A well-designed and constructed septic tank is crucial for preventing groundwater pollution and ensuring long-term functionality. In today’s construction landscape, understanding the intricacies of septic system design is more important than ever. This guide provides detailed insights into building a robust and efficient septic tank.

The Importance of a Strong Septic Tank

It’s essential to recognize the forces a septic tank must withstand. Many tanks, especially those using lightweight concrete blocks with a thin cement layer, are prone to cracking due to ground movement or settlement. A standard tank can hold several tons of liquid, so structural integrity is paramount. Septic tanks should be built from durable materials such as plastics that can flex with ground movement, or reinforced concrete, designed to move as a single entity without losing structural integrity.

Key Design Considerations

• Size: Determining the appropriate size of the tank is the first step. For an average 2-bathroom house with 4 occupants, a tank of approximately 2.9 cubic meters is usually sufficient.
• Chambers: A septic tank should have at least two chambers, but three are better. The second chamber can be about half the size of the first. For instance, a tank might have a first chamber of 1.92 cubic meters and a second chamber of 0.96 cubic meters.
• Depth: A deeper design with a smaller footprint is ideal for smaller land plots. In the example, the depth of the effluent is 1.6 meters.

Construction Details

• Reinforced Concrete: The walls and floor should be constructed using 10 cm (4 inch) thick reinforced concrete to withstand water pressure and ground movement. The reinforcing steel should be an 8 mm mesh with bars spaced 15 cm (6 inches) apart.
• Concrete Mixture: If a mix is not specifically designed for the materials being used, a good mixture is 3 parts aggregate, 2 parts good-quality sand, and 1 part cement. Avoid adding too much water, a slump of 7 is good. The concrete should be vibrated to ensure it is compact without air holes.
• Base: Include a sand bed beneath the concrete floor to accommodate settlement and movement.
• Pipework: Before pouring the concrete, install 15 cm (4 inch) diameter PVC pipes for inlet, transfer, and outlet, along with a 7.5 cm (3 inch) ventilation pipe between chambers.

Pipework and Levels

• Inlet Pipe: The inlet pipe carrying effluent should have a 2% slope (2 cm fall per meter) from the toilet to the tank. Ensure a direct flow into the tank, avoiding unnecessary manholes that can cause blockages and smells.
• Pipe Positions: The inlet pipe should be higher than the outlet pipe, and the transfer pipe slightly lower than both.
• Tee Pieces: Use 15 cm (4 inch) tee pieces on inlet, outlet, and transfer pipes to keep them clear of scum and reduce turbulence. The vertical extensions on the transfer and outlet pipes in the upstream chamber take liquid from the middle level, avoiding bottom deposits.
• Ventilation: An open 7.5 cm (3 inch) pipe above the water level connects the chambers to allow gas flow.

Manholes and Vent Pipe

• Top Slab: Use a 15 cm (4 inch) thick reinforced concrete slab for the top of the tank. Include a 2-inch galvanized steel vent pipe at least 2 meters high for gas dispersal.
• Manholes: Incorporate two manholes for maintenance access, ideally sealed with grout to be gas-tight. These should be positioned to allow access to the inlet and outlet pipes. Manholes are essential for pumping out the tank, usually every five years. Use flat surfaces for the manhole covers, rather than tapered joints, for better sealing. Steel handles on the covers are also important.

Final Thoughts

Following these guidelines ensures a robust, leak-proof, and efficient septic tank. Proper construction prevents leaks and groundwater contamination. Paying close attention to levels, materials, and design details will result in a septic tank that works well and requires minimal maintenance. By understanding these key aspects of septic tank design and construction, you can build systems that are both effective and sustainable.