A Composting Toilet Does Not Get You Out of a Septic System

The off-grid plan usually arrives fully formed: solar on the roof, water hauled or drilled, and a composting toilet instead of a septic system. Two of those three are fine. The third is where the budget breaks, because of a definition almost nobody reads until it is too late.

This catches people who have costed a composting toilet as a way of avoiding a permit and a leach field entirely. It is not. It changes which system you need and can reduce its size, but it does not remove the requirement, and both routes go through the same permitting process.

The permit you are actually applying for

On-site wastewater in Arizona runs under an ADEQ Type 4 General Permit, which covers the design, installation and operation of on-site wastewater treatment facilities from 0 to 24,000 gallons per day. Within that, two sub-types cover most rural residential work:

Permit	What it covers
4.02	Septic tank with conventional disposal — the standard system
4.03	Composting toilet
4.04 – 4.23	Alternative systems, for sites where a conventional design will not work

In Cochise County, applications are handled by the Health Department's Environmental Health Services Division rather than by Development Services. Depending on the system, review may sit with the county or go to ADEQ. Permits expire after two years, which matters if you are permitting early and building later.

You cannot design it yourself

The system has to be designed by a qualified Soil and Site Evaluator, and the county is specific about who qualifies. An investigator or evaluator must be one of:

• an Arizona registered professional engineer;
• an Arizona registered professional geologist;
• an Arizona registered sanitarian; or
• an individual holding a certificate of training from a course recognised by ADEQ.

That list is the answer to the most common question we get, which is whether an owner-builder can do the septic design along with everything else. No — and the evaluator you hire will normally have the paperwork you need for the county application, so engaging one early saves a round trip.

What a site investigation involves

A site investigation is a surface characterisation plus a subsurface characterisation, carried out by that qualified investigator. The subsurface part is where rural parcels get their surprises, because it establishes whether the ground will actually accept water.

Percolation testing has a defined shape. The requirement is to select at least two locations in the primary area and at least one in the reserve area, and test holes are excavated in undisturbed soil at least 12 inches deep, either 12 by 12 inches square or 15 inches in diameter if round.

Sequence this before you clear anything

The order that avoids expensive rework:

1. Legal access first. None of this is deliverable to a parcel a truck cannot lawfully reach — see owning land you cannot legally reach.
2. Water decision. Well or hauled changes where things sit and what setbacks apply. Our hauled water versus drilling piece has the numbers.
3. Soil and site evaluation. Before you know where the house goes.
4. Design, then permit, then build. In that order, with the reserve area protected from traffic.
5. Then pad, drive and drainage — our site prep checklist covers the rest.

Clearing and grading before the percolation testing is done is the classic own goal. Compacted ground tests worse than undisturbed ground, and the rule requires testing in undisturbed soil — so a machine that has already tracked over your best disposal area has cost you the best disposal area. If clearing is on your plan anyway, the land clearing guide covers what you may and may not remove.

If you are still set on a composting toilet

They are legitimate, they are permitted under 4.03, and on the right site they are the correct answer. Expect to supply considerably more documentation than for a conventional system: the manufacturer's name and address, the model number, a copy of the warranty and the installation, operation and maintenance specifications, calculations for composting rate, capacity and waste accumulation volume, documentation of listing by a national listing organisation, your vector control method, and your planned method and frequency for disposing of both the composted residue and the drainage from the unit.

Plus the same soil and site work as everyone else, plus the small septic system for the kitchen. ADEQ maintains a list of proprietary products; check your intended unit against it before ordering.

What we could not confirm

• We could not confirm current Cochise County permit fees. The county publishes an Environmental Health summary of fees and the figure we saw referenced was for an earlier fee year. Ask when you apply rather than budgeting from any article.
• We could not confirm the specific setback distances required between a disposal field and a well, a property line or a watercourse. Those live in A.A.C. R18-9-A312(C) and we have not reproduced them here because getting a setback wrong by a few feet is exactly the kind of error worth reading the rule for. Your evaluator will apply them.
• We could not confirm whether a percolation test result has an expiry, or how long a site investigation remains valid if you permit now and build later. The two-year permit expiry is documented; the evaluation's shelf life we could not source.
• We could not confirm which alternative system types Cochise County reviews itself versus which go to ADEQ. The county's guidance says it depends on the system.
• The county fact sheet we relied on carries an update date of 13 March 2023. Rules and fees move; treat the process as current and the numbers as indicative.
• This is not engineering advice. A qualified Soil and Site Evaluator is a statutory requirement, not a formality.

The short version

A composting toilet does not exempt you from a septic system, because Arizona's gray water definition excludes the kitchen sink, the dishwasher and the toilet — so kitchen drainage still needs somewhere legitimate to go. Everything runs under an ADEQ Type 4 General Permit, 4.02 for a conventional septic system and 4.03 for a composting toilet, with Cochise County's Environmental Health division handling the application. The design must come from an Arizona registered engineer, geologist or sanitarian, or someone holding an ADEQ-recognised training certificate — not from you. Expect a surface and subsurface site investigation, percolation testing at a minimum of two locations in the primary area and one in the reserve, and holes at least twelve inches deep in undisturbed soil. Protect that reserve area from the moment you know where it is, and do the testing before the machinery arrives rather than after.

Sulfur Springs Land Management works the ground around all of this — access, pad, drainage and keeping equipment off the area your evaluator just told you is the only place a leach field can go.