Sizing the system may be the most difficult siting and construction issue for composting toilets.   Any established (proprietary) system on the approved Washington State list will offer sizing guidelines based on building occupancy type (day-use vs. residential), frequency of use, number of users, and the likelihood that ideal composting conditions (discussed under the “definition” section) are maintained. 

The Washington State Department of Health also offers this guidance:

“The composting process can result in a significant volume reduction in a relatively short time period under ideal composting conditions. However, under less ideal composting conditions, this would not be the case, and a larger storage capacity would be necessary. For instance, moldering toilet systems (those systems which support psychrophilic organisms, whose optimum temperature is above 41F and below 68F) are sized much larger than mesophilic composting systems (those systems which support mesophilic organisms, whose optimum temperature is from 68F to 112F) to compensate for their increased processing time.”

A composting toilet subjected to temperatures of 41F or less will only not compost until the temperature rises.  At these low temperatures, the toilet will only be storing the waste.  That is why manufacturers state the capacities of their toilets at 65F (comfortable room temperature of an average human-occupied space).

A second siting issue is the requirement for the final compost product to be applied on-site in very specific (regulated) ways.  See the “applicability” section for more information.

One critical construction issue to consider is the sufficient provision of space, typically in the basement level of the building, to house the appropriate size composting vault(s) for the number of toilets being connected to the system.   Housing the entire system within the building envelope prevents the system from triggering horizontal set-back requirements (in the health code) that are designed more for septic systems than for composting toilets.  This is, therefore, preferable.