Understanding oil fouling in forward osmosis process

Abstract

Oil-contaminated water has been produced in domestic and industrial sectors and regarded as one of main water contaminants. Nowadays, membrane-based technologies, particularly a forward osmosis (FO) process have applied in oil-contaminated water separation due to their small footprint, low fouling propensity, and high rejections as well as high water recovery. Although there have been many studies on the oil fouling, it is still the major problem in FO processes due to the lack of proper information related to the build-up and growth mechanism of oil fouling layer on FO membrane surfaces. In this regard, the investigation of fouling mechanisms of oil droplets in the FO process is important to further advance our standing of the FO membrane performance for treating oil-contaminated water. Therefore, the major objective of this research was to understand the build-up and growth mechanisms of oil fouling during the FO process, and to mitigate the oil fouling. To achieve these objectives, the thesis are consisted with the following topics: i) the development of a direct microscopic observation method for the characterization of oil fouling, ii) the investigation and evaluation of the oil fouling mechanism under the various operating conditions and solution chemistries, and iii) the effective strategies for the minimization of oil fouling. A laboratory-scale membrane FO cell was constructed and operated with artificial oil-contaminated wastewater using fluorescent-dyed oil droplets to enable the direct microscopic observation of the attachment of oil droplets. The attachment efficiency derived from direct microscopic observation experiments were calculated in various operating conditions, and the results were combined with a force balance model including permeate drag force ($F_D$), gravity force ($F_G$), crossflow lift force ($F_L$), and interaction force ($F_I$) to elucidate the key physicochemical parameters governing the attachment of oil droplets on the polymeric FO membrane. From the observation, it was found that the flux decline was directly related to the surface coverage of oil droplets and the oil droplets appeared to attach randomly and individually onto the membrane surface. As the increased attachment of oil droplets, individual oil droplets coalesced with other droplets, and finally formed a thicker oil layer on the membrane surface. The decrease in permeate drag force and increased shear force by decreasing the permeation velocity and increasing the crossflow velocity, respectively, caused less oil coverage , thus, resulted the less flux decline. The increase of negative charges both from oil droplets and membrane surfaces by increasing solution pH, increasing anionic surfactant concentration, and reducing $CaCl_2$ concentration provided less oil coverage due to the increase of electrical double layer repulsion between both oil-oil and oil-membrane. Interestingly, fouling layer composed with hydrophilic and negatively charged organic matters such as natural organic matter (NOM) provided less oil fouling due to the increase of electrostatic and acid-base repulsion between oil-membrane explained by XDLVO theory. For oil fouling mitigation, physical and chemical cleanings could not effectively remove the oil droplets in FO process, but high crossflow velocity during chemical cleaning provided more oil coverage and let to more flux decline. The results from this study provided deep insights into oil fouling formation and its dependence on operating conditions and solution chemistries. The methods developed in this study can potentially be useful for a further advance understanding of FO membrane performance in the field of oil-contaminated wastewater treatment.