18 Sep Graywater Systems That Work

Design-Build-Live and Treehouse pulled off yet another fantastic educational networking event last night. This month’s speaker was Dr. Lauren Ross of Glenrose Engineering. She clearly outlined the compelling reasons for reusing graywater and shared specifics that will ensure the creation of a functional, permittable system.

What is Graywater? – Graywater comes from washing hands, washing clothes (diapers excluded) and taking showers. This effluent can be reused with very little, or no treatment. Its primary use is irrigation.

Why is Graywater Hot Now? – Over the last century or so wastewater treatment has made a huge positive impact on human and environmental health. However, we don't need to treat all of the water that goes down our drains in order to use it again, nor is it wise to use potable (drinking quality) water for irrigation purposes. The most obvious reason for graywater reuse in Central Texas though is our severe shortage of water supply. All of Austin's water storage is in two lakes – Lake Travis and Buchannan (rest of the Highland Lakes are constant level) – and they are at nearly historic low levels currently. Predictions of lake levels don't look good for the near future either. The City's consultant says lake levels will likely fall below levels from the worst drought on record (1940’s) by s summer 2015. And by 2019 we may not be able to physically withdraw from lakes anymore.

This means outdoor irrigation may in the next couple of years be absolutely prohibited no sprinklers and no hand-watering! Graywater use for irrigation is a huge opportunity. Estimation models show that the amount of graywater Austinites generate is about equal to the amount of water used in 2013 for all outdoor irrigation in the City.

Also, some say the economics don't work out because water rates are so low. However, if you consider value of landscape, food production and real estate value of trees, there are significant economic drivers.

How to Make It Work – Currently, there are only two graywater systems permitted within the City of Austin. In order to build a graywater system that is functional and permittable the following state and city regulations must be heeded:

• 400 gal /day effluent limit up to 9 bedrooms
• Provisions must be made to avoid cross-connection of pipes with potable water pipes
• Outflow must be under at least two inches of soil, mulch, or gravel
• No kitchen sink, toilets, nor diaper washwater effluent may go to graywater system
• No discharge to open surface (pond)
• No graywater irrigation of vegetables
• No pooling or runoff
• Certain ground slope requirements must be met
• No chlorinated products
• Limit amount of sodium discharged (no baking soda)
• No graywater systems allowed over Edwards Aquifer and Georgetown Limestone formation
• State law requires a surge tank, however best practices strongly discourage storing graywater

Typical System Design – For a three bedroom house with four occupants, daily effluent would be about 160 gal/day. Soil type does have an impact on how big an area you should spread the graywater over. However, a general rule of thumb is that 10 square feet /gal/day will maximize your irrigation and prevent runoff. So, an area of about 1,600 square feet for dispersing graywater outflow is pretty typical. Since the graywater is generally low pressure, a network of hoses with buried outlets serves to deliver the water to the irrigated area.