FOULING OF MICROFILTRATION MEMBRANES DURING FILTRATION OF WASTEWATER EFFLUENT FOR WATER REUSE: MECHANISMS, EFFECTS, AND CONTROL

Abstract

This research investigates the performance of polyvinylidene fluoride (PVDF) microfiltration (MF) membranes during its utilization for direct filtration of wastewater treatment plant (WWTP) effluent for potable water reuse. The research primarily focuses on the fouling of PVDF MF membranes and explores the mechanisms, long-term effects, and strategies for control and mitigation. The overall hypothesis of the research is that irreversible fouling of microfiltration membranes is influenced by specific effluent organic matter (EfOM) – membrane interactions and the progressive accumulation of these foulants plays a primary role in the long-term productivity loss of microfilters. The research demonstrated that fouling of PVDF microfilters by EfOM proceeded via a two-step mechanism involving the initial deposition of relatively hydrophobic organics to the membrane followed by the attachment of relatively hydrophilic organics. It was also shown that the fouling is critically dependent on the attachment of a small specific fraction of EfOM. Clues to the identity of this specific fraction were obtained using high-resolution mass spectrometry and shown to possibly belong to the lipid chemical class. Potential mechanisms of fouling were evaluated via surface characterization techniques (e.g. attenuated total reflection - Fourier transform infrared spectroscopy, scanning electron microscopy, electrokinetic analysis) along with mathematical modeling (constant flux blocking laws). The analysis revealed the detrimental role of internal pore fouling in irreversible fouling with surface fouling working as a secondary dynamic membrane preventing pore exposure. Further, the analysis of backwash performance of different backwashing solutions underscored non-electrostatic EfOM-membrane interactions as the dominant mechanism of irreversible fouling. Investigations on the long-term performance (fouling propensity and water permeability) of the membranes revealed that while multiple chemical cleaning cycles led to no change in membrane productivity, progressive chemically irreversible fouling led to the loss in productivity, confirming the hypothesis of the research. The irreversible fouling was also accompanied by progressive loss in membrane mechanical strength (measured as Young’s modulus and dynamic storage modulus). Additionally, pre-chlorination of wastewater effluent was an effective pretreatment strategy for fouling control. However, fouling control was dose-dependent and transitional in nature with fouling exacerbated at chlorine doses below a threshold dose. This transitional behavior originated from the dose-dependent alterations in EfOM character and was validated by extended Derjaguin-Landau-Verwey-Overbeek theory calculations of EfOM-membrane free energy of adhesion.