Optimization of the energy demand of reverse osmosis with a pressure-exchange system

Energy recovery systems based on turbines activated by concentrate leaving the plant have been in operation in many reverse osmosis (RO) plants for many years. Other design approaches for RO plants save energy by reducing the energy demand for the desalination process itself. One of them is the pressure-exchange system (PES) which is able to achieve considerably lower energy requirements in comparison to the conventional systems. The PES is suited for plants with a permeate production of more than 2000 m³/d. It enables a simple, variable adaptation of discharge volume and has, due to its design and direct transmission of high pressure from the brine to the feed, an efficiency of approximately 98%, with economic energy utilization and short amortization times. This proven system has ben used in the field of mining for more than 20 years, and with volume flows of up to 1400 m³/h and pressures of up to 16 MPa. On one of the Canary islands, a PES is presently under construction in a RO plant for seawater desalination with a permeate capacity of 5000 m³/d for demonstration purposes.

The optimization of the energy demand for the production of drinking water from seawater with RO contributes essentially to the reduction of operating cost and thus to lower prices for the drinking water produced in this type of desalination plant. One possibility is the installation of systems with lower energy demand than conventional plants based on energy recovery. In this regard, PES can contribute to reduce the energy requirements of RO units with a permeate production above 2 000 m³/d since the fluid pressure from the concentrate is directly transfered to the fluid pressure in the feed stream with an efficiency of approximately 98%. Short amortization times for the device itself allow for the installation of PES in new plants as well as for retrofitting of existing plants. A RO plant for the desalination of seawater with a capacity of 5000 m³/d permeate production, installed on one of the Canary islands, is presently equipped with a PES which will serve as a demonstration plant for this advanced technology.