Carbon nanotubes (CNTs) are rapidly influencing the development and applications of membrane technology for water treatment. Passage of CNTs through membrane pores is becoming a fundamental question to water industries, as the toxicity and environmental fate of waterborne CNTs are largely unknown. This study utilized CNTs and membranes with known properties to investigate the applicability of the Ferry-Renkin sieving equation to the rejection of CNTs by porous membranes. The results demonstrate that the hydrodynamic size of CNTs is more important than their physical dimensions for rejection. Moreover, the classical sieving equation provided reasonable predication of the experimental results. Important for water industries, current membranes used in drinking water treatment should be efficient barriers for waterborne CNTs leached from composite membranes or released from wastewater effluents. Further, process streams containing CNTs may be treated using membrane filtration for CNT recovery. However, micron-pore-size membranes used in previous studies for CNT-membrane fabrication may not be efficient in protecting CNT breakthrough. Since the hydrodynamic diameters of waterborne CNTs are usually above 150 nm, as a general rule of thumb, membranes with pore size smaller than 100 nm need to be used to ensure the safety of CNT membranes. (C) 2014 Elsevier B.V. All rights reserved