Generally, micro-filtration membranes, such as the one you are apparently using, does not retain dissolved species, especially not very small ones like NH4+. So the ammonium will pass through the membrane along with the water by convection and therefore the permeate will have the same NH4+ concentration as inside the MBR

Thank you for your response. In case of a dynamic membrane forming on this membrane, how do you think about the possibility of preventing ammonium passing through the membrane?

Hi, using membrane alone to remove ammonium is quite a challenge. Even RO membrane has low rejection of ammonium, let alone 20 um UF membrane. If your feed water is relatively clean, you can try EDR (Electrodialysis + RO), but if you are using MBR for wastewater source, i guess Paul is giving some nice suggestion. Lastly you can also design your system to remove ammonium after your MBR, e.g retrofit a electrodialysis step to treat your MBR effluent.