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Composting Toilet State Pilot Research Project in Tucson, Arizona

A Tucson based nonprofit, Watershed Management Group (WMG), initiated a research project with EPA funding to see if site-built composting toilets could meet user expectations and if so work towards granting the successful designs ‘reference design’ status with the state of Arizona’s Department of Environmental Quality (DEQ), which would allow others in Arizona to build these composting toilets as an approved alternative to an on-site wastewater system. After collecting two years of successful data on user feedback and pathogen destruction of the finished composted material, WMG is still waiting to hear back from the Arizona DEQ about their request for reference design status.

Activities and Accomplishments

In the summer of 2012, as part of this project twenty residential and two institutional settings installed composting toilets (either a double chamber or barrel toilet design). Eight of the twenty residential homes were expert users and advocates of composting toilets; the other twelve were participants in Watershed Management Group’s Soil Steward program. From May 2012 to June 2013 the University of Arizona’s Bureau of Applied Research in Anthropology monitored the toilets for user likeability and maintenance and performance of the system. This included a pre-inspection and post-installation visit with WMG project staff.

All installations were done by users. Users attended an educational workshop hosted by WMG where they assembled their barrel system and then took them home for installation. The three sites that used a masonry double-chamber system installed the complete system on their own.

All participants had to submit quarterly forms about cover materials used (i.e. sawdust, shredded paper, leaves, etc.), detected odors, composting temperatures, maintenance tasks, and general likeability of the system.

How the Initiative is Applied

Homeowners and institutions who participated in the pilot program were allowed to install and use site-built composting toilets in addition to their sewer or septic system during the study, at the end of the study because of positive results participants were granted permits to continue using their composting toilets.

At the end of the second year composted material from the composting chambers that had composted for at least four months was tested and passed EPA’s threshold for Class A classification of biosolids for safe use in the garden. Based on those results Arizona DEQ provided the pilot participants in coordination with local county departments with an individual 1.08 General Permit at no additional cost.

WMG is reapplying to the Arizona DEQ for reference design status for the two composting toilet designs used in this study. If they receive reference design status these designs will be available to people as alternatives to septic systems throughout the state.

Technical Details


Watershed Management Group (WMG) led the 2‐year Composting Toilet Research Action Project as part of an EPA Environmental Education Grant focused on developing desert soil stewardship. WMG describes the goal of the Desert Soil Stewardship program was to “teach urban gardeners and neighborhood activists critical thinking and stewardship practices regarding soil management to address the environmental issues of loss of soil nutrients, food security, waste reduction, and water conservation.”

Lessons Learned so Far

The barrel composting toilet design adequately reduced indicator pathogens like e-coli and fecal coliforms in compost that composted for at least four months. After four months of composting all samples passed EPA’s threshold for Class A Classification of biosolids safe for use in the garden. The two samples that composted for less than 4 months did not pass the Class A threshold.

Public awareness of composting toilets in the wider community grew as seen by increased weekly email and phone inquiries to WMG. Acceptance of composting toilets as a safe and viable alternative grew among users during the project period. WMG created and shared a series of videos, webinars, and info sheets on their website in addition to hands-on workshops. All of these materials are still available in their Resource Library.

Initiative Cost/Benefit

Each household spent 2-5 hours to attend courses to learn about composting toilets, use and maintain them, and how to install their system. Participants spent between $350 to $550 on parts and materials.

Future Outlook and Replicability

For more information Contact David Omick or Catlow Shipek (Watershed Management Group).

Three years after the study ended sixty percent of participants surveyed (18/21 were surveyed) are still using their composting toilets. The University of Arizona BARA and WMG initiated a follow up study three years after the project ended to understand the long-term use patterns of the toilets. Eighteen of the twenty-one individual settings were visited and completed a questionnaire. The two institutional toilets were not visited during this follow up. Both institutional toilets remain in use.

Until the Arizona DEQ decides on the status of WMG’s proposed reference designs, it is unclear how other site-built composting toilet designs would seek reference design status. This study does show that the EPA Environmental Grant program was interested and able to fund research into composting toilets as a method of saving water in desert climates. 35 EPA Environmental Education grants were given out last year and the year before for a wide range of environmental issues. Interested applicants should check out the EPA’s request for proposals to find out more.

Additional Resources:

Arizona Site-built Composting Toilet Pilot Report Overview and Reference Design of Double Chamber Composting Toilet Systems Overview and Reference Design of Barrel Composting Toilet Systems
Arizona Site-built Composting Toilet Pilot Figures Report Watershed Management Group Resource Library -Composting Toilets EPA Environmental Education Grants


Arizona Department of Environmental Quality (ADEQ) approved, on December 28, 2016, for permitting the construction and use of two site-built designs piloted by Watershed Management Group through a two-year process with 22 participating sites. The Proprietary Treatment Product Listing for the two designs now expands the range of composting toilet options available to Arizonans. The two batch-style composting toilet system designs are 1) a 55-gallon barrel system and 2) a masonry chamber system. The document provided here includes sizing information, technical details, and operations and maintenance of both systems. Updated 10 January 2017.

Compliancy Initiative Contacts:

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