As often as not, septic tank problems arise not because of abuse and misuse but simply because the system is too small for the household. When you build a septic tank, there are simple methods of preventing disaster, particularly when your household grows, for example with the addition of children or grandparents, but also to prevent a system that was built too small to begin with.

Believe it or not, it’s a common problem: A newly built home may be clad in the most expensive of finishes, but the last component for which any money is left is the septic tank and as a result the builders scrimp on the size of the hole. And, believe it or not, very few commercial plumbers know enough about septic tanks and drain fields to advise their clients correctly.

The daily volume of effluent emptying into the tank depends, to a large extent, on the size of the household and individual members’ habits. Baths use more water than showers, for example, and household members who stay at home during the day use the loo more than those who go to work. Party-animals will also use more than the average by way of loo flushes.

Working out your size requirements

The minimum size of a standard two compartment septic tank system, where the primary and secondary tanks are apportioned two-thirds to one-third (as pictured above), should have a volume of 2 520 litres and should be adequate for the daily needs of up to six people. Such a tank would have internal dimensions of 1,0m x 2,1 m x 1,2m deep.

For eight people the size should be 1,1 m x 2,2m x 1,3m deep (volume 3 146 litres).

For ten people the size should be 1,1 m x 2,5m x 1,3m deep (3 575 litres).

For 12 people the size should be 1,2m x 2,4m x 1,4m deep (4 032 litres).

For 16 to 20 people the size should be 1,2m x 2,6m x 1,4m deep (4 368 litres).

If your kitchen sink is fitted with an automatic waste grinder these volumes should be increased by 30%.

If building one’s own tank from brick the walls should be at least 230mm thick, and plastered on the inside. The floor, which should be a single-cast concrete slab, should also be plastered. The dividing wall (between the two tanks) can be a single brick thick, with two holes measuring 230mm x 80mm (equivalent to one brick) sited halfway between the top and the bottom of the tank. There should be a small space between the top of this wall and the roof of the tank to allow gas to flow back and forth between the two tanks. The primary and secondary tanks need not be adjoining but can be some distance apart and joined by pipe, depending on the topography and soil conditions, as long as the above criteria are fulfilled. The end of the inflow pipe from the house should be 50mm higher than the outflow pipe to the soakaway or drain field. Both the inflow and outflow pipes should be fitted with a T-piece. One end of the T should extend into the fluid and the other should stick up in the air as a gas escape.

Knowing what to buy

Another common fault is to fit the roof of the tank with only one manhole over the primary tank. Two manholes should be fitted, one over each tank, to enable them both to be emptied completely by pumping. The roof of the tank should be of reinforced concrete, not corrugated iron etc, which will rust and become dangerous. The alternative to building one’s own tank is, of course, to buy a ready-made plastic digester system.

A standard dig-and-drop system, like this one manufactured by Rototank, will be correctly apportioned and suitably piped.

In their most basic form these are merely “dig and drop” containers divided into correctly apportioned tanks fitted with suitable piping. More sophisticated dig-and-drops exist, however, which function as mini-sewage works, cleaning the effluent to a far greater degree than a conventional septic tank and rendering the final fluid suitable for use as irrigation water or for discharge into rivers and streams. These systems are more complex They are divided into five or more chambers, including two which function as primary and secondary septic tanks, another which is filled with plastic media and an air pump and functions in much the same way as a municipal sewage aeration system, and yet another tank from which reactivated sewage sludge, containing a concentration of beneficial microbes, is pumped back hydraulically into the septic chamber as a “booster”.

For more on fluid waste management, read our series on the topic here.

6 thoughts on “How to size, and build, a septic tank

  1. i need a quote for a septic tank that can accommodate 12 people, the project is in dullstroom walkersons estate

  2. I need a contractor who can help me rebuild my septic tank for 3 people in 1 hectare plot in Mooikloof Pretoria east

  3. I need to construct a septic tank for a change house where 100 people will be using the ablution facilities. your input on size would be great.

    1. While size of the tank is the prime consideration, a number of other factors will affect it. Such as, what kind of ablutions will they be performing, and how often? Are the users there permanently, ie using the facilities day and night for all their needs, eg toilet, shower, basin? Will there be laundry facilities and kitchen water in play? Or will they only be there occasionally, and only showers, eg after sport?
      If it’s just grey water, eg shower, basin, kitchen, laundry, you don’t need a septic tank at all, just a grease trap and a simple filter, and you can re-use the water. It’s only really black water, ie from toilets, that needs the full septic treatment.
      Also, the nature of the ground, and its slope, is important in determining the size of the soakaway system. Sandy soil means smaller soakaway. heavy soil (eg clay) or ouklip, requires a much bigger soakaway.

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