Challenge

The Tri-Town Water District serves the towns of Addison, Bridport and Shoreham, Vermont.  The original water treatment plant was built in 1966 to treat surface water from Lake Champlain and produced an average of 765,000 gallons per day (2.8 MLD) with a peak summer demand of about 1,000,000 gallons per day (3.8 MLD). In 1999, the District hired Otter Creek Engineering Inc. to do an evaluation of the facility.  It was determined that the treatment system, particularly the clarification step, needed improvement; and the plant needed more capacity for future growth. The plant capacity was 1.1 MGD (4.2 MLD) and growth projections dictated they expand the plant capacity to 1.9 MGD (7.2 MLD). Accomplishing this task was more difficult since the site ruled out expansion to the existing plant footprint, and the owner needed to produce water during the construction phase of the project. 

The existing plant was equipped with older style flocculator/clarifiers with tube settlers. Poor clarifier performance and limited filtration capacity necessitated a new treatment approach rather than an expansion of the existing process. The project’s greatest design challenge was to integrate the new treatment process into the existing facility while maintaining its treatment capacity. Whereas limited land availability and cost considerations ruled out an addition to the existing building, proper layout of the system was crucial.

Solution

The Microfloc Trident® process was selected due to its operational flexibility and experience with Lake Champlain waters. The new plant capacity would require three clarifier/filter units.  The efficiency of the new upflow clarifier and filter design allowed for construction of two new clarifier/filter units in the cast-in cells occupied by one of the original flocculators/clarifier trains. This new design would double plant capacity while decreasing the footprint of its treatment train from 1680 sq. ft. to 600 sq.ft. with room to spare for a fourth unit. Otter Creek Engineering was impressed with the Trident technology and the small footprint required.  The adsorption clarifier operates at 10 gpm/sq.ft. (24.4 m/hr) followed by mixed media filter, operating at 5 gpm/sq.ft. (12.2 m/hr).

To facilitate coordination and provide single source responsibility, Siemens Control Systems group was enlisted to design, manufacture and start-up the plant controls and SCADA system for Tri-Town Water District. 

Results

The resulting design allowed for future expansion by providing space for a fourth clarifier/filter unit without further additions to the facility. Operational requirements of the facility made phasing of construction critical. By managing the system operation, the facility was able to meet demand with one of the old clarifiers while two of the new units were constructed. Upon completion of the first two units, the remaining clarifier was shutdown to allow construction of a third unit with space allocated for a fourth. Tri-Town Water District got a plant, which produces superior quality finished water, while saving their customers hundreds of thousands of dollars in construction costs.