Impact of natural organic matter foulants on the filtration and chemical cleaning performance of aged membranes

Long-term irreversible changes in membrane performance and characteristics, referred to as membrane ageing, eventually trigger membrane replacement. The research investigates the extent to which natural organic matter (NOM) components in feedwater impact the filtration and chemical cleaning performance of polyvinylidene fluoride (PVDF) based ultrafiltration (UF) membranes as they age. Lab-scale ageing investigations using model NOM components demonstrated that the impact of membrane age on both filtration and chemical cleaning performance was dependent on the type of model NOM component present in the feedwater. When a model protein-like NOM component was present, the permeability of a membrane during filtration declined faster and to a greater extent with age, while the permeability during chemical cleaning recovered at a slower rate. In contrast, the permeability during filtration or chemical cleaning was not impacted by age when a model humic-like NOM component was filtered, and a model polysaccharide-like NOM component had only a limited impact on permeability during both filtration and chemical cleaning of membranes. The faster decline and slower recovery were correlated to membrane hydrophobicity, which increased with age, resulting in an increase in the rate and extent to which model protein-like NOM was retained on aged membranes during filtration and increased the extent of oxidation required to release it during chemical cleaning. Investigations using mixtures containing multiple model NOM components further confirmed that the impact of ageing on both filtration and chemical cleaning performance was predominantly due to protein-like NOM components. Overall, these results indicated that the chemical cleaning duration needs to be increased to maintain consistent cleaning efficacy as membranes age when protein-like NOM components are retained on membranes. The extent of the increase required depends on the proportion of protein-like NOM in the retained mass of foulants. Results consistent with those from lab-scale ageing investigations were observed at full-scale. In particular, results revealed that at some facilities chemical cleaning may be incomplete at full-scale, and the extent to which chemical cleaning was incomplete increased with membrane age. For these facilities, the duration of chemical cleaning needs to be increased with membrane age to enable sustained operation.