Understanding of membrane fouling phenomena with experiments and simulations

• Main Author: Tanis Kanbur, Melike Begum
• Other Authors: Chew Jia Wei
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2020
• Subjects: Engineering::Chemical engineering::Chemical processes; Engineering::Environmental engineering::Water treatment
• Online Access: https://hdl.handle.net/10356/137580
• Institution: Nanyang Technological University

Dramatic increment of the population brings a significant challenge to meet the clean water demands for humanity; and therefore, wastewater treatment processes are pivotal to obtain clean water from the industrial and agricultural systems. Wastewater treatment generally aims to remove the contaminants and pollutants; then, the treated water can be re-used in the facilities. Oily wastewater is a widely encountered wastewater type and it is seen in different industrial sectors such as food industry, petrochemical industry, etc.; thus, their treatment processes have additional importance for the human health and sustainable earth. Currently, there are several wastewater treatment techniques such as solvent extraction, centrifugation, froth flotation, bioslurry, and oxidation, but their high operation costs, low efficiencies, and additional process requirements decrease the application feasibility of those systems in real life. To overcome the mentioned challenges, membranes, which are porous mediums that provide filtration, were developed in the oily wastewater treatment systems, especially for the low concentration oily wastewater streams. Although membrane technology brings big advantages such as low operation cost and high efficiency, membrane fouling is the biggest obstacle for its real-time operations. Membrane fouling is related to the decline of permeate flux over time, and it decreases the treatment efficiency. The pore size distribution of the membrane, operating conditions, and membrane-foulant interactions are the three main criteria that affect the membrane fouling phenomena; therefore, their detailed investigations can increase the membrane performance in real life. For this purpose, this thesis focuses on the understanding of membrane fouling phenomena by using experimental and computational techniques. The impact of pore size characteristics on the fouling phenomena is assessed by two different porosimetry techniques, which are Evapoporometry (EP) and Liquid-Liquid Displacement Porosimetry (LLDP). At first, five polymeric membranes and an inorganic membrane are investigated with EP and LLDP methods. The comparative study inferred that both methods provide close results to each other despite slight differences, and they can achieve accurate pore size characterization. It is also seen that LLDP provides data by measuring the flow-through pores whereas EP measures both dead-end and continuous pores. Following the outcomes of this comparative study, an improved method of EP, the Adaptive Evapoporometry (AEP), is developed to measure only the continuous pores, instead of combined measurement of dead-end and continuous pores. The EP theory, which is based on Kelvin equation, is improved via theoretical development and it is presented that the average pore size diameter and pore size distribution trends are roundly 10% less than the corresponding values of the EP technique. The impact of operating conditions on the fouling phenomena is analyzed via the Direct Observation Through Membrane (DOTM) method, which is a non-invasive in-situ observation method. Three different cross-flow velocities (0.1 – 0.4 m/s) and three different oil-in-water emulsion concentration (250, 500, 750 ppm) are used in the parametric analysis. The effects of surfactant charges are evaluated by using oil-in-water emulsions without surfactant, with SDS surfactant, and with DTAB surfactant in the first DOTM study. The study shows that the emulsion without surfactant has the highest critical flux, which is the desired purpose in real operations, while the low critical flux is observed for the emulsion with DTAB surfactant. In the second DOTM study, the different Tween surfactants are analyzed at various cross-flow velocities and oil-in-water emulsions. It is interestingly seen that the striping phenomena are observed at low velocities while there are no stripes at high cross-flow velocities. Apart from the experimental studies with porosimetry methods and in-situ observation, the molecular dynamics (MD) simulation is performed as a computational method in order to better understand the molecular interactions between membrane and water, water and foulant, and foulant and membrane, respectively. Interaction energy, radial distribution function, surface tension, mean-squared displacement, and hydrogen bonding are the main assessment criteria of the MD simulation. The MD study is carried out for the different surfactant study of DOTM, and it is seen that the presence of surfactant significantly affects the fouling behavior, and therefore the emulsion without surfactant presents the highest critical flux in the comparative study.