Septic Tanks

 

 

Introduction

 

A septic tank treats domestic sewage that is; the outlets from basins, baths, W.C.’s, showers, sinks and other sanitary and domestic appliances.

 

In septic tanks the solids in the sewage settle to the bottom to form sludge.

Relatively clear liquid is left which forms a layer of scum on its surface. Bacteria feed on this liquid and digest some of the matter in it.

The liquid then either passes into another settlement tank before passing to a watercourse or is discharged underground through a network of pipes to filter through the soil in a soakaway system.

The solids that build up at the bottom of the tank need to be removed about once a year.    

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 


History

In 1860 a French man called Mouras built a masonry septic tank for a house in France.

After a dozen years, the tank was opened and found, contrary to all expectations, to be almost free from solids. 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.

 

Digestion

Sewage is allowed to rest in the septic tank for about 16 to 48 hours.

The process of digestion in the septic tank is done by bacteria.

These bacteria can be killed by certain chemicals.

The process of breaking down the organic matter in sewage is called anaerobic digestion since it is largely outside the presence of air.

 

The digestion reduces the amount of sludge and makes the contents of the septic tank less smelly. Normally it would take about two months to break down all the sludge in the tank so a normally used septic tank will only partially break down the contents.

Too much bleach, detergents and other household chemicals may destroy the useful bacteria. As a result the sewage will not be treated fully and may cause pollution problems. Emptying the septic tank regularly will ensure the septic tank keeps working properly. If possible use biodegradable 'septic safe' detergents.

 

Flow of Effluent

The concept is that effluent from the building should enter the tank at one end, be retained in the tank for a period and discharged at the opposite end to enter the soakaway drain.

The septic tank soon fills and as more effluent enters it automatically displaces the same amount out into the soakaway drain.

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. This clear liquor will overflow as new flows come in.

The 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.

The effluent from a septic tank is by no means fit for discharge into a water course.

 

Some solids, such as soap scum or fat, will float to the top of the tank to form the scum layer. Heavier solids, such as human and kitchen wastes, settle to the bottom of the tank as sludge.

Construction

Septic tanks can be block/brick built or made with glass reinforced plastic (GRP).

Access covers should be of durable quality to resist corrosion and must be secured to prevent easy removal. Septic tanks should prevent leakage of the contents and ingress of subsoil water and should be ventilated. Ventilation should be kept away from buildings.

Discharge and Soakaway

The water is discharged into a soakaway or ‘leaching field’ which consists of metres of perforated pipes laid under the soil. To allow the waste water to drain away efficiently a sizeable area is preferred and a soil type which actually allows the water to soak away. For this reason the siting of a septic tank in heavy clay soil may not be suitable. Free draining sand and gravel’s offer the best conditions.

The fall of distribution drains from the outlet should be as shallow as possible, i.e. 1 in 100 to 1in 200 to allow slow percolation. The bottom of the trench of the perforated pipe should be 900mm above the seasonally high water table, or bedrock if possible. If the water table is closer to the surface than 900mm then it may be possible to run the soakaway drain also closer to the surface ensuring that water does not come up to ground level.

The trench in which the discharge perforated pipe runs can be backfilled with aggregate to assist in percolation. The aggregate can be laid inside a wrap of geotextile material to impede the silting up of the soakaway with silt from the surrounding trench.

The soakaway drain should be long enough to allow the water to percolate into the sub-soil. 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.

Notes:

·         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.

·         Limestone substrata will most probably be fissured, enabling septic tank effluent to flow away too freely into the water table below.

·         Boggy or peaty ground is also unsuitable since the percolation rate is very slow.

It is almost inevitable that the soakaway will eventually clog, so it is worth positioning the tank and soakaway so that an alternative soakaway drain can be excavated in future.

BRE digest 151 Soakaways, details construction and sizing of soakaways.

Capacity

The size of the septic tank depends on the quantity of liquid being discharged to it which is dependant on the number of people in the dwelling.

From BS 6297 Small domestic sewage treatment works and cesspools the Septic tank capacity is;

Capacity (m3)        =       Number of residents x 0.14   + 1.8

For a house with four occupants the capacity is;

Capacity (m3)        =       (4 x 0.14)   + 1.8

Capacity (m3)        =       2.36 m3.

 

 

 

Positioning

 

Septic tanks must be sited at least 7m from the habitable part of the building (preferably downslope), within 30m of a suitable tanker access.

The drainage field or mound serving the septic tank must be at least 15m from any building, 10m from any watercourse, permeable drain or soakaway, etc and not be covered by drives, roads or paved areas.

Steep sloping sites should be avoided. Sites should be remote from ditches, streams and wells.

Compartments

Septic tanks are normally divided internally into compartments.

This allows the new effluent to settle and be digested before it is passed into the outlet.

Also, it means that the route from inlet to outlet is not direct, thus ensuring that liquid circulates before reaching the outlet, giving more time for digestion.

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.

Levels

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 become full, the inlet should be slightly higher than the outlet (say 50 - 100mm). This means that there will be a slight cascade into the tank.

 

 

 

 

 

 

 

 

 


To ensure that the scum on top of the liquid neither impedes influent nor escapes as effluent, both inlet and outlet pipes should be fitted with a tee as shown above.

Cess Pits

 

A cess pit is a sealed storage tank into which sewage is drained until it can be removed for disposal.

The sewage is not treated in the tank just stored.

In some areas a septic tank is not suitable, there may be no suitable drainage in the subsoil, and a cess pool is the only answer.

Older cess pits are usually cylindrical pits lined with either brick or concrete. Modern cess pits are made from fibre glass, steel or polyethylene.

Current building regulations require cess pits to be able to hold at least 18,000 litres of sewage. It is estimated that each person produces 115 litres of sewage a day. For a family of four this means that the tank will need emptying about once a month.

 

Seepage Pits

 

Other sewage systems that have been used in the past are seepage pits or large soakaways. These systems typically involve discharging septic tank treated sewage into a deep, cylindrical pit that is open on the sides and bottom. Sometimes these pits can be constructed using honeycombed brickwork, or concrete manhole sections with perforations in the walls. The holes are frequently filled with large stones or gravel and a cover (probably in concrete ) placed over the hole. If the ground strata for the whole depth is good and will absorb the effluent these can be satisfactory, but if not then these can cause problems as the end result will be a large hole filled with septic tank effluent.