The Potential for Residential Greywater for Outdoor Irrigation in Davis

Residential greywater offers up a huge potential for our city to offset potable water use. When the next drought rolls around, and it will, we could be sitting pretty with healthy trees and landscapes using less water from the Sierra than we do now. How could we accomplish this? The answer is greywater, defined in California as the discharge from laundry wash water, showers, and bathroom sinks.

How much greywater does each of us produce each day?

According to the publicly available Urban Water Supplier report, the average Davisite used 57 gallons of water per person per day this past January, the season low; last summer the high in July was 145 gallons per person per day. Winter use is generally considered to be a measure of indoor use, since we aren’t usually irrigating outdoors then.

If you estimate that a bit over half of that 57 gallons used in the winter is what we use for laundry and showers, we each produce about 30 gallons of greywater every single day year-round on average. (Obviously, some of us use more and some use less.)

If every single one of us diverted the water we normally send to the wastewater treatment plant to our landscapes, we could slash our community level use.

How much do we use as a community?

This June, the City of Davis water utility supplied 71,311 individuals with a total community usage of about 9.4 million gallons of potable water per day (282.6 million gallons for the full month), not including water for commercial, industrial, and institutional uses (see link to source below: Urban Water Supplier Report). Do the math, and you find this equates to 132 gallons per person. That number includes outdoor irrigation in a month like June.

If 30 of those 132 gallons are greywater and we had all used that water for outdoor irrigation, we could have reduced that per capita number to roughly 100 gallons per person per day. Of course, those numbers change each month, gradually increasing and decreasing in response to temperatures and rainfall. Plus, not all dwellings have much in the way of landscaping, not to mention the challenge of installing those systems citywide.

Those (relatively hefty) challenges aside, if we use 30 gallons as a rule of thumb and multiply that by 71,311 individuals, we could have saved 64.1 million gallons of water this past June alone as a community.

Benefits abound

At garden scale, many mature fruit trees need about 20 gallons of water a week. So, 30 gallons of greywater, the quantity produced by one average Davis resident, can supply irrigation water to about 10 medium sized fruit trees year-round, fewer if they are large shade trees. If all wash, shower, and bathroom sink water is diverted, plenty of water is produced.

The benefits of reusing water at home for outdoor irrigation are myriad: offsetting potable water use, healthier shade trees, increased yield for food producing trees, groundwater recharge, reduced embodied energy use, and a bit of cost off your monthly bill. While it takes some research and certainly some costs to install a greywater system, the peace of mind you gain from putting to full use Earth’s second most vital resource – next to air – is invaluable.

The easiest system to install at home is called a laundry to landscape greywater system, L2L, for short. L2L systems require no permit from the City (although wouldn’t it be nice if we had a voluntary permit to keep track of our good works), can be installed by a reasonably handy DIYer, and, especially if you use salvaged parts, might run only a couple hundred dollars in parts. On the other end of the spectrum, a shower-based system could cost as much as $2,200 for design, parts, permits, and installation, unless it’s simply included as part of an overall remodel or new construction, for which costs would vary.

The outdoor portion of shower greywater systems is generally installed separately from the indoor portion. A plumber is needed for the indoor portion, while the outdoor portion lies within the domain of a landscaper. Remodels and new construction are the best time to start thinking about incorporating greywater into your home. Laundry to landscape systems can be installed on a retrofit basis in many homes as a lower cost workshop and a plumber is generally not necessary.

Trees need water

One of the lessons learned from our most recent drought was that trees need water. Unlike Bermuda grass, and some other hardy plants, trees don’t always come back after a stressful event like an extended drought. According to City of Davis Urban Forest Manager and Arborist Rob Cain, the City was removing about 130 trees annually before the drought, but in 2013 that number shot up to 293, then almost 400 trees in our town that had to be removed in one year due to the drought and lack of irrigation. To date this year, we’re back down to 148 removals. Greywater is a best practice with statewide code and local support that can provide the irrigation our trees need to stay healthy during drought.

Want more data?

Davis residents who want more detailed hourly and historical data about their water usage should sign up for Water Hawk through the City of Davis. Renters can use Water Hawk, too, with permission from their property owner.

Urban Water Supplier Reports can be found on the California Water Boards web page: Water Conservation Monthly Reports

https://www.waterboards.ca.gov/water_issues/programs/conservation_portal/conservation_reporting.html

Want trees?

The City of Davis and Cool Davis partner Tree Davis recently received funds to plant 1,000 new trees in our community. If you’re interested in participating in the Tree Davis Community Canopy program, visit this website to request a tree. http://www.treedavis.org/city-of-davis-community-canopy/

All trees will be planted in the front yard of properties, in accordance with the City’s tree ordinance. Recipients are responsible for watering and care of trees. Pruning or removal is the City’s responsibility.

Leslie Crenna is Certified Level 2 Greywater Designer through Greywater Action and participated in a QWEL USEPA WaterSense approved greywater training. She is a member of Water Wise Davis and has played a leading role in the three annual Greywater Showcase events held in our community. Her small business, EcoAssistant, is a member of the Cool Davis Coallition. Email solutions@ecoassistant.net for more information about greywater design and hands-on installation workshops or lesliecrenna@gmail.com for general advice and advocacy interest.