All-water tank: operation pros cons and construction costs

The all-water pit is the main element of an individual domestic wastewater treatment system. This is the most widespread alternative solution, in the absence of a network of sewage. Be careful however, the connection to a collective sanitation system passing nearby remains obligatory.

Operation of the all-water pit

The septic tank is a 3,000 to 10,000 liter tank, divided into two unequal volumes by a partition wall, allowing the free circulation of gases at the top and liquid fluids at mid-height. Diametrically opposed, the ducts offeed et evacuation are located at the top of each compartment. Of the visit stamps and of emptying are provided on the upper horizontal surface.

The septic tank collects and stores, during their transformation, the discharges of toilets, sanitary water, kitchen water and other domestic liquids. Solids and fats are liquefied by bacterial degradation. The septic tank is designed to optimize the proliferation of these bacteria.

Process of bacterial degradation in septic tanks

The discharges fall into the first compartment, which is more voluminous and is divided into strata, according to their density:

  • solid matter, which is heavier, sinks to the bottom,
  • liquids, clearer, occupy the intermediate section,
  • the fats float on the surface,
  • the fermentation gases pass through these layers and accumulate in the upper, ventilated space.

The funds, deprived of oxygen, are colonized by a bacterial population anaerobic which transforms, by digesting them, materials solid in liquid and in acid gases. In the upper floor, in the presence of oxygen, microorganisms aerobic convert fat. The intermediate liquids, degreased and partially depolluted, pass into the second compartment through the median openings made halfway up the partition, then flow towards the filtration.

Installation diagram of the septic tank system

The all-water pit must be connected, in a watertight network, to ancillary equipment by Ø 100 mm pipes, respecting a drainage slope of at least 3% before the pit and 1% after.

The rainwater are excluded of the processing volume.

The 6 main elements of a treatment chain

The following list respects the flow direction of the fluids.

1 – The bar screen

The screen is a tank of 200 to 1000 liters, equipped with a grid which retains the large non-degradable impurities (plastics, glasses, etc.). This element optional is not very useful in the presence of a grease trap.

It must be implanted upstream of a possible lifting station.

2 – The grease trap

The grease trap (or decolloid), traps a large part of the fats, oils and other greasy substances in suspension in the wastewater. This equipment is highly recommended, and even becomes obligatory if the septic tank is located more than 10 meters of the building or if it serves a company dedicated to the restoration (restaurant, caterer, etc.). Its capacity must be proportionate to the daily volume of discharges.

The black water/grey water mixing must be done after the degreasing device.

3 – Installation of the all-water pit

The septic tank must be located:

  • over 5 m of the House,
  • over 3 m property lines, trees or large shrubs,
  • over 35 m springs, wells or drinking water catchment areas.

4 – Ventilation

The upper space of the septic tank and the pre-filter must be vented to the open air by means of a Ø 100 mm pipe, opening at least 40 cm above the roof.

These gases, harmless to humans when the pit is functioning normally, give off unpleasant odors and become toxicin case of congestion.

5 – The pre-filter

The pre-filter retains fine sludge particles still in suspension at the outlet of the all-water pit. Whether incorporated into this equipment or independent, it must be the subject of a regular maintenanceby washing with a jet of clear water.

6 – The ultimate purification device

The final purification of discharges takes place during theinfiltration in the subsoil, by degradation in contact with natural ambient bacteria or by a confined filter specific. Residual water, clean and depolluted, is discharged into nature or pumped. A distribution manhole must be placed at the start of the loop and a manhole, at the end of the loop. The choice of the ultimate purification device is often dictated by the characteristics of the terrain.

Two main techniques are used:

Underground spreading

The underground spreading consists of a network of drilled pipes, from Ø 100 to 125 mm and about thirty meters long. These tubes, coated with sand, are placed on a bed of gravel, at a depth of 0,60 m to 1,00 m.

This is the most common ultimate filtration system. It is suitable for terrain permeable et place.

The sand filter

The sand filter requires an excavation of at least 1.40 m deep, over approximately 200 m² of surface, filled with a layer of sand of +/-70 cm, then by a layer of gravels of +/- 30 cm, on which the spreading loop rests. A geotextile and topsoil cover the whole.

Not drainedthe sand filter is confined in a Polyane film, deployed on the bottoms. Drainedan additional layer of gravel, 10 cm thick, is necessary at the bottom of the excavations.

The sand filter is suitable for land clayey or low permeability.

Advantages of the septic tank

  • The septic tank allows, in safely et in an ecological waythe establishment of residential, industrial or commercial buildings in geographical areas without mains drainage.
  • Properly sized and installed, its operating costs are reduced to regular component maintenance.
  • Its mode of operation is transparent, comfortable, efficient and ecological.

Disadvantages of the septic tank

  • The installation is subject to prior authorization and a compliance check. The files are to be submitted to the Public Non-Collective Sanitation Service (SPANC) of the municipality concerned.
  • Soil investigation and installation costs can be high.
  • The implementation must be carried out by a approved company.
  • Interviews, quarterly for the grease trap, annual for the pre-filter and every 4 years for emptying the pit, are binding.

Price of septic tanks

The set-up costs standards or renovation of an existing septic tank are often higher than installing new. However, check with town halls, local authorities, the state or public bodies, because some aides or some zero interest loans, may be granted subject to conditions. The price ranges are very wide, depending on the region, the services or the speakers. Count:

  • Between 500 and 2000 € for the preliminary certified study (soil and sector)
  • between 50 and 200 € for the SPANC file
  • between 450 and 1000 € for a bare pit of 3000 liters
  • Between €5,000 and €10,000 for a complete new installation
  • Between €8,000 and €15,000 for the renovation of a sector
  • Between 180 and 400 € for the annual maintenance of the installation
  • Between 150 and 250 € for an oil change

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