Real time visual characterization of membrane fouling and cleaning

The research presented in this thesis is focused on the phenomena of membrane fouling. Membrane fouling is simply described as the deposition of unwanted matter on the membrane surface during the course of operation, which results in reduction in filtration efficiency. This research is aimed at visually characterizing membrane fouling as well as fouling removal. Typical fouling characterization techniques are either invasive or non-visual. In a bid to better characterize membrane fouling, a novel technique was developed whereby fouling can be observed visually and in real time. Chapter 2 describes the method, with the fabrication of embedded channel membranes using phase separation micromolding (PSμM). Using a model feed solution containing 6 μm polystyrene particles, deposition on the membrane surface was studied. It was observed that initially, there is the build-up of cake towards the channel exit and with increasing local resistance, there is a change in hydrodynamics leading to build-up closer to the channel entrance. To better describe “normal” feed solutions, chapter 3 describes study on fouling using a feed solution containing bidisperse suspension (3.3 and 5.7 μm) of polystyrene particles. Increasing the fraction of larger particles in the suspension resulted in an initial reduction in cake porosity leading to a minimum at number fraction of 0.5. On the other hand, the specific cake resistance showed a continuing decline with addition of larger particles. These results compared favourably with theoretical calculations of cake porosity (using Tokumitsu’s method) and specific cake resistance (using Kozeny Carman relation). However, the experimental results gave lower absolute values which could be due to better ordering of the cake closer to the membrane. There have been reports of feed spacers contributing to (bio)fouling. This is interesting considering that spacers are used in spiral wound modules as “surface shear generators”. In chapter four, we study the flow and (bio)fouling around micro structured membranes designed to mimic spacer nodes. It was observed that the biofilm formation and growth occurred upstream of these structures contrary to reports in literature. Particulate fouling of the structures suggested that biofilm initiation is similar to particulate deposition whereby biomass on contacting the structure, adheres and grows leading to a biofilm. Air sparging is a technique which has been applied in industries for the removal of (bio)fouling from the spiral wound module. In chapter five, a novel sparging technique was compared to typical sparging and forward flush in terms of removal of biofouling from membrane/spacer channels. Dissolved CO2 was applied as a cleaning agent with nucleation of the gas within the channels due to pressure drop as well as imperfections on the spacer surface acting as nucleation sites. It was observed that forward flush resulted in 40 % restoration of channel resistance while the water/N2 sparging resulted in 80 % restoration. Water/CO2 nucleation resulted in 100 % restoration of the channel resistance due to better distribution of the bubbles within the channel.