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 or the nominal pore size. 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 was to develop a systematic performance testing protocol and specification for low-pressure 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, 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 pilot unit was fed with dechlorinated finished water, raw water from the Chattahoochee River, and filtered water. The microorganisms tested were MS2 phage, PRD1 phage and Pseudomonas diminuta. Under clean water conditions (using dechlorinated finished water), the removal of PRD1 phage ranged between 1.2 and 1.6 log, while negligible removals of MS2 phage were observed. Complete removal of Pseudomonas diminuta (5 Log) was observed under the same conditions. Short- and long-term fouling experiments with raw and filtered water showed that cake layer formation and pneumatically-irreversible fouling can induce additional phage removal to a variable extent depending upon the size of the microorganism, the feed water quality and the decline in specific flux. Includes 12 references, tables, figures.