1-1/2 Membrane Stages; an Innovative Application for PX Pressure Exchangers

This paper presents an analysis of single-stage, two-stage and one-and-one-half-stage designs conducted using a membrane projection program and PX device performance characteristics at different feed water salinities. The paper presents simplified operating guidelines and control philosophy for these designs. The author anticipates that multiple-stage designs will be shown as a way to bring the energy-saving benefits of PX technology into the mainstream of BWRO designs. The data in this paper support the following observations and conclusions; one-and-one-half-stage and two-stage designs effectively lower the recovery rate in each stage while maintaining a relatively high overall recovery rate compared to single-stage designs. A one-and-one-half-stage design allows the system designer or operator to set the fluxes of the two stages to be more equal than can be easily achieved with a two-stage design. Permeate quality is essentially independent of the stage design. Approximately three to five percent more energy is required to operate a two-stage design than is required for a comparable single-stage design. Approximately five to seven percent more energy is required to operate a one-and-one-half stage design than is required for a comparable single-stage design.