Evaluation of forward osmotic backwashing of cleaning efficiency and fouling characteristics according to net driving pressure of seawater reverse osmosis (SWRO) processes.

Contrary to general microfiltration (MF) and ultrafiltration (UF), reverse osmosis (RO) processes are affected by various osmotic pressures according to the raw water characteristics because they use semi-permeable membranes. Thus, the net driving pressure applied when operating RO processes changes. This net driving pressure greatly affects energy consumption and operating efficiency and particularly changes fouling tendencies of RO process. In this paper, seawater reverse osmosis (SWRO) processes were operated by changing the salt concentration of raw water and operating pressure to analyze the fouling characteristics at various net driving pressures. Here, the result of the experiment showed that, the flux increased with increasing net driving pressure. When fouling occurred, the operation time was reduced to a flux decline rate (FDR) of 10% according to the net driving pressure, and the accumulation of organic matter on the membrane surface increased. The fouling index increased with increasing net driving pressure, and the specific cake resistance decreased. The fouling layers formed by this decreased cake resistance were considered easy to remove for a short period by cleaning because they had a slightly loose structure. By performing forward osmotic backwashing (FOB) which is a maintenance cleaning methods for RO processes, the cleaning efficiency and organic matter removal efficiency increased in proportion to the net driving pressure. It was confirmed that the organic matter removal efficiency was higher than the cleaning efficiency because most of the fouling layers had a loose structure. In addition, the organic matter forming dense fouling layers that decreased the actual permeate flux was small. The fouling index decreased after FOB, and the specific cake resistance increased. This rendered it easier to remove fouling layers that had a loose structure and low density in FOB and to clean the fouling layers formed at high net driving pressure. [ABSTRACT FROM AUTHOR]