# Production of Biogas and Biomethane as Renewable Energy Sources: A Review
## 3 Results and Discussions
### Anaerobic Digestion and Co-Digestion
As a result, biogas production is greatly improved by co-digestion compared with mono-digestion. Co-digestion of faceted biomaterials with animal manure and other biodegradable organic substances provides sufficient access and potential for microorganisms to promote optimized degradation [96]. The main advantage of co-digestion is the improvement in biogas yield and its methane content. Despite the many benefits of co-digestion, antagonistic effects due to an incompatible raw-material mixing ratio can result in organic overload, acidification and system failure [97]. Some of the major disadvantages of co-digestion that hinder the application of the technology with large-scale commercial reactors include the accumulation of non-digestible solids within the digester, a high nitrogen load and the accumulation of acids from other co-substrates [98]. However, these contests can be addressed by depicting heterogeneous organic compounds in digester feedstocks and understanding their inherent biodegradability patterns [99]. In addition, the synergistic effects of the co-substrate mixture that are caused by the dynamics of the co-digestion process as well as the microorganisms involved outweigh the disadvantages of the technology. With process regulation and control, as well as pretreatment, the benefits of anaerobic co-digestion can be fully realized. Different in situ and ex situ preparation strategies are involved in optimizing products [100,101]. Animal manure is co-digested with other biodegradable materials to increase economic effectiveness while ensuring the stability of the anaerobic digestion system on a commercial scale [102]. The overall environmental impact of anaerobic digestion is considerably lower than that of the composting process, i.e., 6.5% in all categories, with the exception of agricultural soil pollution, where the percentage is higher by 1%. The results proved significant [103]. However, research and development of co-substrate mixing proportions need to be further investigated for a wide variety of co-digestion substrates. Further research on a wide range of co-digestion feedstock combinations and their mixture ratios is needed. Zootechnical Slurry for Biogas Generation  
A wide variety of biomass available on Earth is used as a component for renewable energy production. For instance, wood, energy crops, agricultural residues, food waste, livestock manure and industrial waste are promising solutions to reducing the negative impact of waste on the environment [104]. Particularly, livestock manure and slurry are considered valuable feedstocks for biogas generation [105]. Their conversion to energy using Waste-to-Energy technologies promotes their use as a clean energy resource to decrease the uncontrolled decomposition of animal waste [106,107]. The improper management and disposal of waste from the livestock industry could represent a significant threat to the environment. Unprotected manure, in fact, contributes to the release of a large amount of biomethane and thus to global warming [108]. Furthermore, this material is also a reservoir of pathogens that can spread and enter the surrounding groundwater [109]. Animal waste management involves several key processes, including thermal (incineration), chemical (hydrolysis) and biological (anaerobic digestion) methods. Ammonia and nitrous oxide emissions, as well as the presence of phosphates, heavy metals and pathogenic agents from livestock effluents, can promote ecosystem dysfunction, resulting in adverse effects on animals' and farm workers' health [110,111]. Therefore, controlled anaerobic digestion (AD) is one of the most suitable technologies for the treatment of manure. Nutrient-rich digested manure becomes efficient matter for clean energy source production to reduce the employment of conventional fossil fuels and hazardous gas emission [105]. It is also applied as a natural fertilizer to replace synthetic fertilizers and to improve soil fertility [112]. However, some limitations of anaerobic manure digestion can affect biogas production.