Particle fouling of submerged- and hard-shell, encased low-pressure membranes

The two broad goals were: (1) critically review the fouling mechanisms and process models developed by research community for hollow fiber membrane technology with the aim of showing their practical significance, and (2) systematically investigate the hydraulic techniques to control membrane fouling (backwash, critical flux and aeration) at bench-scale with encased and submerged membrane systems.; The bench-scale design to investigate backwash frequency and duration provides a convenient way to observe fouling rate systematically and useful guidance for selection of the setpoint flux and backwash conditions that minimize chemical cleaning. Backwash frequency was more important than backwash time in control of membrane fouling for the ultrafiltration (UF) membrane system tested. However, long-term fouling could not be avoided for any combination of backwash frequency and backwash time.; Critical flux is determined only from flow rate exiting the end of the fiber whereas flux varies with fiber length and with time of operation. Fouling can occur where the local flux is higher than the average flux upon which the critical flux test is based. Critical flux was shown to be a relative rather than absolute value because it depends upon fiber length, aeration rate, foulant particle properties and filtration time chosen for the critical flux test.; The effects of the initial flux distribution along the fiber and aeration rate on the particle deposition and permeate flux characteristics showed that the axial gradient in particle deposition was much larger when aeration was used for fouling control than without aeration in submerged, hollow-fiber MF system. Aeration was postulated to retard the progress of initial particle deposition formed near the open end of the fiber where the local flux is initially the highest and thus prevent migration of foulants toward lower flux regions of the fiber with time. Aeration was shown to alter cake structure by lowering the specific cake resistance compared to that measured from unaerated flat sheet membrane tests as are typically used to measure the specific cake resistance.