The best tank material for your site depends on a variety of factors including whether the tank is to be installed above ground or underground, your budget, the water quality required, the site configuration, climate, and structural characteristics. Selecting an appropriate tank for your rainwater harvesting system is important because it is most likely the most expensive component of the project.
It is recommended that you speak with a professional rainwater harvesting designer and the manufacturer before purchasing a tank for rainwater harvesting. They can provide you with a cost estimate, shipping information, and an estimate of the lifespan of the tank. Depending on where you live, you may also need to consider the snow load, wind load, and/or seismic rating of the tank.
The following table summarizes various characteristics of commonly used tank materials.
| Metal Tanks | Plastic Tanks | Fiberglass | Bladder | Reinforced Concrete | |
| Material | Galvanized steel or stainless steel | Polyethylene (PE) Polypropylene (PP) | Fiberglass | Polymer alloy | Reinforced concrete |
| Prefabricated or built onsite? | Can be prefabricated or built on site | Prefabricated | Prefabricated | Prefabricated | Prefabricated or built on site |
| Above ground or underground? | Above ground | Above ground or underground | Above ground or underground | Above ground | Above ground or underground |
| Capacity (gallons) | Prefabricated: 150 – 3,000 Site-built: 700 – 700,000 | Above ground: 50 – 12,500 Underground: 250 – 10,000 | Above ground: 50 – 30,000 Underground: 600 – 60,000 | 25 – 200,000 | Pre-cast: 1,000 – 5,500 Site-built: variable |
| Meets potable standards? | Prefabricated: Yes, if a potable grade coating is included Site-built: Yes, if a potable-grade liner is installed | Yes, if a potable-grade material. | Yes, if there is a potable grade coating | Yes, if potable grade material | Yes, if a potable grade coating or lining is included |
| Liner? | Prefabricated: No liner Site-built: Liner | No liner | No liner | No liner | Either |
| Inlet/outlet port installed onsite or at the manufacturing facility? | Prefabricated: On-site Site-built: At the manufacturer | On-site | At the manufacturer | At the manufacturer | At the manufacturer or on-site |
The following sections will discuss each of these materials in more detail.
Plastic Tanks
Plastic tanks are, by far, the most common choice for rainwater tanks. Plastic tanks are suitable for small-scale residential and commercial projects. This is because they are available in many different sizes and are relatively low cost. Plastic tanks also come in multiple colors such as blue, black, green, or beige. Avoid purchasing a clear tank because algae tend to grow in them. Plastic tanks can also be connected in series to achieve more storage.
Plastic tanks can also be installed above ground or underground. If you choose to install a tank underground, ask the manufacturer for the appropriate buoyance calculations and traffic ratings (if applicable).
Although polyethylene (PE) and polypropylene (PP) are some of the most common plastic tank materials, there are other plastic tanks available. Some plastics leach into the water more than others. It is recommended that you avoid tanks made of polyvinyl chloride (PVC) as it is believed that PVC tends to leach into the water. High-Density Polyethylene (HDPE) is the preferred plastic for rainwater tanks and is also the most commonly used material. HDPE has low toxicity, but it can transmit a plastic tank to water.
Metal Tanks
Metal tanks are also fairly common as they tend to be more aesthetically pleasing than plastic tanks. Bolted or welded galvanized steel tanks also create a tank that is strong, durable, and fire-resistant. However, the quality of galvanized steel varies. In general, thicker metal is better than thinner metal. Corrugation indicates thinner metal. A properly installed, high-quality galvanized steel tank can last for several decades. Adding a plastic membrane to the inside of a galvanized steel tank will increase the lifespan of the tank. However, a tank that shows damage to the galvanized material or that is not installed on a firm gravel base can degrade quickly. In severe cases of corrosion, large flakes will slough off inside of the tank. This can cause iron and/or zinc to leach into the water stored in the tank.
Fiberglass Tanks
Fiberglass tanks are lightweight, very strong, and do not corrode. However, fiberglass tanks tend to be more expensive than plastic tanks. Fiberglass tanks are always prefabricated. They can be custom ordered with specific fitting sizes/locations and access ports. Ladders and access ports can also be molded into the tank for easy maintenance. These customization options allow the customer to purchase a tank that meets the project’s needs. If you are purchasing a large fiberglass tank, contact your local transportation department to verify the requirements for transporting oversized loads.
It is recommended that you flush your fiberglass tanks prior to use. This is because there have been reports of high concentrations of solvents found in new fiberglass tanks.
Bladder Tanks
Bladder tanks are a type of water storage where the water is collected within a flexible plastic membrane that is supported by another structure. These systems are shipped collapsed and installed on site. Bladder tanks with a significant amount of storage require a large footprint. Some are installed outside in covered spaces are even in basements or crawl spaces. An advantage of bladder tanks is that it is a cost-effective way to store large amounts of water. It is important to note that the plasticizers in flexible membranes tend to be more toxic than those used in rigid plastic tanks.
Reinforced Concrete Tanks
Concrete tanks are good choices for large, municipal tanks. Concrete tanks built onsite will require steel reinforcement whether they are built above ground or underground. It is important to note that concrete strength and quality can vary, so it is recommended that you consult an expert for guidance on the concrete mixture that is most appropriate for your site.
Other Tank Materials
There are many choices for water tanks. Each material has advantages and disadvantages. It is important to consider whether a particular material is appropriate for your site and project goals.
Pipe Tanks
Pipe tanks are typically large systems that are installed underground. They are typically made of large concrete or metal pipes that are engineered to hold water. Underground pipe tanks are usually constructed for the purpose of storing water for irrigation or capturing water for the purpose of preventing flooding. It should be noted that pipe tanks are buoyant, so they require ballasting or anchoring in areas with high water tables. A licensed professional engineer (PE) should be consulted to ensure that these tanks are designed properly.
Modular Tanks
Underground modular tank systems can provide large storage volumes. Modular tanks are comprised of small, rectangular structures that can be stacked to create a custom tank shape. These systems can be very strong, and some can even be installed under parking lots. R-Tanks are a common example of a modular tank system. Alternatively, geotextile fabric can be used to allow water to infiltrate into the surrounding soil surfaces.
Wood Tanks
Wood tanks were commonly used to store water in the past. In my opinion, they are quite beautiful. They are typically made of wood planks and steel hoops. The wood expands when wet such that the planks create a tight seal within the steel hoop. If you allow the water level to drop too much in a wood tank, the boards will dry out. This can cause the tank to lose its seal.
Brick Tanks
Small, square tanks made of brick are not common in the United States. However, they are more common in small, non-industrialized countries. These tanks are fairly easy to construct and could become more widespread in the United States.
It is also possible to convert a cheap plastic tank into a high-quality, aesthetically pleasing tank by combining it with masonry. The masonry provides complete shade which extends the life of the plastic tank considerably.
Ferrocement Tanks
Ferrocement tanks are constructed from a grid of steel reinforcement covered with a mixture of sand and cement. These tanks are roughly equivalent to concrete tanks in strength and durability. However, they can be built for a fraction of the cost. Art Ludwig, author of “Water Storage – Tanks, Cisterns, Aquifers, and Ponds” states that “ferrocement is arguably the best all-around material for a permanent water tank.” The disadvantage of ferrocement tanks is that you cannot move them. In general, you also have to build them yourself.
Tank Materials To Avoid
- Polyvinyl Chloride (PVC) exposed to sunlight. PVC breaks down in sunlight, which causes a reaction that forms carcinogens.
- Flexible PVC water bladders or trash cans – they can contain toxic plasticizers
- Old tank coatings made of coal tar or lead-based paint
- Western red cedar
- Concrete that contains fly ash
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