Concrete Septic Tank Detailed Guide

What is a Concrete Septic Tank?

A concrete septic tank is a large container made of concrete that is buried underground and used to collect and treat wastewater from a home or building that is not connected to a municipal sewer system. The septic tank is the first component of an onsite septic system.

The main purpose of a concrete septic tank is to separate solids from liquids in the wastewater and provide initial treatment through anaerobic digestion before the effluent moves on to additional treatment and dispersal components. Wastewater from the home flows into the inlet of the septic tank where heavy solids settle to the bottom to form a sludge layer while oils, grease, and lighter solids float to the top to create a scum layer. The clarified liquid in the middle, known as effluent, flows out of the outlet baffle and on to the drain field or other dispersal component.

Inside the septic tank, the sludge and scum undergo anaerobic digestion by microorganisms which helps reduce solids volume. Periodic pumping of the accumulated sludge and scum is required to maintain proper operation and prevent solids from being carried into the drain field which could clog the system. The concrete septic tank provides an enclosed space for this important primary treatment process to occur.

Benefits of Concrete Septic Tanks

Concrete septic tanks offer several advantages over other materials:

Durability and Longevity

Concrete is an extremely durable material that can last for decades with proper maintenance and care. The average lifespan of a concrete septic tank is around 40-50 years, far exceeding that of plastic tanks. Concrete’s high compressive strength allows it to withstand soil pressures and shifting without cracking or collapsing. This gives concrete septic tanks superior longevity compared to materials like fiberglass or plastic.

Resistance to Corrosion and Rust

Concrete is non-corrosive and provides excellent protection against rust when steel rebar reinforcements are used. Plastic and metal tanks can eventually corrode, crack, and leak over time when exposed to caustic waste materials. The alkaline environment inside concrete tanks helps prevent corrosion of the steel rebar. This resistance to corrosion and rust contributes to the long service life of concrete septic tanks.

Strength and Structural Integrity

Properly designed and installed concrete tanks have tremendous strength and structural integrity. Concrete’s high compressive strength allows it to withstand the soil and traffic loads without fail. The monolithic design and steel reinforcement provides superb protection against cracking and fracturing. Concrete tanks are less prone to buckling and deforming under pressure compared to plastic counterparts. This robust structural integrity ensures the septic tank remains intact and functional for decades. The strength and stability of concrete septic tanks make them a wise long-term investment.

Drawbacks of Concrete Septic Tanks

Concrete septic tanks have some potential downsides to consider:

Heavier Weight Than Other Materials – Concrete is extremely heavy and dense. This makes concrete septic tanks more difficult to transport and install compared to plastic tanks. The heavy concrete limits placement options, as a reinforced base is required to support the weight.
Potential for Cracks and Leaks Over Time – While concrete is sturdy, it can develop cracks and leaks over decades of use. Concrete naturally expands and contracts with temperature changes. Ground settling or tree roots over time can also cause the concrete to crack. Any cracks or defects can lead to leaks and seepage.
Difficult to Repair or Replace – If a concrete septic tank develops cracks or other defects, repairs can be complicated. The heavy weight makes replacing the entire concrete tank labor-intensive. Attempted repairs like sealing cracks may provide temporary solutions, but issues can reoccur. Overall, repairs are often costly and short-lived compared to replacing plastic tanks.

Concrete Septic Tank Design and Components

Concrete septic tanks are large containers made of concrete that are buried underground to collect and treat wastewater from homes that are not connected to a sewer system. The key components of a concrete septic tank include:

Inlet and Outlet Pipes

Concrete septic tanks have an inlet pipe that brings wastewater into the tank from the home and an outlet pipe that directs treated water out of the tank. The inlet pipe is typically 4 inches in diameter, while the outlet pipe is smaller, around 3 inches. This allows solids to settle out in the tank. The inlet and outlet pipes are PVC.

Access Ports and Lids

Concrete septic tanks have access ports with lids at the top to allow inspection and pumping of the tank. There are usually two access ports – one over each compartment. The lids are made of concrete or plastic and sit a few inches above ground level. Access ports allow vacuum trucks to pump out sludge that accumulates in the tank.

Baffles and Filters

Inside the tank are baffles, which are walls that separate the tank into two or more compartments. Baffles prevent solids in the wastewater from exiting the tank too quickly and allow the sludge and scum to separate. Many tanks also have an outlet filter to catch solids and prevent them from entering the drainfield.

Size and Capacity

Concrete septic tanks range in size from 900 to 1500 gallons or more. The required capacity depends on the number of bedrooms in the home and local regulations. Tanks need to hold at least 24 hours worth of wastewater flow to allow solids to properly separate. Most are dual compartment, with 2/3 of the capacity in the first compartment.

Installation Process

Installing a concrete septic tank requires careful planning and execution to ensure it functions properly. Here are the key steps:

Excavating the Hole

The first step is to dig a hole for the septic tank. The hole should be dug wide enough to allow at least 12 inches of space on all sides of the tank. The depth depends on the tank size but often ranges from 4-6 feet deep. Sandy or loose soil may require a deeper hole to prevent the tank from shifting. Make sure to pile the excavated soil a safe distance from the hole’s edge.

Leveling and Bedding the Tank

It’s crucial to properly level and bed the septic tank or it may crack under pressure. The bottom of the hole should be lined with a 4-6 inch layer of sand, gravel, or crushed stone. This bedding material should be leveled and compacted. A layer of sand should be smoothed on top, providing a cushioned, level base for the tank to rest on.

Lowering the Tank into Place

When the bedding layer is prepped, the tank can be carefully lowered into the hole by crane or using straps and ramps. The tank should be oriented with the inlet and outlet pipes facing the correct direction. It must be checked for level on all sides. The bedding material may need to be added or removed beneath the tank to achieve level.

Connecting Pipes and Backfilling

Once settled into position, the inlet and outlet pipes can be attached using watertight seals. The tank may have a riser installed over the access holes. Backfill material (like gravel or sand) is shoveled around the tank, taking care not to shift its position. The backfill is mounded over the top and compacted in layers. Finally, topsoil is placed over the mound and landscape grading performed to complete installation.

Maintenance Tips

Regular maintenance is crucial for keeping a concrete septic tank functioning properly. Here are some key maintenance tips:

Inspect and pump regularly – Septic tanks should be inspected annually and pumped every 3-5 years by a professional. Regular pumping prevents solids from building up and clogging the inlet or outlet.
Check for leaks – Inspect the tank, pipes, and soil around the septic system periodically for any signs of leaks or water damage. Catching leaks early can prevent more expensive repairs.
Avoid roots and debris – Tree roots or debris in the pipes can cause major blockages. Don’t plant trees or shrubs too close to the septic system. Use sink strainers and filters to keep debris from going down the drains.
Protect against freezing – Concrete septic tanks are susceptible to cracking and damage from frozen ground in colder climates. Insulate inlet and outlet pipes and consider an insulating septic tank cover. Avoid driving or compacting snow over the tank or pipes.

Regular maintenance like pumping, inspections, and leak detection can help identify issues before they become big problems. Protecting the tank and pipes from tree roots, debris, and freezing can also prevent costly repairs.

Troubleshooting Common Issues

Some of the most common issues that can arise with concrete septic tanks include:

Slow Drainage

If you notice your drains are draining slower than usual, it could indicate a problem with your concrete septic tank. Causes of slow drainage include:

Buildup of solids in the tank – Over time, solid waste accumulates in the tank. If not pumped regularly, it can clog pipes and cause slow drainage.
Clogged inlet/outlet – Objects getting into the inlet or outlet pipes can cause clogs. Tree roots or buildup of fats/grease are common culprits.
Blocked soil absorption field – If the soil field gets clogged, it can prevent wastewater from draining out of the tank properly.
Improper venting – Gases need to vent out of the tank. If the vent pipe is clogged, it can slow drainage.
Non-standard baffles – Baffles separate the compartments in the tank. If missing or faulty, solids can escape and cause drainfield issues.

To troubleshoot, have the tank pumped and inspected. A professional can identify any issues and recommend repairs. Flushing pipes with a sewer jetter may also help clear blockages.

Sewage Surfacing

If you notice sewage pooling in your yard or backing up into the house, it means wastewater isn’t able to exit the tank. Common causes include:

Clogged pipes – Tree roots, buildup, or collapsed pipes prevent wastewater from leaving the tank.
Soil absorption issues – If the soil field is overloaded or compacted, it won’t allow wastewater to disperse.
High water table – During wet seasons, a high groundwater level can prevent proper drainage.
Missing/damaged baffles – Baffles contain solids in the tank. If damaged, solids can clog the soil absorption field.

To fix, have a professional inspect the tank and drainage field. Pipes may need to be cleared or the soil absorption field expanded/replaced.

Odors

Unpleasant sewage odors coming from your septic system are often a sign of problems. Potential causes include:

Improper venting – Gases need to escape through the vent pipe. If blocked, pressure builds up and can push gases out into the yard.
Cracked tank – Cracks allow gases and wastewater to leak out. They indicate structural damage requiring tank replacement.
Blocked/damaged baffles – Baffles contain solids and gases. If faulty, gases can escape into the drainage field and yard.
Overloaded tank – Too much wastewater overworks bacteria, preventing proper breakdown of solids. This allows gases like hydrogen sulfide to build up.

To eliminate odors, identify and repair the underlying issue. Pumping the tank, clearing vent pipes, or replacing damaged components may be necessary.

Backup into the House

If sewage is backing up into sinks, tubs or toilets, the blockage is likely between the house and the septic tank. Causes can include:

Clogged plumbing – Buildup, roots, or collapsed pipes prevent wastewater from exiting the home.
Improper plumbing – Incorrect pipe slopes or connections can cause backup during high usage.
Missing/faulty effluent filter – The filter prevents solids from leaving the tank and clogging pipes. If missing or broken, solids can get into the pipes.
Exceeded tank capacity – Too much wastewater overloads the tank, causing backup into the house.

Fixes usually involve snaking or jetting home plumbing pipes to remove clogs. The effluent filter and baffles should also be inspected and repaired if needed. Expanding the tank may be required for overloaded systems.

Concrete vs. Plastic Septic Tanks

Concrete and plastic are the two most common materials used for septic tanks today. Both have their own sets of pros and cons when it comes to durability, weight, cost, and installation.

Durability

Properly constructed and installed concrete septic tanks are extremely durable. They can easily last 50 years or more before needing replacement. Plastic tanks have improved in durability over the years but still only last 25-30 years on average. The thicker walls and structural strength of concrete gives it a clear edge.

Weight

Concrete tanks are much heavier, with empty weights often exceeding 1,000 pounds. Plastic tanks usually weight a few hundred pounds. The heavy weight makes concrete tanks more stable underground but much harder to transport and install. Plastic’s lighter weight makes delivery and installation easier.

Cost

Concrete tanks are more expensive, usually $2000-$5000 installed. Basic plastic septic tanks run $1000-$2000. However, concrete’s longer lifespan can make it more cost effective in the long run.

Installation

Due to their heavy weight, concrete tanks require excavation equipment and a crane for installation. Plastic tanks can be installed manually with some effort. Concrete also needs time to fully cure before use. The installation process for plastic tanks is faster and doesn’t require special equipment.

In summary, concrete tanks are more durable and stable, but heavy and costly. Plastic tanks are cheaper and easier to install, but have shorter lifespans. Consider your budget, timeline, and longevity needs when choosing between concrete or plastic septic tanks. Proper maintenance and installation are critical for both to function effectively.

Concrete vs Plastic Tanks Difference Infographic

Septic Tank Additives

Septic tank additives contain bacteria, enzymes, and chemicals that are designed to help break down solids and scum in the septic tank. Some key points on septic tank additives:

Bacteria and enzymes – Most additives contain bacteria strains and enzymes that help break down organic matter in the tank. They can help decompose solids faster than anaerobic bacteria alone.
Pros – Additives can help reduce buildup and extend the time between pumpings. Some additives also contain chemicals to help control odors coming from the tank. Using additives may be beneficial when first starting up a new system.
Cons – Additives are not necessary for most properly functioning septic systems. The natural anaerobic bacteria in the tank are usually sufficient for breaking down waste. Additives can be an unnecessary expense and some additives may disrupt the microbial balance.
Recommendations – Most experts recommend against regularly using additives in a septic tank. They are not needed in a healthy system. However, additive use may be warranted if there are signs of slow drainage or buildup. Consult with a septic professional before using additives regularly. Only use septic-safe, natural additives and follow dosage guidelines carefully.

When to Replace a Concrete Septic Tank

Septic tanks are durable, but they don’t last forever. The average lifespan of a concrete septic tank is around 40 years, though well-maintained tanks can last 50+ years. There are a few signs that indicate when it may be time to replace your concrete septic tank:

Backups and clogs: Frequent backups, clogs, and plumbing issues often mean there are problems with the septic tank. Cracked tanks allow soil and roots to enter, clogging inlet and outlet pipes.
Ground sinking or ponding: If you notice the ground sinking or ponding water above the tank and drain field, it likely has cracks or wall failures allowing sewage to leak out.
Foul odors: Strong sewage odors around the tank could mean broken baffles inside the tank or that sludge and scum have built up too high.
Old age: Once a tank reaches 40-50 years old, its concrete walls are more prone to cracking and failure due to age and ground movement over the years.

Replacing an old failing septic tank requires permitting and can cost $3,000-$5,000 on average. The process involves pumping out the old tank, disposing of the contents safely, excavating the tank, installing a new tank, backfilling, compacting soil, reconnecting pipes, and obtaining final inspections and permits. Doing regular septic tank pumping and maintenance can help maximize the lifespan of your system. But once major failures occur, replacement is usually the only option to restore proper function.