Septic Tank Design

Septic Tank Design: Septic Tank Construction & Maintenance 

Definition of Septic Tank 

Septic Tank is a water-tight receptacle that receives the discharge of human waste from the toilets.  It is an underground storage chamber made of concrete, fiberglass, or plastic and is intends to control, manage, and most importantly treat waste material. It also has some partial decomposition is based on anaerobic bacterial reaction solids and organics materials reduce to liquid.

It’s basically a type of biodigester, only that it lacks the functionality of gas collection and waste treatment.  Most septic tanks just separate the solid and liquid waste material. The wastewater will collect at the slung heap up at the bottom. As a result, the tanks will require emptying from time to time.

Septic Tank Design 2-3 Chambers

It is best to construct a single compartment for small capacity septic tanks. Larger capacities are the best fit for a two-compartment tank. The wall separating the two is about two-thirds the length away from the inlet. It, therefore, perform better than the single compartment tank. The two compartments interconnect above the sludge storage using pipes or square openings.

A concrete or plastic tank is the best to use. The floor and side walls are made using concrete and plastered to render a smooth surface. The floor has a slight slope of 10 percent towards the sludge outlet. As a result, slung moves out more easily

It has one end connected to an inlet from the toilets and sometimes an outlet connected to the drain field. Some septic tank design has 2 chambers while others have 3 chambers. A wall with openings between the floor and roof of the tank separates the chambers. As a result, the drainage pipes fill more evenly and preventing premature clogging of the drains, therefore, extending their life.

Some septic tank designs include water pumps so as to increase the volume and velocity of water drainage.

Septic Tank Design

A well-maintained septic system is odor-free and requires less emptying.

In addition, it should last for decades with no problems.

How a Septic Tank Works

A septic tank works in four basic steps. Namely :

  • Receives the wastewater from the house- all the house wastewater goes to the septic tank through one inlet.
  • Separates the solids from the liquids- The solid waste settles at the bottom of the tank. The wall separating the chambers allows for liquid only to move to the second chamber. The solids materials remain for degrading.
  • Decomposes the solids– Bacteria is used to decompose the solid waste into liquid. The bacteria feed on solid waste and free it from pathogens. They further reduce them to water.
  • Send the wastewater out to the drain field- the treated waste is released to the ground or left to percolate into the soil. Some tanks may include a pump to assist in pushing water out of the septic system.

Also Check >> Is Biodigester Quickly Replacing Septic Tanks?

Factors to Consider When Designing a Septic Tank

Septic Tank Capacity/Size

The septic tank capacity depends on the sewage flow and detention period. In addition, space for sludge and scum accumulation before emptying is also considered.  Depending on the type of house and the number of the user, septic capacity can be customized. The table below shows capacity Viz a Vis number of users.

Septic Tank Capacity

Amount of Sewage Flow

The amount of sewage flow highly depends on the number of users. The tank should be able to hold the total waste for all users during the detention period.

If the design does not account for all users, the tank will block more often and require emptying more frequently. This, on the other hand, increases the cost of maintenance.

Sludge Holding Period

The septic tank design should account for the sludge holding period. The minimum sewage detention period in a septic tank is 24 hours. However, due to the total volume requirement for sludge and scum accumulation, a septic tank is designed for a detention period of not less than 48 hours.

Septic Tank Construction

A septic tank construction should be perfect to prevent cracking and groundwater pollution. In addition, it should be strong enough to withstand the Earth’s movements and hold the weight of the waste. Most importantly, it is best to use plastic or reinforced concrete tanks.

Concrete Septic Tank

septic tank Design

A trench is dug and steel is placed where concrete is moulded on both sides to form a 4 inches thick wall. This provides the strength for the tank to hold pressure and also prevents the steel from rusting. The inlet and outlet pipe are put in place before pouring the concrete. It is very important you mix the concrete is well.

Add little water to harden the concrete. Use a vibrator to ascertain that it has compacted properly with no air holes or cracks. After that, connect the pipe to the inlet and outlet and also install the manhole slab.


Sludge Withdrawal and Disposal

Septic Tank design

Emptying a septic tank happens once or twice a year. Individual home tanks can lust for 2 years. Note that disinfection agents like phenyl can kill the bacteria in the tank.

As a result, do not use flush chemicals down the toilet. In addition, non-biodegradable waste material like condoms and sanitary pads cannot decompose when inside the tank. As a result, do not throw them into the toilet because they may lead to blocking or fill the tank sooner.

Septic Tank Maintenance

Similar to any other system, a septic tank may become faulty and requires maintenance.

Septic tanks should be pumped out every five years to keep the disposal system working properly. However, this may need to be done more often, for example, if they overflow or become blocked. The septic tank effluent should be well and carefully disposed of away so as to prevent any nuisance or danger to public health. This is because it has a foul smell and contains a substantial portion of dissolved organic matter and pathogenic organisms.

While making waste disposal,  an exhauster truck fetches the wastewater from the septic tank using hydraulic power. The track then carries it to land set aside for disposing and treating of the wastewater.

The top cover of a septic tank is watertight and very thick. However, a manhole of relative size is created. This allows inspection and emptying of the tanks.

Septic Tank Failure

The main causes for a septic tank failure include:

  • Too much disposal of grease and cooking oils may cause the drains to block. This is because they are difficult to decompose. It can also result in an odour.
  • Non-biodegradable waste will cause the septic tank to fill up and clog. Do not flush these materials down the toilet.
  • Some chemicals can damage the septic tank components. This includes inorganic material like paint and solvents, water softener, pesticides among others.
  • Tree roots protruding may plunge and clog the pipes or even worse rupture the septic tank. The trees tend to grow rapidly as a result of continuous nutrients supply from the septic system.

Other things that can make a septic tank to fail include a high water table, biofilms that develop over time, and covering the system with an impermeable surface


Septic Tank design

A septic tank design has to be well really thought out so that it will never crack and pollute the groundwater. In addition, they should be able to hold the weight of the wastewater. In addition, it is odorless, more importantly; it should be able to stand any kind of earth movement. A septic tank design should be good and accurate. As a result, it can last up to 50 years of course with good use and maintenance.

Problem Signs In Septic Tanks

The septic tank will need to be checked if there are signs that it is not working properly.
Septic Tank Construction

Some signs that a septic tank is not working properly are:

  • The sewage in the toilet or the liquid waste from other fixtures flows away very slowly
  • Liquid waste overflows from the disconnector trap
  • Wet areas are seen at the top of the septic tank
  • There is a strong unpleasant smell near the septic tank
  • The grass around the tank is very green and growing well

Plastic Septic Tank

Advantages & Disadvantages of Plastic Septic Tanks

Plastic septic tanks are septic tanks constructed from polyethylene resins as an alternative to the concrete septic tank.
and are designed for both residential and commercial use.

Polyethylene/ plastic septic tanks are plastic septic tanks designed unaffected by soil chemicals and by chemicals and gases present in sewage. This type of septic tank will obviously not corrode.

Plastic septic tanks are available in the market  either in single or double compartments  and  ready for installation

Plastic Septic Tank

  • Light weight
  • Resist corrosion
  • Easy installation
  • Watertight
  • Has standard dimensions
  • Resistant to cracks
  • Many colors to choose from


Disadvantages  Plastic Septic Tanks

  • Tank will float if groundwater is high
  • Expensive to buy



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