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A septic tank is a vessel usually buried underground, the purpose of which is the collection, storage, and, to some limited extent, treatment of sewage1.
On a simple hierarchy of sewerage2 facilities, the septic tank falls between the 'long-drop pit latrine', the function of which is to store and bury human waste, and a piped connection to an all-singing all-dancing sewage treatment works (STW), the function of which is to convert human waste products into mostly harmless end products.
The invention of the septic tank is credited to Frenchman John Louis Mouras, who, during the 1860s constructed a masonry tank into which was directed various household detritus from a small dwelling in Vesoul, France, subsequently overflowing to an ordinary cesspool. After a dozen years, the tank was opened and found, contrary to all expectations, to be almost free from solids. Subsequent to collaborations with one Abbé Moigno, a priest-cum-scientist of the period, Mouras was able to patent his invention on 2 September, 1881. It is believed that the septic tank was first introduced to the USA in 1883, to England in 1895 and to South Africa (by the British military) in 1898.
A typical septic tank system consists of a tank and a soakaway drain.
The concept is that influent3 liquid (effluent4 from the dwelling) should enter the tank at one end (at the inlet), be retained in the tank for a period and discharged at the opposite end from the inlet (at the outlet) to enter the soakaway drain. When the tank is full of liquid, as it will be once it is operating properly, liquid influent enters at the inlet thereby driving out (by displacement) effluent of the equivalent volume through the outlet and into the soakaway.
Inside the tank, flotsam is called the scum layer, and anything that sinks to the bottom forms the sludge layer. In between there is a fairly clear liquid layer. Essentially, a septic tank acts as an extension to the gut. A natural process of anaerobic decomposition occurs in the tank which reduces the amount of solid matter and provides some treatment of the waste.
The soakaway drain, or percolation trench, is a method of discharging the tank effluent into the surrounding soil. It is noted here that the effluent from a septic tank is by no means fit (in terms of health) for discharge into a water course (eg, a stream or an aquifer) or onto the ground where it could be accessible to animals, humans included.
The system is by no means foolproof. Eventually, the solids built-up inside the tank will require removal, usually by municipal suction-truck (the honey-wagon), and the soakaway drain may become overloaded.
The Tank - Design Considerations
The tank should be sized sufficiently to ensure that any fluid influent (effluent from the dwelling) is retained in the tank for the minimum specified retention time, probably defined by local municipal regulations (a range from 24 hours minimum to around three days is normally considered adequate, but refer in UK to British Standard Code of Practice (BS 6297:1983 with amendment 6150:1990)). Thus the size of the tank depends on the quantity of liquid being discharged to it which in turn depends on the number of people in the dwelling and the way they use water. Consider a household of four wherein each person discharges 400 litres (depends on a variety of factors - washing/flushing/bathing, etc - it all adds up) per day to the drainage system. The septic tank needs a capacity of 1600 litres (1.6 cubic metres) for one day's household effluent. If waste is to be retained for three days before day four influent starts pushing it out the outlet, the capacity will need to be three times 1,600 litres = 4,800 litres5. Let volume be represented by V.
In order for the tank to work properly, it is generally recommended that the liquid depth should be no more than 1.0 metre. Let the depth be represented by D. The tank should be wide enough to facilitate access for maintenance. 1.2m should be adequate. Let the width be represented by W. The minimum length (L) of the tank is therefore defined by the volume divided by the cross-sectional area D x W.
L = V/DW
Thus, the above worked example yields L = 4.8/1.0x1.2 = 4.0, ie, the length of the tank would need to be 4.0 m.
It is normal to 'compartmentalise' the tank internally. This ensures that influent liquid is to some extent stilled before it reaches the outlet. Benefits of this are two-fold. In the first instance, suspended solids are 'dropped' from suspension as the liquid loses velocity. Second, it means that the 'route' from inlet to outlet is not direct, thus ensuring that liquid circulates before reaching the outlet, hopefully optimising the retention time.
Usually, the compartment divider will comprise a 'slotted' wall, such that the ratio of inlet/outlet compartments is 2/3:1/3 (ie, 2:1). If constructed in block or brick, mortar is left out of the vertical joints between the masonry units at about half-liquid depth to make the slotted wall.
It is important that the slots are not open either near the top or near the bottom of the divider wall, as this will encourage sludge/scum to enter the second compartment.
Inlet and Outlet Pipework
The invert level (ie, the level of the bottom of the pipe - the lowest point where the water runs) of the inlet pipe is determined by the level of the drains from the dwelling. These will be dictated by topography and pipe-slopes.
The level of the invert of the outlet pipe fixes the TWL (top water level) of the tank. When the water reaches that level, the tank is full to capacity, and it will overflow by discharge through the outlet.
In order that the inlet pipe does not back-up (ie, become full), the inlet should be slightly higher than the outlet (say 50 - 100mm, or 2-4 inches). This means that there will be a slight cascade into the tank.
Then, in order to ensure that the scum (that naturally forms in the tank like the head atop a pint of stout, but crispier, like a biscuit), neither impedes influent nor escapes as effluent, both inlet and outlet pipes should be fitted with a vertical tee pipe (a tee-shaped pipe turned so that one arm of the tee is vertically upwards, and one is vertically downwards) arrangement rather than being left plain.
A cross-section through a septic tank and an informative essay on Knowing Your Septic Tank is available here.
The Soakaway - Design Considerations
A soakaway drain typically comprises a length (say around 20+ metres) of perforated pipe laid at a 'flattish gradient' (probably along the contour), in a trench backfilled with poorly graded (ie, of a similar size) stone chippings (single-sized aggregate). The idea is that the liquid from the tank will percolate through the stone chippings and into the soil. It is not uncommon for the chippings to be laid inside a wrap of geotextile material, which impedes the silting up of the soakaway with fine particles (silt) from the surrounding trench.
The size of the soakaway drain must be sufficient to absorb the tank effluent. If the drain is too short or the soil is too impermeable the drain will become clogged and will need to be redug. Typical evaluation of the permeability of the soil will include a 'percolation test' to see how quickly liquid will disappear into the soil. Clay soils will be less absorbent than coarser sandier soils
Note: anyone constructing a soakaway should beware of the risk of poisoning local aquifers and water courses:
Beware of a high water table. A soakaway should not be constructed where the ground water table is close to surface.
In fine soil, the penetration distance of bacteria may be around 3m from the soakaway. Coarser soils will enable greater penetration. Coliforms (gut bacteria) reportedly can survive for as much as a month if they reach a source of groundwater.
A limestone or dolomitic geology will most probably be fissured, enabling septic tank effluent to flow away freely. As such, soakaways are unsuitable in areas where this geology occurs.
Even the best septic tank will not go on functioning forever. In time, the tank will get full of solids and will require emptying and cleaning out. Therefore, it is important to ensure that the tank is accessible by one of the municipal 'honey-wagons', mobile bowsers that will suck the tank empty.
From a safety perspective, a septic tank should be considered as a confined space. It will no doubt contain noxious gases (such as CO and H2S) and should be entered with caution, perhaps with a gas-guard or even breathing apparatus. Vent it thoroughly beforehand, and ensure that you always have a 'mate' on hand - don't enter it alone. If you are not a fully-trained professional you should seek advice from the Health and Safety Executive before entering any confined environment.
Likewise, it is almost inevitable that the soakway will eventually clog, so it is worth positioning the tank and soakway so that an alternative soakaway drain can be excavated in future.
Finally, users of septic tanks (indeed any sewage treatment facility) should also be aware that the biological process is impeded by inter alia the following:
Detergents and bleach will kill the bacteria which keep the tank operating properly. If possible use biodegradable 'septic safe' detergents.
Household grease, such as cooking oils, animal fats and dairy products. (consider installing a grease-trap)
Domestic oils, such as car lubricants and petrochemicals (don't connect your garage sump to your sewerage system and don't empty your gearbox down the lavatory).