New insights into organic gel fouling of reverse osmosis desalination membranes

RO membrane fouling is unavoidable as no pretreatment method exists for perfect removal of foulants, which comprise various organic compounds including those from bio-activity. Improved understanding of fouling layer characteristics and correlations of permeation resistance with desalination process parameters (pursued herein) are essential for improving RO-membrane operations. Organic-gel formation is a major type of RO-membrane fouling, in the absence or presence of micro-organisms. Alginates, in dilute brackish-water solutions, are employed as representative gel-forming foulants. Recent experimental results are summarized regarding fouling-layer rheological properties and specific resistance α, focusing on ionic environment and permeation flux effects. Comparative assessment of these characteristics sheds light into the development and structure of gel layers during desalination; in particular, the nature of these layers (i.e. thixotropic material with yield stress) explains their coherence and resistance to shear forces. The investigated relatively thin fouling layers (of practical significance) appear to develop uniformly, with resistance α strongly dependent on flux. Exploiting the rich literature on synthetic alginate hydrogels, a new modeling approach is presented for the resistance α, using measured fouling-gel structural parameters. The fair agreement of predictions with measurements, using no arbitrary constants, warrants further work on model development. Research priorities on membrane fouling are indicated. •Organic layers studied through rheological and permeation-resistance measurements•Alginate-gel layers develop uniformly on brackish-water desalination membranes.•The specific fouling resistance α depends on flux J, i.e. α=γJβ where β≥1.•These layers behave as thixotropic gels with significant yield stress and elasticity.•Modeling resistance α suggested using measured fouling-gel structural parameters