If you’re building, replacing, or just trying to understand the system you’ve inherited, the type of septic tank you choose has a direct impact on how long it lasts, how it handles ground movement, and how easy it is to maintain. This guide covers the main types of septic tanks used in South Africa — concrete, plastic (polyethylene), fibreglass, steel, and aerobic systems — with the trade-offs of each.
Before we get into materials, two general points. First, every septic tank in South Africa should comply with SANS 10400-Q (non-water-borne sanitation) and the relevant local municipal bylaws. Second, the tank itself is only one part of the system — the soakaway or French drain that handles the effluent matters just as much. We’ve covered that in our guide to French drains.
Concrete septic tanks
The concrete septic tank is still the most common type installed in South Africa, particularly for permanent residences on smallholdings and farms. They are cast on site or delivered as precast units, with two chambers separated by a baffle wall.
Strengths:
- Lifespan of 30 to 50 years when properly built and maintained.
- Heavy enough to stay in place even when groundwater levels rise — they don’t float.
- Relatively inexpensive in materials, especially if cast on site.
- Robust enough to withstand vehicles driving over them when buried at the correct depth.
Weaknesses:
- Cracks develop over time, particularly on properties with reactive clay soils that swell and shrink seasonally — common across much of the Cape.
- A cracked tank leaks sewage into surrounding soil and lets groundwater in, which fills the tank and overwhelms the soakaway.
- Steel reinforcement can corrode where the concrete is below the water line for years.
- Heavy and difficult to remove if a replacement becomes necessary.
Most of the common septic tank problems we see on older Cape Town properties involve cracked concrete tanks where the soakaway has been overwhelmed by infiltrating groundwater.
Plastic (polyethylene) septic tanks
Modern plastic septic tanks are made from rotomoulded high-density polyethylene (HDPE) and have become the most popular replacement option for new installations.
Strengths:
- Won’t crack, rust or corrode.
- Watertight by design — no leaks, no groundwater infiltration.
- Light enough for two or three workers to install without lifting equipment, which makes them practical on tight access sites.
- Lifespan of 30 to 40 years when properly installed.
- Available in standard sizes from 2,500 to 10,000+ litres.
Weaknesses:
- Buoyancy. An empty plastic tank in a high water table will float if it’s not properly anchored. Most installations require a concrete base slab and ribbed sides for friction.
- Susceptible to deformation if heavy machinery drives over them — they need to be installed at the manufacturer’s specified depth and protected from surface loads.
- The plastic itself can become brittle after decades of UV exposure on accessible inspection covers.
For most new domestic installations in Cape Town, a properly anchored plastic tank with a concrete base is what we’d recommend.
Fibreglass septic tanks
Fibreglass-reinforced plastic (FRP) tanks sit between concrete and polyethylene in the market. They’re more expensive than plastic but offer some advantages where chemical resistance matters.
Strengths:
- Excellent chemical and corrosion resistance.
- Lighter than concrete, easier to install.
- Long lifespan — comparable to good-quality polyethylene.
- Can be moulded into specialised shapes for tight installations.
Weaknesses:
- Same buoyancy issue as plastic tanks — must be anchored.
- More expensive than equivalent-volume polyethylene tanks.
- Less commonly stocked locally, so lead times can be longer.
Fibreglass tanks make most sense in commercial or industrial settings where chemical loading is higher than a normal household, or in coastal areas where corrosion is an ongoing problem.
Steel septic tanks
Steel septic tanks are largely a legacy product — you’ll find them on older properties, but they’re rarely specified for new installations.
Strengths:
- Strong when new.
- Rapid to manufacture.
Weaknesses:
- Lifespan of around 25 years, often less in moist or saline ground conditions.
- Rusts from the inside out where the metal sits in standing waste.
- The lid is usually the first thing to fail — corrosion makes it dangerously fragile, and there are well-documented cases of people stepping onto a corroded lid and falling into the tank.
If your property has a steel tank that’s older than 20 years, get it inspected. If the lid is rusty, don’t walk over it. In most cases the cost-effective solution is to replace the whole unit with a polyethylene tank rather than patch the steel.
Aerobic treatment units (ATUs)
Strictly speaking, an aerobic system isn’t a “type of septic tank” — it’s a more advanced wastewater treatment system that uses oxygen to break down waste much faster than passive anaerobic digestion in a standard septic tank.
How it differs:
- An air pump (powered by mains electricity) injects oxygen into a treatment chamber.
- Aerobic bacteria, which work much faster than anaerobic bacteria, break down waste to a much higher standard.
- The treated effluent is clean enough to discharge to a much smaller drainfield, or in some cases to be used for sub-surface irrigation.
Strengths:
- Effluent is treated to a much higher standard — useful where soakaway space is limited.
- Smaller drainfield required.
- Suited to sites with high water tables or impermeable soils where a conventional soakaway won’t work.
Weaknesses:
- Two to three times the cost of a conventional septic tank.
- Requires mains electricity and a working air pump — system fails if power is out for extended periods.
- More frequent service intervals (typically annual).
- Not load-shedding-friendly — extended outages risk anaerobic conditions returning and a smelly back-up.
Which septic tank type is right for you?
For most new domestic installations in the Western Cape, the choice comes down to concrete or polyethylene. Concrete is the traditional default and still works well in stable soils. Polyethylene is faster to install, watertight from day one, and the better choice in clay-rich or wet ground — provided it’s properly anchored.
Fibreglass is worth considering for coastal, commercial, or higher-load applications. Aerobic systems are the right answer when your site simply can’t support a conventional soakaway — but they cost more to run and need reliable power.
Whatever you install, the system only works if it’s properly sized, properly built, and properly maintained. We provide septic tank cleaning across the Cape Town metro, and we can inspect and pump any of the tank types above. If you’re not sure what’s already on your property, get in touch and we’ll have a look — knowing what you’ve got is the first step to keeping it working.
