Carbon-Footprint Effects of Biological Process Layout and Landfill Distance

Biological processes partially convert oxygen demand to biomass, which can be digested, thus producing biogas and biosolids, both associated with carbon-offset and carbon-sequestration, respectively. Biogas energy-recovery in co-generation plants offsets carbon emissions otherwise produced when using fossil fuels. Biosolids may be incinerated (i.e., immediate return of the carbon to the atmosphere), applied to land as soil amendment (i.e., retarded carbon return to the atmosphere), or buried in properly managed landfills (i.e., long-term carbon sequestration). Activated sludge layout affects process energy demand and the quantity of biomass produced, therefore the amount of biogas produced and biosolids to be disposed of. The most common forms of municipal wastewater biosolids disposal for many major U.S. cities involve landfill burial and land application. Using energy parameters and carbon equivalent conversion factors provided by the U.S. DOE and EPA, the carbon-footprint of shipment to landfill or land farms was calculated. In this paper we present the effects of biological process layout and of landfill distance on the treatment's carbon footprint. Warm climates show a positive feedback mechanism on biological treatment footprint, but the increased footprint is mitigated when considering the adjusted MCRT for comparable BOD and nutrient removal, and the benefits of increased BOD and nutrient removal at increased wastewater temperatures. Biosolids disposal site distance affects overall carbon-footprint, yet with lower weight than process MCRT.