What we weighed or our partners weighed, with some estimates for the first two years of operation.
We use a rough blended estimate of 33%. Materials collected “shrink” in the composting process, but the process also includes the addition of additional carbon materials. Some materials like compostable service-ware produce little compost, others like food waste more (though even those vary greatly based on water content).
Formula: impact [lbs_compost_produced] = impact [lbs_diverted] * 0.33
According to the US Composting Council, “In a landfill, food can produce the equivalent of 12 metric tons (mT) of CO2 for every dry metric ton of waste (Eleazer et al, 1997). Since food scraps are roughly 80% moisture, that equals about 2.6 tons on a wet weight basis. In addition, not all the carbon gets converted to methane, since a significant portion stays buried and is counted as sequestration, reducing the net emissions.”
Formula: co2 = impact [lbs_diverted] * 2.6
According to the EPA, methane impact is 25 times greater then CO2, pound for pound.
Formula: impact [methane] = co2 / 25
Source 1: “Tomatoes have been cultivated using a wide range of compost application rates of 5 to 70 tons/acre. Lower rates of compost are typically being used as “maintenance applications.” Appropriate compost application rates will be influenced by existing soil conditions, compost characteristics and the nutrient requirements of the crop.”
Source 2: “On average, one acre of tomatoes will produce slightly more than 1,500 25-pound cartons, or 37,500 pounds of red, ripe fruit. About 5,000 tomato plants are required to meet this number.”
5 tons/acre = 5*2,000/acre = 10,000 lbs of compost/acre
5,000 tomato plants / 10,000 lbs = 0.5 plants per lb of compost
Source 3: “One pound of tomatoes equals about 2 large or 3 medium-sized tomatoes."
37,500 pounds of red, ripe fruit / 5,000 tomato plants = 7.5 lbs of tomatoes per plant
7.5 lbs per plant * 2 tomatoes per lb = 15 tomatoes per plant
Formula: impact [tomato_plants] = impact [lbs_compost_produced] * 0.5
Formula: impact [tomatoes] = impact [tomato_plants] * 15
Miles driven equivalent: "The average passenger vehicle emits about 404 grams of CO2 per mile."
Vehicles taken off the road: "The average gasoline vehicle on the road today...drives around 11,500 miles per year."
miles_driven = co2 * 453.59237 / 404 (1 lb = 453.59237 grams)
impact [cars_off_road_year] = miles_driven / 11500
We total up the amount of compost that has been shared with garden partners (though not necessarily delivered yet), then convert it at 1300 lbs per cu. yd., with a market rate of $25/cu. yd.
impact [shared_compost_value] = impact [lbs_shared] / 1300 * 25
Source 1: “[...] the average tree absorbs an average of 10 kilograms, or 22 pounds, of carbon dioxide per year for the first 20 years.”
Formula: impact [trees_planted] = impact [co2e] / (22 * 20)