Operating Cost Modeling for the New York City Biosolids Management Plan

An operating cost model was developed as part of the New York City Department of Environmental Protection's (NYCDEP) new biosolids management plan governing future construction and rehabilitation related to the solids handling processes for the City's water pollution control plants (WPCPs), which currently operate under a common gravity thickening – anaerobic digestion – centrifuge dewatering scheme. The operating cost model developed performs cost-benefit analysis to form recommendations from among eight universal and two plant-specific thickening and digestion process improvements. The model is a self-contained collection of cross-linked spreadsheets that allow input and storage of plant process data gleaned from discharge monitoring reports, performs mass balance and operating cost calculations, and outputs results in graphical and tabular form.Model inputs were supplied from categories of data that are either specific to the plant analyzed (such as flow rates) or systematically common (such as intended operational parameters for a proposed improvement). Initial mass balance calculations were performed in order to determine flows and loads at each process stage and test the viability of each proposed improvement in meeting minimum process requirements. Subsequent mass balance and cost calculations projected the amounts of sludge generated with each improvement and the related costs of operation. Calculations incorporated flowcharts relating process viability to performance projections as well as convergence of a numerical model to quantify the impact of step loading on sludge mass flux. The model was also programmed with Visual Basic controls to automate the simulation of 104 combinations of plants and proposed improvements. In conjunction with the above, model inputs were structured to be conducive to sensitivity analyses and what-if scenarios by allowing overrides.Model results included prepared process flow diagrams for each proposed improvement and operating cost projection tables influenced by WERF Publication 00-PUM-7, “Full Cost Accounting Protocol for Biosolids Management,” with additional customization to emphasize detail relevant to the improvement options evaluated. Cost groupings included factors related to energy consumption and emissions offsets, intra-system transportation, chemicals, maintenance, and biosolids disposal.This paper will present the details of the NYCDEP biosolids management plan operating cost model, highlighting the overall framework, inputs, and test conditions, as well as describing theuser interface. The results will be presented as an example of the innovative use of cost modelingto manage the biosolids of a large system.