Closing the Nutrient Loop

In preindustrial times, human excrement was recognized as a fertilizer; it was gathered up and delivered to farm fields by the “Honey Wagon.”   That was closed-loop nutrient cycling at its best, as the nutrient content food was contained in excrement and returned to the soil from whence it came. This practice was widespread in China until recent times.

With the invention of artificial nitrogen fertilizers, the closed-loop was broken. It was much easier to apply industrial fertilizer than to gather up and shovel out the honey wagon. Artificial nitrogen fertilizers are widely credited with increasing crop yields to feed a global population now approaching 8 billion while minimizing malnutrition. Nitrogen fertilizer is so inexpensive that it is often over-applied, leading to excessive losses of nitrogen to streamwaters and associated water pollution.

It is time to reestablish the closed nitrogen cycle of yesteryear.

Some progress has been made. In many developed countries with wastewater that is treated in sewage treatment plants, a significant fraction of the nutrient content is now captured in biosolids.  Initially, these were landfilled, but increasingly the biosolids are returned to agricultural lands as fertilizer. In the U.S., 55% of biosolids are returned to fields; 30% are landfilled and 15% are burned.

For regions without organized sewer systems and sewage treatment plants, the fate of human excrement is uncertain. Enter Rebecca Ryals and her coworkers of the University of California at Merced, who have developed and tested a container-based household toilet—Ecosan—with composting, that produces an organic nutrient resource for farmland use.

Ryals found that when applied to croplands in Haiti, the organic nutrient material increased crop yields more than artificial fertilizer (here urea) and that the effect lasted for as long as six cropping cycles, versus only 2-4 for biosolids and essentially one year for urea.  Ecosan systems could dramatically improve crop yields and sustainability in the undeveloped world.

Human excrement is a resource too valuable to waste.


See:  Poop – Translational Ecology (


Ellis, E.C. and S.M. Wang. 1997.  Sustainable traditional agriculture in the Tai Lake region of China.  Agriculture, Ecosystems and Environment 61: 177-193.

Peccia, J., and P. Westerhoff. 2015. We should expect more out of our sewage sludge. Environ. Sci. Technol. 49, 8271–8276. doi: 10.1021/acs.est.5b01931

Ryals, R., and 6 others. 2021.  Toward zero hunger through coupled ecological sanitation-agricultural systems.  Frontiers in Sustainable Food Systems doi: 10.3389/fsufs.2021.716140/full

One thought on “Closing the Nutrient Loop

  1. A caveat regarding biosolids concerns pollutants such as PFAS, antimicrobials, flame retardants, pharmaceuticals, etc (and their various metabolites)

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