BioCycle March 2010, Vol. 51, No. 3, p. 58
A paper came out a couple of years ago that compared the predicted versus actual rates of gas capture at 36 landfills in California (Themelis and Ulloa, 2007). One of the landfills had 100 percent gas collection efficiency, another had six percent. Average efficiency was about 33 percent across the landfills included in the survey.
There are two ways to interpret this study: 1) Landfills are spewing methane into the atmosphere; or 2) Landfills are piss-poor places to make methane. The more I learn about anaerobic digestion, the more I lean towards that second interpretation.
Methane is the result of anaerobic decomposition, which is likely to occur to a limited extent wherever there is food and no oxygen. Your home garbage can is one site for methanogenesis. Your intestines are another (try to convince me that you have never heard of anyone “lighting” farts). Landfills are another. Gas collection requirements were put in place in landfills not to harness green energy but to stop the fugitive migration of methane to surrounding properties. It used to be that you would periodically find a story in the newspaper about the house next to the landfill blowing up. That was fugitive methane at work.
But there is a big difference between having enough methane leaving a couple hundred acre landfill and blowing up a house and creating a system to maximize methane production. Methanogenesis – the three-stage process that creates methane – is carried out by a series of microorganisms. This is true whether the process is occurring in your garbage can, your gut, a landfill or a controlled digestion facility.
When couching landfills as major evil-doers, it is easy to view them as methane belching machines with fierce armies of microbes attacking your leftover mac and cheese and People magazines. When you actually start studying what goes on during methanogenesis, the microbes involved and the different feedback loops and sensitivities, you get a much different picture. This month’s column is an overview of how methanogenesis works and the different organisms involved, including what we know and what we don’t know about them.
Decomposing organic compounds in the absence of oxygen is a multistage process that involves a range of chemical transformations that can occur in sequence as well as simultaneously. In all cases, these reactions occur as a means to release energy stored by different compounds. A major factor in these reactions is that when energy is released, electrons are also released. The big deal is where to put these electrons. When making methane, you are talking about different kinds of bacteria. Although these bacteria or this process has been documented at widely ranging temperatures (4° to 110°C and pHs ranging from 5 to 9) they are most commonly found at 30° to 55°C and near neutral pH. The reactions can be broadly separated into three processes: Hydrolysis, fermentation and methanogenesis.
Hydrolysis, the first process, is a term used to describe a chemical transformation that involves water. Hydrolysis takes place using enzymes secreted by microbes; carbohydrates like cellulose and starches (People and spaghetti) are transformed into simple sugars. Breaking down the cellulose takes time whereas the next stage, transforming the sugars, goes really quickly. Proteins are also partially decomposed by hydrolysis into peptides and amino acids. This is a much slower process than decomposing cellulose and the bugs that do this get cranky if there are too many carbohydrates around. Fats also undergo hydrolysis. Lipids (a type of fat) consist of three long chain fatty acids held together by a glycerol molecule. Hydrolysis breaks these components apart; it’s those long chain fatty acids that take time for the next step of decomposition. Mixing helps the microbes hydrolize the lipids more quickly. There is conflicting information on how mixing helps or hurts the breakdown of the resulting fatty acids.
The next stage of the reaction is fermentation. I am grateful for this reaction on many evenings and weekends. There is a wide range of fermentation reactions, although some people divide these into two processes: Acidogenesis, where the products of hydrolysis are fermented to form volatile fatty acids, alcohols, carbon dioxide, hydrogen gas and ammonia; and Acetogenesis, where acetate is produced from the products of hydrolysis and acidogenesis.
Many compounds are quickly devoured by the fermenting bacteria. Sugars are an important example. Other compounds, including long chain fatty acids and amino acids, are much tougher to eat and only a few species are able to break them down.
The final stage of this process is methanogenesis. Unlike hydrolysis and fermentation, only one family of microorganisms, all from the Archaeal kingdom, is capable of producing methane. Three types of methanogenic metabolism have been identified: 1) Reductive methanogenesis, where hydrogen gas is oxidized and the electrons are put on carbon dioxide to make water and methane; 2) Acetoclasts, which rip apart acetate to make carbon dioxide and methane; and 3) Methyltrophic methanogens, which use compounds that contain methyl to reduce hydrogen gas. The methyl group is ripped of these compounds and released as methane.
WATCH YOUR SEATING CHART
The key message from that quick primer is there are many different ways to make methane. For example, at wastewater treatment plants, 60 to 80 percent of the methane collected is produced by the acetotrophic bacteria. However, you need those reductive methanogens around to eat up all of the hydrogen gas because the other bacteria get cranky when there is too much hydrogen around. But it is not always energetically favorable to get rid of the hydrogen. If you manage it so that the microbes that make the hydrogen are right up against the ones that consume it, the reaction works. Otherwise too much hydrogen builds up.
This is the part of the dinner where you have to worry about the seating chart. Although 60 to 80 percent of the methane is from the acetoclasts who eat up the acetate, that acetate would never be formed if it weren’t for the bacteria that eat the hydrogen. In other words, you need to make sure that the seating chart is done properly. Many of the fermentation products can only be broken down if the appropriate microbes are right next to each other.
But wait! It is a little more complicated. It seems that new research is showing that some of the electron transfer is not through hydrogen but through microbially-constructed nanowires that carry electrons and make anaerobic digestion energetically favorable. These are only stable when digesters are not mixed too fast (as Bond would say, “Shaken, not stirred”).
But if you don’t mix, you can accumulate the long chain fatty acids and you really don’t want to do that. You want those digested as they are some of the most energy-rich materials around, creating one liter of methane for each gram of substrate. But only a few types of microbes are around that can digest them because while the reaction releases a lot of methane, it is a very small net energy gain for the microbes that carry it out. No one is exactly sure of which bacteria those are although some have been identified.
Another issue is portion size. If there is too much food around, you can get a build up of volatile fatty acids. That sends the pH down and the guests get upset. I am sure that you have had some familiarity with acid reflux. There is also evidence that the guests get upset over more than just the acidity. But it isn’t really clear yet what is going on.
I hope that this is enough to convince you that anaerobic digestion has much more in common with a fancy dinner party than an all you can eat buffet. And like a fancy dinner party, the caterer gets something wrong, and the whole thing can go sour. Just ask a wastewater treatment plant operator about that. So if you really want energy from wet organic wastes, anaerobic digestion is the way to get it. And a highly managed and controlled process is the way to get the most out of your feedstocks in the least amount of time. You find a highly managed and controlled process in a dedicated digester, not in a landfill.
Sally Brown, Research Associate Professor at the University of Washington (email@example.com), authors this monthly column on the connections of composting, organics recycling and renewable energy to climate change. Diane Nelsen, a graduate student in Engineering at the University of Washington helped a lot on the basics of anaerobic digestion for this column. She is getting her MS on codigestion of fats in wastewater treatment plants.
March 23, 2010 | General
Climate Change Connections: Picky Eaters
BioCycle March 2010, Vol. 51, No. 3, p. 58