Composting toilets may be permitted in Washington State as residential systems, day-use park systems or cottage systems.   This discussion will focus on requirements for residential systems as these are the most relevant for urban settings.  

Local health jurisdictions may issue an installation permit for any proprietary composting toilet included on the current Washington State Department of Health “List of Registered On-site Treatment and Distribution Products (WAC 246-272A-0110(1).”    In Seattle, King County-Seattle Public Health is the permit-issuing agency.    Installers must also be authorized by KCSPH and proposed systems must include provisions for leachate management. 

Management of the final compost product is governed by federal and state law. Under both the federal “sewage sludge” program and the Washington state “biosolids” program, the product of composting toilets qualifies as sewage sludge, which is classified as “domestic septage” by the U.S. Environmental Protection Agency and the Washington State Department of Ecology.

Neither the federal or state sewage sludge/biosolids rules provide any exemptions or allowances for small quantity generators.  So the product from small composting toilet systems are regulated under the same rules applied to the largest of generators (such as municipal wastewater facilities.)

In Washington State, domestic septage can be managed either as sewage sludge/biosolids (e.g., as sewage sludge/biosolids from a treatment plant) or as domestic septage.   Washington State DOH advises, “managing the product of composting toilets as sewage sludge/biosolids is probably not practical for a homeowner because of the stringent regulatory requirements associated with that management option.”
A more practical approach is to manage composting toilet residuals as domestic septage, for which the following requirements apply:

Area of Application:
Compost toilet residuals may only be applied to certain types of land (typically agricultural or forest land) and expressly not to the surface of a “public contact site,” a “lawn” or “home garden” area.

Pathogens: 
The site management and access restrictions of WAC 173-308-270 (5)(a)(i)-(ix) (5)(b)(i)-(iv) must be met as applicable.

Vector Attraction Reduction:
The residuals must be promptly tilled into the soil or buried so that nothing remains exposed to the surface, and this must be done over a given area of land at the rates recommended in WAC 173-308-080(6). 

Recordkeeping:
A clear record of how much material was applied, when it was applied, and where it was applied must be kept and the person applying the compost must own or explicitly control the land where it is applied.

Given the strict regulations governing the final disposition of the compost produced in composting toilets, their applicability may be limited to sites with sufficient available land to receive the resultant material.    Alternative management options for more highly urbanized sites (also allowable under state statute) include the collection and management of the compost by a licensed domestic septage pumper/hauler or the disposal of the material as solid waste, though these options may defeat much of the intended purpose of installing the system. 

Another limitation on the applicability of composting toilets in urban settings is the challenge of stacking the toilets in multi-story buildings in such a fashion that solids from upper floors may still travel to the receiving chamber unimpeded.    A design strategy for addressing this challenge is highlighted in the C.K Choi Building case study.