Feces Conversion Processes

Julien De Mori
December 16, 2011

Submitted as coursework for PH240, Stanford University, Fall 2011

Fig. 1: Cows on a farm. (Source: Wikimedia Commons)


Biomass is a term used to refer to organic material obtained from a living organism that can be used as a renewable energy source. Traditionally, biomass referred to plant products like wood, but nowadays is used more commonly to refer to ethanol, biogas, or biodiesel. Feces are an abundant form of biomass that can be converted into energy via a variety of mechanisms. In the developed world, 75% of the population live in urban areas, causing large amounts of excess waste that the environment cannot naturally assimilate rapidly enough. [1] As a result, it is necessary to implement mechanisms that convert this waste into energy, whilst eliminating or lessening some of the burden on the environment.

General Processes

The two primary manners in which feces can be converted to energy are thermal and biochemical.

Thermal processes involve the transfer of heat to the manure, whether by combustion, pyrolysis, gasification or some other mechanism, to convert the biomass to another chemical form. In doing so, energy can be released by destruction of the large number of carbon and hydrogen bonds present in the feces. [2] Gasification is a promising process because it converts solid waste into a gaseous product, which is readily combusted and turned into energy.

For wet waste, the biochemical process is used. A promising process makes use of anaerobic digestion by enzymes in bacteria, which decompose the natural waste and produce biogas, a mixture of prevalently methane and carbon dioxide, which can in turn be converted to energy. [3]

Which Is Most Viable?

There are a few issues pertaining to thermal conversion by direct combustion of the manure. Primarily, manure is commonly used as fertilizer, and combusting it instead would require that we utilize artificial fertilizers instead, produced by means of costly and energy intensive processes. Also, combustion of manure would release huge amounts of greenhouse gases.

Gasification is a much more effective, albeit more complex, thermal process. It requires that the organic waste be reacted with controlled amounts of oxygen and steam at high temperatures, such that it not combust, and produce syngas, a mixture of gaseous hydrogen and carbon monoxide, which is directly combustible. [4] A major benefit to gasification is that it can be performed on sewage sludge, and thus eliminates a great deal of waste, both liquid and gaseous, and greatly benefits the environment. [5]

Anaerobic digestion occurs when microorganisms break down organic material in the absence of oxygen. It is extremely useful in converting manure to energy since virtually none of the the nitrogen, phosphorous or potassium are removed from the digested manure. [6] Therefore, it could still be used as fertilizer. A recent investigation found that converting all the cow manure produced in the country into biogas, and subsequently electricity, would generate approximately 2.5% of the country's yearly electricity consumption. Also, yearly greenhouse gas emissions would be decreased by nearly 4%. [7]

What Is Being Done?

The use of feces to create energy has been investigated for some time, and in some cases implemented.

Anaerobic digestion is a powerful way of dealing with the vast amounts of un-utilized farm manure. Some farms have already constructed anaerobic biodigesters to convert their cow manure to energy. At Hillcrest Saylor Dairy Farm they are already managing to save $60,000 a year by using the biodigester, which converts feces and wastewater into electricity, fertilizer, and heating fuel. [8]

Anaerobic digestion and other biomass conversion processes are not limited to use on farm animal feces. In Rwanda, some engineers and activists are attempting to put to use the vast amount of feces produced in prisons. Their biogas facilities can take 100 cubic meters of waste and convert them into 50 cubic meters of fuel. [9]

It is important that action not only be taken on small scales. R&D should continue to go into developing large scale, efficient and effective anaerobic digestion systems for manure around the world. Nonetheless, even enough small scale action could generate large scale impact.

© Julien De Mori. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.


[1] L. Yassin et al., "Techno-Economic Performance of Energy-From-Waste Fluidized Bed Combustion and Gasification Processes in the UK Context," Chem. Eng. J. 146, 315 (2009).

[2] Sustainable Bioenergy Development in UEMOA Member Countries," International Centre for Trade and Sustainable Development, October 2008.

[3] U. Marchaim, Biogas Processes for Sustainable Development (Food and Agriculture Organization of the United Nations, 1992).

[4] J. D. Mackaluso, "The Use of Syngas Derived from Biomass and Waste Products to Produce Ethanol and Hydrogen," Basic Biotechnology EJournal 3, 98 (2007).

[5] A. Mountouris, E. Voutsas and D. Tassios, "Plasma Gasiļ¬cation of Sewage Sludge: Process Development and Energy Optimization," Energy Conversion and Management 49, 2264 (2008).

[6] S. Mukhtar and S. Capareda, "Manure to Energy: Understanding Processes, Principles and Jargon," Texas A&M University, E-428, November 2006.

[7] A. D. Cuéllar and M. E. Webber, "Cow Power: The Energy and Emissions Benefits of Converting Manure to Biogas," Environ. Res. Lett. 3 034002 (2008).

[8] J. Bogo, "Cows to Kilowatts: U.S. Farms Save Big Turning Manure to Energy," Popular Mechanics, 1 Feb 09.

[9] C. Farivar, "Human Feces Powers Rwandan Prison," Wired, 16 Jul 05.