Influence of Permeation Relaxation on Particle Dispersion in Wastewater Membrane Bioreactor Processes

Cake dispersion was studied using three pilot-scale submerged membrane bioreactors operated at a series of permeation and relaxation conditions to treat municipal wastewater. The transmembrane pressure changes were used to calculate the rate of dispersion to compare the fouling reversibility. The results showed that dispersion increased with relaxation duration and the air sparging intensity applied during relaxation. Little improvement in dispersion was observed as permeation air sparging intensity increased however there may be an optimal permeation air sparging frequency. The percent of the total reversible fouling resistance detached during relaxation increased with the permeate flux while the absolute rate of removal declined. Particle-membrane interactions initially limited dispersion during cyclic permeation, however, after sufficient cake formation, equal mass was back-transported during each relaxation cycle. Although filtration duration increased in proportion to the mass dispersed during relaxation, operating conditions that were unsustainable with constant permeation were also unsustainable with cyclic permeation when a net permeation-relaxation fouling rate of zero was not achieved. The net accumulation of resistance was observed to be similar at both permeation conditions.