What Is a Vault Toilet and How Does It Work?

Two common waterless toilet solutions are outhouses (aka pit toilets) and composting toilets. Neither is suitable for remote locations visited by large numbers of people, like state and national parks, wilderness areas and freeway rest stops.

Large-scale composting toilets require regular maintenance, and staff may be put off by the unpleasant task of raking the compost. Moreover, composting toilets only work in moderate climates. Pit toilets must be relocated when they fill up, which makes them impractical in the long term for busy locations.

Vault toilets present a third solution, the one favored by the U.S. Forest Service, according to a 2009 analysis by sanitary engineer Brenda Land. Unlike pit toilets, vault toilets don’t release toxins into the ground. The only maintenance they require is occasional pumping, which she says the Forest Service typically contracts to local septic pumping companies.

What works for the Forest Service could potentially work for homesteaders in remote locations with access to a septic pumping service. But installing one can be expensive.

What Is a Vault Toilet?

A vault toilet is pretty much what it sounds like: A buried, rectangular concrete vault that holds human waste. It’s a holding tank, much like a septic tank, but without input or output pipes, so no water flows through it. Instead, waste continues to accumulate until it’s pumped out.

To keep pumping to a minimum, the vault must be large. According to Land, the typical vault installed by the Forest Service holds from 500 to 1,500 gallons. A vault for an individual homestead could be smaller, but a minimum capacity probably wouldn’t be much less than 500 gallons. That ensures adequate ventilation and separation between the toilet seat and the bottom of the vault.

The structure above the vault includes a riser, i.e. a pedestal or bench with an opening fitted with a toilet seat. The seat is enclosed in a shelter, which may be lit and insulated, depending on the climate and availability of electricity. To control odors, a ventilation system exhausts gases from the vault into the surrounding atmosphere.

What Is a Vault Toilet vs. a Pit Toilet?

Landowners looking for a waterless toilet system typically install a pit toilet. They can dig the pit themselves with shovels and other manual tools and construct a wooden structure above it for privacy.

The larger the pit, the longer the toilet can stay in service. But eventually, it fills up. When that happens, the only remedy is digging a new pit, moving the structure and backfilling the old pit.

Because a vault toilet can be pumped out, it never needs to be moved. But installing one isn’t something most people could do themselves. You’ll need an excavator and a crane, driving up the cost. The shelters for Forest Service vault toilets are often large, with concrete floors and walls.

A key difference between pit toilets and vault toilets is the way they manage waste.

A vault toilet isolates the waste from the rest of the environment. But a pit toilet allows it to mix with the surrounding soil, allowing liquids containing pathogens to potentially leach into the groundwater and contaminate it. This isn’t a major problem for a toilet used by a single family. But for one servicing the public, it’s a major environmental concern.

How Do Vault Toilets Work?

Conceptually, vault toilets are simple: Waste remains in the vault until it’s pumped out, and a vent system exhausts gases outdoors to keep the shelter relatively odor-free.

In a 2003 Forest Service publication, Land discusses design tips to avoid potential problems:

• Location: The critical airflow zone around a vault toilet is a circle with a radius of 20 feet. Ideally, the zone is free of trees and other vegetation to allow odors to dissipate. When that isn’t possible, selective pruning of tree branches, with an eye toward the direction of the prevailing wind, may be necessary.
• Venting: Most vault toilets are passively vented through a pipe that extends at least three feet above the roof of the shelter. Air enters the vault through the riser or a wall vent and exits through the roof vent, creating a draft that keeps air circulating. In heavily forested areas with restricted airflow, Land recommends installing a fan to assist circulation. It’s important to cover the vent openings with screens to prevent birds and small animals from falling in.
• Drainage: The floor of the shelter should be sloped toward the door to prevent water from seeping under the riser and into the vault. The manhole for pumping outside the shelter needs to be covered, with the rim of the opening raised a few inches from the ground to keep out water. Land cautions against putting the manhole under the edge of the roof, to prevent falling rainwater from seeping under the cover and getting into the vault.

About the Expert

• Brenda Land is a sanitary engineer with the San Dimas Technology and Development Center in San Dimas, California. Over the last 30 years she’s written multiple articles on waste management for the U.S. Forest Service.

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