Designing autonomous sanitation for a remote eco-community

Sanitation is one of those building systems nobody thinks about until it stops working. For a centrally connected building, you hire a plumber and move on. For an autonomous building in a remote location with no municipal sewage, the question becomes a design problem with real constraints.

This is the situation we faced with Unity Hub, a three-story net-zero community centre being built at Tepla Gora, an eco-community in the Carpathian region of Ukraine. The building will serve 4-6 people daily and host gatherings of up to 40. It sits at the end of a 1 km rocky mountain road. There is no sewage connection. There never will be one.

We had to design the sanitation system from scratch. Here is what we found.

The initial proposal: a biogas-centred closed loop

Our first design used a HomeBiogas 4 anaerobic digester as the core. Human waste and kitchen scraps feed a reactor installed in the boiler room (where the temperature stays above 30C year-round), which produces cooking gas and liquid fertilizer. Waterless urinals on all three floors handle the majority of visits. A Separett incinerating toilet provides backup.

On paper, it closes the loop nicely. Waste becomes energy and nutrients. Nothing leaves the site. The system runs without electricity (except the backup incinerating toilet). Permaculture people love this kind of thing, and it makes for a good story.

We shared this proposal with the Tepla Gora team. One of the partners pushed back hard.

"Why not just use a bio septic?"

His argument was simple. He uses a concrete ring septic system at his own property. His friend, who runs a small hotel in the Carpathians, uses the same type of system. It works. No smell. Barely any maintenance. You can irrigate with the outflow. And since the Tepla Gora community already avoids harsh chemical cleaning products, a biological septic would perform well.

In Ukraine, concrete ring septic systems are everywhere. Two or three chambers built from precast concrete rings, buried below the frost line. Anaerobic bacteria break down organic matter. The final chamber drains into the soil through a gravel filter. No electricity. No moving parts. Lasts 40-60 years. Total installed cost for a system serving 10-20 people: roughly $1,700-3,200.

Our biogas system is based on a HomeBiogas 4 unit with one toilet, purchased in 2022 for $1,577 including shipping. The unit is already on hand, so the remaining cost is installation: custom gas plumbing, ventilation upgrades, and gas detectors — roughly $1,500-1,600. Total installed cost lands in a similar range to the concrete ring system.

We took the challenge seriously.

Breaking down the comparison

Economics

The concrete ring system wins on longevity and simplicity. It lasts 3-4 times longer, requires no electricity, and runs on $50-100 per year (bacterial cultures and occasional pump-outs). Installation takes one to two days with a backhoe and a crane.

The biogas system produces cooking gas and liquid fertilizer, which have real but modest value. With 4-6 daily users, realistic gas output covers 30-60 minutes of stovetop use per day from human waste alone. More with food scraps added. The liquid fertilizer is useful for Tepla Gora's gardens. But neither output changes the economics much.

The HomeBiogas unit was already purchased in 2022. That shifts the calculation, though it does not change the operational comparison.

Environmental impact

Both systems are biological and chemical-free in normal operation. Both die if you pour bleach or antibiotics into them. Tepla Gora already uses eco-friendly cleaning products, so both approaches benefit equally there.

Where they differ is treatment quality and risk.

The biogas reactor is a closed system. Waste goes in, gas and liquid come out. No groundwater contact. But the digestate still contains pathogens (mesophilic digestion does not fully sterilize), and you are putting methane and hydrogen sulfide inside an occupied building. Gas detectors and reliable ventilation are not optional.

The concrete ring system removes 30-70% of biological oxygen demand in the tank, then relies on the soil to finish the job. This works when soil conditions cooperate and the groundwater table is deep. When they do not, you are leaking partially treated sewage into the ground. The drainage chamber is designed to leak. In a location where nobody has tested the hydrogeology, that is an assumption we are not comfortable making without data.

Earthworks for a concrete ring system are moderate: three pits, about two metres deep, around 10-15 square metres of disturbed ground total. The earth is backfilled and recovers within a season. The biogas system avoids outdoor digging but requires indoor modifications: boiler room reconfiguration, gas lines through the building, upgraded ventilation.

Maintenance

The concrete ring system has the advantage of being boring. Nothing to check weekly or monthly. The only recurring task is calling a pump-out truck every 6-12 months at Unity Hub's usage level.

The biogas system requires weekly attention (emptying incinerating toilet ash, wiping urinals), monthly reactor checks, and quarterly filter inspections. If the boiler room cools below 20C during a power outage, the reactor stalls and needs 2-4 weeks to restart. The concrete ring system, buried below the frost line, works through power outages and cold snaps without interruption.

Winter performance

Ukrainian winters are a serious design constraint. The Carpathians see sustained temperatures of -10C to -25C.

The biogas reactor needs 30-38C for optimal operation. Our proposal puts it in the heated boiler room, which works as long as the boiler room stays warm. A prolonged power outage in winter breaks that assumption. If the reactor cools below 20C, recovery takes weeks.

The concrete ring system slows down in winter. Anaerobic bacterial activity drops to 10-20% of summer levels below 10C. But it does not stop. Sludge accumulates faster in cold months and gets digested during the warmer season. Buried 1.2-1.5 metres below the surface, insulated by the earth, and kept above freezing by the biological heat of continuous use. No external energy input.

The site constraint that changes everything

Tepla Gora sits at the end of a kilometre-long rocky road with steep sections. In winter, it is snow-covered and icy. High-clearance 4x4 vehicles manage it. Standard vehicles often cannot.

The most common pump-out trucks in Ukraine use GAZ-3307/3309 chassis: rear-wheel-drive, 7.85-8.2 tonnes gross, with standard road clearance. A 4x4 option exists (GAZ-33086 'Земляк' chassis, 265 mm ground clearance, 8.18 tonnes) but is far less common. Even the all-wheel-drive variant, loaded with 4 tonnes of liquid, would struggle on the steep icy sections. Most operators would refuse to try.

The concrete ring system requires pump-out service every 6-12 months. A system that depends on a truck that might not reach the site is not a maintenance plan. It is a design flaw.

There are workarounds. A tractor-mounted vacuum tank could handle the terrain. Oversizing the system and scheduling a single pump-out during dry summer months is another option. But these add logistics and uncertainty to what was supposed to be the simpler solution.

The biogas system produces no waste that requires vehicle removal. Liquid fertilizer goes directly to the garden. Incinerating toilet ash is handled by hand. Urinal output drains to a sand filter on-site. Nothing ever needs to leave Tepla Gora by truck.

This is where the partner's argument, which was winning on most practical metrics, runs into the specific reality of the site. His system works well at his property, presumably because a pump-out truck can reach his house.

What the community sees

Tepla Gora is an eco-community. Its members practice permaculture, yoga, and intentional living. They care about environmental impact. They also care about things working without drama.

The biogas concept appeals to the part of the community drawn to circular economy thinking, to the idea of producing cooking gas from waste. It makes for a good workshop topic and impresses visitors.

The concrete ring septic appeals to the part of the community that just wants a toilet that works. The partner who challenged our proposal lives this daily. His system works. He does not think about it. His friend's hotel runs on the same technology.

We keep seeing this split in eco-communities: the appeal of innovative systems versus the reality of what people actually maintain over years. Biogas installations look good in presentations. A reliable septic is what you want at 6 AM in December.

What investors and funders see

Grant funders and innovation programmes respond to novelty. "A circular biogas sanitation system for an autonomous building" reads well in a proposal. "We installed a standard concrete septic" does not get funded.

Practical investors and business-minded partners respond to reliability. Proven technology, low risk, long service life.

These are different audiences with different priorities. The question is whether a system can be structured to satisfy both without compromising either.

The question of innovation

We had to ask ourselves: does every subsystem of an innovative building need to be innovative?

Unity Hub's innovation is its integrated design. Net-zero energy, fossil-free operation, autonomous systems, a replicable community model. The sanitation subsystem needs to work. It does not need to be the headline.

Biogas technology is often perceived as cutting-edge, but anaerobic digestion has been used for over a century. Our proposal does not solve the core challenge of cold-climate biogas. We side-step it by using the boiler room. That is a workaround, not a research contribution.

The real applied research value would be in documenting the system's actual performance in Ukrainian conditions: temperature logs, gas output measurements, maintenance records, failure modes. That data is useful regardless of whether the system is primary or supplementary.

The direction that emerged

The research and community input point in the same direction: separate reliable sanitation from experimental energy recovery.

Primary sanitation would use proven technology that works year-round without electricity, without external service dependencies, and without gas safety concerns in an occupied building.

The HomeBiogas reactor, already purchased, would be fed with kitchen and food waste rather than connected to the toilet system. It still produces gas and fertilizer. But if it stalls in winter, nobody loses toilet access.

This separation also simplifies the building design considerably. No toilet-to-reactor plumbing through the building. No gas lines running from a human-waste reactor to a kitchen. Simpler ventilation. Lower safety burden.

The specific primary sanitation technology — whether concrete ring septic, aerobic treatment unit, constructed wetland, or composting system — depends on site investigations we still need to complete: groundwater level, soil percolation rate, available space at the required setback distances, and whether any vehicle-dependent system is realistic given the road. We will update this article as the design progresses and the system is built.

What this taught us

The partner's challenge was useful because it came from years of daily use, not theory. It forced us to evaluate our own proposal against a system that millions of people in Ukraine rely on. Some of what we found confirmed our original thinking. Some of it did not.

Daily sanitation and experimental energy systems should not share a single point of failure. If connecting them means that a stalled reactor leaves people without a toilet, the design is wrong regardless of how good the reactor is on paper.

Site constraints are design constraints, not footnotes. The mountain road to Tepla Gora eliminates any system that requires regular heavy vehicle access. Autonomous means autonomous, including from service trucks.

And the partner's trust in his own concrete ring system, built on years of quiet daily operation, is a form of data that specifications cannot replicate. We build within a community. If the community does not trust the system, the system has failed before it starts.

We are still finalizing the primary sanitation technology. The site investigations will determine that. But the architecture of the decision, separating what must be reliable from what we want to learn about, is settled. That distinction is worth more than any single technology choice.

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This investigation is part of the Unity Hub sanitation system design within the UA Unity Hub project at Tepla Gora.