Criteria for the proper design and operation of the bio-toilet have not been established either by the manufacturers or by the authorities. Based on the results of experimental research already reported by us, criteria for the proper design and operation of the bio-toilet system were established. The establishment of operation criteria led to the formulation of an operation scheme where three main zones are distinguished: i) green zone, where the best composting performance is expected; ii) yellow zone, where biodegradation can be conducted but performance is not the most efficient; iii) other zones where operation of the bio-toilet is not recommended because odour problems and human health risks will develop, or in the worst case, biodegradation of faeces will not occur. In the design criteria, the surface area required for evaporating the water loads due to contributions of urine and faeces was determined by applying concepts of drying theory. This theory was also applied to estimate the mass and volume of dried sawdust required for dealing with the critical water loading. However, the accumulation of stabilised solids transpired to be more critical and, finally, mass and volume of dried sawdust were estimated based on this factor.

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