Solutions to the momentum and diffusion equations are obtained for rotating disk systems with an imperfect semipermeable interface, with direct application made to the reverse somosis or hyperfiltration process of salt water purification. The equations are solved exactly, and a new technique for solving the momentum equations is described. An approximate solution to the diffusion equation is also obtained which is also applicable to the energy equation, and is shown to be accurate for Prandtl and Schmidt numbers ≥ 1, for a wide range of interfacial mass transfer, for all wedge-type flows as well as the rotating disk system.