p-Nitrophenol (PNP) is one of the most important industrial raw materials and can be found as a pollutant in industrial wastewater streams and groundwater resources. Since PNP contamination can pose a significant environmental and public health risk, the US EPA has listed PNP as a priority pollutant and recommended restricting its concentration in natural waters and industrial effluents. There is therefore a demand to effectively remediate industrial wastewaters and groundwaters that are contaminated with PNP. Biodegradation or bioremediation can be a viable technology for remediating PNP-contaminated waters, since substantial information is available that bacteria are capable of mineralizing PNP under aerobic conditions. However, PNP biodegradation bioreactors that have been reported so far are still limited and the PNP-degrading populations within bioreactors are still poorly understood.