In-vessel composting units are generally made up of an insulated, reinforced plastic, fibre glass or stainless steel vessel with a capacity in the range of 1-3 m3. The majority of off-the-shelf systems are batch-flow type units – where materials are loaded into the top of the vessel, and pasteurised materials are harvested from a grate on the bottom of the unit. A motorised auger is sometimes provided to mechanically mix the compostable organic material and assist in size reduction. The major advantage of in-vessel composting system is that the key parameters affecting the rate of composting can be controlled and optimised.
In addition, the odour control units allow the systems to be stored in locations closer to people. Batch-flow units do require at least two units to be installed on-site to deal with a continuous organics feedstock. Figure 4 shows two different styles of induced aeration in-vessel batch-flow composting units. The in-vessel composting units are tolerant of a wider range of feedstocks than vermiculture systems, but still require specialised management to ensure consistent processing performance.