Human waste is an excellent substitute for polluting fertilizers, scientists find

Scientists in Germany found that urine and feces can be recycled for use as new fertilizers that are safe and productive.

  Syrian migrant puts the fertilizer in the molding in Giza, Egypt November 3, 2022 (photo credit: REUTERS/FATMA FAHMY)
Syrian migrant puts the fertilizer in the molding in Giza, Egypt November 3, 2022
(photo credit: REUTERS/FATMA FAHMY)

Our sewage doesn’t have just to be disposed of in treatment plants.

Scientists in Germany found that urine and feces can be recycled for use as new fertilizers that are safe and productive, on condition that there is no risk that harmful microbes or traces from pharmaceuticals end up in the consumed crops. 

To tackle the climate crisis, biodiversity loss and pollution, mankind will have to move to a circular economy in which all resources are recycled.? Most nutrients needed for plant growth occur in human urine and feces. Urine is especially rich in nitrogen and potassium and also contains trace amounts of metals such as boron, zinc, and iron. Feces could, in theory, supply other nutrients such as phosphorus, calcium and magnesium or valuable organic carbon to soils.

Now, a new study in Frontiers in Environmental Science has shown that modern ‘green’ products recycled from human excreta are excellent – and importantly, safe – fertilizers for agriculture.

Human urine, feces products are 'viable and safe'

Franziska Häfner, a doctoral student at the University of Hohenheim in Stuttgart said: “Here we show that products derived from recycling human urine and feces are viable and safe nitrogen fertilizers for cabbage cultivation. The fertilizers from nitrified human urine gave similar yields as a conventional fertilizer product, and did not show any risk regarding the transmission of pathogens or pharmaceuticals.”

 A visitor pretends to defecate on a display toilet at the the Unko (''poop'' in Japanese) museum in Yokohama, Japan, April 17, 2019 (credit: REUTERS/KIM KYUNG-HOON) A visitor pretends to defecate on a display toilet at the the Unko (''poop'' in Japanese) museum in Yokohama, Japan, April 17, 2019 (credit: REUTERS/KIM KYUNG-HOON)

The combined application of nitrified urine fertilizers and fecal compost led to slightly lower crop yields, but they may increase soil carbon content in the long term, promoting climate-resilient food production, he added.

Häfner and colleagues compared the marketable crop yield of white cabbage grown between June and October 2019 at the Leibniz Institute of Vegetable and Ornamental Crops on plots with sandy, loamy or silty soil enriched with four recycled fertilizers, applied gradually over the growing season. As a benchmark fertilizer, the researchers used commercially available organic vinasse (a byproduct of the sugar or ethanol industry using sugarcane or sugar beet) to produce crystalline sugar, pulp and molasses.

They also tested two so-called nitrified urine fertilizers (NUFs), modern products synthesized from human urine that has been collected separately from feces in which nitrogen-bearing compounds are converted by microbes into valuable ammonium and nitrate

The first NUF was an organic compound named aurin, recently approved for use in human agriculture in Switzerland, Liechtenstein and Austria. The second was CROP (combined regenerative organic food production), developed to recycle wastewater on Moon or Mars bases. Unlike aurin, which is already on the market and for which most pharmaceuticals are filtered out and most pathogens are killed during the production process, the CROP filter system is still under development. For this reason, Häfner used CROP fertilizer derived from sterile synthetic urine in the present study. The effects of the NUFs on cabbage growth were tested both when applied separately to the soil, or in combination with fecal compost, recycled from dry toilets.

The marketable yield, defined as the parts of the cabbages that can be sold, ranged from 35 to 72 metric tons per hectare. This yield was highest for plots fertilized by Aurin, CROP or vinasse, lowest for plots fertilized by fecal compost alone and intermediate for fecal compost augmented with NUFs. Yield was highest on sand, intermediate on silt and lowest on loam. Soils fertilized with NUFs are as productive as those supplied with widely used commercial vinasse.

The authors also screened for the presence of 310 chemicals in the fecal compost, from pharmaceuticals to rubber additives, flame retardants, UV filters, corrosion inhibitors and insect repellants. Only 6.5% of these were present above the limit of detection in the compost, albeit at low concentrations, including 11 pharmaceuticals – and only the painkiller ibuprofen and the anticonvulsant and mood-stabilizing drug carbamazepine were detectable in the edible parts of the cabbages, at markedly low concentrations/This means that that more than half a million cabbage heads would need to be eaten to accumulate a dose equivalent to one carbamazepine pill.

In general, the risk for human health of pharmaceutical compounds entering the food system by means of fecal compost use, seems low,” concluded the authors.

Could we recycle our own feces?

Lead author Dr. Ariane Krause, a scientist at the Leibniz Institute of Vegetable and Ornamental Crops in Großbeeren, said: “Our study results demonstrate that nitrified urine fertilizers such as aurin and CROP have a huge potential as fertilizer in agriculture. They argue for greater use of these recycled products in the future.”

“If correctly prepared and quality-controlled, up to 25% of conventional synthetic mineral fertilizers could be replaced by recycling fertilizers from human urine and feces. Combined with an agricultural transition involving the reduction of livestock farming and plant cultivation for fodder, even less synthetic fertilizer would be necessary, resulting for example in lower consumption of fossil natural gas.”