Effects of aluminum hydrolysis products and natural organic matter on nanofiltration fouling with PACl coagulation pretreatment

Abstract

Coagulation is one of the most frequently applied pretreatment processes for nanofiltration. Polyaluminumchloride (PACl) has been commonly used as a coagulant for drinking water treatment. When used upstream of nanofiltration, PACl hydrolysis products formed during coagulation can act as foulants due to their retention on the membrane surface. Three distinctive PACl hydrolysis products were formed at different pH conditions: Al137+ and other polymeric Al at low pH; Al-hydroxide, i.e., Al(OH)3 at neutral pH; and aluminate ion, i.e., Al(OH)4- at high pH. In addition, humic acid was added to investigate effects of natural organic matter on the fouling by PACl hydrolysis products. The degree of fouling was characterized through analysis of flux decline behavior and fouling mechanisms were examined with mathematical models. The hydrolysis products at a low pH resulted in the most rapid flux decline regardless of the presence of humic acid. At a high pH condition, the aluminum (Al) species alone resulted in substantial flux decline with a fouling mechanism of pore constriction; while the combined foulants (Al species+humic acid) resulted in a relatively reduced flux decline that was well described by a cake formation mechanism. It should be noted that proper control of coagulation conditions is necessary to avoid residual polymeric Al or aluminate ion when used in the coagulation pretreatment before nanofiltration membrane process.