A sound septic tank design is the difference between a system that quietly treats your household wastewater for decades and one that blocks, leaks or contaminates the surrounding soil. Whether you are building on a smallholding outside Cape Town or replacing an ageing tank, understanding how a septic tank is sized, built and drained will help you get it right the first time. This guide covers septic tank design, sizing, materials and the drainage field that makes the whole system work.
What is a septic tank?
A septic tank is a watertight, underground chamber that receives all the wastewater from a property that is not connected to the municipal sewer. Inside the tank, solids settle to the bottom and anaerobic bacteria break them down, while the clearer liquid in the middle – called effluent – flows out to a soakaway or drainage field for further treatment in the soil. It is a simple, self-contained way to manage sewage in areas without reticulated sewerage, which is why septic tanks are so common on farms, estates and rural properties across South Africa.
How septic tank design works
A well-designed septic tank holds wastewater long enough for separation and digestion to happen. The tank is sized so that sewage is retained for at least 24 hours, giving the bacteria time to liquefy the solids and leaving a small amount of settled sludge at the bottom. The effluent then leaves through the outlet and is absorbed into the soil. If the tank is too small, wastewater passes through too quickly and untreated solids reach the drainage field, where they clog the soil and cause failure.
Sizing the tank
Tank capacity is based on the number of people using the system and how often the sludge will be removed – typically every two to three years. A common design allowance is around 130 litres per person for smaller households, increasing the total volume as occupancy rises. Always size up rather than down: a slightly larger tank gives solids more time to settle and reduces how often you need a honeysucker pump-out.
A two-chamber layout
Most modern septic tanks use two chambers separated by a baffle wall. The first, larger chamber receives the raw sewage and does most of the settling and digestion. The partially treated effluent then passes into the second chamber for further settling before it leaves the tank. This two-stage design produces cleaner effluent and protects the drainage field far better than a single-chamber tank.
Construction and materials
Septic tanks are built from materials that stay watertight under constant load: reinforced concrete, brick rendered with waterproof cement mortar, or factory-made polyethylene (plastic) tanks. For a brick or concrete tank, both faces of the wall and the floor are plastered with a waterproof cement render, and the internal corners are rounded to prevent sludge from lodging. A waterproofing additive is mixed into the mortar and the floor concrete to keep the chamber sealed.
The floor is laid with a slight slope towards the inlet so that sludge collects where it can be pumped out easily. The tank is topped with a reinforced concrete slab fitted with access covers for inspection and cleaning. Inlet and outlet pipes are fitted with submerged elbow or T-pieces that draw liquid from the middle layer while holding back the floating scum. A vent pipe fitted with an insect-proof mesh releases the gases produced by decomposition. For a deeper look at the build, see our guide to septic tank construction, materials, design and installation.
How to build a septic tank correctly
If you are planning to build a septic tank, the key principles are: make it watertight, size it for the household, give the effluent somewhere to drain, and keep it accessible for pumping. In South Africa, on-site sanitation must comply with the national building regulations (SANS 10400) and any local municipal requirements, so confirm the rules before you start – our overview of septic tank regulations in South Africa is a useful starting point. The South African guidance on on-site sanitation also sets out separation distances from boreholes and watercourses.
Septic tank drainage: the soakaway
The tank only does half the job; the drainage field does the rest. Effluent leaving the tank is dispersed through a network of perforated pipes laid in gravel-filled trenches, or into a soakaway pit, where it filters down through the soil. The soil acts as a natural filter, removing remaining pathogens before the water rejoins the groundwater. The size of the drainage field depends on the soil’s ability to absorb water, which is measured with a percolation test. Heavy clay soils drain slowly and need a larger field; sandy soils drain quickly. A failing or undersized soakaway is one of the most common septic tank problems we are called out to.
Keeping your septic system healthy
Even the best septic tank design needs care. Avoid pouring bleach, strong disinfectants or chemical drain cleaners down the drains, as these kill the bacteria that keep the system working. Spread out laundry and water use so you do not overload the tank, and have it inspected and pumped on schedule. To understand the different configurations available, read our guide to the types of septic tanks.
Drain Blasters designs, services and pumps septic systems throughout the Cape Town area. If your tank is overdue for a clean or you suspect a drainage problem, our septic tank cleaning team in Cape Town can help.
