Electrically released iron for fouling control in membrane bioreactors: A double-edged sword?

• Electrically released iron affects fouling propensity of soluble microbial product.
• Iron and alginate interact through two regimes based on available binding sites.
• Certain amount of iron and alginate could form stabilized colloidal particles.
• Critical iron demand exists for the purpose of fouling suppression.

Iron injection through electrolytically mediated oxidative dissolution of an iron electrode has been proposed as a convenient means of inducing coagulation of wastewater contaminants and thereby reducing the extent of membrane fouling in membrane bioreactors (MBRs). In order to investigate the effect of electrically released iron on membrane fouling propensity, alginate was selected as a model soluble microbial product, and the impact of electrolyzed iron on the specific resistance to filtration (SRF) quantified for a range of electrolysis times. It was confirmed that ferrous iron (Fe(II)) was initially released with concentration increasing proportionally with time of application of electric current in agreement with Faraday's Law. The fouling propensity of alginate increased at early loading times due to the formation of alginate gels and highly stabilized colloidal species. The presence of calcium ions was found to exacerbate this gelation process as a result of i) alginate bridging by Ca2 + ions, and ii) the reduced capacity of the alginate to bind iron and stabilize iron oxide particles. Significant reduction in fouling propensity was observed at higher iron loadings as a result of the formation of substantive amounts of iron oxides which, most likely, adsorbed alginate and prevented gel formation.

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