A composting system is provided that uses gravity and natural thermal convection to yield a compact, modular, plug-flow compost reactor requiring minimal aeration and agitation energy. The compost reaction takes place in a self-supporting containment unit which is mounted at an angle with respect to its supporting base pad such that minimal external energy is required to mix and transport the composting material during its residence time within the container. The system uses natural convection to supplement external energy in the introduction of air into and through the material. Furthermore, the configuration of the containment unit and its supporting structures allow rapid deployment of compost facilities with minimal permanent civil work and minimal space requirements in a manner that enables subsequent relocation of the equipment.