In the circular agitated in-vessel composting bioreactor, the composting material is fed from the top. Oxygen is supplied to the microorganisms by forced aeration through the augers which rotate around the centre of the reaction vessel. The set of augers (as shown in Fig 1) is connected to a bridge which is attached to a central pivoting system. The bridge is rotated slowly to move the augers. As the augers are hollow and perforated on the edges, oxygen is forced into the composting material to maintain porosity. High temperatures are used to ensure that the limit of pathogens is controlled. If the retention time is less than three weeks, then the discharged material must be windrowed until stability is reached i.e. the curing phase. (Tchobanoglous et al., 2002)
In the rectangular agitated in-vessel composting bioreactor, the waste is placed in the infeed conveyor. The extraction conveyor moves to mix the compost. To prevent heating up and aerate the compost material, oxygen is supplied by forced aeration through the plenum and blowers at the bottom of the bed. Tumbling or mixing occurs at regular intervals to agitate the material, break up clumps and maintain porosity before discharging it in the outfeed conveyor (FAO, 2003). After a retention time of 6-12 days in the bin, the material is windrowed for 1-2 months i.e. the curing period until stability is reached. The horizontal rectangular tank is well suited for sewage sludge, MSW and mixtures of MSW and sludge (Tchobanoglous et al., 2002).