Developing Reliable and Defensible Wastewater Load Projections for San Francisco’s Major Capital Program

San Francisco Public Utilities Commission (SFPUC) recently launched the Sewer System Improvement Program (SSIP) to initiate a phased program of large capital improvement projects (CIP) to update, replace and install new infrastructure to meet the City’s level of service goals for the planning horizon. To support this program, wastewater flow and loads projections were needed to provide key planning parameters impacting facility sizing and project cost. Conventional methods for projecting flows and pollutant loads using residential population projections, per-capita loading rates and/or assumed wastewater pollutant concentrations were identified as deficient due to dynamic changes in population/employment, water conservation efforts and varying wastewater characteristics. Therefore an original, defensible and reliable methodology was needed to provide projected flows and loads. The concept of “Equivalent Population” (EP) was developed and incorporated into a model to account for these changes. Using various sources of data and the EP concept, projected flows and loads were developed and then used in conjunction with validated plant operating data to develop sludge loadings rates used in the design of a new solids handling facility. The concept, data sources, approaches and comparisons with conventional methods are reviewed.To support the planning of a new SFPUC biosolids facility, sludge solids projections were also developed. Initial efforts resulted in very high projected peak loads, due to anticipated high growths in the planning horizon and high apparent peaking factors derived from the process dataset. To avoid unnecessarily conservative facility sizing, the process dataset was examined in detail, and robust screening criteria were developed for data validation. The resulting peak solids projections more realistically correlated with facility operation, ensuring that the subsequent facility design parameters are strongly defensible.