KCI conducted a study and provided design services for the clearwell at the Annapolis Water Treatment Plant. The initial study for this project reviewed options for the rehabilitation of the clearwell including replacement, covering the existing unit and the effects on final effluent pumps. Part of this study included the evaluation of an existing elevated water storage tank, which was no longer in use. The study ultimately determined whether this tank could be refurbished and used as a temporary clearwell or used to increase the storage capacity of the existing backwash storage, or both.
Important features included compliance with all regulations and improved compliance wherever possible; providing for a clearwell that is structurally sound, reliable, and protected from environmental hazards and terrorism; providing for additional backwash storage with adequate hydraulic characteristics; ensuring that hydraulic conditions in the plant (pump curves, capacities, suction head, etc.) were not negatively affected; and developing a cost-efficient, operations- and maintenance-friendly solution. Follow-on design improvements included the design of two 16-foot SWD (Side Water Depth) x 103-foot Diameter, 1.0 MG (million gallon) capacity, pre-stressed concrete tanks (PCT) for an overall capacity of 2.0 MG.
Our team of engineers determined that the existing clearwell could only be filled to one-half of its elevation by gravity. To use the full capacity of the clearwell, the flow from the filters had to be pumped. Through pump tests and analysis of the existing pumps, KCI determined that the clearwell level is critical to the operation of the distribution pumps. To provide greater capacity under these conditions, a clearwell with similar elevation and a larger footprint was required.
Following a detailed hydraulic analysis and topographic surveys, the KCI team proceeded with the selection and design of the PCTs and associated pipeline connections. Critical to the design was establishing the floor elevation of the tanks to achieve required hydraulic operations. The preliminary layout design identified all new pipeline and interconnections with the existing facility.
The elevated water tank was evaluated for possible reuse. One consideration was to relocate this tank to the treatment plant for use as temporary clearwell during construction of new facilities. Another use could be to replace the existing backwash tank at the plant. The tank was inspected for structural integrity and presence of lead paint, and was found to be structurally sound and free of lead paint and suitable for relocation. Research into the cost of relocating the tank found that due to prevailing labor costs and steel prices at the time it was not practical to relocate or have the tank demolished at that time.
Through detailed hydraulic water treatment plant analyses and assessment of normal operating procedures, our solution of two one-million-gallon finish water storage tanks was able to reduce overall operations and maintenance costs by $2 million.
Timothy W. Wolfe, PE, BCEEVice President, Regional Practice Leader
In addition to clearwell design, tasks also included assessing and preparing a hydrologic analysis for the project to satisfy local and state stormwater management regulations, erosion and sediment control permitting, and a comprehensive geotechnical investigation including soil borings, laboratory testing, details analysis and designing reinforced soil stabilization using geosynthetic material.