Application-oriented mini-plant experiments using non-conventional model foulants to evaluate new hollow fiber membrane materials

Abstract

Most of literature reporting on performance of ultrafiltration membranes was performed in bench-scale experiments employing flat-sheets at operating conditions irrelevant to full-scale application. Subsequently, there is a critical need for an appropriate lab-scale testing protocol employing reliable fouling surrogates. Common individual foulants, e.g., humic acid, bovine serum albumin and sodium alginate, were extensively reported; however, their ability to resemble organic fouling by real surface waters remains questionable. In this study, potting soil extracts were investigated as reliable and inexpensive model foulants for testing the performance of new modified polyphenylene sulfone (PPSU) hollow fiber membrane modules. Assessment experiments were performed in comparison with corresponding polyethersulfone (PESU) membranes modules using mini-plant testing unit operated at full-scale conditions. At comparable total organic carbon content, aqueous extracts of four different potting soil types had different dissolved organic carbon compositions and turbidity. Humic substances were the main constituents; however, hydrophobic organic carbon (HOC) and biopolymers varied significantly. An application-oriented testing protocol comprising 24 filtration-cycles was developed. Potting soil extract feeds were emphasized to imitate chemical composition and characteristics of moderately organic-loaded surface waters, at high reproducibility, and induce typical organic fouling within experiment time. Feeds exhibiting high biopolymer content (or modest portions of biopolymer and HOC) were able to distinguish explicitly between fouling propensity of different modified membrane materials. Besides regular performance measures, fouling propensity was quantified as hydraulic irreversible fouling index (HIFI) that was related to turbidity removal; however, no consistent correlation was found with humics and low molecular weight organic fractions. Furthermore, impact of feed solution type on the interplay between membranes’ characteristics and HIFI values was investigated. Good correlation between PPSU-based membranes’ characteristics and HIFI values was noticed upon using feeds containing considerable biopolymers and HOC portions. Nevertheless, in case of PESU-based membranes, impact of improvement of pure water permeability (due to modification) was dominating.