Rainwater Harvesting for Drylands and Beyond by Brad Lancaster

Detergent Considerations When Harvesting Greywater

The following is reprinted courtesy of City of Tucson Water Department. Bracketed text in [italics] is my addition. It is important to note that the information is based on research conducted in the early 1990s. Detergent companies may have modified their formulas since then, thus you may want to contact the manufacturer to inquire about current ingredients and concentrations. In addition, there are many other detergents and soaps not included in the study. For now, this is the best thing I’ve seen on the topic.

This is intended for those conservation-minded people who would like to use washing machine water (greywater) to irrigate their landscapes. Before using greywater, make sure that you are aware of the appropriate methods to operate and maintain a greywater system and the local regulations regarding its use. Information can be obtained from the Pima County Department of Environmental Quality. It should be used only for subsurface irrigation, and human or animal contact must be kept to a minimum. If you plan to use washing machine water to irrigate, you should be aware of the elements present in this water, which may affect your plants or soils. Detergents and other clothes-washing products use a variety of chemicals to aid in cleansing. Some of these ingredients can be harmful to your plants. Because labeling on detergent and other clothes-washing products is often incomplete, a study was conducted to evaluate some critical product characteristics which may adversely affect the landscape, including alkalinity, boron, conductivity, sodium, and phosphate.


(return to top)

Alkalinity refers to the relative amounts of alkaline chemicals in a solution. Sodium, potassium, and calcium are alkaline chemicals; they often are combined with carbonates, sulfates, or chlorides. Plants do not tolerate high concentrations of alkali salts.


(return to top)

Boron is considered a plant micronutrient, required in only very, very small amounts. Most soils provide adequate amounts of this chemical. Concentrations only slightly higher than those considered beneficial can cause severe injury or death to plants.


(return to top)

Conductivity is a simple measure of the amount of dissolved chemicals in a solution. These chemicals can be beneficial or harmful. The higher the conductivity, the more dissolved salts and minerals are present. In general, the higher the concentration of dissolved salts and minerals in the water, the greater the potential for adverse affects on the environment and plant health.


(return to top)

Sodium can act as a plant poison by reducing the plant’s ability to take up water from the soil. Too much sodium can destroy the structure of clay soils, making them slick and greasy by removing air spaces and thus preventing good drainage. Once a clay soil is damaged by sodium, it can be very difficult to restore it to a viable condition.


(return to top)

Phosphate is a plant food and is added to soil as a fertilizer. Soils in the Tucson area are typically low in phosphate; thus, there may be some benefit to plants if phosphate is present in greywater. This should not be relied upon, however, since many forms of phosphate are not readily usable by plants and soils.

Is Biodegradable Better?

(return to top)

The word biodegradable means that a complex chemical is broken down into simpler components through biological action. Do not be confused by the word biodegradable, which often is used to imply environmentally safe. Harmful chemicals as well as beneficial ones may be biodegradable.

A Note About Chlorine

(return to top)

Although chlorine in bleach and detergents is generally expended in the washing process, some may be left in the greywater that reaches plants. Chlorine should not be used in the garden because it may substitute for similar nutrients, blocking normal metabolic processes. The addition of chlorine to water used for irrigation should be kept to a minimum. Choose your detergent and clothes-washing products keeping in mind that it is better for your plants and soils to have a low alkalinity, boron, conductivity, and sodium content in the water. Personal preference may affect your choice of products, since higher levels of these constituents may add to their cleansing ability.

Click this link for a downloadable PDF of an extensive spreadsheet of detergent brands and their properties.

What Can I Irrigate?

(return to top)

Greywater can be used to irrigate fruit trees, groundcovers and ornamental trees and shrubs. Salt-tolerant plants and native desert plants are well-suited to irrigation with greywater. Avoid using greywater on plants that prefer acid conditions, such as:

Sandy soils are less vulnerable to damage than clay soils because they drain better. In very low rainfall areas, apply fresh water [or better yet, harvested rainwater] occasionally to leach out accumulated salts. Be aware that some harmful effects are not always visible immediately and may take one or two years to appear. In any case, you should always pay attention to the health of the plants being irrigated and discontinue using greywater if some signs of stress are observed.

About The Study

(return to top)

All the detergents and related clothes-washing products were purchased in Tucson during May, 1992. The amounts used were based on the manufacturers’ recommended levels for a cool to warmwater wash in a top-loading [vertical-axis] machine. Distilled water was used as a source to minimize the effect of widely-varying salt and mineral levels in tap water. The list is presented in alphabetical order and is intended as a basis for comparison only. No endorsement of any product is intended. This study was based in part on research conducted by the Pima County Extension Service, and was prepared by the Office of Arid Lands Studies, in cooperation with the Soil, Water and Plant Analysis Laboratory, University of Arizona, and sponsored by Tucson Water.

Additional reports
“Long Term Effects of Landscape Irrigation Using Household Greywater” by Water Environment Research Foundation (werf.org), 2007.
Identifies information gaps for future research on the long-term use of graywater for irrigation of residential landscapes, particularly as it relates to human health, landscape plants, and/or the environment.

Drops in a Bucket Blog

 »Read all blog posts...

Sign up for the Newsletter

Upcoming Events

  1. Water Harvesting Design Certification: Fall 2021

    November 8 - November 20
  2. Water Harvesting Design Certification: Spring 2020

    February 28, 2022 - March 12, 2022

Umbrella Newsletter

The Umbrella: Summer 2020

THE UMBRELLA: A catch-all of resources, events, media, and more from Brad Lancaster In this time of Covid-19 and spending more time at home to be safe, I’ve been grateful for the solace, inspiration, and bountiful sustenance my water-harvesting gardens, landscape, and neighborhood forest has provided me, my family, friends, and neighbors. Record summer heat […]

 »Read all newsletters...

Like what this website offers?

Donations are greatly appreciated as they enable us to continuously update this expansive resource and generate new content. Thanks!