Residential Graywater Reclamation Systems

Last week, Erin Reilly wrote a piece on graywater reclamation and conservation that touched on a few new technologies making it easier for homeowners to save water and money. Since then, we’ve learned even more about these systems and the Water Legacy residential graywater reclamation system below. We’ve also spotted a nice video on the subject called “Why Flush Good Water Down the Toilet”, which features Water Legacy technology.

The Water Legacy Residential Graywater Reclamation System conserves potable water by recycling spent water typically released to the dwelling’s sewer discharge. Wastewater generated by residences can be classified into two categories: blackwater and graywater. While blackwater is contaminated to levels that prohibit residential reuse, graywater can be treated and stored for non-potable reuse. Water savings are achieved by using reclaimed water instead of potable water, where permitted. In the average USA household, this can account for 12-24,000 gallons per year in waste water alone!

A typical residence discharges blackwater from toilets, dishwashers, washing machines, and kitchen sinks. Graywater is discharged from showers, baths, and hand sinks. The amount of contaminants in graywater is far less than blackwater. The Water Legacy functions on the simple premise that graywater can be treated to a level safe enough for reuse, using a multi-barrier approach in non-potable applications, mainly toilet flushing. Residential users can save valuable potable water by flushing toilets with spent graywater that would otherwise be sent directly down the drain, despite still having beneficial use potential. The Water Legacy system treats graywater for reuse in toilets without exposing humans or pets to E. coli.

So how does this graywater treatment system work and how does it remove the E. coli bacteria that could make people sick? The Water Legacy consists primarily of a water filter, ultraviolet light disinfection system, hydrogen peroxide (H2O2) disinfection system, and a 55-gallon water storage tank.  Hydrogen peroxide and ultraviolet light are commonly used for wastewater disinfection.

Graywater is collected from showers, baths, and hand sinks via a graywater plumbing system (separate from the blackwater system). The water collected from the graywater plumbing is first filtered by the Water Legacy. The graywater is then dosed with a disinfecting agent; namely hydrogen peroxide, which disinfects graywater by releasing oxygen molecules to neutralize pollutants. Finally, graywater passes through an ultraviolet light disinfection system, which neutralizes living organisms by altering their DNA structure. Disinfected graywater is stored in a 55-gallon drum for reuse until a toilet is flushed, creating demand for the treated water. Treated water is piped to the toilet via a separate plumbing system. In the event graywater production exceeds demand, treated water safely overflows the storage tank to the dwelling’s sewer discharge. Residual hydrogen peroxide mixed in the treated water prevents contaminants from reactivating in the storage and/or toilet tanks. Treated water is also cycled through the ultraviolet light system prior to delivery to the toilet.

The showers in a typical newly-constructed home discharge an amount of water approximately equivalent to the water used for toilet flushing. Research shows that showerheads discharge approximately 10.33 gallons per person per day, and toilets consume 8.16 gallons per person per day. A typical dwelling unit has, on average, 2.62 residents. A typical dwelling unit can therefore save approximately 7,800 gallons of potable water per year by recycling graywater from the showers alone.

The Water Legacy is not intended to treat graywater to drinking water standards or for direct human contact; it is designed to treat graywater to a level that is hygienically acceptable for toilet reuse. Graywater has the potential to contain numerous water-borne pathogenic microorganisms, and possible transmission through inhalation, contact, or indirect ingestion by humans and pets has raised some concerns. In response, Water Legacy has evaluated the capability of hydrogen peroxide and ultraviolet light disinfection systems to reduce E. coli in the treated graywater (E. coli being an easy-to-measure indicator species). Accompanying presences of pathogens is a possibility when E. coli is detected.

A laboratory study was conducted by Christie Chatterley and Dr. Karl G. Linden of the University of Colorado at Boulder’s Department of Civil, Environmental, and Architectural Engineering. The laboratory study analyzed the effectiveness of the Water Legacy’s disinfection systems in reducing E. coli. Four principle subjects were evaluated:

  1. Does the hydrogen peroxide lose effectiveness over time?
  2. Does hydrogen peroxide kill E. coli in graywater?
  3. Does ultraviolet light kill E. coli in graywater?
  4. Does the combination of hydrogen peroxide and ultraviolet light kill E. coli?

The Laboratory Study concluded the following:

  1. Hydrogen peroxide does not lose effectiveness over time.
  2. Hydrogen peroxide decreases the amount of but does not completely eliminate E. coli.
  3. Ultraviolet light disinfection decreases E. coli below detectable levels.
  4. A combination of hydrogen peroxide and ultraviolet light disinfection is most effective at reducing E. coli.

The Laboratory Study also added that graywater examined in the laboratory likely has higher E. coli concentrations than graywater generated by homes.  Home-treated water will receive additional E. Coli reduction from the final pass through the ultraviolet disinfection system before discharge to the toilet; hydrogen peroxide residual will help prevent the reactivation of E. Coli in the Water Legacy-treated water storage and toilet tanks.

According to the 2009 version of the Uniform Plumbing Code (UPC), reclaimed water is defined as, “Nonpotable water that meets or as a result of treatment, meets federal requirements for its intended uses. The level of treatment and quality of the reclaimed water shall be approved by the Authority having jurisdiction.” E. coli removal standards for jurisdictions vary. The laboratory study result of non-detect meets the E. coli water reuse standard for reclaimed water reuse in Colorado (126 cfu per 100 mL).

Water Legacy strives to provide a cost-effective way to reduce potable water demand in our homes. The Water Legacy Residential Graywater Reclamation System satisfies that goal by capturing water that would typically be wasted, treating it, and returning it to our toilets for beneficial use.