FRP Guides

FRP Septic Tanks: How They Work, How to Size One and How to Install It

In short: A septic tank is a buried, watertight tank that holds wastewater long enough for solids to settle as sludge and fats to float as scum, so that clarified liquid can flow on to a drainfield or further treatment. The US EPA notes that septic tanks are usually made of concrete, fiberglass or polyethylene. Fiberglass (FRP) tanks are light, so they are easy to transport and install, and they do not corrode. Tank size depends on the number of users and local rules; the EPA gives the example of a 1,000-gallon (about 3.8 m³) tank for a three-bedroom house with four occupants and no water-saving fixtures. Household tanks are typically pumped every three to five years. FIBROVX builds FRP septic tanks to the capacity and configuration your project needs.

FRP Septic Tanks: How They Work, How to Size One and How to Install It – In short summary: key points at a glance

Table: Why fiberglass? – FRP Septic Tanks — Why fiberglass?. Summary diagram of the table in this guide.

How a septic tank works

Diagram: Installation – FRP Septic Tanks — Installation: summary diagram from this guide.

According to the US EPA:

All wastewater leaves the building through one main drain into the septic tank.
In the tank, heavy solids settle to the bottom as sludge, and oil and grease float to the top as scum.
Compartments and a T-shaped outlet keep sludge and scum from leaving the tank.
The liquid effluent flows to the drainfield, where the soil filters and treats it.

The EPA fact sheet on septic tanks says the tank should retain wastewater for at least 24 hours, and that up to 50 percent of the retained solids decompose while the rest accumulates as sludge that must be pumped out periodically.

A septic tank provides primary treatment only. The drainfield, or a secondary treatment unit, completes the treatment. Local regulations decide what is required after the tank.

Why fiberglass?

Factor	FRP septic tank	Notes
Weight	Light	Easier transport and handling, smaller lifting equipment
Corrosion	Does not rust or spall	Resists the corrosive conditions inside septic tanks
Watertightness	Molded or wound one-piece or sealed shells	Watertightness is essential to protect groundwater
Installation	Fast placement	Must be anchored where groundwater is high
Shape	Cylindrical or custom	Can be made with integral compartments

Concrete tanks are heavy, which helps resist uplift but makes transport harder. Polyethylene tanks are also light. The right choice depends on site access, soil, groundwater and local approval rules.

How to size a septic tank

Tank capacity is set by the expected daily flow and local regulations. The EPA lists the main factors as the number of bedrooms, the number of occupants, the size of the home and whether water-saving fixtures are used. Its example is a 1,000-gallon (about 3.8 m³) tank for a three-bedroom house with four occupants and no water-saving fixtures.

Diagram: Common mistakes – FRP Septic Tanks — Common mistakes: summary diagram from this guide.

For commercial sites, camps and multi-unit buildings, size the tank from the design flow and any pretreatment needed. The EPA notes that commercial inputs should be evaluated case by case and may need pretreatment to remove oil, grease or solids.

In Europe, prefabricated septic tanks for up to 50 population equivalent (PT) are covered by EN 12566-1. Other countries have their own standards and approval schemes. Always check local rules before you specify a tank.

Single or multiple compartments?

A second compartment improves solids retention because effluent passes through a second settling zone before leaving. Many codes require or recommend two compartments, or two tanks in series, for larger flows. We build FRP septic tanks with the number of compartments your design specifies.

Installation

Excavation and bedding. Place the tank on a level, compacted bed as specified in the installation instructions.
Backfill. Use the specified backfill material and compact it in layers so the shell is evenly supported.
Groundwater and buoyancy. An empty light tank in high groundwater can float. Use the anchoring method in the installation instructions, such as straps to a concrete deadman or base slab, wherever groundwater may rise above the tank base.
Burial depth and traffic. The tank must be designed for the burial depth and any surface loads. Do not drive over a tank unless it is designed for traffic.
Access. Provide risers and covers for inspection and pumping.
Venting and connections. Follow local plumbing codes for inlet, outlet and vents.

Maintenance

The EPA recommends that household septic systems are inspected at least every three years by a professional and that household tanks are typically pumped every three to five years. Pumping frequency depends on household size, total wastewater, the volume of solids and tank size. The EPA says a tank should be pumped if the bottom of the scum layer is within 6 in of the bottom of the outlet, if the top of the sludge layer is within 12 in of the outlet, or if more than 25 percent of the liquid depth is sludge and scum.

Customization

FIBROVX manufactures FRP septic tanks to order. We can adjust capacity, diameter and length, number of compartments, inlet and outlet sizes and positions, access openings and risers, and anchoring points. See FRP septic tanks and the Tanks, Pipes and Covers collection.

Need a quote or a drawing check?

FIBROVX manufactures FRP products in our own factory in China and builds every item to order: sizes, resin system, color, molds and OEM branding. Send your application, dimensions or drawing, quantity and destination port.

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Common mistakes

Undersizing. Size for the real number of users and future growth.
No anchoring in high groundwater. Empty tanks can float.
Wrong backfill. Poor backfill can deform the shell.
No access for pumping. Provide risers and covers.
Ignoring local approval. Many regions require approved tank designs.

Key terms

Sludge: Settled solids at the bottom of the tank.
Scum: Floating fats, oils and grease.
Effluent: Clarified liquid leaving the tank.
Drainfield: Soil absorption area that treats effluent.
Population equivalent (PE/PT): A unit used to express wastewater load per person.

FAQ

How long does an FRP septic tank last?

Service life depends on design, installation and maintenance. FRP does not rust, but the tank must be correctly installed and maintained.

What size septic tank do I need?

It depends on users and local code. The EPA example is 1,000 gallons for a three-bedroom house with four occupants and no water-saving fixtures.

How often should a septic tank be pumped?

The EPA says household tanks are typically pumped every three to five years.

Can an FRP septic tank float?

An empty tank in high groundwater can. Anchor it as the installation instructions specify.

Is a septic tank enough on its own?

No. It provides primary treatment. A drainfield or secondary treatment completes the process.

Can you build custom sizes?

Yes. Send the capacity, compartments and connection details.

Related guides

Sources

US EPA, How Septic Systems Work: https://www.epa.gov/septic/how-septic-systems-work
US EPA, How to Care for Your Septic System: https://www.epa.gov/septic/how-care-your-septic-system
US EPA, Septic Tank fact sheet EPA 832-F-00-040 (24 h retention, 50% solids, 1,000-gallon example): https://www.epa.gov/sites/default/files/2015-06/documents/septic_system_tank.pdf
CEN, EN 12566-1:2016, Small wastewater treatment systems for up to 50 PT, Part 1: Prefabricated septic tanks (CEN standard; available from national standards bodies)