Application of ultrasound to mitigate calcium sulfate scaling and colloidal fouling

• CaSO4 scaling in FO process is almost fully reversible. However, colloidal fouling causes more severe flux decline and is harder to clean than CaSO4 scaling.
• Ultrasound and hydraulic pressure increased initial permeate flux. The permeate flux decline rates in the experiments of the CaSO4 scaling and colloidal fouling decreased when FO was assisted by ultrasound.
• When FO assisted by pressure and ultrasound together, synergy effects of initial flux increase were observed in FO performance. Flux decline rate by CaSO4 scaling and colloidal fouling was minimized.

Forward osmosis (FO) has been considered a novel technology for desalination. However, one of the major problems is permeate flux decline by fouling. This paper investigates the mitigation of calcium sulfate (CaSO4) and colloidal (particulate) fouling in FO process by application of ultrasound (and/or pressure). Saturated CaSO4 solution of 2000 mg/L and silica (SiO2) colloid of 5000 mg/L with average diameters of 10–20 nm were used as a model scalant and a model colloidal foulant, respectively. The results showed that CaSO4 scaling in FO was almost fully reversible. On the other hand, colloidal fouling caused more severe flux decline and was challenging to clean. The application of pressure (5 bar) to feed side, increased the initial flux by approximately 20% but subsequently resulted in a higher flux decline. Combination of ultrasound and hydraulic pressure to the FO system showed promising result (a significant increase of permeate flux and lower flux decline by CaSO4 scaling and colloidal fouling).