Microbial passage in low pressure membrane elements with compromised integrity

The main objective of this study was to evaluate microbial passage through membrane elements with various levels of compromised integrity while installed within full-scale UF/MF systems. Challenge tests were performed for this purpose with the microbial contaminant surrogates Bacillus subtilis spores at two drinking water treatment plants. The experimental units used were one of several parallel racks part of each plant in which one of the elements was installed with various levels of compromised integrity achieved by precutting several hollow fibers and inserting removable pin-plugs atthe corresponding ends of the broken fiber segments. The UF rack was operated with 38 elements online (the rack has 50 elements) and was designed for an inside-out operation, and the MF rack included 50 elements and was operated in an outside-in mode (with a permeate outlet at one fiber end only). Spore removals observed for both the UF and MF racks with all precut fibers plugged were equal to or greater than 99.9992%, and as expected the removal efficiency deteriorated with an increasing number of unplugged fibers. Predictions made with a model based on the use of the Hagen-Poisseuille equation for laminar flow and the Darcy-Weisbach expression for turbulent flow inside broken fibers were found to provide an adequate conservative representation of experimental results. Additional simulations performed with the verified model revealed the occurrence of a greater microbial passage for an inside-out configuration compared to an outside-in mode. A lower microbial passage was predicted for the outside-in element configuration with one permeate outlet as compared to an element with permeate outlets at both fiber ends. The model offers a useful tool that together with other considerations such as membrane fouling, cleaning, and durability would assist in the selection of low-pressure membrane element configuration.