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Septic tanks are rectangular chamber can be either with single compartment or multiple compartments usually constructed below ground level for treating waste water.


The functions of the septic tanks are as follows:

  • They receive black water or excreta from the toilets.
  • Heavier solids settle to the bottom of the septic tank where they are decomposed by anaerobic bacteria;
  • The digested sludge is stored at the bottom,
  • The lighter solids such as hair, soap and grease float to the surface forming scum.
  • The gases like carbon-di-oxide, methane and hydrogen sulphide are produced during anaerobic decomposition and released to the atmosphere through vent.
  • To start with, the tank is filled with water for a depth of about 100mm and seeded with cow dung or sludge from other septic tank.
  • Septic tank is to be cleaned once in two to four years to remove the accumulated digested sludge.

Three compartment septic tank

One design modification is to use three compartments; where toilet wastes are discharged into the first compartment, sullage is discharged directly into the second compartment, while the third compartment provides additional and more quiescent settling for solids.

The first compartment should be designed, on the basis of 0.15 m3 of volume/ user, so that desludging could be done once in two years. The second and third compartments should be sized each for one day detention.

Design Criteria for Septic Tanks

Design criteria of septic tanks are as follows:

Number of souls per house hold                          = 5

Number of fixture per house hold                    = 1

% of units discharging simultaneously           = 60

Discharge from each unit                                     = 10 Lpm at peak hours

Surface area for sedimentation                         = 1 m2/10 Lpm of peak discharge

Minimum depth for sedimentation                  = 30 cm

Minimum detention time                                    = 24 hours

Volume of fresh sludge                                         = 0.0005 m3/capita/day

Digestion period                                                      = 45 days

Digested sludge                                                       = 0.03 to 0.07 m3/capita/annum

Minimum total volume of septic tank            = two times daily design flow

Cleaning interval                                                     = 1 to 4 years

Length to width ratio                                            = 2 to 3:1

Construction Aspects

The Septic tanks shall be constructed of sound, durable, watertight materials either masonry or concrete that are not subject to excessive corrosion, or decay. They shall be designed to be watertight, and to provide settling of solids, accumulation of sludge and scum, and be accessible for inspection and cleaning.

Following aspects should be considered during the construction of septic tanks:

  • A baffle or a sanitary tee pipe at the tank inlet slows down the incoming rush of wastewater, so the sludge is not stirred up. This baffle or tee is to penetrate at least 150mm below the liquid level, but the penetration is not to be greater than that allowed for the outlet device.
  • A baffle or a sanitary tee pipe located at the tank’s outlet prevents floating scum from leaving the tank. For tanks with vertical sides, outlet baffles or sanitary tees shall extend below the liquid surface a depth equal to approximately 40% of the liquid depth.
  • Many newer systems have a removable filter at the outlet to prevent finer solids from entering the disposal field.
  • Inspection pipes at the top of the tank are provided for inspecting the inlet and outlet pipes, baffles and tee pipes.
  • No tank shall have an excess of three compartments.
  • Septic tanks having more than one compartment can produce better quality effluent. The
  • effluent solid concentration is considerably lower in case of double compartmeht tank.
  • The first compartment is usually twice the volume of the second compartment.
  • The liquid depth is 1.0 to 1.8 m
  • The distance between the inlet and outlet of the tank is at least equal to the liquid depth of the tank.
  • All septic tanks may have an effluent filter installed at the outlet of the tank. The filter shall
  • prevent the passage of solid particles larger than a nominal 3mm diameter sphere.
  • A partition wall is used to form a multi-compartment tank; an opening in the partition may serve for flow between compartments and the minimum dimension of the opening is 100mm, and the mid-point is below the liquid surface at a depth approximately equal to 40% of the liquid depth of the tank.
  • The outlet of each successive tank shall be at least 50mm lower than the outlet of the preceding tank,
  • Inlets and outlets should be located on opposite ends of the tank. The invert of flow line of the inlet shall be located at least 50mm (and preferably 75mm) above the invert of the outlet to allow for momentary rise in liquid level during discharge to the tank.
  • All baffles shall be constructed from sidewall to sidewall.
  • All inlet and outlet devices shall be permanently fastened in a vertical, rigid position. Inlet and outlet pipe connections to the septic tank shall be sealed with a bonding compound that will adhere to the tank and pipes to form watertight connections, or watertight sealing rings.
  • The top of the baffles or sanitary tees must extend at least 150mm above the liquid level in
    order to provide scum storage, but no closer than 25 mm to the inside top of the tank.
  • Scum storage volume shall consist of 15% or more of the required liquid capacity of the tank and shall be provided in the space between the liquid surface and the top of inlet and outlet devices.
  • Access manholes to each compartment of the tank shall be provided to facilitate inspection and cleaning through properly placed manhole openings not less than 450 mm (preferably 550mm), in minimum horizontal dimension.
  • Access to inlet and outlet devices shall be provided through properly spaced openings not less than 300 mm in minimum horizontal dimension or by means of an easily removable lid section.
  • The top of the tank shall be at least 150mm below finished grade.
  • All manholes shall be extended to within at least 100mm of the finished grade. The manhole extensions shall be constructed of durable, structurally sound materials

3 Responses to “SEPTIC TANKS”

  1. Wa Salus says:

    You say: “To start with, the tank is filled with water for a depth of about 100mm…”.

    I would think that for a single chamber tank, the tank should be filled with water up to the bottom of the outlet T. Then, as sewage is added, the scum layer will be above the bottom of the outlet T and thus cannot enter the drain field.

    Please clarify. Thanks, Wa

  2. Jessie H says:

    Thanks so much for explaining how septic tanks work and why they should be cleaned.

  3. Iris Smith says:

    Thank you for clarifying that the septic tanks must be made of solid, long-lasting materials like concrete or masonry that won’t corrode or deteriorate excessively. My sister says she has to have her home’s septic system pumped. I’ll advise her to get septic tanks made of sturdy and waterproof materials.

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