Researchers have discovered how to use willows (Salix viminalis) to recover essential nutrients from wastewater and remove carbon dioxide (CO2) from the atmosphere. The willows can then be converted into a special type of hydrothermal charcoal that can be spread on agricultural fields, where it will continue to sequester CO2 and slowly release nutrients into the soil.
The world faces numerous problems.
The atmosphere has too much CO2, and more is on the way. Phosphorus is urgently needed for use in fertilisers to feed the global population. In addition, the world is also struggling to purify wastewater to avoid polluting more than necessary.
All these problems are inherently challenging, but solutions may be near.
New research shows that willows can both purify wastewater and recover phosphorus to be reused in agriculture. Willows also sequester considerable CO2 in biomass, and if this biomass is not degraded by bacteria, it can be almost permanently removed from the global CO2 accounting.
The research also calculates the effects and shows how several problems can be solved at once.
“All over the world, researchers are trying to find solutions to global problems, but our study proposes a ready-to-implement solution that uses wetland engineering to solve several problems at once,” explains a researcher behind the study, Andrés C. Acosta, Department of Biology, Aarhus University, Denmark and German Biomass Research Centre, Leipzig, Germany.
The research has been published in the Chemical Engineering Journal.
Numerous global problems
Researchers behind the new study are trying to solve large and complex problems.
CO2 emissions are problematic, because as more CO2 enters the atmosphere, the Earth becomes warmer, and this can lead to all sorts of disasters.
One way to counteract this accumulation of atmospheric CO2 is to get trees to sequester it as biomass.
This requires, however, that the trees are not burned or otherwise degraded, because the sequestered CO2 would return to the atmosphere.
Another problem is purifying wastewater. The global consumption of water is increasing, and this requires more and better wastewater treatment. Trees can solve part of this problem: for example, willows are very good at purifying wastewater so that many harmful substances do not end up in nature, and the water can be reused in agriculture.
Finally, phosphorus is in short supply and can only be mined in very few places, and the entire global food system therefore depends on supplies from countries such as Morocco.
Further, phosphorus is a nonrenewable resource that may be depleted if it is not recycled and kept in circulation.
Willows may solve several problems
The researchers examined ways of solving several problems simultaneously.
The first step involves willows purifying wastewater and recovering the phosphorus in the wastewater from human faeces and urine.
The human digestive system absorbs only about 1% of the phosphorus present in a banana. The rest ends up in faeces and in wastewater, where willows can recover it and solve this second problem.
Growing willows can also solve a third problem by extracting CO2 from the atmosphere. One willow forest will not solve all global climate problems but contributes to a solution.
“If the willows are incinerated, much of the sequestered phosphorus is converted into a very crystalline and insoluble form that makes the phosphorous difficult to recover. If the willows are turned into woodchips and spread on the ground, much of the sequestered phosphorus will leach out and potentially be lost. In both cases, the sequestered CO2 is released back into the atmosphere,” says Andrés C. Acosta.
Sequestering CO2 in soil for millennia
Andrés C. Acosta and colleagues have investigated various methods for avoiding phosphorus being wasted and CO2 ending up in the atmosphere again.
The willow biomass can be processed in various ways so that instead of being released into soil, it can sequester CO2 for the foreseeable future.
One method is called hydrothermal carbonisation: immersing the biomass in water and exposing it to pressure and temperatures that form a carbon-like material called a hydrochar. The second method slowly heats the biomass by pyrolysis to form a pyrochar.
The researchers investigated how these types of willow-based chars differ in retaining CO2 and releasing phosphorus into the environment.
The idea is to convert biomass from willows used for wastewater treatment into char and to spread it on fields.
“Our studies show that the two types of char have different properties. Hydrochar will remain in the soil for decades or centuries, releasing 10–40% of the sequestered CO2 along the way. Pyrochar, in contrast, can sequester CO2 for millennia, and phosphorus from both types of char can be released into the environment,” explains Andrés C. Acosta.
Different properties for different crops
Andrés C. Acosta says that char also solves other problems than the three major ones described previously.
Char enables the soil to retain water better, which is good for the crops growing in the soil.
The researchers also used X-ray μ-computed tomography to investigate the characteristics of the types of char in detail and found that pyrochar has a porous structure that retains water well.
Finally, the study also showed that the willow-based pyrochar does not release toxic chemicals into the environment, such as heavy metals, which could be a concern when the willows grow in wastewater.
“With hydrochar, more phosphorus will be available for crops, whereas pyrochar is cleaner but its form of phosphorus is also more stable and less soluble. This can affect decisions about where to use each type of char. This study enabled deeper understanding of the potential of enabling willows to purify wastewater, converting the biomass into char and using it as a soil conditioner in agriculture without releasing the sequestered CO2 back into the atmosphere,” concludes Andrés C. Acosta.
