A concrete septic tank (often called a cement septic tank) is still the most common type of septic tank on South African smallholdings, farms and older homes that are not connected to a municipal sewer. It is heavy, strong and can last for decades, but it also has specific weaknesses that show up as it ages. This guide sets out the real pros and cons of concrete septic tanks, how they compare with plastic tanks, and what to check if you already have one.
A quick note on where this advice comes from: Drain Blasters does not sell or install septic tanks. We empty and service septic tanks across Cape Town, which means we see the inside of a lot of concrete tanks at 10, 20 and 40 years old. That is the perspective behind this article.
What is a concrete septic tank?
A concrete septic tank is an underground, watertight chamber (usually two chambers separated by a baffle wall) that holds household wastewater long enough for solids to settle and bacteria to break them down. Clarified effluent then flows out to a soakaway or drain field. If you want the full process, see our guide on how septic tanks work.
There are two ways a concrete tank ends up in the ground:
- Precast concrete septic tanks are manufactured off site in standard sizes, then delivered and lowered into the excavation by crane truck.
- Cast-in-situ tanks are built in the hole, either as reinforced concrete or as plastered brick or block walls on a concrete floor slab. Many older Western Cape tanks are of this type.
Both must be sized and positioned to comply with SANS 10400-P and your municipality’s by-laws. Our article on septic tank regulations in South Africa covers the legal side.
Advantages of a concrete septic tank
Weight keeps it in the ground
Concrete’s biggest practical advantage is mass. An empty plastic tank in a high water table can lift out of the ground if it is not properly anchored. A concrete tank is heavy enough to resist this in most conditions, which matters in low-lying and sandy areas where the water table rises sharply in a Cape winter.
Strength under load
A properly designed concrete tank and slab can be specified to carry surface loads, including vehicles where the design allows for it. Plastic tanks generally need a separate reinforced slab above them before anything heavy can pass over.
Long service life
A well-built, well-maintained concrete septic tank commonly lasts 30 to 50 years. Concrete does not deform, and the walls do not become brittle from sunlight the way exposed plastic lids can.
Can be built to fit the site
A cast-in-situ tank can be built to whatever capacity and shape the site needs. That is useful on larger properties, on awkward sites, or where a delivery truck and crane cannot get close to the excavation.
Disadvantages of a concrete septic tank
Cracking and leaks
Concrete is rigid. If the ground settles, if roots press against the walls, or if the tank sits in reactive clay that swells in winter and shrinks in summer, it can crack. Poorly cured concrete and badly sealed joints between precast sections are other common leak points. A cracked tank lets effluent out into the surrounding soil and, just as often, lets groundwater in. If your tank fills up again soon after being emptied during the rainy season, groundwater ingress is a likely cause. See what to do when your septic tank leaks.
Acid attack above the liquid line
This is the failure most owners never hear about. Bacteria in the tank produce hydrogen sulphide gas, which collects in the air space above the liquid and is converted into sulphuric acid on the concrete surface. Over the years this eats into the roof slab, the upper walls and, especially, the baffle wall and inlet and outlet fittings. The process is known as biogenic sulphide corrosion. The concrete below the liquid line can look perfect while the lid is quietly losing its strength.
Weight works against installation
The same mass that stops a concrete tank floating makes it harder to install. Precast units need crane access and a level, well-prepared base. Cast-in-situ tanks take longer to build and must be allowed to cure before use.
Hard to repair properly
Hairline cracks can sometimes be sealed from the inside once the tank has been emptied, but a tank with a structurally failed roof slab or widespread cracking usually has to be replaced. There is no practical way to reline a badly damaged septic tank the way you can reline a drain pipe.
Concrete vs plastic septic tanks
For most new domestic installations in the Western Cape, the choice comes down to concrete or plastic (polyethylene). Here is how the two compare in practice:
| Factor | Concrete septic tank | Plastic (polyethylene) septic tank |
|---|---|---|
| Flotation in a high water table | Resists flotation by weight | Must be anchored, usually to a concrete base slab |
| Surface loads | Can be designed for loading | Needs a separate slab for vehicle loads |
| Typical lifespan | 30 to 50 years | 30 to 40 years |
| Typical failure | Cracks, joint leaks, acid attack on lid and baffles | Deformation if backfilled badly, brittle lids with sun exposure |
| Installation | Crane truck or on-site construction and curing time | Light, fast to install, watertight from day one |
| Best suited to | Stable soils, sites needing weight or load-bearing | Reactive clay and wet ground, provided it is properly anchored |
Packaged aerobic (“bio”) systems are a separate category altogether: they treat effluent further but depend on electricity, pumps and more frequent servicing. Our guide to the types of septic tanks covers fibreglass, steel and aerobic units as well.
What affects the cost of a concrete septic tank?
Prices vary widely by region and supplier, so treat any single figure you see online with caution. The things that actually drive the cost of a concrete septic tank installation are:
- Capacity, which is set by the number of people the tank serves. Undersizing to save money shortens the emptying interval for the life of the tank. Our septic tank sizing guide explains how capacity is worked out.
- Precast versus built on site, including crane hire and delivery distance for precast units.
- Excavation conditions. Rock, collapsing sand or a high water table that needs dewatering all add cost.
- The soakaway or drain field, which is often as much work as the tank itself.
- Plan approval from the local municipality before any work starts.
Ask for an itemised quote from a registered builder or installer that separates the tank, the excavation and the soakaway, so you can compare quotes like for like.
How to make a concrete septic tank last
- Empty it on schedule. Most domestic tanks need emptying every three to five years; our guide to septic tank pumping intervals explains how to set the right interval for your household. Letting sludge build up pushes solids into the soakaway, which is far more expensive to fix than the tank.
- Use the emptying visit as an inspection. The only time you can see the inside of the tank is when it is empty. Ask the crew to check the baffle wall, the inlet and outlet fittings, the underside of the lid and the upper walls for crumbling concrete or exposed reinforcing.
- Keep vehicles and heavy loads off the tank unless it was specifically designed for them.
- Keep trees and large shrubs away. Roots follow moisture and will exploit any crack or joint.
- Keep stormwater out. Downpipes or yard drains connected to a septic system overload it every time it rains. See how heavy rain can affect your septic system.
- Do not rely on additives. They do not replace emptying. Our article on the pros and cons of septic tank additives explains why.
Signs your concrete septic tank is failing
- Wet, sunken or unusually green ground around the tank
- Persistent sewage odours near the tank or lid
- Toilets and drains that are slow throughout the house, or back up at the lowest fixture
- The tank refilling quickly after being emptied, especially in winter
- Cracked, spalled or crumbling concrete visible at the lid or access openings
Any of these warrants an inspection. Our guide to common septic tank problems explains what each symptom usually points to.
Is a concrete septic tank a good choice?
Yes, in the right ground. A concrete septic tank is a sound choice on stable soils, on sites where weight is needed to resist a rising water table, and where the tank must carry surface loads. It is a weaker choice in reactive clay, where ground movement tends to crack rigid concrete, and on sites a crane truck cannot reach. In those conditions a properly anchored plastic tank is often the better option.
Whichever material you have, the biggest factor in its lifespan is maintenance. If your tank is due for emptying or you want the inside checked for cracks and acid damage, contact our team about septic tank cleaning in Cape Town. For general background on septic systems, the Wikipedia entry on septic tanks is a useful technical reference.
