Converting Gray Water to Green Water

The demand for water to supply irrigated agriculture is increasing as the need for food production increases.   To meet this demand and achieve sustainable outcomes we will need multifaceted solutions that include improved efficiencies, creative irrigation management strategies, and expanded source supplies.   Expanding the amount of gray water used for irrigated agriculture is a solution with a sustainable outcome.     Gray water will expand irrigation source supplies by using water more than once and it can be achieved safely with proven approaches and creative management strategies.   Additionally, gray water irrigation will improve water quality with additional treatment in the soil system. This approach reduces the amount of energy used in conventional water treatment and by reducing use of potable (blue) water sources.  

Gray water is defined differently by water management and environmental agencies and within the scientific community.  For this discussion, we define gray water as naturally occurring non-potable fresh water and any water that has been used by humans but has undergone a level of treatment to make it safe for the intended agricultural irrigation use.  Examples of gray water irrigation sources include process water from food production, production water from natural resource extraction, storm runoff, treated wastewater and non-potable fresh water sources. The quality of water from these sources varies considerably from nutrient rich, to highly saline, to near potable quality.  

Gray water sources are underutilized mostly because of negative public perception and easy availability of potable water. Excellent environmental stewardship and crop production are achieved with gray water systems with proper knowledge and management.  For the most effective gray water irrigation, consideration of water quality and quantity, soil type, climate, cropping practices, irrigation method, required level of treatment, and site management are recommended.  Important water quality considerations include nutrients, salts, metals, solids, and other constituents.   

Numerous gray water irrigation systems covering a wide range of locations, soils types, climates, crops and water quality has proven that reuse is effective when properly designed and managed.   The most limiting water quality constituent is used to determine the amount of land and if treatment is required.  Matching the water quality, soil type, and crop will achieve a sound environmental, soil, nutrient, and use approach.    An effective irrigation system design includes incorporation of gray water quantity and physical characteristics, and use of supplemental potable water may be needed.  Billions of acre-feet can be utilized to expand agricultural production under well designed and managed systems.

Several example systems will be presented.   Example systems will include industrial and agricultural process water, and treated municipal waste water.