From green waste to sunscreen

Research associate Gijs van Erven and his colleagues are investigating the most suitable way of tackling this. Essentially, this is about taking a fit-for-purpose approach, making smart choices about which types of biomass and technological processes to use for different applications.

What happens to the valuable substances in plant residues when we burn or compost them? In that case, they provide energy or compost, but they aren’t used as raw materials for new products. "This means we are only using a fraction of their potential," says research associate Gijs van Erven of Wageningen Food & Biobased Research. He is investigating how these valuable raw materials could be recovered from such waste streams after all.

This is becoming an increasingly compelling proposition, not just because the Netherlands is working towards a circular economy and aims to reduce its use of fossil-based raw materials, but also because securing greater independence in raw material supplies is becoming more important. "Given all the geopolitical tensions, it helps if we can make use of raw materials that are available here," says Van Erven.

He does, however, add a note of caution. We don't actually import huge quantities of plant-based raw materials that could be directly replaced with grass from Dutch roadside verges. However, we do use a lot of fossil-based materials and products, such as plastics and adhesives. "If some of that can be produced locally or in Europe from available biomass, it will reduce our dependence on fossil-based raw materials from elsewhere."

If plant material is to be used as a raw material, it often has to be broken down first. This process is known as biorefining. "Biorefining is essentially the process of breaking down plant material into its constituent components, so that you can create added value," explains Van Erven. The aim is to make full use of the plant material: rather than extracting just one valuable substance, the goal is to put as many of the plant's components as possible to good use.

Paper is a familiar example. It's made by extracting cellulose from wood. The cell walls of plants also contain hemicellulose and lignin. These three substances are closely intertwined and give the plant its structural strength. Researchers are using biorefining to try and separate them from one another so that all three can be used as raw materials.

Biorefining is a viable option for relatively clean waste streams, such as wood or wheat straw. But many waste streams aren't actually very clean at all. Those are the ones that Van Erven is focusing on. Municipal leaf waste, for example, may contain leaves from a wide variety of trees, mixed with sand, plastic and other waste. Reeds from a canal may contain not only plant material but also silt and plastic.

According to Van Erven, it is precisely these types of waste streams that are of interest, as they still contain a great deal of untapped biomass. But from a technical perspective, they're a lot trickier. Sand might sound harmless, but when tonnes of biomass are processed in a factory, it can cause machinery to wear down or even seize up. Before refining, the material must therefore first be cleaned, crushed, washed and sieved. This is followed by a series of chemical processes to separate cellulose, hemicellulose and lignin from one another.

There's a growing focus on lignin in particular. Lignin is a natural component of plant cell walls and helps give plants their structural strength. Traditional paper production has always focused mainly on cellulose. Lignin was more of a hindrance than a benefit in this process and was therefore usually burnt to generate energy for the refining process. Van Erven emphasises that this does have its benefits - but it is also a missed opportunity, as lignin has so much more to offer.

New techniques make it possible to extract lignin from biomass while better preserving its original structure, for example by using less aggressive chemicals or carrying out the processing in several stages. Van Erven and his colleagues are helping to develop these kinds of techniques and are testing how they can also be applied to more challenging biomass streams, such as reeds.

The structure is important, because lignin is not just a single substance. It's actually a whole family of substances, each with slightly different properties. At a molecular level, the lignin in a conifer has a different structure to that in a deciduous tree, and both differ in turn from the lignin found in reeds. The structure can even be influenced by external conditions such as drought while the plant is growing.

These differences play a part in determining what you can do with lignin. Lignin from softwood, for example, is better suited to certain adhesives. Lignin from hardwoods, such as willow and oak, may possess properties that make it useful as an antioxidant, for example in personal care products. Other types of lignin, on the other hand, may be suitable in asphalt production to replace some of the bitumen - a binding agent that is currently usually made from petroleum.

This is what Van Erven refers to as 'fit for purpose': understanding the properties of a raw material and matching them to a suitable application. A significant part of his research is focused specifically on that. His team is investigating the lignin structures found in different plants, as well as which biorefining process is most effective at extracting lignin, and how that process affects the structure. This is because the manner in which lignin is extracted from the plant also affects its properties. "As our understanding improves, we can make better choices in this area."

What happens to lignin during processing therefore determines how it can subsequently be used. This is particularly evident in traditional paper production, where the process is entirely focused on cellulose. Van Erven describes the residual lignin as "black sludge". When it comes to asphalt, that dark colour needn't be a problem. But for cosmetics, it would be. "If you were to use that lignin in sunscreen, you’d end up with face paint."

The extracted lignin is sometimes further broken down into components, each with their own properties. Even after that, researchers can still adapt the material for specific applications. "We are investigating how we can further improve these properties, for example by modifying the structure using chemical processes, enzymes or fungi," says Van Erven.

Applications range from asphalt and adhesives to cosmetics and personal care products. Some types of lignin can help block UV radiation and could therefore be useful for sunscreen. The efficacy would depend on the structure of the lignin. Many of the UV-protective substances currently in use are derived from fossil fuels or are not biodegradable, says Van Erven. A bio-based alternative that is also biodegradable could therefore be an attractive option. Research into such applications is being carried out in collaboration with industrial partners. But not every waste stream can be turned directly into sunscreen overnight.

The fact that it is technically possible to extract valuable substances from plant waste does not necessarily mean that it is also a sensible thing to do. If it requires a great deal of energy, water and chemicals, the environmental benefits may be lost. And if the process is too expensive, companies will be reluctant to invest in it. That's why the researchers are assessing both economic and ecological feasibility from the early stages of development.

Using a techno-economic analysis, they're calculating factors such as equipment, labour and production costs, and how much revenue the final product needs to generate to be commercially viable. They're also assessing the environmental impact, for example by examining the use of energy, water and chemicals, and CO₂ emissions. "Technological feasibility is only part of the puzzle," says Van Erven. "The process must also be economically viable and have the smallest possible environmental footprint."

The transition from the laboratory to the factory is also a challenge. For example, processing reeds into a usable raw material requires various processing steps, which must be coordinated on a large scale. Van Erven calls this a cascade. Biorefineries already exist, but setting up a facility capable of processing contaminated waste streams or waste streams of varying composition through every stage on a large scale is no simple undertaking. That's why the team is working in collaboration with industrial partners across the entire supply chain, from raw material suppliers and processors to companies that manufacture the end products.

As part of the current Knowledge Base project, Van Erven and his colleagues are comparing various biorefinery processes for separating cellulose, hemicellulose and lignin. They are investigating which method best extracts the substances whilst keeping their structure as intact as possible. They are testing this on relatively clean elephant grass, for example, but also on more contaminated waste streams such as reeds. According to Van Erven, the initial results suggest that the process can also handle more challenging waste streams with variable compositions, such as reeds. They are still investigating which method is ultimately the most suitable.

The aim of the two-year project is not to produce a ready-made factory, but to create a blueprint that will enable researchers and companies to develop the technology further. This involves not only technical considerations, but also economic and broader social issues. Where are waste streams processed, how do you organise the supply chain, and where does the value created end up? The team is therefore also exploring how to integrate the technology more widely, working with other researchers and industry partners to ensure it delivers real added value.

Moreover, according to Van Erven, this knowledge could extend beyond the Netherlands. "Technology developed here for grass-like biomass such as reeds and elephant grass could potentially be adapted for use with rice straw or sugarcane residues." Countries such as India and Brazil generate those waste streams on a large scale, and they are often incinerated. Local processing could provide new raw materials and help to reduce environmental problems caused by incineration.

Initial results show that even more challenging waste streams, such as reeds, can provide useful raw materials. Van Erven and his colleagues are still investigating which processing method is best suited to this and how it can be applied on a larger scale. But he sees plenty of opportunities: "There's definitely potential."