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Septic Systems

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Septic systems handles the waste from drain systems 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.

Septic Tanks

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 Capacities
Septic Tank VolumeMaximum Capacity
gallonsm3Single family units
(number of bedrooms)
Multiple family units - one bedroom each
(number of units)
750 2.8 1 - 2   15
1000 3.8 3   20
1200 4.5 4 2 25
1500 5.7 5 - 6 3 33
2000 7.6   4 45
2500 9.5   6 60
2750 10.4   7 70
3000 11.3   8 80

Grease Interceptors

Required grease interceptor capacity:


Leaching Fields

The size of the leach field are determined by two main factors.

  • the loading rate or the amount of wastewater that goes out to the leach field on a daily basis
  • The soil percolation or absorption rate - the rate at which the liquids will pass through the soil

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.

Soil Absorption Capacities
Type of soilTypical 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
Sandy loam/clay 2.5 100
Fine sand 4 160
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.

Example - Required Leach Field

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


Leach Lines

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

  • the maximum length of each line should not exceed 100 ft (30 m)
  • the spacing between lines should not be less than 10 ft (3 m)
  • the minimum depth of the leach line should not be less than 12 in (0.3 m) or deeper if the soil freezes during wintertime
  • the elevation rate should be approximately 1%
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Related Topics

Sanitary Drainage Systems

The purpose of the sanitary drainage system is to remove effluent discharged from plumbing fixtures and other equipment.

Related Documents

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Cleanouts provides access to the sewer for rodding.

Content in Horizontal - or Sloped - Cylindrical Tanks or Pipes

Volume of partly filled horizontal or sloped cylindrical tanks or pipes - online calculator.

Draining Systems - Main Vents

Vents in draining systems protects traps against pressure differences that could cause them to siphon or blow out .

Drains and Sewers - Capacities vs. Size

Drainage Fixture Units (DFU) connected to building drains and sewers.

Fixtures Trap Sizes

Recommended drain trap sizes for different types of fixtures

Sanitary Drainage Systems - Volume Flow

Calculate expected volume flow load in sanitary drainage systems.

Sanitary Piping - Drainage Fixtures Unit Loads

Maximum Drainage Fixture Unit - DFU - loads in sanitary piping systems.

Sewer Pipes - Capacities vs. Slope

Carrying capacities of sewer and wastewater pipes - gpm and liter per second.

Waste Water Piping - Flow Capacities vs. Dimensions

In sewage piping and pumping systems the fluid flow rate must be kept within certain limits to avoid operating problems.

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