Biofuel/Alternative Energy

Transformative advancement in renewable energy production by anaerobic digestion (AD) of waste streams requires an inexpensive, simple, and scalable pretreatment to increase the conversion of organic wastes into biogas. (Zamri et al. 2021, Atelge et al. 2020) Production of biogas by AD offers a proven, readily-scalable, and well-understood mechanism for energy production and disposal of organic wastes. However, inefficient conversion of waste into biogas, typically 30-40% in mesophilic digesters without pretreatment (Liu et al. 2021, Atelge et al. 2020, Tabatabaei et al. 2020, Rico et al. 2011, Nasir et al. 2012), makes it difficult for AD to be an economically viable source of renewable energy. Improving the economic viability of AD in the renewable energy market therefore requires a low-cost, efficient pretreatment that consistently and significantly increases the fraction of biomass converted into biogas. (Sevillano et al. 2021, Atelge et al. 2020, Cheah et al. 2020, Anukam and Berghel 2020, Carrere et al. 2016)

Pretreatment of organic wastes prior to AD by physical (e.g., mechanical pulverization, cavitation, and limited pyrolysis), physicochemical (e.g., steam explosion and ammonia fiber explosion), chemical (e.g., acid hydrolysis, alkaline hydrolysis, high temperature organic solvent pretreatment, and oxidative delignification), biological (e.g., lignin degradation by white- and soft-rot fungi), and electrical methods, and various combinations thereof, have existed for several decades, but are energy inefficient and are often not economically viable (Atelge et al. 2020, Anukam and Berghel 2020; Vyas et al. 2017; Kumar and Sharma 2017; Lee et al. 2016). To date, the only economically successful pretreatment method for increasing degradation and biogas production is the thermal hydrolysis process (THP), in which the influent is heated to 130-180°C for 30-60 minutes. THP of sewage sludges increases biogas yield by 50%, decreases viscosity, allowing higher loading rates, decreases effluent chemical oxygen demand (COD) by 50%, improves dewatering, and provides sterilized, odor-free compost. (Liao et al. 2014)

The optimum system for waste pretreatment depends on the physical and chemical characteristics of the waste being treated, and for some wastes, a pretreatment that uses a thermophilic biological component may provide many of the same advantages as THP at less cost. A biological pre-digestion process is more energy efficient then THP because it operates at lower temperature and pressure. However, for some wastes, the optimum pretreatment may be to add thermophilic biology post-THP, which could be done with no additional energy cost because the influent is already heated. Such a combination of compatible pretreatments may provide a significant increase in performance over THP or biological pre-digestion alone for some wastes.

Many wastes are recalcitrant for AD because the organic solids are large, polymeric molecules, e.g. lignocellulose, that are not directly accessible to methanogens (Atelge et al. 2020, Sayara and Sanchez 2019). Hydrolysis of these polymeric materials into small, soluble molecules or ions makes them readily accessible for methanogenesis and improves the rate and efficiency of conversion of the substrate into biogas by AD.

Figure 10 shows a schematic diagram of how a biological pre-digestion would be implemented in a commercial plant for producing biogas from an organic waste stream.