Septic, Choice of the Appropriate Option, Size Calculation

Design
Before starting construction, it is necessary to make calculations. So, it is believed that in a well-maintained house, 200 liters / day of water are consumed per person. In this case, the volume of the septic tank should be such as to accommodate a three-fold rate. We get that for a family of 4 people it is required (200 * 4) * 3 = 2400 liters. Add 500 liters for a few guests and get almost three cube of drains.
But do not think that the volume obtained above is not the volume of the septic tank, the internal volume of the chamber will be greater. Why it becomes clear below.
The septic tank is a flow settler. It must be airtight and have one inlet pipe and one outlet. That is, water is poured into the chamber, settled and leaves on another pipe.
Common Mistakes

• The device septic tank without a bottom - it is wrong to call this structure a septic tank, it is a cesspool, with the only difference that not only solid waste gets there but also gray waters from the bathroom and the kitchen. An analogue of this structure is an ordinary country toilet.
• The device of a septic tank without an outlet pipe - as in the above case, it is illegal to call such a structure a septic tank - it is a storage device. The container is gradually filled with drains and then it has to be pumped out.

Pipes for septic tanks
So, in the septic tank there is an incoming pipe, through which drains enter, and outgoing, along which the settled dirty water leaves. To ensure that when filling the tank, the water does not go back, the outlet pipe should be installed no less than 10 cm below the incoming pipe. For transportation of drains in own sewerage, a pipe with a diameter of 110 mm should be used. Material - red plastic, cast iron or asbestos cement pressure pipes.
Form of septic tank
Now it is worth to touch the form of the septic tank. It does not really matter what the shape of the tank is, it can be either circular or square - water will still fill the entire volume. The only advantage of round capacity is that this form allows you to save on the thickness of the material. Because the pressure forces of the ground go along the tangent to the surface of the septic tank and do not create such a strong pressure as when interacting with flat walls. Between the septic tanks of one volume is worth preferring to one that has more depth. That is, it is preferable to position the vessel deep into the interior, rather than breadth - within reasonable limits of course. But, going from reasoning to figures, let's say the following. The minimum useful depth of the septic tank should be one and a half meters, and then it is already possible to consider how it is more convenient to place the container, dig down or to the sides.
The septic material
In fact, the shape of the pit is determined by the septic material. If you use a finished plastic or metal container, then there is no choice. In this case, you need to fix the container in the ground, so that it is not squeezed out. This is done with a concrete slab. When using reinforced concrete, the rings have to be dig deep, and to produce the required volume, several chambers must be made. If you fill the septic tank from the concrete in place, then it is easier to make a pit rectangular. It's easier to dig, it's easier to count.
Volume and height of the septic tank
Now returning to the volume of the septic tank - 3 cube. The minimum useful depth is the distance from the bottom to the bottom surface of the discharge pipe. Dividing the volume by the height, we get that the inner area of the septic tank should be 2 meters square. For example, for a rectangular septic tank, you can take a width of 1 meter, and a length of 2 meters.
Add to one and a half meters still the outside diameter of the pipe (depends on the material). We also assume that the supply pipe should be 10 cm above the outlet pipe - we add them to the height. Then, to install the tee on the ends of the pipes it is necessary to add about 20 cm. Therefore, in our version, when using asbestos-cement pipes, we obtain a height of 1930 mm (outer diameter of the pipe is 130 mm). And the internal volume will be equal to 3.86 cubic meters.