The City’s water treatment plant is a 30 mgd lime softening facility that treats water from the collector well adjacent to the Missouri River. The raw water is softened in the solids contact clarifiers and re-carbonated before filtering in the high rate filters (anthracite and sand). The high rate filters require periodic backwashing and this became a limiting factor for the water treatment plant during periods of high demand. Individual filters were surcharging during backwashing operations and desired settling times weren’t being achieved between successive filter backwash cycles.

The City of Bismarck hired Apex to evaluate the filter backwash cycle and investigate the hydraulic constraints. Apex developed a hydraulic model based on field measurements and record drawings. The hydraulic model was calibrated to the existing backwash conditions and used to identify potential hydraulic constraints in the existing system.

The modeling effort showed that the associated pipe sizes are adequate and that the hydraulic constraints affecting the filters are primarily caused by insufficient storage volumes in the backwash water reclaim basins. Apex identified several alternatives to correct this issue including reducing backwash flow rates and volumes with air scour, re-purposing other basins, and constructing a new reclaim basin.