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REVIEWS AND ANALYSES

Greenhouse Gas Balance for Composting Operations

^a College of Forest Resources, Box 352100, Univ. of Washington, Seattle, WA 98195
^b Center for Sustaining Agriculture and Natural Resources, Washington State Univ., 1100 North Western Ave., Wenatchee, WA 98801
^c Environmental Credit Corporation, 101 S. Fraser St, Suite 201, State College, PA 16801

^* Corresponding author (slb{at}u.washington.edu).

Received for publication August 24, 2007. The greenhouse gas (GHG) impact of composting a range of potential feedstocks was evaluated through a review of the existing literature with a focus on methane (CH₄) avoidance by composting and GHG emissions during composting. The primary carbon credits associated with composting are through CH₄ avoidance when feedstocks are composted instead of landfilled (municipal solid waste and biosolids) or lagooned (animal manures). Methane generation potential is given based on total volatile solids, expected volatile solids destruction, and CH₄ generation from lab and field incubations. For example, a facility that composts an equal mixture of manure, newsprint, and food waste could conserve the equivalent of 3.1 Mg CO₂ per 1 dry Mg of feedstocks composted if feedstocks were diverted from anaerobic storage lagoons and landfills with no gas collection mechanisms. The composting process is a source of GHG emissions from the use of electricity and fossil fuels and through GHG emissions during composting. Greenhouse gas emissions during composting are highest for high-nitrogen materials with high moisture contents. These debits are minimal in comparison to avoidance credits and can be further minimized through the use of higher carbon:nitrogen feedstock mixtures and lower-moisture-content mixtures. Compost end use has the potential to generate carbon credits through avoidance and sequestration of carbon; however, these are highly project specific and need to be quantified on an individual project basis.

Abbreviations: COD, chemical oxygen demand • GHG, greenhouse gas • HRT, hydraulic retention time • MSW, municipal solid waste • TVS, total volatile solids