An outhouse or pit toilet is primarily a hole dug into the ground, into which biological waste solids and liquids are introduced, similar to a cesspit. If sufficient moisture is available, natural bacteria within the waste materials begin fermentation. Earthworms, amoebas, molds, and other organisms in the surrounding ground soils and flying insects entering the privy hole also consume nutrients in the waste material, slowly decomposing the wastes and forming a compost pile in the base of the pit. Bacteria form a complex biofilm on the wastes and in the surrounding exposed soils around the perimeter of the pit and feed on the wastes splashed or dropped into the pit.
The pit operates differently from a septic tank in that the pit is not normally filled with standing water. The solids act as a sponge to retain moisture but also are exposed to open air, allowing for insects and earthworms to feed on the wastes which would not be possible within a septic tank. Septic tanks also tend to contain only organisms that can survive anaerobic conditions (conditions where Oxygen is not readily available), while the open outhouse pit can sustain both aerobic and anaerobic organisms.
The process of decomposition is slow due to the layering of waste materials but is generally effective if the input of new wastes does not exceed the decomposition rate of the bacteria and other organisms. Small amounts of moisture from urination are absorbed by existing decomposed wastes in the base of the pit. In soils where the percolation rate of water through the soil is slow and where there is not a large amount of waste entering the pit, the wastes can slowly decompose and be rendered harmless without causing groundwater contamination.
In soils with a fast rate of percolation such as sandy soils, or where the base of the pit penetrates topsoils and clay going directly down to underlying gravel and fractured substone, waste liquids entering the unlined pit may quickly seep deep underground before bacteria and other organisms can remove contaminants, leading to groundwater pollution. This fast percolation of liquid wastes out of the pit can be slowed or prevented in newly dug outhouses by lining the base of the pit and the walls with a layer of absorptive organic material such as a thick mat of grass clippings. This material then decomposes and becomes part of the compost pile lining the pit that continues to act as a moisture sponge.
In most pit designs, the privy hole is covered by a small building. The primary purpose of the building is for human comfort, so that the user does not get wet when it is raining or cold when it is windy. However the building has the secondary and (possibly unintended by the builder) effect of protecting the privy hole from large influxes of water when it is raining, which would flood the hole and flush untreated wastes into the underlying soils before they can decompose.
On flat or low-lying ground, the privy hole can be further protected from rain and flood waters by constructing a small raised hill or berm around the edge of the hole, using material from the hole when the pit is first excavated, to raise up the outhouse foundation. This helps falling rain and surface water to flow away from the sides of the outhouse so it does not enter the pit and lead to groundwater contamination.
Rain and surface water flowing into a low-lying open pit will also lead to soil erosion around the edges of the pit that may eventually undermine the building foundation, and potentially lead to collapse of the structure into the enlarging hole.
Eventually over a period of many years, the solid waste forms a growing pile that fills the pit. A new pit is dug somewhere nearby, and soil is used from the new pit to cover and cap off the old pit. Underground organisms such as earthworms continue decomposition of the old pit until the material is indistinguishable from other ground soils.
In locations where a pit must service a large number of users, the single pit may be extended to form a long covered trench or as a series of separate pits, so that the waste inputs are spread out over a larger surface area. The fastest waste decomposition generally occurs in the uppermost layer of solids exposed to the air. Decomposition continues slowly in deeper layers but relies on diffusion of air into the solids to sustain life for the organisms within the solids.
A deeper pit may appear to provide additional capacity but a thick layer of fresh solids deposited by many users may exceed the natural decomposition rate of the organisms in the pit, leading to increased potential for waste seepage out of the pit. A deep pit may also penetrate upper slow-percolation surface soil layers, and allow entry of contaminated waste liquids into the underlying fast percolation subsoils.
Decomposition may be accelerated by stirring or turning the pile, which breaks up the pile and introduces air pockets and air channels into the pile to allow faster organism growth within the bed of solids. There are also commercial products such as PORT from Southland Organics that add a mixture of bacteria, fungus and nitrogen to give a spurt of activity and accelerate the composting activity within the pit.
Pit toilets are becoming far less ubiquitous in America’s parks than they once were. Environmental regulations and the improved sanitary benefits of vault toilets mean that those facilities are gradually being replaced by the similar but more advanced vault.