Last month, Western Growers Science and collaborators published a peer-reviewed review article in the Journal of Food Protection examining the use of reclaimed water for produce irrigation. The paper reviews the available science on water treatment, microbial and chemical hazards, regulatory requirements, irrigation practices and remaining research needs.
As water availability becomes an increasing challenge for agriculture, reclaimed water, also known as recycled water or purple valve water, is receiving greater attention as a potential supplemental irrigation source. But what does the available research say about its safety?
What does the paper conclude?
The research reviewed in the paper indicates that properly treated and appropriately managed reclaimed water can be safely used for producing irrigation. Secondary and tertiary wastewater-treatment processes can substantially reduce microbial hazards, including bacteria, enteric viruses and protozoan parasites.
The quantitative microbial risk assessments reviewed in the paper generally found that reclaimed water does not appreciably increase public health risk from enteric pathogens. In some scenarios, treated reclaimed water may present a lower microbial risk than conventional irrigation sources that receive less treatment or monitoring.
However, not all reclaimed water is the same, and its suitability depends on the treatment level, applicable state requirements and intended use. The paper emphasizes a multiple-barrier approach that includes effective treatment and disinfection, routine monitoring, well-maintained storage and distribution systems, appropriate irrigation methods and sound on-farm water-management practices.
What do we still need to learn?
Although the overall evidence supports the safe use of properly treated reclaimed water, the review identifies several areas where more research is needed. The potential presence of pathogens in wastewater depends partly on the population and activities contributing to the wastewater system. For example, Cyclospora cayetanensis would need to be introduced by infected individuals shedding the parasite. Therefore, the risk would be low in communities where cyclosporiasis is not commonly circulating, although unrecognized or travel-associated infections remain possible. However, the possibility of unrecognized cyclosporiasis is true across all public water systems at all times and yet, domestic (non-food borne) disease rates have historically remained very low in the United States.
Although direct data on Cyclospora are limited, existing US wastewater treatment processes are designed to substantially reduce microbial contaminants, and reclaimed water programs operate under established treatment and regulatory frameworks. Additional research on Cyclospora would help validate treatment performance for this specific parasite and improve methods for determining whether detected organisms are viable and capable of causing illness. These efforts would enhance confidence in existing systems and support continued science-based risk assessment, rather than indicate a deficiency in current reclaimed water practices.
Purple valve water is one of the most highly regulated irrigation water sources. They identify the dedicated distribution system for reclaimed water and are separated from the potable water infrastructure. In California under title 22, the level of treatment for irrigation requires a filtration and disinfection process and has strict microbial criteria and monitoring requirements. This treatment barriers discussed in the articles are part of why properly treated reclaimed water can be used safely to irrigate crops.