A fundamental study of membrane fouling using a novel microfluidic filtration system

Abstract

Membrane filtration has been widely employed in dairy manufacturing, wastewater treatment, and beverage clarification. However, the issue of membrane fouling significantly reduces the filtration performance by increasing the capital and operating costs. The mechanisms of membrane fouling during filtration processes are yet to be fully understood. This thesis details the development and application of a novel microfluidic filtration system for directly visualizing and measuring membrane fouling. Detailed, real-time characterizations of particle deposition during crossflow filtration have been examined over a range of solution properties and operating conditions. An empirical model considering both effects of interactions and hydrodynamics has been developed for describing the dynamic deposition behavior.

This thesis comprises three main sections of work. First is a description of the development of a novel microfluidic filtration system for studying membrane fouling. The system incorporates microfluidics, confocal microscopy, and a dual syringe flow control setup. Time-resolved, high-resolution images of particles on the membrane surface and three-dimensional images of the filtration channel can be obtained. The effectiveness of the system was demonstrated by using model particles. Analysis of images obtained from the system enabled the particle deposition behavior to be described in relation to dynamic surface coverage and particle deposition volume.

Secondly, the effect of solution properties on membrane fouling has been studied in order to assess the effect of the key fundamental physical parameters on membrane fouling. Detailed, real-time deposition processes during crossflow filtration of 0.4 μm model particles was investigated in relation to variations in pH, ionic strength, and particle feed concentration. The detailed structures and distributions of the deposited layer were reported. Experimental results were interpreted using DLVO theory, Van der Waals attraction energy, and a modified Hogg-Healy-Fuersteneau (HHF) formula combined to account for the interplay between membrane-particle interactions and particle-particle interactions to understand the overall deposition behaviour. Overall, solution properties significantly influence the dynamic deposition process. The initial deposition behaviour was primarily governed by membrane-particle interactions, while long-term deposition was largely determined by particle-particle interactions.

Thirdly, the role of operating conditions on membrane fouling during crossflow filtration was investigated by varying permeate flux/velocity and crossflow velocity. Their influence on membrane fouling was characterized by the deposition probability, which can be determined from hydrodynamic forces exerted on particles in the crossflow. Building on the experimental results obtained from various solution properties and operating conditions, an empirical correlation was developed for describing the dynamic deposition volume in terms of interaction energies, deposition probability, and filtration time. A greater amount of deposition was observed with greater permeate flux and lower crossflow velocity. A critical crossflow velocity was observed, over which, particle deposition significantly decreased as velocity increased. A correlation between the deposition behaviour and both the solution properties and operating conditions was developed.

In conclusion, the novel microfluidic filtration system developed in this work, is used in a comprehensive experimental program to provide detailed measurements of membrane fouling over a range of solution properties and operating conditions that revealed new insights into particle deposition. The overall deposition behaviour is found to be governed by the fundamental interactions of membrane-particle and particle-particle (primarily influenced by varying pH and ionic strength), and hydrodynamic forces acting on particles (primarily influenced by varying crossflow velocity and permeate flux). Membrane-particle interactions had more impact on the initial deposition behaviour, since for short-time filtration, particles directly deposit on the membrane surface. As filtration time increased, the membrane surface occupied by deposited particles increases. New arriving particles tent to deposit on the already-deposited-particles, thus particle-particle interactions affected the long-term deposition behaviour. The influence of hydrodynamic forces on particle deposition is characterized by the deposition probability. An empirical correlation was developed and the predictions shown to be in good experimental results. It has been the first time for a correlation to describe the dynamic deposition behaviour integrating both the influence of interaction energies and hydrodynamic forces, which can be used for setting up filtration parameters and better predicting the fouling process over a range of conditions.