1 Introduction
Natural biogeochemical cycles of the Earth such as the carbon, nitrogen, and phosphorus cycles, as well as the water cycle are disturbed by human activities. The resulting changes in the balance of natural cycles have led to sustainability challenges like global warming, eutrophication, soil salinization, and a decline in available freshwater resources (e.g., The Royal Geographical Society
1998; Vörösmarty et al.
2010). While providing humanity with food, agriculture is a major actor imposing strains on natural cycles and resources (Cambpell et al.
2017). Limited arable land and freshwater resources, climate change, and a growing human population are endangering global food security. On current trends, maintaining food security will become increasingly difficult, if present agricultural practices are not adapted to mitigate their effects on natural cycles (Calicioglu et al.
2019; Pretty et al.
2010; Vermeulen et al.
2012). The questions of food security and the environmental impacts of agriculture are well recognized and studied, and there is a need for a shift to a more action-oriented research agenda (Campbell et al.
2016).
Recently, a lot of research has focused on utilization of CO
2 into added-value products such as hydrocarbons (Godoy et al.
2017; Khunjar et al.
2012). Hydrocarbons can be produced using bacteria employing the Calvin cycle, in which carbon atoms from CO
2 are used to build three-carbon sugars, such as most species of H
2-oxidizing bacteria (Kuenen
1999). One focus of previous research has been power-to-X (PtX) technologies to produce hydrocarbons from renewable electricity via water electrolysis and CO
2 from different sources (Koj et al.
2019; Chehade et al.
2019). There is no formal definition for PtX applications, but commonly, they refer to technologies producing something from renewable electricity through water electrolysis and additional processes (e.g., Koj et al.
2019; Uusitalo et al.
2017; Zhang et al.
2017). This definition of PtX is used in this paper. The research interest towards PtX is partly due to the forecast rapid growth of renewable energy capacity using energy generation resources such as solar and wind power, and due to environmental challenges, such as anthropogenic climate change and eutrophication, humanity has to solve to move towards sustainable development. The increasing share of renewables does not happen without problems as they cause fluctuation in energy generation resulting in occasional oversupply. To overcome this problem, different demand response solutions are proposed (e.g., Aghaei and Alizadeh
2013; Zehir et al.
2016). Fortunately, PtX applications can be designed to utilize electricity as a demand response, when electricity prices are low, and to balance the grid (Uusitalo et al.
2017; Zhang et al.
2017). Several life cycle assessment studies have shown that PtX processes in most cases lead to reductions in climate change impacts compared to fossil hydrocarbons (e.g., Uusitalo et al.
2017; Zhang et al.
2017; Sternberg and Bardow
2015). Power-to-gas is one example of a PtX application and it is seen as a promising technology for large-scale and long-term energy storage (Zhang et al.
2017).
When considering agricultural products, it is possible to produce bacterial-based protein-rich biomass, also called microbial proteins (MP), for feed and food purposes using a PtX approach. H
2-oxidizing bacterium can produce protein-rich biomass suitable for feed or food purposes by utilizing H
2, O
2, and CO
2 with additional substances. H
2 and O
2 can be produced via water electrolysis and CO
2 can be provided from sources such as air. (Sillman et al.
2019) Here, the approach is called a power-to-food (PtF) application. The main components of the PtF approach consist of a CO
2 source, bioreactor, water electrolysis, and post-processes for separating biomass from the cultivation medium and for drying. The possibility to produce biomass using the H
2-oxidizing bacterium
Cupriavidus necator has gained interest in previous studies due to high electricity-to-biomass efficiencies (e.g., Liu et al.
2016; Yu et al.
2013; Yu
2014). In addition, unlike traditional protein production, production of MP is seen as climate independent as the climatic conditions do not influence on the growing conditions of a closed production system and bacterium has a fast growth rate (Upadhaya et al.
2016; Srividya et al.
2014).
The nutritional value of some MP sources, such as MP from
C. necator, is comparable to nutritional recommendations and to traditional protein sources such as fishmeal and soymeal based on essential amino acids that must be supplied in feed, as the animals themselves cannot synthesize them (Srividya et al.
2014; Volova and Barashkov
2010; WHO/FAO
1973.). MP from
C. necator has been shown to be useful for 25–50% of the diet depending on the species and age of the animals (Volova and Barashkov
2010). Protein content of bacterial MPs from 50 to 83% is found in literature. However, the usable protein content is usually lower than the absolute raw protein content of the bacterial biomass. (Anupama and Ravindra
2000; Kunasundari et al.
2013) There are three main types of MP sources, which are fungus, yeast, and bacterial protein. The doubling time of bacterial protein is the fastest among the types of MP sources. (Srividya et al.
2014.) Quorn, spirulina, UniProtein®, and FeedKind® are examples of MP-based products available in the market
.
Although bacterial MP is seen as an environmentally sustainable alternative to conventional protein sources, there exist only few MP-related LCA studies focusing on food or feed production. (e.g., Cumberlege et al.
2016; Knudsen et al.
2016). LCA study has been conducted for a bacterial MP known as FeedKind®, which is produced by the biotechnology company Calysta. FeedKind® is a bacterial protein source produced for feed purposes. The bacterium uses methane to build up its biomass, thus it is called MP via methane in this study (Cumberlege et al.
2016). Another example of a MP source that has undergone LCA is microalgae. Their use as food or feed has gained interest in recent years, but the research has mainly focused on their utilization as a raw material for biofuel production (Aresta et al.
2005; Mata et al.
2010; Sander and Murthy
2010; Quin and Davis
2015). Pikaar et al. (
2018) used the MagPie model (Pop et al.
2010) to simulate avoided cropland expansion areas, greenhouse gas emissions, and nitrogen pollution impacts of several bacterial MP production pathways. The biggest avoided impacts were gained by using MP via water electrolysis, which is a similar kind of pathway to produce MP than the studied PtF application has. In addition, it has been shown based on quantitative literature review that it is possible to produce MPs with less direct land occupation area and freshwater use than conventional protein production by using renewable energy, in situ water electrolysis, direct air capture technology, and post-processes to separate microbial biomass from cultivation medium (Sillman et al.
2019). However, to the extent of our knowledge, there are no LCA studies comparatively evaluating MP production via different PtF approaches, even though there are several LCA studies focusing on different PtX technologies (Koj et al.
2019). Different PtF approaches have many technological system modifications, of which the energy sources used, origin of substances needed in the production processes, the bacterium species used, and the selected process optimization are a few examples. These technological system differences influence different categories of the environmental life cycle impacts of the production processes; thus, it is essential to know which kind of technological choices should be preferred in terms of environmental sustainability.
As the overall environmental impact of various system modifications of PtF applications is not known, it is necessary to compare how different system modifications impact LCA categories and which approaches have the least environmental impacts. The sustainability can be evaluated by investigating categories related to the planetary boundaries presented in Steffen et al. (
2015). The concept of planetary boundaries defines a safe operational zone for humanity for nine environmental activity categories. Water use, land use, biodiversity loss, climate change, and nutrient flows are examples of activities in which agriculture has a major role and which have either exceeded or are close to exceeding safe operation spaces (Cambpell et al.
2017; Steffen et al.
2015). The selected categories for evaluating the sustainability of different modifications of the PtF process are related to climate change, land use, freshwater use, and eutrophication. As regards impacts related to biodiversity loss, there are severe limitations to including biodiversity impacts in LCA methodology (Notamicola et al.
2017). Therefore, biodiversity impacts are not assessed in this study.
The aim of this study is to investigate whether a climate-independent PtF technology can be designed to produce protein-rich biomass that has minimal sustainability impacts compared to other protein-rich sources and, furthermore, to establish which PtF system modifications are the most environmentally sustainable. The hypothesis is that protein via PtF application can be designed to cause less environmental impacts than comparable protein sources. The comparable protein sources are soybean and a few other MPs. Soybean is chosen as it is a widely used plant-based protein source and the nutritional value is comparable with protein via PtF application. Other protein sources are selected to compare the sustainability of PtF to other MPs. If the hypothesis is true, the knowledge can be used for mitigation of the impact of food systems on the natural environment. For instance, by substituting protein sources with higher land use impact with ones having lower land use impact, the saved land could be used, e.g., as sinks for atmospheric CO2. This study provides novel information about how food production can be integrated with electrical power production via PtF applications and information about the environmental impacts of PtF applications.
4 Discussion
The LCA in this study shows that compared to soybean production, bacterial biomass can produce protein many times faster with less water use, lower land area requirements, less eutrophication, and lower GWP impacts. Especially the best-case setups of the studied technologies can produce high-quality bacterial-based protein with significantly reduced environmental impacts. Even when best-case setups of PtF applications are not used, the environmental impacts in the studied categories are in many cases smaller than the other protein sources studied (Table2; Figs.
2,
3,
4, and
5). The exception is blue-water consumption, especially when solar energy and DAC is used, but then again, the direct water consumption is not so great. Therefore, the PtF technology has many design options causing relatively small environmental impacts. The flexible design can be beneficial from the perspective of optimal design for local resources and local climate conditions. For instance, the production system can be designed as a closed system, and as such, it will not cause nutrient runoff to the environment. In addition, the production is location and climate independent (Srividya et al.
2014).
The life cycle assessment consists of the major material and energy flows of the PtF applications based on secondary data found in literature and by manufacturers. The impacts of amine consumption of the DAC process are based on estimates of generic organic chemicals; thus, the impacts of precise amine-based chemicals should be investigated. The facilities for MP production, minor nutrients in the cultivation medium and minor unit processes were omitted from the study. In addition, the safety-related aspects, such as contamination and pH control, might slightly cause impacts during the maintenance, which were also omitted. The cumulative impacts of these omitted materials, unit processes, and facilities should be investigated, to gain a better understanding of the lifetime impacts of the PtF applications. The energy and material requirement of the studied bioreactors were laboratory-scale reactors; thus, the material and energy requirement of bioreactors with larger capacity should be tested, although scaling up the capacity would appear to be unproblematic (Reed et al.
2015). Overall, this study gives valuable information when designing sustainable PtF systems.
The countries of the EU import millions of tons of soymeal and soybeans for food and feed purposes. Most of the imported soybeans and soymeal comes from the United States of America and South America. Imports from South America are problematic as there are many sustainability challenges related to soy crop production, for instance, challenges related to soybean farming in former rainforest areas (Barona et al.
2010; Fearnside
2001). By substituting imported soymeal and soybeans produced in South America with protein produced via a PtF system, many environmental impacts can be alleviated, and the food production system can move towards remaining within planetary boundaries as regards climate change, nutrient flows, water use, and land use. It should be noted that the results of this study give an overview of the impacts but do not account for all indirect impacts in transition from one protein source to another. In addition, the amount of substitutable protein is limited and protein from soybean is not the only source the MP via PtF can substitute. For example, Pikaar et al. (
2018) estimate that approximately 10–19% of the protein content in feed is substitutable. However, the MP via PtF is not yet commercialized; thus, the production process must undergo several safety-related tests before it can be used either for food or feed purposes (Dominique et al.
2016).
Although biodiversity is a major category in environmental impact analysis, it is not quantitatively researched in this study. Biodiversity is not commonly studied in life cycle impact assessments due to difficulties measuring biodiversity impact reliably without knowledge of local conditions (Notamicola et al.
2017). However, as pesticides and herbicides are not used in the PtF production processes (Srividya et al.
2014) and there is a possibility to use non-arable land for production facilities, there is a strong indication that the biodiversity impact of MP production is minimal compared to traditional protein production in agriculture. For instance, the worldwide reduction in insects is one alarming indicator of the collapse of our surrounding biodiversity. The main drivers of insect reduction are intensive agriculture and widespread use of pesticides (Sánchez-Bayo and Wyckhuys
2019; Geiger et al.
2010). Furthermore, the majority of soybeans and soymeal imported to the EU originates from South America, mainly Brazil and Argentina. These areas have been identified as being at risk of loss of biodiversity due to increased pressures from soy production (WWF
2014). Future research is needed on how PtF for MP affects biodiversity and its potential to free land from crop production for other purposes, for example, as carbon sinks by afforestation.
When utilizing nutrients from waste flows, as suggested, for example, by Matassa et al. (
2015) and Matassa
a et al. (
2016), questions remain regarding safety aspects of product sterility (Ritala et al.
2017). However, according to results gained from LCA, using wastewaters as P and S sources causes only a small reduction in the studied impact categories, and thus, there is no significant environmental benefit gained by using wastewaters. In the case of the ammonia or ammoniac source, most of today’s NH
3 is produced with the Haber-Bosch process using natural gas as an energy source and to provide H
2 to the process. Thus, only NH
3 from natural gas was considered in this study. Current practice for NH
3 production is fossil dependent and has high environmental impact (Udvardi et al.
2015). However, it is possible to produce NH
3 by supplying H
2 using alternative technologies, which may reduce the environmental impacts of NH
3 production (e.g., Murakami et al.
2005). For example, ammonium sulfate can be recovered from biogas digestate at the sanitation phase. The process has lower systemic energy cost than NH
3 production with the Haber-Bosch process (Törnwall et al.
2017). NH
3 can also be recovered directly from the biogas digester through a semi-permeable membrane, which not only produces ammonia but also improves the digester efficiency (Lauterböck et al.
2014). In view of these alternative NH
3 production methods, the possibility of reducing the impacts of PtF by using novel production practices for NH
3 supply should be investigated.
Electricity generation and the unit process consuming most of the electricity, i.e. the bioreactor with electrolysis, have a major effect on the studied environmental impacts. Thus, for the PtF application to be more sustainable than other comparable protein sources, the source of electricity should be chosen carefully. For example, FImix using the grid mix in Finland as an electricity source for the PtF application causes higher GWP, land occupation, and blue-water consumption values than soybean production, even though a major part of the Finnish grid mix consists of renewables and nuclear energy. As regards the electricity-to-biomass efficiency of the bioreactor, the use of external water electrolysis can result in lower energy consumption than using in situ electrolysis, but there are safety aspects that need to be considered. For example, the gases fed to the reactor may ignite, when they are in contact with measurement instruments in the bioreactor, causing an explosion (C&EN
2016). In addition to the electricity source and bioreactor efficiency, the source of CO
2 has a pronounced effect on the overall sustainability of the PtF process. If there are no reasonable point sources of pure CO
2, using DAC can be beneficial. DAC can separate water from air, making the process produce more water than it consumes. Water separation could be advantageous in areas having high water demand. However, using DAC increases the environmental impacts by approximately 10% as the unit process consumes energy and amines. Nevertheless, a PtF setup with a DAC unit process may have less environmental impact than other sources of protein.
Different PtX applications are usually energy-intensive technologies (Koj et al.
2019; Sternberg and Bardow
2015) and PtF is not an exception. It could be argued that the PtX technologies with the least environmental impacts and the least energy-consuming solutions should be preferred to limit the increase of energy demand (e.g. Sternberg and Bardow
2015). However, there are several aspects that should also be considered. For instance, what products from PtX technologies should replace and what different kinds of environmental impacts should be considered, when making the choices. In the case of protein from PtF technology, there are several impact categories that are relevant in the agricultural sector. Land use, water use, fertilizers use, and biodiversity related impacts can each be the most important impact category depending on what product and where the product is produced. Another aspect is that is the limit of possible renewable energy an issue. It is known that the potential of renewables exceeds many times the energy needs of humankind; thus, it is theoretically possible to construct 100% renewable energy systems (e.g., Barbosa et al.
2017; Connoly et al.
2016).
Electricity consumption in MP production via the PtF approach is higher than in soybean production. However, the trend of the price of renewable energy is falling and production of bacterial MP could be balanced according to the varying production and load of the grid, leading to reduced electricity costs, and/or incentive payments (Zehir et al.
2016). A possible future increase in the cost of food might transform production costs in favor of MP production. Thus, a topic of great interest would be to research the critical tipping point for the economic feasibility of PtF for MP production. Such research should also include techno-economic assessment to establish the best economical setup of PtF application in different locations.
Publisher’s note
Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.