Abstract

The Allegheny County Sanitary Authority (ALCOSAN) is in the process of developing and implementing a Regional Long Term Wet Weather Control Concept Plan (RLTWWCCP) to control combined sewer overflow (CSO) and separate sewer overflow (SSO) discharges into receiving water bodies. The total cost of implementing this plan is estimated to be between US$2 and US$3 billion spread over the next 50 years. ALCOSAN and the participating municipalities hope to reduce this cost by refining the design of conveyance and control facilities, which can be achieved by better characterizing the hydraulic and hydrologic responses of the service area during the development stages of the RLTWWCCP. In order to develop and evaluate alternative conveyance measures and control facilities to manage the amount and frequency of CSO and SSO discharges, ALCOSAN has developed detailed hydraulic and hydrologic computer simulation models. These models have been and will be used to quantify the amount of flow that needs to be conveyed and treated by ALCOSAN and supplement the decision making process by evaluating various alternative solutions and help choose the most viable and cost effective solution.

ALCOSAN is a large authority, and development of the models requires a large level of effort, a considerable cost investment, and a long period of time. ALCOSAN's strategy has been a long-term approach where the level of model complexity increases with the level of detail and refinement needed for planning and design. When model support for concept planning was needed to consider and evaluate numerous alternatives for control strategies, a simplified model was used which provided short run times and quick turn-around for the screening process. When model support for preliminary planning was needed to refine selected alternatives, additional model complexity was added. When detailed facility planning commences to optimize selected conveyance, storage and satellite treatment facilities, the required model complexity will be added. This tiered or incremental approach to model development satisfies the changing needs of large-scale planning projects during the entire project period, answering questions as they arise throughout the life of the project (Smullen et al., 2002).

In order to develop and implement a successful long term or short term CSO and SSO control plan, it is imperative to quantify and characterize the flow that needs to be conveyed and treated. As part of implementing the RLTWWCCP, ALCOSAN is evaluating the need to increase its current treatment capacity. To help quantify the volume and peak wet weather flow that can be conveyed to the ALCOSAN waste water treatment plant (WWTP) with the future RLTWWCCP conveyance improvements in place, a simplified planning level system-wide model was developed. The level of complexity and detail in the model was governed by the specific needs of the planning study.

While developing the simplified preliminary planning system-wide model a highly refined level of detail was used to quantify and characterize the amount of flow entering the conveyance system, during both dry weather and wet weather conditions. The simplification was in the reduction of the complexity of the hydraulic elements of the system which allowed increasing the simulation time step from 5 seconds to 60 seconds without computational instability. The simplified preliminary planning system-wide model was used to simulate long-term system response under various WWTP wet-well operational configurations. The simulation results provided ALCOSAN with information necessary to proceed with the refinement of the RLTWWCCP in terms of increasing the treatment capacity of the plant and mitigating the CSO and SSO discharges into the area water bodies.

This chapter describes the development of the simplified preliminary planning system-wide model, the validation effort and presentation of the model simulation results. The chapter also shows how the level of model complexity, and the corresponding level of effort and cost, were matched to the specific needs of the planning study.

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