Biogas from Sewage and Landfills, Glamorous No but a Renewable Yes

In the last of the “Mad Max” movies, the one that prominently featured Tina Turner, Mad Max: Beyond Thunderdome, the post-apocalyptic world depended on “pig sh*t” for energy.

That idea, albeit the invention of a Hollywood screenwriter, may not, however, be as outlandish as it might originally appear. In India, for example, there are myriad small methane producers throughout the country that produce just enough methane for home uses, using small methane generators powered mostly by cow dung.

In September of 2008, San Antonio, Texas set into motion an ambitious plan that would make it the first US city ever to convert sewage into methane gas on a large scale. The plan calls for the complete recycling of at least 90% of the approximately 140,000 tons a year of “biosolids” produced by the citizens of San Antonio into water for agricultural irrigation, solid compost and now, methane gas, which will be used for the generation of electrical power.

The facilities for extracting methane gas from sewage will be built by Ameresco, a Massachussetts based natuiral gas company.  Steve Claus, the CEO of the San Antonio expects the city to realize about $250,000 annually when the facilities are completed in 2010. The facilities are projected to generate between 900,000 and 1.5 million cubic feet of natural gas per day.

The idea of sewage derived biogas is being tried, on a significantly smaller scale in a number of other localities throughout the nation including Berkeley, California and West Lafayette, Indiana near Purdue University. The Indiana facility started up on June 6, 2009 and is part of an $8.2 million project to upgrade the plant’s digesters. The digesters, in turn, process sewage, grease and food waste into methane which is then used to run two microturbines. The microturbines produce a portion of the electricity needed to run the plant, representing an eventual savings of $18,000 a year for the city.

As part of its “Green Switch” program the Tennessee Valley Authority (TVA) extracts methane from the City of Memphis’ wastewater treatment facility. The methane is “co-fired” with coal to produce eight megawatts of power at the TVA’s Allen Fossil Plant. The TVA estimates that the methane produces energy equivalent to 20,000 tons of coal per year.

The municipality of Lille, France began running city buses on a form of modified diesel fuel derived from sewage sludge treated at the Lille-Marquette municipal sewage treatment plant in 1994. Approximately 12.5% of the energy derived from the biogas produced is used for refinining. The initial costs of the project were 4.7 million French Francs (FF) for the biogas scrubbing unit (that cost included design and research costs) and 3.4 million FF for bus engine conversion (PDF file). The first bus cost 600,000 FF to convert.

Based on an operational estimate of 4000 hours per year, the estimated cost per unit of biogas is 2.9 FF per cubic meter of biogas, which although higher than the cost of natural gas currently should become more competitive if, as expected, natural gas, diesel and gasoline prices rise in the future. Fuel efficiency of the biogas burning bus engines was approximately 60% that of diesel burning engines.

Landfill Gas

Another source of methane emissions that shows steady growth, both in the source and it’s utilization, is landfill gas.  As the name suggests, landfill gas is methane produced by the anaerobic decomposition of waste into methane and by-products such as carbon dioxide, sulfur and other chemicals which are often referred to as volatile organic compounds.

Although somewhat more controversial than using sewage as a biogas source, proponents point out that landfills will release methane regardless of what we do. It is better, they argue, to convert that gas primarily into energy and carbon dioxide than to simply allow it to enter the atmosphere. The fact that methane is 21 times more powerful a greenhouse gas than carbon dioxide and that half the gas emitted by landfills is, in fact, methane would seem to clinch that argument.

Furthermore, landfills produce more man-made methane than any other source in the US. If, in fact, landfill gas production can properly be classified as a renewable energy project, it has quietly become the one of the most prolific and widespread use of renewables. Texas alone, a relative newcomer to landfill gas utilization has 24 landfill gas energy projects and, according to the EPA, sites suitable for such projects at another 57 locations.

The total production figures for landfill gas can be difficult to ascertain as the EPA combines production figures for landfill gas with production figures for the direct burning of municipal waste.  While properly classified as a renewable energy source, burning municipal waste without safeguards presents a potential source of airborne toxins including heavy metals which can hardly be considered green.

According to those figures five states, Florida, Pennsylvania, New York, Massachussetts and California all produced greater than 1 billion kWh of electrical power from the combination of the two sources in 2006. Of those, the greatest production came from Florida which produced 1.9 billion kWh that year.

Other links:

• Power Scorecard – Electricity from Landfill Gas
• US EPA – Landfill Methane Outreach Program (LMOP)
• Wikipedia – Biogas
• Landfill Gas Data and Information – US Government