Home Gray Water Systems

Lauren Ross

The flow of water across soapy hands, the splashy fun of a bubble bath, or clothing swishing back and forth in the washing machine; all of these are gray water.  This water could nourish geraniums or flush the toilet, but instead we treat it like sewage.  We pump it through miles of pipe, screen, aerate, clarify, and chlorinate it, and then dump it into the river.  At the same time we pull fresh water from the river, draw down lakes, treat, chlorinate, and pump another batch of water just to keep those geraniums blooming.

On a water-rich planet, in an energy-rich culture, treating gray water like sewage is simple and safe.  But on a hot July afternoon in Austin, water levels in our lakes often plummet, while half of our domestic water is used just to keep lawns green.  Power plants are stretched to keep up with peak summertime demands – it takes 2 watt-hours of electricity to clean a gallon of water and pump it to our faucet.  Maybe it is time to try a better idea.

The average U.S. household uses about 70 gallons of water for each person every day.  More than half of that is gray water that could be reused onsite.  In the heat of summer, on a warming planet, we could craft a new relationship with this gray water.  It is simple and affordable; and also protects our garden soil, waterways, and aquifers.

The Value of Gray Water

The value of water is slippery.  Austin spends a lot of money to design, construct and maintain drainage systems to get rid of it during storms and floods.  As an essential life nutrient, after 8 to 14 days without it, the value of water is priceless.  In between, the value of water depends on how much we have, how easy it is to get it where we need it, and its chemical and biological purity.

Austin water arrives at our faucets relatively pure and with enough pressure to turn the sprinkler head and water the garden on the uphill end of our yards.  A typical home would generate about 3,000 gallons of gray water per month.  The cost for that amount of water on an Austin utility bill would range from $9 to $36, depending on the total amount of water used.  Most Austin water customers would save between $100 and $200 per year on their utility bills with a simple gray water system.

But the bigger value of gray water is coming into alignment with the reality that we live in a place without enough water; in a place that is heating up and drying up as a consequence of global warming. The value of that alignment is priceless.

A Simple Gray Water System

If you live on rocky soil, on top of the Edwards Aquifer, or close to a stream, creek, or wetland, you have a special responsibility to design and maintain your gray water system carefully.  Nutrients and chemicals in gray water make leafy green lettuces, sweet peaches, or tangy limes. In a stream, however, they make slimy green algae.  If you cannot apply gray water to at least 2 feet of soil and 50 feet from the nearest surface water, send it into the sewer.

All of the gray water should flow to your garden beds within 24 hours.  Store it longer than that and nutrients in the water will begin to smell.

Gray water may contain pathogens.  However, according to research of Center for Disease Control records since 1950, not a single instance of gray-water transmitted illness has occurred in the U. S.   Most pathogens only flourish in the narrow range of conditions found in the human body.  They are quickly overcome by the billions of bacteria in every teaspoon of healthy soil.  Keep gray water away from pets and children by distributing it underneath a layer of soil or mulch.

The gray water application area must be tailored to your gray water production.  The area should be big enough to allow gray water to soak into the soil without pooling or running off.  Make it too big, though, and you won’t meet the water needs of your plants.

Your gray water production will depend on which fixtures you can plumb to your garden, how many people use those fixtures, and their water use patterns.  For example, a typical published number for washing machine gray water is 15 gallons per person per day. Your actual use, though, might be much less.  A top-loading washer uses about 40 gallons per load; a front-loading washer about 20.  Count your washloads in a typical week, multiply the number of loads by the water use of your machine, and divide the total for the week by 7 days to get the daily value.  Collecting individual data on showers and baths is more difficult.  For these fixtures, it is easier to use the published water use value of 25 gallons per person per day.

The gray water application area must also be tailored to the type of soil receiving it.  The best way to figure the size of your application area would be to have the soil tested.  (Two testing labs are listed on page 37.)

Absent a test, you can size your system based on the type of soil.  A sandy clay soil can absorb 100 gallons each day in an area of 60 square feet.  A heavy clay soil will need an area of 120 square feet to absorb the same volume.

The gray water system should be designed with a three-way valve to send gray water to the City wastewater system when the ground is frozen or during a long rainy spell when water is pooling on the ground.

Carefully consider any gray water system with pumps or filters.  Unless you are an astronaut in space, the water you save may not be worth the money or environmental cost to install and maintain the system.  If you cannot use gravity flow, consider hand-carrying buckets, or building an outside shower.

Take care with soaps and detergents. Sodium and boron in laundry detergent are plant nutrients; but at high concentrations they damage soil and slow the growth of plants.  Liquid laundry detergents are generally lower in sodium than powders.

Chlorine bleach is definitely damaging for gray water applications.  Remember those billion helpful bacteria in every teaspoon of soil?  Chlorine will kill them.  If you must use chlorine bleach, divert the gray water to the sewer.

Gray water tends to be alkaline rather than acidic, like Austin soils.  Our landscapes tend not to have acid-soil loving plants like azaleas, camellias, rhododendrons, and blueberries.  But if you have these plants and you want to water them with gray water, you will need to be particularly careful with the soaps you use.  Stay away from bar soap, which tends to be more alkaline than liquid soap.

Do I Need a Gray Water Permit?

Texas, Arizona and New Mexico are leaders in reasonable gray water regulations.   Texas law allows residential gray water systems for up to 400 gallons of water per day that meet simple requirements to be installed without a permit.   Within city limits, however, gray water systems typically fall under cities’ plumbing code ordinances.  One city leader in the permitting of residential gray water systems is San Francisco, California. Laundry-to-Landscape gray water systems can be installed there without a permit.  San Francisco also provides resident-friendly design information for both permitted and unpermitted gray water systems.

Until recently, gray water permitting in Austin was not that easy due to impractical requirements in the plumbing code and a bureaucratic permit process.  However, recent code amendments have eliminated the impractical and inefficient design requirements.

Austin now provides a One-Stop gray water permitting process.  Residential customers will not have to interact with all of the different City departments with an interest in gray water regulation.  Instead, Austin residents should just dial (512) 974-2199 say the words “gray water” and be transferred to the proper city staff person to guide you, as if on a magic carpet, through the process.  So go ahead and call, and begin your gray water adventure!

For More Information

Video showing a laundry-to-landscape installation from “This Old House”: https://www.youtube.com/watch?v=oNNnhCJGY38

San Francisco Graywater Design Manual – www.sfwater.org/modules/showdocument.aspx?documentid=55

Oasisdesign.net – Information about graywater around the country, design plans, and videos for Laundry to Landscape.

Dr. Lauren Ross, with Glenrose Engineering, is a consulting engineer in Austin.

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