Wastewater is a useful medium for growing lettuce in hydroponic systems

  • Food
  • July 10, 2024

Urban agriculture has the potential to improve food security through local, efficient, and sustainable food production. Examples of urban food systems include hydroponics, which grows plants in a nutrient solution without soil, and aquaponics, which combines hydroponics with fish farming in tanks.

A new study from the University of Illinois Urbana-Champaign explores the use of aquaponics wastewater as a growing medium for lettuce in a hydroponic system. This practice has the potential to create a circular ecosystem for recycling organic waste and food production.

The researchers tested effluent from two aquaponic systems in combination with liquid residue from hydrothermal liquefaction (HTL), an emerging technology that converts wet biomass into biocrude oil through a high-temperature, high-pressure process. HTL produces wastewater called hydrothermal liquefaction aqueous phase (HTL-AP), which is rich in nutrients and has potential as fertilizer.

“We wanted to see if naturally occurring microbes from fish waste in aquaponic systems could help convert the nutrients in HTL-AP into forms that plants can absorb. We focused on using wastewater to germinate lettuce seeds. Ultimately, we will observe different stages of crop growth, including mature lettuce and other crops,” said lead author Liam Reynolds, a doctoral student in the Department of Agricultural and Biological Engineering (ABE), part of the College of Agricultural, Consumer and Environmental Sciences and The Grainger College of Engineering at Illinois.

Reynolds conducted the research as an undergraduate student in ABE. He won second prize for the paper in the KK Barnes Student Paper Award Competition at the 2023 American Society for Agricultural and Biological Engineers Annual International Meeting in Omaha, NE.

For the study, Reynolds placed Buttercrunch lettuce seeds in Ziploc plastic bags on paper towels soaked in wastewater treatment products for 10 days, and measured the seeds’ germination and growth.

He tested 32 test solutions combining various percentages of HTL-AP with wastewater from aquaponic systems at the U. of I.'s Bevier Café and the Chicago High School for Agricultural Sciences. The tests also included standard hydroponic fertilizers and deionized water as control solutions.

“We have previously shown that it is possible to grow lettuce hydroponically using treated wastewater; however, it does not grow as quickly or effectively as it could. There are probably some toxic substances that hinder plant growth, and there are also not enough nutrients in a form available to plants,” said co-author Paul Davidson, an associate professor at ABE.

In previous studies, Davidson's research team used 2.5% HTL-AP; however, Reynolds tested solutions ranging from 1% to 10%.

“We found that solutions with up to 8% HTL-AP are still viable for plant growth, at least in the germination stage. This is a higher percentage of HTL-AP than anyone has shown before,” Davidson said. “This allows us to recycle a waste stream that would otherwise go to a wastewater treatment plant, consuming resources, or be discharged into the environment and causing pollution.”

While the researchers found no evidence that the fish waste microbes benefited the lettuce during the seed germination stage, they expect to see effects as the lettuce grows. For now, they concluded that a combination of HTL-AP and aquaponic wastewater did not inhibit lettuce seed germination.

Aquaponic effluents could eventually supplement or even replace standard liquid fertilizers, though more work is needed to ensure the right combination of wastewater to provide sufficient nutrition for hydroponic crop production. Davidson’s team will also address food safety concerns, as some sources of wastewater can contain heavy metals that are toxic to humans.

The researchers conclude that identifying alternative sources of nutrients is critical to increase the circularity of global food production systems and reduce reliance on chemical fertilizers derived from fossil fuels.

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