A Staff Report from the Straight Dope Science Advisory Board

Can animal (including human) manure be used as fuel?

February 21, 2006

Dear Straight Dope:

The other day, I was reminded of the fact that many cultures use dried cow, yak and buffalo manure as fuel. I was wondering: Can the feces of other species (including humans) be used for fuel as well? The heating bills are going to start piling up as winter comes, and I thought a bit of recycling might be in order . . .

The waste products of most animals can be used as fuel, and likely have been at one time or another. This isn't just in so-called primitive cultures, either--many technologically modern nations have studied and implemented both animal and human waste combustion.

Although you might wonder why anyone would burn hippopotamus manure or human waste given the many other fuel sources available, two very good reasons drive their utilization. First, animal and human waste is generally considered a renewable energy source and thus can be burned as "green power," giving the entity combusting the waste (as well as the country in which it resides) a political or treaty-fulfilling credit for reducing greenhouse-gas emissions. Second, some cities and regions have limited landfill or wastewater treatment capacity, and burning the waste can reduce the load on their waste disposal facilities. Other benefits from burning these waste products can include tax credits for using renewable energy sources and good public relations karma. Still another benefit is that burning animal waste can prevent its decomposition into methane, a greenhouse gas that's much worse for the environment than carbon dioxide.

That said, waste combustion presents some significant problems, and one of my many jobs is assessing just what those problems are, the degree of risk involved, and what can be done to minimize the risk.

Human waste combustion needs to be discussed separately from animal waste because the human variety is more hazardous to burn, although not for the reason you might imagine, namely the potential spread of disease or parasites. Outside of college pranks or conceptual art exhibits, few seriously consider burning human waste in its original state. Instead, what we're almost always talking about is burning sewage sludge, the dewatered mass of organic and other compounds produced by sewage treatment plants. The problem is that everything dumped down the drain, both at home and at factories, ends up in the sewage treatment system. This includes household soaps and chemicals, industrial oils and solvents, heavy metals such as mercury and cadmium, and even some mildly radioactive materials. While some fraction of these is separated out during the sewage treatment process, and other items decay or de-volatize, much of the stuff ends up in the sludge.

As you can imagine, burning sewage sludge will release many of these toxins into the environment. Even the best emissions control systems can't remove all the heavy metals, dioxins, and other unpleasant items from the gases going up the smokestack. Whether sludge combustion is more hazardous than, say, coal combustion depends on the specific properties of the sludge, but the thing is, there's more to it than just throwing another buffalo chip on the fire. In the United States sewage sludge typically isn't considered an "opportunity fuel" unless it has a minimal level of hazardous materials. Even then, a driver such as a lack of landfill space or tax incentives of Biblical proportions is generally needed to make it feasible.

Determining to what extent sewage sludge is used as fuel is a challenge. The European Commission estimates that in 2005 approximately 21% of the sludge produced by its member countries was incinerated. Most of this wasn't used for power production, however, and was incinerated simply to dispose of it. In the United States, sewage sludge combustion is uncommon, and is even more rarely used for power generation--the only reliable figures I've been able to find are for 2002, when approximately 0.007% of total electric power production came from sewage sludge combustion. I can tell you from personal experience that while many U.S. utilities have considered sewage sludge combustion, the fear of increased heavy metal (mainly mercury) and dioxin emissions has shut down all the sewage sludge projects I've worked on.

Unlike humans, animals generally don't have a convenient centralized waste collection system (highly-trained toilet-using cats excepted). That means the animal waste fuel supply is often afflicted with low "energy density,"  a frequent problem with alternative fuel sources. Unlike fossil fuel and some biomass resources, animal and human waste often requires substantial ongoing investment in collection and transportation in order to achieve economies of scale. For example, while a single farm may produce a score or more tons of cow manure per day, a typical coal power plant burns 500 tons of coal an hour. Add to this the fact that for some processes there's just no practical way to collect the waste and prepare it for transport to the power facility. A small-scale plant can be located near a convenient supply, but opportunities for these are limited, and small-scale plants typically are expensive for what you get.

Another problem common to all animal (and human) waste is its unsuitability as a fuel. Manure generally has a high moisture content when first produced, and this moisture must be removed by either drying in the environment or industrial drying. Drying in the environment involves noxious emissions and a lot of time, while industrial drying requires energy, which can reduce the efficiency of the system below the break-even point. Even after the waste is dried, its heat output is typically poorer than the worst coal commercially available.

In case you were wondering, the heat content of human waste is highly variable due mostly to its moisture content, which can be 90% or more in situ and is often still 50-75% after pressing and vacuuming. When totally dry, sewage sludge has about 4000-7000 Btu/lb, which is equivalent in heat to lignite coal--also, appropriately, called "brown coal." On a wet basis, it can have less than 1000 Btu/lb, which is too little to sustain its own combustion.

What sorts of animal wastes are or could actually be used as fuel? There are as many types of waste as there are animals, but I'll focus on the ones of greatest economic and engineering interest. Volume is a big factor here. In the United States alone, nearly 300 million tons of wet animal and feedlot waste are produced per year, the equivalent of about 110 million tons of dry waste. Many schemes are underway to utilize not just manure but slaughter and production waste for fuel. For the sake of limiting scope creep in this article, I'll focus on manure from the largest contributors.

Poultry litter (a mixture of excrement, feathers, straw and bedding material, uneaten food, sawdust, etc.) is among the more promising animal energy sources. It's especially attractive due to its dryness and the relative ease of collection from large poultry processors. Litter production is said to be approximately 1.25 tons per 1,000 birds brought to market, according to the Natural Resource, Agriculture, and Engineering Service (NRAES). Given that 8.4 billion birds were processed in the U.S. in 2001, that means 10.5 million tons of litter per year to dispose of. One power plant using poultry litter for combustion is Thetford in Norfolk, England. This plant is reputedly the largest of its kind in the world, burning about 420,000 metric tons of chicken litter per year to generate a peak of 38.5 MW of electric power.

Turkey litter production is estimated by NRAES at between 8 to 14 tons per 1,000 birds brought to market, substantially more per bird than poultry litter. With nearly a quarter billion turkeys raised in 2003, that's a lot of litter. Turkey litter has been in the renewable energy news recently with the announcement of plans to produce fuel from the waste through "pyrolysis," a method of heating an organic compound to release its volatile components, many of which can then be condensed into oil for use in motor vehicles. Turkey litter is also used as a fuel for electricity production at a few facilities such as one in Benson, Minnesota.

The 9 million milk cows in the United States as of 2005 represent a large source of potentially combustible waste. Worldwide there are roughly 125 million milk cows (2003 data), each one happily producing manure in addition to milk, so clearly there's potential here. However, as anyone who has been around a dairy farm can see, cow manure is typically high in moisture, and it doesn't burn well unless dried extensively. It can be gasified or digested to produce methane with more success--Panda Energy plans to gasify one billion pounds of cattle manure per year to power an ethanol production plant in Kansas. Steer manure from feedlots has also been considered and tested as a fuel source, as have buffalo chips, but I know of no large-scale commercial efforts. In lesser-developed countries, cow and steer manure is used for cooking fires, but the quality and heat of the flame leaves much to be desired--it's often the fuel of last resort.

In 2004 there were approximately 60.5 million hogs and pigs in the United States and close to a billion worldwide, each wallowing in potentially usable filth. Pigs are high waste producers--a typical hog can produce more than 10 pounds of manure per day. In addition, according to Mechanical Engineering Magazine:

The conversion of swine manure to oil, it turned out, was easier than converting other biomass to oil because the manure contained less lignin, which is difficult to decompose. In a sense, the hogs turn biomass animal feed into a form that is more suitable for energy conversion.

However, outside of some relatively small projects to create methane from gasified pig manure, or to use pyrolysis to create "swine oil" (as with turkey litter), most of this waste isn't utilized due in large part to the horrifying smell. Yes, the very smell of pig waste, which can be truly breathtaking, has dissuaded clients of mine from seriously considering its use, economics be damned.

Sheep, goats, kangaroos--each of these has been the subject of numerous studies to see if burning their waste or the gasified product thereof could be useful, but as far as I can determine none has been commercially successful absent large tax credits or other subsidies. Some have even studied zoo manure as a resource for power generation, but I don't know of any successful efforts, and I suspect, given the relatively small amount of waste produced at even a large park like the Kansas City Zoo, that the effort would mainly be for public relations. I suspect as well that most zoos can find easier ways to get exposure. Would you want to climb into the lion enclosure to collect their waste? That would truly be a case where, as some say in the industry, "going green can result in a lot of red."

References

BCO (Bioenergy--Climate Protection--Oil Reduction) Newsletter, March 2005, Environmental and Energy Study Institute

"Disposal and Recycling Routes for Sewage Sludge," Economic Sub-Component Report, 29 January 2002, European Commission, DG Environment, B/2.

"Economic and Technical Feasibility of Energy Production from Poultry Litter and Nutrient Filter Biomass on the Lower Delmarva Peninsula," Final Report, prepared for the Northeast Regional Biomass Program, August 2, 1999.

Energy Power Resources website--"Overview," Thetford power station: http://www.eprl.co.uk/assets/thetfor d/overview.html

Kansas Ag Connection, "Kansas Manure to Power Ethanol Plant":

http://www.kansasagconnection.com/story-state.cfm?Id=531&yr=2005

"Renewable Energy Trends 2003, With Preliminary Data For 2003," U.S. Energy Information Administration, Office of Coal, Nuclear, Electric and Alternate Fuels, July 2004

"State works to boost use of biomass; plant to create energy from turkey litter," Gregg Aamot, Associated Press, in St. Paul Pioneer Press: http://www.realcities.com/mld/twincities/news/12668590.htm

"Resuspension of coal and coal/municipal sewage sludge combustion generated fine particles for inhalation health effects studies," Fernandez A, Wendt JO, Cenni R, Young RS, Witten ML, The Science of the Total Environment, 2002 Mar 27;287(3):265-74

Agricultural Statistics 2005, National Agricultural Statistics Service, U.S. Department of Agriculture

Yuanhui Zhang, Kim Ocfemia, and Malia Appleford, "Swine Oil," Mechanical Engineering Magazine (online), American Society of Mechanical Engineers, vol. 1, no. 2, June 2004

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