Eco-friendly cationic celluloses for fouling control in anaerobic membrane bioreactors

The anaerobic membrane bioreactors (AnMBRs) present an efficient method for wastewater treatment, but the membrane fouling poses a primary barrier to sustained, large-scale operation. This study addresses the fouling mitigation in AnMBR by developing bio-based coagulants from microcrystalline cellulose (MCC) and rice straw (RS). The cationic derivatives (cMCC and cRS) were synthesized through a two-step process-periodate oxidation of cellulose, followed by cationization with Girard's reagent T. The resultant cationic celluloses exhibited hydrodynamic diameters of 231 nm (cMCC) and 148 nm (cRS) and zeta potentials of + 38 mV (cMCC) and + 26 mV (cRS). The structure of cationic celluloses was confirmed by ¹H NMR and the nitrogen elemental analysis revealed a degree of substitution of 0.6-0.8 and a high cationicity index of 2.98-3.41 mmol/g. A single dose of 5 ppm cMCC and 7 ppm cRS was added at the beginning of a 60-day continuous AnMBR trial. These coagulants effectively neutralized the negative charges on the sludge particles through electrostatic patch interactions. The enlarged flocs (~ 367 μm) reduced pore blocking and formed a loosely bound cake layer, as confirmed by scanning electron microscopy of the membrane surface. With coagulant addition, the permeate flux remained stable for nearly 50 days and the fouling rates declined by almost 37% in the cRS reactor and by 53% in the cMCC reactor, while achieving 92% COD removal efficiency with no adverse impact on the reactor performance. These biomass-derived cationic coagulants offer a non-toxic, eco-friendly alternative to commercial additives to improve membrane performance in AnMBR systems.