There are many different septic tank designs and although they may look different or be different shapes and sizes they all work basically the same way. Below you will see a picture of the basic internal design of a septic tank.
2 Compartment Septic Tank
The effluent comes in through the pipe on the left and is deposited into the main chamber of the septic tank. If the solids are denser than water they will fall straight to the bottom of the tank and the less dense solids and the greases will float at the surface. The solids that sink to the bottom right away are digested by bacteria and the same goes for the solids that float on top. Bacteria can digest most of the organic matter in human effluent but they cannot digest all of it. The materials that they cannot digest settle to the bottom of the septic tank and we call this material sludge. It is the sludge that is pumped during routine septic tank maintenance. Grease and other insoluble materials will stay afloat on the surface of the tank. The water in the tank is not pure water, it is called gray water because it still contains organic materials that need to be filtered out. As more water enters the effluent pipe coming from your house the water level inside the septic tank rises and gray water will exit through the sewage pipe on the right and head towards the drainage field.
There are generally three access ports at the top of the septic tank. The access port in the middle is what a septic pumping service would open when they are going to pump out the septic tank. The other two ports are located directly above the incoming pipe and the outgoing pipe. They provide easy access in case one of the pipes is blocked or clogged. Your septic tank may also have two different compartments. It is designed this way so that the sludge remains predominantly on one side of the tank so that it does not make its way into the sewage pipe that goes the drainage field. If the sludge builds up high enough so that it does enter the drainage tube and goes the drainage field, you are going to have some major septic tank problems in the near future.
Below you will see a schematic of a septic tank and a drainage field.
Septic System & Drain Field
As you can see in the diagram, the tube that leaves the septic tank goes to a pipe system which is called the drainage field. The pipe system is perforated to allow the gray water to exit and it exits into a specifically created soil bed of crushed rock and other ingredients. If the sludge from the septic tank makes its way into the drainage field it can clog these perforated tubes and end up causing major damage. This can lead to septic system failure which is something that you definitely want to avoid.
In the diagram below you will see what happens to the water when it leaves the drainage field.
Septic Tank Greywater Filter
The gray water that leaves the perforated tubing in the drainage field either goes up to the surface in small amounts and is evaporated or filters its way through the ground. As the water makes its way through the ground it becomes more and more clean and by the time it reaches the groundwater it is once again drinkable. You will notice in the diagram that there is a well and that is where the filtered water will end up. Anyone who lives on a well should have their water tested regularly for bacteria because if there is bacteria in the water that means your septic system is failing. If you discover that your septic system is failing you need to address the problem immediately.
There are other septic tank designs, most notably an aerator septic tank. The design of the aerator septic tank is basically the same as a traditional septic tank. The major difference is that there is a mechanism that introduces air into the gray water inside the septic tank. This allows for aerobic bacteria to digest the human effluent rather than anaerobic bacteria that are found in traditional septic tanks. The reason this is important is because aerobic bacteria work a lot faster than anaerobic bacteria. See the diagram below for an example of a basic aerobic septic system design.
Aerobic Septic System
Essentially the septic tank is just a holding area for human affluence while it is being degraded. So it could even be in the shape of an airplane, as long as it does what it is supposed to do; the shape and actual design of the septic tank does not really matter. The design of the standard septic tank is usually a concrete box because that is the easiest to build and cheapest to manufacture and it works. Considering the job it is supposed to do and the fact that it is buried in the ground the actual look of the septic tank is of no consequence.