As available drinking water supplies are increasingly strained, use of low pressure membranes (LPMs1) for wastewater reuse has become more widespread. Control of viruses in reclaimed water is critical to the protection of public health. The interaction between viruses, water chemistry and membrane properties plays an important role in the organism's removal, especially when its size is smaller than the size of reported membrane pores. Using MS2 bacteriophage as an indicator organism, the log removal value (LRV2) of the virus in waters containing secondary effluent organic matter increased with filtration time and concentration of high molecular weight organic foulants. The LRV increased from 2.1 to 3.0 for high fouling water, while removal in low fouling water ranged from 0.8 to 1.7. In comparison, a LRV of 1.0 was achieved in model water prepared to simulate a non-fouling condition. Addition of equal ionic strength of either sodium or calcium to model water reduced the LRV from 2.5 to 1.6 for sodium and to 0.9 for calcium. Mechanisms are proposed to explain the complexity of the observed membrane virus exclusion. The data in this study show that the use of pretreatment to reduce membrane fouling may ultimately impair virus removal efficiency. (C) 2012 Elsevier B.V. All rights reserved.