Conversion of organic materials to energy can proceed along three main pathways: thermochemical, biochemical, and physiochemical. Useful forms of energy that can be produced include heat, steam, electricity, renewable natural gas, and fuels. Currently, all three pathways are utilized or have been demonstrated using mixed or separated municipal solid wastes (sometimes in combination with industrial or petroleum refining residues).
Biochemical conversion processes include anaerobic digestion (which occurs in landfills and controlled reactors or digesters) and anaerobic fermentation (for example, the conversion of sugars from cellulose to ethanol). Biochemical conversion proceeds at relatively low temperatures and low reaction rates. Higher moisture feedstocks are generally good candidates for biochemical processes. The lignin fraction of biomass cannot be economically converted by anaerobic biochemical means. Therefore, a significant fraction of woody and some other fibrous feedstocks exits the process as a residue that may or may not have market value. The residue, called digestate, can be composted or used as a fertilizer.
Thermochemical conversion processes include gasification and pyrolysis. Thermochemical conversion is characterized by higher temperatures and faster conversion rates. It is best suited for lower moisture feedstocks. Thermochemical routes can convert all of the organic portion of suitable feedstocks and create marketable co-products (biochar and syngas). The inorganic fraction (ash) of a feedstock does not contribute to the energy products and may contribute to fouling of high temperature equipment, increased nutrient loading in wastewater treatment and disposal facilities, and increased disposal costs. Inorganic constituents may also accelerate some of the conversion reactions.
Physiochemical conversion involves the physical and chemical synthesis of products from feedstocks (for example, biodiesel from waste fats, oils, and grease or FOG) and is primarily associated with the transformation of fresh or used vegetable oils, animal fats, greases, tallow, and other suitable feedstocks into liquid fuels or biodiesel. The March 2007 Biofuels from Municipal Wastes-Background Discussion Paper (PDF download) provides additional information on biofuels.
- Digesting Urban Organics Residuals (DUOR): A Forum on Technology, Economics & Permitting
- Permitting Anaerobic/In-Vessel Digestion Facilities and Operations
- Current Anaerobic Digestion Technologies Used for Treatment of Municipal Organic Solid Waste (PDF download), Updated June 2014
- Conversion Technologies Status Update Survey (PDF download), April 2009
- New and Emerging Conversion Technologies: Report to the Legislature (PDF download), June 2007
- Biofuels from Municipal Solid Waste (PDF download), March 2007
- List of Anaerobic Digestion Projects in California, Updated May 2021
- US EPA: Co-Digestion Economic Analysis Tool (CoEAT)
- US EPA: Anaerobic Digestion Informational Page
- American Biogas Council: What is Anaerobic Digestion?