Greywater (Gray Water) Harvesting Systems

Greywater Harvesting Systems designed and built by Water Harvesting Solutions in USA & Canada


Greywater harvesting (also called graywater, grey water and gray water), is differentiated from other harvested water sources like rainwater or condensate in that it has already been “gently used”– usually as water from showers and sinks, but it could also be sourced from ponds, pool backwash or process rinse water. 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 and kitchen sink waste water is termed “black water” and is not normally considered for harvested water systems.

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.

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.

Complies with NSF/ANSI Standard 350 for Safety and Effectiveness

Water Harvesting Solutions utilizes a number of leading-edge filtration, sanitation and monitoring steps to bring the water to near-potable quality, eliminating health and aesthetic concerns while meeting the regulatory requirements of most communities. Our GW-Series of processing systems and filtration skids have been field tested and are compliant with the National Sanitation Foundation NSF/ANSI-350 Standard for commercial graywater processing systems. Learn more about NSF/ANSI-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.

Greywater Harvesting System Overview

Greywater Harvesting Skid, Graywater treatment train
Wahaso Greywater Treatment Skid – Brooks Range, AK


Greywater is collected from showers, baths and sinks through a plumbing system that is separate from toilets and urinals – which produce “black water”. Greywater should be treated almost immediately to stabilize it and prevent it from going septic. Untreated greywater should never be stored more than a few hours.

The first filtration step is designed to quickly stop biological activity and remove the larger particulates inherent in shower and sink discharges. This is accomplished in a settling tank stage, where chlorine or another oxidizing agent is added to immediately begin reducing bacteria in the greywater. After a set contact and settling time, the treated greywater is pumped through the GW-Series filtration skid.

For commercial systems, the next filtration stage uses a self-cleaning disk filter that can effectively handle hair and other greywater solids. Particulates greater than 80 microns in size are removed and sent to the sewer system. The next stage uses a multi-media filter that removes all particulates greater than 5 microns in size. The 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. Activated charcoal is the final filtration step, “polishing” the water to make it crystal clear and free of any odors. Treated water is then ready for reuse and stored in the Processed Water Holding Tank (PWHT) awaiting pressurization to flush toilets, irrigate landscaping or support evaporative cooling systems.

Water quality in the PWHT is usually maintained by adding a small amount of chlorine so that a residual will protect the water in the tank and downstream to fixtures. (See Sanitation below).


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.


Because of the higher contaminants inherent in greywater, residual sterilization capacity is valuable in keeping the entire system clean. Chlorine sanitation is the most common method to achieve this. Depending on the type of system, we will recommend chlorination delivered to the system in 50-gallon drums by a local chemical supplier. The chemical is similar to that used in a municipal water treatment systems.

Ultra Violet Sanitation

Our 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. If the water is cloudy, as is common in untreated greywater, U.V. is ineffective as a sanitizer.


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 Water Storage page.

All storage systems have a connection to a municipal source so that toilet flushing and other uses 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.

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 polyethylene tanks are rated for potable water even though we are not using this water for drinking purposes.


A repressurization system is required to move the greywater 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.

Greywater Control Systems, Harvesting System Controls, Wahaso Control System
Wahaso WCS-100 Control System with built-in BAS connectivity.


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 most building automatic systems and other alarm and condition monitoring including Modbus™ or BACNET™.

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.


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. These systems do need to be checked on a regular basis, with consumables replaced as needed and periodic cleaning of automated components. Please see our Maintenance Guidelines & Support page.

Equipment skids are built using industrial-grade UL listed 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.

To learn how greywater harvesting might be used in your building project, please Contact Us.