Assessing the performance of the membrane distillation bioreactor (MDBR) for wastewater reclamation

• Main Author: Goh, Shuwen
• Other Authors: Anthony Gordon Fane
• Format: Theses and Dissertations
• Language: English
• Published: 2014
• Subjects: DRNTU::Engineering::Civil engineering::Water resources
• Online Access: http://hdl.handle.net/10356/56101
• Institution: Nanyang Technological University

The membrane distillation bioreactor (MDBR) is a high retention system and has the potential to produce high quality water. It is a relatively new technology and there are limited studies on the MDBR. Thus, this thesis investigated membrane wetting and fouling phenomenon in the MDBR, their impact on MD performance and the effectiveness of existing control methods for mitigating fouling. Compared to the MD system, the biodegradation of organic carbon and nitrogen (particularly protein) led to 1.7~3.6 times delay in membrane wetting in the MDBR system. Greater deposition of organics onto the hydrophobic membrane surface reduced the hydrophobicity of the MD membrane, resulting in faster membrane wetting in the MD system. Prior to membrane wetting, the overall organic carbon removal efficiency in the MDBR system was as high as 99.9%. Greater fouling in the MDBR accounted for the lower flux (8% lower than that observed in MD). Like the MBR system, the fouling layer could exert a mass transfer resistance to flow and may cause some degree of pore closure or obstruction. Unlike the MBR system, the fouling layer could also provide some degree of heat transfer resistance in the MDBR system. Since the biofouling layers observed in this thesis are typically less than 20 µm, the heat transfer resistances exerted by such thin layers are unlikely to be the major reason contributing to the significant flux declines observed during fouling in MD/MDBR systems. The driving force in the MD system is vapour pressure gradient. It has been shown for the first time that the hydrophilic microporous structure of a deposit layer (biofilm) with pore size distribution (PSD) in the 10 nm range can reduce vapor-pressure by as much as 20-36%. This has been explained by the Kelvin effect. Thus the biofouling in an MDBR could lead to flux decline by mass transfer and thermal resistances and by decreasing the vapor pressure driving force. Methods which had shown success in flux improvement and fouling control proved effective in the MDBR. Measures such as periodic membrane cleaning and increase in air flow rate minimized fouling in the MDBR and successfully improved the MDBR flux to 10 L/m2.h, which was maintained for over 130 hours. The findings from this thesis, particularly on the fouling mechanism, its effect on the MDBR flux and fouling control, are of relevance to other MD applications such as seawater desalination. The novel approach employed for biofilm characterization by evapoporometry would be useful for studying fouling in other membrane operations, for example, in reverse osmosis and MBRs. Lastly, the brief studies conducted on areas such as biological nitrogen removal provided information useful for MDBR operation and identifying areas for future work.