When a house is not connected to a sewerage system, it has to depend on an onsite decentralized system to treat and dispose the waste water generated. A common mechanism is a Septic Tank and a Soil Absorption system combination.
A septic tank works on the principle of natural decomposition of solids through anaerobic digestion aided by bacteria and treatment of the liquid effluent through the layers of soil that it passes through. The septic tank system is optimal for less densely populated communities, individual households where the soil is permeable, ground water table is not too shallow and is suitably away from any surface water source.
The drains from the house are all connected to the Septic tank which is a water tight structure buried underground. The Septic tank aids in the settlement and anaerobic decomposition of solids in the waste water and the separation of scum and grease. The clarified liquid effluent flows out of the outlet pipe of the septic tank and is connected to the soil absorption system or a drain field. Generally a shallow trench serves the purpose of the soil absorption system. The main design parameter of the absorption system in addition to its dimensions is that there should be at least 2 – 4 feet of unsaturated soil layer to treat the waste water that flows down before it hits an impervious soil layer or rocks or shallow water table. These trenches are half filled with gravel through which the outlet pipe from the septic tank flows through. This pipe will be perforated so as to allow for the effluent to be distributed in the trench. A synthetic fabric (geo-textile) is placed over the gravel layer before backfilling with the excavated soil. The synthetic fabric will prevent the soil from entering the gravel layer and clogging the perforated pipes.
The nutrient rich effluent will help in replenishing the soil and recharging the ground water aquifers after being treated by the soil layers above. Also, it avoids the need for capital intensive centralized treatment systems which are also high on energy requirement. The septic tank soil absorption system relies on natural (soil and bacterial) treatment and thereby does not require energy.
Some of the design considerations (not in detail) are:
- The size of the septic tank in relation to the household size
- The material of the septic tank (between Polyethylene, Concrete, Fiber Reinforced Plastic)
- The location of the tank considering distance from the nearest water source, plumbing lines
- The baffle walls or T-shaped inlet/ outlet in the septic tank to force the incoming solids to the bottom of the tank and to prevent the escape of scum/ grease through the outlet which can result in the clogging of the drain field.
The proper design of the system is important to ensure the
- Long term durability of the system (easily more than 20 years if designed and maintained well)
- Prevention of contamination of surface and/ or ground water
- Prevention of drain field getting clogged and resulting in bad odors, back flow into the house
Illustration: Layout of Septic Tank and Drain Field
Illustration: A Polyethylene Septic Tank
No wonder that almost 30% of the households in the United States have an on site treatment such as the Septic Tank Soil Absorption system thereby eliminating the need for an energy guzzling and capital intensive centralized treatment system, positively contributing to soil health and also replenishing the ground water aquifers.
- Image Courtesy – http://www.jgallagherseptic.com/