David Powelson

Methane oxidation in landfill cover soils, is a 10% default value reasonable?

We reviewed literature results from 42 determinations of the fraction of methane oxidized and 30 determinations of methane oxidation rate in a variety of soil types and landfill covers. Both column measurements and in situ field measurements were included. The means for the fraction of methane oxidized on transit across the soil covers ranged from 22 to 55% from clayey to sandy material. Mean values for oxidation rate ranged from 3.7 to 6.4 mol m(-2) d(-1) (52-102 g m(-2) d(-1)) for the different soil types. The overall mean fraction oxidized across all studies was 36% with a standard error of 6%. The overall mean oxidation rate across all studies was 4.5 mol m(-2) d(-1) +/- 1.0 (72 +/- 16 g m(-2)d(-1)). For the subset of 15 studies conducted over an annual cycle the fraction of methane oxidized ranged from 11 to 89% with a mean value of 35 +/- 6%, nearly identical to the overall mean. Nine of these studies were conducted in north Florida at 30 degrees N latitude and had a fraction oxidized of 27 +/- 4%. Five studies were conducted in northern Europe ( approximately 50-55 degrees N) and exhibited an average of 54 +/- 14%. One study, conducted in New Hampshire, had a value of 10%. The results indicate that the fraction of methane oxidized in landfill greater than the default value of 10%. Of the 42 determinations of methane oxidation reported, only four report values of 10% or less.

Methane oxidation in water-spreading and compost biofilters

This study evaluated two biofilter designs to mitigate methane emissions from landfill vents. Water-spreading biofilters were designed to use the capillarity of coarse sand overlain by a finer sand to increase the active depth for methane oxidation. Compost biofilters consisted of 238-L barrels containing a 1: 1 mixture (by volume) of compost to expanded polystyrene pellets. Two replicates of each type of biofilter were tested at an outdoor facility. Gas inflow consisted of an approximately 1: 1 mixture (by volume) of CH4 and CO2. Methane output rates (J out; g m-2 day-1) were measured using the static chamber technique and the Pedersen et al. (2001) diffusion model. Methane oxidation rate (J ox; g m-2 day-1) and fraction of methane oxidized (f ox) were determined by mass balance. For methane inflow rates (J in) between 250 and 500 g m-2 day-1, the compost biofilter J ox, 242 g m-2 day-1, was not significantly different (P = 0.0647) than the water-spreading biofilter J ox, 203 g m-2 day-1; and the compost f ox, 69%, was not significantly different (P = 0.7354) than water-spreading f ox, 63%. The water-spreading biofilter was shown to generally perform as well as the compost biofilter, and it may be easier to implement at a landfill and require less maintenance.