Biofilm and membrane biofouling control with microbubbles

• Main Author: Ashutosh Agarwal
• Other Authors: Liu Yu
• Format: Theses and Dissertations
• Language: English
• Published: 2014
• Subjects: DRNTU::Engineering::Environmental engineering::Water treatment
• Online Access: http://hdl.handle.net/10356/61673
• Institution: Nanyang Technological University

Bacterial attachment onto surfaces is the first step towards the formation of complex 3-dimensional biofilm, while it is also responsible for development of membrane biofouling which results in significant decline in water flux and increase in trans-membrane pressure. To mitigate membrane biofouling, physic-chemical cleaning methods have often been practiced. However, chemical cleaning of membrane not only degrade the membrane surface structure, but also generate carcinogenic byproducts upon reaction with the organic matters present in water. Hence, this study explored the potential of self-collapsing microbubbles (MBs) for biofilm/biofouling control. MBs with diameter less than 50 µm have been known for their ability to generate pressure waves through shrinking and subsequent self-collapsing phenomenon. The potential of air MBs for biofilm detachment from membrane surface in comparison to chemical cleaning with sodium hypochlorite (NaOCl) was investigated. It was found that about 88% of fixed biomass detachment was removed from membrane surface after 1 h air microbubbling, while only 10% of biofilm detachment was achieved in the control experiment without MBs. Microscopic observations clearly showed that nearly all extracellular polysaccharides and proteins in biofilms were removed from the membrane surface upon the treatment by MBs, indicating a complete disruption of the extracellular polymeric matrix of biofilms. It was further demonstrated that microbubbling is much more efficient than chemical cleaning with 0.5% NaOCl solution in terms of removal of fixed biomass, extracellular polysaccharides and proteins. Since the growth stage of biofilms closely determines their properties and structure, understanding of detachment of different age biofilms becomes crucial. In the second phase of this study we thus investigated the responses of different age biofilms grown on membrane surfaces towards self-collapsing air MBs. Changes in fixed biomass, extracellular polysaccharides and proteins were determined for 3-h, 12-h, 18-h and 24-h old biofilms before and after treatment with MBs. The resistance-in-series model was further applied for analysis of various resistances after treatment of different age biofilms with MBs. The results showed substantial flux recovery after 1-h MBs treatment for stationary phase biofilms in comparison with initial and exponential growth phase biofilms, which was consistent with the relatively larger percentage reduction in fixed biomass, extracellular polysaccharides and proteins for stationary phase biofilms. The experimental data were further supported by CLSM and SEM images. Collapsing MBs appear to be an alternative green chemical-free technology for mitigation of membrane biofouling. It appeared from the first two phases study that the forces generated through uncontrolled self-collapse of individual MBs would not generate forces strong enough, as a result, a long microbubbling time was essentially required for substantial detachment of biofilms. In the third phase of the study, we thus explored intermittent low-intensity ultrasonication-triggered MBs bursting for improving the biofilm removal. This was done in such a sequence of that MBs were continuously introduced into the reaction vessel for 15 min, while US was activated for 2 s after every 2 min of microbubbling. It was found that fixed biomass, extracellular proteins and polysaccharides of 24-h old biofilms grown on membrane surface were reduced respectively by 75%, 79% and 72% after the treatment by the proposed US+MBs method. Fourier transform infrared (FTIR) analysis further revealed that the chemical compositions of biofilms were not altered by the US+MBs treatment, suggesting biofilms were removed through physical forces due to the generation of shock wave and high-speed water jet through US-triggered bursting of MBs. The proposed method can be considered an efficient chemical-free technology for cleaning of biofilms over the use of self-collapsing MBs. In conclusion, this study clearly showed that collapsing microbubbles would be a new chemical-free alternative for mitigation of membrane biofouling.