The power of manure

When thinking about renewable energy sources, images of windmills and solar panels often come to mind. Now add to that picture livestock manure. Researchers from the MIT Joint Program on the Science and Policy of Global Change have found that the implementation of climate policies in the United States could hasten adoption of anaerobic digesters as a source for renewable electricity.

Anaerobic digesters break down organic waste using methane-producing bacteria. This methane can then be captured and burned to generate electricity. But anaerobic digesters have several other benefits besides production of renewable energy.

Traditional livestock-manure-management techniques include storing manure in anaerobic pits or lagoons, which release methane emissions into the atmosphere. In the United States, these emissions account for 26 percent of agricultural emissions of methane, a potent greenhouse gas. Diverting these emissions toward electricity generation thus reduces total U.S. greenhouse gas emissions and may qualify for low-carbon energy subsidies and methane-reduction credits. Anaerobic digesters can also reduce odor and pathogens commonly found in manure storage and digested manure can be applied to crops as a fertilizer.

In collaboration with the University of Wisconsin, researchers used the MIT Emissions Prediction and Policy Analysis model to test the effects of a representative U.S. climate policy on the adoption of anaerobic digesters. Currently, support for anaerobic digesters has been limited and the economic value of most systems is insufficient to promote widespread adoption.

The lack of widespread use of anaerobic digesters is not due to lack of availability; the researchers estimate that cattle, swine and poultry manure deposited in lagoons or pits currently has the potential to produce 11,000 megawatts of electricity. (For scale, one megawatt can power 1,000 homes for one instant.) The main reason for the lack of anaerobic digesters is that they compete with electricity from cheaper, traditional sources. However, under a climate policy that puts a price on all emissions, electricity produced from fossil fuels becomes more expensive, and low-carbon energy sources become more competitive.

The study found that, under a representative climate policy, anaerobic digesters are introduced in 2025 when the price of carbon-dioxide equivalent, or CO₂e, is $76 per ton. (CO₂e refers to the concentration of carbon dioxide that would cause the same amount of radiative forcing as a given greenhouse gas. Because different greenhouse gases have different global warming potentials, carbon dioxide is used as a reference gas to standardize the quantification of multiple greenhouse gas emissions.) By 2050, use of anaerobic digesters would contribute 5.5 percent of national electricity generation and would mitigate 151 million metric tons of CO₂e, mostly from methane abatement. These mitigated emissions would also allow the livestock operations to sell emissions permits, adding economic value to the process.

Overall, the researchers identified a win-win situation, where incentives to reduce greenhouse gases would result in both market benefits (cheaper energy generation and sale of emissions credits) and non-market co-benefits (environmental and health gains, fertilizer uses) from adoption of anaerobic-digester operations. Such incentives, in the form of climate policies that provide methane-reduction credits and increase the costs of electricity from fossil fuels, provide the opportunity for a novel linkage between agriculture and energy production.