The septic tank is par excellence the “all-terrain” system for sanitizing toilet waste water. It gives the full measure of its usefulness in rural areas without a collective wastewater collector. Prohibited for sale in new or renovation since the law on water of December 30, 2006, these devices are replaced by the septic tanks, also able to treat domestic wastewater (grey water). Septic tanks installed at a date prior to these provisions are however tolerated, except in the event of the sale of the property.
Principle of operation of the septic tank
The septic tank comes in the form of a buried 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 litres. For single-family houses, we resonate conventionally in number of main rooms reported 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 is destructive to the environment, it is desirable to anticipate from the construction stage the foreseeable increase in the building’s reception capacities.
The decisive role of bacteria
Sewage naturally contains a large number of bacteria capable, under certain conditions, of feed waste in which they live. Hence the qualifier “septic” given to these tanks whose role is to create and maintain the best environmental conditions for the protection and multiplication of microorganisms producing the enzymes assets. The amount of decomposed material is directly proportional to the density of the microbial population. The difficulty of allowing the coexistence, in the same volume, of organisms requiring the presence of oxygen (aerobic) and those living in an oxygen-deprived environment (anaerobes), is naturally regulated by a phenomenon of strata: aerobic bacteria break down the light elements and the fats constituting the upper layers (foam), while anaerobic microbes digest, liquefy and gasify deep sludge made up of heavier papers and solids.
Typical composition of the sewage treatment circuit
Each sewage treatment system consists of a number of buried 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 actual septic tank, divided into two chambers. The upper part has one or more visit stamps
- The prefilter, located at the outlet of the septic tank. Optional, but widely used, this thin tub is filled with pozzolan, a porous volcanic material. A sort of bacteria incubator, it rids the tributaries of some of their solid residual impurities
- A 100 mm diameter vent must overhang the roof by at least 40 cm. It is used to decompress the falls of toilets and to evacuate the gases produced by the pit. Ventilation also helps to supply oxygen to aerobic bacteria
- The absorption and the final 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 manhole (distribution box) at the entrance to the drain and a 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 weak 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 some examples:
The absorbent bed
Evolution of spreading by drains, the absorbent bed requires moving the soil over a large surface 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 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 discharges. Comparable to spreading beds, they provide better decontamination effluents and require less land area
- Drained sand filters are indicated for impermeable basements or to avoid underground dispersion of water. 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 purified, can be directly evacuated by gravity towards a ditch, a watercourse, a sump or any other outlet located below.
The principle of filtration remains identical for each of these solutions. The excavation covers a minimum area of 20 m² over a depth of at least 1.70 m. It is filled with superimposed layers of washed silica sand from 0 to 4 mm and from rolled washed gravels from 10 to 40 mm in which the spreading pipes are embedded. The land is leveled with topsoil. As for a spreading, a inspection window must be installed at each end of the filter. There is a variant based on the principle of drained sand filters, in which are planted shrubs with high water absorption, like Thujas. Releases can then be zero or significantly reduced.
Septic tank maintenance
The septic tank and its accessories or filtration systems require little maintenance. However, some stations must be checked at regular intervals, otherwise malfunctions or overflows with particularly unpleasant odors:
- Be careful not to exceed the number of users planned for your installation
- Seed from time to time with a biological activator powder or granules
- Make sure that there is no backflow of effluent or gas when flushing the toilet, a sign of a clogged pipe
- Be sure to use biodegradable paper and especially never wipes
- Never pour in quantity into the bowl: bleach, non-biodegradable detergents, solvents, medication, etc. under penalty of destroying the bacterial flora, risk of blockages and overflows
- Hose down the pozzolan pre-filter increases its longevity
- Check the sludge level in the septic tank annually. Attention danger: let degas a few minutes after opening the inspection plug
- Have the pit emptied every 4 years by a professional, to eliminate excess sludge, remembering to add a sachet of organic reactivatorgic in the pipes
Finally, remember that rainwater must not, under any circumstances, flow into the septic tank.