Forward osmosis membrane fouling and cleaning for wastewater reuse

Membrane fouling properties and different physical cleaning methods for forward osmosis (FO) and reverse osmosis (RO) laboratory-scale ﬁ ltration systems were investigated. The membrane fouling, with respect to ﬂ ux reduction, was lower in FO than in RO when testing an activated sludge ef ﬂ uent. Cross-ﬂ ow velocity, air-scouring, osmotic backwashing and effect of a spacer were compared to determine the most effective cleaning method for FO. After a long period of fouling with activated sludge, the ﬂ ux was fully recovered in a short period of osmotic backwashing compared with cleaning by changing cross- ﬂ ow velocity and air-scouring. In this study, the osmotic backwashing was found to be the most ef ﬁ cient way to clean the FO membrane. The amount of RNA recovered from FO membranes was about twice that for RO membranes; biofouling could be more signi ﬁ cant in FO than in RO. However, the membrane fouling in FO was lower than that in RO. The spacer increased the ﬂ ux in FO with activated sludge liquor suspended solids of 2,500 mg/L, and there were effects of spacer on performance of FO – MBR membrane fouling. However, further studies are required to determine how the spacer geometry in ﬂ uences on the performance of the FO membrane.


INTRODUCTION
In pressure-driven membrane processes, such as microfiltration (MF), ultrafiltration (UF), nanofiltration and reverse osmosis (RO), water is filtered mainly by using hydraulic pressure as a driving force. The use of hydraulic pressure in such systems requires a large amount of energy and leads to significant fouling on the membrane surface.
Thus, the biggest challenge of pressure-driven membranes processes is control of fouling, which is inevitable, and many studies have consequently focused on the control of Over the past decade, there has been growing interest in osmotically driven membrane processes, such as forward osmosis (FO), as highly efficient, sustainable processes that can replace the pressure-driven membrane processes (McCutcheon et al. ; Lutchmiah et al. ). In the pressure-driven membrane process, water is filtered by applying hydraulic pressure; in the FO process, water is filtered by using osmotic pressure as the driving force. The main advantages of the FO process are no need for hydraulic pressure and a high removal rate exhibited for a wide range of contaminants (Cath et al. ), and lower susceptibility to membrane fouling than pressure-driven membrane processes (Mi & Elimelech ; Mi & Elimelech ; Kim et al. ). The FO process in water treatment is applicable to leachate and wastewater treatment, water reuse, brackish groundwater and seawater desalination (Kravath & Davis ; Cath et al. a, b; Phuntsho et al. ). Interest in FO technology has grown significantly as the commercialization of membranes designed for FO process is possible, but it has been slower for wastewater applications (Lutchmiah et al. ).
Membrane bioreactors (MBRs) are often used in processing wastewater for reuse. Activated sludge used for biological wastewater treatment processes has a high mixed liquor suspended solids (MLSS) content and extracellular polymeric substances and causes significant membrane fouling. Therefore, the membrane biofouling is an important factor when FO is considered for wastewater treatment. FO processes can be suitable for application in the MBRs because of the low membrane fouling characteristics (Achilli et al. ). The wastewater reuse process involves the serial connection of the membranes to a conventional activated sludge process, or the connection of the RO membrane to the MBR process. Integrated/hybrid membrane processes may be necessary to meet stringent water quality standards for water reuse and could reduce water scarcity (Ang et al. ).
The FO-MBR process uses the FO membrane instead of the MF or UF membranes that are commonly used in existing MBR systems. The FO-MBR process is an advanced technology for wastewater reuse; it requires low energy and has a good removal efficiency for organic matter (Cornelissen et al. ; Achilli et al. ). The major advantage of the FO-MBR process is its long filtration time due to the lower level of membrane fouling, long cycle of backwashing, high removal rate of foulants, and, in contrast to membranes with an angstrom pore size, its suitability for osmotic backwashing. There are two approaches to osmotic backwashing. Feed water may be substituted for the second draw solution at a higher concentration than the draw solution. The other approach is that the flow of the draw solution can be changed from one side (permeate side) of the membrane to the other (feed) side of the membrane, thus removing the foulants. FO membrane cleaning methods and steps, which are required for FO to reach wastewater application and water reclamation standards, have been reviewed recently (Lutchmiah et al. ; She et al. ). However, there are limited studies investigating cleaning methods such as cross-flow velocity, air-scouring, osmotic backwashing and use of spacers in wastewater reuse in FO and compared to RO.
Thus, the purpose of this study is to understand membrane fouling properties by evaluating physical cleaning methods in FO-MBR. We first analysed the fouling characteristics in FO and RO processes using activated sludge or secondary effluent. We then evaluated the cleaning efficiency of various physical cleaning methods, such as the shear force of increasing cross-flow velocity, air-scouring, osmotic backwashing and spacers.  Table 1.

FO membrane and surface characterization
The roughness of the membrane surface was determined by atomic force microscope (AFM) analysis (PUCOStation AFM, Surface Imaging Systems, Herzogenrath, Germany) and was quantified in terms of the root mean square (RMS) roughness, which is the RMS deviation of the peaks and

Model test solutions
Silica (SiO 2 ) particles in a powdered form (particle diameter 1-5 μm) were used as model particulate foulants. The silica particles were diluted with DI water to the concentration of 5,000 mg/L and were sonicated for 3 min to prevent particle aggregation before use.
Commercial Aldrich humic acid was used as a model col-  approximately 20 W C. The water bath temperature was maintained by circulating chilled water through a stainless-steel coil immersed in a water bath.

Osmotic backwashing
The FO membrane, which uses osmotic pressure without hydraulic pressure, is suitable for osmotic backwashing. In the operating mode, the draw solution flowed on the permeate side. Water is filtered from feed water by using the osmotic pressure itself as the driving force. In the cleaning mode, the flow of the draw solution is changed from one side (permeate side) of the membrane to the other (feed side), and cleaning solution that has a higher concentration than the draw solution flows on the feed side (Figure 2(a)).
Osmotic backwashing is a cleaning method involving a flow of water from permeate to feed side by means of osmotic pressure.

RNA analysis on feed water and fouled membranes
Total RNA was extracted from the samples of feed water and fouled membrane surface by using a PowerWater ® RNA iso-  effective prior to cake forming compaction (Mi & Elimelech 2008); therefore, increasing cross-flow velocities, as tested in this study for the FO membrane in which the cake layer had already formed, was not effective in recovering the flux.
Air-scouring was also performed at the cross-flow velocity of 17 cm/sec. The levels of air injection increased gradually in three steps (0%, 12.5% and 25%, air of the total channel volumes) and were applied to the membrane surface. There was no significant improvement in the flux recovery rate when the air injection increased from 0% to 12.5% channel air. However, there was some improvement in the flux when the air injection increased from 12.5% to 25% channel air.       (Figure 2(c)). The membrane surface in FO is expected to have a more loosely compressed cake layer than the membrane surface in RO. Therefore, the cake layer can be easily removed through a short period of physical cleaning, and the flux can recover. On the other hand, the membrane in the RO mode is not expected to be removed easily due to the compressed cake layer.

Effect of spacer on membrane fouling
The role of spacers in FO systems for membrane fouling was investigated: the foulants were 5,000 mg/L silica particles (diameter 1-5 μm) and 200 mg/L humic acid in DI water.  therefore, it was also necessary to test whether the spacer could affect the performance of FO in MBR systems using activated sludge. Figure 9 shows the spacer effect on the fouling of FO membrane using activated sludge of MLSS 2,500 mg/L: water flux decreased rapidly without the spacer but decreased gradually with a spacer, and the gradient flux decrease with a spacer was significantly different. Therefore, membrane fouling was influenced by the spacer in FO-MBR systems. However, further research needs to be carried out to determine how FO membrane performance is influenced by the spacer geometry and the role of the spacer at different MLSS concentrations. MBR systems often run under high concentrations of MLSS 2,500 to 10,000 mg/L; therefore, the effect of the spacer could be different.

CONCLUSIONS
The flux of RO for activated sludge and secondary effluent in a wastewater treatment plant was only half that for FO. The removal efficiency of RO for TN, NH4-N and TP was lower than that of FO in the same initial flux conditions. Microbe activity, as measured by RNA, for FO was about double that for RO, and the biofouling could be more significant in FO than that in RO. However, the membrane fouling in FO was lower than that in RO, as shown in the flux reduction.
A number of physical cleaning methods were compared in order to discover the most effective cleaning method to restore the performance of FO. For the air-scouring method, there was a rapid improvement in the cleaning,