There is nothing better than a reliable system in the house. Water flows freely on demand, the flick of a switch illuminates the house, and a press of a button clears the toilet tank.
However, in urban centers, there are sewerage systems that ensure such reliability for all home owners. Rural houses, suburban villages and off-grid structures require individualized systems that manage, process, and dispose of waste produced.
Most of these septic systems comprise of two main units namely: septic tank and soak pit. The two elements differ in their purpose, construction and design. This article discusses the features, parameters and essentials of designing a septic tank and a soak pit.
Definitions
Before moving on to the size and calculation of the two units, it is essential to define their purpose and describe their function. Septic tank and soak pit are two components of an onsite sewage system that involves gravity settling of solids and aerobic treatment of effluents before disposal into the environment.
What Is a Septic Tank?
A septic tank is an underground water-tight chamber constructed of reinforced concrete, brick, plastic, fiberglass or any other approved material. It receives and holds sewage from the household plumbing system including the bathroom, kitchen, and laundry.
The tank ensures primary treatment of the effluent through sedimentation of solids and flotation of greases. It comprises of three layers: the bottom layer consists of settled mineral matter referred to as sludge.
The top layer is formed by floating oils and fats called scum. In between the sludge and the scum there is clear liquid that flows out of the tank into the leaching pit.
This tank provides anaerobic digestion of organic matter and a degree of biological cleansing of the effluent. The liquid effluent is led into the soak pit where it is absorbed and filtered through the soil to remove impurities.
What Is a Soak Pit?
A soak pit, also called a leach pit, is a shallow excavation with a perforated pit at the center. It is a chamber constructed of concrete blocks or other approved material with openings that allow effluents to seep out and be filtered by the top layer of the subsoil.
The liquid from the septic tank is discharged into the pit where it is broken down by microbial action before being absorbed into the ground. The soak pit is always placed at a lower elevation than the septic tank and is connected to it by an appropriate sewer line.
It is important to note that soak pits should be located with care because they may cause pollution of water wells or other watercourses if constructed incorrectly.
Design Criteria: The Essentials of the System
Septic tank and soak pit system should not be designed on a guesswork basis. There are specific design criteria to septic systems that, when followed, prevents odor, clogging of the drainage field, tank or pit overflow, or ground and water pollution.
Septic Tank Design Criteria
Retention Time
The retention time or the detention period is the time that the effluent wastewater stays in the septic tank before being discharged to the leaching pit. For domestic use, the recommended retention time is 24 – 48 hours.
Capacity and Sizing
The septic tank size is determined by the number of house occupants and the daily wastewater generation. Larger families and those with higher water use will need bigger tanks. Most building codes set out the minimum liquid volume of tanks applicable to all home sizes. For instance, a code might recommend that a septic tank serving up to 5 people should have a minimum liquid volume of 1300 liters or more. The liquid volume increases with the number of occupants.
The recommended length to width ratio for a septic tank is 2:1 or 3:1. A two compartment tank is usually recommended, where the first compartment holds 2/3 of the volume and the second holds 1/3.
The tank should have a liquid level between 1.2 – 1.5m. The additional volume (above the liquid level) allows for the storage of scum. A minimum of 0.3 – 0.5 m of freeboard is required above the liquid level for inspection, desludging and safety purposes.
The tank should be accessible and inspectable and have inlet and outlet ports that have appropriate fittings to prevent the entry of scum and sludge.
Soak Pit Design Criteria
Soil Percolation Test
The borehole soil test determines the rate at which the ground soaks up water. A soak pit works by relying on the suction power of the sub-stratum to draw off effluent from the pit. Different types of sand and clay will absorb water at different rates. The soak pit may not be suitable for soils with an extremely high water table.
Depth and Size
A soak pit can be circular or square and constructed of concrete blocks with clean gravel or stone. The stones should have a maximum size of 20 – 50 mm and the blockwork should be of open-jointed design. The soak pit should be excavated to a depth below the highest water level. The pits should be located at least 1.5 – 2 m away from water wells, buildings, property lines, and depressions where water might accumulate.
In addition, the soak pit should not be placed in the path of surface water run-off or connected to the municipal sewerage system.
Frequently Asked Questions
1. How often should a septic tank be cleaned?
Averagely, a septic tank that services 5 – 6 people is required to be desludged every two to three years. Depending on the number of occupants and the volume of the septic tank, the cleaning period may vary.
The sludge should not exceed the recommended level and if possible allowed to settle. Too much sludge reduces the retention time of the effluent and may lead to clogging in the leaching pit.
2. Can rainwater or roof drains be connected to a septic tank?
No, rainwater or roof drains should never be connected to a septic tank. The tank is designed to receive and process wastewater from the house plumbing system. Large volumes of water from roof gutters and other surface water drains may overwhelm the tank, disturbing the sludge and causing environmental pollution. Surface water should be kept away from the septic tank and connected to a separate drainage system.
3. What happens when a soak pit is blocked?
When a leaching pit gets blocked, wastewater can back up and cause wet spots in the area. A blocked soak pit may be evident by wet patches, odors or marshy ground around the pit. The cause of blockage can either be due to the sub-soil type or an overload of the system. Blockage may also occur if the pit is not appropriately sized for the volume of effluent flowing into it.
The pit is blocked due to the accumulation of organic matter in the pores in the blocks or stones. The accumulation of sludge, oil or silt may also be the cause. It is important to regularly clean the septic tank because it retains most of the solids and grease from the effluent. If the pit is blocked, it may need to be cleared and enlarged after a survey of the site.
Conclusion
Septic tanks and leaching pits are essential components of onsite sewage systems that protect public health and the environment. Their presence in a house enables proper disposal of domestic waste as well as reduces pollution of underground water. The design and installation of these systems depend on the size of the house, the number of occupants, and the site’s sub-stratum. The soak pit should always have an appropriate size and slope to ensure that the effluent wastewater does not overwhelm the system. The correct soak pit size is obtained through soil examination. Periodic desludging of the septic tank is essential to control the amount of sludge.