Fouling due to CaSO4 scale formation in forward osmosis (FO), reverse osmosis (RO), and pressure assisted forward osmosis (PAFO)

Forward osmosis (FO) is a water treatment technique that utilizes a semi-permeable membrane to remove dissolved solutes from water by the use of a salinity gradient. Unlike reverse osmosis (RO), the driving force of FO is not the hydraulic pressure and thus FO does not need high energy to pressurize feed water. However, the efficiency of FO may be deteriorated by the occurrence of fouling induced by the precipitation of inorganic scales if the feed water includes salts such as CaCO3, CaSO4, or silica. The focus of this study was the investigation of FO membrane fouling caused by CaSO4 scale formation. The changes in flux, saturation degree, and the turbidity of the brine were measured as a function of operation time in a laboratory scale FO system. The fouling characteristics of FO were compared with those of RO and pressure-assisted FO (PAFO) under similar filtration conditions. A theoretical model considering different scale formation mechanisms was also applied to quantitatively analyze FO fouling.