A GREENHOUSE GAS EMISSIONS ACCOUNTING MODEL FOR BIOSOLIDS MANAGEMENT PLANNING

Biosolids management planning is subject to an increasing number of constraints and criteria driven by raised public awareness. Sustainability will be one of them and will be measured, to a large extent, in terms of ‘carbon footprint’ or greenhouse gas (GHG) emissions. The authors of this paper have developed a model to estimate equivalent CO2 emissions rate per dry ton that could be of potential use in evaluating the relative benefits of biosolids management practices. The GHG emissions equivalent is not a direct translation of energy consumption, as it also includes the embedded energy of chemical inputs, potential for carbon sequestration and offsetting of chemical fertilizers, and fugitive methane emissions. The model incorporates some of the calculations and default values that are being developed through the United Nations (U.N.) Clean Development Mechanism under the Kyoto Accord and other tools that evaluate GHG emissions reduction credits for environmental projects.The process is initiated by entries for the wastewater treatment facility: primary and secondary solids production, percent solids, and energy cost of sidestream treatment. It moves through a series of modules including alternatives for thickening, liquid stabilization, dewatering, cake stabilization, odor control, transport, and final use or containment. Each module generates ‘outputs’ which quantify energy and chemical consumption, sidestream treatment demands, and air volumes for odor control, as well as inputs to the next module in the treatment train. Some modules generate energy offsets, fertilizer offsets, and carbon sequestration. Finally, all inputs and outputs pass through the GHG equivalency module to generate a GHG equivalent ‘bottom line.’ A large number of default values are needed to complete the modules. These have been entered from literature searches and project experience. They are subject to change from plantspecific factors, local mix of energy sources on the grid, transport distances, and other factors. This version of the model includes parameters from biosolids management operations in Merrimack, NH and New York City.Ultimately, this will be an open-ended project designed to incorporate values from other contributors.The presentation will include the overall conceptual framework, user interface, default values, and examples of hypothetical projects illustrating sensitivities of GHG emissions to planning decisions.