Treated wastewater from aquaponic systems can support lettuce cultivation, according to US researchers, which is good news for advocates of urban agriculture.

Aquaponics is a food production system that couples aquaculture - raising aquatic animals such as fish and prawns in tanks, with hydroponics - cultivating plants in water. The concept means nutrient-rich aquaculture water can be used to irrigate hydroponically grown plants, which could have an important role in developing urban agricultural facilities.

The study from the University of Illinois Urbana-Champaign examines the use of aquaponics wastewater as a growth medium for lettuce in a hydroponic system, and could potentially create a circular ecosystem for organic waste recycling and food production.

The researchers tested effluent from two aquaponic systems in combination with liquid residual from hydrothermal liquefaction (HTL), an emerging technology that converts wet biomass to biocrude oil through a high-temperature, high-pressure process. HTL yields a wastewater called hydrothermal liquefaction aqueous phase (HTL-AP), which is rich in nutrients and can potentially be used as fertiliser.

“We wanted to see if the 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 for lettuce seed germination.

"Eventually, we’ll observe different stages of crop growth, including full-grown lettuce and other crops,” said lead author Liam Reynolds, a doctoral student in the College of Agricultural, Consumer & Environmental Sciences and Grainger College of Engineering in Illinois.

For the study, Reynolds placed Buttercrunch lettuce seeds in Ziploc plastic bags on paper towels saturated with the wastewater treatments for 10 days, measuring seed germination rate and growth. He tested 32 trial solutions combining different percentages of HTL-AP with wastewater from aquaponic systems at the UoI’s Bevier Café and the Chicago High School for Agricultural Sciences. The trials also included standard hydroponic fertiliser and deionised water as control solutions.

“We’ve previously shown that it’s possible to grow lettuce hydroponically using treated wastewater; however, it doesn’t grow as quickly and effectively as it could. There are likely to be some toxic compounds inhibiting plant growth, and there are also not enough nutrients in a plant-available form,” said associate professor Paul Davidson.

“We found that solutions containing up to 8% HTL-AP are still viable for plant growth, at least in the germination phase. This is a higher percentage of HTL-AP than anybody has demonstrated before.

"This makes it possible to recycle a waste-stream that would otherwise go to a wastewater treatment plant, which takes up resources, or it would be discharged into the environment causing pollution.”

While the researchers did not find evidence the fish waste microbes benefited the lettuce at 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 does not inhibit the germination of lettuce seeds.

Aquaponic effluent could eventually supplement or even replace standard liquid fertilisers, although more work is needed to ensure the right combination of wastewater to provide adequate nutrition for hydroponic crop production. Davidson’s team will also address food safety issues, as some sources of wastewater may contain heavy metals that are toxic to humans.

The researchers conclude it is crucial to identify alternative nutrient sources to increase the circularity of global food-production systems, as well as decrease the reliance on chemical fertilisers derived from fossil fuels.

The paper, Investigating the Impacts of Wastewaters on Lettuce Seed Germination and Growth, is published in Agriculture, and was partially funded by the USDA National Institute of Food & Agriculture.