An intermodal container converted to a tank for processing wastewater has a process wall detachable from a hatchway in a wall of the container. The process wall is bolted to an interior facing flange in the container wall and is installed so as to sandwich a gasket and a lip between the process wall and the flange. When the process wall is at a container end wall, first components for interface with water to be treated are sealingly fixed into one part of the process wall designed to be below the level of contained water. Second process components such as those associated with communication cabling, power cabling or piping to be run through the converted container are sealingly fixed into an upper part of the process wall above a design level of contained water to be treated.