Cake dispersion for a submerged membrane bioreactor treating municipal wastewater
The objective of this thesis is to quantify the cake dispersed under permeation relaxation for short term fouling in submerged membrane bioreactors used for municipal wastewater treatment. Experiments were conducted using a ZW-10 membrane module submerged in an activated sludge tank by varying permeate fluxes, aeration rates, aeration frequencies for different mixed liquor samples. Stepwise permeate flux decline method was devised to quantify total mass and rate of cake dispersion by monitoring the temporal variation in transmembrane pressure. Multi-variable linear regression analyses showed that total cake dispersion increased with increasing temperatures, cTOC concentrations, and soluble carbohydrate concentrations and with decreasing permeate fluxes. The overall cake dispersion rates increased with decreasing cTOC concentrations, permeate fluxes, aeration frequencies and with increasing bound carbohydrate concentrations. However, the cake dispersion rate during the first flux decline stage increased with increasing aeration rates and bound carbohydrate concentrations and with decreasing the percentage of aeration and cTOC concentrations. Increasing bound protein concentrations caused a decrease in cake dispersion rates during the third flux decline stage, while the percentage of aeration and cTOC concentrations caused a decrease in cake dispersion rates during the fourth flux decline stage. Increasing permeate flux had the greatest impact on cake dispersion by reducing both cake dispersion rate during relaxation and the total cake removed after relaxation. Mixed liquor with high carbohydrates concentrations and cTOC concentrations had higher total cake removal but mixed liquor with high cTOC concentrations required longer relaxation times. Shorter aeration frequencies and continuous aeration during permeation formed the cakes that were more difficult to disperse.