The energy loss characteristics of sealed-end parallel flow hollow fiber membrane aerators were evaluated in this study. The membrane aerators were used successfully to oxygenate water without producing bubbles. Typically, a membrane aerator consists of a large number of sealed-end fibers in an external shell. Pressurized pure oxygen is maintained inside the hydrophobic gas permeable fibers at a pressure below the bubble point, and the water to be aerated is pumped over the outside of the fiber. Due to the high concentration gradient, oxygen diffuses through the gas filled membrane pores and dissolves directly into the water. Friction between the fibers and the water is the major energy loss mechanism in this type of modules. The gas supply manifold acid other fittings are responsible for minor losses. Pilot scale modules with two different gas supply manifold designs were operated and the energy requirements of the module were measured over a practical range of design and operating conditions. The major and minor energy loss characteristics of the module were correlated with the physical module parameters and the flow conditions in the pipe. The friction factor for the fibers was found to be about a factor of 2 higher than the corresponding friction factor for pipe flow under similar flow conditions.