Graywater Reuse Isn’t Always As Simple And Safe As It Sounds.

The procedure of separating graywater from black water and sending it out to the yard isn’t always as simple as it seems.

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QUESTION: What is graywater and should I use it on my garden?

ANSWER: This is a question I hear more often these days. I do quite a bit of work in the southwestern part of the country – Arizona, New Mexico and Nevada. It is not surprising that the question of graywater reuse is raised in these areas due to regional long-term drought conditions over the past decade. Graywater is water generated in a residence from the bath, sink, shower and washing machine.

Through the years, separating graywater from the black water, which includes toilet waste, has been talked about as being beneficial for the septic system in terms of reducing overall flow into the soil treatment area. Unfortunately, many homeowners are working under the misguided assumption that since graywater does not include urine and feces, it is somehow benign and does not need to be treated before it is released into the environment. This led in many places to homeowners looking to reroute graywater from the septic system to another point in the backyard. In my state, that was to the road ditch or behind the lilac trees in the tall grass. In the Southwest it is under the Palo Verde trees and into the dry wash or arroyo.

My answer to using or putting graywater elsewhere is that it needs to be treated before being discharged into the environment. Even though the graywater does not include toilet waste, it still has a significant number of fecal coliform present to create the potential for disease transmission. In septic tank effluent, there are roughly a million colony-forming units of fecal coliform bacteria per 100 milliliters of sewage. In graywater, the number is lower but still in the neighborhood of 100,000 or more. In addition, there are solids and other organic material that can cause odors if discharged directly to the environment. So in my view direct discharge is out of the question.

READ THE SMALL PRINT

When I am giving talks on this subject in Arizona, someone in the audience always points out that because of the water situation in Arizona there are tax credits for using graywater, and there is no special permit required if the flow is less than 400 gallons per day. I have not looked deeply into requirements in the neighboring states of New Mexico and California, but at least on the surface they are similar. These people are right in their comments, but if you look closer at the Arizona rules, you find some interesting things.

There is a fairly lengthy list of requirements for the use of reclaimed or graywater and most people are not aware of them. I am not going to list all of them but will discuss a few that I think are important to the consideration or use of graywater to water the garden.

Graywater used for irrigation is generated within the residence on the property and is used for irrigation of the lawn, landscaping and garden, and it must be kept on the property so there is no runoff onto other properties or discharging into the arroyo. There are also similar requirements in terms of separation distance to groundwater and other setbacks as required for placement of the septic system components.

Nothing other than graywater generated by the residence with characteristics of domestic sewage should be added. So it’s the same prohibition on hazardous materials as for septic systems. No solvents, no paint, no oils from petroleum products, no car wash water are allowed.

When used for irrigation, graywater is managed so there is not any surface ponding, and any tanks used to collect and distribute the graywater for irrigation are covered to prevent odors and breeding of mosquitoes. Surface-applied graywater cannot be put on any food crops other than citrus trees, so it would not be used on a vegetable garden.

ON THE SURFACE

The southwestern states require systems that allow routing graywater back into the septic system if the need arises. Septic system designs do not give credit for the reduction of flow caused by diverting graywater. Systems need to be sized with the assumption that graywater will be routed through them. Also, graywater is required to go through a filtration system to remove any solids.

Graywater applied by surface irrigation must not contain water used to wash diapers or similarly soiled or infectious garments unless the graywater is disinfected before irrigation; and surface irrigation by graywater is only by flood or drip irrigation, spray irrigation is not allowed. So since washing clothes will involve soiled or infectious garments – such as underwear – graywater will need to be disinfected before it is used for irrigation if applied to the surface. This can presumably be avoided if it is applied subsurface through drip irrigation.

Needless to say, these requirements change the discussion dramatically. So while recycling household graywater for gardening – other than vegetables – and landscape use is an excellent way of saving water, it cannot be done without the expense of putting in place the components to provide for treatment and safeguards to public health and the environment.

In more humid and frigid areas such as Minnesota, putting in place a graywater system for irrigation would require the ability to switch back and forth on a seasonal basis and would require draining the tanks and irrigation system the same as a lawn irrigation system.



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