Credit: Dave Kosling, U.S. Dept. of Agriculture, CC BY 2.0

The worst drought in the United States in a half-century, which has battered the country’s agricultural industry, brings to bear a potential foreshadow of our water-stressed future. And it also makes relevant the consideration of permaculture farming techniques like wastewater irrigation.

As the U.S. drought continues to play out, severe water shortages are presenting a crippling dilemma for farmers in particular. The water shortage affects not only the country, causing food shortages and elevating grain and crop prices and food prices, but also reverberates around the world, as many other countries depend on U.S. grain and food exports.

In many parts of the world, farmers are beginning to reuse water in order to maintain food production, often by directing wastewater into irrigation. In Israel, for example, treated wastewater provides more than half of the water used in agriculture. The practice isn’t limited to the driest parts of the planet. Even where water is still reasonably plentiful, many small farmers prefer to use wastewater because it contains a powerful amount of nutrients such as nitrogen and phosphorus that can reduce or eliminate the need for expensive chemical fertilizers.

In essence, the reuse of wastewater for agriculture is doubly green: it’s recycling and it eliminates the need for the kind of chemical fertilizers that are often associated with environmental damage. In addition, in the parts of the world without modern wastewater treatment facilities, using the water for irrigation is often the safest way to dispose of it.

However, there can be health risks from improper (or even proper) use of treated wastewater in agriculture. The water can be chock full of pathogens, which, in some cases, can live long enough to be transmitted to humans, particularly through raw fruits and vegetables.

A study by the National Research Council found that the potential pathogens in wastewater can be divided into four categories:

• Bacteria (Salmonella, Shigella, Mycobacterium, Klebsiella, Clostridium)
• Protozoan parasites (Entamoeba, Giardia, Trichomonas)
• Helminth parasites (Ascaris, Toxacara, Taenia, Trichuris, Enterobius)
• Viruses (Picornaviruses, Adenoviruses, Rotaviruses)

The International Water Management Institute, which works in India, Pakistan, Vietnam, Ghana, Ethiopia, Mexico and other countries where wastewater irrigation is common, conducts research to assess and reduce the risks of the practice. The organization advocates a multiple-barrier approach to farmers, outlining a number of ways to reduce risk. These include ceasing irrigation of crops several days before harvesting to allow sunlight to kill any pathogens, watering plants very carefully to avoid contaminating the plant’s leaves and other edible parts and even cleaning vegetables with disinfectant after the harvest.

The group also recommends that farmers allow the “fecal sludge” they use as fertilizer to dry in the sun before use, a practice that will also kill pathogens with sunlight.

Although cooking fruits and vegetables grown with wastewater can largely eliminate bacteria, it won’t do anything with contaminants such as chemicals and heavy metals. According to a report published by the environmental sciences division of the provincial government of Alberta, Canada, wastewater used for irrigation can also have elevated levels of salt, sodium and other chemical constituents that can damage plants and restrict the types of crops that can be grown. The report found that treated municipal wastewater has salt or sodium levels that should completed exclude the water from use in irrigation, due to the harmful effects to both crops and the surrounding ground water.

Then, there are human and animal antibiotics. Wastewater, carrying antibiotics not consumed, and which are mostly unchanged in organic waste, is routed to treatment facilities, which often do not have the technology to completely remove common them.

These consistent concentrations of antibiotics put into soil via irrigation have the potential to increase the antibiotic resistance of naturally occurring soil microorganisms. This essentially creates whole farms breeding drug-resistant bugs against which humans have no defense. And aside from potential harm to humans, constant applications of antibiotics to crops can also cause damaging changes to the good, helpful bacteria living in the soil and plants, according to scientists at the Water Resources Research Center at the University of Hawaii who are conducting a research project designed to study the impact of crop uptake of microcontaminants from recycled wastewater.

Researchers in the Netherlands have compared the antibiotic resistance of soil bacteria in samples taken and stored between the 1940s (when the world first began to use antibiotics) and 2008. The scientists concluded that “…there is growing evidence that resistance to antibiotics is increasing both in benign and pathogenic bacteria, posing an emerging threat to public and environmental health in the future.”

A new study may help ease some of those fears, however. A group of researchers in Israel recently set out to discover whether long-term irrigation with treated wastewater created resistant-enough soil microbes that followed into produce, where it could then mingle with normal bacteria and create “superbugs.” The research, which was funded in part by the Israeli Ministry of Agriculture and Rural Development and the U.S.-based Environmental Health Fund, appears in the American Chemical Society’s Environmental Science & Technology publication.

While a number of scientific studies have reported high levels of antibiotic resistance in soil irrigated with wastewater, said the Israeli researchers, their conclusions have failed to take into consideration the naturally occurring antibiotic resistance in soil bacteria.

There are a whole lot of antibiotics in the world’s water supply, it turns out. According to the American Society of Agricultural and Biological Engineers, resistance to multiple antibiotics — including tetracycline, daptomycin and erythromycin — shows up in soil regardless of whether the water source is fresh or reclaimed.

The Israeli study echoed these findings, concluding that in most of the soil samples analyzed, antibiotic-resistant bacteria and antibiotic resistance gene levels in treated wastewater-irrigated soils were largely the same (and lower in some cases) compared with those in soils irrigated with fresh water. The study also concluded that the resistant bacteria that enter the soil through treated wastewater are unable to compete or survive in the soil environment. (Not everyone is convinced, including a blog that questions how the Israeli researchers were able to find any agricultural plots with no history of exposure to human-originated bacteria or drug-resistance genes.)

Still, the findings are positive for a practice that is likely to become more widespread as the world becomes more scarce of fresh water. Many farmers and municipalities continue to follow World Health Organization guidelines for wastewater handling for crop irrigation.

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