Onsite wastewater treatment systems (OWTS) serve approximately 25 % of all homes in the United States. Conventional OWTS often require a lot of space to work properly and there are concerns over OWTS treatment efficiency for nutrients and pathogens. These concerns about OWTS has led to an increased use of pretreatment units that are less land demanding and produce effluent of a higher quality than of a septic tank in order to protect drinking water sources and human health. One option for pretreatment units is membrane bioreactors (MBR). MBRs are suspended growth activated sludge treatment systems that relay upon membrane equipment for solid separation, in one single unit. This master thesis was a part of a research program at Colorado School of Mines. The objectives of this master thesis are threefold. Initially, the purposes were to increase the state of the knowledge using MBR in wastewater treatment and evaluate a pilot-scale MBR during start-up and steady state conditions, in respect of nutrient and organic compounds removal and overall system performance. A second objective was to evaluate the performance of the MBR during different stress conditions (power failure, high loading and high flux) in respect of nutrient and organic compounds removal and overall system performance. A third objective was to investigate permeate quality with respect of nutrients and organic compounds after infiltration through soil columns. The purpose was to assess if the effluent quality was suitable for direct discharge to the underlying groundwater or to surface water. The result showed that the MBR was able of more than 90 % removal of COD, independent of the COD in the influent. Nitrification was over 99 % after the start up period. The denitrification was limited by carbon and due to this the Swedish requirements of 10 mg/l nitrogen for discharge of treated wastewater to surface water was not fulfilled. The Swedish requirement of 0.3-0.5 mg/l phosphorous for discharge of treated wastewater to surface water was not fulfilled for the MBR. Phosphorous removal was not expected, since the MBR not was designed for it. From a water quality stand point the MBR is a viable pretreatment process to OWTS, since it removes COD, ammonia and fecal coliforms good and also nitrate to some extent. When enough carbon for high denitrification efficiency was available in the incoming wastewater the effluent quality was suitable for direct discharge to surface water. Total-P is not removed by the MBR, but was easily removed in the soil as the soil column experiment depicted and also the research performed at Mines Park test site showed.