Improving sewer operational management

A dynamic hydraulic model of the San Diego Metropolitan Sewer System has been developed to predict flow and depths in major trunk sewers and interceptors. In addition to the more traditional sewer capacity planning, the model has been used to analyse and develop strategies for scenarios related to operation, maintenance and rehabilitation.

The Metropolitan Sewer System (Metro System) serves a population of 1.8 million. It conveys wastewater via a network of trunk sewers and two major interceptors. These terminate at a pump station (PS2), discharging the effluent to the Point Loma Treatment Plant (PLWTP).

The city of San Diego's Metropolitan Wastewater Department (MWwD) is responsible for the planning, design, construction, operation and maintenance of the Metro System. MWwD has been involved in applying dynamic hydraulic models to the analysis of system-wide planning applications.

HydroWorks was used for the hydraulic analysis of the Metro System. The selection of the software was based on the need for an accurate and robust simulation engine, data viewing facilities, and real time control (RTC) modelling capabilities. Three case studies were examined, focussing on the benefits of dynamic hydraulic modelling:

1. Force main shut-down analysis

PS2 discharges flow through two parallel force mains that need inspecting and maintaining. The task requires operating the system using only one force main during the inspection of the other main. The hydraulic model was used to: l establish existing and forecasted peak flows entering PS2.

- determine maximum capacities of each force main
- estimate the time when future flows would exceed these capacities
- investigate alternative operating procedures

2. Rehabilitation of interceptors

The project involves replacing the existing PVC lining which covers the crown and the top of the pipe. Work can only take place when the depth of flow is under 50.8 cm. The hydraulic model was used to:

- determine for how long the desired depth of flow can be maintained
- estimate the number of days during which no work will be possible to develop a realistic project budget and schedule.

3. Wet weather flows diversion study

Pending completion of a new ocean outfall in 1998, the International Boundary and Water Commission requested that MWwD temporarily accept some effluent. The effluent would be discharged to the San Ysidro Trunk Sewer (SYTS), tributary to the city's South Metro Interceptor (SMI). The model was used to:

- determine if capacity is available in the SYTS and SMI
- develop an operating plan to control the temporary discharge and minimise environmental impacts.

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