Steady-state pressure-retarded osmosis (PRO) performance of a pilot-scale (72 m(2)) cellulose triacetate (CTA) hollow fiber (HF) module using 0.5 M NaCl and tap water as the draw solution (DS) and feed solution (FS), respectively, was evaluated by a PRO test system with a DS regeneration unit under a wide range of operational conditions. We investigated the effects of varying operational parameters, such as DS and FS inlet flow rates and hydraulic pressure difference, on PRO performance (water flux, pressure drops on the FS and DS sides, and power density). To estimate the module performance, we used conventional membrane area-based power density (W-area), as well as proposed and used a novel estimable parameter, power output per module volume (W-vol), which promises to be useful when designing full-scale PRO plants. The obtained Warea of the CTA HF module was 0.14 W/m(2) and was approximately one sixth of the W-area reported for a thin-film composite (TFC) polyamide spiral wound (SW) module [Y. C. Kim et al., Environ. Sci. Technol, 2966-2973]. However, the W-vol of the CTA HF module was 1.11 kW/m(3)-module and was at least 45% higher than those of the SW modules (0.75 kW/m(3)-module).