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One of the key decisions in designing a water harvesting system is to determine how much water should be stored and how that water will be stored. Select a storage system that is too small, and there may be lost opportunity to capture and store water between uses. That may mean that excess rainwater is being directed to the stormwater system instead of being retained for flushing toilets or irrigating landscaping. Select a storage system that is too large and the result can be expensive wasted capacity that is not needed.
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A system should be sized to take into account all the water sources that are being accessed - the average amount of rainfall per rain event, for instance. Concurrently, how much demand will there be for the harvested water? Average demand is easily calculated if the water is being used to flush toilets. More guesswork is required when the water is to be used for irrigation or cooling tower make-up. It is usually best to err on the side of too much capacity rather than not enough.
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| You can use our to estimate the amount of rainwater available to your building, or we are happy to help you scope your project at no charge. |
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| Once the targeted storage capacity has been determined, there are many options for storage vessels, each with its own advantages and disadvantages. |
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LARGE STORAGE CAPACITY: 5,000+ GALLONS
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Large storage capacity is most often required when the system is design to retain stormwater off very large building roofs and parking lots. These capacities can range from 20,000 gallons to millions of gallons. The most efficient method for storing these large quantities of water is in subterranean vessels.
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| CONCRETE VAULTS |
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Concrete vaults are available in standard prefabricated sizes, or can be custom fabricated to meet the unique requirements of any property. They can be designed to be water tight so that all harvested water can be recycled, or they can include outflows that slowly return the water back into the surrounding soil to recharge the groundwater.
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Advantages: Unlimited capacity; very long life; high strength under load bearing surfaces like parking lots
Disadvantages: Expensive to build and install.
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| ATLANTIS® SYSTEM |
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This relatively new system was developed in Australia, and permits unlimited underground storage at a very competitive price. Polypropylene Ranitank modules are snapped together at the building site and stacked vertically and connected horizontally to create a storage space of unlimited size and shape. The system can also support non-porous parking areas. The tank system is wrapped in a geotextile filter medium and stabilized with compacted washed sand. For water retention and reuse, the excavation is lined with a 300 ml poly liner that contains the water in the cistern. For temporary detention purposes, the retention liner is omitted, allowing the water to infiltrate into the ground which can aid in recharging local groundwater.
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Wahaso recommends the Atlantis system for these applications. For more information please
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Advantages: Unlimited capacity and shape; natural biological water filtration and purification; integrated capture of surface water; much lower cost than sealed systems like vaults and tanks; no heavy equipment required for installation; 95% void means smaller excavations and less fill than cylinder and pipe systems. "Living system" does not require sanitation in the tank or mechanical recirculation to maintain water quality like a sealed tank system. Eligible for extra LEED points - made from 85% recycled plastics and manufactured in the Mid-West U.S.
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Disadvantages: More time and man-hours required for installation than a cylinder tank; made from petroleum-based material.
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MEDIUM STORAGE CAPACITY: 1,000 – 20,000 GALLONS
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Most commercial water harvesting systems fall into this range of storage capacity. Supplies of greywater or rainwater and their respective uses can be maintained in balance with less than 20,000 gallons of storage in most cases. There are many storage options from dozens of manufacturers in this range as well. Tanks can be stored in inside basement or utility areas, outside in a visible or concealed area, or buried underground.
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| FIBERGLASS TANKS |
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Fiberglass tanks come in many standard sizes and can quickly be installed on a site inside or out. Available in sizes up to 50,000 gallons, these tanks are suitable for burying or above-ground use.
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| Advantages: Large capacities at lower costs than metal tanks.
Disadvantages: Mostly restricted to standard sizes and shapes; more costly in smaller sizes; utilitarian in appearance
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| POLYETHYLENE TANKS |
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Polyethylene tanks are available from numerous manufacturers in many shapes and standard sizes up to 20,000 gallons. Suitable for above-ground use inside or outside a building. Polyethylene tanks became quite expensive recently with the surge in oil costs.
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| Advantages: Relatively inexpensive – lowest cost for this size; no assembly required on-site.
Disadvantages: Petroleum based material with high cost swings; restricted to standard sizes and shapes; utilitarian in appearance; requires special UV inhibitors for outside usage.
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Polyethylene Tanks are Ideal for Smaller to Medium Sized Storage Capacity and are Readily Available in Many Standard Sizes
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| STEEL TANKS |
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Steel tanks are ideal for outside storage of rainwater in buildings that wish to display their water conservation efforts and where internal or subterranean storage is not practical. CorGal tanks come in a variety of standard sizes and have a plastic liner to keep them water tight.
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Wahaso recommends CorGal tanks for these appllcations.
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| For more information, |
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Advantages: Attractive; long lasting; made in USA; cost per gallon is lower in larger tanks.
Disadvantages: Expensive cost per gallon; high assembly cost at site; requires custom concrete base pad.
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CorGal Steel Tanks Offer an Attractive Option for Outside Storage |
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| SMALL STORAGE CAPACITY: 50 – 5,000 GALLONS |
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Depending on a building’s size, occupants and water requirements, a smaller storage capacity may be all that’s needed. This might be true in a residential building where greywater is being used to flush toilets. In that case, the supply and demand of water are closely aligned, reducing the need for large storage capacity. Or the system might collect rooftop rainwater to flush toilets in a smaller shower house or comfort station.
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Smaller sizes often permit interior location of tanks in basements or utility areas which eliminates concerns with freezing temperatures or the expense of burying tanks. New tank designs offer even more placement options (see below).
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| POLYETHLYENE TANKS |
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Poly tanks are available in many small capacity sizes and shapes. See above for advantages and disadvantages.
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| WATER HOG® SYSTEM |
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This new tank system offers the ultimate in small storage flexibility. Each tank holds 50 gallons and can be combined with innumerable additional tanks to expand capacity. Tanks can be located horizontally under a floor or deck, or vertically along the outside or inside of a wall. The trade off is high cost per gallon of storage.
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Wahaso recommends the Water Hog system for these applications. For more information, please
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The Water Hog® System Provides Modular Units Combined for Small Storage Capacity |
| STEEL TANKS |
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Steel tanks are also available sizes as small as 700 gallons. However, the cost per gallon in these small sizes is very high. See above for advantages and disadvantages.
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Let Wahaso scope your project and help you determine the optimal water harvesting storage system for your building. We are happy to provide project scoping at no expense. Please
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