A Short Review of Membrane Fouling in Forward Osmosis Processes

Abstract

Interest in forward osmosis (FO) research has rapidly increased in the last decade due to problems of water and energy scarcity. FO processes have been used in many applications, including wastewater reclamation, desalination, energy production, fertigation, and food and pharmaceutical processing. However, the inherent disadvantages of FO, such as lower permeate water flux compared to pressure driven membrane processes, concentration polarisation (CP), reverse salt diffusion, the energy consumption of draw solution recovery and issues of membrane fouling have restricted its industrial applications. This paper focuses on the fouling phenomena of FO processes in different areas, including organic, inorganic and biological categories, for better understanding of this long-standing issue in membrane processes. Furthermore, membrane fouling monitoring and mitigation strategies are reviewed.

Keywords: forward osmosis; fouling monitoring; membrane cleaning; membrane fouling; membrane surface modification.

Figure 1

Increase in publications on forward osmosis between 2005 and 2016. The number of publications is calculated based on the Scopus database using keywords “forward osmosis” and “pressure retarded osmosis”.

Figure 2

Illustration of FO, PRO, and RO processes: (a) FO process where no pressure is applied on the high concentration solution. Water flows from the low concentration side to the high concentration side; (b) PRO process where applied pressure on the high concentration solution is less than the osmotic pressure difference across the membrane. Water flows from the low concentration side to the high concentration side; (c) RO process where applied pressure on the high concentration solution is greater than the osmotic pressure difference across the membrane. Water flows from the high concentration side to the low concentration side; (d) Classification of FO, PRO, and RO in a flux versus pressure plot. Reprinted from [1,25,26], with permission from Elsevier.

Figure 3

Illustration of both internal concentration polarisation (ICP) and external concentration polarisation (ECP) through an asymmetric FO membrane in (a) active layer facing the feed solution (AL-FS) and (b) active layer facing the draw solution (AL-DS) orientations. ICP occurs within the membrane support layer, and ECP exists at the surface of the membrane active layer. C_feed, C_draw, $\Delta {\pi }_{eff}$, J_s and J_w represent the feed solution concentration, draw solution concentration, effective driving force, reverse salt flux and water flux, respectively. Reprinted from [40] with permission from Elsevier.

Figure 4

Schematic illustration of the fouling and cleaning with and without hydraulic pressure. (a) Loose alginate fouling layer due to the lack of hydraulic pressure, which permits effective physical cleaning; (b) Compact alginate fouling layer formed under hydraulic pressure, which results in low cleaning efficiency. Reprinted from [5], with permission from Elsevier.

Figure 5

Illustration of (a) A smoother and non-porous FO membrane dense layer with less fouling; and (b) A rougher and more porous FO membrane porous layer with greater pore plugging and subsequent fouling. Reprinted from [66], with permission from Elsevier.

Figure 6

Confocal laser scanning microscopy (CLSM) orthogonal view of Pseudomonas aeruginosa biofilm structures developed on (a) FO and (b) RO membranes (scanning electron microscopy, SEM image) after biofouling for 24 h. The RO biofilm morphology followed the deformed membrane structure, as observed in the cross-sectional SEM images. The top insets are matching enlargements of the biofilm layer in side view; (c) Schematic illustration of biofouling formation in FO and RO processes, respectively. Reprinted from [87], with permission from Elsevier.

Figure 7

(a) A photograph of the developed membrane fouling simulator (MFS), external dimensions of 7 cm × 30 cm × 4 cm); (b) Biomass concentration over the length of the MFS in comparison with the test rig and full scale module. Reprinted from [95], with permission from Elsevier.

Figure 8

(a) Optical coherence tomography (OCT) 2-D image of a grown biofilm on a membrane surface without feed spacer in an area of a 3.83 mm × 0.85 mm. The membrane is shown at the bottom of the figure. The biofilm had a heterogeneous structure containing voids; (b) 3-D reconstruction of a biofilm grown on the surface of a membrane and feed spacer in a flow cell; the image was obtained after processing OCT 3-D scans in an area of 6 mm × 6 mm × 1.08 mm. Spacer filaments contained most of the biomass detected; (c) Spatial distribution of oxygen concentration (mg/L) at the inlet side of the MFS on day 0 and after 5 days of biofilm development. The arrow indicates the water flow direction. The scale bar represents oxygen concentration (mg/L). The imaged area is 4.0 mm × 3.5 cm. Biofilm accumulation started on the feed spacer. Reprinted from [96], with permission from Taylor & Francis Online.