Food production is a very water-intensive business, but an international team of researchers is investigating whether growing algae in the drainage and wastewater systems of greenhouses could save this precious resource, and lower costs.
Agricultural greenhouses, like many other facilities, need to reduce their demand on freshwater sources in response to UN forecasts that global freshwater resources will decline by 40% by 2030. By growing microalgae in greenhouse drainage water, say the researchers, total fresh water usage can be reduced as the water is cleaned up without chemicals or expensive equipment, and can be reused in a closed loop.
Food producing countries such as Portugal, Spain and Italy are already experiencing water shortages, exacerbated by droughts. Due to global warming, such extreme weather events are predicted to become even more frequent, which heralds the need to redesign agricultural and industrial processes that require high volumes of fresh water.
Growing a kilogram of tomatoes in a greenhouse – although more resource-efficient than cultivation in fields – still requires 4-20 litres of fresh water. However, direct reuse of the water from the greenhouse's own drainage system risks spreading plant diseases that can destroy entire harvests - and high-tech disinfection and recycling systems can be costly.
"Recycling irrigation water is a cost. With microalgae, this cost can be turned into revenue."
In addition, wastewater discharged by greenhouses can cause environmental problems. Nitrogen, phosphorus, and other nutrients added to the irrigation water to encourage fast and healthy plant growth can be washed into rivers and lakes.
The high levels of nutrients in these waters then cause plants and algae to grow excessively, causing rapid deterioration of the water quality. Dead zones arise, in a process called eutrophication, which means only little oxygen is freely available to fish and other aquatic creatures, and many do not survive.
Planned and controlled growth of microalgae, cultivated in open channels known as raceways, can help address both problems. It makes the water discharged by greenhouses, reusable within the facility - closing the loop so it does not reach the water environment. This in turn reduces demand on freshwater sources, freeing them up for other uses, including crop irrigation and drinking water.
Excitingly, the approach can also result in new microalgal-based products for agriculture and aquaculture - transforming waste into value - and even generating extra income streams. The research project aims to test this concept by developing new low-cost cultivation systems expected to halve the cost of microalgae production.
The research project is called REALM - reusing effluents from agriculture to unlock the potential of microalgae - and is funded by Horizon Europe with an €8.6 million grant. It started in July 2022 and will run for four years in a collaboration between companies and institutions in Belgium, Finland, Germany, Netherlands, Portugal, Spain and the UK.
"Recycling irrigation water is a cost. With microalgae, this cost can be turned into revenue," explains microalgae specialist Alexandre Rodrigues from Necton, the company that coordinates REALM. "By having microalgae production facilities close to the greenhouses, microalgae producers will be getting free water and nutrients.
"They will clean that water and transform it into valuable microalgae, which can be used in innovative products to grow plants faster or to grow fish healthier."
"We will use the drain water as a source of both water and nutrients. We will use the sun as the main energy source."
Microalgae are rich in essential nutrients and produce compounds that can boost the health of plants and animals. Together, greenhouse farmers and microalgae producers can grow microalgae to manufacture aquaculture feed or agrichemicals like bio-pesticides and bio-stimulants.
As microalgae only need sunlight, carbon dioxide, water and nutrients, their growth is very environmentally friendly, say the researchers.
"The production of microalgae can be very sustainable," says biotechnologist Mariana Carneiro from Necton. "In REALM, we will use the drain water as a source of both water and nutrients.
"We will use the sun as the main energy source, and we will try to validate a direct air capture system that concentrates CO2 from the air, because microalgae perform photosynthesis and transform carbon dioxide into oxygen. Microalgae can be a natural and sustainable source of interesting compounds, which is why we should keep exploring its potential and improving its production."
Within the next four years, the researchers want to establish automatic, low-cost open-pond systems that grow microalgae next to greenhouses. Sensors in the system will monitor the growth of the microalgae and the removal of nitrogen.
The researchers anticipate to decrease the nitrate content in the drain water to below the EU limit, which is currently set at 50 milligrams per litre. At the same time, the system is expected to reduce the costs of growing microalgae by up to 50 percent.
According to the team, a production price of less than €10 per kilogram (dry weight) is plausible, which is an important step forward for the industry. Reduced production costs can help microalgae producers reach more sectors and lower priced markets.
This can boost entrepreneurship to fully seize the potential of microalgae for biofuels, sustainable food and animal feed and medicine. Microalgae is currently only available in niche markets for a few day-to-day products, such as smoothies, chocolate bars and cosmetics. However, they can make a much wider contribution.
When greenhouse farmers and microalgae producers adopt the REALM concept, they can help collaborate to overcome some of the biggest challenges they face., while increasing their competitiveness in a sustainable way – for a better future in which freshwater is not a luxury item.
Participating companies and institutions
Belgium: Interuniversitair Micro-Electronica Centrum
Finland: Turun Yliopisto
Netherlands: Wageningen University, University of Twente, Stichting Imec Nederland
Portugal: Necton (coordinator), GreenCoLab, S2AquaCoLab, Hubel Verde
Spain: Biorizon Biotech, University of Cadiz, University of Almeria, Grupo Empresarial La Caña
UK: Phycoworks Ltd, Decerna Limited