Assuming you mean membrane bioreactor, its biggest advantage is the low space requirement and high solids removal efficiency.
Usually land is relatively plentiful and cheap in a rural location. MBRs can however give very high solids removal efficiencies, which can be desirable if discharging to a high-grade water body, as is nmore commonly the case in rural locations.
MBRs are more expensive to purchase and to run than the simpler activated sludge treatment for anything other than the highest degrees of treatment.