Greenhouse horticulture is an innovative sector that produces high-quality fruit, vegetables and plants, whilst using many of its resources very efficiently. Take for example the recirculation of water and nutrients within the greenhouse. Despite this high efficiency, supply chains are mostly linear, not circular. Like many other sectors, greenhouse horticulture relies on finite natural reserves spread out across the globe: for example, natural gas for energy and CO2; or phosphate rock (P) and potash (K), which are mined to make fertiliser; basalt and peat for substrate; or crude oil for plastics.
Precious raw materials
At some point, these natural reserves will run out. For some, such as phosphate rock, this reality is relatively close. Because of this, prices could become unstable. Not only that – the extraction and distribution of these raw materials have a considerable environmental impact. We all know about recycling, but most of these raw materials end up in the soil, rivers, oceans and air, often with negative consequences for the environment and human health. Also, once these resources have been released into the environment, they are difficult to recover, because they have become severely dispersed. Recovering phosphate from an ocean or river is like trying to recapture a spoonful of sugar after dissolving it in a swimming pool.
This is why it is crucial to think of improvements together, so that in 2050, we still have a blooming greenhouse horticulture that not only contributes with the end product, but also valorises its residual flows. Examples include leaves, stems or substrate, but also residual flows from other sectors could be upcycled by greenhouse production.
A circular future?
With insights gained from the knowledge base research (kennisbasisonderzoek in Dutch, or KB) and in collaboration with the Club of 100, we have designed a guiding vision of greenhouse horticulture in a circular economy. We see a lot of opportunities in the transition towards circular horticulture, for parties in this sector and others, to extract more value from their products and residual flows whilst producing in a cleaner way. We work together with both the public and private sector on innovations that can be applied short-term. We also look at redesigning production systems, to allow circularity to be integrated long-term. With the new knowledge from KB research, we want to work towards a circular pilot project or living lab, where innovations are shown and further developed.
The KB research focuses on 6 material flows that are used and/or generated during the production of fruit, vegetables and plants in greenhouses: Fertiliser, Substrate, Water, CO2, Plastics and Biomass. Each of these flows has its own story, with specific challenges and opportunities.
In many Dutch greenhouses, fertiliser is already recirculated within the irrigation system, resulting in an extremely high efficiency. However, the supply chain itself is not circular yet, because the raw materials for most (artificial) fertilisers come from mines that will eventually be depleted.
For nitrogenous fertilisers, things are slightly different. This is because nitrogen is stripped from the air and converted to ammonia through the Haber-Bosch process. However, the energy used for this process usually comes from natural gas. Then again, over the past few years considerable investments have been made to reduce the energy consumption and CO2 emissions in Dutch ammonia synthesis plants, by further optimising the process.
The nutrients in fertilisers leave the greenhouse as part of the end product, like a tomato, or as organic waste such as the stems or leaves (biomass). After a tomato is eaten, many of these valuable nutrients end up in the environment, where they disturb ecosystems through eutrophication. Once these nutrients are released into rivers and eventually end up in the sea, they are very difficult to get back. Imagine trying to recover a teaspoon of sugar after having dissolved it into a swimming pool.
Developing fertilisers based on other (organic) sources such as manure, food and other organic waste as well as the purification of sewage water and sludge is a challenge, but also provides opportunities. These new fertilisers must be applicable in greenhouse production systems, whilst guaranteeing a high quality to ensure efficiency and (food) safety.
Several materials are used for substrate in greenhouse horticulture, of which many come from natural reserves, like basalt for stone wool and peat for potting soil.
Although basalt is a very common type of stone, and global reserves are virtually endless, mining and transport have a significant environmental impact. The construction sector aims to recycle stone wool, used for insulation, as much as possible. In greenhouse horticulture, stone wool mats are not yet made into new mats.
Peat as a raw material for substrate is under pressure, because peat soils store carbon, which is released as CO2 (a greenhouse gas) during excavation. Exploitation of peatlands can also lead to the loss of ecosystem services such as water filtration and water storage. An EU-wide ban on the excavation and import of peat seems to be getting closer and closer.
Advantage of soilless systems
The advantage of soilless systems (in substrate or directly in water) compared to growing in soil is that water and fertiliser can be applied and controlled very precisely. Collecting and reusing water and fertiliser (recirculation) also becomes a lot easier. Because of this, substrate plays an important role in today’s greenhouses: not only to use water and fertiliser efficiently, but also to limit discharge into the soil and ditches.
When it enters the greenhouse, substrate is fairly pure. But during the growing cycle, it gets mixed with fertilisers, water, roots and root exudates, making it no longer a ‘mono-stream’. A mixed material flow is usually much harder to recycle than a mono-stream. Substrate also often gets mixed with plastic. For example, stone wool mats are wrapped a in plastic bag, and potting soil sits in plastic pots.
Still, in the Netherlands, stone wool mats get collected on a large scale and processed into new raw materials such as stone wool granulate for the production of bricks, and plastic for bin bags. Other substrates such as coconut fibre and perlite are also converted into new raw materials or products. The substrate in potted plants ends up in the consumer’s organic waste, and gets (partially) processed into compost or a soil amendment. Circular principles are therefore already regularly being applied. Despite this, every year, many tonnes of primary raw materials are imported to make substrate for greenhouse horticulture. These chains are therefore not completely closed for all types of substrate.
Finding alternative sources
The challenge, but also opportunity, is to find alternative (biobased) sources that can replace these raw materials – and to recycle material flows in a high-quality manner so they can be made into substrate again. Extending the lifespan of substrates in greenhouses can also be investigated, so that fewer raw materials are needed in the first place. Redesigning cultivation systems could mean that, for some crops, substrate could be eliminated altogether to grow soillessly, for example by suspending the roots directly in water or even in air.
All over the world, CO2 is famous for being a greenhouse gas that contributes to global warming. Although high CO2 emissions and its resulting impact on the climate are widely agreed upon, CO2 remains an essential part of the atmosphere for plant growth. Over the past 50 years, outdoor CO2 concentrations have risen from 325 ppm to about 410 ppm. In greenhouses, CO2 concentrations are increased up to 1000 ppm, to increase crop growth. This leads to a much higher tomato yield per square metre, for example.
CO2 in horticulture
Most of the CO2 used in greenhouse horticulture comes from the combustion of fossil fuels, mainly from natural gas in combined heat and power (CHP) systems. Part of the sector gets its CO2 through an elaborate network of pipes. Part of this CO2 is a by-product of the petrochemical industry, and part of it comes from a bioethanol plant.
Only a fraction of the CO2 supplemented in greenhouses is actually taken up and fixated by the crop. The rest (>90%) is emitted into the atmosphere, since it leaves through the vents into the outside air. The loss of CO2 through the windows can be reduced by up to 80% through more intelligent dosing based on sensors and control technology. A (semi-)closed greenhouse with mechanical ventilation can considerably improve efficiency. However, efficient as it may be, this does not make it circular.
Alternative sources for CO2
As the energy transition progresses and the use of fossil fuels decreases, so does the production of CO2. Alternative sources must therefore be considered if CO2 is still to be used in greenhouses. One possibility is to use the CO2 released during the incineration of waste – but this source will also disappear long-term, because in a circular economy, there will be virtually no waste to incinerate.
Developing innovative solutions for CO2 supplementation in greenhouses is a challenge, but also provides opportunities. The atmosphere can play an important role in this as a source. This is because CO2 can be extracted directly from the air using ‘Direct Air Capture’ technology. At the moment, this is too expensive to be applied in greenhouse horticulture. Indirectly, plants are a method to capture atmospheric CO2, since they fixate CO2 from the air through photosynthesis. If residual organic flows like pruning waste and manure are processed in bioreactors, this CO2 gets re-released and could be used in greenhouses.
Tension between circularity and climate change
When it comes to CO2, it is important to consider the tension between circularity and climate change. From a circularity perspective, all the CO2 that is taken out of the atmosphere is released back into the atmosphere again, leading to zero net emissions, a.k.a. ‘climate-neutral’. However, there is a growing call for reducing the amount of atmospheric CO2 to mitigate climate change. This is known as ‘climate-positive’, and would require the long-term storage of CO2 from Direct Air Capture or bioreactors. Examples include underground reservoirs, but also biochar or even trees for carbon capture and storage.
In a greenhouse, plants grow by converting water, fertiliser and CO2 into ‘biomass’. Biomass is organic material produced by plants and animals. All parts of the crop, such as the roots, stems, leaves, fruits and flowers are biomass. For some crops, the end product includes the entire plant, such as for orchids. For other crops, only part of the total produced biomass goes to the consumer, like with tomatoes. The roots, leaves and stems of a tomato plant are mostly regarded as organic waste.
Organic waste from greenhouse horticulture is currently partially processed into low-grade compost, but this biomass will mean much more in the transition to a circular economy. After all, biomass can be processed into all sorts of high-quality raw materials. One of the hurdles in doing this is that this flow currently contains not just biomass, but also plastic and metal. Clips, rings and nylon rope used to support the plants get mixed in with the residual biomass.
Making full use of biomass from greenhouse horticulture
Making use of residual biomass from greenhouses is a challenge, but also provides opportunities to set up new value chains. So much can be obtained from biomass: salts for fertilisers, organic matter or biostimulants for substrate, fibre for packaging and building materials, proteins for food and feed, aromas and dyes for the cosmetics industry and medicinal compounds for the pharmaceutical industry. Research needs to be done to determine which plant residual flows are appropriate for which applications, and how the hurdles can be overcome or eliminated. On top of this, it is just as important to involve growers and entrepreneurs in collaboratively setting up these new value chains and business cases.
Plastic plays an important role in the packaging of fruit, vegetables, plants and flowers from greenhouses. But the ‘plastic footprint’ of the final product is more than just its packaging. During the crop cycle, many more plastic products are used.
Plastic footprint in greenhouse horticulture
An obvious example are plastic pots. But there are also foils that cover the floor and cultivation tables, used to prevent weeds and reflect (or absorb) sunlight. There are also foils used in the ceiling and walls to save energy or provide shade. Substrate mats are wrapped in plastic to reduce evaporation, algal growth and the risk of pathogens. Plastic string, clips and sticks are used to physically support the plants and avoid damage during the crop cycle.
All of these products have a function in the greenhouse and are beneficial to yield and quality. But the current way plastics are used comes with considerable disadvantages as well and is not circular. Nearly all plastics are derived from two finite raw materials: crude oil and natural gas.
Plastics stand in the way of innovation
Dependence on oil and natural gas will have to decrease in the long term, but plastics get in the way of innovation short-term as well. For example, for popular greenhouse-grown vegetables such as tomatoes, cucumbers, sweet peppers and aubergines, plastic clips, rings and string stay behind with the stems and leaves. This plant biomass is often regarded as ‘organic waste’, but is actually valuable biomass, a resource in itself that can be valorised. The contamination of this biomass with plastic makes the plant material hard to process, making it a less useable resource. Environmental plastic pollution remains another big issue. This includes the spread and accumulation of microplastics in the soil and water.
Including plastic use across the value chain in the transition to a circular economy
Including plastics in greenhouse horticulture across the whole value chain in the transition to a circular economy is a challenge, but also provides opportunities. The first step in this process is to critically consider how important these products are, and whether they really serve an important purpose. If they do, then perhaps this purpose can be served by a biobased material, to reduce our dependence on oil and natural gas. New products could be biodegradable, so that organic waste becomes easier to valorise. Supply chains can also focus on plastic products that can be infinitely recycled after collection, without a decrease in quality.
Water is essential for plants, and so also for growers. The majority of the water used in greenhouses for irrigation is rainwater. To a lesser extent, groundwater is used, and a handful of crops also use surface water. Tap water is another possible source, but because of its costs and quality, it is rarely used.
Rainwater most often used for irrigation
One of the reasons rain water is preferred is its high quality. For most (substrate-based) crops, it is important that irrigation water contains as little sodium as possible. Sodium barely gets taken up by the crop, which can lead to it accumulating in the irrigation system as water gets recirculated. High sodium concentrations can compromise the absorption of fertiliser by the crop. Ground-, surface- and tap water often contain too much sodium, which has to first be removed through techniques like reverse osmosis. An additional downside of surface water is that it may contain pathogens.
Emissions-free growing by 2027
Greenhouses use water very efficiently and in a very circular way. This is because most growers use drip irrigation, combined with the recirculation of drain water. Still, water is occasionally discharged to natural surface water or the sewage system, either between crop cycles or because of the excessive accumulation of sodium. The greenhouse horticulture sector is working towards emissions-free growing by 2027, after which discharging nutrients and plant protection agents will no longer be allowed. But if water goes into a greenhouse, how does it go out?
The vast majority of irrigation water is taken up and transpired by the crop. Because of this, the air inside the greenhouse becomes more humid, and this air subsequently leaves the greenhouse during ventilation. This water is very clean and poses no threat to the environment. A fraction of the original irrigation water leaves the greenhouse in the produce. For example, tomatoes are about 95% water.
Availability depends on the climate
Broadly speaking, the water cycle of a greenhouse is very comparable to part of the natural water cycle: rain water is taken up by plants, which then transpire it. The transpired water condenses in the atmosphere and comes back as precipitation. A circular process. But unlike other material flows, the availability of water strongly depends on the climate.
Although the Dutch are no stranger to precipitation, climate change also plays a role. Heavier showers in the winter and hot, dry summers mean precipitation is less evenly distributed over the year. This leads to a higher risk of flooding and problems with water storage. The demand for water from greenhouses is higher in the summer, and storage capacity usually does not account for virtually rainless summers.
Finding synergies for a common water management strategy
Making sure enough high-quality water remains available for greenhouse production is a challenge, but also provides opportunities for collaboration. The provision of drinking water is at the top of the list, but also nature reserves need water, and groundwater- and soil quality must be safeguarded. In a circular economy, consumers of water from different sectors, water purification companies and regional water authorities will have to find synergies for a common water management strategy. An option currently being explored is to purify waste water from other (economic) processes to use it in greenhouses.
Mapping it out
Although every material flow comes with its own challenges and opportunities, these different flows are far from independent and innovations may often require an integral approach. To foster integral solutions, we have made a number of quantitative diagrams for the material flows in different crops within the KB research programme. These crops include tomatoes, phalaenopsis and roses.
Overviews like these show the materials that enter the greenhouse, and how they leave. They also show the relative size of each flow, and which flows are mixed during the crop cycle. Arranging and visualising this knowledge forms a foundation, from which to build towards circular greenhouse horticulture.
Cross-overs: Towards Circular Greenhouse Horticulture Together
Circularity is more than just one’s own sector or field of expertise. If we want to reduce our dependence on primary raw materials from natural reserves, these will have to be replaced with alternative resources. One type of solution is to connect greenhouse horticulture with different sectors. The goal of these connections is to make the outgoing flows of one (food) production system the inputs for the next. We call this concept “cross-overs” and within the KB research programme, a few cases are being investigated.
A ‘recirculating aquaculture system’ (RAS) is, just like a greenhouse, a food production system in which water is recirculated. In the Netherlands, this is a fairly new commercial method to cultivate fish or shellfish, but worldwide, it is a fast growing industry.
High degree of control over production factors
The cultivation of fish in a RAS is characterised by a high degree of control over factors like temperature and water quality, as well as a high water and feed efficiency. In a RAS, water gets filtered, pumped around and continuously re-used. The fish are kept in a separate cultivation area and fed a special feed. The fish use oxygen (O2), excrete ammonium (NH4+) and carbon dioxide (CO2) via the gills, and also excrete floating and solid waste (sludge). A mechanical filter removes the floating and solid parts. The next filter, the biofilter, converts the ammonium (NH4+) via nitrite (NO2-) into nitrate (NO3-). Aeration ensures a sufficiently-high oxygen concentration and removes carbon dioxide, after which the water is fed back to the cultivation area, closing the loop.
That said, the loop is not completely closed, because water regularly has to be discharged. This is because nitrate (NO3-) accumulates in the RAS as more and more feed is added. To keep the fish healthy, nitrate concentrations must remain below a certain level. The sludge filtered by most aquaculture companies is also in most cases a waste stream, the disposal of which through the sewage system involves costs.
Opportunities for aquaponics
This is where aquaponics comes in. Aquaponics is a cross-over production system that consists of hydroponic vegetable cultivation and aquaculture.
There are many ways to combine these production systems, but to ensure optimal conditions for both the plants and the fish, a so-called (semi-)decoupled aquaponic system is most effective. In a decoupled system, each subsystem has its own recirculating water, but the nitrate-rich waste water from the RAS goes to the greenhouse instead of being released into the environment. Although too much nitrate can be problematic for the fish, it is an essential nutrient for plants, and a shame to throw away. Other techniques exist as well, to make nutrients such as phosphate and potassium from sludge available to the plants.
An important aspect of a decoupled aquaponic system is that the nutrient-rich water from aquaculture has to first be processed before it can be used in the greenhouse. Nutrients can be added that the plant needs, but that are lacking in the water coming from the fish. The pH can be corrected. And, if necessary, the water can be disinfected (with UV, for example) and filtered to remove small floating particles. The water that is eventually used in the greenhouse for irrigation does not go back to the RAS, since the greenhouse’s irrigation system is already built for recirculation.
Being a cross-over between greenhouse horticulture and aquaculture, aquaponics fits in agriculture’s transition to a circular economy. Apart from water and fertiliser, this combination presents other opportunities for exchange of energy and CO2. We are investigating the potential and opportunities for further improvements to sustainability and want to collaborate with companies to develop solutions to make the circular production of food a possibility.
Just like a greenhouse, a pig farm has all sorts of material flows going in and out. Take feed, water, bedding material (straw, sawdust or alfalfa), manure and CO2. Many of the outgoing flows are not re-valued enough into new resources.
Using products from pig farming in greenhouse horticulture and vice versa
Manure can be processed in aerobic or anaerobic bioreactors to produce biogas, CO2 and fertiliser (e.g. the nitrogen, phosphate and potassium components). These are all products that greenhouse horticulture – after the necessary processing steps – can make use of. In the other direction, used substrate at the end of a crop cycle might be of value as bedding material in a pig-shed. Some green residual flows, like the leaves and stems of tomato and cucumber plants, may be of value as part of the pigs’ feed.
We are investigating the potential of a cross-over between greenhouse horticulture and pig farming, and want to cooperate with companies to develop solutions to make the circular production of food a possibility.
Want to know more?
Do you have a question about circular greenhouse horticulture, cross-overs or specific material flows? Get in touch with ir. AT (Alexander) Boedijn.
We are also curious to hear your thoughts:
- Which opportunities or hurdles do you see for your company in the transition towards a circular economy?
- Do you have data from practice that could be shared to improve or expand our knowledge?
- What are you getting out of the results of knowledge-base research towards Circular Horticulture?
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