An Approach to Forecasting Typical-Year Wastewater Flow Rates Under Future Conditions

Li Zhang, Fang Cheng, Robert Herr, Gregory Barden, Hunter Kelly and Edward Burgess


It is often necessary to forecast flow with model simulations of future conditions to meet regulatory requirements for system improvement plans. It is very challenging to estimate flow in future conditions, especially for extremely long-term planning horizons. Overestimating of future flow will result in unnecessary cost to the system owner, while underestimating of future flow will result in inadequate performance of the sewer collection system. This paper presents a systematic approach to estimate both dry-weather flow (DWF) and wet-weather flow (WWF) in future conditions to avoid over-estimating or under-estimating future flow rates. The performance of sewer systems is often assessed on the basis of a typical year approach to represent the long term average. The paper also developed a unique approach to estimate typical year groundwater infiltration (GWI) for characterizing system performance with typical year simulations of future conditions.

The City of Columbus 2005 Wet Weather Management Plan (WWMP) provided projections for the planning horizons through 2025 for the combined sewer system and through 2050 for the sanitary sewer system. These projections needed re-visiting with development of new and improved model and updated population data. As part of the City’s Sewer System Capacity Model Update (SSCM) 2006 project, DWF, composed of base wastewater flow (BWF) and GWI rates, and WWF (rainfall-derived inflow and infiltration, or RDII) were projected to year 2050 from 2010 rates. To re-evaluate the WWMP proposed system solutions, the SSCM 2006 model was modified to produce a Reduced Pipe Model (RPM) 2010, and further updated to represent both system and flow conditions for years 2012 and 2050.

This paper begins by documenting the approaches and results of the flow projections, including BWF, GWI and RDII, and follows with the associated analyses used in the flow projection calculations, including population projection, future flow characteristics and typical year GWI analysis. To develop a future condition model, the SSCM 2006 estimated future flow rates for the year 2050. Population projection to year 2050 was achieved by utilizing population growth trends, their geographical locations and population data for years 2010 and 2035 provided by Mid Ohio Regional Planning Commission (MORPC).To represent the DWF and WWF characteristics in future conditions, nine of the most recently developed flow monitor basin areas in the system were selected based on the analysis of pipe age, and flow characteristics for GWI, BWF and RDII were developed using these nine basins.

To characterize seasonally-variable GWI for typical year simulations, GWI rates were determined on a long-term monthly average basis by analyzing relationship of GWI and precipitation using long term wastewater treatment plant (WWTP) effluent data and long term precipitation data.

The RPM 2010 model network was modified to generate estimated flow rates in 2050. The end product of the RPM 2050 model will be applied in evaluation of sewer collection system performance with future improvements for the City’s system.

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