Graywater, Urine, Solids: Why the Rules Treat Waste Streams Differently

A composting toilet changes the waste equation, but it does not make every drain disappear. It handles toilet waste — the solid stream and, in a urine-diverting system, the liquid stream from the toilet. But sinks, showers, laundry, and kitchen drains still produce wastewater that needs an approved handling plan. Understanding how regulators categorize each stream is what makes it possible to plan a complete, legal system.

Note:  This post is educational, not legal advice. Graywater rules vary significantly by state and county — always confirm requirements with your local authority.

Toilet waste is only one part of a building's wastewater. Graywater from sinks, showers, laundry, and kitchen use still needs a legal disposal or reuse plan — and how your jurisdiction treats each of those streams directly shapes what a complete off-grid or alternative sanitation installation looks like.

Some jurisdictions allow graywater reuse under specific rules. Others require a septic system, sewer connection, or engineered onsite wastewater plan for all non-toilet waste. The range is wide, and the definitions that determine which category applies are not always intuitive.

Ohio's onsite wastewater rules offer a clear illustration of why this matters: when a composting toilet is used for toilet waste, other household sewage — including graywater — must still discharge to an approved treatment system. Ohio is not unusual in this regard. Most state codes were written around conventional plumbing systems and require the full wastewater picture to be addressed, not just the toilet portion. Knowing this going in means the planning conversation is complete from the start.

What Counts as Graywater?

Graywater is wastewater generated by household activities that do not involve toilet use. The sources most commonly included are:

• Bathroom sinks and lavatories

• Showers and bathtubs

• Laundry (washing machine discharge)

Kitchen sink water occupies its own category in many jurisdictions. Because it contains food particles, grease, and detergents, it is often treated more like blackwater — requiring the same level of treatment as toilet waste — even though it does not originate from a toilet. Some states explicitly define kitchen sink discharge as blackwater. Others include it in graywater but with additional treatment requirements. This is worth confirming specifically, because it affects whether a simplified graywater system will cover all non-toilet sources or only some of them.

In terms of volume, graywater typically represents 50 to 80 percent of a household's total wastewater output. A family of four with efficient fixtures still produces 80 to 120 gallons of graywater per day. That volume needs somewhere to go — and planning what that looks like is as central to an alternative sanitation installation as choosing the toilet itself.

What Counts as Blackwater?

Blackwater is wastewater that contains fecal matter or has come into contact with toilet waste. In a conventional plumbing system, this means everything that goes through the toilet — urine, feces, flush water, and toilet paper — combined into a single stream.

A composting toilet changes that picture. Feces go to the composting chamber, where they are managed as a solid biological material rather than a liquid waste stream. In a urine-diverting system, urine is separated at the source and handled independently. Neither stream becomes part of a mixed liquid waste that flows to a septic or sewer.

The complication is that most plumbing and sanitation codes were written with conventional toilet systems in mind. The legal definitions of "blackwater" and the treatment standards that accompany them predate the widespread adoption of composting and source-separating systems. That means the rules do not always map cleanly onto how a composting toilet actually works — and local interpretation by health inspectors fills the gap. When you talk to your health department, be prepared to explain the system in plain terms and bring the manufacturer documentation that shows what happens to each stream.

Where Urine Fits

In most regulatory frameworks, urine is classified as part of blackwater because it originates from the toilet. That classification was established for conventional flush systems, where urine and feces always mix. In a urine-diverting composting toilet, they do not.

Separated urine is a materially different stream from mixed sewage. It carries far lower pathogen levels than combined toilet waste, and in healthy individuals it exits the body essentially sterile. These properties are why urine source separation has attracted attention in sanitation research and why some jurisdictions have begun to treat diverted urine differently than mixed blackwater.

In practice, the regulatory treatment of the separated urine stream varies considerably:

• Some jurisdictions allow diluted urine to be applied as fertilizer to non-edible garden plants — a practice sometimes called "liquid gold" — with no permit required for small household volumes.

• Others treat diverted urine as wastewater regardless of source separation, requiring it to be discharged to an approved system.

• Many codes simply have not addressed the separated urine stream explicitly, leaving it in a grey area that the local health department interprets case by case.

Ask your health department specifically how diverted urine is classified in your jurisdiction. It is a narrow enough question that most inspectors can answer it directly, and the answer shapes how the liquid side of the system needs to be managed for compliance.

Why Low-Flow Does Not Mean No-Flow

A common assumption when switching to a composting toilet is that the water infrastructure question largely goes away. It does not. Toilet waste is removed from the liquid waste equation, but every other water use in the home — showering, hand-washing, dishwashing, laundry — still produces wastewater that needs management.

Even a small off-grid household with efficient fixtures and low daily usage generates meaningful graywater volume. Two adults taking short showers and using water-saving fixtures will still produce 40 to 60 gallons of graywater per day from non-toilet sources alone. That volume needs a permitted outlet.

The good news is that graywater systems are generally simpler and less expensive than full septic systems, and several states have developed streamlined permitting pathways for them. Subsurface irrigation systems, mulch basin systems, and laundry-to-landscape configurations are all examples of permitted graywater disposal methods that are significantly lighter-footprint than a conventional drain field.

The right approach depends on your jurisdiction, your water sources, your soil conditions, and the volume you are managing. What matters at the planning stage is not finding the final answer — it is knowing to ask the question. A complete sanitation plan addresses the toilet waste stream and the graywater stream. Planning both from the beginning keeps the project on track and avoids a permit conversation midway through installation that delays everything.

Questions to Ask Your Inspector

When you speak with your local health department about a composting toilet installation, the graywater conversation is a separate thread from the toilet approval. Come prepared to address both. These questions cover the ground most inspectors will want to explore:

• How is graywater defined in your jurisdiction — specifically, does kitchen sink water fall under graywater or blackwater rules?

• Is graywater reuse permitted on this property type, and if so, what methods and volumes are allowed?

• What is the required disposal method for graywater if reuse is not permitted — drain field, dry well, connection to existing system?

• How is diverted urine from a source-separating toilet classified — as blackwater, graywater, or is there specific guidance?

• If there is an existing septic system on the property, can it remain in place to handle graywater only, and does its capacity need to be re-evaluated?

• What system sizing requirements apply — is the graywater volume calculated on fixture count, bedroom count, or daily flow estimates?

• Are there specific setback requirements for graywater disposal components — distance from wells, property lines, structures?

These questions move the conversation from the general (can I have a composting toilet?) to the specific (what does the full system look like?). Inspectors respond well to applicants who have thought through the whole picture. It signals a serious installation and often results in a more direct and efficient review process.

Plan the full system before you start:

→ Download the Before You Install Checklist

Frequently Asked Questions

What happens to graywater when you use a composting toilet?

A composting toilet handles toilet waste only. Graywater — wastewater from sinks, showers, laundry, and typically kitchen drains — is not managed by the toilet system and must be handled separately. Depending on your jurisdiction, that may mean connecting to a municipal sewer, maintaining or installing a septic or graywater system, or using a permitted reuse method such as subsurface irrigation. Planning the graywater side of the system is as important as planning the toilet side.

Can shower water go outside?

In most jurisdictions, discharging graywater directly to the ground surface is not permitted without approval. However, many states have graywater reuse programs that allow shower water to be directed to subsurface irrigation systems — typically for non-edible plants, away from water sources and property lines, with no standing water at the surface. The rules vary significantly: California, Arizona, and New Mexico, for example, have well-developed frameworks for permitted graywater reuse. Other states are more restrictive. Check with your local authority before directing any graywater outside.

Can urine go into graywater?

In a urine-diverting system, the separated urine stream and the graywater stream are kept separate by design. Whether diverted urine can legally be directed to a graywater disposal system depends on how your jurisdiction classifies it. Some treat it as graywater once separated; others classify it as blackwater regardless of origin. The most practical approach is to ask your health department directly — the answer is usually clear and saves guesswork about what configuration is permittable.

Do I still need a septic system if I have a composting toilet?

Possibly, but not necessarily a full one. A composting toilet removes toilet waste from the liquid waste stream, which can meaningfully reduce the required capacity of any onsite system. In some jurisdictions, a composting toilet paired with a permitted graywater system can fully replace a conventional septic. In others, an existing septic may need to remain for graywater, though it may be allowed to operate at reduced capacity. New construction on land with no sewer connection will need some form of graywater management regardless of which toilet system is used. The question is not whether a system is needed — it is what the simplest legal option looks like for your property.

Does the composting toilet output need to be disposed of separately?

Yes. The composted solids produced by the toilet chamber are a separate consideration from liquid waste streams. After a curing period, the output is typically suitable for application around non-edible plants or for further outdoor composting. Some jurisdictions have specific guidance on what composting toilet output can legally be used for. When in doubt, the safest approach is to treat it as a soil amendment for ornamental landscaping rather than food gardens, and to confirm any local rules with your health department.

The categories that regulators use — graywater, blackwater, diverted urine, composted solids — exist because each stream has different properties and poses different public health considerations. A composting toilet simplifies the toilet waste side of that picture considerably. It does not simplify the graywater side on its own.

The practical takeaway is straightforward: plan both streams from the beginning. Know what your jurisdiction requires for graywater before the installation conversation starts. Ask the specific questions about how diverted urine is classified. And build the full system design before you finalize your purchasing decisions — because the complete picture is what gets permitted, not just the toilet.

Use the Before You Install checklist to plan the full system:

→ Download the Before You Install Checklist