# Recent Advancements in Anaerobic Digestion and Gasification Technology
## 4 Recent Studies on Anaerobic Digestion Focusing on Biogas Production  
Globally, approximately 50 million small-scale domestic digesters, 132,000 commercial digesters, and 700 upgrading biogas digesters have been installed. These digesters are fully functional and operate using anaerobic digestion technology. For instance, in Europe, to be specific, 15,000 commercial biogas digesters are in full operation [9]. In terms of biogas production, up to 31.24 billion cubic meters (bcm) were reported in 2015 from Europe. This number is predicted to increase by 70 bcm by 2030 because of the readily available resources [10]. On another note, the total global energy production resulting from biogas was approximately 2.8 \(\times\) 10\({}^{5}\) TJ in 2000; fourteen years later, it increased to 1.3 \(\times\) 10\({}^{6}\) TJ [NBD 2016]. According to the world biogas association report released in 2019, 10.5 GW of electricity generation came from the 17,783 biogas digesters in Europe. Although the specific time was not disclosed, it was indicated that Germany is leading with 10, 971 biogas digesters, followed by Italy (1655), France (742), Switzerland (632), and the United Kingdom 613 [11]. [PERSON] et al. [12] provided a comprehensive review of anaerobic digestion as an effective technology for biogas. The review finds that anaerobic digestion achieves a higher energy production, especially the co-digestion process, whereas the two-stageanaerobic digestion process helps in increasing methane production. Hence, the authors recommended the use of food waste as a suitable substrate for biogas digester because of its excellent biodegradability and high-water content. In another study, [PERSON] et al. [13] emphasized that the European agricultural sector is known for small-scale anaerobic digesesters. This is because of the small-scale individual farm sizes and land productivity, which are insufficient to meet the feedstock requirement of large and medium-scale plants. The study reveals that there is a need for national support which will aid in adopting small-scale anaerobic digesesters within the European countries. Nevertheless, there is still an overriding obstacle hindering the full commercialization of small-scale digesesters, and this has to do with upfront investment costs. In addition to this, substrate type as well as a mode of utilization is another factor of consideration for efficient biogas production. Mono-digestion is characterized by imbalanced nutrients, lack of microbial communities, and fluctuation in operational parameters. In a bid to address these challenges, [PERSON] et al. [14] conducted a study focusing on the anaerobic digestion of rice straw with livestock manure. Rice straw is high in cellulose and, hence, requires pre-treatment before being fed into the biogas digester. Evidently from the study, it was revealed that rice straw when co-digested with biological waste (livestock manure) for anaerobic digestion showed a better degradability potential. Therefore, the condition for optimum pre-treatment, operational parameters, and mixture proportion with the substrate improves methane production, according to a study by [PERSON] et al. [14]. Various factors, such as anticipated toxicity, lower pH (6.6), and low carbon C:N ratio (12:2), are said to be responsible for the challenges faced in using only textile waste (sludge) for energy and nutrient recovery. To this effect, [PERSON] et al. [15] conducted a study on the anaerobic co-digestion of textile aerobic sludge with food waste and cow dung. This was assessed through a biochemical methane test under a mesophilic temperature of 35 \({}^{\circ}\)C. From the result, cow dung mixed with textile sludge in a ratio of 1:1 generated 524.4 mL/g of biogas, whereas using textile sludge only did not produce any biogas. In addition, the mixture of food waste and textile sludge resulted in biogas production of 288.3 mL/g, which is lower compared with that of cow dung with textile sludge.