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Authors: JIN LE
Keywords: backwash, biofilm carrier, membrane fouling, pore size, SCMBR, surface roughness
Issue Date: 18-Jan-2011
Abstract: Membrane fouling, the key disadvantage that inevitably occurs continuously in the membrane bioreactor (MBR), baffles the wide-scale application of MBR. Ceramic membrane, which possesses higher chemical, biological, mechanical and thermal resistance compared to the polymeric counterparts, has seldom been used in MBR to treat municipal wastewater due to the high price involved in membrane fabrication process. Four ceramic membranes with the same materials but different pore size, ranging from 80 to 300 nm, were studied in parallel using four lab-scale submerged MBRs in Phase 1 (i.e., one type of ceramic membrane in one MBR). Total chemical oxygen demand (COD) and ammonia nitrogen removal efficiencies were observed to be consistently above 94.5% and 98%, respectively, in all submerged ceramic membrane bioreactors. The experimental results showed that fouling was mainly affected by membrane¿s microstructure, surface roughness and membrane pore size. An in-depth study of fouling mechanisms for different pore-sized ceramic membranes in MBR were conducted in Phase 2 with four different pore¿sized membranes being placed in one SCMBR in order to have a same mixed liquor condition for all the membranes. Results showed that M80 was able to reject more organics and experienced more serious initial fouling within the first 7 days. However, from Day 10 onwards, significant TMP increase was observed for all the membranes, where M300 has the highest TMP increase rate (dp/dt) of 2.31 and M80 has the lowest dp/dt of 0.96. Based on the LC-OCD, EEM spectra and HPLC results of the membrane permeates and mixed liquor supernatant of the sample after 1 h and 14 d operation duration, it was found that a bioflm/biocake layer was formed on the membrane surface after a period of time. This biofilm/biocake layer enhanced the organic rejection for ceramic membranes, especially for M300. The biofilm/biocake development on the membrane surface was also monitored by scanning the membrane surface using a high-resolution scanner at each sampling point. Obtained images analyzed by ISA-2 software showed that the overall biofilm/biocake coverage ratio increase rate on the membrane surface was related to the membrane surface roughness, which was M300 > M100 > M200 > M80. This finding indicates that the biofim/biocake coverage ratio could be a good indicator for membrane fouling. Membrane fouling has been noted as the main obstacle for the widespread application of MBR. In order to alleviate membrane fouling, in Part 1 of Phase 3 a porous suspended biofilm carrier was introduced into submerged ceramic membrane bioreactors (SCMBR) treating domestic wastewater. Results showed that biofilm carrier addition was very efficient in mitigate cake formation on the membrane surface, resulting in lesser fouling in SCMBR with carrier. SCMBR without carriers had 5 times higher cake resistance and 2.5 times higher total resistance than that of the one with carriers. In addition to the biofilm carrier addition, three times higher backwash flux than that of the membrane permeate was adopted in R2 (with backwash) in Part 2 of Phase 3. Results showed that backwash applied was very efficient in alleviating membrane fouling by partially removing the biocake/biofilm on the membrane surface and foulants in the membrane pores.
Appears in Collections:Ph.D Theses (Open)

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