How a Septic Tank System Works

Septic tank systems

Septic tanks are classic ‘bio-digestors’ that rely on anaerobic bacteria to break down human sewage wastes over a period of (at least) 21 days.

6000 litre jojo septic tank

6000 litre JoJo Conservancy/Septic Tank

Septic systems are used to treat sewage from (usually) single dwellings in districts where municipal Wastewater Treatment Works are not available. Larger systems are also used by rural industries to cater for the disposal of staff effluent.

Conventionally ‘in-the-ground’ septic tanks will run smoothly for many years when they are respected and not overloaded

Critical points

To perform satisfactorily septic tanks must be:

  1. Large enough for the load they receive;
  2. Constructed properly; and
  3. Not abused with chemicals and solids.

Some septic tank history

The septic tank is thought to have originated in France during the early 1800s where it was developed to deal with human wastes generated in the new towns that expanded rapidly to support the industrial revolution following the regime of Napoleon Bonaparte. Until that time, the wealthy used buckets in an ablution room within their house and the staff (chambermaids) emptied the contents when necessary into the same cesspit latrine adjoining the house that they used themselves.

Some larger buildings, country houses, abbeys etc. had quite sophisticated sewers that drained from within the house to a cesspit and associated reed bed in the grounds. Faecal solids were flushed along the pipes (by household staff) using random buckets of wash water.

Large cities had networks of brick or stone built gravity sewers that drained all effluent to the river around which the town had developed. Such disposal methods – along with the vermin that flourished upon the discarded refuse – gave rise to frequent outbreaks of illness, because rivers, such as the Thames and Seine, became contaminated with sewage.

What makes up a septic system?

Septic systems have two parts:

  1. Septic tank
  2. Soakaway or leach field

1.  Septic tank

A septic tank has three main functions:

  1. Separation of sewage solids from liquid (the faecal solids float to form a scum);
  2. Reduction of scum (stools) and dissolved COD (mostly urine) by anaerobic bacteria (in both chambers); and
  3. Storage of inorganic solids and minerals (septage) as a fine silt on the floor (to be removed by spade or suction periodically – usually every 10-25 years).

Natural anaerobic bacteria and associated microorganisms living within the septic tank adapt to degrade human sewage to simple biogasses, water and inorganic radicals (for example, trace minerals in food).

For this to happen the tank must be:

  • Properly sized for the load it is receiving (i.e. number of people resident in the dwelling);
  • Constructed correctly (i.e. always with inlet and outlet ‘baffles’ – usually ‘T’ junctions on their sides – and at least two chambers: a primary (solids digestor) and a secondary (maturation) tank; and
  • Only receive black (sewage) water, not grey (wash) water

2.  Soakaway

Also known as a French drain, drainage field or leach field, the soakaway is an integral part of the septic tank system. It allows partially treated effluent from the septic tank to ‘soak away’ into the surrounding soil where it is progressively ‘polished’ back to potable quality as it seeps ever further downwards through the ground towards the natural water table.

Typical soakaway

A soakaway under construction

The efficiency of the soakaway depends upon:

  1. The porosity of the soil;
  2. The length of the drainage trench in relation to the number of residents in the dwelling; and
  3. How it’s constructed – it must be built along the contour without any gradient. It need not be straight, but it must follow the contour.


1) Where soil conditions do not favour soakaways there are two alternatives:

  • A storage conservancy tank, which is routinely pumped out (usually) by the municipality; and
  • A small package plant (another name for a sewage treatment plant), which refines the effluent to discharge quality. Several septic tanks may be connected to a common sewer manifold served by one package plant. This is often referred to as a ‘small bore system’.

2) Older publications, pre circa 1990, advocate the inclusion of grey water in the septic tank. That was when tanks were larger and generally before the ‘off-the-shelf’ roto-moulded plastic tanks that are commonly installed today, were freely available. Also, in those times the excess N and P, found in laundry water especially, were seen as advantageous to the development of the faecal-degrading biomass in the primary chamber of the tank. However, modern (post 1990) foodstuffs frequently contain high levels of refined micro-nutrients, which fulfill the same task in the smaller volume tanks that economy usually dictates the builder installs today. Many of the elements in modern food contain complex molecular structures which, emulsified in waste water,  take time to break down. Many elements resist degradation which is why high performance biological additives (such as our BIO-SYSTEMS STR) are becoming ever more important in balancing the equation.

How septic tanks work

Septic tank

Diagram of a typical septic tank

Septic tanks rely on naturally-occurring anaerobic microbes that degrade organic wastes (faeces and urine) into simpler organic compounds: natural gasses, water and inorganic radicals. They work without input of energy other than that provided by gravity (water flow).

  1. Septic tanks should have at least two separate chambers, separated by a common wall that is perforated two thirds below the surface of the liquid to allow effluent to move from the primary chamber to the secondary.
  2. The inlet and outlet chambers are at the same level in the ground and both fitted with side mounted ‘T’ junctions open at their two ends (see diagram above). This is important as it allows incoming wastes to enter the primary chamber beneath the crust and prevents floating solids escaping from the secondary to foul the soakaway, at the same time permitting rodding from the inspection cover frame above in the event of a blockage.
  3. The incoming flush of raw sewage displaces an equal quantity of semi-treated effluent from the primary to the secondary and in turn from the secondary out to the soakaway or leach field.
  4. There are at least two inspection covers (manholes) – one over each ‘T’ junction – to facilitate rodding and cleaning. Larger (multi-chambered tanks) should have inspection covers above the inlet and outlet to each chamber. These covers must be a gas tight fit and strong enough to withstand passing traffic (heavy duty, cast iron lids are necessary in roadways). The grooves in the frame must be free of soil – grease is usually applied to ensure an airtight seal.
  5. Large tanks are multi-chambered so the perforated cross walls provide support for the lid.
  6. Biogas generated by the microorganisms during the degradation of the sewage escapes back up the inlet sewer and is vented at the dwelling. Large (multi-chamber) tanks should be vented above the waterline to allow for this.
  7. The primary tank should have an operating surface crust 10–50mm thick floating on the surface.
  8. The secondary tank should resemble the sky on a dark, cold night (i.e black water with a myriad tiny ‘stars’ – bubbles of biogas).
  9. A healthy septic tank will not generate odour.

The processes involved…

  • The primary receiving tank holds sewage solids floating on flush water and urine. Microbes are sourced from the ‘starting’ biomass – commonly natural latent spores attached to the internal tank surfaces during construction and a small percentage from the human digestive tracts, passed in faecal sewage. It is here that the sewage solids are broken down into simpler substances. The chamber should have a thin light brown scum (15–50mm) floating on the surface.
  • The secondary tank is a maturation chamber where the partially-treated effluent is further organically ‘cleaned’ by the microbes prior to discharge to the soakaway. Here there should not be any surface scum.
  • The dividing wall retains sewage solids and scum whilst permitting the solid-free, partially-treated effluent migrating to the secondary tank for maturation.
  • The degradation in COD within the septic tank is commonly 1,000–2,500 ppm, depending on residence time.
  • The faster the flow through the tank, the less the residence time permitting the biological breakdown and the greater the load on the soakaway.

Source: Bio-Systems SA (contact Bob Hadley for more technical information: info@biosystemssa)

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