Part 3: Use Graywater to Irrigate Landscape Plants

Irrigating landscapes with gray water is fairly to the mainstream population of California. This article will define what gray water is and discuss how it can be used to irrigate the landscape while conserving water and answer questions about irrigation water sources on the California Supplemental Exam for landscape architects.

This article is the third in a series on irrigation water. If you missed the first two articles, you can find them here:

Gray Water Irrigation Has Finally Gone Mainstream

You don’t have to be a hippie in Humboldt County anymore to love irrigating with graywater. California’s recent severe drought finally encouraged the state’s political leaders to adopt common-sense graywater regulations. Landscape architects can now legally design graywater irrigation systems.

What is Gray Water?

Gray water is untreated household waste water which has not come into contact with toilet waste. Gray water includes used water from bathtubs, showers, bathroom wash basins and water from clothes washing machines and laundry tubs. It shall not include waste water from kitchen sinks, dishwashers or laundry water from soiled diapers. Gray water is different than black water since black water contains human waste. Black water is never safe to use for irrigation and is not included in this discussion.

Summary of California Gray Water Regulations

Gray water is untreated household waste water which has not come into contact with toilet waste. Gray water includes used water from bathtubs, showers, bathroom wash basins and water from clothes washing machines and laundry tubs. It shall not include waste water from kitchen sinks, dishwashers or laundry water from soiled diapers.

Gray water is different than black water since black water contains human waste. Black water is never safe to use for irrigation and is not included in this discussion.

California recently added gray water regulations to the California Plumbing Code. Permits are required for gray water systems that produce more than 250 gallons per day. The following rules apply to utilizing gray water for residential landscape irrigation:

  • No permit is needed for a washing machine system if the system follows health and safety guidelines outlined in the California Plumbing Code
  • No permit for a single fixture (one shower), if guidelines are followed
  • Mulch, gravel, or soil basins are allowed for covering release points
  • Depth of discharge is 2” under mulch
  • Systems must minimize contact with humans and pets
  • Water containing hazardous chemicals or used to wash diapers or infectious garments cannot be used
  • Water cannot come from kitchen sinks or toilets

Are There Limitation on Gray Water Use for Irrigation?

There are a few safety regulations that landscape architects must follow when designing gray water systems. Some limitation on gray water use include:

  • Gray water shall not be used in spray irrigation
  • Gray water allowed to pond or runoff and shall not be discharged directly into or reach any storm sewer system or any surface body of water
  • Human contact with graywater or the soil irrigated by graywater shall be minimized and avoided, except as required to maintain the graywater system
  • Graywater shall not be used to irrigate root crops or edible parts of food crops that touch the soil

Two Types of Gray Water Systems

Systems are separated into simple and complex depending on the quantity of water and there are fewer requirements for simple systems.

Complex Gray Water Systems

Complex gray water irrigation system are defined as  “gray water systems that discharge over 250 gallons (947 L) per day. Complex systems shall require a construction permit, unless exempted from a construction permit by the Enforcing Agency. The Enforcing Agency shall consult with any public water system (as defined in Health and Safety Code, Section 116275) providing drinking water to the dwelling before allowing and exemption from a construction permit. A complex system shall be designed by a person who can demonstrate competence to the satisfaction of the Enforcing Agency.”

Simple Gray Water Systems

A simple system is a gray water system serving a one- or two-family dwelling with a discharge of 250 gallons (947 L) per day or less. Simple systems exceed a clothes washer system and/or a single fixture system. Simple systems shall require a construction permit, unless exempted from a construction permit by the Enforcing Agency. The Enforcing Agency shall consult with any public water system (as defined in Health and Safety Code, Section 116275) providing drinking water to the dwelling before allowing and exemption from a construction permit. The design of simple systems shall be acceptable to the Enforcing Agency and shall meet generally accepted graywater system design criteria.

Gray Water and Ground Water

Somethings just don’t mix. Gray water and ground water are two types of water that should stay apart.

Gray water cannot come in close contact with ground water. The plumbing code states, “no irrigation or disposal field shall extend within three (3) vertical feet (915 mm) of the highest known seasonal groundwater, or to a depth where graywater contaminates the groundwater, ocean water or surface water. The applicant shall supply evidence of groundwater depth to the satisfaction of the Enforcing Agency.”

Using Gray Water In The Landscape

There are three ways that gray water can be used to irrigate the landscape:

  • Mulch Basin
  • Irrigation Field
  • Disposal Field

Mulch Basin

Gray water can be used for irrigation if directed to a mulch basin. A mulch basin is a method of using gray water as flood irrigation. As defined by the California Plumbing Code, “a mulch basin may be used as an irrigation or disposal field. Mulch basins shall be sized in accordance with Table 16A-2 and of sufficient depth, length and width to prevent ponding or runoff during the graywater surge of a clothes washer, bathtub or shower. Mulch must be replenished as required due to decomposition of organic matter. Mulch basins will require periodic maintenance, reshaping or removal of dirt to maintain surge capacity and to accommodate plant growth and prevent ponding or runoff.”

Irrigation Field

An irrigation field uses gray water to irrigate plantings using a pressurized drip irrigation or micro-sprinkler system. Gray water is collected in a tank. When irrigation is needed, a pump pressurizes the irrigation system and delivers the water to the landscape. The irrigation equipment used for gray water irrigation fields is similar to a typical drip irrigation system design and installation. The plumbing code requires “filters used in graywater irrigation systems shall be as specified by the manufacturer’s installation instructions for the design flow rate and intended use. The filter backwash and flush discharge shall be contained and disposed of into the building sewer system, septic tank or, with approval of the Enforcing Agency, a separate mini-leachfield sized to accept all the backwash and flush discharge water. Filter backwash water and flush water shall not be used for any purpose. Sanitary procedures shall be followed when handling filter backwash and flush discharge or graywater….The system design shall provide user controls, such as valves, switches, timers and other controllers, as appropriate, to rotate the distribution of graywater between irrigation zones….All drip irrigation supply lines shall be polyethylene tubing or PVC Class 200 pipe or better and Schedule 40 fittings. All joints shall be properly solvent-cemented, inspected and pressure tested at 40 psi (276 kPa), and shown to be drip tight for five minutes, before burial. All supply piping shall be covered to a minimum depth of two (2) inches (51 mm) of mulch or soil. Drip feeder lines can be poly or flexible PVC tubing and shall be covered to a minimum depth of two (2) inches (51 mm) of mulch or soil….Where pressure at the discharge side of the pump exceeds 20 psi (138 kPa), a pressure-reducing valve able to maintain downstream pressure no greater than 20 psi (138 kPa) shall be installed downstream from the pump and before any emission device….Each irrigation zone shall include a flush valve/antisiphon valve to prevent back siphonage of water and soil.”

Disposal Field

Gray water not used directly for irrigation can be directed into a disposal field which works like a septic leach field. California’s plumbing code dictates, “disposal systems shall be not less than three (3) inches (80 mm) in cross sectional dimension and shall be constructed of perforated high-density polyethylene pipe, perforated ABS pipe, perforated PVC pipe, leaching chambers or other approved materials, provided that sufficient openings are available for distribution of the graywater into the trench area. Material, construction, and perforation shall be in compliance with the appropriate absorption fields drainage standards and shall be approved by the Enforcing Agency. (B) Filter material, clean stone, gravel, slag, or similar filter material acceptable to the Enforcing Agency, varying in size from three-quarter (3/4) inch (19.1 mm) to two and one-half (2-1/2) inches (64 mm) shall be placed in the trench to the depth and grade required by this section. The perforated section shall be laid on the filter material in an approved manner. The perforated section shall then be covered with filter material to the minimum depth required by this section. The filter material shall then be covered with untreated building paper, straw, or similar porous material to prevent closure of voids with earth backfill. No earth backfill shall be placed over the filter material cover until after inspection and acceptance.”

Estimating Daily Gray Water Output

Before you can design an irrigation system using gray water, you need to determine how much gray water the residence will likely produce. Here are some guidelines from the plumbing code to use when sizing a system in the design phase:

  1. Determine the number of bedrooms in the dwelling. The plumbing code assumes that two people occupy the first bedroom and one additional person is present for each additional bedroom. For example, a three bedroom house would theoretically have four people. So, the gray water system is designed for four people.
  2. Now, determine what gray water sources will be used for the system.

Showers, bathtubs, and wash basins are assumed to produce 25 gallons per day/per person. So the family of four will produce 100 gallons of gray water from these sources.

Laundry gray water is estimated as 15 gallons per day/per person. Our theoretical family of four produces 60 gallons of laundry gray water per day.

Once you have estimated the quantity of gray water produced daily, you can determine what type of system can effectively and safely use the gray water in the landscape. gray water regulations are fairly new and changing frequently. Check the plumbing code regularly for updates and changes to the regulations listed above.

The Future of Gray Water

With demand for water growing and the state’s water resources shrinking, gray water use is likely to increase in the foreseeable future.

In conclusion, three types of water are commonly used for irrigation in California. Potable water is the most common and precious type of water. Recycled water and gray water can also be safely used for irrigation in safety precautions are followed and the systems are properly designed.

 

Note: The quoted text was taken directly from the latest version of the California Plumbing Code.

About

John is a landscape architect who is currently preparing to take the California Supplemental Exam to become licensed in California. He is currently a licensed professional landscape architect in Georgia and Florida. John graduated from California State University, Pomona with a BSLA degree in landscape architecture in 1997 and has extensive practice experience in residential and commercial landscape design.

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