Key factors for site-selection of biogas plants in Sweden

Biogas production through anaerobic digestion is an integral part of the transition toward a biobased and circular economy and its expansion is foreseen in many parts of the world as well as in Europe. In Sweden, a governmental inquiry suggested biogas production to be increased from about 2 TWh today to 7 TWh by 2030. This rapid expansion would require installation of several new biogas plants across the country. However, the location of biogas plants can greatly affect its business performance and there are several geographic and socio-political factors that would limit the choice of location. Through dialogue with existing biogas producing companies and a few other related actors, we identified 12 factors that are commonly considered in the site-selection of biogas plants in Sweden or are considered to be important in the years to come. These factors are grouped into those related to supply and demand (feedstock supply, biogas demand, digestate demand, and carbon dioxide demand), infrastructure and synergies (available infrastructure, adjacent existing industries), land-use and zoning (nearby housing, zoning, and historic preservation sites), and socio-political context (political strategies and goals, organizational capability, and local social acceptance). We discuss how these factors can be used under rapidly transforming conditions in Sweden through different site-selection logics and highlight the importance of spatially explicit analysis for individual or coordinated decision making in future. Our method of enquiry and analysis, and to a certain degree the factors, can be also relevant for other countries, particularly in Europe. This study paves the way for more in-depth investigation of the question of site-selection of biogas plants in Sweden; both in the direction of detailed analysis at the local level, or screening analysis on the regional or national level for improved coordinated actions.


Introduction
Biogas production through anaerobic digestion plays an indispensable role in the transition toward a biobased circular economy (Fagerström et al., 2018).It is a versatile and technologically mature way of extracting energy and nutrients from biomass, including low grade organic residues and wastes which are typically very wet, bulky, and of low economic value.The produced biogas contains about 60% methane (CH 4 ) which can be used for heat and/or power generation or can be upgraded to almost pure biomethane and used as transport fuel or in industrial applications (Wellinger et al., 2013).Digestate contains most of the nutrient content of the feedstock in a more mineralized form which makes it a good fertilizing product (Lukehurst et al., 2010).Other products such as carbon dioxide (CO 2 ) might also be obtained from upgrading of raw biogas.Consequently, biogas solutions have been utilized at industrial scale in many parts of the world, including European countries, and are part of green economy expansion plans (Gustafsson and Anderberg, 2021;Scarlat et al., 2018).
Excluding biogas plants that are part of wastewater treatment facilities or those that extract biogas from landfills, there are 97 biogas plants in Sweden: 36 co-digestion plants, 54 farm-based plants, and 7 industrybased plants.In total, about 2.2 TWh biogas is produced in Sweden.Codigestion plants have the highest share in biogas production (52%), while farm-based and industry-based plants have a smaller share (3% and 6% respectively: Swedish Gas Association, 2020).A governmental inquiry has suggested a biogas production target of 7 TWh per year by 2030, which would be a significant and rapid increase from the current production (Westlund et al., 2019).The 10 largest co-digestion plants in Sweden have an average capacity of 61 GWh/plant (Swedish Energy Agency, 2017).So, although existing plants can contribute to the suggested expanded biogas production target, creating an additional 1 TWh would require at least 15 new large-scale plants in different parts of the country, given the type of production systems the country currently uses.Where to build such plants, to maximize the multiple benefits that can be offered by biogas solutions, is not an easy task.Coordinated action among biogas plants is needed to avoid sub-optimal development.For example, the suitable placement of biogas plants across the country can facilitate the redistribution of nutrients from where they are spatially concentrated and are available in excess (e.g.manure from livestock production) to areas where they are highly demanded (e.g.crop production regions) (Metson et al., 2021a(Metson et al., , 2021b)).If plant locations are not coordinated, then there is a chance that plants that are built at a later date cannot effectively contribute to nutrient recycling because transport distances are economically unviable.
Like any bioenergy project, multiple factors can contribute to the success and sustainability of a new biogas plant, and many of them are related to the physical location of a plant.These include the sufficient availability of biomass supply, demand for products and byproducts, cost-efficient logistics (i.e.transportation), and fulfillment of institutional requirements (Hiloidhari et al., 2017).Determining a suitable location for a production facility is sometimes referred to as a "Facility Location Problem" (Tagliabue et al., 2021) or a "Site Suitability Analysis" (Akther et al., 2019;Kurka et al., 2012), and we refer to it as 'site selection' or 'site-selection analysis.'When parameters (factors) for site-selection of a single (or group of) biogas plant(s) are known, and geo-spatial information about the region is available, site-selection analysis can be assisted by mathematical approaches such as optimization (e.g.Metson et al., 2021aMetson et al., , 2021b) ) or quantitative multi-criteria analysis (e.g.Akther et al., 2019).However, these types of analyses can only be effective and produce realistic results if the majority of important factors for site location of biogas plants are known and translated into constraints or criteria.
Determining which factors should be considered in models of biogas plant site-selection can (and should be) grounded in real-world experiences of actors, not only theoretical ideas of site-selection.These factors are diverse given how institutional conditions (e.g.legal requirements to obtain a permit) and other contextual factors vary across the landscape (both within and among countries).The integration of actor and practitioner experiences increases the relevance and legitimacy of academic findings (Hage et al., 2010;Lang et al., 2012); these actors are the ones that can point to those contextual factors that are used to make decisions in diverse cases.In short identifying and understanding the important factors for site-selection of biogas plants is a prerequisite for in-depth mathematical approaches to site-location and these factors can only be reliably learned through interactions with existing practitioners, as well as referring to the governing regulations for placement of industrial facilities in a particular place.Meeting the ambitious goals for green energy generation, climate impact reduction, and transition toward circular and biobased economy calls for coordinated actions locally and at the national level, which requires frameworks and datasets that facilitate such coordination.
The aim of this paper is to fill part of the said knowledge gap by synthesizing experiences of existing Swedish organizations-mainly biogas producers, but also expert organizations that work with them-about the most important factors that they have considered in deciding on the location of large-scale biogas plants.In addition to synthesizing how previous decisions have been made, we examine how future changes in the Swedish context might change the prioritization (and even list) of factors to consider for selecting new biogas plant locations.Our contribution is to provide a future-oriented list of factors that can be used for more effective collective actions supported by spatially explicit analyses.

Method
We focused on the Swedish context for large-scale plants producing 10 or more GWh per year.In terms of production, these plants are the dominant type since they produce more than half of the national biogas production (Swedish Energy Agency, 2017).We maintained both a retrospective perspective (historical conditions that led to the site-location of existing biogas plants) as well as a forward-looking perspective (current and forthcoming conditions with regards to site-selection of new biogas plants).
We relied on multiple sources of data to get a broad understanding of relevant factors and perspectives and to increase the validity of our results.The following type of information was gathered: (1) published literature, focusing on site-location of industrial facilities and in particular biogas plants, (2) interviews with experts from biogas industry, (3) Environmental Impact Assessments (EIAs) that are required for obtaining a permit for building a new biogas plant, and finally (4) focus group discussions with biogas-related actors (Fig. 1).

Fig. 1.
Overview of the structure of the paper: aim, scope, methods for data collection and analysis, and key results.

Literature study
Existing literature (mainly articles and reports) were consulted to gain insights on site-location problem and serve as scaffolding for our more specific investigation of biogas production in Sweden.Therefore, the aim of literature study was not to provide a systematic and comprehensive overview of the existing knowledge but to be able to sufficiently ground our study in existing knowledge and to increase its quality and relevancy.
We used databases such as Google Scholar, Scopus, Web of Science, and DIVA which is a major Swedish digital archive for scientific publications.The initial search was performed through combinations of keywords such as anaerobic digestion, co-digestion (plants that digest several types of substrates), biogas plants, biogas logistics, bio-fertilizer, biogas localization, localization issues, site selection, and digestate.We proceeded by selecting a few sources that were more closely related to the issue of site-location of biogas plants, with the priority being given to Nordic countries, but to a lesser degree Europe and the rest of the world.These sources were not limited to Sweden.

Interview study
Our primary method of data collection was interviews with experts from the biogas industry, mainly representatives from biogas producing companies in Sweden.Through these interviews, we wanted to learn about the history of establishment of biogas plants and the factors that were considered as important in the decision making regarding their site-selection and get their perspectives regarding future developments in Sweden with regards to the issue of site-location of new biogas plants.As a starting point, we benefited from the existing network of biogasrelated actors who are formed around Biogas Research Center (BRC, biogasresearchcenter.se), but in effect we considered all 34 co-digestion plants in Sweden (the current number is 36).Based on the possible flexibility or degree of freedom of actors in selecting the location of their biogas plant, we placed them into three provisional groups (Table 1): • Restricted: actors that are associated to a particular site, e.g.nearby farms that would build a biogas plant.• Partially restricted: actors that can only choose a site-location within a specific region or municipality; typically associated with the fact that in Sweden municipal waste management in a municipal responsibility (Avfall Sverige, 2021).• Unrestricted: actors that are not attached to any specific municipality or region.
The interviews were performed during the spring of 2018 as part of a Master's thesis work (Johansson and Wretman, 2018).We contacted between two and four representatives from each site-location group and eight responded positively.We also added two consultancy firms with extensive expertise in relation to the development of biogas plants in Sweden.We conducted a total of 11 interviews from 10 organizations ( 8Swedish co-digestion plants, and 2 research/consultancy organizations) (Table 1).For each organization, we requested interviewees that had good knowledge around the history of the plant's establishment and development over time.Preferably, we asked to interview those that were working at the time that the biogas plant was being located and installed.
We drafted an interview guide (see Appendix, which built on the literature study) and performed semi-structured interviews which were voice recorded and later transcribed.The interview guide included questions regarding the story of the establishment of the biogas plant, the actors involved, the proximity to feedstock supply and demand for biogas and biofertilizer, barriers and facilitators, and present and future perspectives with regards to site-location of biogas plants.

Document study (EIAs)
Performing an EIA is a mandatory requirement of the Swedish Environmental Code for obtaining a permit to build a new biogas plant in Sweden (if it is going to handle more than 2500 tons of feedstock per year).It should provide information on the direct and indirect environmental effects of the new installation and demonstrate that the suggested site is indeed suitable for hosting the new plant, including alternative sites for comparison.Therefore, these documents could provide additional information regarding the important factors (mainly in the forms of constraints) for the site-location of biogas plants.
We reviewed the Environmental Impact Assessments (EIA) of almost all existing co-digestion plants in Sweden that were not included in the interview study to complement the information acquired from the interviewees.We studied 21 EIAs which were obtained through the county administrative boards who are responsible for their archiving.As for the governing regulations related to the permitting process for establishing biogas plants, we considered the situation before 2018 (Swedish EPA, 2017) as this was the year that this part of study was performed.

Focus group discussions
Finally, to obtain a more updated future-oriented outlook and reflect upon the changing Swedish conditions (since 2018), we convened two focus-groups (early in 2021) comprised of representatives from the biogas sector and a few relevant public organizations.A summary of the findings from the interview and document study regarding the most important factors for site-selection of biogas plants in Sweden were presented to the attendees.Participants were then asked to discuss the possible factors that were missing from the list, suggestions for improvement, possible similarities or differences between the Swedish context and other countries.We also asked them to reflect on possible changes and trends in the last three years since the first draft of the list of factors was compiled.All the participants in these focus groups were associated with the BRC.In the first focus group 15 participants were present which (in addition to the university) represented 8 public or private biogas-related organizations.The second focus group involved 20 participants which (in addition to the university) represented 8 public or private biogas-related organizations.Together these two focus group meetings involved 17 experts representing 13 distinct organizations (excluding the university).Four of the participating biogas producers were also involved in the interview study which was performed in 2018.

Analysis
We used reflexive thematic analysis to guide our data handling and interpretation (Braun andClarke, 2006, 2019).This approach uses six steps (familiarization, coding, generating themes, reviewing themes, defining and naming themes, and writing up) to focus in on patterns across the data and create a meaningful narrative(s).The method did not force us to choose between inductive and deductive approaches to coding, the process was iterative and organic.Literature, EIAs, and interview transcripts were used for data familiarization, and subsequently coded by two master's students (JW and her thesis partner).The generation, refinement, and naming of themes was done over several meetings with the supervisors (GM, RF), and eventually the examiner (JA) in the context of their thesis work.Preparing for the focus groups provided another opportunity for all authors to further reflect on the factors given developments in the field (and their participation in other projects).We did not record and transcribe focus group discussions; it was rather detailed rapporteur and session facilitator notes which were coded and thematically interpreted following the same steps as with our other data sources.Focus groups were run by RF and JA, first familiarized, and coded by RF, and then refined with GM.Of course, this process was building on the codes and themes already identified through other data sources, with the focus now on validity of existing factors, novel themes, as well as how priorities might have changed over time among actors.
The 'final' factors, and specifically the wording we use to summarize them, are thus the result of a reflexive process over almost four years and are akin to themes in the reflexive thematic analysis terminology.These themes are a result of the reflective process of authors at different career stages (master's student to professor) as well as industry actors (looking at our interpretation of factors and moving thinking further in the focus groups).Theme development also emerged from the authors interest in sustainable solutions.By this we mean that the motivation of the work was not simply a description of what biogas plants do, but rather how can organizing what current actors do (and think is important) help researchers and actors collect and use data to increase the role biogas production plays in sustainable waste management and energy system.
Our results likely have some generalizability to other contexts in terms of types of factors considered, but it is essential to keep in mind that Sweden is a small and wealthy country, our number of engaged stakeholders is limited, and that the positionality (Roberts et al., 2020) of the researchers all influence how one should interpret and use the findings presented here.Although the nature of our research aim is not explicitly linked with issues of race, gender, or equity (where positionality is particularly important to acknowledge), no work is devoid of the influence of the authors' background.In fact, this is why we favored a reflexive thematic analysis method, over other qualitative pattern generating methods, which centers the idea that the interpretation of the data is subjective and can none the less be robust through reflexivity and transparency (Braun andClarke, 2021a, 2021b).In our case, disciplinary backgrounds and previous research projects played a central role in how themes were identified, in addition of culture and demographics.RF has background in industrial ecology and systems analysis and is interested in the range of the ways in which biobased solutions-such as anaerobic digestion-can contribute to resource efficiency of the actors and society on different scales.GM on the other hand has not focused on biogas solutions in her work.She is a sustainability scientist focused on socio-ecological systems and looks for human and non-human factors that influence natural resource management (predominantly nitrogen and phosphorus), especially spatially-explicit considerations.JW approached interviews and the data with fresher eyes, with less internalized assumptions about what would be important to site-selection.And finally, JA has a background in corporate environmental management and has experiences as an environmental consultant working with EIAs.He has conducted different types of sustainability analysis studies of biofuels, mainly biogas, during the last decade.

Results and discussion
We identified 12 key factors influencing to the localization of biogas plants in Sweden (Table 2).In the sections below we describe each factor, how we have grouped them, and how these factors may be useful in guiding the expansion and development of new biogas plants on the Swedish landscape and beyond.

Feedstock supply
Feedstock supply refers to economically feasible access (which is more than just a short distance) to a relatively secure supply of biomass that can be used for biogas production.All our data sources pointed toward the important role that feedstock supply can play on the sitelocation of a biogas plant.In fact, some actors considered it as "the" most important factor, as it was the first thing they would think about when determining the location of a new plant.According to all interviewees, the relative proximity to the feedstock supply increases the likelihood of accessing large amounts of it without increasing the logistics cost.The importance of feedstock supply is supported by many published studies (e.g.Epp et al., 2008;Ma et al., 2005;Wellinger et al., 2013).
"Substrate supply is number one.The location of desired substrates will almost always come in first hand when locating the plant, in all of our projects" [interviewee no.9].
"The location of our biogas plant was selected based on the availability of substrates and to keep the transportation of it to a minimum.It's the transports that kill the deal when using manure as a substrate" [interviewee no.3].
While it is theoretically clear that the feedstocks with high moisture content (such as manure slurry) cannot be economically transported over long distances, the interviewees did not find it realistic to provide a generic quantitative estimate on a distance that is considered to be "acceptable" for all types of feedstock.However, some estimates have been made based on cost, or sometimes energy content of the feedstock.For instance, using energy content, Berglund (2006) considered the maximum transport distance of manure to be about 200 km and for food waste and slaughterhouse waste about 600-750 km.Bojesen et al. (2015) state that wet and bulky feedstock such as manure slurry cannot be economically transported further than 40 km; and there are other studies that provide shorter estimates (e.g.Epp et al., 2008).According to Björnsson and Lantz (2010;as reported in Westlund et al., 2019) the specific transportation cost of an energy-rich feedstock such as slaughterhouse waste can be about 6-8 times less than that of liquid manure (e. g. about 6 vs. 37-46 Euro/MWh biogas produced).Therefore, although long-distance transports are costly, feedstock such as food waste can be transported over different municipalities depending on e.g.availability of biogas plants and treatment fees.
Of course, the economically feasible distance is highly contextspecific and depends on type of transport, fuel prices, and labor cost.Despite these variations in the estimates on the accepted distance for transportation of feedstock, there was a consensus among all our sources of information that, in most cases, feedstock is a very important factor for site-location of the biogas plant and proximity to its supply becomes particularly pronounced for biomass with high moisture content.

Digestate demand
Digestate demand refers to the possibility of delivering the produced digestate in economically reasonable ways to its users; typically, in the form of biofertilizer in agriculture.All sources of information that we relied on expressed the importance of a steady digestate demand, which preferably would be located close to the biogas plant.Digestate has a very high moisture content (about 95% of it is water) which makes its handling and transportation a relatively costly issue.Therefore, a few of the interviewees considered a minimized transport distance to digestate demand to be a more important factor in site-location of biogas plant than its distance to the feedstock supply.For most biogas producers, digestate management is a cost that should be minimized.In Sweden, digestate is commonly applied within distance of 10-30 km from the biogas plant (Vestman et al., 2014).
Generally, the respondents considered the use of digestate as biofertilizer as the desired method of its application.Indeed, the use of digestate in agriculture is the most common and almost 90% of the produced digestate in Sweden is used as biofertilizer (Swedish Gas Association, 2020).
"The sites outside the cities rarely make the disposal of digestate a problem.There are farmers within a relatively close proximity as soon as you leave the city basically.This is good since the farmers have the manure and want the digestate back as fertilizers.A kind of natural symbiosis.So, the plants are usually located nearby both substrates and areas for digestate use" [interviewee no.9].
For most of the interviewed actors, the use of digestate as biofertilizer was very positive and went hand-in-hand with the use of farmrelated feedstock such as manure, agricultural residues, or energy crops.According to one of the interviewees, the main rational for site-location of one of the biogas plants was the added value that synergies between digestion of manure and use of digestate as biofertilizer provided.However, for a few others, the use of digestate was challenging because their plant was in an urban area where digestate had to be transported over longer distances to be utilized.To overcome this site-location challenge, these actors considered further digestate processing (e.g.solid-liquid separation) and alternative applications such as the use of solid fraction for covering landfills.

Biogas demand
By biogas demand we refer to an economically justified utilization of the produced biogas at the scale that is large enough to match the production.According to the interviewees, biogas demand is among the most influential factors in deciding the site-location of a biogas plant, although its relative importance against other important factors can vary depending on what site-location logic is adopted.
Nearby users are clearly preferred, as this reduces distribution costs of the biogas.Most of the interviewees considered the sale of upgraded biogas as vehicle fuel to a single customer with sufficiently large demand as the most desirable utilization.Furthermore, being able to secure longterm contracts to sell biogas was highly desirable as this would increase the stability (Ersson et al., 2015).In Sweden such characteristics are associated with the use of buses and taxis (Ammenberg et al., 2018).This means, that from a biogas demand perspective, sites close to cities are commonly desirable.Despite this, many indicated that the utilization of the biogas can be handled after the site-location of the biogas plant is determined based on other factors.According to one of the interviewees: "The produced biogas was at the start a secondary product as the nutrient re-circulation was in focus.When we started, there was no market for biogas as a vehicle fuel as there is today.Initially, the main purpose of the biogas was electricity generation and district heating."[interviewee no.4] Furthermore, placing a biogas plant in a region that does not use much biogas can be an advantage.Several interviewees emphasized the advantage of being the first biogas producer in a region which allows the producer to establish themselves in an emerging local market for biogas as vehicle fuel.
"There was a new public procurement of bus services operator in progress within region X.A new contract of ten years was to be established.This was the reason that this biogas plant was built in the first place, to secure this market and a long-term income from the sale of biogas to bus traffic."[interviewee no.3] In areas with no gas grid, it is common in Sweden to transport the upgraded biogas in compressed form (CBG) with trucks carrying pressurized containers.Due to its high specific transport cost this limits the distribution range.However, the steady growth in interest for liquefied biogas (LBG) in the country could mean that the produced biogas can be affordably sent farther (Gustafsson et al., 2020).Still, building an LBG facility is only economically justified if the new biogas plant is relatively large (60 GWh of biogas or more per year according to some of the interviewees).

CO 2 demand
Large volumes of CO 2 are generated in the upgrading step which aims to increase the methane share to about 97% by volume (Bauer et al., 2013).So far, almost all the (biogenic) CO 2 is released to the atmosphere.Capturing and utilizing this CO 2 stream can provide additional income for the biogas plant and significantly further improve the climate performance of the produced biogas-and can turn some biogas production systems into net carbon sinks (Li et al., 2017).One application can be production of CH 4 from CO 2 and hydrogen (H 2 ), where there is access to cheap renewable electricity for producing H 2 .Swedish island of Gotland in the Baltic Sea may be a suitable place for such a system (Mohseni et al., 2017).Also, CO 2 from biogas plants would be suitable for utilization for carbonation of soft drinks or beer, cooling and freezing of food (Laursen and Strandmøllen A/S, 2020).
While CO 2 demand has never been an important factor in sitelocation of biogas plants in Sweden, many actors believed that it could become important in the future.If so, reasonably short distance to industrial users of CO 2 could influence the decision on site-location of biogas plants in future.

Available infrastructure
By infrastructure we refer to all (mainly physical and often public) structures and facilities that have a fixed location and are required, or can greatly facilitate, the operation of biogas plants, and improve the logistics.
All interviewees considered availability of suitable infrastructure as an important factor to be considered in the site-location of biogas plants.This is also highlighted in the EIAs and is in line with several studies in the literature.Examples include road networks and gas grids, but can also include specific solutions such as a pipeline for transportation of manure, also mentioned in several site-location studies (e.g.Ma et al., 2005;S ¸ener et al., 2011).Although Sweden has extensive road and rail networks across the country, having access to roads with sufficient bearing capacity can be an issue.Most respondents believed that such considerations are more relevant when looking at specific sites, as opposed to trying to decide in which region or municipality to place a R. Feiz et al. plant.In fact, they stated that building additional infrastructure such as local access roads may be justified from the business perspective.
Local governing bodies can assist with construction of longer connection roads or pipelines in certain cases.Again, this supports the notion that lack of certain infrastructure may not be a deal-breaker when selecting a new biogas plant site, but still is important for planning.
"Some infrastructure was already existing at the site but then the municipality built the needed connection roads so we could handle the expected transports in a reasonable way" [interviewee no.1].
In some instances, the availability of a gas grid (in relation to the 'biogas demand') can be a decisive factor: "One important thing to consider is how the biogas will be utilized.Upgraded or not?If you want to upgrade the biogas, one would like to localize adjacent to a larger natural gas grid, and connect to the grid" [interviewee no.7].Berglund (2006) stressed on the importance of proximity to gas grids as it provides opportunities for relatively cheap and secure distribution of the biogas.However, Ottosson et al. (2020) indicated that being close to larger gas networks may not always be an advantage, as competition from abroad under imbalance regulations-on different sides of a border-might put the Swedish producers connected to the grid in an unfavorable situation.Larsson et al. (2015) have argued that the history of biogas development in Sweden demonstrates that it is possible to develop a biogas market without having a comprehensive gas grid-at least not initially.This view was complemented by some of the actors that we spoke to: The expansion of LBG production and use can affect the relative importance of proximity to gas grid-at least for some actors.
While available infrastructure is mostly viewed as an aspect that can limit economic performance, it can sometimes act as a geo-spatial constraint that deterministically excludes some site options.For example, conflicts with underground utility installations (pipes or cables) needs to be avoided, or some area buffer area around infrastructures such as major roads or electricity grids may need to be excluded due to safety (or other) reasons (e.g.Trafikverket, 2010).

Existing industries
'Existing industries' refers to the importance of adjacency to existing industrial sites for determining the location of the new biogas plant.This also ensures that basic infrastructures such as electricity, water, or roads are present.Some actors mentioned that such proximity or co-location makes it much easier to obtain permits for building a new plant.
The geographical proximity of biogas production with other industrial actors can have several mutual benefits such as facilitating the development of industrial symbiosis (Lindfors et al., 2020;Martin and Eklund, 2011), increasing local waste management capacity of a large food processing industry (Feiz et al., 2021), and enhancing byproduct management of a biorefinery (Hagman and Feiz, 2021).Interviewees also pointed out that cooperating with near-by waste treatment facilities (sewage treatment, waste separation & recycling, composting, landfill, etc.) can also be beneficial for integrated permits, and support the collective ownership of the expensive technology.
But the possibilities of co-location are not limited to waste-related sites.Sometimes an actor already owns an industrial activity and wants to place the biogas as close to it, and in relation to it.For example: "The aim was just to localize the biogas plant as close to our existing farming facilities as possible" [interviewee no.2].Some actors warned that proximity to existing (or former) industries can also be a burden.For example, space limitation may be an issue if a certain plot is to be shared.Older sites may also have some land/soil contamination which can become a problem for the newly installed biogas plant to solve.Others (in the focus group) mentioned that some industries may be skeptical that a nearby biogas plant can complicate their business (e.g.affecting their operational routine, or the permitting terms).

Land-use and zoning
Any site-location project must follow (or lobby to change) existing zoning plans and navigate potential tensions (or synergies) with adjacent land-uses regardless of legal requirements.Statements from the actors with talked with agreed that municipal land-use plants were of the utmost importance for the specific site-location of biogas plants.
Even lack of such plans can become problematic as it can create a long process to clarify is a location is suitable or not.
Demarcations of industrial (which supports biogas plants) and residential (to avoid) areas are particularly important.Fitting in to existing zoning plans makes the process easier.The proximity to residential areas (nearby housing) was mentioned by most of the actors, and in EIAs, as a constraint on site-selection.This can manifest itself formally in zoning plans (i.e., residential areas with some margins are excluded), but often involves a more complex set of conditions to consider.Problems can appear near cities and densely populated areas (see paragraph below) thus many interviewees considered agricultural areas2 to be better from this perspective: "Nearby housing might not be a critical factor in agricultural areas if there are not that many neighbors.The actors must consider the increased road usage as well.It might not be a strategic move to transport digestate through a city for example" [interviewee no.10].
Several studies have also stressed that issues such as increased local odor, interferences with traffic in public roads, and such issues (or their mention) can create resistance towards a new biogas plant (Epp et al., 2008;Khan, 2003).
In addition, the interviewees mentioned the relevance of identifying protected areas-including nature reserves, military zones, and historical preservation sites-where one cannot build.Presence of such areas create additional constraint in choosing a location that otherwise is considered desirable.Encountering historically protected sites-or objects found at the site-would urge a costly and time-consuming investigation.Early identification of such issues that may arise in association with certain locations would minimize the risk.
It was also mentioned that sometimes the expansion of an existing site contradicts the existing zoning plan.According to Wellinger et al. (2013) the site-location should not be selected merely on the basis of present conditions and possibilities of expanding in future should also be considered.Others have made similar points in their site-location studies (e.g.Kigozi et al., 2015;Sultana and Kumar, 2012).Other site characteristics, not related to zoning, but pervious land uses or site conditions such as inclination or sub-soil conditions can also be grounds for rejecting a site (e.g.During Filho et al., 2016;S ¸ener et al., 2011).

Socio-political factors
Socio-political including formal and informal actors' capacity, goals, and interests were highlighted by focus group participants, especially in early phase site-location planning.Epp et al. (2008) also identified such factors and referred to them as favorable soft requirements consisting of political support, available know-how for biogas operation in the region, and committed project developer.
Local governments often have their own goals (e.g.achieving climate neutrality by 2030), which can facilitate the development of new biogas plant sites (including recruiting biogas companies, building infrastructure, or help with zoning).In addition, certain areas within a municipality may be prioritized for economic development-e.g.biogas production in relation to local food/feed production, green job creation in a district, or other socio-economic benefits (e.g.Lavrencec, 2010).
Several focus group participants and interviewees emphasized the importance of organizational issues.The presence of competent and engaged people within organizations can greatly influence how siteselection decisions are made.A group of aligned actors can create an organizational drive, influence internal or external competitions, and weaken local resistance.Also, aligned and cooperating actors-such as a group of farmers who are willing to provide crop residues and manure as feedstock to the biogas plant and in return receive digestate as biofertilizer-can influence the site-location and facilitate the implementation process.In short, the importance of dedication and courage, along with more formal requirements, was viewed as central to solving issues related to infrastructure access and avoiding getting stuck in long administration processes.
Finally, the general attitude of the nearby community and the public opinion toward the installation of a biogas plant in a certain area was deemed to be very important (which is also the case of other industrial sites, Mwirigi et al., 2014).Like organizational learning, prior failed or bad experiences in a community with regards to biogas development can create both more knowledge of what to do better, and general mistrust or resistance.Local acceptance toward biogas production varies in different places (Bojesen et al., 2015), which can affect site-location but can also change over time.

Making use of the factors
The factors we identified fit well within existing frameworks of generic site selection problems.Generic site location issues for industrial facilities (known as the Weber problem) often involve balancing distances from suppliers and consumers (Kaplan, 2011).The objective may be to minimize transportation costs (Chen et al., 2014).In our case, those factors related to supply and demand (see Table 2) would match.Secondary generic factors could include access to skills (labor) and knowledge (Feldmann and Olhager, 2013), but these were not mentioned in our data collection about biogas plants.Still, we identified other types of 'secondary factors' for strategic biogas plant site-selection around how such a site sits within existing industry, land-uses, and political and community preferences.
The factors are also in agreement with other frameworks that are specific to biogas plant location; they remain unique in that they sit at a useful intermediary level of geographic, temporal, and organizational scales to be useful for individual organizations and coordinating agencies, and flexible enough to inform future planning.Epp et al. (2008) have suggested a three-tier site-selection approach.In the first tier, a suitable region is selected (radius 15 km) in which issues such as the supply of feedstock and demand for digestate are considered.In the second tier, the neighborhood is determined (radium 1 km) by considering the possibilities of selling the biogas, electricity, or heat, at a higher price and with favorable conditions.Finally, in the third tier, the actual site is determined.3This last step would require the size of the biogas plant to be known and a detailed analysis of the factors such as proper road access, site features, potential nearby conflicts, and property rights associated with the selected site should be performed.Our results point to the factors mentioned by Epp et al. (2008), but incorporates those more generic factors that are essential for strategic planning at national and regional levels.That is, to meet national goals for biogas expansion, decisions for sites needs to happen at scales above 15 km, involve the coordination of different types of actors, and account for changing future circumstances.

Categorizing identified factors
The identified factors can be summarized with regards to how they can be formally operationalized-even if we consider that there are different logics for site-location.Based on their exclusionary or nonexclusionary nature, we (roughly) grouped factors into (1) constraints which are land-use-related factors that tend to limit the choices for the site-location and are often governed by regulations and regional landuse planning, and (2) contributors which are factors that can improve to the economic or business viability of the biogas plant in the selected site (Table 2).It should be noted that this grouping is flexible and fluid; and many factors have both aspects at the same time.It is important to note that these factors are interconnected; some factors mediate others another, while some could be viewed as subservient to another or working at different geographical levels.For example, a decisionmaking process about site-location can begin by focusing on broadly defined regions that are promising for feedstock supply and digestate demand, and then geospatially narrowed down through considerations related to available infrastructure and existing industries.

Diversity of logics for site-selection
The rationale that governs the selection of a subset of the factors as relevant, the priorities that are given to them, and the geographical scales that those factors are applied shape the "site-selection logic."It was clear from our investigation that there is no single site-selection logic that can be applicable to all types of actors and situations.Still, there are shared considerations that help explain why certain actors use particular logics (i.e., the relative importance of factors or the order in which said factors are considered).
One prominent consideration is how geographically bounded-either legally, or for some practical reasons-a biogas actor considers themselves to be.For example, a municipality which is considering building a new biogas plant could be legally bound to its' borders.In that case, the actor may be interested to identify a few candidate sites-all within its political borders-and try to evaluate them against each other.Since Swedish municipalities are responsible for waste management (Avfall Sverige, 2021), municipality-related biogas installation could very well be established in proximity and collaboration with waste treatment and the zones that are already allocated to it (e.g.landfill, recycling centers, wastewater treatment facilities, incineration plants, and so on).
Furthermore, it is rarely the case that the investigation around sitelocation begins with a "blank slate."It is very common that those who decide regarding the site-location, due to their priori knowledge, already have some ideas on potentially interesting areas or specific sites (possibly associated with favorable establishments) and the investigation builds up on that knowledge.However, as biogas solutions expand, it may be the case that large spatial scale planning needs to take place to select sites and thus a need for more systematic inclusion and exclusion site criteria.

Changing priorities over time
A major observation by the interviewees from large-scale co-digestion plants (in 2018) was the shift of the Swedish biogas market over time.Prior to 2010 a larger share biogas was used for heating, but the share of upgraded biogas has been rapidly increasing since 2005 and most of the produced biogas in Sweden is upgraded now, of which, most is used as vehicle fuel (Swedish Gas Association, 2020).
The focus group discussions highlighted the electrification of taxis, buses in public transport, and private vehicles as ongoing transformation (cf.Lundström et al., 2019).This electrification could imply a limit on the demand for biogas as vehicle fuel for certain forms of transportation (e.g.Mutter, 2019).However, many interviewees considered the use of biogas as fuel in heavy transport and shipping as major expanding markets (cf.Dahlgren, 2020).The opening of these large markets to liquid biogas (LBG) and natural gas (LNG), which can be mixed in different proportions, would mean a tremendous opportunity (and challenge) for the biogas industry.
Rapid shifts in the biogas market over the last decade highlights the importance of being proactive in decision making with regards to both site-and technology-selection.The expansion of the LBG market (noted as a transformation in both interviews and focus groups) is easier for large biogas plants to adapt to, while some actors might be more restricted because of technology investments.If a shift to LBG is possible then the relative importance in selecting a site for its proximity to biogas demand would decrease.Perhaps such a shift would increase the importance of being closer to feedstocks or digestate demand.

Role of biogas solutions and tools for coordinated action
While biogas solutions are multi-functional (Lindfors et al., 2019), it is often the case that one of their functions is prioritized for a specific actor or plant.If we consider three common functions of biogas solutions4 : is the "biogas producer" primarily viewed as renewable energy provider, recycler-supplier of nutrients, or waste manager?(Fig. 2).Surely it can be a combination of all, but the weight that is given to these different roles can vary.
Individual actors are often clearly aware of these weights for their own biogas production systems, but it is important to think about these functions beyond the scope of individual actors.By doing this, the suggested expansion of biogas production in Sweden from 2 to 7 TWh (by 2030) could be done in a way to try and maximize these multiple benefits.Coordinated action among stakeholders (from multiple biogas companies, to municipal governments and farmers) can benefit from geo-spatially explicit systems analysis of site-locations to screen for strategic locations for biogas production given stated priorities (e.g.Metson et al. (2021aMetson et al. ( , 2021b) which looked at maximizing nutrient recycling and minimizing transport costs).Considering the influential role of land-use planning documents on biogas plants, and the (often) slow update process of such documents, explicitly integrating biogas solutions would facilitate win-win expansion across these functions.
Still, as we observed in our investigation, every case has its idiosyncratic features and thus requires a tailored approach.The list of factors compiled here, and their characterization as constraints or contributors, can be a way to bridge across geographic scales.In fact, geo-spatially explicit systems analysis of site-locations can be used to operationalize the specific constraining or contributing factors that are relevant in a relatively small geographical scale (Metson et al., 2022).

Conclusion
Our study indicated that the site-location of biogas plants in Sweden are affected by 12 main factors which we grouped into those related to supply and demand (feedstock supply, biogas demand, digestate demand, and CO 2 demand), infrastructure and synergies (available infrastructure, adjacent existing industries), land-use and zoning (nearby housing, zoning, and historic preservation sites), and socio-political (political strategies and goals, organizational capability, and local social acceptance) aspects.
These factors may have overlapping implications and should be viewed flexibly.Some of them contribute to an organization wanting to place a plant in a particular location because these factors are important from an economic/business perspective, while others constrain or exclude where a plant can be located.Our data indicate that no single site-selection logic is applicable to all types of actors or situations; still, many of these factors are, and will continue to be, important for decision-making.In addition, the site-location logics can be affected by the dynamics of the biogas industry.For example, the increasing relevance of producing liquefied biogas (LBG)-which can be transported over long distances-can increase the relative importance of 'feedstock supply' and 'digestate demand' for some actors.
We have used multiple types of information sources to increase the likelihood of capturing the most important issues related to our aim, but all methods and data have their limitations.While we have tried to include recent developments in the biogas sector, there are still several rapid and multi-faceted developments we could have missed.For instance, there are many new domestic and international actors entering the Swedish biogas sector and these stakeholders might have different ways of approaching site selection (and may affect how existing actors select sites).At the same time, conducting our research in one specific country (focused on a particular size of plant) can limit applicability to other contexts.Still, our method of enquiry and analysis can be applied to countries other than Sweden as well, especially those in Europe.In addition, it is likely that our findings including several of the identified factors and how they can be used via different site-location logics are also applicable to other countries.
Despite the variations and fluidity of how the factors can be prioritized or conceptualized, we believe the set of 12 factors we identified are generic enough (at least in the context of Sweden) to be operationalized to help inform coordinated actions among actors and across contexts.In Fig. 2. Biogas solutions are multi-functional and depending on the weights that are given to these roles, different national or regional logics for their sustainable development can be adopted.

Table 1
List of the interviewees and the type of their organizations (anonymized).

Table 2
List of the identified factors for site-selection of biogas plants in Sweden.
a These factors were added after the focus group discussions.R.Feiz et al.