Food security is a global concern. Experts estimate that farmers will need to produce more food in the next 35 years than ever before in human history. This is because the global population has increased significantly. According to the United Nations (UN), it is three times larger than it was in the mid-twentieth century. Medical advancements have led to a decline in mortality and an increase in longevity, which has resulted in the increase. Climate change is also affecting food production in some countries. These changes affect crop growth differently depending on the location. For example, some regions experience ideal growing conditions that propagate crop growth. However, some regions experience extreme weather conditions such as floods, droughts, and extreme temperatures, which can cause crop loss.

Extreme weather can destroy huge crop yields, resulting in shortages of certain foods in some locations. In 2010 and 2012, warm evening temperatures affected corn yields across the U.S. Corn Belt. Additionally, premature budding due to a mild winter caused $220 million in losses of Michigan cherries in 2012.

The increased demand for food can also contribute to climate change. As crop losses increase, the need for food imports also increases. This elevates the food miles on some produce as well as the global carbon footprint. Furthermore, it may also lead to the elevated use of nitrogen-based fertilisers to propagate crop growth, polluting the atmosphere.

Additionally, crop losses or the destruction of farmland caused by extreme weather can result in the expansion of farmland by cutting down forests and destroying habitats and ecosystems.

Recently, researchers from the U.S. published an article in the Open Access journal Agriculture exploring urban agriculture methods, more specifically the use of aquaponics waste water as a growth medium for lettuce in hydroponic systems.

What is urban agriculture?

The United Nations (UN) defines urban agriculture as:

“Practices that yield food and other outputs through agricultural production and related processes (transformation, distribution, marketing, recycling), taking place on land and other spaces within cities and surrounding regions.”

Urban agriculture has great potential as it allows localised crops to be produced in densely populated areas year-round. However, it does have a variety of challenges to overcome, including high capital requirements, especially for vertical farming and controlled-environment agriculture, and being energy intensive due to the requirement of artificial lighting and fossil fuel-based synthetic fertilisers.

Urban agriculture can take many forms, from creating gardens in built-up areas via community, rooftop, or back gardens to vertical farms and hydroponics. Vertical farms are ideal for built-up areas as they are designed so that crops can grow on top of each other. Moreover, they can grow inside tray systems or towers, which utilise the best of a small space.

Hydroponics for sustainable crop growth

The U.S. Department of Agriculture (USDA) defines hydroponics as:

“Hydroponics is the technique of growing plants using a water-based nutrient solution rather than soil and can include an aggregate substrate or growing media, such as vermiculite, coconut coir, or perlite. Hydroponic production systems are used by small farmers, hobbyists, and commercial enterprises.”

Plant researchers have used this technique for centuries to study plant physiology. Research suggests that the technique could date back to the Hanging Gardens of Babylon, built in 600 BC. It’s a sustainable solution to propagating crop growth as it uses less water and requires less land. In addition, it reduces the need for pesticides and herbicides, reducing pollution into the atmosphere. Furthermore, it reduces water pollution and waste by decreasing runoff and nutrient leaching.

Aquaponics for sustainable crop growth

Aquaponics is a process that can be traced back to indigenous communities across the globe.

It is a sustainable, highly engineered water-based agriculture system that combines rearing fish in tanks and hydroponics. The water used to rear the fish is rich with nutrients, so it is used as a natural fertiliser for the plants. In return, the plants purify the water for the fish.

Using wastewater to propagate crop growth in a hydroponic system

The study mentioned about investigated the prospects of developing a hydroponic system that uses wastewater to propagate the growth of buttercrunch lettuce (Lactuca sativa L.). Three different sources generated the wastewater: Chicago High School for Agricultural Sciences (CHSAS), Bevier Café, and the University of Illinois (UIUC) hydrothermal liquefaction (HTL) plant.

The researchers used aquaponic effluents from CHSAS and Bevier Café. The UIUC HTL plant processed the third wastewater source.

HTL processes wet biomass into sustainable fuel sources in the form of biocrude oil. The wet biomass is converted thermochemically via a hot pressurised water environment, breaking the solid biopolymeric structure into liquid components and producing biocrude oil. This process also results in by-products, including the aqueous phase, or Hydrothermal Liquefaction Aqueous Phase (HTL-AP), which has potential for use in crop production systems.

Researchers suggest that HTL-AP has potential for crop production because it has been thermally treated to eliminate pathogens and bacteria via HTL while retaining essential plant nutrients

“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 Professor Paul Davidson

The researchers sealed buttercrunch lettuce seeds in Ziplock bags along with paper towels saturated with the aquaponic effluent samples from each source as well as the HTL-AP and two controls. The study aimed to investigate if wastewater could propagate the germination of the lettuce seeds.

“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 Liam Reynolds lead author on the paper.

Results

The study identified that the CHSAS aquaponic effluent performed better than Bevier Café. CHSAS has a bigger aquaponic system generating effluents with higher concentrations of nitrate and ammonia. The researchers also identified that the seed germination was not inhibited by solutions containing 2‒8% HTL-AP. They noted that these solutions performed similarly to one of the controls, standard hydroponic fertiliser. Furthermore, they also suggest that solutions of higher concentrations may result in inhibitory effects in plants, and lower concentrations may not have enough nutrients to sustain plant growth. In addition, they state that it’s clear that further research is needed regarding the combination of wastewaters to provide a reliable and sustainable fertiliser.

“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.” – Professor Davidson

Alternative nutrient sources are needed to increase the circularity of global food production systems as well as decrease the reliance on chemical fertilisers derived from fossil fuels or mined from the earth.

If you would like to read more on hydroponics and sustainable agriculture or would like to submit research in this area, please see the Special Issue in Agriculture: Innovative Hydroponic Systems for Sustainable Agriculture.