The traditional method for determining whether a membrane is considered an ultrafiltration (UF) or microfiltration (MF) membrane is based upon the molecular weight cutoff of the membrane. Microbial removal performance does not currently play a quantitative role in determining whether a membrane is classified as MF or UF. This leads to ambiguity in the classification of low-pressure membranes. The overall goal of this AwwaRF-funded research is to develop a systematic performance testing protocol and specification for MF and UF membranes with respect to removal of viral and submicron bacterial pathogens with various water qualities and operating conditions. Pilot studies were conducted at the Atlanta Fulton County Water Treatment Plant in Georgia, with a continuous microfiltration (CMF) unit (hollow fiber, PVDF membrane) to evaluate its capability to remove microorganisms under selected water quality and operational conditions. The microorganisms tested were MS2 phage, PRD1 phage and Pseudomonas diminuta. The pilot was fed with dechlorinated tap water (finished water) or raw water from the Chattahoochee River. Pseudomonas diminuta was removed to the detection limit of its assay (5-6 Logs) while using dechlorinated tap water. When fed with raw water and after allowing cake to form on the membrane, the system achieved an average removal of 0.7 Log (MS2) and 2.3 Log (PRD1). In comparison, low removal of MS2 and 1.4 Log-removal of PRD1 were observed while using dechlorinated tap water. Under these conditions, the flowrate appeared to have very little effect, if any, on the removal of MS2 and PRD1 phages. The results obtained at bench-scale were comparable to the pilot-scale results. The highest viral removal was observed under the hydraulically/pneumatically irreversible fouling conditions (PRD1=4.1- 6.1 Log; MS2=0.5-5 Log). This can be attributed to a greater specific flux decline during the extended operation of irreversible fouling conditions. Includes 9 references, tables, figures.