Frutas y hortalizas como alimento funcional

Biogas is a flammable gas consisting of methane (50-75%), carbon dioxide (25-50%), and minor amounts of other components such as water, oxygen, sulphur, and hydrogen sulphide (da Costa Gomez, 2013). It is generated through a sequence of biochemical processes in which microorganisms break down biomass in oxygen-free environments - a process known as AD. The biomass feedstock types typically used for the generation of biogas can be grouped based on their origins and include energy crops (e.g., maize and elephant grass), agricultural wastes (animal manure and slurry and vegetable by-products and residues), industrial wastes (organic wastes from biofuel production, food industries, breweries, etc.) and municipal wastes (source-separated household waste, sewage sludge, the organic portion of municipal solid waste, and food waste) (Al Seadi et al., 2013).

The generation of biogas from organic waste products or purpose-grown crops helps to reduce the amount of greenhouse gases that become emitted during energy generation. It contributes both to the replacement of conventional, fossil-based fuels and the prevention of atmospheric methane emissions stemming from storing organic wastes in open spaces (see Vasco-Correa et al. [2018] for a review of studies estimating the greenhouse gas reduction potentials of different AD system types). The use of biogas systems also provides a number of additional benefits. For instance, they help avoid bad odour from openly stored wastes and reduce the biological and chemical oxygen demand of

waste that re-enters the environment. The digestate, i.e., the material remaining after the AD

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While the various microorganisms involved in the AD process and the environmental conditions under which they operate are generally understood, the different groups of organisms interact in complex ways with each other and their environment (Achinas et al., 2017; Murphy and Thamsiriroj, 2013). This makes it difficult to control the AD process with precision in industrial applications. In turn, this highlights the importance of design features that create optimal conditions for the microbial activity, that allow system operators to monitor system performance, and that allow operators to intervene in the environmental conditions whenever necessary.

The core of the biogas system is the AD unit where the organic input material becomes transformed into biogas and digestate. The variety of available AD reactor designs is large (e.g., see Mao et al., 2015). Typically, they comprise a substrate feeding system, agitation mechanisms to bring the microbes in contact with the substrate and to prevent sludge settlement at the bottom of the reactor, temperature control measures, and outlets for the generated biogas and digestate (Bachmann, 2013). Box 1 provides an overview of the most common medium to large-scale reactor types in Thailand, including brief descriptions of their key defining characteristics.

Box 1: Common designs for industrial-scale AD reactor units in Thailand and other countries in Southeast Asia. Sources: Rahayu et al. (2015) and data collected for this thesis.

In addition to the reactor unit, biogas plants also include a variety of other technological equipment (see Figure 5). Some plants include substrate pre-treatment processes to make the feedstock suitable for treatment in the digestion unit. This can include screening to remove large or non-biodegradable objects, mixing, pH

Common designs for industrial-scale AD reactors in Thailand and the wider Southeast Asian region

 Anaerobic Filter Reactor

Tank-based reactor including a fixed medium inside to which microorganisms attach in order to prevent washout

 Continuously Stirred Tank Reactor

Tank-based reactor including agitation equipment such as hydraulic or mechanical mixing  Modified Covered Lagoon Reactor

Covered lagoon involving some form of mixing mechanism such as baffles or bottom-feeding  Simple Covered Lagoon Reactor

Covered lagoon not involving any mixing mechanisms  Upflow Anaerobic Sludge Blanket Reactor

Reactor design allowing microorganisms to grow in aggregations to prevent washout despite rapid inflow of substrate through bottom-feeding

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neutralisation, and/or temperature adjustment (see Bochmann and Montgomery [2013] for a more complete overview of available pre-treatment options). The digestate is stored in a separate location and can subsequently be used as fertiliser due to its nutrient-rich composition. The generated biogas is either stored in the digester itself or in a separate container.

Figure 5: Typical processes involved at industrial biogas plants. Based on Rahayu et al. (2015).

The generated biogas can be used to heat boilers and/or to drive gas engines that are linked to generators for electricity production. It can also be used as fuel for gas vehicles or can be injected into gas grids. Some projects primarily invest in AD systems to prevent atmospheric methane emissions from waste storage in open spaces and use flares to burn off the biogas that they generate. For instance, some of the CDM projects analysed as part of the present thesis use this method to generate Carbon Emission Reduction credits by preventing atmospheric methane emissions from storing wastes in open lagoons.

Depending on the utilisation of the biogas, some plants also include biogas cleaning and upgrading equipment to condition the biogas for use in subsequent energy conversion equipment. This can involve dehumidification, compression, and/or hydrogen sulphide removal in scrubbers (Beil and Beyrich, 2013; Petersson, 2013).

Monitoring and control devices are used throughout all of these processes to ensure that temperature, pH values, liquid and gas flows, and gas pressures stay within ranges that allow for a stable and safe operation of the plant (Holm-Nielsen and Oleskowicz-Popiel, 2013).

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The optimal design of a biogas plant is primarily determined by the composition and amount of feedstock that the plant will treat (Bachmann, 2013). While in principle any biodegradable matter can be used, the technical feasibilities and economic potentials of substrates differ, depending on the amount of feedstock available, its dry matter content, degradation rate, contaminant and inhibition risk, etc. (ibid.). Aside from the characteristics of inputs and the utilisation of outputs, the choice for a plant design also depends on a number of practical considerations, for example, the availability of space, funds, and infrastructure. In general, the goal of designing a plant is to make optimal use of the resources available for a particular project (Bachmann, 2013). The choice of equipment, dimensions, and layout needs to be tailored to the context of each plant, which means that no two biogas projects are exactly alike.