Fertilize crops with human waste? Let me explain, researcher says

Water flows through a number of grid chambers.
Water flows through a number of grid chambers at the Metropolitan Wastewater Treatment Plant in St. Paul.
Evan Frost | MPR News

Sewage. Wastewater. Ew. In most places, both are things to discard and avoid. But what if we could use them to replace something we already spend a lot of time and energy making, like synthetic fertilizer for crops?

That's the vision Jeremy Guest, a professor of civil and environmental engineering at the University of Illinois, is pursuing.

Guest said more work is needed to engage both farmers and wastewater treatment plant managers on the possibilities, but he says now is the perfect time to talk about it. Communities are facing expensive wastewater maintenance and upgrades and stricter regulations for the water they release into rivers and lakes. And locally sourced fertilizer could benefit farmers, too, he said.

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How do most cities treat wastewater?

In general, the way we manage wastewater is by reducing the pollutants in it. We focus on minimizing the hazards to the water it will be discharged into, and then use biological treatment processes — bacteria that eat contaminants — to reduce the concentration of contaminants in the water.

From there, we make sure it's safe to release into a stream or a lake or other kind of water body.

What happens to the solid waste?

One of the first things that happens when wastewater arrives at a treatment plant is that the solids are separated from the liquids. The solid parts usually wind up at a landfill.

What are the consequences of dealing with wastewater the way we do now?

Wastewater trash
Trash is screened out of the wastewater stream before the water is treated at the Metropolitan Wastewater Treatment Plant in St. Paul.
Jennifer Simonson | MPR News file

Cities' current approach to managing everything that gets flushed and poured down the drain is to treat everything in the water as a pollutant that has to be removed or mitigated, Guest said.

This has worked well when it comes to the organics and pathogens and even solids in the water, he said, but now many small towns and larger utilities are facing the challenge of removing nutrients like nitrogen and phosphorus, which can lead to cyanobacteria blooms like those in Toledo, Ohio, in 2014.

This focus on managing everything as pollutants leaves water utilities with limited options for treating nutrients — and most of those options are prohibitively expensive. And even once the nutrients are removed from the treated water, they need to be discarded. Often, that means that nitrogen goes back into the atmosphere and phosphorus ends up in a landfill or on other land.

Instead, the industry has been working on re-orienting its thinking about nutrients, Guest said, and moving from nutrient removal toward nutrient recovery. That's especially true when it comes to nitrogen, phosphorus and even potentially potassium, all of which are the types of nutrients that help plants grow.

What are some ways to repurpose the waste?

Guest said that if we were to shift to nutrient recovery instead of nutrient removal, we might be able to grow crops by using a technique called fertigation, which is when a farmer uses the byproducts of treated water to grow crops.

It's already possible to grow algae as a soil additive or for other purposes, and it's even possible to recover crystal products, which look and feel like synthetic fertilizers.

Is that happening already in some places?

Many utilities have tried to be proactive about the way they treat water, Guest said. They've been separating the nutrients from the water and applying them to agricultural land nearby. And some companies have started up to do the same. One product, called struvite, comes from extracted nutrients, and works like the crystal fertilizers farmers already use. Other companies are in the process of scaling up to cultivate algae, which can be sold as a soil additive or turned into plastic-like materials.

The most common way to recover nutrients from wastewater, he said, is by producing biosolids. It's when microorganisms grown during wastewater treatment are stabilized — made safe for application on soil — so they can be applied to agricultural lands as a nutrient-rich fertilizer.

Many treatment systems create biosolids, but the latest developments take that process a step further, using technology that turns the recovered nutrients into fertilizer products that are closer to what farmers might already be buying on their own.

Why isn't this method of using wastewater nutrients to feed crops more common?

It's largely a business question, Guest said: Utilities need to make sure the process is cost-effective — and they need to make sure they're creating fertilizer that farmers want.

Ultimately, he said, no single solution will work for every utility, and each wastewater treatment organization will have different needs, depending on location, size and local agriculture. "Bigger utilities have the advantage of economies of scale," Guest said. "For small utilities, it's a different challenge and they may pursue more local recovery options, which requires engagement with local stakeholders."

Does recovering nutrients from wastewater make financial sense?

"Especially in larger cities," Guest said, "the recovery of nitrogen and phosphorus is cost-competitive with its removal."

So, at large wastewater treatment plants, there's not a huge financial incentive to move toward nutrient recovery. That won't likely happen, Guest said, until the technology evolves.

"We need a portfolio of technological solutions that can help different utilities, various locations and sizes," he said, so the process is accessible and financially useful for any utility that might want to use it.

We're already on the path to making reusing human waste for farming a more common and viable option, Guest said. "When you see the impacts of nutrients in the aquatic environment and, in particular, cyanobacteria blooms, the way to manage them is to reduce the amount of phosphorus that goes into our fresh waters," he said. "Agriculture has a role to play in that, without a doubt, but utilities are the ones that are most heavily regulated."

It's wastewater utilities that have to adapt the most, he said, particularly as municipalities and states continue to make their rules about the amount of phosphorus allowed in their waters stricter.

Guest is in St. Paul Tuesday night to give a talk at an event hosted by the Freshwater Society.