adsorbent pre-deposited on a woven mesh or membrane. Our previous work showed that when heated aluminum oxide particles (HAOPs) are the adsorbent in such systems, substantial natural organic matter (NOM) removal and fouling reduction can be achieved. The present study investigated the effects of feed water chemistry and operational conditions on μGAF performance. Parameters investigated include the pH, ionic strength, and concentration of divalent cations in the feed, the flux through the adsorbent layer, and the pressure drop across both the adsorbent layer and the membrane. In the range from pH 3 to 9, lower pH promoted removal of NOM in general, and transparent exopolymer particles (TEP) in particular, by HAOPs. These enhanced removals led to increased fouling of the HAOPs layer and reduced fouling of a downstream membrane. Higher ionic strength exacerbated fouling of the bare membrane but had little effect on the ability of HAOPs to collect foulants from the feed. Increased concentrations of divalent cations reduced membrane fouling slightly, possibly due to the agglomeration of polysaccharides caused by divalent cations. Increasing the flux through the HAOPs layer had essentially the same effect as lowering the pH of the feed – it enhanced removal of NOM and TEP by, and increased fouling of, the HAOPs layer, thereby reducing fouling of the downstream membrane.The HAOPs layer was incompressible, even when some NOM had accumulated on or in it, suggesting that NOM molecules adsorb on the HAOPs throughout the layer and do not form a continuous gel layer on top of the HAOPs layer. Particulate and colloidal matter were inferred to be the dominant foulants in μGAF units, whereas soluble NOM (including soluble TEP) was the key foulant in the downstream membrane units.
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