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Human waste could be used as sustainable fertilizer

To solve global issues such as the climate crisis, biodiversity loss, and pollution, humanity will need to move to a circular ecology, where most resources are recycled. According to a new study published in the journal Frontiers in Environmental Science, human waste products such as urine and feces may also be used to create products that could act as excellent and safe agricultural fertilizers, since they contain important nutrients, including nitrogen, potassium, boron, zinc, iron, calcium, and magnesium.

“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 transmission of pathogens or pharmaceuticals,” said study lead author Franziska Häfner, a PhD student in Agricultural Sciences at the University of Hohenheim.

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

The scientists compared the marketable crop yield of white cabbage grown between June and October 2019 at the Leibniz Institute of Vegetable and Ornamental Crops (IGZ) on plots with sandy, loamy, and silty soil enriched with four recycled fertilizers. As a benchmark fertilizer, they used the commercially available organic vinasse, which is produced through the fermentation of biomass residues from bioethanol production.

Moreover, the researchers also tested two types of “nitrified urine fertilizers” (NUFs) – products synthesized from urine collected separately from feces, such as Aurin or CROP – in which nitrogen-containing compounds are converted by microorganisms in ammonium and nitrate. The effects of NUFs on cabbage growth were assessed both when applied separately to the soil and in combination with fecal compost.

The analysis showed that the marketable yield ranged from 35 to 72 metric tons per hectare, and was highest on plots fertilized by Aurin, CROP, or vinasse, lowest on plots fertilized only by fecal compost, and intermediate for fecal compost augmented with NUFs.

To test the risk of contamination with various products contained in human waste, the scientists screened for the presence of 310 chemicals in the fecal compost, including pharmaceuticals, flame retardants, UV filters, corrosion inhibitors, rubber additives, and insect repellants. They found that only 6.5 percent of such substances were present above the limit of detection, and only the painkiller anti-inflammatory drug ibuprofen and the anticonvulsant and mood-stabilizing drug carbamazepine were detectable in the edible parts of the cabbages, at insignificant concentrations (1.05 to 2.8 μg per kg).

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

“Our study results demonstrate that nitrified urine fertilizers such as Aurin and CROP have a huge potential as fertilizer in agriculture. They argue for a greater use of these recycled products in the future,” said study lead author Ariane Krause, a researcher at IGZ.

“If correctly prepared and quality-controlled, up to 25 percent of conventional synthetic mineral fertilizers in Germany 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,” she concluded.

By Andrei Ionescu, Staff Writer

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