Plain Water: Septic Tanks

Somewhere under the lawn of a house at the end of a lane, a few metres from the back door, is a concrete or plastic box the size of a small car, buried, with a lid at ground level that the owner has probably never opened. Everything that goes down the drains of the house goes into it: the toilets, the shower, the kitchen sink, the washing machine. The box has no pump, no power and no moving parts, and from one side of it a pipe runs out to a line of trenches under the grass, where the water the box lets go seeps into the ground. It is a septic tank, it has been the sanitation of the countryside for a century and a half, and in the countries with the best sewers in the world, a fifth of the houses still have one.

The after you flush article on this site followed the water from a city toilet to the plant. This one follows it when there is no plant: into the tank, into the soil, and, when things go wrong, into the well and the bay.

A box with two chambers

The septic tank was patented in France in 1881, by a man named Mouras who had noticed that a sealed cesspool under his house, left for years, held far less solid matter than it should have, and who guessed, correctly, that something in the sealed tank was eating it. What was eating it was bacteria that live without oxygen, and a septic tank is a vessel designed to give them time. It holds two or three days of the house's water, three to four cubic metres for a family, usually in two chambers in series. The water comes in at one end through a baffle that stops it disturbing the surface; the solids that can sink sink to the bottom as sludge, the fats and oils that can float float to the top as a crust of scum, and the water in the middle, clearer than it came, leaves through a baffle at the other end. In the sludge and the scum the bacteria work slowly, breaking the solids down to gas and a smaller mass of residue, and the residue accumulates for years until the tank has to be emptied.

What the tank does to the water is less than its name suggests. It takes out most of the solids and perhaps a third of the organic load, the BOD of the article of that name, and almost none of the nitrogen, the phosphorus or the bacteria. The water that leaves the tank is still sewage, settled and greyer, and it would kill a stream if it reached one. The tank is the first stage. The second is the ground.

The soil

From the tank the water goes to a drain field: a set of trenches, usually half a metre wide and a metre deep, filled with gravel or plastic chambers around a perforated pipe, under which lies the unsaturated soil, the metre or two of earth between the trench and the water table that is damp and full of air and alive. As the water trickles down through it, the bacteria and fungi that live on the soil grains take the organic matter and eat it with oxygen, as the aeration tank of a sewage works does with a blower; the soil filters out the solids and most of the bacteria and viruses, which die in days outside a body; the phosphorus binds to the iron and clay; and at the bottom of the trench a thin black layer of biological growth called the biomat forms, which slows the water and does much of the work. A metre of good soil, loam over sand, with the water table two metres down, is a treatment plant as effective as most of the ones this site has described, for a single house, for nothing.

The soil is also where the design can fail, and the failures are of three kinds. Clay does not let the water through fast enough, the trenches fill and the sewage rises to the surface of the lawn or backs up into the house. Coarse sand or fractured rock lets it through too fast, before the soil has done its work, and the sewage reaches the groundwater nearly as it left the tank. And a high water table, in a flat wet country or on a coast, means the soil under the trench is saturated, with no air for the bacteria, and the water goes into the groundwater sideways. The regulators test the soil before a house is built, by digging a hole and timing how fast water drains from it, and the house that fails the test gets a mound of imported sand above the ground or a small mechanical plant instead; the houses built before anyone tested, which is most of them, have whatever the ground gave them.

Nitrogen

The thing the soil cannot do is take out the nitrogen. The ammonia in sewage is turned to nitrate by the bacteria in the aerated soil, as the nitrogen article on this site describes, and nitrate dissolves and moves with the water to the groundwater and on, and a septic suburb is a field of nitrate sources a few dozen metres apart. The water in the private well of the house next door, which the article on wells describes, draws from the same ground, and nitrate in rural wells is in large part the neighbours' tanks. At the scale of a coast the sum is an estuary. On Cape Cod, where tens of thousands of houses on sandy ground drain into the aquifer that feeds the bays, the regional plan found that septic systems were the source of most of the nitrogen that was turning the bays green with algae, and the towns are now spending billions on sewers, on tanks that remove nitrogen, and on shellfish beds to eat it. The lagoons of Florida's east coast, where the manatees died in the hundreds in 2021 when the seagrass was lost, tell the same story, and so do the bays of Long Island.

The truck

Every few years the tank is full of sludge and a truck comes, with a vacuum pump and a hose, and takes two or three cubic metres of the blackest water on this site away. Where it goes depends on the country. In the rich ones it goes to the nearest sewage works, which takes it into its sludge line, as the sludge article describes, and charges for it. In the cities of Africa and South Asia, where most of the people are on tanks or pit latrines rather than sewers, the truck is the whole sanitation system, and what it does with the load has been the subject of a decade of work under the name faecal sludge management: whether the trucks can reach the lanes, what they charge, whether there is a plant to take the load or a riverbank, and what the men who empty the pits by hand, where the trucks cannot go, are exposed to. About two and a half billion people live on this system, more than on sewers, and the plants that treat what the trucks bring, in Dakar, Durban, Kampala and Dhaka, are among the most important and least visited works in the world.

Why it stays

For all of that, the tank is the right answer for most of the houses that have one. A sewer to a village at the end of a lane costs more per house than the house, and carries the water tens of kilometres to be treated with energy and chemicals and discharged to a river, when the ground under the lawn would have done it for free. The failures are failures of the ground, the density and the neglect, and each has a fix: a soil test before the house is built, a limit on how close the tanks can be, a tank that removes nitrogen where the bay needs it, and the truck every three to five years, which is the one thing owners skip, because the tank is silent until it is not.

What it teaches

The septic tank is the plain water article about the treatment plant that most of the world owns and nobody runs. A box in the ground settles the solids, the soil under the lawn does the biology, and the water reaches the groundwater clean or not depending on what the ground is made of and how many neighbours are doing the same. The flushable wipes article on this site described what a wipe does to a sewer; in a tank it fills the box and blocks the baffle, and the truck comes early. The tank asks almost nothing of the owner. The ground does the rest, and the bay keeps the score.

Two days in the tank, a metre of soil, a truck every few years, and a bay that knows how many tanks there are.