Biologically induced mineralization in anaerobic membrane bioreactors: Assessment of membrane scaling mechanisms in a long-term pilot study

Abstract Herein, we report on a mechanism of inorganic fouling observed in a pilot-scale anaerobic membrane bioreactor (AnMBR) treating high strength leachate from domestic food waste. Long-term operation (around 700 days) of the AnMBR encountered frequent, sudden irreversible fouling events driven by biologically induced mineral scaling which required intense chemical cleaning to recover membrane permeability. Mineral scale formation occurred on the surface and within pores of 100 kDa ultrafiltration (UF) membranes. The UF membrane rejected phosphorus, calcium and magnesium up to 97%, 92% and 60%, respectively, which suggests that these ions either precipitated ahead of the UF membranes and existed in a colloidal state and/or precipitated and were retained within UF membrane pores. Microscopic and spectroscopic analyses combined with geochemical modeling confirmed hydroxyapatite, dolomite and struvite were the prevailing mineral precipitates coming from the methanogenic digester; these minerals were embedded in the cake layer and found within membrane pores. Geochemical modeling suggests that slower-growing minerals such as dolomite might precipitate within the membrane pore structures possibly causing membrane pore constriction and/or blocking. Combined use of HOCl (to oxidize and remove organic matter) and citric acid (to dissolve minerals and chelate divalent ions) proved the most effective cleaning regime to recover membrane permeability.