Filtration field for a septic tank do it yourself: schemes and calculations

Suburban sewage system has some features that affect the sanitary condition of the entire land. The filtration efficiency of sewage effluent depends on the design of the autonomous treatment plant.

As a final stage of cleaning, a filtration field for a septic tank is often used, which is necessary for the purification of a liquid. Find out how to make calculations and build a field with your own hands.

The content of the article:

  • Filtration field as part of sewage
  • Structural features of the PF
  • Typical device layout
  • Filter field design
    • How to choose a scheme and choose a place?
    • Dimension calculations and budgeting
  • Installation Instructions PF
    • Stage # 1 - Trenching
    • Stage number 2 - laying of perforated pipes
    • Stage №3 - ventilation device
    • Stage №4 - backfilling and further maintenance
  • Are there any other solutions?
  • Conclusions and useful video on the topic

Filtration field as part of sewage

Without a main part that performs the initial processing of sewage waste, that is, a septic tank, the filtration field is not used, since its purpose is the purification of already cleaned liquid. To make it clearer, consider how VOC works.

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The cleaning process begins in the drive, where sewage is separated into different fractions: solid mineral waste falls as a sediment, the fat floats up and forms a film, some of the substances remain in the water in the form of suspended matter If air supply is not provided, the process of decomposition of a certain part of the waste occurs due to the vital activity of anaerobic bacteria.

Scheme of sewer system

Diagram of a sewage system consisting of an internal sewage system, a two-chamber septic tank with air supply (accumulator + aerobic compartment), a collector and a filtration field

Then the liquid flows into the next compartment, equipped with ventilation, where aerobic microorganisms are engaged in the treatment of wastewater. They form activated sludge, which can later be used as a fertilizer. The result of a two-stage cleaning is a slightly turbid liquid, not yet suitable for use.

It turns into technical water or simply gets into the ground (ditch, reservoir) after undergoing purification, which is performed in the following way:

  • on the filter field;
  • in the infiltration tank;
  • directly in the ground;
  • in the filter well.

A typical multistage system, which has dozens of variants of performance, is good in that it effectively cleans sewage waste minimizes cooperation with waste pumps and keep the environment clean backyard plot. And now we will dwell on the construction of the filter field.

Structural features of the PF

The filtration field is a relatively large area of ​​land on which secondary cleaning of the liquid takes place.

This cleaning method is purely biological, natural in nature, and its value is in cost savings (no need to buy additional devices or filters).

PF size

The size of the PF depends on the area of ​​the free territory and the landscape of the garden plot. If there is not enough space, instead of a PF, an absorbing well is arranged, which also filters the liquid before it enters the ground.

A typical filtration field device is a system of parallel-laid drainage pipes (drains) that extend from a collector and are placed at equal intervals in ditches with a thick sand-gravel layer.

Previously, asbestos-cement pipes were used, now there is a more reliable and economical option - plastic drains. A prerequisite is the presence of ventilation (vertically installed risers providing oxygen access to the pipes).

The system design is aimed at ensuring that the liquid is evenly distributed over the selected area and has the maximum degree of purification, so there are several important points:

  • the distance between the drains is 1.5 m;
  • length of drainage pipes - no more than 20 m;
  • pipe diameter - 0.11 m;
  • intervals between ventilation risers - no more than 4 m;
  • height of risers above ground level - not less than 0.5 m.

In order for the fluid to move naturally, the pipes have a slope of 2 cm / m. Each drainage is surrounded by a filtering “cushion” of sand and pebbles (rubble, gravel), and is also protected from geo-fabric ingress from the earth.

Well filtration field

One of the complex options of the device: after cleaning in the filtration field, water enters the accumulation well, from where it is pumped out with the help of a pump. Its further way is to a pond or a ditch, and also to the surface - for irrigation and technical needs.

There is one condition, without which the installation of a septic tank with a filtration field is impractical. Requires special properties of the soil, that is, on loose coarse and fine-grained soils that have no connection between the particles, it is possible to construct an after-treatment system, and dense clay soils, particles of which are connected in a consolidated manner, for this purpose do not will fit.

Typical device layout

Whatever the overall size of the filtering field, its design consists of the following parts:

  • collector (control well, distribution well);
  • plastic drain networks (drainage pipes with holes);
  • ventilation risers;
  • filtering "pillows".

Traditionally, the drainage layer is poured out of sand and gravel (rubble, pebbles). To protect the drains using geotextiles. The sewage system with PF looks like this:

The thickness of the filter layer

Pay attention to the thickness of the drainage cushion. The minimum indicator is considered to be the total thickness of 1 m, in this scheme it is larger: crushed stone - 0.3-0.4 m, sand - 0.8-1 m

When building a filtration field with your own hands, it is not necessary to construct a collector yourself - on sale you can find plastic sewage tanks of the required volume.

Often do without the distribution well, connecting directly to the septic tank and pipe system - but it is convenient for small-sized PF.

4m x 3.75 m filtration field scheme

The scheme of the filtration field is 4 mx 3.75 m. The distance between the drains is 1.5 m, each drainage pipe is equipped with a ventilation riser. As an underground filter - “cushion” of sand and rubble with a layer of geotextile

Sometimes instead of the PF use ready-made plastic devices - infiltrators. They help out when there is a shortage of free space, and the ground has no layers of loam with sandy loam and has sufficient carrying capacity.

If desired, you can install multiple infiltration devices connected by pipes in series.

Infiltration system diagram

Scheme of the local sewer system with infiltration. Flower gardens are not recommended for breaking in the filtering fields, as the root system can damage the pipes. For the infiltrator, on the contrary, the decor of flowers is the most acceptable option.

Next, we consider how to design and install the PF correctly.

Filter field design

Drafting project - a mandatory step before any major construction. It is necessary to accurately make the markup, make calculations, estimate the budget, prepare materials, take into account all the nuances.

Professionally drafted project will save from errors that are peculiar to inexperienced beginners.

How to choose a scheme and choose a place?

The choice of the scheme depends on three factors:

  • type of septic tank;
  • availability of free territory;
  • cleaning requirements.

The fact is that the degree of purification differs in different septic tanks. For example, biological treatment stations (Topaz, Astra, Eurobion) do not need a filtration field at all: 98% purified water immediately enters the drainage trench or reservoir.

Septic tanks built independently of concrete rings, bricks or tires, on the contrary, do not by themselves are effective sewage treatment plants, therefore the liquid leaving them requires additional aftertreatment.

We recommend to read the instructions for self-installation of various types of septic tanks:

  1. Septic tank of concrete rings: device, diagrams + step-by-step installation process
  2. How a drain pit is constructed of bricks: options and methods of construction
  3. How to make a septic tank of their own hands: a step by step instruction
  4. How to make a two-chamber septic tank of concrete rings: instructions for construction

As a rule, all elements of the sewage system are located in one line, that is, they are arranged alternately in one direction from the house - first a septic tank, then a filtration field.

The scheme of a septic tank from 3 chambers

3-cell septic tank scheme (settling tank + anerobic cleaning chamber + storage well), constructed of concrete rings with a filtration field equipped with a ventilation system

This means that when setting up a septic tank, it must be borne in mind that a part of the free territory behind it will be required for the construction of the PF (or, at a minimum, the installation of an infiltrator).

When volumetric discharge of wastewater, the principle works: the “branching” and longer the network of drainage pipes, the more efficient the cleaning.

It is necessary to initially pay attention to the specifics of the device filter fields:

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Optimal enclosing soils

The device of the filtration field is possible only in soils with high filtration ability, which include sands of various sizes, gravel, rubble and pebble deposits

Ditch with constructed filtration field

The depth of laying the drainage pipes of the field is on average 0.8–1.0 m. Moreover, between the roof of the water-saturated horizon and the conditional bottom of the soil after-treatment system there must be at least 1 m

The area allotted for the filtration field

The organization of the filtration field requires the allocation of a fairly large area of ​​land that is not stuck or used in agricultural work.

Ground filtration system with turns

It is undesirable for the soil filtration system to have a large number of turns, each of which represents a potential hazard to blockages.

Optimal enclosing soils

Optimal enclosing soils

Ditch with constructed filtration field

Ditch with constructed filtration field

The area allotted for the filtration field

The area allotted for the filtration field

Ground filtration system with turns

Ground filtration system with turns

Dimension calculations and budgeting

To correctly calculate the size of the field, it is necessary to take into account the daily amount of wastewater and the composition of the soil. If you know exactly the features of the soil, you can push off from the volume of the septic tank.

In the calculations of the filter field will help the table.

Pipe length calculation

Suppose the volume of your septic tank is 8 m³, and the composition of the soil is coarse-grained homogeneous sand. Therefore, at least 4 m of perforated pipes (or 2 pipes of 2 m each) will be required for effective cleaning of the liquid from the septic tank.

But these are approximate calculations. There are tables that allow you to more accurately determine the size of the "working" area. They are based on consideration of such qualities as the permeability of soils.

Here is a variant of such a table, which can be useful for owners of suburban areas with clay or sandy soils.

Table with soil types

According to the table, we can conclude that areas with clay soils are not suitable for the device field filtering, and the most suitable are sandy areas with medium-grained and coarse-grained by sand

Peat indicators correspond to the data on silt sand, and pebble and gravel have maximum water permeability: their filtration coefficient is 100-200 m / day. For them, there are no allowable load norms, since such a loose composition is able to skip any volume of liquid.

Determining the size of the field, you can count the number of pipes, ventilation risers (an average of 1-2 for each drainage), backfill (gravel, pebbles, rubble, sand), geotextiles, and then derive the approximate cost of all materials.

Installation Instructions PF

In addition to the listed materials, a tool will be required for excavation (shovels, buckets, wheelbarrows). Trenches designed for drains are not as deep as a pit for a septic tank; therefore, construction equipment can be avoided. However, several pairs of workers will speed up the process.

The cycle of works on the filtration field device is conditionally divided into a number of standard steps:

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Stage 1: Laying the drainage pipes to the bottom of the pit

For the device of the filtration field, we develop a pit, the bottom of which is filled with rubble to improve filtration. Top lay drains - perforated pipes through which the treated wastewater will flow into the ground

Stage 2: Supply the sewer pipe to the system

To the trench with laid on the bottom of the drains we bring the sewer pipe out of the septic tank under the slope for the natural movement of wastewater

Stage 3: Assembling and installing ventilation risers

We connect drains to the sewer pipeline. At the opposite edge of the perforated pipes we put the ventilation risers

Stage 4: Filling of a ditch with a system of soil

After checking the system, i.e. after filling with water and control of work, we fall asleep to the filtration field with the soil rolled off during the development

Stage 1: Laying the drainage pipes to the bottom of the pit

Stage 1: Laying the drainage pipes to the bottom of the pit

Stage 2: Supply the sewer pipe to the system

Stage 2: Supply the sewer pipe to the system

Stage 3: Assembling and installing ventilation risers

Stage 3: Assembling and installing ventilation risers

Stage 4: Filling of a ditch with a system of soil

Stage 4: Filling of a ditch with a system of soil

Stage # 1 - Trenching

At the first stage it is necessary to prepare a place for laying the perforated pipes. There are two ways: you can dig one big pit, and then it will be more convenient to arrange drainage and collect construction of pipes, but you can make several trenches (according to the number of drains), which significantly shortens the time construction.

The depth of the pit should be such that the liquid in the pipes does not freeze during the cold season, that is, the extensive pipe system must be placed below the ground freezing level. When constructing ditches, one should remember about a small slope that allows the liquid to move naturally - by gravity. The slope is 1.5-2 cm / meter of pipe.

Drainage structure

The drainage structure usually consists of 2–3 or more branches. This is explained by the fact that the length of one branch should not exceed 20 m, and 20 m is not enough to ensure filtration of the entire volume of water.

At construction PF adhere to strict geometry. The pit, as a rule, has a square or rectangular shape, and the trenches are the same in length. Suppose you need a total length of pipes of 60 m - you can make 4 branches of 15 m each or 6 branches of 10 m each. The length of one drain is the distance from the inlet pipe (or collector) to the last vent "Fungus".

The lower part of the trenches is covered with coarse sand (from 10 cm to 1 m), then by 0.4-0.5 m gravel (rubble, pebbles). If catchment drains are needed, they are located in the ground under the sand, but not less than 1 m above groundwater.

Filtration field device

One of the options for the filtration field device. The only thing that does not matter - the composition of the soil, which is used to fill the installed drainage system. This may be the sand left over from the “cushion” and the soil that was removed during the excavation process.

The drainage pipes lead to a storage tank located in the opposite side of the septic tank.

Stage number 2 - laying of perforated pipes

Drainage pipes made of plastic are laid on the prepared base. The process itself is quite simple, the main thing - to choose the right pipe.

You can buy ready-made - smooth or corrugated, with perforations and a textile layer, or you can take ordinary sewer pipes and drill holes in them with a drill. The recommended diameter of drains is 100-110 mm.

Perforated corrugated pipe

Corrugated pipe with perforation and filter of geotextiles. Production material - PND, diameter - 110 mm, depth of laying - at most 5 m. Is on sale in bays, the cost of running meter - 140-160 rub.

Together with the pipes, it is necessary to purchase a set of fittings for connecting different elements. It will take corners and tees. The process of laying drainage pipes is described in more detail in this material.

Stage №3 - ventilation device

The ventilation system is necessary for oxygen to enter the tubes, without which aerobic bacteria lose their viability. For ventilation risers, you can use ordinary gray sewer pipes, covering them from the top with covers to protect them from debris.

Installation of ventilation pipes

If the drains are not longer than 4 meters, the ventilation pipes are installed at the end of the branches. Longer pipes are equipped with 2-4 fungal risers that cut into drains with the help of tees.

The minimum height of the ventilation pipes above the ground is 0.5 m. Usually they are tried to be made more accurate or decorated to maintain the aesthetic appeal of the garden landscape.

Stage №4 - backfilling and further maintenance

After laying the perforated pipes, backfilling is necessary. From the sides and from above, each branch is covered with rubble (the top layer is about 50 mm), then it is covered with a layer of geotextile and a finishing layer of soil. Geotextiles are used to prevent pipe silting. The ground above the drains should be compacted, but not to damage the pipes.

The filtration field is included in the work with the septic tank. Special actions for the maintenance of drains are not provided. It is believed that the PF functions flawlessly for 6-7 years, after which it is necessary to disassemble the structure and replace the gravel filter. To increase the service life of the filter, geotextiles are laid under the layer of gravel (crushed stone).

As an easy-to-install alternative to the traditional filtration field, a modern version has been developed - an infiltrator cassette, the installation of which will be introduced by the following photo selection:

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Step 1: Trench Development for Infiltrating Cassette

For the withdrawal of clarified and disinfected wastewater in the system of infiltrators we are developing a trench. The production should have a slope of 0.002 - 0.003 in the direction of the ground treatment to ensure natural flow

Step 2: Laying the geotextile to the bottom of the trench

The tamped bottom of the trench is covered with geotextiles, the edges of which are set up behind the septic tank and raised to the sides of the mine

Step 3: Filling rubble to the bottom of the trench

The bottom of the trench we fall asleep with fine rubble, trying to observe the slope created during the development

Step 4: Install and connect the infiltrators

Install the infiltrators on the prepared base and connect them to the septic tank. The device installation scheme may vary; their location does not affect the system’s performance.

Step 5: Filling the trench with rubble infiltrates

Fill the trench with infiltrators with gravel. We make the dumping in layers, without tamping, so as not to damage the system and the connection of components

Step 6: Hiding the geotextile system created

Trench with infiltrators and gravel poured from above is covered with geotextile

Step 7: Filling the trench with the primer

Fill the trench with rolled off soil during development. As a result, a low mound of 10–20 cm should remain above the trench; she will sit down within a couple of weeks.

Step 8: Landscaping with autonomous sewage

We equip the site with the system of infiltration of treated waste water so that over it the movement of transport is excluded

Step 1: Trench Development for Infiltrating Cassette

Step 1: Trench Development for Infiltrating Cassette

Step 2: Laying the geotextile to the bottom of the trench

Step 2: Laying the geotextile to the bottom of the trench

Step 3: Filling rubble to the bottom of the trench

Step 3: Filling rubble to the bottom of the trench

Step 4: Install and connect the infiltrators

Step 4: Install and connect the infiltrators

Step 5: Filling the trench with rubble infiltrates

Step 5: Filling the trench with rubble infiltrates

Step 6: Hiding the geotextile system created

Step 6: Hiding the geotextile system created

Step 7: Filling the trench with the primer

Step 7: Filling the trench with the primer

Step 8: Landscaping with autonomous sewage

Step 8: Landscaping with autonomous sewage

Are there any other solutions?

Not everyone can use the filtering field as a way to purify sewage. What to do to those who own a piece of clay or built a house in an area with a high level of groundwater?

Recommendations for the selection of septic tanks for high GWL are described. in this article.

Scheme of operation of the biological treatment station

Scheme of operation of the biological treatment station. After passing through several tanks equipped with aerators, air lifts and filters, the water becomes 98% clean. The main function of waste treatment, as in septic tanks, is performed by anaerobic and aerobic bacteria.

You can also create a sewer system with filter well, but its installation also requires a number of conditions (for example, non-clay soil and the location of groundwater one meter below the conditional bottom of the well).

If you just install a septic tank without additional treatment, insufficiently clarified and disinfected water will flow into the ground and an unpleasant smell may appear.

Conclusions and useful video on the topic

Videos with useful information about the device treatment facilities.

Septic Rostock with PF:

Theory of arrangement of a septic tank with filtering fields in pictures:

You can build a filtering field with your own hands if you correctly make calculations and fulfill all installation conditions. To determine the type of soil or choose a septic tank, you can contact the experts. A complete wastewater treatment system is a guarantee of environmental cleanliness and, consequently, comfort.

After studying our article, there are questions about the arrangement of the field for filtering, please write them in the block with comments and we will try to answer them promptly.

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