Modeling Greenhouse Gas Reduction as an Economic Factor in NYC Biosolids Management Planning

In considering sustainability impacts, such as greenhouse gas emissions, the typical optimization of cost efficiency is often expanded to account for the perceived value of mitigating adverse environmental effects. This both increases community and societal acceptance of engineering projects and lessens actual costs incurred through design, construction, and operation. Stakeholders are usually in favor of overall cost minimization, but elasticity exists in the potential willingness to absorb additional economic costs in exchange for incremental reductions of environmental impacts. For the New York City Biosolids Management Plan, greenhouse gas emissions at 13 water pollution control plants were valued relative to incurred operating costs from nine biosolids treatment process scenarios. These were evaluated using a developed model that reported emissions levels as carbon dioxide equivalents. The model also applied cost factors derived from carbon credit market performance and projected market pricing to assign a monetary value to the emissions. Model results were used in conjunction with separate traditional evaluations of capital and operating costs involved in implementing selected process scenarios at each plant to justify planning recommendations.Over the course of the biosolids management plan, recommendations were made separately for future sludge handling upgrades at the 13 plants by the examination of more than 500 process scenarios. The scenarios were defined by combinations of various sludge thickening and stabilization processes, which were screened for applicability and optimality at each of the plants. Quantifiable characteristics, such as flow, concentration, and nutrient density for each process scenario step were input into a version of the greenhouse gas model developed by Gould and Bandi (2008), customized specifically for the project. This model analyzes energy and chemical consumption, sidestream electrical treatment demands, and other factors in the production of emissions sources and sinks at each process stage to yield a net greenhouse gas equivalent value as a function of the mass of solids processed. For this project, the model was modified to allow rapid evaluation of all applicable process scenarios at each of the 13 plants simultaneously. Equivalents were assigned a current and future value comparable to operating cost by applying cost factors from domestic carbon credit markets and projections for market pricing, respectively.Modeled greenhouse gas emissions valuations were analyzed in conjunction with calculated capital and operating costs for implementation of each process scenario at each plant (assembled separately over the course of the biosolids management plan). The analyses supported recommendations for future process implementations at all of the plants. In using the model, it became possible to better align process selection with cost by accounting for the value of greenhouse gas reduction. By exposing the interactions of different process parameters as components of the net emissions levels, the overall methodology is hoped to inspire data collection practices amenable to continued performance tracking, a core component of sustainable design standards.