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A Comparison of the Carbon Footprint of Aerobic and Anaerobic Digestion
Author: John Willis, Michael Radcliffe; Cliff Amett (Columbus Water Works, GA); Bill Toffey (Philadelphia Water Department PA)
Date: 4/108
Residuals 2008 Conference (March)

In the United States, anaerobic and aerobic digestion are two of the most commonly used practices for treatment of sludge to produce biosolids for land application. Both processes are specifically referenced in the 40CFR, Part 503 regulations as Processes to Significantly Reduce Pathogens (PSRPs or Class-B processes). This paper compares these technologies at two medium-sized POTWs in Georgia. In particular, the anaerobic digestion cases describe two versions of an operating mesophilic digestion plant as Plant 1A (as currently configured and operated with use of methane for vessel heating and the rest of the digester gas being flared); and Plant 1B (with planned enhancements that are currently under construction to convert to a Class-A, temperature-phased anaerobic digestion process with the addition of engines for co-generation). The Plant 2 (aerobic digestion) case study describes an operating plant that currently land applies a liquid, Class-B biosolids. The carbon footprint of the anaerobic digestion with cogeneration is approximately 200 metric tons per year (MTpy) of CO2 per mgd smaller than anaerobic digestion alone and 325 MTpy of CO2 per mgd less than the aerobic digestion.