This decision may depend upon local regulatory requirements, and whether it is desirable to use surface or subsurface final onsite disposal for the greywater portion of the wastestream. Depending upon site conditions and the type of disposal system used for the greywater portion of the wastestream, if kitchen wastes are included in the greywater it may be necessary to modify the design to provide for greater pretreatment prior to final soil disposal. Composting toilets are a relatively well-developed, and fairly commonly used technology as compared with other alternative wastewater technologies. The use of composting toilets allows for a significant reduction in water consumption, and may reduce nitrogen and pathogen loading to a subsurface disposal system. In order to ensure that the composting process is properly maintained in the composting toilet, the waste material must remain adequately mixed, with the moisture content in an acceptable range.
Odors can result from anaerobic conditions if too much moisture accumulates in the unit.
The fairly regular attention required for composting toilet units that receive much usage may be objectionable to some home owners. Retrofitting an existing home with a composting toilet unit may require structural modifications and be fairly costly. Electrical power is required for most composting toilet units. In order for the residuals from a composting toilet unit to be used as a soil amendment at a residence, the federal rules require that testing be conducted to demonstrate adequate pathogen reduction. In addition, it must be demonstrated that certain metals concentrations are below an acceptable level.