Findings from a Capital Expenditure Analysis Timing Evaluation Performed for the Fairfax County NCPCP

Fairfax County, VA commissioned Hazen and Sawyer to complete the Primary/Secondary Program, a comprehensive evaluation of infrastructure rehabilitation and reinvestment strategies for the primary and secondary processes at the Noman M. Cole Jr. Pollution Control Plant (NCPCP). Several factors drove the need for the Program, including the age of the existing infrastructure, the need to achieve future process treatment goals, and the anticipated growth of flows and/or loads over the 30-year planning horizon. These goals are defined by the numerical limits set by the Virginia Pollutant Discharge Elimination System (VPDES) permit. Table 1 presents the current VPDES discharge permit limits for the NCPCP, which is designed to treat a 67-mgd annual average daily flow. Based on the past and more recent regulatory climate, it is assumed that the existing mass limits for cBOD5, TSS, TN, TP and concentration limits for NH3-N will remain unchanged for the future 80-mgd annual average design flow. Because meeting these static mass limits becomes more challenging as influent flow or load increases, it is common for utilities to explore, and where advantageous, leverage other options for nutrient management. As such, the implementation of various treatment infrastructure components under the Primary/Secondary Program will be triggered by one or more of the following: flow, load, capacity, infrastructure condition or regulatory based factors. A capital investment financial analysis tool was developed to evaluate the impact of implementing different primary, secondary, and tertiary treatment capital investments and phasing under different future condition scenarios. This financial analysis tool takes a holistic, whole plant (liquids) approach to evaluate and quantify the anticipated costs of improvement scenarios based on anticipated increases in nutrient loading. This analysis was used to determine the optimal timing and type of infrastructure investments related to the NCPCP primary/secondary and related tertiary processes.