**8.1 Advantages of biogas technologies**

The production of biogas through anaerobic digestion (AD) offers significant advantages over other technologies of bioenergy production. It has been acknowledged as one of the most energy-efficient and green technology for bioenergy production [82]. For many reasons, it is a versatile renewable energy source [158], it can be produced when needed and can easily be stored [61], biogas can be easily upgraded to remove undesired components thus producing a higher fuel standard (Biomethane) with high specific caloric value [189]; it combines energy (gas) storage with generation [214]; the feedstock source is often a waste or problem product, and hence its use for energetic utilization resolves waste management problems [214]; biogas technology provides an excellent opportunity for mitigation of greenhouse gas emission, improving air quality, and reducing global warming [6, 215–217]. Biogas technology also has potential to mitigate climate change and eutrophication [218]; it can be used as an alternative to fossil fuels [158]; it can easily co-digest a range of feedstocks, thus providing an integrated waste management service; it provides valuable co-products such as nutrient-rich bioslurry [214]; biogas production is a treatment technology that generates renewable energy and recycles organic waste into a digested biomass, which can be used as fertilizer and soil amendment [82, 189]; Methane-rich biogas (biomethane) can also replace natural gas as a feedstock for producing chemical materials [82]; biogas is considered to be the future of renewable and sustainable energy [219]; noise levels generated by methane-powered engines are considerably lower than those of diesel engines, a plus in congested urban environments [12]; biogas technology has an important role to play in the waste management, renewable energy, water, and nutrient (food security) sectors [214]. The development of a national biogas sector contributes to increase the income in rural areas and creates new jobs [189].

The benefits of using co-digestion techniques for optimizing biogas production yields which including dilution of potential toxic compounds, improved balance nutrients, synergistic effect of microorganism, increased load of biodegradable organic matter, and higher biogas yield [82, 220, 221]. In small-scale installations,

worldwide, the gas is primarily utilized for lighting and cooking. In larger units, the gas can be used for co-generation (generation of heat and electricity), as vehicle fuel or as fuel in industrial processes [58].

### **8.2 Disadvantages and problems**

Biogas production from anaerobic digestion (AD) suffers from several technical limitations. The social acceptance of biogas is usually hampered by health and environmental concerns. There are undesired and harmful substances contained in biogas which considered as biogas pollutants (such as H2S, Si, volatile organic compounds (VOCs), siloxanes, CO, and NH3). H2S and NH3 are toxic and extremely corrosive, damaging the combined heat and power (CHP) unit and metal parts via emission of SO2 from combustion [11, 160, 215]. The existence of H2S not only influences the quantity and quality of the biogas produced which can limit its application, but also generates harmful environmental emissions and corrodes the engines of biogas purification machinery [23, 163]. It also causes excessive corrosion and expensive deterioration of lubrication oil [21]. Moreover, the presence of siloxanes in biogas, even in minor concentrations, is associated with problems. It is well known that during combustion silicone oxides generate sticky residues, which deposit in biogas combustion engines and valves causing malfunction [11, 160]. Biogas produced by AD still contains impurities. Therefore, the systems used in the production of biogas are not efficient [189]. The quality and quantity of biogas usually requires pretreatment to maximize methane yields and post-treatment to remove H2S, which involves considerable energy consumption and higher costs [163]. There are no new technologies yet to simplify the process and make it abundant and low cost. Similar to other renewable energy sources (e.g., solar, wind) production of biogas is also influenced by the climate. The optimal temperature required for bacteria to digest waste is about 37°C. In cold weather, digesters need heat energy to preserve a fixed biogas supply [189]. The greatest challenge encountering the utilization of biogas as a truck and bus fuel has been the restricted driving range that it provides, meaning that drivers must refuel much more often than they would in petrol- or diesel-powered cars [12].
