Fouling Control in Anaerobic Membrane Bioreactors by Flux Enhancer Dosing

Abstract

Anaerobic membrane bioreactor (AnMBR) technology is increasingly researched for wastewater treatment in a circular economy scenario to recover nutrients, water, and biogas. AnMBR couples the advantages of anaerobic digestion, such as low sludge production, no aeration requirement and biogas production, with the benefits of membrane technology, that is, complete solids removal and a high removal degree of pathogenic organisms. Nevertheless, membrane fouling remains the major operational challenge, limiting the economic feasibility and applicability of AnMBRs. Membrane fouling is responsible for lower flux, higher transmembrane pressure, the need for intensive biogas sparging or increased crossflow velocities for membrane scouring, and increased frequency of membrane cleaning and membrane replacement; consequently, increasing energy and operational costs. Researchers extensively studied the causes and mitigation of membrane fouling in both aerobic and anaerobic membrane bioreactors. Membrane fouling mitigation strategies have focused on optimisation of membrane operational variables, such as: gas sparging, crossflow velocity, filtration relaxation cycle, permeate flux and frequency and intensity of chemical cleaning. Although optimisation of operational variables might be suitable when the sludge has good or moderate filterability, it may not be adequate or sufficient when fouling is caused by a sludge with poor filterability. The application of flux enhancers for fouling control has been extensively investigated. Flux enhancers are adsorbents, coagulants and flocculants that decrease fouling by changing the sludge characteristics, thereby improving sludge filterability. Particularly, cationic polymers have been successfully applied as flux enhancers in short term tests on large scale aerobic membrane bioreactors (MBRs), whereas in AnMBRs research is scarce, and so far, only done at lab scale. Results from MBRs cannot be directly translated to AnMBRs because the extent and nature of membrane fouling under anaerobic and aerobic conditions are different. This thesis studies the feasibility of dosing cationic polymers into large scale AnMBRs for fouling mitigation, focusing on long term effects, possible side effects, optimal dosing strategy and variation of required dosage.