## 3 High Retention Membrane bioreactors (HRMBR)
### Osmotic Membrane bioreactor (OMBR) for OMPs Removal  
One of the promising options is to integrate submerged forward osmosis membrane with bioreactor known as an osmotic membrane bioreactor (OMBR) was materialized as an attractive alternative for sewage treatment and reuse [96; 97]. OMBRs can produce clean permeate and efficiently degrade nutrients and OMPs as well as toxic pollutants like phenols. Another advantages are low fouling propensity of FO membranes [98; 99]. OMBRs present certain benefits such as low fouling propensity, less numbers of cleaning cycles and therefore less operating cost and energy intensive process [58; 97]. Further, it is possible to offer longer contact time for refractory compounds in OMBR due to the size exclusion mechanism it can retain any compound having molecular cutoff weight (MWCO) larger than forward osmosis membrane pore size [82]. For now, salt accumulation from draw solute to feed solution is a major issue in OMBR operation. FO membrane is semipermeable and hence salt leakage is an unavoidable phenomenon that relates molecular weight of draw solution. OMBR operation at high sludge retention, coupling with MF/UF membrane could be an option to mitigate salinity build up [100]. Recently combining OMBR with electrodialysis (ED) was suggested [53]. As previous research suggests, OMBR offers excellent OMP removal due to synergy between FO rejection followed by biotransformation in the bioreactor at long HRT and SRT. FO membrane rejection mechanism for OMP removal can be attributed to electrostatic repulsion, hydrated radius, molecular weight cut-off, retarded forward diffusion, Henry's law constants and hydrophilic (log D). Further, in the bioreactor OMP removal involves biodegradation in presence of diverse microbial communities and sludge sorption [101]. Table 4 shows some of the recent work of OMBR in OMP removal. [PERSON] et al. (2012) successfully achieved >96% removal of 4 pharmaceutical compounds in OMBR that showed varying biologic process removal efficiency. For instance, ibuprofen showed highest removal (>90%) and below detection limits of the instrument followed by 40-90% moderate removal for naproxen. Nonetheless, diclofenac had too low removal of \(<40\%\) and almost no biodegradation was observed for very persistent carbamazepine (0%) [102]. [PERSON] et al. (2014) used real wastewater as feed to the UF-OMBR (hybrid ultrafiltration-osmotic membrane bioreactor) system to examine OMP removal and deduced that among 20 OMPs, the UF-OMBR system was found efficient to remove 15 OMPs well below the analytical instrument detection limit. Forward osmosis membrane has shown higher removal of hydrophilic OMPs than hydrophobic OMPs. They noted that when UF system was offline for two weeks removal of bisphenol A, DEET, TCEP and sulfamethoxazole decreased, due to salinity build-up [36]. In another OMBR-RO operation performed by [PERSON] et al. (2017), salt accumulation has negatively affected process performance by both reducing and changing microbial consortia thereby increased forward osmosis membrane fouling due to increased SMP and EPS [5]. \begin{table}
\begin{tabular}{c c c c c c c} \hline \multirow{2}{*}{**Wastewater**} & \multirow{2}{*}{ \begin{tabular}{c} **Membrane with** \\ **Operating** \\ **Conditions** \\ \end{tabular}  } & \multirow{2}{*}{ \begin{tabular}{c} **Flux** \\ **(LMH)** \\ \end{tabular}  } & \multirow{2}{*}{ \begin{tabular}{c} **Salinity** \\ **(g/L)** \\ \end{tabular}  } & \multirow{2}{*}{\ \begin{tabular}{c} **Micropollutants** \\ **Investigated** \\ \end{tabular}  } & \multirow{2}{*}{ \begin{tabular}{c} **\%** \\ **Removal** \\ \end{tabular}  } & \multirow{2}{*}{**References**} \\  & & & & & & \\  &