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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.

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.

Wahaso offers a free analysis of the potential supply and demand for a system. As part of our Scoping stage, we look at all available sources and applications of on-site water sources and evaluate options for efficient system design. A key component of that step is our Cistern Optimization Modeling. Our model looks at supply and demand on a daily basis and uses daily rainfall information for your location for the past six years, modeling the value different cistern size options so that we can recommend the right amount of water storage. Usually we see a point of diminishing returns for storage where we continue to save more water each year, but the amount of storage needed to achieve that savings is not economically viable. Cistern Optimization Modeling is just another example of how Wahaso works in your best interest to design and recommend efficient water harvesting systems. Contact us for more information.

Once the targeted storage capacity has been determined, there are many options for storage vessels, each with its own advantages and disadvantages.

Wahaso Cistern Optimization

Wahaso’s Cistern Optimization modeling helps clients understand the value of different cistern sizes by evaluating hypothetical supply and demand using six years of actual daily rainfall activity for a location. The point of diminishing returns occurs when the cost of adding more storage capacity outweighs the value of additional water savings



Large storage capacity is most often required for properties with large rooftops and high demand for non-potable water. 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 subsurface retention systems.


Storm Capture Install Concrete storage systems are strong, versatile and space-efficient. Concrete cisterns can be poured in place – but that process is expensive and time consuming – especially at a busy construction site. In recent years, prefabricated systems have been offered by a number of manufacturers. Oldcastle Precast offers the Storm Capture® system comprised of standard linear dimensions of various depths. Modules can be placed at a site in a few hours and be ready later the same day for vehicle traffic. Systems are easily partitioned to support stormwater detention as well as retention for harvesting reuse.


Unlimited capacity; very long life; high strength under load bearing surfaces like parking lots; hold-down systems not required


Expensive to fabricate on site; require heavy equipment for installation


Storage Atlantis Assembly This relatively new system was developed in Australia, and permits unlimited underground storage at a very competitive price. Polypropylene RainTank 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. Wahaso recommends the Atlantis system for these applications. For more information please Contact Us.


Unlimited capacity and shape; passive 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.


More time and man-hours required for installation than a cylinder tank; made from petroleum-based material.


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.


Storage Large Fiberglass TanksFiberglass 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.


Large capacities at lower costs than metal tanks.


Mostly restricted to standard sizes and shapes; more costly in smaller sizes; utilitarian in appearance


Poly Tanks

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.


Relatively inexpensive – lowest cost for this size; no assembly required on-site.


Petroleum based material with high cost swings; restricted to standard sizes and shapes; utilitarian in appearance; requires special UV inhibitors for outside usage.


Storage Corgal Building
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. Wahaso recommends CorGal tanks for these applications. For more information, Contact Us.


Attractive; long lasting; made in USA; cost per gallon is lower in larger tanks.


Expensive cost per gallon; high assembly cost at site; requires custom concrete base pad.



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.

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).


Poly tanks are available in many small capacity sizes and shapes. See above for advantages and disadvantages.


Storage Corgal StarbucksSteel tanks are also available sizes as small as 700 gallons. New options are available for tanks that come fully assembled from the factory. The cost for these tanks is very competitive with fiberglass or polyethylene and can offer an attractive, long lasting option for tanks located outside. Contact us for more information about small systems using corrugated steel tanks. See above for advantages and disadvantages.

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 Contact Us.

  • Water Harvesting Solutions, Inc.

    P.O. Box 279
    Hinsdale, IL 60522
    Call Toll Free: 800-580-5350

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