I saw a couple articles recently about anaerobic digestion and how it can make energy and help improve the environment. What exactly is anaerobic digestion, and if it’s so great how come I don’t hear about it more often? — Enrique J. Fenmas, Corona del Mar, CA
Enrique, that’s a great question. There are many different technologies out there that can provide homegrown renewable energy, as well as other benefits, but only a few of them seem to garner the lion’s share of attention, dollars and government support.
First, let’s review the basics. Digestion is a naturally occurring process whereby bacteria consume organic material. When this process happens in the absence of oxygen (ie, anaerobically), one of the by-products is methane gas, the principal ingredient of natural gas. Anaerobic digestion (AD) occurs naturally in the environment: for example, plant and other materials decay at the bottom of a swamp and you get swamp gas, or trash decomposes in a landfill producing landfill gas. Humans have witnessed this process since the early days of recorded history, and have tried to find some way to harness it.
Anaerobic digesters are devices that help capture, control and enhance this process, and have evolved from rudimentary covered pits to highly sophisticated devices designed to carefully manage the reaction for maximum efficiency.
Digesters are most often used in agricultural settings. Large-scale dairy and swine farms produce large quantities of manure; traditionally this manure is collected and kept in a large open-air lagoon, where it remains for some period of time before being spread on the fields as a nutrient aid. Throughout this process bad things can happen: digestion naturally occurs in the lagoon releasing methane and other pollutants into the air, and when spread on the fields rainfall can cause the manure to run off into waterways causing water quality issues. (Oh, and on hot days it smells really, really bad).
Utilizing anaerobic digesters provides two principal benefits: production of energy (in the form of biogas) and environmental benefits. In this case the manure, rather than being left in a lagoon, is kept in a closed anaerobic digester where the biogas is captured. Biogas is composed of about 60% methane, with most of the rest being carbon dioxide. Biogas can be used as is or cleaned up to be used as a renewable substitute for natural gas, propane or other fossil fuels.
In addition to the clean energy produced, there are also numerous environmental benefits. The capture of greenhouse gas emissions that would otherwise have been emitted into the atmosphere can help combat global warming (methane is 18 times more powerful a greenhouse gas than CO2). Other air quality improvements include significant reduction in odor, ammonia and particulates, and water quality improvements can include reduction of nutrient run-off issues (phosphates, nitrates, BOD materials).
So if AD is so great how come they’re not all over the place, you ask?
Until recently, use of AD in the U.S. has been limited to old-generation, small-scale technology; however, recent advances in technology have made AD more cost competitive with other types of energy. While viability is well established at operating facilities, large-scale deployment requires support comparable to that provided to competing sources of energy in order to expand and develop the potential market into one that can truly impact the country’s needs for alternatives to fossil energies.
AD use is much more prevalent in Europe where higher energy prices and government incentives have spurred widespread adoption of this technology. In the U.S., by contrast, historically low energy prices have retarded development of alternatives. Unfortunately, current government policies don’t treat biogas as favorably as other renewable fuels. Biogas has never had the sort of political support or constituent base to mobilize action in Washington.
Let’s examine the playing field a little more closely. A variety of renewable (as well as non-renewable) fuels receive subsidies, which can be expressed in a variety of terms, making direct comparisons difficult (per gallon of fuel produced, per kwh of electricity generated, etc.). In order to compare these subsidies it is useful to express them all in terms of dollars of subsidy per units of energy (mmbtu) produced. When expressed this way, it is apparent that some renewables receive preferential treatment. For example, biodiesel from agri-fuels receives $8.55 per mmbtu, ethanol receives $6.16 and biodiesel from waste oil receives $4.27. Renewable power generators get much less: solar and wind receive around $2.00 per mmbtu.
Currently, biogas only gets a credit if the energy is used to produce electricity, and then it’s only half of what wind and solar receive! Even in the narrow arena of animal waste to energy, gamesmanship is alive and well.
For example, it was a surprise to many when the Udall Renewable Energy Standard passed as an amendment to the House Energy Bill. It was an even bigger surprise to the biogas industry when it was stipulated that the only qualifying use of animal waste is if it is gasified (a different, and as yet economically unproven, technology). No one yet knows who is behind the narrow definition but it is inconsistent with every other existing state Renewable Portfolio Standard. One hopes that this will be corrected in the process of conferencing the bill between the House and Senate.
Until the playing field is smoothed out so that different technologies can compete in the market based on their own merits—rather than the strength of their political lobbies—we may not be able to develop a broad enough portfolio of renewable energy options to start tackling climate change and make our country more energy independent.
Al Morales has held a number of leadership positions in the renewable energy and finance fields, and most recently was COO and EVP of Environmental Power Corporation/Microgy, a publicly-traded developer of commercial-scale biogas facilities.