Utilization of unrejected ammonium for fouling prevention in a forward osmosis process tested with real primary wastewater from Hong Kong

Abstract

Forward osmosis (FO) process is known to have high rejection towards foulants and ions. However, the polyamide surfaces of FO membranes are usually designed to be negatively charged, which results in low ammonium rejection. This study proposes to utilize the unrejected ammonium in the draw solution (DS) of FO processes by adding OCl− to generate monochloramine, which can diffuse back towards the feed side and prevent. Tests were performed to identify the optimal FO operating conditions, such as DS concentration, crossflow velocity, and flow direction. Optimal conditions were determined to be a DS concentration of 1.0 M, a crossflow velocity of 5.4 cm/s and counter-current flow mode. A 72 h fouling tests were also conducted with real wastewater from the Siu Ho Wan Sewage Treatment Works in Hong Kong as the feed. The existence of monochloramine alleviated membrane fouling. The novelty of this study lies on the fact that the chloramine is generated using the unrejected ammonia, and consequently utilizing the generated chloramine that is reversely diffusing towards the feed to prevent the fouling formation. Findings from this study suggest that the generation of monochloramine from unrejected ammonium ions can lengthen membrane life while endowing self-cleaning abilities that prevent severe fouling.