Are composting toilets legal and practical in a house?

Arizona's composting toilet permit is numbered 4.03. It sits in the state's on-site wastewater rules, A.A.C. R18-9-E303. Its title is "4.03 General Permit: Composting Toilet, Less Than 3000 Gallons Per Day Design Flow." The rule defines the device as a manufactured system that "receives human waste from a waterless toilet directly into an aerobic composting chamber." So a bucket in the corner or a homemade box does not fit. It has to be a manufactured turnkey or kit system.

The rule lists when an applicant may use one. Any one of these is enough. They are limited water, environmental limits near a sensitive area, too little space for other systems, severe site limits, or, in the rule's words, "The applicant desires maximum water conservation." That last reason is broad. You do not have to prove hardship.

Three conditions matter most for a homeowner:

• Wastewater from everything else in the house still has to be managed. The rule allows a composting toilet only if wastewater is handled as the section provides and "soil conditions support subsurface disposal of all wastewater sources."
• Any reuse of separated gray water must follow Arizona's gray water rules. Our page on whether gray water is legal in Arizona covers those.
• The unit "shall only receive human excrement unless the manufacturer's specifications allow the deposit of kitchen or other wastes into the toilet."

The rule also caps use at "no more than 50 persons per day," which a single household will not reach.

The same rule spells out what goes in the Notice of Intent to Discharge. You submit the manufacturer's name and warranty, the model number, and the composting rate and capacity math. You also state how you will control vectors and how often you will dispose of the composted residue. The rule also requires "Documentation of listing by a national listing organization" showing the unit meets its stated loading and treatment performance. Units on the state's approved list or in an approved reference design are exempt from that listing. EPA's fact sheet names ANSI/NSF 41 as the governing standard, so look for that listing.

The rest of your wastewater needs a design too. The rule calls for an interceptor for non-toilet wastewater and a trench or bed for subsurface dispersal. It also requires at least 5 feet of vertical separation to a limiting condition and a 100-foot setback from wells. So a composting toilet does not end septic-style design work. It shrinks the load, and you still have a system to permit.

Maricopa County lists "composting toilet" as an alternative system. It says "Alternative OSWTFs must be designed by and stamped with the seal of a Professional Engineer (P.E.), registered to work in the state of Arizona and with expertise in the on-site wastewater field." It adds that "Maricopa County may have additional requirements or restrictions." In another county? Ask that county's environmental health office how it handles the permit.

### Questions to ask before you buy

Before you spend money on a unit, get answers to these. Each one comes straight from what the state permit asks you to submit:

• Is the unit a manufactured system with a national listing for loading and treatment performance?
• What is the manufacturer's installation, operation and maintenance specification?
• How will vectors such as flies be kept out?
• Where will the finished compost go, and how often?
• Where will the liquid drainage from the unit go?

If a seller cannot answer these, the permit office will not accept the application either.

The state permit does not clearly cover a home on city sewer. The 4.03 permit is written for on-site wastewater, meaning a home that treats and disposes of its own. It is not a route for swapping the toilet in a home on a city sewer. We did not find a Phoenix-area code section that covers that case, so this page does not answer it. Ask your city's building safety or plan review office before you buy anything. Not sure which you have? Start with how to tell whether you have septic or city sewer.

EPA's fact sheet covers both sides. On the plus side, "Composting toilet systems can be used almost anywhere a flush toilet can be used," and they cut water use because nothing is flushed. EPA says a properly sized and maintained unit breaks waste down to 10 to 30 percent of its original volume.

The downsides are the practical part. EPA lists these among the disadvantages:

• "Maintenance of composting toilet systems requires more responsibility and commitment by users and owners than conventional wastewater systems."
• "Composting toilet systems must be used in conjunction with a graywater system in most circumstances."
• "Most composting toilet systems require a power source."
• Improperly installed or maintained systems can produce odors and unprocessed material.

EPA lists accepting kitchen waste as an advantage, but in Arizona the rule allows it only if the manufacturer's specifications say so, so check before you toss scraps in. You will also add bulking material such as sawdust or leaves, and empty the finished compost. EPA says the end product is removed "anywhere from every 3 months for a cottage system to every 2 years for a large central system," and that the humus "legally must be either buried or removed by a licensed septage hauler in accordance with state and local regulations." Arizona's permit makes you state your disposal method and frequency up front, so plan for it before installation.

A composting toilet suits a remote cabin, a lot with poor soil, or an owner who wants to cut water use and does not mind the upkeep. It is a poor fit if you want a fixture nobody thinks about. Want to use less water in a normal house? A modern efficient flush toilet gets you much of the way. See how a dual flush toilet works and our guide to conserving water inside your home. If you are weighing a replacement, how to know when to replace a toilet walks through it.

Here's the practical order. First, confirm your property's wastewater setup. Second, call your county or city before you buy a unit. Third, get the engineered plan and the Notice of Intent moving before anything gets built. Skipping the order is how people end up with a unit they cannot legally use.

On a septic system already? How often to pump a septic tank covers the upkeep you have now. For a standard toilet install or repair, HQ Plumbing & Air handles toilet repair and replacement, and we are 24/7 if a toilet is overflowing. This page is general information, not legal advice, so confirm the current rule text with ADEQ and your local permitting office before you build.