Effect of water-swollen organic materials on crossflow filtration performance

Membrane filtration device for studying compression of fouling layers in membrane bioreactors

A filtration devise was developed to assess compressibility of fouling layers in membrane bioreactors. The system consists of a flat sheet membrane with air scouring operated at constant transmembrane pressure to assess the influence of pressure on resistance of fouling layers. By fitting a mathematical model, three model parameters were obtained; a back transport parameter describing the kinetics of fouling layer formation, a specific fouling layer resistance, and a compressibility parameter. This stands out from other on-site filterability tests as model parameters to simulate filtration performance are obtained together with a characterization of compressibility. Tests on membrane bioreactor sludge showed high reproducibility. The methodology’s ability to assess compressibility was tested by filtrations of sludges from membrane bioreactors and conventional activated sludge wastewater treatment plants from three different sites. These proved that membrane bioreactor sludge showed higher compressibility than conventional activated sludge. In addition, detailed information on the underlying mechanisms of the difference in fouling propensity were obtained, as conventional activated sludge showed slower fouling formation, lower specific resistance and lower compressibility of fouling layers, which is explained by a higher degree of flocculation.

Modeling cake buildup under TMP-step filtration in a membrane bioreactor: cake compressibility is significant

Fouling is inevitable in membrane bioreactors (MBRs) due to the complex nature of activated sludge, which contains a broad variety of potential foulants. Filter cakes that build up from sludge particles are traditionally highly compressible due to both the deformation of the individual sludge particles and the rearrangement of these particles in the cake. However, this phenomenon has been little examined in studies of fouling mechanisms in MBR systems. This study examines the properties of the cake layer, modeling the cake buildup and specific cake resistance (α), including compressibility, in terms of pressure-dependent α. The changes in fouling resistance during transmembrane pressure (TMP)-step filtration in an MBR setup were simulated using an empirical pressure dependence of the specific cake resistance and a simple mass balance model. The total change in fouling resistance in each TMP step could be divided into an initial rapid change in specific cake resistance due to filter cake compression followed by simple cake buildup. By including cake compression in this simple model, the model fitted the data with high precision. We demonstrated that compressibility should be considered when describing cake fouling in MBRs.