Biofilters cut old landfill carbon footprint

Researchers in the US are testing biofilter systems as a viable alternative to releasing methane from passive landfill vents into the atmosphere. The technology could reduce the overall impact of old landfills on global warming. Details are reported in the current issue of the International Journal of Environmental Engineering.

Organic matter rotting in smaller, old landfill sites generates a slow trickle of the potent greenhouse gas, methane, into the atmosphere, amounting to just 2 or 3 kilograms per day per vent. In contrast to controlled methane generate for biofuel from modern, managed landfills, tapping this slow stream of the gas is not viable technologically or economically. However, methane has an infrared activity 21 times greater than carbon dioxide and so represents an important anthropogenic source of this greenhouse gas when attempting to balance the climate change books. Indeed, landfills contribute 12% of worldwide anthropogenic methane emissions due to the decomposition of organic waste.

Old landfills typically have passive gas vents. Methane is simply released into the atmosphere from these vents, or if the rate of emission is high enough it can be burned, or flared. According to Tarek Abichou and Jeffery Chanton of the Florida State University, Jose Morales of Environmental and Geotechnical Specialists, Inc., Tallahassee, Florida and Lei Yuan of Geosyntec Consultants in Columbia, Maryland, methane oxidation has recently been viewed as a more benign alternative to venting or flaring of landfill methane.

The researchers tested two biofilter designs capable of oxidizing methane gas to carbon dioxide and water. Both are packed with so-called methanotrophic bacteria, microbes that digest methane. They found that the radial biofilter design gave a much higher methane oxidation rate than a vertical biofilter. The higher surface area exposed to methane flow led to greater oxygen penetration into the biofilters, essential for microbial digestion. The radial biofilter has a surface area of well over 1.2 square meters whereas the vertical biofilter amounts to just 0.3 square meters area.

The team also found that the average percent oxidation rate of 20% and higher for the radial biofilter was possible when the air temperature was 20 to 36 Celsius, indicating the optimal soil temperature for methanotrophic bacteria to oxidize methane. Vertical biofilters averaged a little over 12% oxidation.

Abichou, T., Yuan, L., Chanton, J., & Morales, J. (2011). Mitigating methane emissions from passive landfill vents: a viable option for older closed landfills International Journal of Environmental Engineering, 3 (3/4) DOI: 10.1504/IJEE.2011.041354

Yet another supernova

Just when you’d given up hope of another starburst, a third type comes along unannounced! This third class of previously unidentified supernova could help explain some anomalous observations in the night sky and even how our bodies come to contain so much calcium.

Until recently, astronomers had assumed there were just two types of supernovae. The first two types of supernova are either hot, young giants that explode on to the scene violently as they collapse under their own weight, or old, dense white dwarves (type a1) that undergo a thermonuclear explosion to briefly add their light to the night sky.

However, a third class appeared in telescope images in early January, 2005 and scientists, seeing that it had recently begun the process of exploding, started collecting and combining data from different telescope sites around the world, measuring both the amount of material thrown off in the explosion and its chemical composition.

Avishay Gal-Yam and colleagues at the Weizmann Institute in Israel and teams in Canada, Chile, Italy, UK, and USA, soon realised that the new supernova was neither old and dense nor young and hot.

There was too little material being ejected by the 2005 supernova for it to be an exploding giant, but its remote location from stellar nurseries suggested it was old. Moreover, its chemical makeup did not match the second type of supernova. The scientists turned to a computer simulation to see if they could figure out what kind of stellar processes could give rise to this anomalous kind of starburst.

Type Ia supernovae are primarily composed of carbon and oxygen as seen in their spectra, but the newly discovered supernova has unusually high levels of calcium and titanium which derive from nuclear reactions of helium not carbon and oxygen. However, the astronomers were initially at a loss to explain the source of the helium. Their simulations suggested that a pair of white dwarves might have been involved, with one assimilating helium from the other. When the thief star’s helium load rises past a certain point, the explosion occurs. “The donor star is probably completely destroyed in the process, but we’re not quite sure about the fate of the thief star,” says Gal-Yam.

Helium theft may have led to a third class of supernova that gives rise to the calcium in your bones and the titanium in a replacement hip! (Credit: Gal-Yam, Weizmann Institute of Science.

These new supernovae are relatively dim, so may not be as rare as they at first seem. This might explain why calcium is so prevalent in the universe and so in life on earth. The existence of radioactive titanium from these supernovae might also preclude the need for exotic explanations, such as invoking dark matter, of positrons at the heart of our galaxy. “Dark matter may or may not exist,” says Gal-Yam, “but these positrons are perhaps just as easily accounted for by the third type of supernova.”

Links

Avishay Gal-Yam homepage

The slow rise of The Andes

The Andes is the world’s longest continental mountain range and the highest outside Asia, with an average elevation of 4000 metres. The question of how quickly the mountains reached such heights has been a contentious one that University of Michigan paleoclimatologist Christopher Poulsen and graduate student Nadja Insel working with Todd Ehlers of the University of Tuebingen in Germany, believe they may now have settled with a new interpretation of isotopic data. Their work suggests that the rise of the Andes was a very gradual process.

Poulsen’s uplifting work suggests that previous interpretations of the evidence have misconstrued changes in oxygen isotope ratios as being due to a rapid rise of the mountain range whereas the more likely explanation is that the changes are due to shifts in ancient climate.

“In the modern climate, there is a well-known inverse relationship between oxygen isotopic values in rain and elevation,” Poulsen explains.
“As a rain cloud ascends a mountain range, it begins to precipitate.
Because atoms of oxygen-18 are more massive than those of oxygen-16, it is preferentially rained out. Thus, as you go up the mountain, the precipitation becomes more and more depleted in oxygen-18, and the ratio of oxygen-18 to oxygen-16 decreases.” Geologists use the ratio of these isotopes, preserved in rock, to infer past elevations and so the rate of rise of a mountain range.

“If the ratio decreases with time, as the samples get younger, the interpretation would typically be that there has been an increase in elevation at that location,” Poulsen adds. He points out that that is the precise conclusion drawn by a series of papers on the uplift history of The Andes published over the past four years. On the basis of oxygen isotope ratios determined by analysis of carbonate rocks, the authors of those papers suggested that the central Andes rose about 2500 to 3500 metres in a mere three million years, Other geologists had assumed that the rise to those heights took place over tens of millions of years.

Unfortunately, elevation is not the only thing to disturb oxygen isotope ratios in precipitation. “It can also be affected by where the vapour came from and how much it rained,” says Poulsen. “More intense rainfall also causes oxygen-18 to be preferentially precipitated.” He and his colleagues were skeptical of the rapid-rise scenario, and so performed climate modelling experiments to investigate whether something other than altitude might have given rise to the shift in ratio observed in carbonate deposits.

Andes: Credit to http://www.flickr.com/photos/atyt/

“The key result in our modelling study is that we identified an elevation threshold for rainfall,” Poulsen says. “Once The Andes reached an elevation greater than 70 percent of the current elevation, the precipitation rate abruptly increased. In our model, the increased precipitation also caused the ratio of oxygen-18 to oxygen-16 to significantly decrease. Our conclusion, then, is that geologists have misinterpreted the isotopic records in the central Andes. The decrease in the ratio is not recording an abrupt increase in elevation; it is recording an abrupt increase in rainfall.”

This conclusion is backed up by geochemical and sedimentological data, Poulsen said. “There is evidence that the central Andes became less arid at the same time that the isotope records show a decrease in the ratio of oxygen-18 to oxygen-16.”

Links

Christopher Poulsen

Science Express, 2010, online