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Membrane bioreactor (MBR) technology is considered a well-established, mature technology with many full-scale plants around the world treating municipal and industrial wastewater. However, membrane fouling and energy consumption still remain serious obstacles and challenges in the wider spread of the MBR technology. Therefore, considerable research and development efforts are still underway. Recent developments are primarily focused on aspects related to energy reduction, fouling control and novel configurations for enhanced process performance. This review addresses the recent work on the above mentioned aspects and it discusses the overall life cycle of MBRs and the market prospects for MBR technology. Novel MBR configurations and integrations with other technologies are also reviewed. Finally, the challenges that need to be addressed in order to facilitate market penetration of MBR technology are highlighted.

Nitrous oxide (NO) is an important pollutant which is emitted during the biological nutrient removal (BNR) processes of wastewater treatment. Since it has a greenhouse effect which is 265 times higher than carbon dioxide, even relatively small amounts can result in a significant carbon footprint. Biological nitrogen (N) removal conventionally occurs with nitrification/denitrification, yet also through advanced processes such as nitritation/denitritation and completely autotrophic N-removal. The microbial pathways leading to the NO emission include hydroxylamine oxidation and nitrifier denitrification, both activated by ammonia oxidizing bacteria, and heterotrophic denitrification. In this work, a critical review of the existing literature on NO emissions during BNR is presented focusing on the most contributing parameters. Various factors increasing the NO emissions either per se or combined are identified: low dissolved oxygen, high nitrite accumulation, low chemical oxygen demand to nitrogen ratio, slow growth of denitrifying bacteria, uncontrolled pH and temperature. However, there is no common pattern in reporting the NO generation amongst the cited studies, a fact that complicates its evaluation. When simulating NO emissions, all microbial pathways along with the potential contribution of abiotic NO production during wastewater treatment at different dissolved oxygen/nitrite levels should be considered. The undeniable validation of the robustness of such models calls for reliable quantification techniques which simultaneously describe dissolved and gaseous NO dynamics. Thus, the choice of the N-removal process, the optimal selection of operational parameters and the establishment of validated dynamic models combining multiple NO pathways are essential for studying the emissions mitigation.

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