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.
본 학위논문에서는 실험실 규모 정삼투 막에 형광 염색한 유분오염 폐수 모사용액을 투과시켜, 실시간으로 유분의 막에 흡착을 관찰하였고, 수투과도 감소와 연관지어 유분 막오염 기작을 밝히고자 하였다. 현미경 관찰을 통해 운영 조건, 용액 조성 및 막 표면 특성을 변화시키는 등, 다양한 운영조건에서 유분의 정삼투 막에 부착 효율을 계산하였고, 이를 힘평형 모델에 적용하여 유분 부착에 주요한 영향을 미치는 물리화학적 변수를 밝히고자 하였다. 유분이 막에 부착할수록, 각각의 유분이 다른 유분과 합쳐지고, 결과적으로 막표면에 더 두꺼운 유분 막을 형성함을 관찰하여, 수투과도 감소는 유분의 표면 덮힘과 관련 있음을 밝혀냈다.
정삼투 공정 운영 조건에 있어, 투과속도 감소로 인한 투과힘의 감소 및 막면유속 증가로 인한 전단력의 증가는 유분 덮힘 현상을 감소시켰으며, 이로 인해 수투과도 감소가 지연되었다. 하지만 유분 막오염 세정 시에, 높은 막면유속은 더 많은 유분 덮힘 현상을 야기하였으며, 이는 수투과도 감소로 이어졌다. 용액 조성이 유분 막오염에 미치는 영향을 살펴본 결과, 용액 pH 증가로 인한 음전화의 증가, 음이온 계면활성제 농도 증가 및 $CaCl_2$ 농도 감소는 유분-유분 및 유분-막 간의 전기 이중층의 반발력 증가로 유분 덮힘 현상을 감소시켰다. 정삼투 막 표면 특성의 경우, 친수성 및 음전하의 유기물의 막오염 층은 XDLVO 이론으로 설명되는 유분-막간의 정전기적 및 산-염기 반발력의 증가로 유분 막오염 현상을 저감시켰다. 본 연구 결과를 통해 정삼투 공정에의 유분 막오염의 형성 및 유분 막오염 현상의 운영조건, 용액 조성 및 막 표면 특성에 대한 영향을 확인할 수 있었다. 또한 본 연구에서 개발한 분석방법은 정삼투 막에서 유분 오염 폐수를 처리할 때에 막오염 현상을 이해하는데 활용될 수 있을 것으로 기대된다.
