i.Reduced cost of waste disposal

The production of biogas offers solutions, for the treatment and management of digestible animal and plant waste leading to sterilization and production of useful manure and sewage with significant volume and mass reduction which effectively reduces the cost of waste treatment and disposal [30, 64, 86]. According to the French Agency for Environment and Energy Management (ADEME), using manure as raw for anaerobic digestion reduces methane emissions from the storage and use of manure, and biogas reduces fossil greenhouse gas emissions (with an effective reduction in greenhouse gases of more than 751,000 tons of CO2 equivalent). The global warming potential of carbon dioxide emissions from biogas is smaller than that of methane emissions from fossil fuels. Therefore, biogas production and use reduce greenhouse gas emissions in accordance with the Kyoto protocol and Paris agreements. The negative effect is that reduced use of manure increases use of chemical fertilizers for fertility or nutrient replenishment to the soil [13, 80].

## **2.4 The biogas cycle**

Biogas is generated by anaerobic digestion of biomass with the most exploited feedstock being agricultural wastes like manure, poultry litter, hay, and straw, stillage from ethanol production, activated sludge from wastewater treatment plants, etc. [87]. The biogas cycle is demonstrated in **Figure 1**.

**Figure 1** shows the biogas cycle which consists of biodegradation of biomass mainly from energy crops and wastes from animal husbandry, biofuel process products, crop harvesting waste, industrial waste and food consumption wastes which are fed to anaerobic digester whose main products are biogas and biofertilizer which is taken back to the soils to support biomass regeneration [88]. The CO2 released in combustion is absorbed by the vegetation through photosynthesis to form biomass. The organic nitrogen rich residual sludge from the digester is used as fertilizer for the soil [87]. Biogas can be treated and enriched and used as natural gas, but can also be combusted for production of thermal energy or electricity through different generation prime movers [89, 90].

The anaerobic digesters have been used widely to supply biogas by many households [87, 91]. The new trend in biogas production and utilization is the biorefinery concept which mainly uses renewable biomass as energy source and combines with production of chemicals, like plastics, solvents, and synthetic fuels [79, 89, 91]. A typical example is the Danish Bioethanol Concept which comprises the ethanol production from lignocellulosic biomass while biogas is produced from stillage and cellulose waste. Then residual cellulose waste is also recycled after wet oxidation for additional conversion into biogas [39, 87, 92]
