Septic tank: operation and maintenance of this sanitation system


The septic tank is par excellence the “all-terrain” system for sanitation of toilet drainage water. It gives full measure of its usefulness in rural areas without a collective wastewater collector. Forbidden for sale in new or under renovation since the water law of December 30, 2006, these devices are replaced by the all water pits, able to also treat domestic wastewater (gray water). Septic tanks installed at a date prior to these provisions remain however tolerated, except in case of sale of the property.

How the septic tank works

The septic tank comes in the form of a underground tank, concrete or synthetic materials. Its volume, proportional to the quantity of waste to be treated, is theoretically not limited. In practice, it very rarely exceeds 10,000 liters. For individual houses, we conventionally resonate in the number of main rooms compared to a number of usual inhabitants. For a building with four main rooms or less, the minimum capacity of the septic tank is set at 1500 liters + 500 liters per additional room.

As this is heavy work that destroys the environment, it is desirable to anticipate the foreseeable increase in the building’s reception capacities from the start of construction.

The determining role of bacteria

Blackwater naturally contains a large number of bacteria capable, under certain conditions, of feed waste in which they live. Hence the term “septic” given to these tanks whose role is to create and maintain the best environmental conditions for the protection and multiplication of micro-organisms producing the enzymes active. The amount of decomposed matter is directly proportional to the density of the microbial population. The difficulty consisting in allowing the coexistence, in the same volume, of organisms requiring the presence of oxygen (aerobic) and those who live in an environment deprived of oxygen (anaerobic), is naturally regulated by a phenomenon of layers: aerobic bacteria break down the light elements and the fats constituting the upper layers (foam), as anaerobic microbes digest, liquefy and gasify the deep sludge made up of papers and heavier solids.

Typical composition of the sewage treatment circuit

Each sewage treatment system is made up of a number of underground elements. They are connected by rot-proof pipes 100 to 125 millimeters in diameter. To ensure the natural flow by gravity, it is agreed to provide a slope of 2 to 4% to the septic tank, then about 1% to the spreading circuit. The system must include:

  • The septic tank proper, divided into two rooms. The upper part has one or more visit stamps
  • The pre-filter, located at the outlet of the septic tank. Optional, but widely used, this thin tank is filled with pozzolana, a porous volcanic material. A sort of bacteria incubator, it rids the tributaries of some of their solid residual impurities
  • A ventilation of 100 mm in diameter must overhang the roof by at least 40 cm. It is used to decompress the toilet scraps and to evacuate the gases produced by the pit. Ventilation also helps supply oxygen to aerobic bacteria.
  • The absorption and the last filtration of the residual tributaries can be ensured by a simple spreading made up of pipes 100 to 125 mm in diameter perforated in their lower part and embedded in a bed of sand. It is possible to increase the absorption capacities of the system while reducing its length by arranging the drains in parallel networks (loops). The regulations require the presence of a inspection window (distribution box) at the entrance to the drain and an inspection manhole (loopback box) at its end.

Alternatives to linear drains

The particular constraints of the site, the available surface, the high permeability or the too low absorption capacities of the ground and many other criteria can impose a different filtration system or more efficient, or even replace it with a device dynamic drying. Here are a few examples:

The absorbent bed

Evolution of spreading by drains, the absorbent bed requires moving the soil over a large area to a depth of 80 cm. The bottom of the excavation, covered with geotextile receives a layer of sand, then gravel in which the spreading tubes are embedded. A new geotextile is covered with the topsoil.

Sand filters

There are two types of sand filters like:

  • Undrained sand filters can be used when the soil is sufficiently permeable to allow the natural dispersion of the purified waste. Comparable to spreading beds, they provide better decontamination effluents and require a smaller land area
  • Drained sand filters are indicated for impermeable basements or to prevent underground water dispersion. They can be confined in a sealed volume (casing or polyethylene sheet). Their bottom is always covered with an impermeable film intended to collect the treated water which, perfectly cleaned, can be directly evacuated by gravity towards a ditch, a watercourse, a sump or any other outlet located below.

The filtration principle remains the same for each of these solutions. The excavation covers a minimum area of ​​20 m² and at least 1.70 m deep. It is filled with superimposed layers of washed siliceous sand from 0 to 4 mm and rolled washed gravel from 10 to 40 mm in which the spreading pipes are embedded. The land is leveled using topsoil. As with spreading, a control manhole must be installed at each end of the filter. There is a variant using the principle of drained sand filters, in which are planted shrubs with high water absorption, like the Thuja. Discharges can then be zero or significantly reduced.

Maintenance of the septic tank

The septic tank and its accessories or filtration systems require little maintenance. However, some workstations must be checked at regular intervals, under penalty of malfunction or overflow with particularly unpleasant odors:

  • Be careful not to exceed the number of users planned for your installation
  • Seed occasionally with a biological activator in powder or granules
  • Make sure there is no backflow of effluents or gas when flushing the toilet, sign of a blocked pipe
  • Be sure to use biodegradable paper and especially never wipes
  • Never pour a large quantity into the bowl: bleach, non-biodegradable detergents, solvents, medication, etc. under penalty of destroying the bacterial flora, risk of blockages and overflows
  • Clean the pozzolan prefilter with a water jet increases its longevity
  • Check the level of sludge in the septic tank annually. Warning danger: leave to degas a few minutes after opening the inspection cover
  • Have the pit emptied every 4 years by a professional, to remove excess sludge, without forgetting to add a bag of biological reactivatorgeek in the pipes

Finally, remember that rainwater must not, under any circumstances, flow into the septic tank.

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