Natural organic matter and colloidal fouling in crossflow membrane filtration

Although crossflow membrane filtration has evolved as a viable water treatment process, its application is often restricted by a gradual decline in permeate flux due to fouling. Among the wide spectrum of foulants, dissolved natural organic matter (NOM) and suspended colloidal particles are considered to be major causes of membrane fouling. The general objective of this work is to investigate systematically chemical and physical aspects of NOM and colloidal fouling in crossflow membrane filtration.; In the first part, the role of chemical and physical interactions in NOM fouling of nanofiltration membranes is systematically investigated. Results of fouling experiments with three humic acids demonstrate that membrane fouling increases with increasing electrolyte (NaCl) concentration, decreasing solution pH, and addition of divalent cations (Ca{dollar}sp{lcub}2+{rcub}{dollar}). At fixed solution ionic strength, the presence of calcium ions, at concentrations typical of those found in natural waters, has a marked effect on membrane fouling. Divalent cations specifically interact with humic carboxyl functional groups, and thus, substantially reduce humic charge and the electrostatic repulsion between humic macromolecules. Reduced NOM interchain repulsion results in increased NOM deposition on the membrane surface and formation of a densely packed fouling layer. In addition to chemical effects, results show that NOM fouling rate increases substantially with increasing initial permeation rate. It is demonstrated that the rate of fouling is controlled by an interplay between permeation drag and electrostatic double layer repulsion; that is, NOM fouling of NF membranes involves coupling between physical and chemical interactions. The addition of a strong chelating agent (EDTA) to feed water reduces NOM fouling significantly by removing free and NOM-complexed calcium ions. EDTA treatment of NOM-fouled membranes also improves the cleaning efficiency dramatically by disrupting the fouling layer structure through a ligand exchange reaction between EDTA and NOM-calcium complexes.; In the second part, a series of membrane filtration experiments are performed to investigate systematically the dynamic behavior of permeate flux decline in crossflow membrane filtration of colloidal suspensions. Results are analyzed by a transient flux model developed based on hydrodynamics and thermodynamics of colloidal suspensions. Experimental results show that permeate flux declines much faster with increasing particle concentration and transmembrane pressure. It is also demonstrated that the rate of permeate flux decline increases as the ionic strength of the feed suspension increases. This phenomenon is attributed to the formation of a more densely packed cake layer resulting from reduced electrostatic repulsion among the accumulated particles. The results also suggest that, under laminar flow conditions, crossflow velocity (shear) has a very small effect on permeate flux. Finally, it is shown that the formation of particle cake layer is reversible, indicating no irreversible adsorption of particles to the membrane surface at given chemical and physical conditions.