Indoor agriculture is touted as sustainable, but it comes with its own big environmental side-effects. One of these is all the materials that go into it. Think about rockwool, binding material, and plastic pots.

Not only does this require a lot of energy to produce – it also produces lots of unrecyclable waste. All of this is why the circular economy is a good idea.

How should these materials be sourced, and what should their fate be after they have been used?

At the NVTL’s annual conference in February, Christiaan Bolck from Wageningen Food & Biobased Research gave a presentation on biobased and biodegradable materials in greenhouse horticulture.

The Ellen MacArthur Foundation‘s diagram on circular materials, edited by Christiaan to include biobased materials.

Materials and Products

Biodegradable plastic is plastic that can be broken down by micro-organisms. But this isn’t the same thing as biobased plastics. Not all biobased plastics are biodegradable, and not all biodegradable plastics are biobased.

For example, PET can be made from biobased ethanol, but it is not biodegradable. On the other hand, fossil-based PBAT is biodegradable.



An overview of fossil and biobased plastics, and which of these are biodegradable. Courtesy Christiaan Bolck.

Bioplastic gets a lot of attention – but it is important to know whether the plastic you are using is biodegradable in the context in which it will be used. There are plenty of standards and certificates for this.

Biodegradability

The biodegradability of a material depends on its environmental conditions – for example, a material that will degrade on land might not degrade in the sea. Some factors affecting this include:

Presence of micro-organisms

Oxygen availability (affects anaerobic or aerobic digestion)

Water availability

Temperature

Chemical environment (pH, electrolytes, etc)

In general, degradation happens in the following order: polymers → oligomers → monomers → biochemicals → minerals. This is what ‘downcycling’ is in an ecosystem – for example, in aquaponics, when fish waste is mineralised by a biofilter to create nutrients for plants. Energy is lost and waste is broken down into simpler parts.

Downcycling in a diagram. This is the sequence in which complex materials get broken down over time. Courtesy Christiaan Bolck.

Degradation can happen in a controlled way – like in a diodigester – or in an uncontrolled way – such as if the material is disposed in the environment. There are so many options.

Plenty of options to dispose of biodegradable materials! Some are controlled, which allows us to use the energy in these materials. Others are uncontrolled or even make use of natural ecosystems.

Making Use of Biology

Although not all biobased materials are biodegradable, using biology could have its own advantages.

When making plastics out of fossil hydrocarbons, we are adding functionality to a fairly simple substance. With biobased plastics, on the other hand, we are taking something very complex – plant matter, for example – and taking away its complexity and functionality.

The existing functionality in biomass could be used. See, for example, some of Wrath of Gnon’s fascinating Twitter threads on the properties of wood:

"Those using wooden cutting boards were less than half as likely as average to contract salmonellosis, those using plastic or glass cutting boards were about twice as likely as average to contract it. Cleaning the board regularly after preparing meat made no difference." pic.twitter.com/mDOVjVRfsg — Wrath Of Gnon (@wrathofgnon) February 19, 2020

Making Cardboard From Greenhouse Crops

Turning high-quality fibre into low-quality fibre is easy, but the other way round is hard. Christiaan showed us the European Polysaccharide Network of Excellence’s cellulose matrix. This diagram shows where lignocellulose can best be used, based on its quality and availability. Christiaan then went through some of the specific properties of cellulose and how they related to different materials.

EPNOE‘s Cellulose Resource Matrix, showing the uses of fibre based on their quality and availability.

One of the more well-known examples of using fibres from greenhouse crops is using tomato stems and leaves to produce cardboard for packaging. Have a look at the majestic video below to see how it’s done.

The economics of this are important of course. Although using stems and leaves to make packaging is far more useful than composting it, the separation required to do this is expensive. That said, the good news is that tomatoes produce enough fibre to make 100% of their own packaging.

Coincidentally, at the time of writing, there is panic about toilet paper shortages. Localising the production of some of these materials can’t be too bad of an idea, even when ignoring the obvious environmental benefits!

Closing remarks

Glad to see research being done on making all those materials for indoor agriculture more sustainable. Not only that, but it was interesting to see the narratives of upcycling/downcycling and the complexity of biological materials being brought up. Many thanks again to the NVTL and Christiaan.

Courtesy Christiaan Bolck.