Interactions between membrane surface, dissolved organic substances and ions in submerged membrane filtration

Interactions between dissolved organic substances (dissolved organic matter and two model substances), ions (Na+ and Ca2+) and the membrane surface were investigated using a submerged membrane module. The membrane permeability, the fouling formation and the rejection were significantly influenced by the solution’s ionic strength and especially by the presence of calcium ions. An increase of the ionic strength led to a decrease of the DOMrejection of the membrane from 75% to about 50% if Na+-ions were added and to about 15% if Ca2+-ions were added. This was due to neutralization of charges of the membrane surface and due to shielding of negative charges of DOM molecules. The ionic strength and pH of the cation solutions were kept at similar values. As the solution ionic strength increases, DOM molecules can permeate more easily through the membrane pores, since their shape and molecular size change. Adsorption of DOM on the membrane surface and in the membrane pores was only observed by filtration of solutions with Ca2+. The results showed that in the presence of Ca2+, adsorption played a mayor role in flux decline and fouling formation.

It could be clearly shown that DOM of natural origin (fraction <0.45 µm) is a principal contributor to membrane fouling. A higher flux decline is expected if solutions with high DOM-concentrations are used. Fouling mechanisms such as pore blocking or adsorption that cannot be avoided by membrane backwash largely contribute to fouling and its corresponding flux decline. Raw water properties such as the presence of Ca2+ ions play an important role in the flux decline. Results clearly show the presence of Ca2+ ions in high-DOM water leads to an increase in flux decline. Adsorption of DOM–Ca2+ complexes and small colloids within the membrane or on its surface may explain this effect. The general DOM-rejection of the investigated membrane in the absence of major salt content is much higher than expected. Electrostatic repul- DOC [mg/L] Adsorption: 41,7 mg DOC/m membrane surface Adsorption time [days] Fig. 7. DOM-adsorption on the membrane surface in the presence of Ca2+ ions (0.61 mmol/L).