Environmental Engineering Reference
In-Depth Information
Calculation
Due to rounding, performing the calculations given in the equations below may not
return the exact results show.
(0.125 mmBtu/gal) × (71.35 kg CO
2
per mmBtu) × (1 metric ton/1000 kg) = 8.92 ×
10
-3
metric tons CO
2
per gallon.
(8.92 × 10
-3
metric tons CO
2
per gallon) × (8500 gal per tanker truck) = 75.82 metric
tons CO
2
per tanker truck.
Number of Tree Seedlings Grown for 10 Years
Forest application of biosolids is extensively practiced in the Pacific Northwest.
Thus, we need to be concerned about the carbon footprint involved. A medium-
growth coniferous tree, planted in an urban setting and allowed to grow for 10 years
sequesters 23.2 lb of carbon. This estimate is based on the following assumptions:
•
Medium-growth coniferous trees are raised in a nursery for one year until
they become 1 inch in diameter at 4.5 feet above the ground (the size of tree
purchased in a 15-gallon container).
•
The nursery-grown trees are then planted in a suburban/urban setting; the
trees are not densely planted.
•
The calculation takes into account “survival factors” developed by the U.S.
Department of Energy. For example, after 5 years (1 year in the nursery and
4 in the urban setting), the probability of survival is 68%; after 10 years,
the probability declines to 59%. For each year, the sequestration rate (in
pounds per tree) is multiplied by the survival factor to yield a probability-
weighted sequestration rate. These values are summed over the 10-year
period, beginning from the time of planting, to derive the estimate of 23.2
lb of carbon per tree.
Please note the following caveats to these assumptions:
•
Although most trees take 1 year in a nursery to reach the seedling stage,
trees grown under different conditions and trees of certain species may take
longer—up to 6 years.
•
Average survival rates in urban areas are based on broad assumptions, and
the rates will vary significantly depending upon site conditions.
•
Carbon sequestration depends on growth rate, which varies by location and
other conditions.
•
This method estimates only direct sequestration of carbon and does not
include the energy savings that result from buildings being shaded by urban
tree cover.
To convert to units of metric tons CO
2
per tree, multiply by the ratio of the molecular
weight of carbon dioxide to that of carbon (44/12) and the ratio of metric tons per
pound (1/2204.6).
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