**4.3 Pretreatment technologies used in biogas production**

A number of various treatment processes presently exist for the organic waste management; some more technologically modernized than the others, and some more founded in some countries where the legislation and policy promotes for certain environmental goals. The four alternative systems are currently applied worldwide, i.e., Landfilling, Aerobic Composting, Incineration and Anaerobic Digestion (AD). Anaerobic digestion of organic waste is the most desirable management method and this research is going to discuss it in detail [50, 115, 116].

Organic waste is considered extremely heterogeneous, whereas its moisture content as well as level of impurities differs significantly. Hence, pretreatment before an aerobic digestion is a main process. Diverse types of pretreatment technologies have developed and are successfully installed in many anaerobic digestion plants in the worldwide. The organic waste pretreatment is considered as the major process step in biogas generation plants ensuring flexibility to treat different types of organic waste, efficient extraction of contaminants, high availability of AD plants, substrate homogenization, wear resistance, high biogas yields, energy efficiency,

and production of high-quality fertilizers. Organic waste almost includes contaminations, such as glass, metals, stones, and sand; additional systems are also required to deal with such heavy contaminants of the waste. Thus, pretreatment techniques, may be mechanical (e.g., milling), chemical (e.g., acid or alkali treatment), or thermal methods (e.g., steam explosion), are usually applied. Novel pretreatment methods are emerging, which focus on ionic liquid or supercritical CO2 to solubilize and collect lignin, both increasing biogas production, while also providing additional revenue through lignin collection. Regardless of the pretreatment technique used, this step is an essential consideration for improved biogas production from lignocellulosic feedstocks [50]. In any AD application treating organic waste, a mechanical pretreatment is installed.

A critical unit in industrial biogas plants is mechanical pretreatment; it possibly contains pulpers and shredders. These apparatus are utilized to improve the surface area of tough solid substrates [such as municipal solid wastes (MSW), cardboard, mixed industrial wastes, bulky refuse, waste tires, waste wood and waste papers, etc.] via crushing and breaking down, leading to their more efficient digestion and improved AD process. Different pretreatment technologies are available to reduce the size of the organic waste and to separate the plastic and packaging material from the biodegradable fraction of the waste. Thereby, plants are generally highly flexible to treat all kinds of organic waste without any quality restrictions. Sewage sludge or agricultural biomass, e.g., straw are difficult to degrade an aerobically due to their rigid structure. Therefore, organic waste thermal treatment at high pressure and temperature values is more familiar when treating such types of organic biomass. In contrast, food waste can be efficiently converted in anaerobic digestion systems to biogas by mechanical processing. An efficient pretreatment of organic waste also ensures the production of high-quality fertilizers and hence, the recycling of valuable nutrients back into the natural cycle is achieved and additional, expensive digestate processing after AD can be avoided. High biogas yields in anaerobic digesters are achieved, whether the biodegradable organic material is well crushed in the pretreatment and a large surface are for microbial degradation is achieved [50].

The composition of organic waste come from different areas (commercial, municipal, industrial) differs significantly. The most critical criteria for the selection of an appropriate pretreatment technology are waste composition. Moreover, for the selection of the most appropriate pretreatment system, it is significant to know which kind of AD, i.e., dry or wet digestion systems, should be used to treat the organic waste. Wet anaerobic systems use pretreatment technology to take out the undesirable pollutants before the anaerobic digestion process and are operated at a lower solid concentration. The digestate after anaerobic digestion may be used immediately as high-quality fertilizer and no further digestate treatment (compost refining, post-composting, etc.) is usually required. Amount of biogas generation is high due to the efficient organic waste pretreatment. The preferred technologies to treat wet organic waste such as food leftovers, food waste, packaged food, and organic fraction of MSW are Wet anaerobic digestion systems. Dry anaerobic systems use simpler pretreatment technology before the anaerobic digestion process and are operated at higher solid concentrations. As the efficiency of impurity separation is not enough to utilize the digestate immediately as high-quality fertilizer, further digestate treatment (i.e., compost refining, post-composting) is normally necessary to know whether the input substances are polluted.

Dry anaerobic technology are mostly utilized to know whether the organic waste involves a high percentage of garden waste and also after a mechanical extraction process can be used to treat the organic fraction of MSW [50]. Dry anaerobic digesters are higher solids loading and biomass retention, controlled feeding and

### *Recent Advances of Biogas Production and Future Perspective DOI: http://dx.doi.org/10.5772/intechopen.93231*

spatial niches, pretreatment is simpler, but it has complex and expensive transport and handling of waste, material handling and mixing is difficult, and only structured material can be used [117]. For processing dilute organic slurry with a total solid content of maximum 10–15%, the wet systems are designed. Substrates consist of total solid higher than 15% will be co-digested with co-substrates of lower total solid content, or usually diluted with recirculated or fresh process water. Various ranges of low solid substances have been successfully treated by wet AD technology, involving food industrial effluents and sewage sludge. In contrast, in solid-state fermentation processes, also called dry digestion, the substrates used have high solid content (25–40% TS), thus an essentially different technical approach regarding the waste handling and treatment is needed [50, 118]. Due to the high viscosity in the dry digestion systems, heat and nutrient transfer is not as efficient as it is in wet processes, therefore mixing is very important to prevent local overloading and acidification [119]. In spite of that, conventional mechanical mixers are not appropriate for solid-state processes; instead, recirculation of the waste or re-injection of the produced biogas is often used in these types of reactors to solve the mixing problems [50, 120]. The main benefits of wet anaerobic digesters is dilution of inhibitors with fresh water, but it has some drawbacks including scum formation during crop digestion, high consumption of water and energy, short-circuiting, and sensitive to shock loads [117].
