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How Does A Septic Tank Work?

Anaerobic Bacterial Action of a Septic Tank

In many parts where mains sewers are not available sewage is disposed off by sending it to a two chamber septic tank. In the tank the waste is broken down by naturally occurring bacteria. Semi purified water drains from the septic tank into a leach field where the rest of the impurities get broken down by microbes and exposure to the elements.

Here we look at how the bacterial process works, the design of a septic tank and we answer questions and provide tips on how to get your septic tank working correctly and the things that may stop your septic tank working correctly.

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The design Of a 2 chamber septic tank

How Does A Septic Tank Work?

Septic tanks are small scale sewage treatment systems. They are simple and, when operating correctly, very effective. By the way sewage is, in a household setting, the output from your toilet and sewerage is the infrastructure that carries it. A sewer is a pipe that carries sewage. Note that in American English sewage and sewerage are used interchangeably.

Black Water

Septic tanks are designed to process black water as opposed to grey water. Black water, foul water or sewage is, for domestic purposes, water containing human waste particularly fecal matter and/or urine.

Grey Water

Greywater is water discharged from washing processes and, as we shall note later, should be kept separate from sewerage systems. Because it doesn't contain bacterial contamination it can be treated in a different way.

The Bacterial Process in a Septic Tank

The First Stage - An Anaerobic Bacterial Process

Let us start by looking at the process that occurs in a septic tank, this will help us to understand what is important about the tank's design and how we can make sure a septic tank operates correctly and effectively.

Raw sewage enters the first chamber of the septic tank and solid particles fall to the bottom of the tank where bacteria starts to consume the solid waste.

The bacteria that make your septic tank work are naturally occurring bacteria which establish themselves in the tank, they reproduce at a rapid rate. They do not need oxygen and, as they act on the sewage, they convert it into liquid and gas.

Most of the solid waste particles are broken down by the bacteria, leaving a small percentage that remain at the bottom of the tank as a sludge. The gas produced rises to the top of the water leaving a thick crusty scum that collects on the surface. This first stage of processing normally takes 3 or 4 days.

This is an anaerobic process which means that no oxygen is required (as opposed to an aerobic process which involves nubile young bodies dressed in leotards frolicking around to loud music, panting, grunting and gulping in masses of oxygen).

By this stage we should expect the "brew" to be 60 to 70% processed.

The Second Stage - Second level bacterial action

The semi processed liquid now moves on into a second chamber of the tank where a second round of sediment settlement and bacterial action occurs and particles are broken down further. Again sludge settles in the bottom of the second chamber.

The final stage - killing the good bacteria

From the second chamber the now, almost fully processed liquid leaves the tank through a discharge pipe. In many countries the law decrees that this discharge is sent to a final process which allows the bacteria to finish its work and die off.

The Drainage or Leach Field

Normally this is a drain field, otherwise known as a seepage or leach field. The discharge is fed into a network of pipes buried close to the surface of the ground. The pipes are perforated with many seepage holes which allow the discharge to soak away into the soil. Here an aerobic process takes place where UV light and naturally occurring bugs (that do need oxygen) break down the final traces of contamination and kill off the septic tank bacteria in what is now fairly pure water. This water can now safely percolate through the ground to the water table below. Both septic tanks and drainage fields need to be correctly designed if they are to operate correctly.

There are other ways of dealing with septic tank discharge water, it may be fed to biofilter systems; wetland areas where plantlife, snails or other life forms clean the water; or other systems that may sterilise the discharge water perhaps by using chemical sterilisation such as chlorine (this is not the same as swimming pool chlorine) or ultra violet (UV) light.

The Absence of an effective final stage causes problems.

In many parts of the world, particularly cities in underdeveloped countries that do not have the luxury of piped sewage systems, septic tanks are used in crowded urban areas where there simply aren't the large areas of land available that are required for effective drainage or leach fields. Without other options available septic tanks are simply allowed to drain into the surrounding ground. This is the case in many towns and cities throughout Indonesia and many other parts of South East Asia.

This is why ground water is often contaminated and, because in such places water supply pipes may often be cracked or leaking thereby allowing contaminants to enter the water supply, why tap water is a common source of illness.

Piped sewerage systems are, therefore, an important initiative to improve public health.

The importance of healthy bacteria in your septic tank

A septic tanks works best when the bacteria is healthy and the most effective bacteria to break down human excrement is the bacteria that naturally occurs in our bowels. Just like our bowels a septic tank will not work correctly if the bacteria is out of balance, it may start to smell and it may fill up quickly because the waste is not fully breaking down.

Do not put disinfectants in your septic tank.

Disinfectants can kill the bacteria and stop your septic tank working.

Do not use additives in your septic tank.

In some parts of the world additives are being sold to "enhance" the performance of septic tanks. Septic tanks do not need additives, they work with naturally occurring bacteria that reproduces rapidly to deal with the waste in the tank. Whatever additives sales people might say scientific evidence tell us that additives can, in fact, interfere with the natural process and should not be used.

Additives may contain bacteria of different strains that may put your septic tank out of balance. Remember that every time we go to the toilet a good healthy dose of bacteria is added to your septic tank we do not need to add anything else.

Do not put yeast in your septic tank

Yeasts are not bacteria and can compete with and actually inhibit the action of healthy bacteria. There is a condition known as candidiasis in which a bowel that has lost its healthy bacterial balance (often from taking antibiotics) is taken over by yeasts (candida albicans). It can take many years for people with this condition to get back to a healthy balance in their bowel. It follows that it is unwise to put yeats in your septic tank.

Why does a septic tank have two chambers?

As we can imagine raw sewage entering the tank can mix with the already partially processed water in the tank. The reason why a septic tank has 2 chambers is to create a two stage process. Raw sewage entering the first chamber will displace the partially processed contents which are pushed into the second chamber thereby separating out the raw sewage from the partially purified water.

Understanding this it becomes obvious that we need to try and avoid turbulence as raw sewage enters the tank and we also need to allow partially processed water to move gently from the first tank to the second.

Maximising the efficiency of a septic tank?

There are a number of things we can do to have our septic tank operating as effectively as possible and maximise the level of purification it can achieve.

1. The first thing we can do is to make sure our septic tank is working as efficiently as possible. It must be designed properly and must not leak. It should be built strongly enough so that it will not crack with earth movement.

2. We can ensure that the design minimises turbulence both as the sewage enters the tank and as it transfers from one chamber to the other. This is done by putting tee pieces which act as baffles on the inlet, transfer (between the first and second chambers) and outlet pipes.

3. We can keep the liquid in the tank long enough to be fully processed:
a. Make sure the septic tank is large enough.
b. Only send black water (sewage) to the septic tank,
c. Do not allow groundwater or rainwater to get into the tank.
d. Pump out the sludge from time to time to maintain the tank capacity.

4. Another very effective measure we can take is to add a third chamber, effectively a third level of purification, to the septic tank.

5. Do not put disinfectants down your toilet. The bacteria might do us a nasty if we ingest them but they are, after all, our friends. We can keep them healthy by making sure that bacterial disinfectants are NOT put into the toilet or septic tank. Grey water containing antibacterial soaps, disinfectants, caustic floor cleaner, bleach, water softener and the like can kill our precious bacteria and actually inhibit the effectiveness of our septic tank.

6. Do not put anything that the bacteria cannot break down (such a sanitary napkins) down your toilet these may clog your pipes and fill up your septic tank.

An efficiently operating septic tank can be expected to fully break down the sewage that enters it and, when a septic tank is working correctly, there should be no smell.

With that we can close the lid (thank goodness) and forget about the thing for another 5 years.

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Copyright © Phil Wilson October 2014
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