Concrete vs. Plastic vs. Fiberglass Septic Tanks: Which Do You Have, and Does It Matter?

Most Illinois homeowners never think about what their septic tank is made of until something goes wrong — a lid that won’t sit flush anymore, a tank that seems to have shifted in the ground, or a pumper who says “huh, that’s an old one” when they pop the lid. The material matters more than most people realize, because it drives how long the tank lasts, how it fails, and in some river-bottom parts of Illinois, whether it stays in the ground at all.

The U.S. EPA notes that septic tanks are usually made of concrete, fiberglass, or polyethylene (plastic). Each one holds sewage the same way — a buried, water-tight container that lets solids settle and scum float while relatively clear water flows out to the drain field — but they age very differently.

How do concrete septic tanks fail?

Concrete is the traditional choice across Illinois and still the most widely accepted material by county health departments. It’s heavy enough to resist floating and, when properly built, can hold up for 40 years or longer.

The failure most people hear about is hydrogen-sulfide corrosion: bacteria in the sewage produce hydrogen sulfide gas, and above the waterline — where the gas meets oxygen — it can convert to sulfuric acid and slowly eat away at the concrete, showing up as surface pitting, crumbling mortar, or exposed aggregate near the lid. In practice this kind of chemical corrosion is uncommon in residential tanks; most concrete tank problems that look like corrosion actually trace back to a poor concrete mix at the time of manufacture, not ongoing acid attack. The more common real-world issues are hairline cracks from ground shifting or heavy equipment driving over the tank, and baffle failure — the internal walls that direct flow can corrode or break loose over time, letting solids drift toward the drain field.

A cracked concrete tank can often be patched with hydraulic cement or a specialized sealant rather than replaced outright, which is one advantage concrete has over the other materials.

Why do plastic septic tanks float, and is that a risk in Illinois?

Plastic (polyethylene) tanks are rotomolded as a single piece, which means no seams to leak, and at 200–400 pounds they’re light enough to set by hand or with a small machine — no crane needed. That keeps installation cost down, which is why plastic tanks are popular in remote or hard-to-access sites.

The tradeoff is buoyancy. An empty or partially empty tank weighs far less than the water it displaces, so when groundwater rises around a plastic tank, the upward pressure can be enough to lift a poorly anchored tank out of the ground — sometimes just enough to crack pipe connections and cause a slow leak, sometimes enough to visibly heave the lid or tank body upward. Installers address this by calculating the buoyant force against the tank weight, the water inside it, and the weight of the soil covering it, then adding mechanical anchors or extra burial depth when the math doesn’t clear a safety margin on its own.

This matters more in parts of Illinois that sit close to a river. Homes near the Mississippi, Rock, or Galena Rivers — think Savanna, Galena, and low-lying stretches near Fulton or Nauvoo — can see the seasonal water table climb close to the surface during spring snowmelt and heavy rain, especially in years with significant flooding. If your property is in one of those areas and you have (or are installing) a plastic tank, ask your installer directly whether it’s anchored for your soil and water conditions — requirements and typical practice vary by county, so check with your county health department for what your jurisdiction requires.

What about fiberglass tanks?

Fiberglass sits in between the other two. At roughly 250–350 pounds it’s still light and easy to place, and it’s rigid enough to resist the flotation risk that plastic carries, plus it’s fully corrosion-proof. The catch is availability and price — fiberglass tanks are less common in Illinois, so they’re often special-ordered, and the material itself tends to cost as much as or more than concrete without concrete’s weight advantage against flotation. Long-term failure in fiberglass tends to show up as surface crazing (a web of fine surface cracks) or separation at seams and fittings after many years in the ground, rather than the corrosion or cracking patterns seen in concrete or plastic.

Does tank material change how pumping and inspection work?

The pumping process itself — evacuating liquids and solids from every compartment — is the same regardless of material. But the material changes what a pumper or inspector has to watch for while the lid is open:

• Concrete: the crew checks for interior cracking, crumbling near the access lid, and whether the baffles are still solid. A concrete lid that’s cracked or spalling is a safety concern — it can collapse under weight — and is worth flagging during a routine inspection.
• Plastic: the crew looks at whether the tank walls have bowed or deformed (a sign of soil pressure or groundwater stress), whether pipe connections at the inlet and outlet are still tight, and whether the tank has visibly shifted or tilted in the ground — an early warning sign of flotation stress even if the tank hasn’t fully surfaced.
• Fiberglass: the crew checks fittings and seams for separation and the surface for crazing or soft spots.

A general routine pump-out in Illinois runs $250 – $400, and adding a full inspection typically adds $150 – $250 to that visit. If your tank is old, of unknown material, or you’ve never had one, that combined visit is the cheapest way to find out what you’re dealing with before it becomes an emergency.

How do you tell what material your tank is?

1. Check your county health department’s permit file. Most Illinois counties keep the original septic permit on record, and it usually lists tank material, size, and installation date.
2. Look at the lid. Concrete lids are heavy (often requiring a pry bar or two people to lift) and gray. Plastic and fiberglass lids are noticeably lighter, sometimes green or black, and can flex slightly underfoot — never stand on a lid you haven’t confirmed is rated for it.
3. Ask when it was installed. Homes built before the 1970s in Illinois are almost always concrete, since plastic and fiberglass tanks weren’t widely used until later. Newer installs, especially in remote or hard-to-access spots, are more likely to be plastic.
4. Have a pumper confirm it on the next visit. The crew will know within seconds of opening the lid, and it costs nothing extra to ask them to note it for your records.

If you’ve never had your tank located or opened, that’s worth fixing before you have an emergency. See our guide on how often to pump a septic tank for a starting schedule.

Does material affect what a repair or replacement costs?

Yes, in a few ways. A cracked concrete tank is often repairable in place, which can be far cheaper than replacement. A plastic tank with a cracked wall or failed fitting is harder to patch reliably and is more often replaced outright. And because concrete requires a truck-mounted crane or hoist to set, while plastic and fiberglass can often be placed with lighter equipment, labor costs on a new install can run higher for concrete even though the disposal-side work is identical.

If a pumper tells you your drain field is failing regardless of tank material — standing water over the field, slow drains throughout the house, or sewage odor outside — that’s a separate system from the tank itself. Drain field repair in Illinois typically runs $800 – $2,500 for a partial repair, well below full replacement. See our drain field repair page for Ottawa for what that process looks like.

The bottom line

Concrete lasts the longest and resists flotation on its own weight, which is why it remains the default in most of Illinois. Plastic is cheaper to buy and install and works fine on dry, well-drained sites, but it needs proper anchoring anywhere the water table can rise close to the surface — a real consideration for river-adjacent towns in northern and western Illinois. Fiberglass splits the difference but costs more and can be harder to source locally. Whatever material you have, the maintenance schedule doesn’t change: get it pumped every 3–5 years and have a pumper flag any material-specific wear while the lid is open.

Frequently Asked Questions

Related reading

• How often should you pump your septic tank?
• How much does septic pumping cost in Illinois?
• 10 signs your septic tank is full
• Septic backup during spring thaw in Illinois
• Emergency septic pumping in Stockton, IL
• Septic inspection in Galena, IL
• Drain field repair in Ottawa, IL
• All Illinois service areas