Plastic septic tanks are commonly installed systems for effectively and efficiently treating household wastewater in homes not connected to a public sewer system. Septic tank systems work by allowing solids to settle out and separate from wastewater before the effluent flows out into a drain field. The septic tank provides preliminary treatment by holding wastewater long enough for solids and greases to rise to the top as scum, while heavier solids settle to the bottom as sludge.
Plastic septic tanks have become a popular option for many homeowners due to their durability, resistance to corrosion, lightweight, and relative affordability compared to concrete tanks. Polyethylene plastic is commonly used to manufacture these tanks in a seamless molding process. The material’s smooth interior surface prevents buildup and allows for easy cleaning over time. Plastic tanks have the added advantage of being easier to transport and install than heavy concrete tanks. They are an effective and reliable septic system component for rural and suburban homes.
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What are Plastic Septic Tanks Made Of?
Plastic septic tanks used in households today are usually made from polyethylene plastic. This is the same type of plastic used to make recyclable containers and bottles. Polyethylene is popularly utilized in septic tank manufacturing because it is highly durable, resistant to corrosion, and extremely lightweight compared to materials like concrete or steel.
The polyethylene used to construct septic tanks is a high-density version of the plastic. It provides an ideal balance of flexibility and structural integrity needed for underground septic tank installations. The relative softness prevents cracking while the density gives septic tanks the requisite strength. Polyethylene’s chemical resistance also allows tanks made from it to withstand exposure to the wide variety of substances that get flushed into septic systems.
Benefits of Plastic Septic Tanks
Plastic septic tanks offer several benefits that make them a popular choice for homeowners:
Economical – Plastic septic tanks are very affordable compared to other materials like concrete or metal. This makes them accessible to most homeowners. The low cost makes them a practical option.
Durable and Long-Lasting – These tanks are extremely durable thanks to being constructed out of polyethylene plastic. They are resistant to cracks and leaks, unlike concrete tanks which can develop cracks over time. Their durability means they can last for decades.
Lightweight – Weighing much less than concrete tanks, plastic septic tanks are easier to transport and install. This lightweight characteristic also allows them to be installed in areas where very heavy concrete tanks cannot.
Rustproof – Plastic materials like polyethylene do not rust or corrode. This gives plastic septic tanks an advantage over metal tanks which can eventually rust and fail. Owners do not need to worry about corrosion issues.
Resistant to Chemicals – Polyethylene plastic is unaffected by household chemicals that go into the septic system. Acids and bleach that destroy concrete do not damage these plastic tanks.
Challenges with Plastic Septic Tanks
Plastic septic tanks do come with some challenges that must be considered before installation.
Can Float
One of the main challenges with plastic septic tanks is that they can float out of place if not properly anchored. This is due to their lightweight design and buoyancy. Plastic tanks have a specific gravity of 0.97, meaning they weigh less than the equal volume of water. This makes them prone to floating up, especially when empty or not yet filled with wastewater. Proper anchoring is critical during installation to prevent movement.
Anchoring Required
Since plastic septic tanks can float, they require anchoring or weighting systems to keep them securely in place underground once installed. Anchors are attached to the tank and buried underground to prevent upward movement. Some common anchoring options include:
- Concrete weights placed over the tank
- Metal or concrete anchor straps tied into the ground
- Soil anchors drilled into the ground around the tank
Anchoring is an essential extra step when installing plastic septic tanks. Proper anchoring following local regulations helps ensure the tank remains where it was installed.
Heavy Weight Can Damage
While plastic septic tanks are durable, extremely heavy weight sitting directly on them can potentially cause damage or deformation over time. This is why extra caution is needed when installing plastic tanks under driveways or roadway areas with heavy vehicle loads. Proper installation depth and/or traffic ratings are important to prevent the plastic from cracking under thousands of pounds of weight. Regular inspection is also advised in high-vehicle traffic areas to check for damage.
Installation Considerations
Proper installation of a plastic septic tank is critical to prevent issues like floating and shifting. Because plastic tanks are so lightweight, with a specific gravity of 0.97, they can easily shift or even float up out of the ground if not anchored properly.
To prevent floating, plastic septic tanks require anchoring systems to be installed. Some common anchoring options include:
- Concrete anchors – These involve placing the tank in a concrete anchor ring or cradle dug around the tank. The concrete anchors the tank firmly in place underground.
- Soil anchors – These anchor the tank with steel cables or bars driven into the ground around the tank. The cables are then attached directly to the septic tank or a concrete anchor ring.
- Metal hold-down straps – Straps are bolted over the top of the tank and staked deep into the ground. The straps prevent upward movement.
Proper placement and installation depth are also important to prevent floating. The tank must be placed on undisturbed or compacted soil, and deep enough to allow for at least 12 inches of soil cover over the top of the tank.
Following anchoring and placement guidelines will help ensure the plastic septic tank stays securely in place underground. This prevents costly damage from shifting and floatation.
Maintenance Requirements
Regular maintenance is crucial for any septic system, including plastic septic tanks. Here are some key maintenance requirements:
Pumping – Septic tanks need to be pumped out regularly to remove sludge that builds up at the bottom of the tank. For plastic septic tanks, pumping should generally occur every 3-5 years. This helps prevent sludge from flowing into the drain field.
Inspections – Annual inspections allow you to visually check the septic tank and make sure all components are functioning properly. Look for any signs of cracks, leaks, clogs, or root intrusion. Inspecting access ports and manholes can reveal issues before they become major problems.
Cleaning – Plastic septic tanks are relatively easy to clean. Simply scrub the sides and inside surfaces with a long brush and water. Avoid using harsh cleaners or chemicals. Focus on removing any grease or debris buildup.
Replacing Filters – If your plastic septic tank has an outlet filter or screen, this will need to be cleaned or replaced periodically. Outlet filters help keep solids from entering the drain field. Replace them according to manufacturer instructions, generally every 3-5 years.
Regular maintenance extends the lifespan of plastic septic tanks. Following the manufacturer’s recommendations for pumping frequency, inspections, cleaning, and filter changes will help prevent premature failures or issues.
Cost Comparison
Plastic septic tanks tend to be more affordable than concrete septic tanks. On average, a plastic septic tank costs $1,000-$1,500 installed while a concrete septic tank runs $1,500-$2,500 installed. The price difference comes down to a few factors:
- Materials – Plastic resins are cheaper than concrete and rebar. This makes the raw material costs for plastic tanks lower.
- Manufacturing – Plastic tanks are made through a rotational molding process that is highly efficient. Concrete tanks require more manual labor to construct and cure.
- Weight – Plastic tanks weight 1/4th of what a concrete tank weighs on average. This significantly reduces transportation costs.
While concrete tanks have a longer overall lifespan, plastic tanks can still last 20-30 years with proper maintenance. For many homeowners, the lower upfront cost of a plastic tank is more manageable, even if a replacement is needed decades down the road.
Overall, plastic septic tanks provide an affordable option without sacrificing too much on lifespan. The cost savings over a concrete tank can be 20-50% in many cases. For budget-focused homeowners, plastic tanks are generally the most economical choice.
Lifespan: How Long Do Plastic Septic Tanks Last?
Plastic septic tanks are known for their durability and long lifespan compared to other materials. Properly installed and maintained plastic septic tanks can last over 50 years.
The lifespan of a plastic septic tank depends on several factors:
- The quality of the polyethylene plastic used in manufacturing. High-density polyethylene (HDPE) plastic is more durable and resistant than lower density polyethylene. HDPE plastic tanks have a typical lifespan of 50-100 years.
- Thickness of the plastic walls. Thicker plastic walls (1/4 inch or more) increase strength and prevent cracking or buckling under soil pressure over time.
- Proper installation and maintenance. Ensuring proper bedding, backfill and compaction during installation prevents settling and damage. Regular pumping and inspections help maintain the tank and prevent premature wear.
- Climate and soil conditions. Colder climates can damage plastic tanks over time. Installing insulation helps prevent frost damage. Aggressive soil conditions can also deteriorate plastic tanks quicker than ideal soil.
- Usage and loads. Heavy use from large households or excessive solids/grease loads can cause earlier failure. Proper use and maintenance is key.
With ideal conditions and proper maintenance, plastic septic tanks can easily outlast their concrete counterparts. While steel tanks may last longer, plastic tanks provide the best balance of durability, cost and ease of maintenance for most homeowners.
Environmental Considerations
Plastic septic tanks are often touted as an environmentally friendly option compared to concrete tanks. This is due to several factors:
- Plastic is lighter weight, requiring less energy to transport and install. Concrete tanks are extremely heavy, requiring heavy machinery and more fuel for delivery and installation.
- Plastic tanks have a longer lifespan, meaning fewer tanks need to be manufactured over time. The average plastic tank lasts 25-30 years, while concrete tanks last 15-20 years.
- Plastic tanks are 100% recyclable at the end of their lifespan. The polyethylene plastic can be melted down and reused in manufacturing new plastic products. Concrete tanks are not recyclable.
- Plastic tank manufacturing has a smaller carbon footprint. The plastic molding process requires less energy and resources compared to concrete casting.
- Plastic tanks take up less space in landfills. At the end of life, plastic tanks weigh much less than concrete versions.
However, plastic tanks are not without environmental downsides:
- Plastic production utilizes petroleum resources and chemicals. More plastic in the world means more oil and gas extraction.
- Plastic tanks have more risks of leaking chemicals into the ground if damaged or cracked. Concrete tanks are less prone to leakage.
- Plastic tanks may still end up in landfills if not recycled properly. Proper end-of-life recycling is essential.
Overall, plastic septic tanks compare favorably to concrete in terms of environmental impact. But care must be taken to install, maintain, and recycle plastic tanks responsibly.
Conclusion
Plastic septic tanks offer a cost-effective and durable option for wastewater treatment. Their lightweight polyethylene construction makes installation straightforward while providing corrosion resistance and longevity. However, special considerations are required to prevent floating, and plastic tanks may be damaged by extremely heavy weights.
When weighing the pros and cons, plastic septic tanks excel in economy and lifespan with few downsides beyond float potential. With proper anchoring and care to prevent overload damage, plastic septic tanks are a smart choice for most homes. Their nearly maintenance-free nature and resistance to chemicals and corrosion make these septic tanks a favorite for decades of reliable service.
Unless space constraints, soil conditions, or local regulations dictate otherwise, plastic septic tanks are recommended for their balance of affordability and low maintenance. With periodic inspections and pumping as needed, homeowners can anticipate a long service life from these durable polyethylene septic systems. Plastic tanks have stood the test of time as a wise investment in rural and suburban wastewater management.
