Biofouling on forward osmosis system

Fouling is an inevitable issue that all membrane systems have to face. The presence of membrane fouling causes membrane systems (such as reverse osmosis and forward osmosis) to suffer the increase of resistance thus reducing the efficiency of the systems. This raises concerns about the osmosis technology as it also reduces the system and membrane lifetime while increasing the maintenance costs. From previous papers and literature review, polysaccharides were found to be the main contributor to membrane fouling. The literature explains the polysaccharides that caused the membrane fouling were alginate, BSA, AHA, xanthan and others however, only alginate and xanthan were tested in this research project. The mixing interaction of other cations such as Ca2+ with some of the aforementioned polysaccharides (salt in the form of CaCl2 and NaCl were also tested to see the changes in fouling effects when both are combined. Throughout the experiments, a fixed amount of NaCl and CaCl2 and the polysaccharide were kept constant. The draw solution (NaCl mixed with DI water) was always retained to be saturated. These experiments were designed in this way to examine the differences between each polysaccharide and its combination towards fouling behaviour, since alginate and xanthan have different chemical characteristics. The results show that xanthan causes a higher resistance compared to alginate. In the case where NaCl and CaCl2 were present in the feed solution, the resistance of both polysaccharides greatly increases thus resulting in lowering the flux and ultimately decreasing the system efficiency. Out of all the experiments, the xanthan with salt resulted in highest flux decrease while the alginate only had the least flux decline (excluding the baseline experiment). Further analysis was done using the total organic carbon (TOC) and confocal laser scanning microscopy (CLSM). These examinations demonstrated the characteristics and properties of the polysaccharide layers that were formed on the membrane surface. The CLSM result was compared with the flux and resistance movement and it was found that they supported each other (and the findings were closely related). Since CLSM analysis is able to show the x, y and z dimension, the thickness can be found within each CLSM images. Therefore the thickness of the polysaccharide (fouling) layer (from CLSM images) was thick and/or dense, the (a higher resistance was achieved) higher the resistance would be and vice versa