California Department of Resources Recycling and Recovery (CalRecycle) 

Conversion Technologies

Biochemical Conversion Processes

Biochemical conversion processes include anaerobic decomposition or digestion and anaerobic fermentation, and occurs at lower temperatures and lower reaction rates compared to thermochemical processes.

 Higher-moisture feedstocks, such as food waste, are generally good candidates for biochemical processes.

Energy products from biochemical routes include biogas (also landfill gas) and ethanol (sometimes referred to as bioethanol). Biogas can be used to produce electricity, fuel, or renewable natural gas. According to the U.S. Department of Energy, other biofuels, such as biobutanol and dimethyl ether, are being investigated and could become important biofuels.

For biomass feedstocks, the lignin fraction currently cannot be economically converted biochemically. The lignin and other stabilized residue from biochemical conversion may be suitable as a compost product or could be used for energy through thermochemical processes, perhaps to supply the energy needs for the biochemical conversion plant. If the residue is of poor quality or highly contaminated (i.e., the feedstock came from mixed waste rather than clean source separated material), it may not have a market value and would likely end up in the landfill.

Anaerobic Digestion

Anaerobic digestion (AD) is a fermentation technique that operates without free oxygen and results in a biogas containing mostly methane and carbon dioxide but frequently carrying impurities such as moisture, hydrogen sulfide, ammonia, siloxane, and particulate matter. Anaerobic digestion occurs in landfills, manure lagoons (covered or not), controlled reactors, or in-vessel digesters.

Biogas, primarily methane and carbon dioxide, is the principal energy product from AD processes. Biogas can be burned directly for heat or steam or converted to electricity in reciprocating or gas turbine engines, steam turbines, or fuel cells. Biogas can also be upgraded to biomethane and used as a vehicle fuel, injected to the natural gas transmission system, or reformed into hydrogen fuel. The digestate from AD (lignin and other stabilized residue) may be suitable as a compost product. 

Fugitive greenhouse gas (GHG) emissions resulting from the anaerobic decomposition of organic wastes in landfills have been identified as a significant source of emissions contributing to global climate change. Reducing the amount of organic materials sent to landfills by diverting those materials to anaerobic digestion and compost facilities is part of the AB 32 (California Global Warming Solutions Act of 2006) Scoping Plan. For more information on the connection between the waste sector and California’s GHG emission reduction goals, please see CalRecycle’s Climate Change page.  

AD systems have been used in Europe for over 20 years to treat the biodegradable fraction of solid waste prior to landfilling in order to reduce future methane and leachate emissions and recover some energy. As a consequence of the European Commission Landfill Directive, installed AD capacity in Europe has increased sharply and in 2013 there were 244 AD plants with more than 8 million tons of annual capacity.

There is a growing number of AD facilities in California that use the organic fraction of municipal solid waste (MSW) as feedstock, including some WWTPs that co-digest food waste and wastewater. With the passage of AB 341 and AB 1826, the development of a viable anaerobic digester infrastructure in California that uses our food waste and other urban organic wastes is one of CalRecycle's highest priorities. To find AD facilities in California, use the Facility Information Toolbox (FacIT). For more information see the regulations for in-vessel digestion of compostable materials.

For an in depth review and discussion of AD systems for MSW, see CalRecycle report Current Anaerobic Digestion Technologies Used for Treatment of Municipal Organic Solid Waste, March 2008.
Anaerobic Digestion facility permitting information is available.

Anaerobic Fermentation

Anaerobic fermentation (i.e., hydrolysis followed by fermentation to alcohols) is generally used industrially to convert substrates such as glucose to ethanol for use in beverage, fuel, and chemical applications and to other chemicals (e.g., lactic acid used to produce renewable plastics) and products (e.g., enzymes for detergents).

Fermentation of starch- and sugar-based feedstocks (i.e., corn and sugar cane) into ethanol is fully commercial but not yet for cellulosic biomass because of the expense and difficulty in breaking down (hydrolyzing) the materials into fermentable sugars.

Cellulosic feedstocks, including the majority of the organic fraction of MSW, need hydrolysis pretreatment (acid, enzymatic, or hydrothermal hydrolysis) to break down cellulose and hemicellulose to simple sugars needed by the yeast and bacteria for the fermentation process. With the possible exception of acid recycling and recovery, acid processes are technologically mature, but enzymatic processes are projected to have a significant cost advantage once improved.

Lignin in biomass is a byproduct of fermentation processes and is typically considered for use as boiler fuel or as a feedstock for thermochemical conversion to other fuels and products. Hydrolysis of lignocellulosic feedstocks is the subject of intense research.

Alcohols, such as ethanol and butanol, are the primary energy product from hydrolysis and fermentation processes. There are no known hydrolysis and fermentation systems operating on MSW feedstocks in the world.

Additional information on anaerobic digestion and fermentation can be found in New and Emerging Conversion Technologies: Report to the Legislature, June 2007.