What is a Septic System?

Basics of Septic Systems Function

A septic system consists of two basic parts: a septic tank and a soil absorption field or drainfield. Wastes flow from the house into the septic tank where most solids are separated to the bottom and are partially decomposed by bacteria to form sludge. Some solids float and form a scum mat on top of the water.


The liquid effluent from the septic tank, carrying disease-causing organisms and liquid waste products, is discharged into the soil absorption field.


In the absorption field, the water is further purified by filtration and decomposition by microorganisms in the soil. The semi-purified wastewater then percolates to the groundwater system.


Septic Tank Functions

The typical septic tank is a large, buried, rectangular, or cylindrical container made of concrete, fiberglass, or polyethylene. Wastewater from the toilet, bath, kitchen and laundry flow into the tank. Heavy solids settle to the bottom where bacterial action partially decomposes them to digested sludge and gases. Most of the lighter solids, such as fats and grease, rise to the top and form a scum layer.


Septic tanks may have one or two compartments. Two compartment tanks do a better job of settling solids and may be required for new systems. Tees and baffles are provided at the tank's inlet and outlet pipes. The inlet tee slows the incoming wastes and reduces disturbance of the settled sludge. The outlet tee keeps the solids or scum in the tank. All tanks should have accessible covers for checking the condition of the baffles and for pumping both compartments. If risers extend from the tank to or above the ground surface, they should be secure to prevent accidental entry into the tank.


Solids that are not decomposed remain in the septic tank. If not removed by periodic pumping, solids will accumulate until they eventually overflow into the drainfield, leading to costly repairs or replacement. Remember that retention time, or the time available for solids to settle out of wastewater, decreases as the sludge layer increases in your septic tank.



















It's always surprising to learn that most people who are on a septic system do not really know how they are designed or how they operate. Even those people who have been on septic systems their whole lives are not real sure about its function.

First off, we'll start by stating that today's septic system are not like Grandpa's on the farm. Back in the day, the only waste water that went into the system was from the bathroom. Everything else went out on the lawn or into a pit somewhere else. Today systems are designed to treat ALL waste water expelled from the home.

Septic Systems are really your own personal, private sewer systems. They operate much in the same way as a municipal sewer plant with the exception that the only source is you. Wastewater is transported from the house via your sewer pipes out into the septic tank. The tank is generally a two compartment tank. The first compartment is to hold and treat the majority of the solids; the second compartment is another opportunity for settling before the effluent water is transported out to the absorption field via more pipes. In the absorption field so much of the effluent water leaches into the ground where it gets treated yet again and then eventually into the ground water and the rest is evaporated into the air.

Let's visit about the importance of each component of the septic system.

First the pipes from the house to the tank. These pipes need to be clear and properly connected. With the new homes they use plastic PVC pipe which is wonderful. Older homes may have cast iron, orangeburg, or clay tile. All of these for obvious reasons have been abandoned for PVC in newer homes for many years. Cast iron will corrode over time and the opening will get smaller and smaller. You can rout them out with a line machine, but once the corrosion has started it will come back pretty quickly. Orangeburg pipe is rolled-up tar paper and over time becomes brittle and will break and allow roots to get in. Same with clay tile pipe. If you own a home with any of these types of piping, best consider replacement with PVC.

The septic tank itself is an interesting animal. There have been many, many designs over the years, but they always seem to come back to a concrete box. There are some plastic and fiberglass versions out there that function as well as the good old concrete box, but usually the price of the concrete wins out for installation.

The "average" tank today is a 1250 gallon, two compartment tank. Some have 2 lids, some have 3 or more lids. We'll talk about "average" right now and address "exceptions" later.

The way that a tank works is that liquid waste enters the tank on the inlet side and falls either due to a baffle or sanitary tee into the system. A baffle or tee is recommended to keep the incoming rush of water from disturbing the bacteria action that floats at the top of the effluent water. This chamber of the tank holds most of the solids and it is here that the bulk of the processing occurs. All waste water contains certain amounts of bacteria and enzymes. These bacteria and enzymes work to digest the solid waste that enters the tank and get it into as much of a liquid form as it can.

As the wastewater arrives into the tank anything that is heavier than water sinks to the bottom create a "sludge" layer - generally inorganic material, unprocessed solids (such as table scraps), etc.  Anything that is lighter than water floats to the top to create a "scum" layer - generally your fats, oils, and greases - not just cooking grease, but greases from soaps and detergents also.

Once in the tank anaerobic bacteria begin to do their work munching away at the materials deposited trying to digest or liquefy it as best they can.  Anything that the bacteria and enzymes can't liquefy falls to the bottom of the tank and adds to the "sludge" layer. It is also interesting to note that bacteria doesn't live forever and when it dies it actually sinks to the bottom of the tank and adds to the "sludge" layer.

The water that separates the "sludge" and "scum" layers is call "effluent water". It is this treated water that eventually makes it's way to the absorption field.

Before we go any further with how the system works it is important to try to get a visual as to what happens when a system is not pumped out periodically. If you think about it, over time the sludge layer is going to get thicker and thicker as the inorganic (non-digestible) material and the dead bacteria stack up on the bottom of the tank. The scum layer is also going to get thicker and thicker as the bacteria multiply and grow munching away on the organic (digestible) waste that has entered the tank. Eventually, that middle layer that should be fairly clear waters gets smaller and smaller. But if this is allowed to go on too long eventually it gets so thick that it's actual scum and sludge that is floating out into the absorption field. When this happens the pores of the ground get clogged, the field becomes saturated and you have a major system failure on your hands.  In tanks with two compartments the settling process (scum on top/sludge on bottom) occurs in the second compartment as well and if allowed to continue too long will also be allowing scum and sludge to float out into the absorption field causing the same major system failure.

Brief history on tank size and exceptions:

Way back when tanks were home made they came in a variety of sized and materials. In the 40' & 50's they started building the concrete box, single compartment, but it did standardize the tank a bit. In the late 70's they came to adopt the standard size of 1000 gallons and put in a wall and divided the tank for better settling. Most counties in Alabama determine the needed size of the tank by the number of bedrooms (supposed to equate back to average number of people in the home). Today's standard is 3 bedrooms require a minimum tank size of 1000, 4 bedrooms a 1250 and 5 or more would require a 1500 gallon capacity tank. Most builders will install a 1250 tank even if the home is only 3 bedrooms. This gives the owner a little wiggle room for expansion and addition of a bedroom in the basement.  But, the true size of the system is Not the size of the tank, but the size of the soil treatment area, or leach field.

Exceptions occur when there is a specific need for a larger tank, more compartments, more treatment is needed or a larger soil treatment area. This is usually determined by the engineer after soil tests are done and percolation rates are calculated, lot size is evaluated, ground water location, etc. If anything falls into the exception category then the engineer designs a system that will function within the prohibiting conditions. These systems usually require more attention to the maintenance than the standard gravity fed system.

The outlet lines also need to be clear and properly connected. Again, today new homes use PVC pipe. If your home is older, then you may consider replacing any of the old types of pipe, or you may have yourself many headaches.

Now onto the absorption field. With a basic gravity fed septic system, there are primarily two types of fields. A gravel bed, which consists of an area that has been excavated or dug out, gravel laid down, perforated pipes laid on top of the gravel and recovered with soil.The effluent water enters the bed at one end and gravity allows it to flow to the other end seeping out of the holes and into the ground. There is also another type of system that consists of trenches and dome shaped plastic chambers. This system works in much the same way as a gravity gravel bed, but is much easier to install and less costly.

If the terrain, size of lot or percolation rates dictate and absorption field, and possibly the whole system, may have to be designed by an engineer. These would be the exception systems discussed above and could possibly have pumps, sprayers, filtration beds, jets, dousing systems, etc. Again, if any of these components are used, it is even more critical that the home owner maintain the system and have it pumped and inspected regularly.



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Septic System Drainfield

The drainfield receives septic tank effluent. It has a network of perforated pipes laid in gravel-filled trenches, or beds in the soil. Wastewater trickles out of the pipes, through the gravel layer, and into the soil. The size and type of drainfield depends upon estimated daily wastewater flow and soil conditions.


The soil below the drainfield provides the final treatment and disposal of the septic tank effluent. After the effluent has passed into the soil, most of it percolates downward and outward, eventually entering the groundwater. A small percentage is taken up by plants through their roots, or evaporates from the soil.


The soil filters effluent as it passes through the pore spaces. Physical and biological processes treat the effluent before it reaches groundwater, or a restrictive layers, such as hardpan, bedrock or clay soils. These processes work best where the soil is somewhat dry, permeable, and contains plenty of oxygen for several feet below the drainfield.


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