March 29, 2011

Beer Waste to Biofuels

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Beer-making generates more than just tasty brew – it makes a lot of wastewater. Using a little publicized renewable energy technology that has been around for over 20 years, breweries can coax methane from the wastewater, which then can be  burned to generate heat within the breweries.

Making methane from wastewater takes a microbial village. The water is pumped into giant, oxygen-free tanks called anaerobic digesters, where complex communities of bacteria feast on leftover carbon sources from the water. Eventually, single-celled organisms called archae turn broken down products into burnable methane.

Research associate Jeffrey Werner and Prof. Lars Angenent, biological and environmental engineering, studied microbial communities living in nine digesters in a major brewing company across the country for a year.

“We had some questions about how microbial communities and their structure can be related to useful function we can get out of them. Anaerobic digesters that produce methane from brewery waste water have very complex communities,” said Werner.

In addition to understanding how the members of the microbial communities related to the functioning of the digesters, the researchers were interested in seeing how their populations changed over time.

Their findings were published in the Proceedings of the National Academy of Sciences this month.

Werner and his colleagues sequenced DNA from brewery waste digester samples to identify the types of bacteria present at any one time. They obtained community-wide profiles of their samples with a new technology called deep-sequencing that captures the wide variety of bacterial species living in a sample. These “species snapshots” allow scientists to gauge the complexity of a community more broadly than possible using older, Petri-plate methods.

From their digester samples, Werner identified distinct types of bacteria that indicated nearly 5,000 species in all. He then compared his snapshots of the microbial communities over a year to monitor how the communities changed and how they related to the conditions of each of the nine digesters.

Werner found that the role of bacteria relates to their stability within the digester communities. “We found that the syntrophs, which are bacteria with very specialized function and very small genomes that can only do one or two metabolic processes, have very stable populations and are very resilient.”

Because of their special job in the communities, which is to make only a few small compounds, they are less susceptible to population fluxes that other generalist bacterial groups might be. The engineers used “machine learning” to turn their data set into useful predictions. Werner fed his community data into statistical models that were repeatedly refined by a computer.

From the 5,000 species present in the samples, the models pared down the data to relate data to species. One such way Werner used machine learning was to identify species that were unique to individual digesters.

“If you gave me a sample from one of these nine digesters, I could tell you with 97 percent accuracy which one it came from using our predictive model,” Werner said.

The communities found in brewery waste digesters develop naturally from bacteria already found in waste. Angenent’s research group is interested in these kinds of natural communities, without genetic engineered bacteria. “We’re working with undefined cultures, mixed cultures. These are open cultures,” Angenent said. “Even though a lot of bacteria come in, they shape themselves into this community, and they’re pretty resilient.”

Angenent, along with Werner and Matt Agler grad study mixed bacterial communities in order to find ways to produce fuels other than methane. In a lab a few miles off of campus, small digesters transform biomass waste, like corn fiber, into precursors of precious liquid fuels.

One of the most important elements of the group’s brewery waste work is honing the tools they used: deep-sequencing technology and complex computational analysis to make sense of a vast number of community data.

According to Werner, “These tools aren’t that useful for optimizing brewery waste digesters because they’re all ready really efficient, but the fact that we can come up with these quantitative relationships helps us to ask new questions about ‘engineering’ communities with new functions.”

Angenent hopes to answer questions about other fuel-making systems with the approach used for the brewery waste digesters: ones that rely on diversity and complexity. “Can we use these undefined mixed cultures, with thousands of species, to shape them in a way so that they will do what we want them to do?

Original Author: Daina Ringus