Rainwater Harvesting, Water Recycling & Onsite Water Reuse

It is becoming clear that one of the greatest challenges that the United States faces over the next decade will be ensuring an ample supply of water.
 
Oct. 2, 2009 - PRLog -- http://www.bnm-us.com

As the cost of providing potable water to the public continues to rise and with limited water resources it is becoming evident that the current model is not sustainable. Several European countries, parts of Asia, Australia, volcanic islands and coastal areas throughout the world often have no choice but to rely on rainwater harvesting and water reuse programs because of limited surface water and groundwater supplies which are often susceptible to saltwater intrusion. However the current situation in the United States is that the majority of the houses and businesses continue to use potable water for non-potable needs.  About 1/3 of all fresh water demand in the USA is for irrigation and this places a heavy burden on existing water treatment infrastructure.

There are 3 main sources of water for residential and commercial properties, the municipal or well potable supply, the rainwater falling onto the property catchment area and the wastewater discharged from the property. The historic approach has been to use the potable supply for all water uses, seek ways to divert the stormwater to prevent flooding of the property and to regard the used grey & black water as a waste.

Perhaps the easiest of these water conservation concepts to implement is the recycling of harvested rainwater which, although not a new concept, has become increasingly practical and affordable in residential and commercial applications.  The technology required to capture, filter, store and reuse rainwater has made significant advances worldwide as cost effective components have become available and a better understanding of the water quality requirements and definition of “fit for purpose” is provided by regulators.

Understanding the local regulatory requirements is an essential first step in implementing a water reuse system. US regulation for water reuse is very fragmented and managed at a state or county health department level. The definition of fit for purpose is important in balancing the health based risk assessments with the desire for an affordable solution. Many states are introducing standards that reflect this tiered approach to performance standards. An example of this is wastewater treated for use as irrigation water using a subsurface drip system which limits any health exposure risk compared to water recycled for use in toilets which poses a greater risk. The discussion of treatment standards for reclaimed water focuses largely on whether the treatment system is capable of consistently achieving an appropriate water quality, that is fail safe in operation and that is “fit for purpose”, which in turn is dependant on the exposure levels associated with the water use. It is becoming recognized that the key to ensuring safe and reliable reuse water is to implement multiple barriers which control microbiological pathogens and contaminants that may enter the water supply systems.

Having determined the regulatory requirements, deciding what the water will be used for, the quantities required, and determining the quantities of water available are the next steps in the design of a water reuse system.

Typical potable water demands which could be accomplished with non-potable water (harvested rainwater) include Landscape Irrigation, Agriculture needs, Industrial including Cooling Towers & Boilers, Fire Suppression / Protection, Laundry Washing, Pool / Pond Filling, Toilet Flushing and Vehicle Washing.

From the home or business owners perspective the system needs to be easy to maintain and have a reasonable return on investment. There are six fundamental components of a rainwater harvesting system.

1.  Catchment Surface: Determining whether the proposed project is a system retrofit or new construction, and identifying the type of catchment surface eg roof type are important considerations in system design and component selection.
2.   Gutters and downspouts: These channel water from the roof to ground level and need to be routed to a collection point on route to the storage tank.
3. Filters, Leaf screens, first flush diverters and/or roof washers:  These are components which remove debris, dust and dirt from the captured rainwater which can be either inserted into the downspouts, filter the water prior to the storage tank or be incorporated into the storage device.
4.  Rainwater Storage/Containment Device or Cistern: The typical storage tank size ranges    from 300 to 5,000 gallons for residential systems and 5,000 to 100,000 gallons and up for commercial systems. Most tanks are typically in a vertical cylinder or low-horizontal cylinder configuration. Depending on the site suitability storage tanks can be located above ground or below ground and are constructed of various materials including Plastic, Fiberglass, Pre-Cast Concrete, Metal or Wood.
5.   Re-use treatment/purification: Filters and other methods of treatment include but are not limited to screen mesh, sand filtration or membrane for non-potable reuse.     UV disinfection and/or reverse osmosis may allow potable reuse dependant on local regulation.
6.     Conveyance & System Controls: Typically either gravity fed or pumped to the end use.  Residential systems sometimes use gravity (above ground storage) or various types of pumps depending on the complexity of the system. It is important to ensure that the system functionality is consistent with local plumbing standards.  A pneumatic or remote sonic device that measures the water level in the tank is typically mounted either at the tank or in the building. System controls that monitor the water level in the tank and switch automatically to municipal supply and/or well water intake are common for larger more complex systems. This feature allows the water level in the tank to be maintained regardless of the amount of rain collected and can monitor the flow of total water used versus harvested water and replenishment water. This configuration requires a backflow preventer or an air gap to ensure communication does not occur between the harvested rainwater and the water mains.

Rainwater harvesting programs have been highly successful in European and Australian markets where rainwater harvesting is becoming widely used for toilet flushing and clothes washing. Where irrigation is a key use of water for a homeowner and supply is sufficient, a reasonable target would be to achieve a 70% reduction in potable water usage. Examples include school applications which have achieved close to 100% reduction in non-potable water consumption where rainwater is collected on the roofs and used in toilets, urinal flushing and wastewater is reused for subsurface drip irrigation on the playing fields which are automatically regulated using soil moisture tensiometers. This approach can also provide the nutrient requirements in these grassed or landscaped areas without the need for additional fertilizer.

Other water conservation solutions include incorporating membrane treatment into urban settings. The Solaire, Battery Park City, NY, is the first residential water reuse project in the United States. The water reuse system services a 293-unit residential high-rise apartment building. Of the more than 25,000 gallons per day that are recycled, 9,000 gallons per day are used to flush toilets, 11,500 gallons per day go to the cooling tower, and 6,000 gallons per day are used for landscape irrigation within an adjacent park and the projects green roof.

Residential scale membrane units are also being developed. Initiatives such as LEED certification and the National Association of Homebuilders “National Green Building Standard” are being promoted to rate buildings on their water conservation credentials which will continue to highlight the benefits of water conservation.

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Bord na Mona provides odor biofiltration, wastewater and water reuse solutions for municipal & residential applications including membranes, MBBR, SAF, peat biofilters and rainwater harvesting.
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