Environmental resources are currently at stake due to overwhelming anthropogenic influences, notably from agriculture (Kala Mahaswa, Widhianto, & Hasanah, 2021). Broadly, these include heavy extraction, pollution, land use change and other modes and methods of resource use and environmental management that dominate modern human civilization. These practices disregard the intrinsic value of nature, contribute to climate change and other planetary boundary tipping points, endanger economic factors that allow societies to thrive and grow and contribute to socioeconomic inequities. Agriculture is imperative for human survival; thus, it is increasingly evident that agricultural practices must transform to take advantage of techniques that use all resources as efficiently as possible and eliminate the notion of waste. One such example of this, is the use of agricultural biomass from lignocellulosic, vegetable and fruit byproducts through the integration of three different ‘clean’, renewable processes in what is called the biorefinery approach (Fermoso et al., 2018).
While the biorefinery approach is broad, the sustainability benefits include the maximization of resource efficiency use through the joint application of three processes that valorize the ‘waste’, as reviewed for the first time by Fermoso et al. (2018). The approach systematically integrates the extraction of high-value compounds, anaerobic digestion and composting of agricultural byproducts, as illustrated in Figure 1. Using these methods together in a system, rather than on their own, would have the potential to scale up the bio-based economy, increase profits, avoid common trade-offs associated with the practices on their own and overall maximize benefits of agricultural management. The end goal of a biorefinery is to optimize the use of biomass in order to produce fuels, power, materials, chemicals and other value-added products while simultaneously optimizing costs, resource use, environmental protection, carbon balance and a range of socioeconomic aspects including food security, decreased pollution, livelihood improvement for farmers, etc.
Figure 1: This dynamic model illustrates how the three processes influence one another in a circular mechanism that maintains the sustainability and symbiosis of the biorefinery process.
Understanding the processes in a ‘biorefinery system’ can be comparable to a petroleum refinery, however the main inputs would be renewable, sustainable, regenerative and ecologically sensitive (Bridgwater, 2013). Firstly, the recovery and extraction of high-value compounds would serve as the main profitable output, while also partially detoxifying the waste. The extraction of compounds is a valuable process; however, it could be the costliest of the biorefinery processes, and it requires high energy consumption. On the other hand, the recovered compounds could also bring the highest income, resulting in profitability and investment opportunity for the biorefinery. Where the cyclical solution comes in for the energy use requirements, is with the second function of the system, the anaerobic digester. Anaerobic digestors generate methane from organic waste, which is an effective waste treatment technology that simultaneously achieves pollution reduction and energy recovery (Ariunbaatar, Panico, Esposito, Pirozzi, & Lens, 2014). Anaerobic digestion of agricultural waste is widely reported on (Fermoso et al., 2018), and not only does the process deliver biogas energy, but it retains the nutrients contained in the biomass. Lignocellulosic biomass and vegetable and fruit byproducts are proposed as inputs for the anaerobic digester, which are then used for the production of biofuels. The biofuel is then what powers the energy needed for the extraction of high-value compounds, as well as other energy requirements for the biorefinery. The common barriers and trade-offs for adoption and scaling up of anaerobic digesters include high investment costs, low stability and low product value (Kim et al., 2016; Fan Lü, Wang, Zhang, Shao, & He, 2021). However, these could partially be supplemented through the financial gains from selling the recovered compounds. Finally, the application of the wet residue called ‘digestate’ produced from the anaerobic digestion process, can be inputted back into agricultural soils, closing the nutrient cycle and returning nutrients from wastes back to the soil. However, since a direct application is not always suitable or safe, as it could spread pathogens in the environment in which they are applied, composting is introduced in the final biorefinery step. Composting is an important process that biologically stabilizes the organic matter and turns it into stabilized humic substances, which recovers nutrients from the digestate and minimizes environmental pollution risks. Altogether, these processes complement one another in the biorefinery system, creating a bio-based value chain that contributes to food security, environmental protection, waste reduction and economic progress.
The biorefinery approach exemplifies a value chain that fits into the model of the circular economy, which is a concept that challenges the design of currently dominant linear economy models that ‘take, make, dispose’ (Rhodes, 2017) instead of managing resources with a sense of their current and future roles in natural environments and human communities in mind. The circular economy is loosely defined, however it is characterized by methods that consider sustainable management for retaining resources in the economy to the longest extent possible (Korhonen, Honkasalo, & Seppälä, 2018). The circular economy rejects economic models that produce exorbitant amounts of waste and use resources without care, therefore the biorefinery value chain fits within the circular economy because it valorizes what would otherwise be considered organic waste and turns it into a useful and productive source that produces multiple benefits (Rhodes, 2017).
The biorefinery approach does not only hold economic, sustainability and environmental benefits, but human health benefits as well. Figure 2 exhibits a table included in the review by Fermoso et al. (2018) that notes the added benefits that come from the extraction of valuable compounds from fruit and vegetable byproducts. These include pharmaceutical, cosmetic and food industry benefits, as well as the multiple human health benefits including “antioxidant, cardiovascular, antihypertensive and antiproliferative effects” (Fermoso et al., 2018, p. 8452).
Figure 2: It is noted that most vegetable and fruit byproducts present similar health benefits to the vegetable itself, thus inciting the interest in “their use as a source of phytochemicals and bioactive compounds” (Fermoso et al., 2018, pp. 8454-8455).
Human populations cannot thrive if agricultural systems do not thrive. In this lens, agricultural ‘waste’, or as it fits into the circular economy, the opportunities for agricultural biomass, are going to be consistently available and abundant. Thus, this concept is sustainable because the resource is going to in a state of continued renewal. To upscale this value chain, there must be continued efforts towards comprehensive cost-benefit analysis of each biorefinery process, both on its own and as it operates within the entire system. For example, although extracted compounds will be the main profitable deliverable of the biorefinery, prices of extracted compounds are in constant flux, thus there is an urgent need to study profitability of the biorefinery in this aspect, since many of the benefits derive from the supposition that this will be a profitable aspect of the biorefinery (Fermoso et al., 2018). Other efforts for scaling up this system include the exploration of alternative uses of methane from anaerobic digestion; further reduction in investments and operating costs and the study of each raw material to determine appropriate production conditions; continued efforts for enhancing the biodegradability of lignocellulosic agricultural byproducts and the biomethanization of fruit byproducts through effective pretreatments; correct monitoring of anaerobic digestion of fruit to avoid acidification and possible need for addition of nitrogen source; etc. (Fermoso et al., 2018). There are currently a number of efforts for improving and further studying specifically anaerobic digesters, such as developing methods to improve resilience of anaerobic digestive systems (Amha et al., 2018); the recovery and harvesting of not only biogas but other value added products such as lactic acid (Kim et al., 2016) and volatile fatty acids (Zhu, Leininger, Jassby, Tsesmetzis, & Ren, 2021); the removal of inhibitory compounds such as volatile fatty acids (Yuan & Zhu, 2016) and ammonia (F. Lü, Luo, Shao, & He, 2016) and more broadly, the larger scale production and stable in-situ extraction of anaerobic digestion bioproduction (Fan Lü et al., 2021). According to Guo, Song, and Buhain (2015), the renewable biogas energy produced through anaerobic digestion could potentially provide a quarter of the world’s natural gas demand, and 6% of primary energy demand (Fan Lü et al., 2021), and the research being conducted on the capacity of anaerobic digesters to fulfill energy needs and effectively use biomass, particularly food waste, is steadily increasing (Komilis et al., 2017). These encouraging steps toward biorefinery components can be used as a window of opportunity for the introduction of a systems process that recognizes agricultural biomass as a valuable resource.
In conclusion, while it can be observed that the biorefinery approach can be a leading example for the energy sectors’ transition to the circular economy, there are still many improvements to be made in order for the system to be scaled, equitable, cost effective and sustainable. The communities involved in research and development of anaerobic digesters must work closely and in collaboration with those involved with the extraction of valuable compounds as well as the research efforts towards more efficient and cost effective composting methods. Farmers and others involved in agribusiness supply chains must be kept in consistent communication about developments in biorefineries, as the agricultural biomass from their lands will serve as the most relevant input. Key stakeholders from sectors in energy and human health and nutrition must also be involved, as their expertise will guide and promote the development of biorefineries as beacons for sustainable and renewable energy with the bonus of improving human health from decreased pollution, as well as nutrition benefits from improved soil health. Lastly, experts in climate action must be key stakeholders in the development of biorefineries, as it will be crucial to investigate the footprints of biorefinery plants into the future.
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