From Conventional Activated Sludge Process to Membrane-Aerated Biofilm Reactors: Scope, Applications, and Challenges

In the midst of increasing global production of domestic wastewater over the years, the treatment capacities did not show promising increments to keep up with it. Traditionally, biological treatment systems such as wetlands, conventional activated sludge (CAS), trickling filter processes, and rotating biological reactors were used to treat these wastewaters. The capital and operation and maintenance (O&M) costs of the process play a critical role in the final system selection. During the past decade, membrane bioreactor (MBR) has progressively replaced these biological wastewater treatment systems. For example, the most advanced form of MBRs called membrane-aerated biofilm reactors (MABRs) could be operated with higher energy efficiency of 70% compared to CAS process. Moreover, even at a low footprint, MBRs could achieve a high volume of treatment in existing area with records of up to 50% extra capacity. Following these MBR systems, the next technological innovation was membrane-aerated biofilm reactor (MABR), which uses the bubbleless aeration through the lumen of fiber membrane. The bubbleless aeration, in fact, assists the smooth growth of microorganisms compared to the bubbled aeration in CAS process which often interferes with the microbial growth in the system. Apart from providing diffused aeration, the membrane also serves as attachment medium for microorganisms that consume organics and nitrogen, thereby purifying the wastewater. Thus, within a single reactor, simultaneous nitrification and denitrification are achieved. The MABRs have been successful in the treatment of variety of pollutants such as landfill leachate, pharmaceutical wastewater, ammonia-rich wastewater, domestic wastewater, and anaerobic digestion liquor. In addition, their applications have flourished for the treatment of high carbon and nitrogen wastewater, volatile organic compounds, and xenobiotic components. However, the major limitation of this process is maintaining optimal biofilm thickness on the membrane surface and scaling-up mechanisms to real scale plants.