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3400 Gallon Underground Rainwater Storage Tank for Large-Scale Systems
TheGraf Carat S 3400 Gallon Underground Cistern is built for serious rainwater harvesting. Whether you're managing water for commercial irrigation, flushing systems, or large residential setups, this high-capacity underground tank delivers unmatched strength, clean water protection, and long-term savings. Engineered to never collapse when empty, it’s the go-to solution for anyone who needs reliable underground water storage that adapts to harsh ground conditions.
With a patented modular design, precision-sealed dome, and 15-year warranty, the Carat S 3400 is your best bet for long-term performance and peace of mind.
Features and Benefits of the Graf Carat S 3400 Gallon Modular Tank
✓ Massive Capacity, Fully Usable – 3400 gallons of real, usable water storage makes this tank ideal for large-scale rainwater harvesting and backup water systems.
✓ Won’t Collapse When Empty – Unlike most tanks, the Carat S is engineered to stay stable even when drained, when installed per manufacturer instructions.
✓ Modular, Tool-Free Assembly – Installs in minutes with a patented quick-connect system. No screws needed. Stackable and easy to transport.
✓ Built to Handle Groundwater – Can be buried 30" to 42" in standard setup or up to 70.9" deep with optional risers. Stays stable even in saturated soils.
✓ Vehicle-Load Ready (Optional) – Upgrade for parking lots or driveway installs with a 2.2-ton axle load capacity.
✓ Advanced Filtration Compatibility – Designed to work with optional Optimax Pro® internal filter system for clean, safe water collection.
✓ Sealed Against Contamination – Includes multiple seals and lip-seal fittings to block dirt, prevent seepage, and keep water safe for non-potable use.
✓ Includes Critical Fittings – Comes with (2) 2" lower balancing bulkhead fittings and (2) 2" upper vent bulkhead fittings for multi-tank or high-flow systems.
✓ Rotating and Adjustable Dome – Aligns independently for easier pipe connections and includes a telescoping riser with 5° tilt capability.
✓ Long-Term Value – Made from high-quality, recyclable Duralene and backed by TÜV-tested safety and a 15-year warranty.
Specifications for the Graf Carat S 3400 Gallon Underground Tank
Capacity
3400 Gallons (Fully Usable)
Construction
High-Density Duralene Plastic
Manway Opening
31.5 inches
Riser Included
14-inch Telescopic Riser
Lid Type
Childproof PE Locking Lid
Fittings Included
(2) 2" Lower Balancing, (2) 2" Upper Vent
Groundwater Stability
Up to middle of tank (standard), top with proper risers
Vehicle Load Rated
Optional, up to 2.2 tons axle load
Installation Type
In-Ground, Frost-Proof
Adjustability
Dome rotates and tilts (up to 5°)
Warranty
15 Years (Manufacturer)
Easy and Safe Assembly
The patented quick connection (illustrated in green), allows the Carat tank to be assembled without screws in only few minutes. Disassembly is possible at any time.
The first-class EPDM material used in the profile seals (illustrated in yellow), has been laboratory tested to last more than 25 years.
The centering bolt (illustrated in orange) assures the accurate and easy assembly of the two half shells preventing any leaks
Flush with ground level (2 seals)
The Carat S underground tank has numerous seals to effectively stop dirt getting into the tank. This means that seepage water cannot get into the tank and, thus, dirt particles cannot contaminate the rainwater. The seals are in the intersection between the tank and the tank dome and between the tank dome and the telescopic dome shaft. All supply pipes connected to the tank dome are also sealed with five lip seals as standard.
Application
The global innovation – the new tank generation of the Carat rainwater underground tanks have been specially developed for rainwater harvesting
Frost-proof installation underground
Groundwater stable up to the middle of the tank due to extremely rigid construction
The tank that turns its head for you
The GRAF Carat S underground tank has a rotating tank dome. The tank dome can be aligned with the connections independently of the tank – this makes installation much easier! All installation pipes are connected using the five standard lip seals. The Carat S telescopic dome shaft connects the system to the ground surface. The height of the tank can be smoothly adjusted to suit the local conditions and it can be tilted by 5°. The whole system is flush with ground level.
Logistical Advantages of the Carat Tank
The Carat tank consists of two half shells which can be stacked on top of each other, allowing multiple tanks to be stacked on a single pallet for shipping. For example, it is possible to stack five 1,700 gallon Carat tanks on a single pallet. This allows a standard 40 ft. shipping container to house a total tank volume of up to 34,000 gallons! The tanks' unique stacking feature directly reduces transport costs and environmental impact from vehicle emissions.
How to Assemble a Carat Tank Video
Vehicle Loading Option
Includes heavy duty telescopic dome shaft and cast iron lid for vehicle loading up to a maximum axle loading of 2.2 tons.
Is this easy to use? I haven’t had a system like this before.
Yes, it’s designed to be simple and beginner-friendly. If you’re just using a tank or a basic bundle, it works passively, rainwater flows from your roof into the tank through a downspout diverter. There’s no need for electricity or complicated setup. If you need to add a tank to an already established rainwater system, it’s just as easy, most tanks can be connected in series or used as additional storage with a few basic fittings. We can help you choose the right components to make the expansion seamless.
How do I know what tank size I need for my home or garden?
A simple way to estimate the right tank size is by calculating how much rainwater your roof can collect. Multiply your roof’s surface area (in square feet) by your average annual rainfall (in inches), then multiply by 0.62, this gives you the total gallons you could collect in a year. For example, a 1,000 sq ft roof with 30 inches of rainfall can collect around 18,600 gallons annually. From there, we can help you choose the right tank size based on your water use and how much storage you want on hand, whether you're using it for irrigation, home use like toilets or laundry, or even potable water with proper filtration.
Can I bury this tank underground?
Yes and no, not all tanks are designed to be buried. Only tanks that are specifically rated for underground installation should be placed below ground. Always check the product specifications or reach out to us before burying any tank to ensure it’s safe and properly supported.
Is this tank safe for long-term water storage?
Absolutely. Our tanks are made from high-quality, UV-stabilized, food-grade polyethylene or reinforced materials designed for long-term rainwater storage. They resist algae growth and won’t leach chemicals into your water.
Can I collect rainwater from just one downspout?
Yes. Many basic systems start with a single downspout diverter, especially for smaller tanks. This setup is easy to install and still collects a significant amount of water, especially in areas with frequent rainfall or large roof sections.
What’s the difference between above-ground and underground tanks?
Above-ground tanks are easier to install, inspect, and move. Underground tanks save space and are often used when visual impact or limited yard space is a concern. We offer both, and we’re happy to help you decide what fits your space and needs best.
Will the tank overflow if it fills up?
No worries, each tank includes an overflow port that directs excess water safely away from the tank when it’s full. You can connect this to a drain, second tank, or garden area to make use of the overflow.
Can I connect this tank to a pump later on?
Yes. All of our tanks are compatible with add-on pumps, filters, and automation systems. You can start simple and upgrade later with no problem, just let us know your goals and we’ll help future-proof your setup.
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