Workshop Speech: Seawater Desalination by RO membrane

Abstract

Among desalination technologies available today, RO is regarded as the most economical desalination process for wide range of water salinity. The development of crosslinked aromatic polyamide composite membrane in 1980's was a significant affair, and many types of membranes, such as low-pressure types for brackish water desalination and high-pressure types for seawater desalination, have been developed after that. The permeate flow rate and water quality of RO membranes have been greatly improved, especially high pressure type SWRO membranes have enabled us to carry out the 60% of high recovery rate operation and low cost seawater desalination, which is called Brine Conversion Two Stage Seawater Desalination System and they have been already running in the world. Recently the development of high-performance SWRO membrane is required since the regulation for water quality is getting severer and severer. In particular, boron removal is one of the most important issues to be conquered in seawater desalination. WHO has established severe guideline value in the boron concentration in drinking water because boron is known to kill citrus trees and cause infertility. Boron exists as boric acid in seawater, and its concentration is 4 to 7 ppm. Boric acid is a very small molecule, which has about 0.4 nm in diameter because it is not ionized and not hydrated in a neutral region. So boron is the typical substance, which is difficult to be removed by RO membrane.