Evaluation of the anaerobic digestion process for low and micro-scale biogas production: a review

A theoretical analysis of biogas production technologies classified in two subgroups; low and micro scale, was carried out using the methodology of formal concept analysis (FCA) together with the VOSviewer and Conception Explorer tools for information processing, a series of attributes that each subsystem must comply with in terms of the sustainability of each technology were analyzed. The information selected for the development of the analysis required a quality study, based on the percentile of the journals in which the manuscript was published. The search for information was carried out in web sites such as: Scopus, Web Of Science, Dimensions and Lens among others. In total, 105 sources of academic information were analyzed, selecting 56 articles and 18 book chapters for the development of the work. In conclusion, it can be affirmed that low-scale biogas plants present more favorable characteristics than micro-scale plants; however, both technologies have a high potential for application worldwide in isolated or rural areas, supplying electric or thermal energy to less favored communities. Finally, it is important to point out that the methodology developed has proven to be a relevant tool for the classification of information and serves as a first step to group and relate search concepts by means of a deterministic set of attributes.


Introduction
The world is currently undergoing an energy crisis due to the continuous exploitation and use of fossil fuels, which is why different renewable energy sources such as solar, wind and biomass have been explored [1].Additionally, the exploration of this type of energy has been an urgent global mission due to the fact that fossil fuel reserves are limited and their accelerated use has negatively impacted human health and climate change [2].Understanding the above, biomass resources are an attractive alternative with high potential for renewable energy production.Biomass, including plant 1299 (2024) 012008 IOP Publishing doi:10.1088/1757-899X/1299/1/012008 2 and waste generated by animal and human activities, is a renewable organic source that can be reused [3] [4].
Globally, digesters have been planned, designed, constructed and maintained, incurring active applications and technological developments of low and micro-scale biogas [5].Currently, there are a large number of unreported biodigesters, however, indicators relate that there are more than 4000 small-scale (decentralized) digesters in Germany, 350 in Austria, 72 in Switzerland and 65 in the United Kingdom, related to Europe alone [6] [5].In Asia, China has about 30 million biodigesters, India 3.8 million, Nepal 0.2 million and Bangladesh about 60,000.In the United States and Canada, there are a large number of centralized installations [7].In Africa and Latin American countries, the related facilities in the literature are scarce, there are several technologies to improve biogas, but they have disadvantages such as high energy consumption or high initial investment costs [8].For this reason, the scientific community has developed studies to improve the efficiency of the processes.In 2020, Roubik and Mazancová presented a project for the application of biogas from cattle manure to meet the energy needs of households located in rural areas [9].A year later, Arutyunov et al. presented a proposal for the application of low-scale biogas plants for biogas production, as well as liquid biofuels for application in agricultural areas [10].In turn, Lohani et al. presented a case study in Nepal where biogas is used for cooking, benefiting several poor communities [11].From another angle, O'Connor et al. presented a review of the current status of small-scale biogas production systems on farms in Europe, discussing the current status and production capacity on the continent [12].
Currently, alternative processes are being developed in order to improve the potential of biogas in terms of CH4 percentages, which include the use of excess CO2 in the presence of H2O for absorption system [13].Angelidaki et al, developed a study where they transform H2 into CH4, considered as appropriate and as an alternative for utilization as natural gas [14].Other methods are under development, such as biogas recirculation inside the biodigester allowing in situ biogas upgrading, being more cost-effective and simpler than commercial upgrading processes [15].
The objective of this study is to identify the technologies and processes for biogas production at low and micro scale, taking into account sustainability characteristics that allow evaluating, according to the context, their possible implementation.The information analysis will be developed under the application of the formal concept analysis method [16] [17] [18] and the use of the Concept Explorer [19] and VOSviewer [20], software, relating a series of objects and attributes, which will facilitate the processing of the information.With this approach in mind, the document is organized as follows.Section 2 presents the methodology, defining the attributes and assigning each biogas production technology an object for the development of the formal concept analysis.Section 3 covers the literature review, focusing on the level of compliance with each of the attributes.Section 4 presents the critical review using the Concept Explorer tool and the formal concept analysis method, evaluating small and micro-scale technologies.Finally, Section 5 presents the most relevant conclusions of the work.

Methodology
With the intention of evaluating technologies that produce biogas anaerobically at low and micro scale, Figure 1 presents the methodology for the development of this study, initially a search for information is performed using scientific databases such as Dimensions, Lens, Scopus and Web Of Science, among others, with the following search string; "biogas* production*" OR "Anaerobic* Digestion*" OR "Low* Scale" AND "Review*", where manuscripts were extracted and subsequently analyzed by VOSviewer software allowing to establish a bibliographic coupling network that correspond by means of keywords and authors with higher citations (Figure 1).The criteria for selecting the information were defined by the year of publication and the percentile of the journals in which the manuscript was published; 56 articles and 18 book chapters were selected.In the application of the FCA methodology, objects and attributes must be defined [21], for this study the objects were defined in micro-scale and low-scale biogas production technology and the attributes that each biogas production technology must meet are: A1.Life cycle: will meet the object with a more favorable life cycle assessment (LCA).A2.Implementation area: meets the object with the lowest environmental impact on land use for operation, with high performance levels.A3.Application of resources or solid wastes (Waste) for its production.A4.Efficiency of the system: will meet the object with the highest level of efficiency.A5.Cost: the object with the lowest initial investment cost will meet the criterion.A6.Technology complexity: lower maintenance and operation requirements.A7.Maturity: the object with the highest level of maturity will meet the criterion.Taking into account the objects of this study, a literature review is carried out according to the compliance to a greater or lesser extent of each attribute.This will result in a matrix that will allow a qualitative analysis.The data analysis will be performed through the open access software "Concept Explorer" [19], evaluating all existing relationships, grouping technologies and attributes.

Low-and micro-scale biogas production
This section presents theoretical concepts to classify the types of biodigesters, different biogas production technologies and the respective classification according to energy production.

Anaerobic digestion
It is called biomethanization or biogas production.Generally, an anaerobic process is a highly complex microbial process that takes place in the absence of oxygen, where organic matter is decomposed by the action of microorganisms, generating a mixture of gases (biogas) consisting mainly of methane and carbon dioxide [22].The organic matter is decomposed by different groups of microorganisms into simple compounds, which become volatile fatty acids.These acids are consumed by methanogenic microorganisms that result in the production of methane and carbon dioxide.This whole process is carried out in a biodigester or reactor [23].Table 1 shows four different stages in the anaerobic processes

Biogas production technologies
There are several technologies used in the biogas system [24] [25], Additionally, for decentralized systems Rasimphi et al. classifies plants into fixed dome, floating drum, balloon, horizontal, earthen pit and ferrocement [7] [26].Henceforth, the term decentralized system refers to small-scale biogas plants, linking small-and micro-scale systems into one category [6].The designs are generally supported by geographical location, by the availability of materials in the local area for technological implementation [26][25]28].

Anaerobic contact (ACR)
Can mix the tank with sludge recycle [27]

Batch
Widely used in small-scale farms. [27]

Hybrid bioreactor
Weaknesses of batch reactors are channeling and clogging. [27] Source: Author

Plant size classification
Based on the size of the combined heat and power (CHP) system biogas plants can be classified into micro, small, medium and large scale.Micro-scale has a CHP of less than 15kWe.Small scale is between 15kWe and 99kWe.Medium scale is between 100kWe and 299kWe and large scale exceeds 300kWe.The difference of the scale is also different in the design of the systems.The first is the passive system which is defined as a plant where the biogas recovery unit is in the existing biomass.
The temperature is below 20 •C.The second is the low rate system, where the digesters need more time to digest.The third system is the high rate system which requires digesting liquid in a short period and keeping the solid in a longer period, but still shorter than the low rate system [12].Table 3 presents a classification of biogas plants according to their energy capacity.It is limited to the treatment of small quantities of organic waste.
-Domestic use

Cogeneration electric power between 15 -99 kW
Uses biomass generally available on farms for heat and power production.

Cogeneration electrical capacity between 100-299 kW
Linking plants between small and large scale.

Large scale Cogeneration Electrical power> 300 kW
Generally, have a feedstock consumption exceeding 5000 tons per year.
-Medium and large communities Source: Author.Adapted from [32][12] Small-scale biogas technologies provide a number of benefits to communities in isolated areas [33].These benefits range from socioeconomic to environmental areas, and include improved health and air quality in the environment where the system is implemented, as well as improved fertilizer quality [34].Additionally, it generates employment by using local labor for its manufacture and operation over time, as well as diversification in the supply of electricity for the area [35].Micro and small-scale biogas technology is crucial for sustainable development through circular economy schemes since they are generally fed with animal manure, energy crops and solid wastes to address the lack of energy in impoverished rural communities, small-scale biogas is a viable alternative to remedy this problem to some extent [36] [37].
It is important to mention that farmers must take into account the leaching of biogas production into the aquatic environment in order to generate systems that are sufficiently sustainable in environmental terms, i.e., the use of soils is an important item for the proper management of the by-products of the process [38] [39].Namely, the size of the small-scale system can generate greater or lesser impact on soil use, however, the issue should focus on the proper management of leachate regardless of whether the plant has low or micro-scale characteristics, i.e., the impact can be the same regardless of the size of the system [39] [7].
The economics of small-scale systems comprise lower investment costs with respect to centralized systems [40], however, the size of the plant directly influences the initial cost of investment, maintenance and operation [41], that is to say, the micro-scale system will have a lower investment value than low-scale plants [42].This economic factor is evaluated taking into account that the use of raw material for these two types of systems is free, and comes, for example, from animal manure, water, treated sludge, solid waste, agricultural waste, among others [43].
In terms of efficiency, both small and micro-scale plants vary in efficiency based on the use of resources in the area [44]; the type of feedstock used in the process [45].The feedstock varies according to the geographical area of implementation and the available resources [46].However, lowscale systems, having a higher production capacity, present higher efficiencies [47].In terms of life cycle and maturity, low-scale systems are generally manufactured with higher strength materials than micro-scale plants and have a larger number of applications worldwide [48] [49] [50].
Finally, Table 4 presents the matrix of attributes for the two types of biogas plants; low and micro scale.The attributes that meet each attribute are identified with an X in the matrix.

Results
Figure 2 represents the graphical diagram of the formal concept analysis performed in the Concept Explorer tool to evaluate low and micro-scale biogas plants, based on the information provided in the attribute's matrix in Table 6.This diagram relates the small-scale plants; classified in low and micro scale with those 7 attributes proposed to identify their advantages and disadvantages as an alternative technology.The different nodes shown in Figure 2 are identified as follows: i. Blue-white: contains only attributes ii.
Blue-black: they are containers of micro and low scale technologies.iii.
White: connector node Considering the above, the analysis of the diagram is simple.The low and micro-scale biogas plants analyzed are related to all the attributes that connect them from top to bottom, up to the node of origin.Specifically, attribute A2 "Area of implementation" and A3 "Application of resources or solid waste for production" are shared by the two types of biogas plants, i.e., they comply with this attribute.Additionally, low-scale biogas plants comply with three attributes individually; A1 "Life cycle", A4 "System efficiency" and A7 "Maturity".On the other hand, micro-scale biogas plants do not comply with the above mentioned tributes, however, they comply with two attributes individually; A5 "Cost" and A6 "Complexity of the technology".From this analysis, it is evident that both types of biogas plants have favorable characteristics to provide energy solutions at a decentralized level for isolated or rural areas.As an alternative to the theoretical analysis approach, we can state that low scale systems are more suitable for small scale biogas plant applications, however, micro scale biogas plants can overcome the attributes that it does not meet, through the development of new systems, the linkages of new design materials and biomass application in a suitable way in the medium and long term.

Conclusions
The information was analyzed under the FCA methodology, evaluating characteristics such as costs, efficiency, land use, life cycle, maturity, among others, contributing to overcome current problems for a specific problem.The most relevant conclusions of the work are presented below: i.Most of the literary information does not present information on the classification of biogas plants based on their size, so there is a need for a classification at a global level.ii.
There is a high interest by different authors in the characterization of solid wastes or biomass used in the process of biogas production at low and micro scale.Animal manure and solid wastes (garbage) are used by different authors and reach acceptable biogas production levels for each specific analysis presented in their publications.iii.
Low-scale biogas plants comply with 5 of the 7 proposed attributes, showing greater applications than micro-scale systems.However, both systems are highly applied in small North America, Europe, Asia and especially in Canada, for electric energy supply.iv.
In general, small-scale biogas plants are an alternative for the production of energy and the use of environmentally polluting matter, i.e., they solve waste treatment problems and produce biogas that can be used as a source of energy.However, the treatment of the leachate produced in the process must be considered for the system to be environmentally sustainable.Translated with www.DeepL.com/Translator(free version)the technological development of lowscale and micro-scale commercial systems, which allows generating a system with rural application at low cost and with acceptable efficiency, generating a socio-economic impact in impoverished communities that lack energy resources; heating and electricity.The current centralized systems are highly expensive, so there is a challenge that involves reducing costs in design, construction and implementation, as well as maintenance and operation over time.

Figure 2 .
Figure 2. Methodology used in the development of the study.

Figure 3 .
Figure 3. Diagram of information analysis FCA methodology

Table 1 .
Stages of the anaerobic digestion process.

Table 2 .
Type of reactors for biogas production

Table 3 .
Sizing classification of biogas plants by anaerobic digestion

Table 4 .
Selection matrix of attributes with respect to objects