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Greywater harvesting (also called graywater, grey water and gray water), is differentiated from other harvested water sources like rainwater or condensate in that is has already been "gently used"-- usually as water from showers and sinks, but it could also be sourced from the rinse water in a commercial washing machine or dishwasher. It is most often harvested to flush toilets in a building, but the cleaned water can also be used for irrigation and other applications. Toilet waste water is termed "black water" and is not normally considered for harvested water systems.
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The most important advantage of greywater over rainwater for harvesting is that in a residential building - where there are lots of uses of showers and sinks - the greywater provides a constant supply of harvested water for flushing toilets. Our experience is that the greywater supply 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.
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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 to "black water", resulting in unpleasant odors, colors and health hazards if not treated correctly.
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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 aesthetic concerns while meeting the regulatory requirements of most communities.
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The following section provides a general overview of the system components of a commercial greywater harvesting system from Wahaso.
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Schematic for Wahaso's proprietary greywater ultra filtration system. In this high-capacity 25 GPM system, two of these systems provide redundant filtration as back-up to ensure uninterrupted potable water supply.
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| FILTRATION |
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Greywater is routed from showers, baths and sinks through a plumbing system that is separate from toilets and urinals – which produce black water.
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• The first filtration step is designed to quickly remove the larger particulates inherent in shower and sink discharges. A settling tank will naturally separate the heavier and floating material where it can be sent to the sewer system. We like this option because the cleaning step occurs automatically, saving maintenance efforts and costs
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• A duplex basket strainer can also serve the purpose of pre-filtration. Typically 1/16” screen baskets remove large particulates, and redundant strainers ensure constant filtration and easy cleaning. When the differential pressure rises to a changeover point, the duplex baskets can be alternated to a fresh basket while the first chamber is cleaned and made ready for its next cycle.
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• Depending on the final usage of the harvested water, additional filtration is provided to further clean the water. Bag filters can be used to reduce the particulates to 100 microns or less as a final filtration step, or to reduce the burden on the more sophisticated final filtration step.
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Wahaso's proprietary ultra-filtration skid cleans greywater to .02 microns. This is a small system capable of delivering 1.25 GPM, but can be scaled to any output requirement. |
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• When a high standard of cleanliness is required, Wahaso uses a proprietary step. Capillary tubes remove particulates down to .02 microns which is below the level of many pathogens. We believe ultra filtration is better than sand filters because it is both more effective at removing smaller particulates and more efficient in overall water savings.
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| STERLIZATION |
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It is necessary to sterilize the water to keep algae, viruses, bacteria and other organic contaminants from forming in the storage tanks. There are several technologies available for this purpose.
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• Chlorination. Because of the higher contaminants inherent in greywater, residual sterilization capacity is valuable in keeping the entire system clean. Chlorine sterilization is the most common method to achieve this. For Wahaso projects, we prefer chlorination using calcium hypochlorite in the form of solid briquettes. 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.
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Harvested water can be sterilized in a number of ways. This unit injects sodium hypochlorite into the system and constantly monitors levels. |
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Hypochlorite briquettes are added to a hopper in the dosing unit where they are combined with water to make a high concentration chlorine liquid. An automatic free chlorine analyzer continuously monitors the parts per million (ppm) level and proportionally controls a chemical injection pump to dose in more chlorine as necessary to maintain the preset level.
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• Other sterilizing agents that can be used include Ozone and Chlorine Dioxide. These sterilizing agents are very powerful and are typically only used on greywater systems. They are more hazardous to handle and require on-site generation of the Ozone or Chlorine Dioxide gas plus an injection system.
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New chlorine dosing technology uses dry calcium hypochlorite briquettes and patented spray technology to produce fresh liquid chlorine solution as needed. |
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• Some greywater systems use ultraviolet sterilizers that expose the water to a specific wavelength of UV light that destroys the DNA of organisms present and keeps the water sterile. Sizing of the UV system is critical to maintain sufficient exposure rates to keep 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. It's 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.
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| STORAGE |
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Storage of the treated greywater is determined by the demand and uses for the water, available greywater volume and turnover frequency, and space to locate the tanks. For more information on the options for storing rainwater and greywater, see our
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• 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 should a supply problem develop in the greywater harvesting system. In typical applications of a greywater system for toilet flushing, there is more than enough supply of greywater from showers and baths to meet flushing requirements.
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• 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 tanks are NSF-61 rated for potable water even though we are not using this water for drinking purposes.
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Storage capacity is matched to the supply and demand for greywater. |
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• Larger underground storage tanks are usually considered when the system also incorporates rainwater from roofs and parking lots. These systems are typically only considered for water intended for irrigation.
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| APPLICATION OF GREYWATER |
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A repressurization system is required to move the rainwater to toilets or 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; if a high demand situation occurs, both pumps can be made available. And 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.
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Duplex pump skid provides required pressure and GPM for the harvested water to toilets, irrigation or other applications.
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| GREYWATER SYSTEM MONITORING AND CONTROL |
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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 web page for maintenance or educational purposes. Wahaso can use the remote access to help a building maintenance staff diagnose potential system problems.
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| Sophisticated systems monitor and control the harvesting and storage process. These systems can be integrated into the building's automation systems. |
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| Ongoing Maintenance |
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Wahaso's greywater harvesting systems are engineered to meet the rigors of commercial and institutional use. Fully automated, Wahaso's greywater harvesting systems are designed to operate independently and efficiently. The ultrafiltration membranes will consistently produce high quality water with automated self-cleaning cycles. 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 qualified plumber or maintenance staff.
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| To learn more about how ultrat filtration works, And to lean how greywater harvesting might be used in your building project, please |
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