Tips to help optimize process performance. Membrane bioreactors (MBRs) are a viable treatment alternative when a wastewater utility has footprint constraints, a market for reclaimed water, or needs to produce a high-quality effluent. Because MBRs can handle higher mixed liquor suspended solids (MLSS)concentrations and don't need secondary clarifiers, their footprint is much smaller than that of a conventional activated sludge process. MBRs can produce effluent with biochemical oxygen demand (BOD) and total suspended solids levels of 2 mg/L or less. They also can provide biological nutrient removal.