Essential Seal

Watertightness is an essential attribute of onsite treatment system tanks, regardless of material and in any specific application.
Essential Seal
Injection-molded parts on plastic septic tanks allow precise wall thickness. A continuous gasket is physically locked in place to ensure watertight conditions.

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Watertightness is critical in onsite treatment system tanks, and in risers, lids and inlet and outlet piping. A septic tank or pump tank that is not watertight can cause a number of problems:

Groundwater or surface water leaking in will cause hydraulic overloading of the drainfield and may flush solids out, eventually plugging the field.

Untreated wastewater leaking out can contaminate water resources, causing a threat to public health – in areas with high groundwater, viruses and other pathogens from wastewater can travel great distances.

Cracks or open seams can allow roots to penetrate and expand the openings.

Whether tanks are made of the traditional concrete or newer materials, watertightness is essential. Fiberglass and plastic tanks have become widely accepted by contractors, designers and regulators, largely because their light weight makes them easy to move around difficult job sites.

Each material has its place and its benefits. Advances in the structural integrity of plastic tanks and new approaches to ensuring watertightness are making them candidates on almost any job. Concrete tanks have been improving as well.

There is an optimal approach and a "right" tank for each project, depending on location, size, budget and application. What is not negotiable in the world of septic tanks is the need to be watertight.

Materials and best practices

Codes related to septic tanks help ensure that minimum standards are met. Several states and provinces have adopted strict requirements on watertightness. Codes typically focus on the end goal of defining watertight test criteria and do not specify manufacturing processes or tank materials.

In most areas, codes still state only that a watertight tank must be provided, but that is beginning to change. Many recent code changes now require testing to ensure tank watertightness, and best manufacturing processes and technologies have been developed to meet the new requirements.

Concrete tanks

The National Precast Concrete Association has developed a Septic Tank Manufacturing Best Practices Manual that helps precasters manufacture high-quality watertight tanks.

According to the manual, "The growing dependence on these systems places a greater emphasis on system performance and component structural integrity. As such, protection from water infiltration and exfiltration is a critical element in the design of tanks used in onsite systems. Regulatory codes and project specifications requiring structurally sound and watertight tanks are becoming the rule rather than the exception, as they should be."

The manual goes on to explain, "With the increasing regulatory demands for structurally sound and watertight tanks, it is critical for precast concrete manufacturers to continually raise the bar on quality. And, that proper installation of the tank is absolutely critical for maintaining structural integrity and watertightness."

Concrete tanks have been the standard for septic tanks and pump tank applications. They are now offered with top-seam joints to minimize the risk of leakage. Additives are necessary in the concrete mix to prevent corrosion from the wastewater environment: If additives are not included, the oxidation of hydrogen sulfide will corrode the concrete. Concrete tanks are also ideal for traffic applications as they can be designed to meet the required loading.

Plastic tanks

Plastic and fiberglass are naturally inert to wastewater constituents and will not corrode. Tanks manufactured with plastic or fiberglass are notably lighter, making them ideal for difficult-access sites.

Injection molding or rotational molding allow the inclusion of corrugations and ribbing to strengthen the tanks. Interior structural bulkheads can be included to increase strength even further. Injection molding is new to tanks in the 1,000-gallon capacity range, but the process has been evolving and offers a variety benefits.

The walls of injection-molded tanks have a consistent wall thickness, and the process enables use of much higher-strength plastic. This yields strong yet light tanks that are manufactured in halves, allowing the tanks to nest for increased shipping density.

In addition, injection-molded tanks are manufactured with high precision, allowing an EDPM watertight gasket to be inserted between the halves to ensure watertightness. Plastic tanks that are rotationally molded do not have seams.

New applications

In septic system applications, the need for compact systems for small lots and for lots in environmentally sensitive areas is serving as a catalyst for tank innovation, including further safeguards to ensure watertightness.

Other applications coming to the forefront of tank design include rainwater harvesting, in which homeowners collect runoff from roofs and other structures to irrigate their yards or garden. It is essential for these tanks to be watertight, as well.

Septic tanks are the heart and lungs of onsite treatment systems, which the U.S. EPA has identified as critical components of the nation's wastewater infrastructure. Technologies are available to provide homeowners and businesses with watertight tanks that protect the environment. The future of tanks is sound: higher quality, higher standards and new technology.


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