## 3 High Retention Membrane bioreactors (HRMBR)  
Organic microppollutants (OMPs) have been largely revealed in sewage, hospital and industrial wastewater at concentrations of up to several micrograms per liter [93]. Though many reports claimed more efficient and reliable OMP removal in MBRs than activated sludge process. However, persistent, high molecular weight hydrophilic OMPs still showed poor removal in MBR treatment [93; 94]. While reverse osmosis can efficiently remove low molecular weight compounds, the OMPs rejected by this technique (RO concentrate) are not always recycled to bioreactor (MBR) to achieve more degradation. Thus, the organic retention time (ORT) of those microppollutants are not independent of the HRT of the MBR process [35].  
Figure 3: Effect of functional groups and hydrophobicity on the removal of OMPs in the AnMBR [89]. Reproduced with permission from [89], Copyright Water Research, 2014.  
Recent progress in advancing sewage purification and reclamation motivated research efforts towards novel technologies of high retention membrane bioreactors (HRMBRs) that showcased advanced wastewater treatment technology. Instead of deploying multiple treatment processes, water reclamation could be achieved in a HRMBR offers small footprint [95]. These mainly include the integration of NF, FO and MD membranes to the conventional MBRs. Some of them are osmotic membrane bioreactor (OMBR), membrane distillation bioreactors (MDBR), bio-electrochemical membrane reactor (BEMR) which can be an efficient and a safer'multiple-barrier approach' in sewage treatment and specifically to achieve high removal of OMPs [81; 82; 95]. In addition to achieving the rejection and prolonging the retention time of refractory OMPs for further biodegradation, HRMBRs are less energy intensive due to natural osmosis phenomena thus alleviating the greenhouse gas (GHG) emission issue during sewage reclamation process [35; 95]. [PERSON] et al. (2018) appraised that OMP showed varying removal efficiency for different membrane technologies (0-100% removal). For instance, carbamazepine an anticonvulsant medication compound is quite persistent, which showed, less than 20% with removal in conventional sludge process and MBR [52]. However, integrated MBR-NF MBR-RO system accomplished 93% and 99% removal, respectively. In recent years both academia and industry shown increasing attention with regards to development of HRMBR [95].  
The combined MBR can produce better quality permeate, lessen membrane fouling and thereby reduced cleaning cycles [82]. Nonetheless, HRMBR technology has certain drawbacks that need to be resolved prior to commercialization. Salt accumulation due to reverse salt diffusion from draw side to feed side, flux decline with time due to salt accumulation and concentration polarization, membrane fouling and membrane deterioration due to bacterial attack are major concerns. Many novel aspects are considered to overcome those shortcomings such as progress in developments in novel diversified bacterial consortia and advanced membranes [95]. In the following sections two major HRMBR systems, namely osmotic membrane bioreactors (OMBR) and membrane distillation bioreactors (MDBR) performance in wastewater treatment was discussed.