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Greywater Harvesting

Greywater (also referred to as grey water, graywater and gray water), is defined as any water that has been “gently used”– usually from showers, sinks and rinse water from washing machines or dishwashers. It should not be confused with black water—wastewater and sewage from toilets—which is not typically considered for reuse. The most common use for greywater harvesting is for toilet flushing, but it can also be applied to irrigation, cooling tower make-up and other uses, depending on a building’s needs.

Greywater harvesting is distinct from rainwater harvesting in that it utilizes a more constant and predictable supply of water for on-site reuse. Residential buildings typically benefit the most from greywater harvesting because showers and sinks are used most often, providing a greater supply of water for reuse. Our experience is that the greywater supply in a residential building can usually meet 100% of toilet flushing requirements. And because the supply is steady and predictable, (unlike rainwater!), the storage requirements are dramatically less, saving storage space and costs.

Harvesting greywater is a relatively new practice in commercial and institutional buildings, and carries many system and regulatory implications not associated with rainwater or condensate harvesting. Unlike other renewable water sources, greywater normally contains biological and chemical contaminants that can quickly turn the water to septic “black water”, resulting in unpleasant odors, colors and health hazards if not treated correctly. For this reason, greywater systems are much more complex in their design.


Patent-Pending Design and NSF-350 Certification for Safety and Effectiveness

Water Harvesting Solutions utilizes a number of leading-edge filtration, sterilization and monitoring steps to bring the water to near-potable quality that is able to eliminate any health and esthetic concerns while meeting the regulatory requirements of most communities. Our GW Series of processing skids is patent pending and currently being certified under the new National Sanitation Foundation 350 Standard for commercial graywater processing systems. Learn more about NSF-350 Certification here.

It is important to note that the term “greywater” refers to untreated water from showers and sinks. Once the greywater has been filtered and sanitized – its classification changes from “greywater” to “on site treated non-potable water” and it is then safe to store and utilize for many purposes. You can see examples of installed greywater systems that we designed and built on our Projects Page.


The following section provides a general overview of the system components of a commercial greywater harvesting system from Wahaso.



Greywater is collected from showers, baths and sinks through a plumbing system and treated immediately to stabilize it and prevent it from turning into black water.


The first filtration step removes the larger particulates inherent in shower and sink discharges with a settling tank, a large mesh screen or a bag filter.


The next filtration stage uses a multi-media filter that removes all particulates greater than 5 microns in size. This filter minimizes maintenance by automatically going through a backwash step that flushes debris to the sewer system and the resets the multiple levels of filtration media.


Our greywater systems also inject liquid chlorine into the greywater stream to immediately kill pathogens that could create odors and foul the filtration system. (See Sterilization below).

Wahaso’s proprietary greywater processing system is efficient and effective in filtering and sanitizing greywater to make non-potable water safe for toilet flushing and irrigation.



It is necessary to sterilize filtered water to keep algae, viruses, bacteria and other organic contaminants from forming in the storage tanks. There are several technologies available for this purpose, and we use different methods depending on a project’s specific needs.



Chlorine sterilization is the most common method to keep the harvesting system clean. Our team prefers chlorination using calcium hypochlorite in the form of solid briquettes because the chemical is similar to that used in a municipal water treatment systems but is in a form that is safer and easier for building maintenance staff to handle.


UV Sterilization

Some greywater systems use ultraviolet sterilizers that expose the water to a specific wavelength of UV light, destroying the DNA of


Wahaso’s chlorination system—WCS-100—injects liquid chlorine solution into the treatment stream and monitors residual chlorine levels to ensure that harvested water is safe for application.

organisms present and keeping the water sterile. The advantages of UV sterilization are that it uses no chemicals to kill pathogens, requires minimal maintenance and is significantly less expensive than chlorination systems. Its biggest disadvantage is that the UV light can only kill pathogens that are directly exposed to the tubes – and there is no residual killing capability like chlorine. That can lead to fouling of filters and growth of bacteria in other parts of the system.


Storage of the treated greywater is determined by its demand and uses, available greywater volume and turnover frequency, and space to install the tanks.


All storage systems have a connection to a municipal source so that toilet flushing can occur even if there is not an adequate supply source of greywater. An air gap inlet can automatically be turned on to make-up water in the treated tank in case a supply problem develops in the system.

Wahaso tank systems can be pre-mounted on skids for ease of installation with all internal piping manifolds and sensors mounted and pre-tested at our fabrication center. All of our


Storage capacity is matched to the greywater supply and demand.

tanks are NSF-61 rated for potable water quality, even though we are not using this water for drinking purposes.


A repressurization system is required to move the greywater to toilets and other applications throughout the building. In commercial systems, pump skids are duplex, with two identical commercial grade pumps in tandem with each rated at 70% of peak demand. The operating system alternates demand between the two pumps, and if a high demand situation occurs, both pumps can be made available. If one pump should fail, the system will continue to provide harvested water.


We work closely with building engineers to properly size the pumps so that adequate pressure (PSI) and volume (GPM) is available at the farthest – and highest altitude – end point in the system.


Duplex pump skid provides required pressure and GPM for the harvested water to toilets, irrigation or other applications.



Water Harvesting Solutions employs programmable logic controllers (Allen-Bradley as standard) that fully automate and control the entire process for greywater harvesting. Our proprietary software is customized for each application and provides the capability of interfacing with building automatic systems and other alarm and condition monitoring.

In addition to monitoring the system mechanicals, the control system can track the amount of water in each tank and track and display the monthly amount of water harvested. An interface allows remote monitoring via a


Sophisticated systems monitor and control the harvesting and storage process. These systems can be integrated into the building’s automation systems

web page for maintenance or educational purposes. Wahaso can use the remote access to help a building maintenance staff diagnose potential system problems.


Fully automated, Wahaso’s greywater harvesting systems are designed to operate independently and efficiently while meeting the rigors of institutional building use. The chlorination system is easy to operate and maintain utilizing NSF 61 approved dry chlorine pellets. Equipment skids are built using industrial-grade UL and NSF approved components. We warrant the entire system for one year. Should a component need replacement beyond that period, it can usually be replaced with a readily available part by any competent plumber or maintenance staff.

If you would like to download an informational sheet on our patent-pending greywater system, please click here. To learn how greywater harvesting might be used in your building project, please contact us.

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