It is not always possible to connect to a centralized sewage system, so the only alternative is a septic tank, which is also called a treatment plant. In this article, we will talk about the general principles of septic tanks and their purpose, as well as give recommendations for the independent construction of a septic tank from turnkey reinforced concrete rings.
Sewer drains contain various organic substances that, if released into drinking water or food, can cause serious problems. Therefore, before the drains go into the ground, they must be cleaned and made safe. This is exactly what a treatment plant does, where the organic content of fecal and sewage masses is separated into harmless sludge and relatively clean water, which can be poured onto the ground or used to water the garden. The septic tank itself does not affect the contents in any way, but it creates favorable conditions for the work of bacteria, which decompose any organic matter into harmless components.
How to choose a seat
The main document that regulates the choice of a place for installing a septic tank is SNiP 2.04.03-85 "Sewerage. External networks and structures". In addition, it is necessary to obtain a hygienic certificate that allows you to build a septic tank in the selected location, this is written in SanPiN 2.1.5.980-00 "Hygienic requirements for the protection of surface waters". According to these documents, a sewage treatment plant must be built no closer than 50 meters from a well or a well, but water from a septic tank must be discharged at a distance of at least 30 meters.
Also, the septic tank must be placed at a distance of at least 4 meters from the house and the water pipes passing by it, as well as at least 5 meters from the road. Having chosen a place for the construction of a septic tank, contact the district administration or the sanitary and epidemiological station (SES) for approval and obtaining a building permit and certificate. It is possible to build without the consent of the local administration, but if violations of any legal requirements are found, you will be bombarded with fines and requirements to move the capital structure to another place.
Calculation of volume and performance
For complete cleaning, the drains must be spent in the septic tank for at least three days, therefore, the volume of each septic tank compartment is determined by the formula V = 3 * A * B, where:
V - The volume of one compartment of the septic tank;
A - Conditional rate of water consumption per person;
B - The maximum number of people living in the house.
Septic tanks built according to this formula are very large, but they provide maximum cleaning of sewage and fecal waste, turning them into water and sludge suitable for irrigating a garden, which can be used as a good fertilizer. The conditional rate of water consumption depends on many factors. SNiP 2.04.01-85 Internal water supply and sewerage of buildings determines the conditional consumption rate of 95-300 liters per person per day. You can try to independently calculate how much water you consume, or use the table from this SNiP. We recommend using the maximum values, as well as increasing the number of residents by half.
This will lead to an increase in the cost of construction, but if guests come to you, the sewage treatment plant will not overflow and flood the ground in front of the house with feces. Some experts use the above formula, but determine from it the total volume of a septic tank from several departments. This approach seriously reduces construction costs, but worsens the degree of wastewater treatment with an increase in the number of residents.
If you dispose of purified water near your home and water intake points (up to 50 meters), then we recommend applying the formula to the volume of one compartment, but if the distance is over 50 meters, then you can apply the formula to the total volume. At such a distance, under-treated drains will not pose a danger.
Types of septic tanks
Several types of septic tanks can be built from concrete rings, which differ in:
- the level of penetration;
- insulation;
- diameter of wells.
The deepening of the treatment plant depends not only on its size, but also on winter frosts, that is, the depth of freezing. The lower the depth of freezing, the lower the septic tank should be, because a strong drop in temperature (below zero degrees) slows down or even stops the processes taking place in each of the departments. Therefore, it is necessary either to deeply deepen all sections of the septic tank, or to insulate them with foam or other materials. It is possible to reduce costs with a strong deepening of the septic tank if you raise not concrete rings, but a brick well above the freezing depth. Indeed, above the depth of freezing, even with good insulation, the processes are still greatly slowed down.
In addition, septic tanks are divided according to the number of elements. Wastewater treatment plants with one element are called cesspools and, they are characterized by minimal efficiency. Maximum efficiency in septic tanks of three elements, a further increase in the number of elements no longer increases the quality of treatment. Three compartments can be placed along, in this case, the whole structure turns out to be very long, or installed in a triangle close to each other, this reduces the area of the pit for installing a septic tank by almost a third, and therefore the volume of earth that needs to be dug. If a small septic tank is needed, then it is made single, and a wall with an overflow hole is laid inside the rings, which divides the space into two chambers.
How to build yourself
Despite the fact that you will have to hire construction equipment, self-construction of such a septic tank will significantly save money. After all, there is nothing complicated in its construction for a person who is at least a little versed in construction and installation, concrete and plumbing works. All calculations are as primitive and approximate as possible, so even a schoolchild can perform them. The main condition is that the drains should flow into the septic tank by gravity, so the optimal slope between the outlet of the house sewer and the inlet of the septic tank is 1-3 cm per meter. The septic tank can also be installed on a hill, but for this you will have to install a pump and valves that will not allow the drains to move in the opposite direction.
Having determined the volume of the septic tank, consider ways to discharge the purified water. It can be pumped into the fields or drained directly into the ground. If you chose the second option, then in addition to the pit for the septic tank, you will also need to dig trenches in which to lay drainage pipes that drain water 10–20 meters from the foundation of the septic tank. Drainage of water near the foundation will result in subsidence and damage to the septic tank. Water should flow through the pipes by gravity, and in the place of discharge you need to dig a hole (naturally below the pipe) measuring 1x1x1 meters (length, width, depth) and fill it with sand or fine gravel. Then lay the pipe and fill it up a meter with the same sand or crushed stone, then fill it with earth or fill it up to the very top, this will be drainage.
It is also necessary to choose suitable rings and start digging a pit. The rings are chosen based on the area that can be allocated for a septic tank and the ability to dig deeply. In areas with rocky soil, it is preferable to use the widest rings, for example, the KS-30-1 (KFK-30-1) brand, the inner diameter of which is 3 meters. If there is a shortage of free space on the site and clay, loamy or sandy soil, it is advisable to make the deepest pits under the KS-10-9 (KFK-10-9) rings, whose inner diameter is 1 meter.
It is also necessary to determine the position of each column in advance, given that the optimal distance between them is 50 cm. This space is needed for insulation and protection in case of ground movements. Having determined the depth of the well, add to it the depth of freezing (if you want to protect the septic tank as much as possible from frost heaving) and cushions of rubble and concrete (approximately 20 and 30 cm). This will be the depth of the pit that you will have to dig. It is known from experience that digging a foundation pit on your own is very long and difficult, so it is easier to hire an excavator that will do all the work in 1-3 hours.
If for some reason it is not possible to hire an excavator, you can use the services of teams carrying out earthwork, the cost will cost the same, but it will take much more time.
Pulling out a hole and leveling the walls, begin to compact the bottom. This is a mandatory procedure on all soils, except for rocky ones. Without it, the foundation of the septic tank will not receive enough strength. It can be compacted using a vibrating plate or a manual rammer. Then pour and compact a layer of crushed stone with a fraction of 40–60 mm and a thickness of 15 cm, on top of it a layer of crushed stone with a fraction of 10–15 mm and a thickness of 5 cm. The surface of both layers of crushed stone must be leveled and checked before and after compaction.
Having finished with a crushed stone pillow, mount the formwork, which in size should exceed the future septic tank by at least 20 cm in either direction. From the reinforcement, lay a double-row mesh with a distance between layers of 10 cm. The mesh size of the mesh is 10–20 cm. All joints of longitudinal and transverse reinforcement must be tied with wire or welded. Having finished with the preparation, pour concrete of a grade not worse than M200 and a class not lower than B15. After pouring concrete, be sure to check the level of the pillow, level it if necessary. You can start installing rings in 25-28 days.
Septic tank installation
Before starting the installation, it is necessary to deliver the rings, floor slabs and all the necessary material, and it is advisable to take the rings with a ready-made hole. In this case, you will have to work less with a puncher. If there are no holes in the rings, then they will need to be drilled using a perforator and a crown of a suitable diameter. It is also necessary to take rings with a lock, they are easier to install and better tolerate ground movements. To mount the rings you will need:
- lifting crane;
- a couple of assistants;
- long staircase;
- sealant, for example, Aquacement, but you can use any other suitable for reinforced concrete.
- Mark the places of all rings on the concrete pad and apply sealant around the perimeter of the future ring.
- Put on the first row of rings and apply a layer of sealant to the locks, then put the next row and so on until the end.
- Having installed all the rings, proceed to punching holes, we recommend using a crown with a diameter of 115–120 mm. It is very convenient to insert PVC pipes into such a hole, from which you will make bypass channels, and the inlet channel should be 5-15 cm higher, due to which the outlet of the drainage pipe will be 15-45 cm lower than the entrance to the septic tank. Both the incoming and outgoing pipes must be turned down and lowered by 15–25 cm, this will significantly increase the cleaning efficiency, because it will not let organics with positive buoyancy into the next chamber. And the processed sludge, due to negative buoyancy, will go to the bottom.
- After installing all pipes, including the drainage, install the covers with holes for the manholes.
After installing the hatches, it is necessary to install ventilation and wells. In the first chamber of the septic tank, ventilation is not needed, because all processes there take place with a minimum amount of oxygen, but in the second, ventilation is necessary. Wells can be made from bricks or rings. Some make wells according to the width of the opening for the hatch, but in such cramped conditions it is very difficult to maintain a septic tank. Therefore, we recommend making a well at least one meter wide. This increases construction costs, but greatly facilitates the maintenance of the septic tank. Cover the well with a stove and a lid. After finishing with the installation, be sure to coat the entire septic tank from the outside with sealant.
Insulation and backfill
For insulation, use either a special foam shell, which you can buy in hardware stores, or cut the foam into small strips and glue it onto the polyurethane foam. You can also cover the entire septic tank outside with polyurethane foam, which will be comparable in price to any other insulation, but it will take many times less time. Read more about insulation with polyurethane foam here (Warming a house with polyurethane foam - walls, roofs with your own hands). Having finished with insulation, proceed to backfill. Try to fill the pit evenly, from all sides, in small portions, this will help to avoid shifting the rings. After leveling the surface of the backfill with the ground around it, wait 2-3 weeks and, when the ground around the septic tank sags, add more. The next top-up will need to be done after the rainy season.
Operation and cleaning
The septic tank can be used after you have finished building the wells. Septic tanks do not require any special maintenance; it is necessary to check the sludge level in the first and second chambers once every 3-5 years. When going down the hatch to check, be sure to use a gas mask, otherwise you may suffocate. If the level of sludge is more than half the distance to the outlet pipe, then it is necessary either to call a suction truck, or to scoop it out with buckets. Sludge from a septic tank can be used as fertilizer for feeding non-food plants.
