Membrane ageing due to chemical cleaning agent

Abstract

Sodium hypochlorite is commonly used as a cleaning agent to remove adsorbed foulants from PVDF-based micro/ultra filtration membranes in water and wastewater treatment applications. Although effective for fouling control, extended sodium hypochlorite exposure can affect the physical/chemical characteristics and hinder the treatment performance of these membranes. In the present study, experiments were conducted to comprehensively quantify the effects of sodium hypochlorite exposure on changes in the physical/chemical characteristics and the filtration performances of blended PVDF-based supported hollow-fiber membranes and identifying the mechanism(s) responsible for the changes. Both the effect of the sodium hypochlorite concentration (C) and the duration of exposure (t) on the membrane characteristics are investigated. The physical/chemical characteristics and the filtration performances of virgin and aged (i.e., weathered due to exposure to sodium hypochlorite) membranes were compared. The membranes were characterized based on chemical composition (FTIR and NMR), mechanical strength (yield strength), surface hydrophilicity (contact angle), pore size and porosity (scanning electron microscopy and challenge test), membrane resistance (clean water permeation test), and affinity of the membrane for foulants (cleaning efficiency). The results indicated that exposure dose and concentration of the sodium hypochlorite used have a significant influence on the membrane characteristics. For the exposure conditions considered, the impact of sodium hypochlorite exposure on the parameters investigated could be most accurately and consistently correlated to an exposure dose relationship of the form Cnt (where, C=concentration and t=exposure time) rather than the Ct relationship commonly used to define the extent of exposure to cleaning agents. For all the parameters investigated, the power coefficient n was less than 1 indicating that time had a greater impact on the changes than did the concentration of the sodium hypochlorite. The results suggest that the use of sodium hypochlorite for chemical cleaning, at concentrations that are higher than those typically used for chemical cleaning would have less of an effect on the characteristics of the membrane materials. Changes in the characteristics were attributed to the oxidation of the hydrophilic additives (HA) present in blended PVDF membranes. A new non-destructive membrane characterization technique to evaluate the amount of membrane ageing is proposed.