Why Separation Matters: Liquids, Solids, and a Cleaner System

Most composting toilets separate liquids and solids because the two materials behave differently and cause odor problems when combined. Keeping them apart controls moisture in the composting chamber, reduces odor risk, makes the materials easier to process while keeping the whole system simpler to maintain.

The fact that some composting toilet manufacturers are still allowing the liquids and solids to combine is borderline negligence in our opinion, since this is certain to cause odor problems and compost quality issues. 

The cleanest composting toilet experience starts before composting begins. It starts with separation.

In a flush toilet, everything mixes immediately. Water, urine, feces, paper, and cleaning products become a single wastewater stream, processed at a treatment plant that may be miles away.

A urine diverting composting toilet takes a different approach: it keeps the two streams apart at the source, so each can be managed appropriately.

That design choice — simple in concept, significant in practice — is what makes the composting process cleaner, more stable, and easier to live with.

What Source Separation Means

Source separation means diverting liquids and solids before they can mix, at the point of use. In a urine diverting toilet, this is accomplished through the shape of the bowl itself: a forward-facing liquid channel guides urine into a separate collection path, while solids fall into the composting chamber below. No valves, no moving parts, no chemicals.

The result is two clean streams. Urine — nutrient-rich but liquid-heavy — goes to its own container or drain. Solids — which need structure, carbon, air, and time to stabilize — go to the composting chamber where conditions can be properly controlled.

Research on urine source separation has drawn attention in sanitation science for good reason. Urine carries a disproportionately high share of the nutrients in household wastewater — including nitrogen, phosphorus, and potassium — despite representing a small fraction of the total volume. Handling it separately preserves those nutrients for potential reuse and removes the largest source of liquid load from the composting side. Urine that hasn't been contaminated with humanure is unlikely to have pathogens present.

Why Moisture Control Matters

Healthy composting requires a specific moisture range — roughly 30 to 50 percent. Below that, microbial activity slows. Above it, the pile becomes waterlogged, oxygen is displaced, and the conditions shift from aerobic to anaerobic. (anaerobic lack of oxygen creates methane + odors)

Urine adds significant liquid volume to any sanitation system. A single adult produces roughly 1.5 liters of urine per day. In a mixed system, that volume alone can push the composting chamber well beyond the ideal moisture range, without any carbon correction keeping up.

When urine is diverted, the compost chamber stays drier by default. Carbon cover material, mixing and active aeration does the remaining work of absorbing moisture from solids and maintaining the right balance for actual aerobic composting that kills pathogens and makes real compost. Separation of liquids and solids enables aerobic composting to happen without odor problems. 

How Separation Supports Odor Control

The compounds most responsible for sewage odor — hydrogen sulfide, nitrous oxide and ammonia — are produced when urine and feces mix and begin breaking down together in low-oxygen conditions. Separation removes the conditions that generate these odorous volatile organic compounds (VOCs) from forming.

Urine can be easily treated with natural compounds like quicklime or citric acid to stabilize the nutrients making it odor-neutral and plant-ready. Solids, covered with carbon material and kept in an aerated chamber, break down aerobically with minimal smell. Neither stream poses a significant odor challenge when handled separately.

The key phrase is "managed correctly."

Raw urine stored in an open-air container in warm climates will quickly begin to break down and produce ammonia odors. That smell is nitrogen fertilizer being lost to the air.  This is why Virro's Urine Odor Stabilizer is useful: it delays that breakdown, keeping the nitrogen stable so the urine is odor free & ready for plant-use.

What Happens to Each Stream

The liquid stream

Urine is collected in a sealed container below the toilet, or using the Smart Drain kit to send it to a plumbing drain. Depending on the installation, it can be directed to a drain, an absorption field, or kept in a removable container for manual disposal.

Diluted at roughly 10 to 20 parts water to one part urine, it can be applied to trees and landscaping plants or even your backyard garden as a nitrogen-rich liquid fertilizer — sometimes called "liquid gold" in composting circles.

Virro's Float Sensor in the uring storage jug monitors the liquid container level and alerts when it is approaching capacity, removing the guesswork about when to empty. The urine storage jug in Renew toilets can be pre-dosed with stabilizer compound after each time it is emptied. In a full-time residential or RV setup most people use the Smart Drain urine diversion kit to drain urine into existing holding tanks or other drain plumbing. If you want to use the urine as fertilizer, stabilizing compounds can be periodically added to the drain to keep the urine odor free and plant-ready.

The solid stream

Solids enter the composting chamber and are immediately mixed and covered with carbon material — typically coconut coir, wood chips, or sawdust. Airflow from the ventilation system keeps the environment aerobic. Thermophilic aerobic microbes immediately begin breaking down the material, eventually resulting in a dry, stable compost that is significantly reduced in volume compared to the original input.

The composting chamber in the Renew system is designed for easy access to empty it periodically. When the chamber reaches capacity — typically every three to four weeks for two adults — the contents are removed for further outdoor curing before eventual land application.

Explore the products that make each stream manageable:

→ Renew Manual Compost Toilet

→ Urine Odor Stabilizer

Why Simple Systems Are Easier to Maintain

A separating toilet creates two predictable, manageable tasks instead of one unpredictable one. The liquid container follows a regular emptying schedule you can plan for. The composting chamber follows a longer cycle that you can check periodically. Each stream gives you clear signals — the float sensor tells you when the urine container is full; the compost chamber level tells you when it is time to empty.

Contrast that with a mixed composting system: moisture fluctuates with every use, odor events are harder to diagnose, and correcting an imbalance often means waiting for conditions to stabilize over days. Separation removes most of those failure modes.

This is also why urine diverting composting toilets have become the standard design in residential off-grid sanitation. The early mixed-chamber designs taught the field what not to do. Separation is what makes the technology reliable enough for everyday use.

Frequently Asked Questions

Why do composting toilets separate liquids and solids?

Because urine and feces behave differently and benefit from different handling conditions. Urine is liquid-heavy and nutrient-rich — it is best collected and managed or disposed of separately. Solids need a controlled aerobic environment with carbon and airflow to compost cleanly. Keeping them apart prevents excess moisture in the composting chamber, reduces odor risk, and makes both streams easier to manage independently.

Is urine sterile?

In a healthy person, urine leaving the body is essentially sterile — bacteria are not typically present when it exits. However, it can pick up bacteria from skin or the collection surface very quickly after leaving the body. For practical purposes, fresh urine is low-risk, and most guidance treats it as safe for diluted garden use on non-edible plants. Urine that has been stored for extended periods without stabilization is a different matter and should be handled with basic precautions.

Does urine go into the compost?

In a good urine diverting composting toilet, the answer is NO! Urine is collected separately and either disposed of or used as diluted fertilizer. Adding urine directly to the composting chamber in volume would disrupt moisture balance and create significant odor problems. 

Why not just mix everything together in a composting toilet?

Mixed-chamber composting toilets exist, but they are significantly harder to manage. The excess moisture from urine makes it difficult to maintain aerobic conditions, which leads to slower breakdown, odor risk, and more frequent interventions to correct the balance. The compounds in urine also create reactions with the compost solids, creating volatile organic compounds (VOCs) otherwise known as odors.  Separation removes the largest variable — liquid load — and makes the composting environment predictable and stable. The maintenance difference in practice is substantial.

Stabilized odorless urine can be combined with finished compost to create nutrient-rich compost, but raw urine should not be combined with raw humanure unless you want an odor nightmare. Anyone who tells you that it's not a problem to combine raw urine with raw humanure in a composting toilet either has a malfunctioning sense of smell, or questionable honesty. Read more here about the work that Rich Earth Institute has done on this topic.

How do I know when the liquid container is full?

Virro's Float Sensor monitors the liquid container level and sends an alert before it reaches capacity. Without a sensor, the only option is visual checking — which is easy to forget in a high-use household. For full-time residential use, a float sensor removes one more thing to track manually.

Separation is not a workaround or a compromise. It is the feature that makes a composting toilet genuinely practical. Two clean streams, each managed on its own schedule, with clear indicators and straightforward maintenance — that is the system.

Every element of the Renew is designed around this logic. The bowl shape that guides streams apart. The carbon chamber that keeps solids aerobic. The float sensor that monitors liquids. The stabilizer that extends the window between empties. Each part serves a specific role because separation created two specific jobs to do.