Applicability  
The Washington State Department of Health document "Water Conserving On-Site Wastewater Treatment Systems", effective 15 May 2000, is a helpful guide with recommended application and design standards.  
   
Some pertinent performance, application, and design standards are:  
Greywater treatment & disposal / reuse systems must provide treatment and disposal at least equal to that provided by conventional on-site systems.
All permitting, installation and inspection requirements are the same as required in Chapter 246-272 WAC.
Greywater on-site sewage systems must provide permanent, year-round treatment and disposal of greywater unless this is already provided by an approved on-site system or connection to public sewer.
Greywater systems are intended to treat and dispose "residential strength" greywater. Greywater exceeding typical residential strength must receive pre-treatment to at least residential strength levels.
Greywater may be used for subsurface irrigation of trees (including fruit trees), shrubs, flowers, lawns and other ground covers but must not be used for watering of food crops or vegetable gardens, any type of surface or spray irrigation, to flush toilets/urinals or to wash walls, sidewalks or driveways.
Systems that use laundry wastewater only are treated as a special case and may be permitted as such by local health officers. Generally, such systems can be simpler and smaller than other greywater systems.
Designers should consider plant water and nutrient needs and limits, salt tolerances, depth of root zones, etc. In general, shade-loving and acid loving plants do not like greywater. Plants that are not suitable include rhododendrons, hydrangeas, azaleas, ferns, magnolias and dogwood. Plants that likely would do well with greywater irrigation include rosemary, roses, oaks, cottonwood and bearded iris.
Most hand soaps and shampoos will not damage plants at low residential concentrations. With detergents, conductivity, alkalinity, sodium, boron and phosphate levels must be considered to determine suitability.