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

How low can you go?

We’re repeatedly advised to switch off electrical devices, like TVs and DVD players at the mains outlet rather than leaving them in standby mode, to turn to compact fluorescent bulbs and to turn them off when illumination is no longer necessary, to do our laundry at lower temperatures, to run the dishwasher only when it’s full, and to avoid using energy-hungry power showers. All those kilowatts add up to a lot of power wasted if we don’t.

According to a new study into energy use in the UK, by following this advice we might be reducing our carbon footprint a lot more than we thought. Conversely, those who don’t follow the advice might be wasting far more energy than the government thinks and so contributing more to carbon dioxide emissions and so anthropogenic global warming and climate change. Writing in the journal Energy Policy this month, Adam Hawkes, of the Grantham Institute for Climate Change at Imperial College London, has calculated that the figures used by government advisors to estimate the possible carbon dioxide reduction possible might be 60% too low.

Hawkes points out that power stations that supply electricity vary in their carbon dioxide emission rates, depending on the fuel they use: those that burn fossil fuels (coal, gas and oil) have higher emissions than those driven by nuclear power and wind. In general only the fossil fuel power stations are able to respond instantly to changes in electricity demand. He says that the government should keep track of changing carbon emission rates from power stations to ensure that policy decisions for reducing emissions are based on robust scientific evidence.

Hawkes used 60 million data points for electricity production each half-hour period by each power station in Great Britain from 2002 to 2009 and calculated the emissions for each different type of generator by examining government data showing their average annual fuel use. He then calculated emissions rates attributed to a small change in electricity demand from these two data sets.

SPT86-montalto-power-station (Credit: David Bradley)
Montalto power station (Credit: David Bradley)

His new study suggests that excluding power stations with low carbon emission rates, such as wind and nuclear power stations, and focusing on those that deal with fluctuating demand would give a more accurate emission figure. Hawkes’ calculations show that, 0.43 kilograms of carbon dioxide per kilowatt hour of electricity consumed is 60 percent lower than the actual rates observed between 2002 and 2009 (0.69 kilograms of carbon dioxide per kilowatt hour), meaning that policy studies are underestimating the impact of people reducing their electricity use.

“One way governments are trying to mitigate the effects of climate change is to encourage people to reduce their energy consumption and change the types of technologies they use in their homes,” Hawkes says. “However, the UK government currently informs its policy decisions based on an estimate that, according to my research, is lower than it should be.”


Energy Policy, 2010, online

Interplanetary storm

A meteoric storm raged over the Earth 13,000 years ago as thousands of pieces of rock each the size of the Tunguska comet rained down over the course of an hour. The end result was a dramatic cooling of the planet, according to astronomer Bill Napier of the Cardiff University Astrobiology Centre.

Writing in the journal Monthly Notices of the Royal Astronomical Society, Napier suggests that the temperature drop was as high as 8 Celsius and interrupted global warming at the end of the last ice age, causing glaciers to re-advance.

Scientists have puzzled over a boundary layer marked by the occurrence of a “black mat” tens of millimetres thick present at sites throughout the United States, which contains high levels of soot from continental-scale wildfires and nanoscopic hexagonal diamonds found only in meteorites or impact craters. The evidence hinted at a catastrophic change at that time caused by the impact of an asteroid or comet 4 km across on the Laurentide ice sheet, which at that time covered what would become Canada and the northern part of the United States.

Napier points out that the cooling lasted a more than a millennium and led to the rapid extinction of 35 genera of North American mammals, as well as the disruption of the Palaeoindian culture. However, the chances of an asteroid impacting the Earth during that period were extremely low. Moreover, the heat generated by the rising fireball would be limited by the curvature of the horizon and could not have led to the continent-wide occurrence of wildfires.

Napier has now devised a model that can account for the evidence.
According to Napier’s model, the Earth ran into a dense trail of material from a large disintegrating comet. He points out that there is compelling evidence that such a comet entered the inner planetary system between 20,000 and 30,000 years ago and has been fragmenting ever since, giving rise to a number of closely related meteor streams and asteroids known as the Taurid Complex.

As the comet disintegrated, the Earth would have ploughed through at least one dense swarm of cometary fragments over an hour-long period.
Thousands of individual impacts would have occurred across what is now continental America, each releasing the energy of a megaton atomic bomb and triggering extensive wildfires.

2005 Hubble Space Telescope image of the breakup of a comet (73/P Schwassmann-Wachmann 3).
2005 Hubble Space Telescope image of the breakup of a comet (73/P Schwassmann-Wachmann 3). Credit: NASA / ESA / H.Weaver (JHU/APL) / M. Mutchler / Z.Levay (STScI)

“A large comet has been disintegrating in the near-Earth environment for the past 20,000 to 30,000 years, and running into thousands of fragments from this comet is a much more likely event than a single large collision. It gives a convincing match to the major geophysical features at this boundary,” says Napier. Indeed, a recent meteorite which may have come from this giant comet progenitor fell on Yukon Territory in January 2000 and has the highest abundance of nanodiamonds of any meteorite so far analysed.

Monthly Notices Royal Astronom Soc, 2010, in press Preprint link
Cardiff staff