Abstract
Pre-coagulation could mitigate the membrane fouling, and thus we used different Al coagulants as a pre-treatment for ultrafiltration to explore their effects on the morphology of the membrane cake layer and fouling. Parallel bench-scale tests, using three different species of Al (AlCl3, PACl15, and PACl25), with and without humic acid, were operated continuously for a long period to investigate the effects of floc aging (~13 days). Specifically, the presence of humic acid affects the cake layer by influencing the rate and extent of floc crystallization, as greater crystallization leads to greater fouling (bio-fouling was excluded in this study). The fouling rate (indicated by the trans-membrane pressure at constant flux) varied with Al species and was found to increase as follows: PACl15 < PACl25 < AlCl3. The presence of humic acid also intensify membrane flouling. The results showed that three species of Al induced different sizes of primary nanoparticles and fractal dimensions of flocs, and therefore produced cake layers with different thickness/structure. Analysis of flocs with different ages indicated a crystallization process in the cake layer. Crystallization exacerbate membrane fouling by decreasing the porosity of the cake layer, and the situation became severe in the presence of humic acid.
Highlights
Different types of pre-treatment before membrane filtration have been explored for many years, such as adsorption, coagulation, and oxidation.[1,2,3,4] Some materials, such as powdered activated carbon (PAC),[5] heated aluminum oxide particles (HAOPs),[6,7] magnetic ion exchange (MIEX)[8] and synthesized zeolite,[9] have been studied as pre-adsorption materials to mitigate membrane fouling
The variation of trans-membrane pressure (TMP) during the operation of ultrafiltration using AlCl3, PACl15, and PACl25 as precoagulants is shown in Fig. 1, together with UF without precoagulant for comparison
While the development of TMP was significantly different in the four systems, from an initial value of ~5 kPa, the extent of fouling decreased in the order: no pre-treatmentå AlCl3å PACl25å PACl15
Summary
Different types of pre-treatment before membrane filtration have been explored for many years, such as adsorption, coagulation, and oxidation.[1,2,3,4] Some materials, such as powdered activated carbon (PAC),[5] heated aluminum oxide particles (HAOPs),[6,7] magnetic ion exchange (MIEX)[8] and synthesized zeolite,[9] have been studied as pre-adsorption materials to mitigate membrane fouling. It was found that the TMP of all three membrane systems with pre-coagulation was again reduced to its initial value, indicating that the cleaning was effective with or without the presence of HA (Fig. 1).
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