Low pressure membranes (LPMs)---ultrafiltration (UF) and microfiltration (MF)---are increasingly being used globally for drinking water treatment to remove particulates and microbiological contaminants. Viruses in drinking water are smaller than the nominal pore size of MF and some UF membranes. Even so, laboratory studies and field experience demonstrate that enteric viruses are indeed removed by LPMs. To perform properly, membranes must remain intact. Imperfections introduced during manufacturing and handling typically cause holes or macro-pores in membrane surfaces. Compromised membranes allow passage of viruses and other contaminants across the membrane barrier directly into permeate.

A particle tracking model was developed to assess the significance of holes and large macro-pores on virus passage through LPMs. Modeling predictions are compared to the results of virus rejection studies using two MF and two UF flat-sheet membranes challenged with MS2 and PRD1 phage. The membranes tested have different characteristics (monomer, nominal pore size, thickness, hydrophobicity, and resistance). Large hole challenge tests were conducted using needle-compromised membranes. A small hole challenge test was conducted on an Excimer laser-drilled UF hydrophilic membrane. Model results agree well with experimental data. Hydrodynamics at the membrane surface at the point of imperfection, and membrane resistance have the greatest effect on virion hole passage. This is the first such model to assess virion passage through small holes at the membrane surface. The implications for full-scale MF and UF systems are discussed.