## 2 MBR Types and Configuration  
Both academia and industries research and development (R & D) efforts are focused on aerobic and anaerobic membrane bioreactor (AnMBR), enzymatic membrane bioreactor and baffled membrane bioreactor [53]. In aerobic MBRs oxygen from supplied air acts as an essential medium for the bacterial growth while anaerobic is done without oxygen (no external air supplied). This leads to different bacteria strain in aerobic and anaerobic processes. Anaerobic process can easily be optimized for wastewater having high organic loading. Yet, maintaining low temperature for huge feed volume in mesophilic range poses a challenge. Moreover, anaerobic processes are not as efficient for high chemical oxygen demand (COD) removal as well as they exhibit more fouling potential than aerobic MBRs [54]. In wastewater treatment organic and nutrient removal are essential. For total nitrogen (TN) removal, denitrification was performed. To achieve TN removal denitrification requires anoxic conditions so anoxic tank is placed before or after aerobic tank. In order to achieve enhanced biologic phosphorus removal, anaerobic tank is incorporated in treatment train [55].  
The two mains basic MBR configurations involve either submerged membranes or external circulation (side-stream configuration) (Figure 2). Submerged MBR configuration operates under subatmospheric pressure instead of hydraulic pressure. In this arrangement, membrane is placed inside the bioreactor and it is known as submerged MBR. In this design, pure water is obtained through MF or ultrafiltration UF membranes from mixed liquor and process is operated under low hydraulic pressure. The second configuration is also known as side stream or external cross flow MBR in which hydraulic pressure is applied. The membrane unit is isolated from bioreactor and an additional recirculation pump is employed to circulate bioreactor mixed liquor forced through membrane and pure permeate is obtained [52]. The side stream configuration deliberately separates bioreactor from external membrane thus reducing membrane maintenance. However, operating cost increases due to mixed liquor recirculation pump installation [54]. For this submerged configuration, flat sheet (FS) and hollow fiber (HF) membranes are ideal choices [52; 56]. The submerged MBR process has less operating expenditure (OPEXm) due to elimination of mixed liquor recirculation pump than the side stream MBR system and this makes submerged membrane bioreactor (SMBR) as attractive option in wastewater treatment [54; 55]. The membrane fouling involves deposition of impurities such as sludge flocs, colloidal particles and inorganic solutes, into membrane pores and onto the membrane surface and forms cake layer [57]. Membrane fouling simply incurs additional operational and repair costs due to rapid pressure drop, increased cleaning cycles and consumption of chemicals and it deteriorates permeate quality and reduces quantity (flux) [58]. Due to membrane fouling permeate flux decreases with time and fouling leads to frequent cleaning which incurs operating cost and process downtime [57].  
Figure 2: Configurations of a membrane bioreactor: (**a**) immersed and (**b**) sidestream [59]. Reproduced with permission from [59], Copyright Journal of Membrane Science, 2011.