Septic systems handles the waste from the drain system in buildings, normally by processing the waste through septic tanks and leaching areas.
Solid waste and grease are retained in septic tanks and effluent waste water are disposed into the ground by leaching systems.
Scum and fat/grease are floated to the top of the tank. Solids are settled in the bottom.
The maximum capacity of a septic tank for a normal single or multiple family house, or as an alternative based on the load - Drainage Fixture Units (DFU) - are indicated in the table below:
|Septic Tank Volume||Maximum Capacity|
|gallons||m3||Single family units
(number of bedrooms)
|Multiple family units - one bedroom
(number of units)
|750||2.8||1 - 2||15|
|1500||5.7||5 - 6||3||33|
The size of the leach field are determined by two main factors.
The adsorption capacity of the soil can be determined by tests which are required in some jurisdictions. Typical absorption capacities for different types of soils are indicated below.
|Type of soil||Typical Absorption Capacity for a 24-hour period|
|Clay with small amount of sand or gravel||1 - 1.5||40 - 60|
|Clay with small amount of sand or gravel||1.5 - 2||60 - 80|
|Coarse sand or gravel||5||200|
The required area can be calculated by dividing the 24-hour amount of wastewater with the absorption capacity of the soil.
The total loading rate from a single family dwelling are 1200 gallons. The required leach field in a soil consisting of fine sand can be calculated as:
A = (1200 gallons) / (4 gallons/ft2)
= 300 ft2
The leach lines are made as trenches filled with washed rock/gravel to flow level with a perforated pipe on the top. Rock is added to cover the pipe and an approved filter material is used to keep soil from filtering down into the rock.
The leach line dimensions depends on the required capacity of the system. As a guideline