A standard 20 ft or 40 ft sea-freight shipping container is configured as a septic tank, or as an aerobic trickle filter station. The shipping-container is lined with a watertight shell or liner. The liner may be a prefabricated separate structure inserted into the container, and supported from the container walls. Or, the liner may be actually constructed inside the container, again taking mechanical support from the container walls. The openable doors at the end of the standard container are retained, a wall being added to the shipping-container a foot or two inside the doors to form the tank, leaving a utility room for pipework, valves, pumps, etc.