What Does It Mean When A Septic Tank Is “Full”?

When observing their septic tank, some homeowners mistakenly think something is wrong when they look into the tank and see that it’s “full”. They see sewage near the top and think that there must a problem and something must be wrong. In fact, it is usually fine in cases like this, and homeowners are simply uneducated on how a septic system works. They misunderstand what the term “full” actually means when talking about a septic tank. That is why it is important to understand how a septic system works and understand exactly what it means to say a tank is “full”. An uneducated homeowner may look into a perfectly fine septic tank and think there’s a problem simply because they see wastewater floating near the top.

What is normal operating level?

The level of the wastewater inside a functioning septic tank is called its normal operating level. Homeowners often see their tanks at this normal operating level and immediately think something must be wrong. These people may be uneducated on how a functioning septic tank works; they don’t know that wastewater is supposed to be at this level. A septic tank at this normal operating level is not considered full. So, how what does it actually mean for a septic tank to be full and how do you know when it is? To start, let’s understand a little more about the normal operating level.

Where is the normal operating level?

For most homes, septic tanks are generally 1000 gallons to 1500 gallons in capacity. The exact capacity depends on the size of your house and a few other factors. The capacity of a septic tank refers to how much water it holds at normal operating level. This level is typically 8-12 inches below the top of the tank; right where the wastewater level reaches the bottom of the outlet pipe in the side of the tank. When wastewater reaches this point, it will then flow out of the septic tank and into the soil absorption system (aka drainfield, aka drywell). The wastewater level in a functioning septic tank always remain at normal operating level. As wastewater flows into the septic tank from the house, this causes effluent to flow out the outlet pipe on the other side of the tank. This keeps wastewater at a uniform level inside the tank at all times.

Note: If a septic tank is above or below normal operating level, that means there may be a problem. This is when it should become a concern and you should contact a septic company promptly. A low level could also that the tank has been recently pumped. If this is the case, most septic tanks fill back up to normal operating level within 4-10 days after pumping. This varies based on water usage inside the house.

So now that we know what the normal operating level is and where to find it, let’s talk about what it actually means for a septic tank to be full.

What causes a septic tank to become full?

It is actually the level of solids in your septic tank that determines when it is full. The main purpose of a septic tank is to separate the solids (solid waste, toilet paper, kitchen sink food scraps) and the liquids (liquid waste, toilet/sink/shower/ laundry water) in the wastewater. The solids settle out of the liquids and either float or sink, depending on their density. This process is called pretreatment. After solids settle out, they become trapped between septic baffles inside the tank and accumulate over time. Septic baffles help solids accumulate by using a wall to trap them, while still allowing to pretreated liquid, called effluent, to flow around it and exit the tank. As untreated wastewater flows into the tank via the inlet, pretreated effluent exits the tank through the outlet. Over time, more and more solids settle out of the wastewater and build up inside the tank. Eventually, the septic tank fills up and cannot store any more solids. This is the point at which a septic tank is considered full. A septic tank should always be pumped before it reaches this point, because damage will occur to the system if it is not!

What happens after a septic tank becomes full?

Most of the times a septic tank fills up, you will never even know it. Most people think their septic will back-up or overflow when it is full, but this is actually false. When a septic tank fills up to capacity with solids, they usually begin to push themselves around the outlet baffle and escape the septic tank through the outlet pipe. So, even though the tank is at full capacity, you won’t notice anything wrong because old solids just push themselves out of the way to make room for new ones. This is one of the worst things that can happen to a septic system and can fail a soil absorption system very fast. It can take years of the lifespan of a system. All of this could be happening, and you won’t even know it. So, how can you tell when your septic tank is full? How do you know it needs pumped?

How can you tell when a septic tank is full?

Unfortunately, there is no easy way to tell. Looking into your septic tank through a cleanout or a manhole riser will not give you this information. Often, by time it is backing-up or overflowing, it is too late and the system is already ruined. At this point, pumping the septic tank will not fix the problem, as your soil absorption system has already clogged and failed. This is why nearly every local government and septic expert have recommended pumping frequencies. Your safest bet is to get your septic tank pumped on a regular basis to remove the solids before they fill up. Depending on household sizes and water usage, pumping is recommended every 1 to 2 years. If you are unsure about a recommended frequency, give your septic company a call and they can help you. Since pumping ($$) costs significantly less than needing a whole new soil absorption system ($$$$$), you are actually saving money by pumping and cleaning your septic on a regular basis because it extends the lifespan of the system.