North Lee County Water Treatment Plant Rehabilitation and Expansion

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Challenge

The North Lee County RO WTP (NLC WTP) was constructed in response to the need for additional capacity in the Lee County Utilities’ (LCU) northern service area. LCU began operating the NLC WTP in October 2006 and since that time has faced several challenges that have resulted in the inability to meet nameplate capacity. The causes for this include:
• Changes in feed water quality
• Irreversibly fouled RO membranes
• Process control deficiencies
• Undersized RO feed pumps

Additionally, several other deficiencies were identified by the LCU staff including:
• Undersized or inadequately controlled infrastructure to support expanded plant flows
• Reliability concerns pertaining to:
o Corroded equipment
o Maintainability of chemical injectors
o The adequacy of sulfide removal

Approach

Using a progressive design-build procurement, Lee County Utilities selected the Carollo Engineers design-build team to complete the required rehabilitation and expansion of the $19 million project within an 18-month project schedule. The project included an RO pilot study led by Carollo to test acid elimination from the RO feed water and the effectiveness of three antiscalants. Carollo addressed the hydraulic issues by increasing RO feed pump sizes from 75 to 250 HP and redesigning the existing and new RO trains to reduce feed pressure. An innovative turbine-assisted, motorized interstage booster pump was included in the new and rehabilitated RO train designs to recover wasted energy from the concentrate stream. Reliability concerns were addressed by improving the control and chemical feed systems and eliminating sulfuric acid. Following completion of construction and successful startup, the City of Decatur began operating and maintaining their facilities while producing effluent quality that was well within the limits set by their governing agencies.

Results

Using the progressive design-build delivery method provided $1.6 million in value engineering savings. Carollo’s design for the project reduced operating costs for LCU by as much as $784,000 per year. This project was substantially completed on schedule and was awarded the 2012 Florida Section Design- Build Institute of America (DBIA) Award for water/wastewater projects.