The integrity of 0.04 mu m polyvinylidene fluoride hollow fibre ultrafiltration (UF) membranes was assessed by challenge tests with citrate stabilised silver nanoparticles. Spherical, zerovalent, nanosilver particles maintained a net negative surface charge (-25 +/- 4 mV) and narrow size distribution (60 +/- 10 nm) in aqueous suspension for 72 h. Nanoparticle challenge testing of intact UF hollow fibre membranes demonstrated rejection efficiency as high as 2.9 log removal value (LRV), without affecting the hydraulic performance of the membranes. Challenge testing of deliberately compromised membranes through a sequence of breaches indicated that the nanoparticles can detect a loss of integrity in the filtration area of 3 x 10(-5)%, equivalent to 3 damaged fibres in 100,000 fibres. The LRV decreased from 2.8 to 1.3 with one pin hole of 100 mu m thickness. Membranes were exposed to hypochlorite in order to mimic membrane ageing; exposure to 2500 and 5000 mg L-1 h of hypochlorite decreased the rejection efficiency of the particles by 0.2% and 0.9% with a corresponding loss in the intrinsic membrane resistance of 19% and 38%, respectively. Deliberate membrane compromise through chemical exposure was useful for generating membrane degradation that represented the initial stages of membrane failure due to routine chemical exposure. Considering the practical limitations of incorporating challenge testing with MS2 bacteriophage in full scale plants, silver nanoparticles have the potential to be an alternative indicator for challenging UF membranes for validation purposes. (C) 2013 Elsevier B.V. All rights reserved