What is the purpose of an energy recovery device (ERD) in a reverse osmosis (RO) system?

The primary purpose of an energy recovery device (ERD) in a reverse osmosis (RO) system is to recover the energy contained in the high-pressure brine stream exiting the RO membranes and transfer it to the incoming feed water, thereby reducing the overall energy consumption of the desalination process. RO systems require high pressure to force water through the semi-permeable membranes, leaving behind the dissolved salts. After the water passes through the membranes, the remaining brine stream still retains much of this pressure. Without an ERD, this high-pressure brine would be simply throttled and discharged, wasting the energy used to pressurize it. ERDs capture this energy and use it to pre-pressurize the incoming feed water before it reaches the high-pressure pump. There are several types of ERDs, including pressure exchangers (PXs) and turbochargers. Pressure exchangers directly transfer pressure from the brine to the feed water using a rotating rotor. Turbochargers use the high-pressure brine to drive a turbine, which in turn drives a pump to pressurize the feed water. By recovering and reusing this energy, ERDs can significantly reduce the energy consumption of RO systems, often by 30-50%, leading to lower operating costs and a reduced environmental footprint. ERDs are particularly beneficial in large-scale seawater RO plants where the energy requirements are substantial.