Human feces get their day in the sun — and field

Researchers explore ways to make fecal matter a safe and transportable soil amendment for agricultural use

Researchers are breathing fresh air upon an aromatic subject that is often quickly flushed from sight.

Human feces has historically been used as a natural fertilizer in many other countries, but its benefits haven’t found major uses in modern agriculture.

Not that long ago, outhouses sat in most backyards and their contents could be carried a short distance to the field or kitchen garden. It was prime example of a circular economic system that recycled waste through soil fertility and food production.

But around the world, the sewage system is now stressed or broken.

“We rarely have the luxury now of a very small circular economy,” said Johannes Lehmann from Cornell University’s College of Agriculture and Life Sciences.

“Nowadays we live in mega cities and the distance between our feces, whether it’s dairy manure or human feces and agricultural fields, are much too large.”

Lehmann, a professor of soil and crop sciences based in Ithaca, New York, said there are disposal issues in many parts of the world where manure has to be shipped hundreds of miles, which is not cost effective.

He sees opportunities to reconnect nutrient cycles involving excrement and agriculture by heating, drying and shipping product where it can enrich soil and reduce other fertilizer costs.

He used the Canadian Light Source facility at the University of Saskatchewan to test his theories.

Using unprocessed human feces as fertilizer is risky because of potential disease-causing pathogens. That risk is exacerbated by use of antibiotics, hormones and endocrine disruptors that appear in all manures.

It is also a transportation issue because 80 to 90 percent of feces and urine is composed of water, which has no economic value as manure and is expensive to transport.

“Who wants to ship large amounts of water with a little bit nitrogen through the countryside because typically after 10 or 20 miles the nutrient value of this kind of liquid is lower than the transportation costs,” Lehmann said.

“In a circular economy and recycling nutrients we need to densify the feces and make the material a higher value.”

While urine by itself is nitrogen-rich, it can contaminate groundwater via runoff. That is a serious problem in developing countries like Kenya.

In 2013, Lehmann and colleague Leilah Krounbi thought it possible to close the waste stream loop by recycling nitrogen from urine that was otherwise being lost to runoff.

Adsorbers have already been engineered using carbon nanotubes or activated carbons to hold gas or liquids, but the Cornell researchers focused their efforts on low-tech materials like human feces during their seven year study.

Dried human feces are high in phosphorus, potassium and some micronutrients, but most of the nitrogen is in urine.

“How can we get the nitrogen out of that liquid part that’s still very diluted,” Lehmann wondered. “Maybe we have this solid material that we pyrolyze and we somehow get the nitrogen out of the liquid onto the solid and make it into a dry benign material that then has a shippable characteristic with a high value.”

He said biological conversions usually don’t reduce the weight and volume but thermal chemical conversions can produce four kilogram of a dry benign material from 100 kg of wet and potentially pathogenic material.

He and Krounbi heated feces to 500 degrees C in the absence of oxygen to produce biochar, which was free of pathogens.

The first attempt to enrich the solid material by adding urine was unsuccessful.

“Then we thought, OK, let’s figure out how we can make the nitrogen into gas form, which is relatively easy because you’ve just increased the pH of the urine and then ammonia gas comes out. We thought let’s use CO2 to condition that solid material and then have the ammonia gas interact with that solid material after we conditioned it,” he said.

They primed the surface of biochar with CO2, which soaked up ammonia, a nitrogen-rich gas given off by urine. By repeating the bonding process, they loaded the biochar with several layers of nitrogen, producing a solid nitrogen-rich material.

The beamline at the U of S Canadian Light Source, also known as the synchrotron, enabled the scientists to study the interactions between nitrogen, the ammonia gas and carbon during the absorption process.

The synchrotron also assisted in showing the nitrogen availability to plants if the material was used as fertilizer.

What started as a search for a solution to problems in Africa has widespread applicability, particularly in agriculture Lehmann said.

“It’s a new way of thinking about the liquid and the solid in creative ways of recycling nutrients from the liquid phase — either dairy manure lagoon or human urine and solid human feces or dairy feces or chicken manure or whatever you have and combining them in ways that makes a commercially competitive fertilizer or shippable material that can be conceived in a circular economy.”

About the author

William DeKay's recent articles


Stories from our other publications