Wahaso provides custom systems for harvesting stormwater for irrigation, toilet flushing and other uses
Federal Stormwater Phase II regulations are the driving force behind the increasing number of underground basins being constructed for retention, detention, and infiltration of stormwater run-off. These regulations protect waterways from being polluted by run-off from impervious surfaces such as building roofs and vehicle parking lots during a storm event and also address the recharging of groundwater aquifers.
Building owners must often spend hundreds of thousands of dollars to construct huge cisterns to hold hundreds of thousand of gallons of stormwater from a rain event and then slowly meter its release to the municipal stormwater system. Ironically, most of these buildings then turn around and purchase millions of gallons of drinking water each year for non-potable uses – like flushing toilets and irrigating landscaping!
TURNING A LIABILITY INTO AN ASSET
At Wahaso, we differentiate between rainwater and stormwater. We define rainwater as the relatively clean water coming from rooftops and stormwater as the water collected from parking lots and other ground level run-off. Stormwater is often contaminated with silt, oil and carbons from automobiles, road salt in northern climates and nitrates and other fertilizers from landscaped areas and more. So additional filtration and cleaning steps may be required, but stormwater can still be an excellent harvested water resource.
Stormwater is most often detained and then released. Stormwater harvesting is the process of retaining stormwater for reuse. Harvested stormwater can be used for any number of purposes – irrigation is the most common use, but this valuable resource can also be used to flush toilets, make-up water in evaporative cooling towers and more. Not only does this harvesting effort save the building owner thousands of dollars a year, but the practice saves the energy needed to purify and transport that water back and forth to the municipal system. At the same time, retaining and harvesting the water at the site reduces the burden on the municipal system.
Economics and Return on Investment
For large stormwater detention systems, the incremental cost for converting the storage from detention to retention for harvesting is a small fraction of the cistern cost. For example, a 400,000 gallon detention cistern might cost $500K or more. Adding a harvesting capability to irrigate the property’s landscaping could be $50K or less – a 10% increase in total system cost — that would yield savings year after year in municipal water and sewer charges. And because the detention systems are usually mandated by local codes, they are a sunk cost when calculating the ROI of the incremental harvesting capability.
Moving from Stormwater Detention to Retention and Reuse
Converting from detention to retention can be as simple as closing the metered outlet to the tank through a control valve. The valve is normally closed and can be opened to meter out water if the cistern reaches filling capacity or when multiple rain events are expected. Another approach is to create both retention and detention areas such that when the retention area is filled, the cistern drains to the metered detention section.
HARVESTING SYSTEMS FOR STORMWATER
The system requirements for harvesting retained stormwater depends on the quality of the retained water and how it will be applied.
It is key for any storage system that the water entering the cistern be properly pre-treated. This means that a filtration process needs to remove debris, sediment and pollutants before the water enters the tank. If the water is coming off rooftops and other non-parking areas, this may be as simple as passing the water through a vortex filter. Contaminants from parking areas need special treatment to remove sediment, oils, hydrocarbons and other pollutants. There are many biological and mechanical methods for this pretreatment that can include bioswales with plantings and soils engineered to remove specific contaminants. For large volumes of water, mechanical separators are a better option. One reference for additional information on treatment is the book Stormwater Treatment by Gary Minton
Stormwater stored in the Atlantis D-Raintank system can be retained for reuse by adding an impermeable liner to the excavation. Water quality is maintained in the cool and dark cistern and improved through capillary action and microbe interaction in the surrounding compacted material. A submersible pump is used to harvest the water for irrigation.
There are many systems suitable for storing treated stormwater for reuse. We like the Atlantis D-Raintank system because of its design flexibility, low cost compared to other options and its interactivity as a storage system. Sand is compacted around the sides and top of the storage tank between the permeable geotextile liner around the tank and the impermeable containment liner. Surface water can enter the tank directly by filtering through the compacted material above the tank, and water in the tank is able to interact with the compacted material on the sides. This action helps keep the water in the tank oxygenated and microbes in the soil layer act on organic material in the cistern to further purify the stored water. Several other methods of storage are available. Visit our Storage Systems link.
Stormwater detention ponds may seem like an inexpensive way to store stormwater, but they take up valuable real estate and create a hazard to people and pets.
By retaining stormwater below ground, recreational green areas or parking space can be freed up above. Water is kept cool, dark and vermin-free below ground – ready for harvesting
Retained stormwater can be harvested for a number of uses – irrigation is most common, but it can also be treated for flushing toilets, making up cooling tower or boiler water and other applications.
Water used for irrigation usually goes through an additional filtration step and is sanitized using an ultra-violet process if it to be used for surface irrigation. Water used inside building may be treated with chlorine as an additional safety measure. It must then be pressurized at a flow rate appropriate for the end-use. For more information on Wahaso’s harvesting systems see this link.
Wahaso is happy to talk with you about how we might help you convert your stormwater detention to retention and reuse through harvesting. Please Contact Us.