Most, if not all, chemical plants generate waste water from their processes. This waste water is often pretreated and then disposed to water courses as agreed with the appropriate authorities. The focus of attention has usually been on the environmental hazards of waste water. This article illustrates that process safety hazard from waste water treatment can also pose significant risks. Five serious events are described associated with the pretreatment, storage reuse, or treatment of waste water from chemical production. They are from three different countries and from a variety of businesses. These include one detonation, a confined vapor cloud explosion, an overpressure from gas evolution and one thermal runaway and fire. Significantly, often waste water sumps span an entire plant and have developed explosions over extensive areas. The article describes the events, lessons learned and some methods and procedures that can be used minimize the risks. (C) 2010 American Institute of Chemical Engineers Process Saf Prog 30: 45-51,2011