FLOW BALANCE FOR A LARGE COMBINED COLLECTION SYSTEM

The Detroit Water and Sewerage Department required that the source and magnitude of all dry weather flow generated be quantified so that suburban customer billing and overall cost allocation of combined sewer overflow treatment facilities could be performed more accurately. An approach to balancing flows within the collection system was developed. This approach integrated data collection with meter maintenance through detailed review of data and continual communication between reviewers and maintenance personnel. Independent review of the flow meters using dye dilution test certification was employed to correct for bias error. Rigorous data selection criteria were used to ensure that flow balance assumptions were valid. In cases where upstream and downstream meters were needed to quantify incremental area dry weather flows, differential metering was used. Within each incremental area, sources of flow were characterized using a geographic information system based on population, employment, significant industrial use, water treatment plant discharges, and dry weather inflow and infiltration. Error associated with differential metering was clarified. Results from these efforts were used to support billing discussions. Furthermore, results were used to identify and resolve flow routing questions and to assess areas of high dry weather inflow and infiltration for water distribution and collection system maintenance personnel. Limitations and recommendations are presented.