New Study Suggests Landscape Plants Do Well Irrigated by Graywater


The Water Environment Research Foundation (WERF), in its Fall 2012 Progress Newsletter, reports that the most landscape plants irrigated with household graywater are as healthy as those watered with freshwater. It based that assertion on a long-term study with Sybil Sharvelle, Colorado State University serving as principal investigator.
Below is the summary of the report as it appears in the fall WERF newsletter that can be accessed at
Droughts, rising energy costs for water transport and treatment, and increased population are driving interest in innovative alternatives to traditional water sources. One approach that is gaining popularity is household graywater reuse for landscape irrigation. In the United States, graywater recycling is not a common practice when compared to countries such as Australia, where many new homes are equipped with graywater recycling systems. Regions of the United States with climates akin to parts of Australia, particularly the southwest and more arid climates, could benefit greatly from having improved information on alternative sources of water.
WERF delved further into this topic with a Long-Term Study on Irrigation Using Household Graywater - Experimental Study (06CTS1COa). This study clarified information on the fate and occurrence of graywater constituents and their potential impacts on soil quality, groundwater quality, and plant and human health, as a result of its application for residential landscape irrigation.
Among the results, the research team found that most landscape plants are as healthy under long-term graywater irrigation compared to freshwater irrigation. Among 22 plant species evaluated, the research team only observed three species (avocado, lemon tree and scotch pine) that were sensitive and showed reduced growth, leaf burning or resulted in small fruit production under
Careful consideration needs to be taken before implementing graywater irrigation programs as there are potential risks to both the environment and human health. There are no documented instances of disease arising from graywater exposure, but the perceived threat to human health does arise from the fact that graywater contains microbial concentrations far in excess of levels established in standards for recycled irrigation, drinking, and bathing.
Placing a mulch layer over drip emitters where graywater is applied appears to be a good control to minimize human contact with graywater irrigated soil. Governance and regulation of domestic graywater use for landscape irrigation will continue to evolve and be better informed as research unveils the actual risks and benefits.
Experimental studies in 06CTS1COa were conducted in three parts: field studies on existing household systems, field studies on new household systems, and greenhouse studies. For the existing household systems, four households were selected in the southwestern U.S. where graywater was applied for more than five years. For the new household systems, graywater irrigation systems were installed at three households in Arizona, California and Colorado.
In all household studies, soil samples were taken and plant tissue analysis was conducted. In addition to the field studies, a greenhouse experiment was conducted to evaluate the impact on plants and monitor leachate from graywater irrigated soils.
The quantitative data collected during these experiments will provide guidance to decision makers, water agencies, regulators, product manufacturers, and consumers so that safe graywater irrigation systems can be installed and operated for household irrigation.
Furthermore, the information contained in this study will provide guidance for lawmakers and regulators on how to govern this evolving practice.