Urine Diversion in Composting Toilets - Yes or No?

What WCTNZ® means by a true composting toilet system

At WCTNZ®, we distinguish a complete composting treatment pathway from a toilet that principally collects human waste for processing somewhere else. This is our practical product-selection distinction, not a separate legal product category called “true composting” in the New Zealand Building Code.

In a conventional urine-inclusive composting system, urine and solids enter the intended composting arrangement together. Moisture, nitrogen, oxygen, bulking material, chamber capacity and biological retention are managed as parts of the same process. It is an integrated sanitation design, rather than simply a dry collection container.

That does not mean that every drop of liquid remains in the compost. Some established systems deliberately evaporate excess moisture or have a designed leachate outlet. Any liquid leaving the treatment chamber still needs its appropriate management pathway.

New Zealand's AS/NZS 1546.2:2008 standard assesses waterless composting toilets against performance requirements, including composted end-product quality. A product description, a compost-like appearance or a statement that material has “started breaking down” is not a substitute for model-specific evidence of that performance.

The smallest genuine treatment systems are not necessarily the smallest toilets

A little urine-diverting toilet may occupy less bathroom space, but physical compactness is not the same thing as processing capacity. The amount of waste the system can treat depends on chamber volume, operating conditions, time and any processing features—not merely the size of the pedestal.

Batching is an important distinction: rotating a purpose-designed composting chamber out of service to finish its treatment cycle is different from taking a small bin of fresh or partly processed waste out of a toilet and tipping it into an unrelated secondary composting bin. WCTNZ® sees correctly selected batching systems as one of the smallest practical routes to an integrated composting process, where the model's retained chambers, operating instructions and evidence support that outcome.

Likewise, a more advanced processor may use heat or mechanical assistance to deliver a defined composting result within a smaller footprint. But features alone do not establish certification. Check what the manufacturer has actually demonstrated for the named model and use conditions.

Urine is not the enemy of composting

In a composting system designed to accept urine, it contributes moisture and nitrogen to the composting mass. Microorganisms need moisture to function, and an overly dry pile may decompose poorly even where the ventilation system is working.

Problems develop when liquid loading becomes greater than the system can manage. Excess free liquid can displace air from the pile, contribute to ammonia-related odours and place additional demands on evaporation or drainage. Cold, damp conditions may make that balance harder to maintain.

The correct response is not to divert urine from every composting toilet. It is to keep the treatment process within the moisture and loading conditions for which the system was designed. Where a urine-diverting option is fitted, the solids may need attention to hydration as directed by the manufacturer.

For a fuller explanation, read our Moisture Balance in Composting Toilets guide.

Urine diversion creates a second wastewater-management responsibility

This is the most important practical point. Urine leaving a toilet through a diverter is not treated merely because it has been separated. A hose from a pedestal does not convert urine into greywater, and a receiving tank does not automatically treat it.

With a urine-inclusive composting system, the manufacturer has designed the treatment and moisture-management arrangement around waste entering the composting chamber. When urine is diverted, the installation now has two distinct pathways: the solids-composting or solids-collection route, and the separated urine route.

• Urine to container: the liquid is contained, not treated. It must be stored hygienically and emptied at a suitable, authorised receiving point.
• Urine to a sewer or existing toilet: can be a practical route where that receiving system is appropriate and the connection or manual disposal arrangement meets the relevant requirements.
• Urine to a septic or on-site wastewater system: requires capacity and a design that accepts this waste stream; it cannot be assumed from the presence of a septic tank alone.
• Urine to land: requires an appropriate on-site wastewater design and the applicable approvals or permitted-activity conditions. A generic soakaway is not automatically an acceptable solution.

For New Zealand on-site installations, the ESR composting-toilet scoping study and Tasman District Council's guidance identify the need for urine or liquid diverted from a composting toilet to enter an appropriate on-site wastewater system designed under AS/NZS 1547. The actual discharge arrangement and consent pathway remain site-specific.

Four situations worth considering

1. Frequent entertaining and unusual peak urine loads

Love hosting friends and family? Parties and gatherings can create a sudden increase in urine volume. A well-sized urine-inclusive composting system may handle occasional peaks without difficulty, but repeated high liquid loads can challenge smaller chambers.

For a genuine batching or continuous composting system, a manufacturer-supported urine-diversion pedestal can be a useful option where expected peak loads justify it. For example, the Nature Loo™ Alectura Comfort range offers a urine-diversion pedestal option for particular higher-urination-use situations. That option is only sensible when the separately collected urine has a properly designed destination.

2. Cold sites where moisture is difficult to control

A composting chamber in a cold, humid or low-evaporation installation can be more sensitive to excess liquids. Depending on the system, urine diversion may reduce that load and help the composting mass remain aerobic.

However, taking away urine also takes away moisture and nitrogen. More effective ventilation, larger chambers, insulation or correctly specified heating may sometimes provide a better solution. Do not retrofit urine diversion to a certified system without the manufacturer's support and a complete receiving-system design.

3. RVs, campervans, caravans and boats

This is where urine diversion is especially common. Small vehicle toilets often use a urine bottle and a removable solids container because separation makes the limited storage volume more manageable. That is a valid design choice for mobile sanitation.

But the solids collected in most such compact configurations will still be fresh or partly processed when the container is emptied. WCTNZ® therefore describes many of these products more accurately as urine-diverting collection toilets, even though the wider market and vehicle guidance often call them composting toilets. They generally rely on further treatment or an appropriate waste-disposal route after collection.

Examples include the EcoLet™ Separera Mini UD-C, Sun-Mar™ GTG™ and CompoCloset™ Cuddy, each of which must be assessed against its own instructions and intended use.

4. Secondary or emergency toilets with an existing wastewater connection

Where a property already has an appropriate flushing toilet and a connected wastewater system, a compact urine-container toilet can offer a convenient supplementary facility. The collected urine may be emptied through the existing toilet where appropriate, rather than requiring a new discharge arrangement.

That can work well for occasional use, a sleepout or a temporary backup arrangement. It does not convert the solids chamber into a complete composting treatment unit, or automatically remove building, installation or disposal requirements.

Composting performance and RV self-containment are two different tests

New Zealand has separate frameworks serving different purposes. Confusing them is one reason so many online composting-toilet comparisons are misleading.

The Plumbers, Gasfitters and Drainlayers Board's guidance expressly discusses vehicle toilets holding partially processed solids that may need subsequent treatment, while still giving inspectors a pathway for assessing the vehicle's self-containment. In other words, a urine-diverting toilet can be useful and acceptable within a certified RV without being a fully completing composting treatment system.

In the residential market, we would not assume that a compact urine-diverting unit has demonstrated composted end-product performance simply because it is advertised as a “composting toilet”. Some products are primarily designed for collection, and their product literature may not provide independent laboratory test evidence or certification against AS/NZS 1546.2 for the relevant model.

For product-level documentation, see WCTNZ® Certified Technology & Product Evidence. For New Zealand vehicle requirements, use the PGDB self-contained vehicle information.

Start with the required treatment outcome—not the urine-diversion label

• Permanent home or bach: begin with a residential composting system selected for the actual occupancy, climate, maintenance routine and expected end-product performance. Compare a purpose-designed rotating batch system or continuous system before treating a small RV container toilet as an equivalent.
• Limited bathroom space: a split-system composting toilet may preserve a compact bathroom pedestal while providing substantial treatment volume below or beside the building.
• On-floor processor: where there is no space for a split chamber, compare the operating capacity and documented end-product performance of suitable enhanced processing systems.
• Genuinely mobile use: a urine-diverting containment system may be the sensible answer, provided the vehicle's approved waste-storage, ventilation and disposal arrangements are understood.
• Optional urine diversion on a true composter: consider it when the manufacturer's design supports it and an engineer or competent project designer has resolved the separate liquid pathway.

For example, the Nature Loo™ Alectura Premium uses rotating composting chambers as part of its integrated batching architecture, with model-specific AS/NZS 1546.2 certification information available from WCTNZ®. That is a fundamentally different operating proposition from a small urine-bottle toilet whose solids are removed before completing a demonstrated composting cycle.

Six questions worth asking

• Does the system actually process waste to a defined end-product standard, or primarily store it for removal?
• Can the manufacturer provide model-specific AS/NZS 1546.2 test or certification evidence, where that performance is being claimed?
• Does the designed composting cycle finish in the system's own active and out-of-service chambers, or require a separate secondary composting facility?
• If urine is diverted, what exactly receives it—and is that system designed and authorised to manage it?
• Is the system sized for full-time users and peak loads, including the site's climate and available service access?
• Are you buying a residential sanitation solution, an RV containment system, or a genuine processing system for another application?

Urine diversion: yes when justified—not as a shortcut around treatment

Urine diversion can be a very useful engineering option. It can help a small collection toilet stay manageable, support a vehicle's limited storage space or reduce unusually high liquid input to a true composting system.

But it should not obscure the most important distinction: where and how does the whole waste stream receive the treatment or safe disposal it requires? Urine sent away from the composting chamber is still liquid waste, and a small solids bin emptied frequently has not necessarily produced completed compost.

Urine diversion and true composting FAQs