The primary measure of landfill performance is the efficient use of landfill airspace. Landfill airspace can be defined as the volume of space on a landfill site, which is permitted for the disposal of MSW. Landfill airspace utilization is typically reported as the “Airspace Utilization Density,” or AUD, which is calculated by dividing the weight of the waste landfilled by the volume of the total airspace utilized.
Food waste has a much higher moisture content than mixed waste. In addition, a significantly higher fraction of its biogenic carbon is converted to landfill gas (LFG) when it is landfilled. For these reasons, it is likely to occupy less airspace than mixed waste after it has been biodegraded in a landfill, and, as a result, it is likely to save only a fraction of the amount of landfill airspace that would be saved—for example—if a ton of mixed recyclables is diverted from landfilling.
To evaluate the potential landfill airspace savings associated with food waste diversion, mass balance analyses were conducted for landfilled food waste and mixed waste, with the results presented respectively in Tables 3 and 4. It should be noted that the assumptions used in these analysis are identical to those used by EPA in its “Waste Reduction Model” (USEPA 2015).
Noteworthy differences between food waste and mixed waste in this analysis include the following:
- Moisture Content: Food waste has a moisture content of 73%, as compared to 20% for mixed waste.
- Dry Mass: Alternatively, the weight of the dry mass of food waste is 27%, while it represents 80% of mixed waste.
- Biogenic Carbon: Biogenic carbon constitutes 51% of the carbon in food waste, while only 42% of the carbon in mixed waste.
- Methane Generation: Only 16% of the biogenic carbon in mixed waste is converted to methane, while 42% of the biogenic carbon in food waste is converted to methane.
- Water Content of Remaining Landfilled Waste: Almost 80% of the mass remaining in the landfill following food waste biodegradation is water. In contrast, water represents only 5% of the mass remaining, following mixed waste degradation.
Based on the data presented in these tables, the dry mass remaining after food waste decomposition in a landfill represents 21% (15% ÷ 71%) of the weight of the mass remaining in the landfill, with the rest of the weight due to moisture. Alternatively, the dry mass remaining after mixed-waste decomposition in a landfill represents 95% (69% ÷ 73%) of the weight of the mass remaining in the landfill, with the remaining 5% of the weight due to moisture. In summary, most of the weight of biodegraded food waste in landfills is due to moisture, while most of the weight of biodegraded mixed waste is undecomposed waste.
The large differences in the amounts of undecomposed waste between food waste and mixed waste do not necessarily translate to differences in the volumes occupied by these wastes. To estimate the impact on landfilled waste volumes, the bulk density of the undecomposed waste—on a dry-weight basis—was assumed to be 1,000 pounds per cubic yard. (As a point of reference, the compacted bulk density of mixed paper is 755 pounds per cubic yard.)
Based on this assumed bulk density as well as the density of water and the different percentages of water in each waste, it is estimated that biodegraded mixed waste would occupy 48% more landfill airspace than that occupied by biodegraded food waste. (See Table 5.)
Also, based on this analysis, it can be concluded that biodegraded food waste occupies significantly less airspace in landfills than biodegraded mixed waste. Put another way, the AUD of food waste is significantly higher than mixed waste. Since the effective airspace utilization density over the long term includes the impacts of waste decomposition, it can be concluded that the diversion of food waste from landfill disposal will likely result in a notable decrease in the AUD of the remaining landfilled waste, with the impact directly tied to the percentage of food waste diverted.
This conclusion has a number of important implications regarding the benefits of food waste diversion programs. One of these is that food waste diversion may not achieve as significant amount of savings of landfill airspace as originally thought.
Another important implication of the decrease in the AUD of a landfill due to food waste diversion is that less waste will ultimately be able to be disposed in the permitted landfill airspace.