Concrete vs plastic wastewater tanks: pros and cons

A concrete tank is not necessarily an old-fashioned septic tank, and a plastic tank is not necessarily a modern treatment system.

Both materials can be used for conventional septic tanks or systems that provide more treatment. WaterFlow offers concrete and plastic septic tanks, as well as concrete and plastic options within the EconoTreat range.

The process inside the tank determines how the wastewater is treated. The tank material affects things such as weight, installation and structural strength.

1. Strong, solid construction

Concrete is a robust option for an underground tank. A properly manufactured and installed concrete wastewater tank can provide decades of service.

Its strength can also be useful where the installation calls for a tank designed to handle greater ground loads. The exact tank specification still matters, including its lid and access covers.

2. Extra weight can help in wet ground

When groundwater rises around a buried tank, it pushes upwards. This can lift or move the tank if the installation has not been designed to resist that force.

Concrete’s weight helps hold it in place. However, even concrete tanks can float under the wrong conditions, particularly when empty. Wet sites still need a proper assessment and installation design.

3. Some models allow deeper installation

Certain concrete tanks can be installed deeper than comparable plastic models. This can help where the wastewater pipe arrives at the tank well below ground level.

For example, WaterFlow’s concrete EconoTreat models offer deeper burial capability than its plastic models. The allowable depth must still be checked for the particular tank.

1. Getting them into position takes more planning

The weight that makes concrete useful underground also makes it harder to move.

Delivery and installation need suitable lifting equipment, with enough access and room to work. A narrow driveway, tight corner or tank location far from the nearest vehicle access can complicate the job.

On an open building site, this may be straightforward. Behind an existing house with established gardens and limited access, it can be a bigger consideration.

2. Transport and lifting can add to the price

The tank price is only part of the cost. Freight, lifting equipment and any work needed to create access can change the overall installation price.

Ask how the supplier plans to get the tank from the road into the hole. That is a useful detail to settle before installation day.

1. Easier to transport and lift

Plastic tanks weigh much less than concrete tanks. This can make delivery and positioning simpler, particularly where larger machinery cannot easily reach.

WaterFlow’s plastic EconoTreat option is designed to help with installations where tighter access means smaller machinery is needed.

2. Useful for awkward sites

A lighter tank can give the installer more options for getting it into place.

That can be helpful when replacing a system beside an existing home, working down a restricted driveway or installing somewhere remote. The tank’s actual dimensions still matter, though. Lightweight does not necessarily mean small enough to fit through every gap.

3. Resistant to corrosion

Plastic does not rust and is resistant to many forms of chemical attack. It is a well-established material for wastewater tanks, rather than simply a cheaper substitute for concrete.

Wet-ground installation needs particular care

A lightweight tank needs careful consideration where groundwater may rise around it.

Depending on the tank and site, the installation may require anchoring, a concrete surround or another specified method of resisting uplift. Those extra requirements can affect both the work involved and the cost.

Always check that the proposed model is suitable for the groundwater conditions.

Plastic tanks have specific installation requirements, including how deeply they can be buried and what support they need around them.

The material used beneath and around the tank is part of that installation. A lightweight tank still needs the right preparation and protection from loads above it.

Concrete tanks also have limits. Neither material gives you permission to add extra soil, paving or vehicle traffic without checking the design.

Only if the complete installation is specifically designed for vehicle loads.

A strong concrete tank does not automatically have a traffic-rated lid. The tank, covers, burial depth and surrounding construction all need to be suitable.

Tell your designer early if the proposed location is near a driveway, parking area or somewhere farm machinery may travel. Do not assume that being underground makes the tank safe to drive over.

There is no reliable winner based on material alone.

Plastic’s lighter weight may reduce delivery and lifting costs. However, special installation requirements in wet ground could reduce that saving.

Concrete may be straightforward to install where a delivery truck and suitable lifting equipment can reach the excavation easily.

Compare quotes for tanks that provide the same required capacity and treatment, then check that both include:

• Delivery and lifting.
• Excavation and removal of surplus soil.
• The base and material placed around the tank.
• Any anchoring or other special installation work.
• Pipe connections, access covers and finishing work.

A cheaper tank is only a saving if it is also economical to install correctly.

The treatment process usually has more influence on routine maintenance than the tank material.

A conventional septic tank needs sludge removed whether its shell is concrete or plastic. An aerated system needs its treatment equipment serviced in either material.

For example, EconoTreat’s owner’s manual covers both concrete and plastic tanks, with maintenance checks for the blower, pumps, filters and sludge levels.

Ask about the tank warranty separately from the warranty and maintenance requirements for pumps and other components.

For an open site with good delivery access, a concrete tank may be a practical choice, particularly where its weight or permitted burial depth is useful.

For a location that is difficult to reach, plastic may make installation much easier.

If the ground gets wet, either option needs closer attention to groundwater and installation requirements.

At WaterFlow, we offer both materials. We look at how the tank will reach its position, how it will sit in the ground and what treatment your home needs before recommending an option.