Graywater and the Hydrology Cycle

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Graywater systems vary greatly depending on the context. A retrofitted system reusing water from the washing machine in an existing house will look completely different from a system designed during the planning phase in an apartment building. Be prepared to tweak it!

Design Considerations for Existing Buildings

What are the graywater system’s goals? Reuse the water in the most efficient way possible or keep it out of a failing septic system?

Site Considerations
Access to drain pipes
Some sources of graywater may not be accessible, such as those from a shower in a house with a slab-on-grade foundation (the pipes are buried in the cement).
Slope of the land
Flat and downward sloping sites usually provide options for gravity-based systems, while sites with uphill slopes need pumped systems. Factor in additional electrical costs when considering a system that includes a pump.
Drainage/Infiltration of the soil
Sites with sandy soils and quick drainage may not need to spread out the water as much as sites that have heavier clay soils.
Proximity to creeks and waterways
Graywater may not be used within 100 feet of a waterway in California. Other states may have different setbacks from waterways.
Quantity of graywater produced
The quantity of graywater produced will determine how many plants can be irrigated, and suggest the most appropriate type of system. If sufficient graywater exists to irrigate an entire landscape, a pressurized system may work best to reach all plants. If there is only enough water for a section of the yard, a simpler system with limited reach may be desirable.
Plant irrigation needs
How much irrigation does the existing landscape need? Does it match up with the amount of graywater produced? If the total need is greater than the amount of graywater produced on-site, are there options for transitioning to less water-intensive plants? If there is more than needed for the existing landscape, are there beneficial plants (such as habitat or shade plants or fruit trees) that could be planted to lower heating needs in the building?
Suitability for existing plants
Simple systems are best suited for irrigating larger plants, such as trees, shrubs, and perennials. Because lawns can be very expensive and difficult to irrigate with graywater they’re often not considered practical.
pH considerations
Graywater is typically basic, or alkaline, so acid-loving plants may not thrive with it. pH neutral detergents, such as liquid laundry detergents do exist, however. If the graywater is from a source with pH neutral products, any type of plants can be watered. If not, acid-loving plants should be avoided.
Desired maintenance level
Some types of systems require regular filter cleaning. If maintenance is needed more than once or twice a year, it’s unlikely that most homeowners will maintain the system properly. If used in nonresidential settings with an on-site facility manager, the chances are much higher that a filtered system is appropriate.
Permit needs and budget
A system requiring a permit will most likely have additional fees and requirements. Now, unfortunately, permit requirements may force the installer to compromise the system’s effectiveness. Some areas in California, such as Oakland, San Francisco, and Willits, have created streamlined permit processes for residential graywater systems Others, including Santa Cruz, San Diego and Santa Monica, may have high fees or unnecessarily complex requirements that make it difficult to install a well-functioning, affordable, and legal graywater system. Following the January 2010 adoption of the new CA Plumbing Standards, Chapter 16, Part A, which governs California residential outdoor systems, graywater permitting standards are currently under consideration in many municipalities.
Design Considerations for New Construction

There are many more options for graywater use when the system is incorporated at the design stage before building occurs.

  • Site the building on the high point of the property. This way, gravity will do the work of transporting graywater to the landscape without the use of electricity needed for pumping.

  • Make sure there is enough room in the crawl space or basement to access the graywater pipes.

  • If a pressurized system is needed, plumb all the graywater together to a centralized location where a surge tank and pump can be located.

  • If separate systems from various fixtures are a more functional option , plumb a graywater stub-out.

  • Design the landscape around the amount of graywater available. Choose plants that are appropriate to for graywater irrigation.