|Ph.D Student||Ronen Avner|
|Subject||Influence of an Antibacterial Feed Spacer on Biofilm|
Development in Membrane Filtration Systems
|Department||Department of Civil and Environmental Engineering||Supervisors||Professor Carlos Dosoretz|
|Professor Emeritus Raphael Semiat|
|Full Thesis text|
Biofouling development in pressure driven-separation membranes is still a major limitation to their wide application in wastewater treatment. Biofouling eventually influences membrane performance and life-span, especially in the spiral-wound module configuration applied in reverse osmosis.
In the current thesis, the concept of suppressing biofouling formation using an antibacterial feed spacer as a generic approach with no influence on the qualities of the membrane, was investigated in a bench scale-cross flow system mimicking the flow conditions in a wound membrane configuration. Conditions of the experimental system were designed to promote biofilm development and performed at laminar flow (Re=200-300). Ultrafiltration membranes (200 kDa MWCO) were used for simplicity.
Two conceptual approaches for spacer modification were evaluated. The first, named 'long-range' antibacterial influence, meaning that antibacterial influence is not only due to direct contact but also acts away from the spacer, such as ion or hydroxyl radical release. The modification was done coating the surface of a commercial polypropylene feed spacer with silver or zinc nano-particles by sonochemical deposition. The second, named 'short-range' antibacterial influence, was based on direct contact with polymeric quaternary ammonium groups covalently bound to the polypropylene surface.
The antibacterial abilities of the modified spacers (MoSps) were evaluated in static conditions and all MoSps were able to reduce bacteria by several orders of magnitude in a relatively short time.
Two sets of flow-through experiments were designed to evaluate the ability of the MoSps to repress biofilm development on membranes. One aimed to evaluate potential biofilm development using a pure culture of Pseudomonas putida S-12. The second aimed to simulate field conditions used a mixed microbial culture enriched from a membrane bioreactor fed with domestic wastewater.
The physical properties of the MoSps and biofilm development were evaluated using high resolution/energy dispersive spectrometry-scanning electron microscopy (HRSEM-EDS), confocal laser scanning microscopy imaging (CLSM) and atomic force microscopy (AFM). HRSEM depicted significantly less bacteria attached to the membranes exposed to MoSps, mainly scattered in a sporadic monolayer structure. CLSM-dead-live staining assay indicated a higher percentage of dead bacteria on the membranes adjacent to MoSps in contrast to live bacteria found in the control samples. The MoSps reduced permeate flux decrease by at least 50% compared to unmodified spacers.
All in all, the influence of the 'long-range' antibacterial MoSps resulted somewhat more effective towards biofilm reduction than the 'short-range', although both were able to decrease biofouling and theoretically increase the life span of the membrane.