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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.

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

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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 field1

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) water2

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.

Septic tank construction

1. Siting on the property

Frequently local bylaws dictate where septic systems may or may not be positioned. Refer to your Title Deeds or local authority before construction commences.

Note: If frost is seasonal, ensure the tank will be below the 'permafrost' level.

2. Dimensions

  • Conventionally septic tanks are a box built with 230mm brick walls on a 150mm concrete in-ground slab. Plastered and (preferably) epoxy painted, they must be water tight and not leak. The lid can be made with a 25mm screed on plastic on abutting pre-cast concrete lintels. Care must be taken to ensure loading strength to withstand passing traffic.
  • Plastic 'torpedo' roto-moulded tanks are also available but, due to the reduced surface area, are not as efficient as brick ones. They should be encased in concrete to avoid movement in the soil, which frequently causes distortion and rupture of the pipe junctions.

  • The effluent chambers are conventionally 1.8m deep with an effluent operating depth of 1.5m. The overall hole in which the box is constructed is commonly 2.2m deep - any deeper is impractical to dig in most soils without the walls caving in.
  • The box has a dividing wall, which is perforated (for example, three missing half bricks) approximately 1.1m below the effluent surface and roughly 200mm-500mm above the floor.

3. Tank sizes

This is easy to calculate on the basis of flushes per person per day. It equates to approximately 60 litres per person (six toilet flushes per person of 10 litres each per flush - the discharge capacity of an average toilet cistern). Taking the recommended 'residence time' of 21 days into account, this indicates a minimum tank size of 1,200 litres per person. So for a family of five, and to ensure a 21-day residence time, you need a tank capacity of 6,000 litres minimum. Remember also the friends that call, especially over weekends. If in doubt rather go for a 7,500 litre unit to be safe - the construction cost is only fractionally more.

Surveys indicate that the average five-person family will use 180-220 litres of water per person per day. That's a total of 1,000 litres (and includes bathing, showering, laundry, cooking, washing up, etc.). So they would need a 21,000-litre septic tank. However, the family would only use an average of 60 litres per person per day for toilet flush. So that's 300 litres per day, which equals 6,500 litres per month. So a normal septic tank of 6,000 litre capacity would be in order. The other 14,500 litres of waste 'grey' water should go directly, via a grease trap, to the soakaway, or, better still, via a grey water treatment system so it can be re-used in the garden.

Starting a septic tank

Demystifying some common myths...

  • Traditionally, many rural dwellers believe that a dead cat or a sheep's head will 'start' a septic tank. This is not a practical option because the bacteria within the body parts of dead animals are not the same as those found in sewage. Technically it would take several weeks for the bacteria to adapt to their new environment and become a sewage-digesting biomass.
  • As with municipal works, 'seeding' with sludge from a nearby system is also not satisfactory as the donor source may well be suffering from a weak biomass. 
  • Often people believe their tank is working because they can see maggots or worms on the surface of the scum. The presence of these creatures has nothing to do with the sewage treatment process other than indicate that the tank is in a poor condition. Bacteria are microscopic and cannot be seen with the unaided eye.

The proper procedure is to introduce the correct sewage-degrading bacteria into the system from the start. This can be done by inoculating the new tank with BIO-SYSTEMS STR or BIO-SYSTEMS SK1 (in cool climates), which will ensure the tank and drain field start efficiently too.

Note: There is an advantage to the builder in constructing the septic tank at an early stage. It negates the need for site toilet hire and will provide the client with an operating system at hand over.


  1. If a septic tank is undersized for the hydraulic load (volume of effluent) it receives then sewage solids are liable to be washed over into the soakaway, blocking it as the tank overflows.
  2. 'Pumping out' the tank only gives a few days' respite - because once blocked the soakaway is very difficult to clean. Our BIO-SYSTEMS SoakAway Cleaner (info sheet available at is a highly effective product in achieving this odious task. 
  3. Repeated 'pumping' – necessary to prevent frequent overflows and the surrounding garden from being soaked in raw sewage – is expensive and not the answer. It results in a depletion in the essential microbial 'biomass' living in the liquid in the tank, leading to sterility and complete failure of the entire system coupled with appalling odour from the fats that are still in the tank and soakaway.
  4. Very few 'vacuum tanks' are fitted with spate pumps necessary to extract fat sludge. These units are expensive (around R1,400 per hour yard to yard).

The soakaway or leach field

A soakaway (also known as a leach field or drain field) receives partially-treated effluent from a septic tank or grey water drain. Its purpose is to complete the job initiated in the septic tank. It cleans the dirty water by organically breaking down the suspended wastes and impurities through the surrounding soil to a degree that makes it suitable for discharge into the underlying water table. From here the 'cleaned' water is collected in boreholes (or artesian wells) and piped to the surface for re-use by others.

Typically soakaways contain a hard, insoluble media such as crushed stone, clay brick ends or old car tyres. This hard substance hosts a bacterial biofilm (slime) that progressively degrades COD (organic impurities) as the effluent passes slowly over and around it and then seeps out to the surrounding soil. Here further natural soil microorganisms, adapted to the fractionally higher organic load contained in the nearly cleaned water, polish the liquid to near potability (ultimately you can drink it). Once in the earth the purification process is gradually continued by bacteria in the soil. The exponentially cleaned water then rejoins the underlying water table. Consequently, the design and position of the soakaway in relation to the slope of the natural ground and the porosity of the soil, as well as the proximity of the underlying water table, are essential to the efficacy of the system.

Design parameters that must be observed:

  1. Porosity of the soil – soakaways won’t work...
    a. in clay soils;
    b. where the natural water table is 'high' such as near or in wetlands;
    c. where underlying bed rock prevents seepage.
  2. Size of the septic tank – if it is to be a combined installation.
  3. Number of people living in the residence.
  4. Nature of the waste water.
  5. Slope of the ground.
  6. Proximity to natural watercourse.
  7. Distance from property boundary.

A soakaway that works properly is designed to cater for the hydraulic load it receives and is a proper (easy-to-build) structure. Essentially, it should be a long, narrow trench build on a rammed earth floor. It must be dead level with walls of dry-packed 'butterfly' bricks (or blocks on their sides) and with a layer of clean crushed aggregate sandwiched between two wraps of geo-textile – the outer one preventing the ingress of soil/ sand; the inner to prevent small particles of media aggregate falling into the central cavity. The whole trench should be covered with loose, pre-cast paving slabs to prevent accidental injuries and for ease of access.

How a soakaway works

Effluent enters the soakaway trench from the septic tank (see above). The water stagnates allowing bacteria washed over from the septic tank to continue the maturation process in association with those latent in the soakaway. Simultaneously, as the water seeps out through the geotextile, organic wastes suspended in the effluent become snared by the mucilaginous slimes that develop on the surfaces of the 'media' (the clean stone or brick ends or tyres). These wastes are progressively degraded as they flow very slowly from the channel propelled by the incoming effluent and the pull of gravity towards the surrounding soil.

Meanwhile partially-cleaned molecules of water slowly soak into the interstices between the soil particles where further bacterial slimes thrive under the damp conditions – the microorganisms gain their 'food' from the remaining COD in the semi-treated effluent. The fluid, under pressure from that flowing into the soakaway, slowly seeps (permeates) further and further into the surrounding soil following the gravitational gradient (minimal though it may be) and is progressively purified by the latent bacteria within the soil. Ultimately, sparkling clean, clear, potable water joins the underlying water table.

It follows that the actual porosity of the soil, clay content, particle size and gradient are prime factors in the speed of bioaugmentation and the ultimate final discharge quality of the treated liquid sewage effluent. It also follows that if too many soakaways are present in poorly performing soils, toxic problems (overload) will result. This is especially true where developing populations in cluster housing use artesian wells (boreholes) that are sunk to tap the underlying water table.

Where soil conditions are not conducive to soakaways or where congestion occurs or is likely to occur, the local authority may decline permission to install soakaways and the septic tank will then have to discharge to a 'conservancy' tank - the contents of which must be pumped out (most authorities have vacuum tankers serving this purpose) at regular intervals; for a fee. An alternative may be to install a small package plant, which does the same job as a soakaway but costs more to install and operate. However, the advantage is that the treated effluent is usually re-usable for agriculture and maybe even potable  purposes.

Grey water considerations

Taken that the average person uses a total of between 120-180 litres of water per day, our family of five will use around 15,750 litres in the same 21-day period. If this all goes to the septic tank, the total hydraulic load is approximately 750 litres per day. Divide into the 6,000 litre tank capacity and this equals eight days’ residence time.

For the conventional roto-moulded, off -the-shelf, 'large' septic tank of 2,500 litre capacity, the factor is  only 2.4 days’ residence time. No septic tank will work this way. Therefore, grey water must bypass the tank to (preferably) its own soakaway; or it can be treated for re-use (see Re-use of grey water).

Maintaining your septic system

Ensure that grey water does not flow to the septic tank, which it must bypass en route to the soakaway or drain field.
Care is needed to prevent hydraulic and organic overload conditions that will otherwise upset the smooth running of the septic system.

The septic tank

  1. Toilet tissue
    Use good quality twin or triple ply. Soft tissue absorbs water and disintegrates readily and, even when used to excess, should not cause a problem in a tank of sufficient capacity. Agricultural and building staff sometimes use coarse paper or rags, this should be prevented at all costs.
  2. No foreign materials. Only human sewage and toilet tissue should be flushed. Cigarette butts, ladies textiles, rubber goods, etc. should never be put in the toilet bowl.
  3. Chemicals. Proprietary 'loo cleaners' available in supermarkets are usually 'safe' in septic systems where BIO-SYSTEMS products are used, but avoid bleach and strong acid or alkali drain cleaners and chemicals. Discolouration is usually caused by minerals in the flushing water, which in some rural districts may not be to urban standards.
  4. Use BIO-SYSTEMS STR (info sheet available at at least once a year. This will keep the system healthy and prevent expensive pump outs.

The soakaway

  1. The soakaway should run smoothly if the tank is respected and not subjected to overloads.
  2. If the seepage reduces – usually due to overloading – and the system 'backs up' causing overflows from the secondary (maturation) chamber of the septic tank, then apply our BIO-SYSTEMS SoakAway Cleaner (info sheet available at This product will gradually degrade the fats that seal up the minute seepage holes, allowing the system to return to normal.
  3. Check for pipe collapse or penetration by roots as these factors can be the cause of problems.

Common septic system problems

Septic tank systems are like cars, they need periodic maintenance to keep them performing at their peak.

Septic tank trouble is caused usually by one or all of the following:

  1. Size - the system is too small for the job it is intended to do.
  2. Fats - an overload of fats causes failure in the soakaway or each field.
  3. Death - the tank 'dies' because it has received a dose of toxic shock from harsh chemicals, bleach, acid drain cleaners or HTH.
  4. Overload - too many people using the loo and/or too often.
  5. Feast and famine - heavy usage over a holiday period followed by long periods of inactivity (commonly experienced at weekend getaways).
  6. Wash out – when grey water (from the bath, laundry, etc.) or storm water (from entry drain points that aren’t 'bunded' properly) enters the tank and causes excessive hydraulic load, preventing the degradation of faecal wastes.
  7. Overflow – when the water from the secondary chamber cannot escape to the soakaway because the latter is gummed up with congealing fats, thereby preventing liquid seepage to the surrounding soil.
  8. Pump outs – also known as 'vacuuming' to remove excess water (usually caused by a malfunctioning soakaway). This is bad practice because it removes the aqueous environment of the beneficial bacteria. The fats – the cause of the problem – remain. Done frequently the tank, devoid of bacteria, dies and the problem worsens. Very few suction tankers are equipped with 'spate' pumps and high-pressure jets that slurry up and remove fats. Rigs that do are very expensive.
  9. Odour - caused by rotting fat and (anaerobic) COD.

The BIO-SYSTEMS solution

First of all, let's start off on the right foot and get the design correct! We offer drawings of both septic tanks and soakaways or  drain fields in email format:

BIO-SYSTEMS has developed a tough blend of hardened microorganisms that degrade the complex components of modern sewage - particularly the refined fats, oils and greases (FOG) that would otherwise congest septic tanks and cloy the walls of soakaways, sealing the essential seepage ducts between soil particles:

  1. BIO-SYSTEMS STR – all septic tanks.
  2. BIO-SYSTEMS SoakAway Cleaner (SAC) – all drain fields.

Click on the BIO-SYSTEMS Product table for a complete list of products. Also see our Soakaway case history.

Go to Pit latrines and Re-use of grey water for more on the domestic disposal system.

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