The effect of hydrodynamic conditions and system configurations on the permeate flux in a submerged hollow fiber membrane system

The present study was undertaken to investigate the effect of the hydrodynamic conditions and system configurations on the permeate flux in a submerged hollow fiber membrane system. Specifically, the study was aimed at establishing the magnitude of the contribution from single-phase (i.e. water only) and dual-phase (i.e. air sparged) cross-flow, as well as that from the lateral sway in multi-fiber modules, on the permeate flux in a submerged hollow fiber membrane system.The reduction in the permeate flux over time could be characterized by an initial short period of fast permeate flux decline, followed by a longer period of slower decline. The reduction in the permeate flux could be effectively modeled using three parameters: a pseudo-steady-state permeate flux, an initial rapid permeate flux decline coefficient and a long term slow permeate flux decline constant. Single-phase cross-flow had a limited effect on the pseudo-steady-state permeate flux. However, dual-phase cross-flow (i.e. with air sparging) significantly enhanced the pseudo-steady-state permeate flux. The pseudo-steady-state permeate flux for dual-phase cross-flow was 20–60% higher than that for single-phase cross-flow. The pseudo-steady-state permeate flux increased with the extent of air sparging for dual-phase cross-flow. However, a plateau was observed above which an incremental increase in air sparging intensity was not accompanied by an increase in the pseudo-steady-state permeate flux.For dual-phase cross-flow, when promoting lateral sway in a multi-fiber module, it was possible to further increase the pseudo-steady-state permeate flux by up to approximately 40%, compared to that for a single fiber module. Also, when promoting lateral sway in a multi-fiber module, a high pseudo-steady-state permeate flux could be achieved at a much lower bulk cross-flow velocity (i.e. air sparging intensity) compared to that for a single fiber module. The results suggest that promoting lateral sway in a multi-fiber module for dual-phase cross-flow resulted in the physical contact between the membrane fibers, which enhanced the permeate flux by mechanically eroding the foulant layer that formed on the membrane surface.No consistently significant trend in the initial rapid permeate flux decline coefficient was observed for the experimental conditions investigated. However, the long term slow permeate flux decline coefficient was observed to be proportional to the inverse of the pseudo-steady-state permeate flux. These results suggest that the long term permeate flux decline coefficient is governed by a time dependent mechanism, such as the adsorption of the fouling material to the inside of the membrane pores.