Washington State Greywater Code

Rather than repeat the whole code here (it is 15 pages long!), here is the link:

Chapter  Greywater Reuse for Subsurface Irrigation

It is set for enactment July 2011.

I have paraphrased som of the code in order to reduce space requirements.

As in California, “ Nothing in this chapter prohibits the adoption and enforcement of more stringent regulations by the local health jurisdiction”.

Light greywater and Dark greywater are defined. Dark greywater may only be used with a Tier 3 treatment system.

“Subsurface irrigation” means applying greywater below the surface of the ground directly into the plant root zone.

The greywater may be used for subsurface irrigation of plants that produce food but must not come into contact with edible portions of any plant.

This is a three tier code.

Tier 1: A gravity fed irrigation system, maximum 60 gallons per day. NO pumps.

No permit is required. However two separate Tier 1 systems (i.e. a total of 120 gallons per day) can be installed—the local authority may require a permit if two Tier 1 systems are installed.

Tier 2: A pumped, non treatment graywater system, up to 300 gallons per day.

A permit is required:

Tier 3: A pumped, graywater treatment system, under 3,500 gallons per day.

Tier 1: Gravity system

The greywater is light greywater.
The total flow of greywater is 60 gallons per day or less.
The greywater originates from a single-family residence.
The single-family residence is served by either a public sewer system or an on-site sewage system.
The greywater is diverted to the subsurface irrigation system through a single diversion point. Flows from fixtures located close enough to each other to be diverted through a single diversion point may be combined.
The greywater is delivered through the irrigation system by gravity distribution and no pumps are used to convey the greywater.
The greywater is not stored.

The total minimum irrigation area available to receive the greywater is adequate based on a calculation of:

  1.  the estimated volume of greywater;
  2. (the evapotranspiration rate in inches per week for the geographic area of the state where the landscape or garden is located; and
  3. the water requirements of the plants, known as a plant factor. A “Greywater System Checklist and Irrigation Area Estimation Tool” is available from the Washington State Department of Health‟s website, [website address to be inserted here].
The system is covered by at least four inches of appropriate material which may include suitable soil or other material such as mulch, humus, or compost. If material other than suitable soil is used, the irrigation field is augmented periodically as needed to maintain adequate cover during the growing season.
The homeowner ensures that the system is properly operated and maintained.
The homeowner maintains a record of the system that shows the location of the system, identifies the fixture or fixtures that are the source of the greywater, describes the system design and the system‟s maintenance requirements, and includes the calculation of the total irrigation area required under subsection (h) of this section.
The homeowner may direct greywater to separate irrigation fields so long as the total flow of greywater to all fields combined does not exceed 60 gallons per day.

This the first code I have seen that requires an evapotranspiration rate & plant factor calculation , and that even a simple gravity system must irrigate a correctly sized area. What is doesn’t do is specify (yet) what the required minimum irrigation efficiency is.

Later in the code, Tier 2 & 3 systems assume a 90% irrigation efficiency.

Tier 2: Pumped untreated graywater, under 300 gallons per day

  • A permit is required, unless a qualified professional designs a Tier 2 system for a single family residence.
  • Must be installed by a qualified professional, except the local health official may allow a resident owner of a single-family residence (not adjacent to a marine shoreline) to design a system, or the health officer may design the system if the office performs the soil and site evaluation.
  • “Qualified professional” means an on-site sewage treatment system designer licensed under chapter 18.210 RCW or a professional engineer licensed under chapter 18.43 RCW who is knowledgeable in irrigation system design.
  • Irrigation rates shall be calculated by using the following equation
Estimated Water Use in gallons per day =
Irrigation Area x Evapotranspiration x Plant Factor x 0.62 divided by Irrigation Efficiency x 31 days per month,where:

Irrigation Area equals the number of square feet to be irrigated;

Evapotranspiration equals the average of either May through September Monthly Irrigation Requirements, in inches, as determined by the Washington State University, State of Washington Irrigation Guide (WIG), 1985 (as amended 1990; 1992 for select western Washington crops), or monthly averages for systems that adjust application rates by the month or based on actual conditions;

Plant Factor equals 0 to 0.3 for low water use plants; 0.4 to 0.6 for average water use plants; and 0.7 to 1.0 for high water use plants; and

Irrigation Efficiency equals 0.9

(if this looks familiar, it is the same formula that we use in this guide. The formula was supplied by us to a member of the committee)

  • Application of the greywater to the plants is even throughout the irrigation field. This is typically achieved through pressure distribution.
  • The subsurface irrigation components of the greywater irrigation system must be installed a minimum of six inches deep and no deeper than 12 inches below the finished grade. The six inch cover layer must consist of four inches of suitable soil and two inches of mulch.(This is bad. Why must the dripperlines be covered with 4 inches of soil? The most efficient location for dripperlines is at the surface with a covering of mulch. The 90% irrigation efficiency factor only applies to sub mulch irrigation, because capillary irrigation effects are stronger in the top 2-3” of soil.Placing dripperline 4” under soil will automatically increase dripperline length by about 300%, labor by 400%,

To get a permit:

  • You need a soil and site evaluation by a qualified professional or local health office. You may be allowed to perform your own evaluation is not on a marine shoreline, if the local health office agrees.
  • The person evaluating the soil and site shall: (a) provide a report to the local health officer that includes:
  • a soil map showing the soils within the project site. The person evaluating the soils must ensure that the soil types of the site are properly identified, correctly used in the hydrologic analysis, and will provide suitable soil capable of supporting healthy plant life. If the original, undisturbed soil will be augmented with additional soil, include a description of the additional soil, how it will be tilled into the original soil, and how the resulting soil will meet the requirements of WAC (1)
  • use the soil names and particle size limits of the United States Department of Agriculture Natural Resources Conservation Service classification system; and (c) determine texture, structure, compaction, and soil characteristics and classify the soil as in WAC, Table II.
  • describe the drainage characteristics of the site and those areas immediately adjacent to the site that contain characteristics impacting the design; (iii) the existence of designated flood plains and other areas identified in the local management plan required in WAC 246-272A-0015; and (iv) the location of existing features affecting system placement, including the items requiring setback, identified in WAC , Table 1, and other features such as: 1. surface water and stormwater infiltration areas; 2. abandoned wells; 3. outcrops of bedrock and restrictive layers; 4. driveways, parking areas, and other impervious surfaces; 5. the on-site sewage system serving the building, if any; and 6. underground utilities.
  • Prior to beginning the construction of a Tier 2 or Tier 3 greywater irrigation system, a person proposing the installation of such a system must provide information to, and obtain a permit to install from, the local health officer. The information provided shall include: (a) the following general information: (i) name and address of the property owner; (ii) parcel number and if available, the site address; (iii) identification of the public sewer system or on-site sewage system serving the property; (iv) size of the parcel; (v) name, signature, and stamp, if applicable, of the person responsible for designing the system; (vi) date of application; and (vii) name and signature of the owner or the owner’s authorized agent. (b) the soil and site evaluation specified under WAC ; (c) a dimensioned site plan of the proposed irrigation field, including: (i) general topography and/or slope; (ii) drainage characteristics (iii) the location of existing and proposed encumbrances affecting system placement, including legal access documents, if any component of the system is not on the lot where the greywater is generated. (d) location, type, and size of the irrigation system components; (e) flow rate in gallons per minute, application rates in inches per hour, and design operating pressure per square inch for each zone; (f) source of greywater (fixtures) and the location of the diversion valve; and (g) any additional information deemed necessary by the local health officer.

Installation and Site inspection

Only a person approved by the local health office may construct and install a Tier 2 or Tier 3 system.

The approved installer must be on the site at all times during the excavation and construction of the system.

The local health office shall either inspect the system before cover, or allow the person who designed the system to inspect, but only if the designer is not also the installer.

Discussion and Analysis of the Washington Greywater Code

Tier 1I do not see how either branched drain, or Laundry to Landscape (L2L) systems can be installed while having any regard for evapotranspiration rates and plant types.

According to the Oasis Design calculator about 10 holes is required to avoid overloading the pump / achieving uneven distribution for a L2L.

A mulch basin for a tree is typically about 4’ in diameter, or 12.6 square feet. The total irrigated area with 10 mulch basins will be 126 square feet.

However, assuming an ETo Value of 5” per month (summer average across many parts of Washington State), the optimum irrigation area is:

Low water use plants             2,256 square feet             (c/w L2L efficiency 5%)
Medium water use plants      1,304 square feet             (c/w L2L efficiency 10%)
High water use plants               903 square feet             (c/w L2L efficiency 14%)

It is possible, if washing is done on a daily or every second day, that capillary irrigation effects will assist, however at best this is likely to double the efficiency.

Branched drain efficiency is worse, as no more than 6 outlets would be installed for a system of this size.

Laundry to Landscape isn’t the only option—some graywater dripperlines can use gravity alone to irrigate at a rate of 1 gallon per minute, per 150 feet of dripperline, covering 700 square feet of garden area with light clay soil. 300 feet of dripperline will therefore cover 1,400 square feet.

If the washing machine is run every day, or second day, the irrigation efficiency is about 90%, so much more water is saved.

This method does require the use of a surge tank (e.g. a 50 gallon barrel) .

Tier 2

Basically, the permit process is onerous. A full house greywater collection, pumping and irrigation system costs about $2,000 installed in Arizona, New Mexico and Texas, with little ongoing maintenance.

The same style of system will cost at least $4,000 under this proposed code, and require much more ongoing maintenance (due to the extra filtration required for in soil dripperlines).

The critical comparison between Tier 1 and Tier 2 (apart from permitting costs) is Tier 1 only requires 4” of mulch , whereas pressurized dripperline must be covered by at least 4” of soil and 2” of mulch.

This simply doesn’t make sense. Supplying 6 gallons a day into a 12 square foot mulch basin (L2L or branched drain) is much more likely to produce surface runoff than applying water via dripperline on the surface at a rate of 0.046 gallons per day per square foot.