Optimization of the operating parameters in a membrane bioreactor operated in direct-flow mode by assessing threshold flux for compressibility

Abstract

Direct-flow filtration (i.e., without membrane aeration during the filtration phase) is a challenging operation strategy in membrane bioreactors (MBRs). This study aims to provide a comprehensive approach for assessing optimal values for the main operating parameters (filtration time between backwashes and permeate flux) through the determination of the threshold flux for compressibility (Jth,c). A pilot-scale tertiary MBR was used for conducting filtration tests. Flux-step trials revealed the crucial role of cake compressibility in fouling deposition and reversibility. Accordingly, the compressible cake model was used to investigate Jth,c under different operating conditions and suspension characteristics. Based on this approach, lower filtration times (5–7 min) are recommended so that sustainable process productivity can be maximized. Specifically, decreasing filtration times from 12 to 7 min results in net permeate flux increase of more than 50%. A significant effect of suspension properties (biopolymer cluster content, BPC) on Jth,c was also found which decreased from 41.9 to 13.0 L/h m2 when increasing BPC from 0.3 to 4.6 mg/L. Long-term tests, under temporized and variable filtration time modes, demonstrate the feasibility of using the Jth,c as a guideline for optimizing filtration performance.