When sewage leaves your house and into your septic system the first component it reaches is the septic tank. All the greywater and waste fills up the tank before it flows out into your absorption area. The septic tank is typically the most obvious structure of your septic system, yet many wonder how it works.
How A Septic Tank Works
When you first open up a septic tank you will notice that the tank is full and close to the top with sewage. The first thought is typically that the tank is ready for pumping. However, this is the level that a tank normally operates. As the tank fills up it spills over into the drain field.
You may wonder why the sewage and other waste doesn’t just empty directly into the drain field. Why would there be tanks to fill up when it just spills over into the field anyway? The septic tank is there for one main function: to allow waste to settle after the bacteria has broken it down. This may seem trivial but it is a pivotal action for a functioning system. If you could slice a septic tank in half you could distinguish three different layers.
- The top layer is the floating solids (or scum). This contains oils, fats, greases, and anything else that did not break down from the anaerobic bacteria.
- At the very bottom is sludge. Both the floating solids and the sludge are the key reasons for pumping out your septic tank regularly. If those solids and semi-solid sludge go out into your drain field the lifespan will greatly decrease.
- Between the sludge and floating solids is the clarified effluent. This is the only waste that can go out into the field, and it should comprise the bulk of your waste in the tank.
The bacteria in the tank is comprised of anaerobic bacteria. Anaerobic bacteria is any bacteria that can grow without oxygen. This is why you can close up a septic tank lid and is still able to break down the waste. The waste can pour into the tank, hold the waste, and exit because of the design of the tank.
A tank is designed with the inlet pipe about 3 to 4 inches above the outlet pipe. This allows the sewage to enter the tank without backing up into the house. The question now arises how the clarified effluent exits the tank without the floating solids coming with it, and this is where baffles come in.
Baffles, although a simple design, play a very important part in the longevity of your septic system. There are two baffles in your septic tank. One is an inlet baffle and the other is an outlet baffle.
The inlet baffle’s job is to direct any waste down into the tank without the waste churning up the solids in the tank. This allows for the tank to settle and the different layers to develop. As the tank fills up and the waste begins to settle the effluent reaches the outlet baffle.
The outlet baffle usually looks exactly like the inlet but has a slightly different use. As the waste in the tank rises it goes up the outlet baffle. The floating solids will be blocked by the baffle exterior while the effluent spills out into the drain field.
Inlet Observation Port
The first part of your tank you may see is the inlet observation port. This is typically a 4 inch pipe with a white cap on top. If you haven’t found it yet your lawn mower will. Although they can be frustrating when mowing they are useful for several reasons.
- They mark where the tank is located. This saves a lot of time when pumping out your tank or inspecting the system.
- If someone is inspecting your sewer line and cannot access it from the home, they can also use the inlet observation port and go back through to the house.
- The inlet observation port is also very useful to check for any unnecessary trickles into your septic system. Make sure it’s been at least 20 minutes since something has drained into the septic system and then check to see if there are any slow trickles coming in. This is something you can do about twice a year to make sure everything is working correctly.
Septic Tank Lid
Just beyond your inlet observation port you will have your septic tank lid. This covers your central manhole of your tank. This is where all pumping should be done. It is a large opening between 18” to 24” and sometimes larger.
Many people don’t like the look of a septic tank lid on their lawn. But, if it is visible, it can save your pumper a lot of time and potentially some money for you. Some pumping companies will charge an extra fee if they have to dig too deep to find the lid. During a home sale, if the lid is too low an inspector will require the lid to be brought up closer to the top of the soil. It’s best to keep track of where your lids are located. This is useful for maintenance and so you can disclose their location if you sell your home.
Knowing where your lids are located is a good way to look for any damage. Concrete lids can crack if they are not placed back gently. Sometimes a piece of the lid will chip off and will create a gap for water and dirt to flow into the tank. This can add extra strain to your drain field over time.
Septic Tank Pumping
Another common question we receive is “how often should I pump my tank?” This has a very simple answer: minimally it should be every two years. There is, however, some important things to look out for when someone comes to pump your septic tank.
Remember, we said that the idea of pumping is to remove the floating solids on top and the sludge on the bottom. To do this a pumper needs to access the manhole in the center of the tank. The manhole gives the pumper enough room to swing his pump hose to grab any floating solids. The central manhole can also help them to see much more of the tank and know if a good portion of the solids are removed.
Make sure that your pumper is not feeding his pump hose through your inlet observation port. This can break your inlet baffle off and the pumper cannot see the amount of solids left in the tank.
If by the end of the pumping many solids are left over. A good pumper will back flush some of what he pumped out to mix up the solids on the bottom and then vacuum up what is left. There will always be a little bit of solids left over, a septic tank is considered clean when there is 1 inch or less of liquid left in the tank.
Once everything is pumped out they can shine a flashlight into the tank and check for any cracks, roots, or deterioration below the previous liquid level.
Different Types of Septic Tanks
There are an array of septic tanks that can be offered. Therefore, it is important to ask the question “What kind of septic tank do I have?” Here are a few that we see the most often.
Primary and Secondary Tanks
In 1997, Pennsylvania required a secondary settling tank to all newly installed systems. This means that if you replaced your drainfield and applied for a permit, you would also need to add a second tank. The reasoning behind this was that the first tank was settling all the solids but there was still some spilling out into the drain field. This second tank provided an extra treatment area that would settle the solids and break down more of the waste. Any solids you can see in the secondary tank would be going out into the drain field if that tank didn’t exist.
The secondary tank is typically located right after the primary tank. Most of the time it’s smaller and has a capacity of about one-half of the primary tank. If you have a completely new septic system installed after 1997 many times your installer used a cheaper method than two tanks. This is called a dual-chamber tank.
Dual-chamber Septic Tank
A dual-chamber tank uses chambers instead of separate tanks. This is one large rectangular tank with a wall in the middle. A common size for a dual-chamber tank is a 1,250-gallon tank. This holds 750 gallons in the first chamber and 500 in the next.
The main benefit to a dual-chamber septic tank is the reduction in price to install. There is only one hole to dig and one tank to place in. These are the go-to when a new system is installed. Unless the primary tank is in great condition, an installer will recommend a dual-chamber tank.
If you have a dual-chamber tank make sure you have both chambers pumped out. They should both be readily accessible for pumping. Sometimes the second chamber lid is buried deeper than the first chamber. This can make the pumper think that there is only one chamber to pump out. Being aware that you have a dual-chamber tank and that there are two lids can help you maintain your system.
Many people use the terms “holding tank” and “septic tank” interchangeably. There is, however, major differences between the two. A holding tank is much larger and has a typical volume of 2000 gallons. The tank has no outlet and it “holds” in all the sewage that enters it. Every bit of wastewater that leaves the house stays in the holding tank. A holding tank has a float switch near the top. When the sewage gets close to the top of the tank, the switch will flip and an alarm will go off. This alerts the homeowner to call a pumper to come out and pump the holding tank. As you can see, this means a holding tank can have drastically more expensive maintenance than a typical septic system. Quite frequently, if you own a holding tank you will have to pump on a monthly basis. Every ounce of water usage needs to be pumped and hauled.
Why would someone use a holding tank? There can be several reasons. Sometimes, the home has no place to put a septic system and no hookup for public sewage. The holding tank is the only option that a home has. A more likely option is that the home doesn’t receive much use. If your home is a vacation home that you only use for part of the year, then an entire septic system might not be worth the money.
If you have a cesspool you may not have a septic tank. This is because a cesspool will act as both a septic tank and an absorption area. They are no longer installed and are an older type of septic system. Cesspools are made by digging a deep pit. An installer built it into a large cylindrical structure with cinder block lining the sides and open dirt on the bottom. The cinder blocks are laid on top of one another without any mortar in between. The lack of mortar allows the sewage to seep between the cracks into the surrounding soil. When the soil on the bottom can no longer drain the cesspool starts to fill up. Over time all of the surrounding soil can no longer hold the sewage and the cesspool fills up to the top. At this point the cesspool is no longer usable.
All the solids that would settle in a tank and eventually be pumped out, instead are kept in your absorption area. The solids will eventually fill up the soil and keep it from draining. You should treat a cesspool as if it were a septic tank and have regular pumping maintenance on it every two years.
In all the tanks we have mentioned so far they all contain anaerobic bacteria to break down the waste before it enters the drain field. The aerobic tank uses aerobic bacteria for treatment of sewage. As we know already anaerobic bacteria thrives in a lack of oxygen. Aerobic tanks promote airflow so that bacteria that uses oxygen (aerobic bacteria) can grow.
The tank has two extra components that allows the aerobic bacteria to grow: a mechanism for producing air supply and propagation media (usually a honeycombed structure). The honeycombed sides create a texture for the aerobic bacteria to grow. The air supply is what adds the oxygen into the tank.
The main benefit of aerobic tanks is that it will help the soils in the drain field to last much longer. The anaerobic bacteria in typical systems adds sludge and can pull oxygen out of the soil which affects its ability to drain.
Septic Tank Problems
Septic tanks are built to be sturdy. They tank manufacturer pours them to be about 3 inches thick. They are meant to last 25 years; which is a long time, but not forever. Eventually the tank starts to show signs of deterioration. This can come in many forms but here are the most common.
As the bacteria starts to break down the sewage in the tank they release gases above the liquid level. The bacteria above the liquid level turns those gases into sulfuric acid. Over time the sulfuric acid builds up to a point where the concrete will break down. Because of this reaction on the top part of the tank; a crucial part of inspecting tanks involves looking above the liquid level to see any structural damage.
This reaction can not only ruin the tank itself, but be the initial cause for the following septic tank problems.
Rebar can become exposed by the concrete eventually deteriorating and showing the rebar. This is a major red flag for septic inspectors. An inspector will deem a tank unsatisfactory if they observe exposed or rusted rebar. Once you see rebar you can deduce that the concrete in the tank has become mushy and is crumbling.
Many tanks have concrete baffles that jut out into the tank. Since baffles have more surface area exposed to the chemical reactions caused by bacteria; they are usually the first component inside the tank to break down.
Once the baffles go you lose their important functioning. The outlet baffle is the most important one. If there is no outlet baffle then nothing is holding back the solids from floating out into the drain field.
Cracks in the Tank
When an installer places a tank in the ground there can be a few feet of soil cover on top. That soil adds a significant amount of weight to the top of the tank. Over time this weight, coupled with the chemical reaction in the tank weakening the structure, will create cracks in the tank. They usually start on the top of the tank. The deeper the tank is buried the higher the chance that a crack will occur.
Many times people will plant trees and large bushes right up next to their septic tanks. They may have planted these to help spruce up their landscape, but have unwittingly risked the structure of their tanks.
The tree roots will start to break through the concrete tanks and cause structural deficiencies. This may seem odd that a tree has the power to go through thick concrete. But, over time the thin roots work their way into tank walls. Once they are through they will start to thicken over time. The expansion of the roots will cause cracking and ultimately a crumbling tank.
This is something a pumper should keep their eye on during regular maintenance pumping. When the roots are still thin you can hire someone to cut them out and pull them away from the tank. As a homeowner it is important to keep trees at least 15 feet away from any component of the septic system.
By now you should have a good base of knowledge for understanding septic tanks. From here you can grasp a little more when an inspector or pumper talks about your tank. This will assist you in maintaining your system and buying/selling a new home.
Be sure to come back to learn more about septic tanks and septic systems!