Introduction
When it comes to managing household wastewater in rural or isolated areas, septic tanks and soak pits play a crucial role. These systems are essential where access to public sewage infrastructure is not available. Understanding how they work is vital for ensuring efficient wastewater treatment, environmental protection, and maintaining hygiene around homes. In this blog post, we will explore the functioning of septic tanks and soak pits, their construction details, and how they help in treating and disposing of wastewater. Whether you’re building a new home or simply looking to improve your knowledge of wastewater management systems, this guide will provide a clear overview of these key components, helping you make informed decisions for sustainable sanitation solutions.
Septic Tank
A septic tank is a crucial system for treating sewage in rural or isolated areas where a public sewer system isn’t available. It operates as a horizontal continuous flow sedimentation tank, where sewage moves slowly. The septic tank serves two main functions: (i) the settling of solids in the sewage through sedimentation, and (ii) the partial or complete digestion of the sludge before it is disposed of.
Septic action occurs within the tank due to anaerobic bacteria, which break down complex substances like proteins, carbohydrates, cellulose, and fats into simpler compounds. Nitrogen in the sewage is converted into ammonia, while colloidal matter undergoes flocculation, liquefaction, and finally digestion. The effluent from the tank is typically discharged into a soak pit or agricultural drains, allowing it to be absorbed by the soil. Here, aerobic bacteria in the soil continue the process of breaking down organic matter, stabilizing it.
It is important to note that effluent from a septic tank should never be released into open drains. The system is designed to prevent a direct flow between the inlet and outlet, typically achieved using pipe-tees at both ends. A baffle wall with openings is placed near the outlet, and the outlet pipe is positioned about 15 cm lower than the inlet pipe. This design helps in controlling the flow and maintaining proper sedimentation. Sludge that settles at the bottom of the tank must be cleaned periodically, usually every 6 to 12 months, and a vent pipe is provided to allow gases to escape.
Figure 1 illustrates a basic septic tank setup. The design ensures efficient sewage treatment while minimizing environmental impact.
Soak Pit
A soak pit is a covered pit dug into the ground, designed to absorb effluent from a septic tank. It is typically constructed in pervious soil that can efficiently absorb the discharged liquid. There are two types of soak pits, as illustrated in Figure 2: (a) lined and (b) unlined.
Lined soak pits are used when the inner volume is kept empty. The lining, which can be made of bricks, stones, or concrete blocks, is built with dry open joints and backed with at least 7.5 cm of coarse aggregate. However, if the inner space is filled with stone or brick aggregates, no lining is required, apart from a masonry ring at the top to support the structure.
Sludge Soak Pit
A sludge soak pit is a pit where sewage effluent from a house drain is discharged directly. The water from the accumulated sewage is absorbed by the surrounding pervious soil, while the sludge remains in the pit and is digested over time. These soak pits are commonly used in areas with highly permeable subsoil. Figure 3 shows a typical section of a sludge soak pit, which is lined with masonry that has open joints. The pit is covered with a rigid slab at the top.
The diameter of these pits typically ranges from 2 to 3 meters, and the depth varies between 2.5 and 4 meters. Each household usually has its own soak pit, which can last up to 20 years for a family of six. When the pit fills up with sludge, the top cover is removed, and the pit is emptied to restore functionality.