A septic tank made of reinforced concrete rings consists of several wells in which the rings are installed on top of each other. The first well is a septic tank and is required to be installed, the rest of the tanks (their number may vary) is a system of filter wells into which the treated effluent from the septic tank will be overflowed.
We calculate the volume of the septic tank
The volume of a septic tank is a value that cannot be taken from the ceiling or on the advice of neighbors. The calculation of a septic tank from concrete rings is regulated by building codes, is individual for each user and depends on a number of factors: not only on the number of residents living in the house, but also on the way of life, the degree of improvement of the building, the composition of the soil, the level of groundwater, the climatic zone. Let's figure it out in order.
In our calculations, we will rely on such official documents as:
- Set of Rules 32.13330.2012 (year) "Sewerage. External networks and facilities"
- STO NOSTROY 2.17.176-2015 (year) "External engineering networks. Autonomous sewage systems with septic tanks and underground wastewater filtration facilities. Design and installation rules, implementation control, requirements for work results", as well as other relevant documents.
The Code of Practice 32.13330.2012 dryly and concisely tells us the following (clause 9.2.13.3):
ECHZh - the equivalent number of residents (in our case, the number of residents in the house).
If less than 25 people live in your house, then you can calculate the volume of a septic tank from concrete rings using the formula: daily water consumption by one person multiplied by the number of people and by THREE days.
As we can see, clause 9.2.13.4 allows us to use a single-chamber septic tank, implying that after "pre-cleaning" our septic tank will be pumped out. Our goal is the operation of wastewater treatment plants without pumping out, i.e. full autonomy. Therefore, it makes sense to use two chambers (1 chamber - a septic tank with a concrete bottom, 2 chamber - a filter well) or even three chambers (1 chamber - a septic tank with a concrete bottom, 2 and 3 chambers - filter wells), despite the fact that SNiP recommends doing this, with a large number of tenants.
Water consumption per person is determined according to the Code of Rules 30.13330.2012 "Internal water supply and sewerage of buildings":
Table A.2 - Estimated (specific) annual average daily consumption of water (effluent) in residential buildings, l / day, per 1 inhabitant
| Residential buildings | Construction and climatic region | |||
| I and II | III and IV | |||
| general | including hot | general | including hot | |
| With plumbing and sewerage without baths | 100 | 40 | 110 | 45 |
| The same with gas supply | 120 | 48 | 135 | 55 |
| With plumbing, sewerage and baths with solid fuel water heaters | 150 | 60 | 170 | 70 |
| The same with gas water heaters | 210 | 60 | 235 | 95 |
| With centralized hot water supply and sitz baths | 230 | 85 | 260 | 105 |
| The same, with baths longer than 1500-1700 mm | 250 | 100 | 285 | 115 |
For most country houses in the Moscow region (II climatic region), this figure will be at the level of 150 liters per day per person. With economical use, the spillway can be reduced to 100 liters per day (possibly less). If you pour it to the fullest, get ready to drain 250 liters of water per day per person.
The climatic region is determined according to SP.131.13330.2012 "Construction climatology". The average monthly temperature in January in Moscow is -7.8 ˚С, in July 18.7 С. Based on this, we define the climatic zone as IIG (DVA-GE). You can read more about this on the page in the information section.
For example, let's say that 3 people live in our house.
The volume of the septic tank in this case will be:
- 3 people * 90 liters / day * 3 days = 810 liters (3 people, economical consumption);
- 3 people * 150 liters / day * 3 days = 1350 liters (3 people, average consumption);
- 3 people * 220 liters / day * 3 days = 1980 liters (3 people, maximum flow).
How many rings do you need?
The calculator for calculating a septic tank from concrete rings provides fairly accurate data on the number of rings to be laid. It should be borne in mind here that the volume of one KS-10-9 ring (Wall ring: inner diameter - 1 meter, wall height - 90 cm) is 3.14 * 0.5 * 0.5 * 0.9 = 0.7 m3 , or 700 liters.
This means that for the manufacture of a tank for a septic tank, we need:
- 2 rings (3 people, economical);
- 2 rings (3 people, average consumption);
- 3 rings (3 people, maximum consumption).
At the same time, the top ring is never filled purely technologically and remains dry. This is due to the fact that the inlet pipe (coming from the house) cuts into the septic tank at a depth of about 70 cm.
This means that one more thing needs to be added to our number of rings:
- 3 rings (3 people, economical);
- 3 rings (3 people, average consumption);
- 4 rings (3 persons, maximum consumption).
This will be your septic tank (the first well shaft), in which the wastewater treatment process is underway. Now, the cleaned wastewater needs to be drained somewhere ..
For this, another mine (filter well) is being built nearby, into which the treated effluent from the septic tank will be overflowed. The main requirement for the second container is that it must have time to release all the water entering it into the ground, and all the drained water (and we considered its consumption higher) must be absorbed into the ground! Only in this situation, we will not have to pump out our septic tank and we will achieve full autonomy.
We calculate the volume of the filter well
IMPORTANT! If your site has a high level of groundwater, then the filter well is not for you, but for your case you need to use aeration fields.
The filter well passes water into the ground through the bottom, as well as through the holes in the walls of the rings. If the filtering well is not arranged correctly, the water will not have time to go into the ground and, as a result, it will have to be pumped out regularly, therefore, if one well does not cope with its tasks, then a second is built next to it (it is not allowed to put more than 4 rings in the mine) ...
Or by conducting a simple experiment:
Remove the top layer of soil (chernozem) and when you reach the bottom layer of soil, roll it into a ball that fits in your palm:
- If the ball collapses when squeezed - this is sandy loam
- If the ball turns into a cake, with cracks along the edges - it's loam
- If the ball turns into a cake, and cracks do NOT form around the edges - this is clay
Having learned the type of our soil, we turn to the table "soil absorption forces" (STO NOSTROY 2.17.176-2015 p. 25-26):
| P / p No. | Name of breeds | Soil filtration coefficient, m / day | Permissible design load per m2 of filtering surface, l / day |
| Clay soils | |||
| 1 | Clay | less than 0.001 | Less than 1 |
| 2 | Heavy loam | 0,001-0,05 | 1-30 |
| 3 | Light to medium loam | 0,05-0,4 | 30-40 |
| 4 | Suspension dense | 0,01-0,1 | 25-35 |
| 5 | Loose suspension | 0,5-1,0 | 45-55 |
| Sandy soils | |||
| 6 | Silty clayey sand with a predominant fraction of 0.01-0.05 mm | 0,1-1,0 | 35-55 |
| 7 | Dusty homogeneous sand with a predominant fraction of 0.01-0.05 mm | 1,5-5,0 | 60-80 |
| 8 | Fine-grained clayey sand with a predominant fraction of 0.1-0.25 mm | 10-15 | 80-100 |
| 9 | Fine-grained homogeneous sand with a predominant fraction of 0.1-0.25 mm | 20-25 | 105-110 |
| 10 | Medium-grained clayey sand with a predominant fraction of 0.25-0.5 mm | 35-50 | 115-130 |
| 11 | Medium-grained homogeneous sand with a predominant fraction of 0.25-0.5 mm | 35-40 | 115-120 |
| 12 | Coarse sand, slightly clayey with a predominant fraction of 0.5-1.0 mm | 35-40 | 115-120 |
| 13 | Coarse-grained homogeneous sand with a predominant fraction of 0.5-1.0 mm | 60-75 | 130-160 |
| Pebble and gravel soils | |||
| 14 | Pebble with sand | 20-100 | 100-170 |
| 15 | Sorted pebbles | more than 100 | - |
| 16 | Pebble clean | 100-200 | - |
| 17 | Gravel clean | 100-200 | - |
| 18 | Gravel with sand | 75-150 | 160-200 |
| 19 | Gravel-pebble soils with a significant admixture of small particles | 20-60 | 105-130 |
| Peat | |||
| 20 | Little decomposed peat | 1,0-4,5 | 55-745 |
| 21 | Medium decomposed peat | 0,15-1,0 | 35-55 |
| 22 | Peat heavily decomposed | 0,01-0,15 | 25-35 |
We select the type of our soil and, at the column on the far right, we determine the amount of "water absorption" in liters per 1 m2 of soil surface per day. In my case, this is loam and 35 liters per m2 per day. Therefore, in order to absorb all the water, we need to distribute it on the following surface of our soil:
- 3 people * 90 (liters / day) / 35 (liters / day * m2) = 7.7 m2 (3 people, economical consumption, loam soil)
- 3 people * 150 (liters / day) / 35 (liters / day * m2) = 12.8 m2 (3 people, average consumption, loam soil)
- 3 people * 220 (liters / day) / 35 (liters / day * m2) = 18.8 m2 (3 people, maximum consumption, loam soil)
The filter well passes water into the ground through the bottom (on which a bottom filter is arranged from a layer of crushed stone ~ 20 cm), as well as through holes in the walls of the rings. In order to gain the required number of square meters of the "absorption" surface, we will count the number of required rings.
- The bottom area of the well is 3.14 * 0.5 * 0.5 = 0.7 m2.
- The surface area of the wall of one ring is 3.14 * 0.9 * 0.9 = 2.5 m2
This means that the bottom ring gives us 3.2 m2 of "absorption" surface, each subsequent ring adds another 2.5 m2, therefore, the number of rings we need is:
- 3.2m2 + 2.5m2 + 2.5m2 = 8.2 m2 (> 7.7 m2) - 3 rings (3 people, economical consumption, loam soil)
- 3.2m2 + 2.5m2 + 2.5m2 + 2.5m2 + 2.5m2 = 13.2 m2 (> 12.8 m2) - 5 rings (3 people, average consumption, loamy soil)
- 3.2m2 + 2.5m2 + 2.5m2 + 2.5m2 + 2.5m2 + 2.5m2 + 2.5m2 = 18.2 m2 (~ 18.8 m2) - 7 rings (3 people, maximum consumption, soil loam)
We also take into account that this is the number of working rings, there will be another ring on top, as in the case of a septic tank. We get 4, 6 and 8 rings, respectively, for the constructed filtering well.
We remember the rule according to which the maximum working depth of the septic tank is 2.5 meters. This means that in the second and third versions, the filter well must be split into TWO wells. In the case of an average consumption, we get 2 shafts with 3 and 4 rings each. In the case of maximum flow, we have 2 shafts with 4 and 5 rings.
Well, here are our results:
Given: 3 people, economical consumption, loam soil, low ground level.
Result: treatment facilities from a septic tank for 3 rings and a filter well for 4 rings.
The cost of installation is approximately ~ 44,500 rubles.
Given: 3 people, average consumption, loam soil, low ground level.
Result: treatment facilities from a septic tank for 3 rings and two filtering wells for 4 and 3 rings.
The cost of installation is approximately ~ 65,000 rubles.
Given: 3 persons, maximum consumption, loam soil, low ground level.
Result: treatment facilities from a septic tank for 4 rings and two filtering wells for 5 and 4 rings.
The cost of installation is approximately ~ 78,500 rubles.
The calculation of a septic tank for a private house is not so difficult, but in any case it is individual for EACH SPECIFIC CASE. Only an individual and correct calculation will give you that treatment system that will work and do not require frequent cleaning.
