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The growth, breakage and regrowth of flocs formed by aluminum sulfate (alum) with humic acid (HA) in water at neutral pH was investigated by jar testing with continuous optical monitoring. Various initial dosages of alum and different breakage shears were investigated to compare the floc strengths and to explore the growth of flocs and regrowth of broken flocs. In all cases there was significant irreversibility of floc breakage when no additional coagulant was added. On the other hand, when a small additional dosage of alum was added to the suspension during floc breakage, the size of regrown flocs was higher than that before breakage. The result did not change with the variation of the initial dosage of alum, and the intensity and duration of floc breakage, provided that the additional coagulant was added shortly before the end of the breakage process. It seems that aluminum hydroxide is better able to form flocs, when newly precipitated, rather than after an extended period of high shear.

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The fouling of ultrafiltration (UF) and nanofiltration (NF) membranes during the treatment of surface waters continues to be of concern and the particular role of natural organic matter (NOM) requires further investigation. In this study the effect of pH and surface charge on membrane fouling during the treatment of samples of a representative surface water (Hyde Park recreational lake) were evaluated, together with the impact of pre-ozonation. While biopolymers in the surface water could be removed by the UF membrane, smaller molecular weight (MW) fractions of NOM were poorly removed, confirming the importance of membrane pore size. For NF membranes the removal of smaller MW fractions (800 Da-10 kDa) was less than expected from their pore size; however, nearly all of the hydrophobic, humic-type substances could be removed by the hydrophilic NF membranes for all MW distributions (greater than 90%). The results indicated the importance of the charge and hydrophilic nature of the NOM. Thus, the hydrophilic NF membrane could remove the hydrophobic organic matter, but not the hydrophilic substances. Increasing charge effects (more negative zeta potentials) with increasing solution pH were found to enhance organics removal and reduce fouling (flux decline), most likely through greater membrane surface repulsion. Pre-ozonation of the surface water increased the hydrophilic fraction and anionic charge of NOM and altered their size distributions. This resulted in a decreased fouling (less flux decline) for the UF and smaller pore NF, but a slight increase in fouling for the larger pore NF. The differences in the NF behavior are believed to relate to the relative sizes of ozonated organic fractions and the NF pores; a similar size of ozonated organic fractions and the NF pores causes significant membrane fouling.

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Membranes prepared from layers of graphene oxide (GO) offer substantial advantages over conventional materials for water treatment (e.g. greater flux), but the stability of GO membranes in water has not been achieved until now. In this study the behavior of GO membranes prepared with different quantities and species of cations has been investigated to establish the feasibility of their application in water treatment. A range of cation-modified GO membranes were prepared and exposed to aqueous solutions containing specific chemical constituents. In pure water, unmodified and Na-modified GO membranes were highly unstable, while GO membranes modified with multivalent cations were stable provided there were sufficient quantities of cations present; their relative capability to achieve GO stability was as follows: Al 3+ > Ca 2+ > Mg 2+ > Na + . It is believed that the mechanism of cross-linking, and membrane stability, is via metal-carboxylate chelates and cation-graphite surface interactions (cation-π interaction), and that the latter appears to increase with increasing cation valency. The instability of cation (Ca or Al)-modified GO membranes by NaCl solutions during permeation occurred as Na + exchanged with the incorporated multivalent cations, but a high content of Al 3+ in the GO membrane impeded Al 3+ /Na + exchange and thus retained membrane stability. In solutions containing biopolymers representative of surface waters or seawater (protein and polysaccharide solutions), Ca-GO membranes (even with high Ca 2+ content) were not stable, while Al-GO membranes were stable if the Al 3+ content was sufficiently high; Al-formed membranes also had a greater flux than Ca-GO membranes. PAPEROriginal content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence.