Where to put the septic tank and soak pit on a small plot

The tank and the pit follow different rules

A septic tank is a sealed, watertight box. Its floor and walls are built so nothing leaks out (IS 2470 Part 1, cl. 4.1), so the code doesn't give it a minimum distance from the house. It asks for three things in cl. 3.3.1: put it somewhere open to the sky, as far from the outside wall of the building as you can, and somewhere a cleaner can reach it. It also keeps the tank out of swampy ground and areas that flood.

The soak pit (the code calls it a seepage pit) and the dispersion trench do the opposite job. They're built to leak: the lining has open joints so the effluent soaks into the soil. Germs travel with that water, and wet soil can weaken a foundation. So the code puts numbers on them in Part 2, cl. 5.3:

• not closer than 18 m to any source of drinking water, such as a well;
• as far from the nearest habitable building as you can afford, but not closer than 6 m;
• further still if the soil calls for it, judged from the percolation test and the soil's bearing capacity;
• no soak pit or trench at all in limestone or fissured rock, where cracks can carry effluent a long way.

A well counts whoever owns it, and so does a house. The code's site survey asks for every well, tank and stream nearby (Part 2, cl. 3.1 d) and warns about harm to "adjoining property" (cl. 3.2 b). A borewell you or a neighbour drink from is a source of drinking water too.

Why a small plot usually fails the 6 m rule

A 30x40 ft plot is 9.1 m by 12.2 m. If the house covers the plot except for a front strip of about 1.5 m and a rear strip of 1 m, no point on the plot is more than 1.5 m from your own wall. The 6 m rule can't be met anywhere, and the houses on either side often stand on the boundary line too.

Before you give up on a pit, check these in order:

1. Is there a sewer in the street? If the local body runs one, connecting to it usually beats any pit. Ask at the ward office what the connection needs.
2. Can you leave a yard of about 7.5 m? On a 40x60 ft plot (12.2 m by 18.3 m), a front or rear yard that deep fits a 1.2 m round pit with its near edge 6 m from the house. The neighbours' walls still have to be 6 m away, so check those too.
3. Is there any well within 18 m, on any plot? Walk the lane and ask. An open well two plots away can rule out the only spot that met the 6 m rule.
4. If nothing fits, the code points to a biological filter or an upflow anaerobic filter where soil absorption won't work or land is short (Part 2, cl. 5.3 and 6.0). A licensed engineer has to design it.

Ground water matters too. If the monsoon water level comes within 1.8 m of the surface, Table 1 rules out a pit and moves you to a trench in a raised mound or a filter. The percolation test guide covers the soil check.

Placing the tank: open sky, a reachable cover and the vent

The tank needs cleaning every one to two years; the code says once in two years at least for a small house tank (Part 1, cl. 5.1). Today that means a vacuum tanker parked outside and a hose dropped through the cover. Cl. 5.3 allows portable pumps, cl. 5.4 sends the sludge into a vehicle, and both say manual handling should be avoided. So place the tank where:

• the access cover is in the open, not under a room, a staircase or a tiled car porch you'd have to break;
• each chamber has its own opening, at least 455 x 610 mm or 500 mm round, with a sealed concrete or cast-iron cover and a lifting handle (cl. 3.4.9);
• a tanker can park close enough for its hose to reach. Ask a local desludging operator how long their hose is before you fix the spot;
• rain from the roof and the yard runs away from it, not onto the cover.

The vent pipe is at least 50 mm across with a mosquito-proof cage on top (cl. 3.4.10). If the tank is 20 m or more from the nearest building, a vent about 2 m high is enough. On a small plot the tank is always closer than that, so the vent must go 2 m above the top of the building. The code also lets you connect it to the house's own soil vent pipe instead.

Worked examples

Tips and tricks

• Draw the plot to scale on squared paper, mark every wall and well within 18 m, then draw a 6 m line round each house and an 18 m arc round each well. Whatever is left is where the pit can go.
• Measure to the edge of the pit, not its centre. On a 1.2 m pit that's 0.6 m you can't count.
• Keep the tank cover clear of anything you'd hate to break: tiles, a ramp, a planter. You'll open it every year or two.
• Put the pit where the ground is a little higher than the yard around it, or build the masonry ring the code shows, so rainwater doesn't run in (Part 2, cl. 5.2.1).
• Write down where the neighbours' wells are and the date you checked. If one is dug after you, the record shows your pit came first.
• Leave the pit area unbuilt. Part 2, cl. 5.3.1 calls it an advantage to have room to build a new absorption system later.

Frequently asked questions

Calculators for this job