# Recent Advancements in Anaerobic Digestion and Gasification Technology
## 6 Feedback for Anaerobic Digestion
### Municipal Solid Waste (MSW)  
Municipal solid waste is waste generated from household waste, commercial waste, and waste from cleaning. Precisely, it consists of a fraction of household and garden waste. The composition of MSW includes paper waste, plastic waste, glass waste, metal waste, organic waste, and textile waste. For efficient digestion of these solid wastes, impurities, such as plastics, glass, and metals, need to be removed, as they affect the operation of the process, especially for biogas production [51; 52; 53; 54]. To lower greenhouse gas emissions and lessen the dependency on the use of external energy, the utilization of source-separated municipal organic waste is needed [55]. In considering the anaerobic digestion of MSW, particle size is one factor that must be considered. The particle size impacts the surface area, which increases and influences the hydrolysis rate during anaerobic digestion and lag phases. These are known as the limiting stage of adaptation of microorganisms to the substrate. [56]. Towards this, [PERSON] et al. [56] mentioned that a particle size of <2 mm is recommended and effective for methane production, having an electrical energy of 2960.4 kWh per week. The recommended particle size of <2 mm is 19% higher than in reactors with larger particles. Thus, the <2 mm particle size indicates lower design cost and maintenance. This is because the smallest particle enables a shorter retention time. Despite the recommended value of the particle size, an optimal range for the particle size still has not been defined. Notwithstanding, the size variability (from the crushing process) is another parameter capable of affecting the kinetics of the anaerobic digestion of MSW. Approximately1.3 tonnes of MSW are generated globally every year. This MSW consists of 46% organic waste, 17% paper waste, 10% plastic waste, 5% glass waste, 4% metal waste, 3% textile waste, 13% inert waste, and 2% miscellaneous waste [57].  
At the continental level, Africa generates the highest organic composition of municipal waste at 66%, followed by Europe (54%), South America (53%), and then Asia (47%). North America and Australia are regarded as the countries with the least composition of municipal waste generation, producing 26% and 25%, respectively. The variation in the organic composition of this waste by continent might be attributed to the rates of the organic portion of waste from low-income countries (50-70%) versus high-income countries (20-40%) [58]. Interestingly, anaerobic digestion of municipal waste provides many benefits, such as the separation of organic biodegradable fractions from total municipal waste, generation of renewable energy, reduction of landfilling, and mitigation of pollution.  
A grouping of the waste category of the feedstock required for anaerobic digestion was discussed earlier, and this is presented in Table 5.  
\begin{table}
\begin{tabular}{p{113.8 pt} p{113.8 pt}} \hline
**Technologies for Industrial Waste** & **Description** \\ \hline Up-flow anaerobic sludge blanket (UASB) & This involves the pumping of waste into the sludge bed, consisting of granular bacteria and a sludge blanket. The UASB essentially separates gas and prevents bacteria sludge from flowing into the waste. \\ \hline Anaerobic fixed-film (AFF) system & The AFF employs the use of a fermentation tank to provide a biological firm as an intermediary on which to fix bacteria, thereby reducing bacteria losses from the system. \\ \hline Continuous stirred tank reactor (CSTR) system & This technology uses a stirring machine characterized by high efficiency of digestion to reduce the hold-up time within the anaerobic digester. \\ \hline Modified covered lagoon (MCL) system & The MCL system is covered by high-density polyethylene (HDPE) plastic sheet as a container for biogas production. This system increases the contact area of bacteria sludge with the waste. \\ \hline \end{tabular}
\end{table}
Table 4: Various technologies employed in industrial waste for anaerobic digestion [36].