By William Beddow, P.E., Jim Bradbury, P.E., M.ASCE, and Mikes Maillakakis, P.E.
To keep pace with increasingly stringent regulations and growing demand, Lee County Utilities in southwest Florida recently constructed a new, larger drinking water facility that treats three separate, and very different, groundwater sources. The 14 mgd Green Meadows Water Treatment Plant near Ft. Myers includes a unique combination of reverse-osmosis, ion exchange, and degasification systems to treat the highly variable water supplies as efficiently and cost effectively as possible.
Modern water utilities face various challenges, including an ever-aging infrastructure, shifting workforce demographics, unreliable or changing water supply sources, evolving regulations, and rising customer expectations. However, for Lee County Utilities (LCU), based in Ft. Myers, Florida, a customized water supply solution offered the only sustainable option to deal with three separate sources of groundwater. By contrast, most drinking water providers typically rely on one water source and one treatment system. At the same time, the LCU, which currently serves more than 254,000 customers in southwest Florida, needed to expand its capacity to accommodate growth demands.
To provide the high-quality, potable drinking water that its customers expect, the LCU recently opened the new 14 mgd Green Meadows Water Treatment Plant (WTP) near Ft. Myers. The first-of-its-kind facility in southwest Florida, the Green Meadows WTP combines new and existing technology to treat water from three aquifers using the latest large-scale treatment technologies. The new facility replaces an aging lime softening plant that had operated for more than 35 years and reached the end of its useful life.
The Green Meadows WTP includes a reverse-osmosis (RO) system to desalinate brackish well water from the Upper Floridan aquifer (UFA), as well as an innovative ion exchange system that uses cation and anion exchange to remove iron, hardness, and organics from freshwater pumped from the local surficial aquifer. In addition, the WTP blends the RO permeate with freshwater from a third source, the Sandstone aquifer. This blend then undergoes degasification, disinfection, and chemical addition. To facilitate additional operational flexibility, the WTP can direct Sandstone aquifer water back to the RO treatment system, if needed.
This blend of traditional and proven, innovative technologies, combined within a single system, provides a dynamic, cost-effective treatment facility that increases the LCU's water supply treatment flexibility and reliability while reducing operational and treatment costs by as much as 60 percent. In addition to the primary treatment components, the project includes a 15,000 sq ft process building, a 5,000 sq ft office and operations building, a standby generator, shared chemical systems, 7 raw-water sand strainers, a backwash waste pumping station, 8 UFA wells, modifications to 27 existing wells, 7 electrical buildings, a 2,873 ft deep injection well to dispose of treatment concentrate, 8 mi of production well piping, and 5 mi of service road leading to the production wells.
In service since August 2018, the Green Meadows WTP serves an estimated 30,000 homes and businesses in south Lee County neighborhoods, as well as the Southwest Florida International Airport and Florida Gulf Coast University.
Treating Florida source water is complicated. High water demand, a sensitive environment, brackish water sources, and complex aquifer water chemistry, including high levels of naturally occurring dissolved organic material, all contribute to a complex water treatment environment. As drinking water demands increase, utilities must seek alternative water sources and more advanced treatment techniques, and explore aquifer storage and recovery as well as other storage options. At the same time, most find it useful to promote water conservation.
The story of the LCU's new greenfield plant began more than a decade ago, in 2007, when the Jacobs Engineering Group, of Dallas—then CH2M HILL—was retained to design an expansion to the existing lime softening plant to meet increasing water needs driven by a regional building boom. The LCU's goal was to find a cost-effective solution to providing long-term sustainability while increasing capacity. Achieving this goal would require meeting the following key objectives:
- compliance with evolving regulatory and treatment requirements
- creation of a facility that could be easily maintained and operated and that would have a long lifetime
- inclusion of an electrical system with Class 1 reliability-that is, two separate electrical systems that could power the facility independently in the event that one of the systems experienced a failure
- creation of a sustainable, low-cost operating facility that met the LCU's finished water goals
- inclusion of equipment that would be standardized to reduce maintenance time and expenses
The LCU's plans to upgrade the existing facility changed abruptly in 2009, amid the burst of the housing bubble. The building boom that had driven up water demand collapsed overnight, hitting southwest Florida and the Ft. Myers area particularly hard, and water demand slumped immediately. So the LCU put the project, which was at the 60 percent design phase, on the back burner until 2011, when the housing market began to improve.
Early in the process, Jacobs had evaluated the condition of the existing facility, worked with the LCU to identify updated treatment goals, examined the renewal and allocation options of the LCU's water-use permit, assessed the availability of source waters, and identified and bench-tested the potential treatment processes. Ultimately, this evaluation confirmed the need to expand the existing lime softening plant. To increase as much as possible the production capabilities of an expanded plant while minimizing operation and maintenance costs, the project team decided to maximize the existing freshwater allocations from the surficial aquifer and Sandstone aquifer at the plant.
However, the project team encountered regulatory resistance. The entity responsible for regional water supply planning, flood control, water quality improvement, and ecosystem restoration—the South Florida Water Management District—did not want an increase in freshwater withdrawals, preferring that the LCU examine such alternative sources as brackish water or recycled water.
Upon further investigation, the project team determined that the expanded facility could augment its existing fresh water sources with the brackish UFA well water, which would best be treated by means of an RO system. The permeate of this RO system then could be blended with the Sandstone aquifer water. This blended stream could then undergo degasification and be combined with the treated surficial aquifer water. Because of the high potential for the surficial aquifer water to foul the nanofiltration and RO membranes, the project team opted to incorporate cation and anion exchange to treat this water source.
Although project participants initially planned to expand the treatment facility and wellfield, they soon realized that the best option when it came to long-term sustainability and lower operational costs entailed design and construction of a new treatment plant. The project team had various reasons for considering a new plant. Ultimately, the primary factors underlying the decision were the need for a more resilient water supply and the difficulty that the lime softening plant would likely face in meeting future regulatory requirements.
The project team also determined that RO concentrate, highly saline water pumped from a planned future well, or a combination of both sources could effectively augment the cation regeneration exchange system to reduce operating costs significantly. The highly saline water would be withdrawn from the Lower Floridan aquifer (LFA) by means of the planned backup injection well, the primary purpose of which would be to retrieve this water to augment the process of regenerating the cation resin.
Jacobs used an in-house benefit-cost tool to define the treatment system. Using the company's proprietary water-quality and treatment-modeling tool, called Source, staff conducted a desktop analysis examining the different approaches for blending three different groundwater supplies, identifying quickly how the blending approaches would affect treatment, and optimizing the options for conducting further pilot testing. Jacobs's proprietary Conceptual and Parametric Engineering System tool was used to generate detailed conceptual designs and quantify the necessary materials to provide the team with cost estimates for several treatment variations and capacities, with the ultimate goal of helping the LCU define the best-value option. Another proprietary tool helped provide 3-D visualization of the conceptual, parametric design early in the design process, which facilitated faster design decisions. Dynamic simulation of the hydraulics, controls, and processes was also conducted to optimize energy efficiency and confirm the hydraulic characteristics of the wellfields.
A combination of existing and new wells and their associated piping withdraw and convey raw water from the three separate sources to the new Green Meadows WTP. They are:
- fourteen existing surficial aquifer wells with approximate depths of 40 ft below land surface (BLS) and a combined design production capacity of 4.2 mgd, along with 5 mi of pipeline to convey the raw water from the wellfield to the WTP
- twelve existing Sandstone aquifer wells with approximate depths ranging from 160 to 235 ft BLS and a combined design production capacity of 4.8 mgd, along with 5 mi of pipeline to bring the raw water from the wellfield to the WTP
- eight new UFA wells with approximate depths ranging from 720 to 870 ft BLS and a combined design production capacity of 7.0 mgd, along with 2.8 mi of new high-density polyethylene pipeline to bring the raw water from the wellfield to the WTP
Some existing wells were retrofitted with new water-level transducers. The water-level signals, as well as existing well instrumentation, programmable logic controllers, and variable frequency drives, were connected to the new control system by means of a fiber-optic cable. Two existing UFA test wells, which were converted to production wells, and six new UFA production wells, which were located within the existing wellfield access right-of-way, were equipped with pumps, variable-frequency drives, flow meters, generators, and instrumentation and controls systems. Electrical gear was housed in individual prefabricated buildings at each well site.
Two new raw-water transmission pipelines were included for two existing UFA wells, and an existing transmission main—which was used to convey groundwater from the Sandstone aquifer and the UFA via new and existing wells—was extended. The design also called for a new raw-water transmission pipeline dedicated to the Sandstone aquifer and a new building to house membrane treatment processes. Other spaces comprised a control room, a laboratory, and storage and office areas.
Pretreatment for the groundwater from the Sandstone aquifer and UFA entails sand straining, pH adjustment, and scale inhibition. As an additional pretreatment step, the membrane feed water from the Sandstone aquifer and UFA wells flows through a 5 μm cartridge filter system before being pumped into the membranes. Having a combined 9 mgd design capacity, the three cartridge filters each hold 224 individual spiral-wound cartridge filters, for a total of 672 cartridge filters.
Three customized RO trains reduce total dissolved solids and hardness concentrations in the brackish water from the UFA to produce up to 7.5 mgd of permeate. Each RO train contains 60 pressure vessels, each of which has seven RO membrane elements. Combined, the 1,260 membrane elements have a total filter area of 504,000 sq ft, or 11.6 acres.
The RO system is designed to treat water from the new UFA wells or treat a blend of the UFA water and water from the Sandstone aquifer, the latter of which has a total dissolved solids concentration of approximately 2,000 mg/L and could potentially have a concentration of up to 6,000 mg/L in the future. To help reduce operating costs, each train includes an energy-recovery device as part of the inter-stage booster pump. As noted earlier, the RO train permeate is blended with water from the Sandstone aquifer ahead of degasification. The waste concentrate from the RO process is disposed of in a deep injection well at the site.
The ion-exchange system can treat up to 4.2 mgd of surficial aquifer water and consists of five cation exchangers to remove iron and hardness, five anion exchangers to reduce organic compound concentration and color, and a regeneration and clean-in-place facility. The cation exchangers contain a total of approximately 5,500 cu ft of resin, and the anion vessels are loaded with approximately 4,500 cu ft of resin. The system is designed to operate in series, with treated water from the cation vessels feeding the anion vessels.
The water treated by the ion exchange system is blended with degasified RO permeate and water from the Sandstone aquifer wells at the clear well. The ion exchange system is used to store salt, produce brine, and regenerate the resin and can recycle the anion regeneration. The ion exchange system was designed and constructed so that it also could use highly saline water from a future on-site LFA well to help regenerate cation resin and minimize the use of bulk salt. This will further reduce operational costs, should the LCU decide to pursue this option.
After undergoing filtration in the sand strainers, up to 4.8 mgd of Sandstone aquifer water is either treated with RO or blended with RO permeate and then degasified by means of two packed tower degasifiers that remove hydrogen sulfide and carbon dioxide. The 33 ft tall, 13 ft diameter degasifiers have a combined 16 mgd design capacity. The blended and degasified water then is mixed with water from the ion exchange system. This ultimate flow stream is chlorinated and disinfected in the clear well, which has a minimum storage volume of 55,500 gal. and a 16 mgd design treatment capacity.
Blending the Sandstone aquifer water with the treated flow streams helps reduce finished water chemical usage, decreasing the WTP's overall operational costs. The finished water is pumped approximately 1 mi to tanks at the Airport Haul Road storage facilities before distribution.
Construction on the project began in July 2015, after the LCU selected Garney Construction, which has its headquarters in Kansas City, Missouri, as its construction manager at-risk (CMAR). This type of delivery method commits the construction manager to deliver the project within a guaranteed maximum price (GMP) on the basis of the construction documents and specifications at the time of the bid, plus any reasonably inferred items or tasks. Garney held 13 subcontracts but did not perform any work itself. At peak, the construction workforce numbered more than 100 personnel. Fully 75 percent of the subcontractors were local, and 300 local residents are estimated to have worked at the site.
Usually a CMAR is engaged at the 30 percent design stage. In the case of the Green Meadows WTP, the LCU decided on the CMAR approach once design was complete. Value engineering was conducted, and a second, redundant injection well was removed from the contract, saving $6 million. Although not implemented as part of this project, the second injection well can be incorporated in a later phase. The old lime softening plant was closed once the new plant was complete, but demolition plans have not yet been announced.
Of the three GMP contracts, the first involved the construction of the UFA production wells and one deep injection well that would be used to dispose of the RO concentrate. Begun in December 2015, the second GMP contract covered most of the remaining construction operations. The third GMP contract comprised the electrical systems, instrumentation and controls, and miscellaneous civil work.
For example, the subcontractor had to provide the electrical supply for the wellfield and the new treatment components associated with the facility, including standby power generation facilities. Site work improvements included grading, parking, access roads, and stormwater facilities, as required by Lee County's building codes and regulations issued by the South Florida Water Management District and the Florida Department of Environmental Protection. Existing underground process piping and utilities had to be identified and relocated to accommodate the new plant within the existing site. A new septic tank system also had to be located on-site.
Although Jacobs served as the prime engineer, the company, under its contract, teamed with various local consultants for some specialized elements. The Jacobs team and Garney had a previous project collaborations and similar corporate cultures focused on relationships, safety, and client satisfaction.
Because the Green Meadows WTP is adjacent to environmentally sensitive areas, special consideration was given in the permitting, design, and construction phases to minimizing disturbances to sensitive species of the nearby Imperial Marsh.
Some checks were done to locate existing underground pipes, and some buried asbestos cement piping required removal. The site, surrounded by wetlands, was built up by 2 ft so that it would be above the local floodwater level. Otherwise, construction involved conventional footings and slab on grade.
The two main buildings are the process area building, which is a prefabricated steel structure, and an attached operations center, which was built using load-bearing masonry construction. Both are constructed to withstand the 170 mph winds that can occur in Florida during hurricanes. Two small fiberglass buildings were constructed to house chemical systems.
The top concrete slab of the chlorine contact basin required special reinforcement to prevent cracking and to support the degasifier towers and their blowers, as well as five high-service 100 hp variable-frequency drive pumps. During design, engineers used MicroStation software, from Bentley Systems Inc., of Exton, Pennsylvania, to avoid conflicts among pressure piping, conduits, and stormwater pipes.
As part of the development of the Green Meadows WTP, the project team sought to address the challenges faced by operators and maintenance staff. For example, a new supervisory control and data-acquisition system was added to enable operators to monitor and control the new wells and the WTP.
Additionally, accessing the remote wells during the wet season would prove challenging because portions of the wellfield road would be submerged during sheet-flow rainfall events. At these times, access would require a special wetland vehicle. At times waist-deep in water, field-survey crews were on-site early in the project, collecting more than 10,000 survey points. The initial road, comprising natural sand and crushed limestone, was constructed at grade so as not to interfere with the natural sheet-flow pattern that occurs during the wet season. At such times, the wetland water levels increase, submerging flow channels and flowing in a generally southwest direction.
So a 5 mi long, paved, single-lane road was constructed. This required installing 67 culverts with diameters of 36 in. Of these, 24 were necessary between wells 4 and 5 alone. Another 12 culverts were required between well sites 11 and 12, at Imperial Marsh, to help maintain sheet flow and enable the LCU staff to inspect and maintain distant production wells. The many culverts enabled the road to be raised safely above the high-water level.
Despite numerous obstacles, including multiple wildfires, Category 3 Hurricane Irma, and the wettest January on record, the project team persevered to deliver this state-of-the-art WTP under budget and on schedule, returning approximately $1.5 million to Lee County. The overall construction contract cost totalled $75.4 million. Other than the nearby $78-million JetBlue Park, the spring training home of the Boston Red Sox, the new Green Meadows WTP marks the most expensive project undertaken in Lee County during the past 10 years and the third largest of all time in the county's history.
William Beddow, P.E., is a senior project and client account manager and Jim Bradbury, P.E., M.ASCE, is a senior resident engineer for Jacobs Engineering Group, of Dallas. Mikes Maillakakis, P.E., is a senior project manager for Lee County Utilities.
Owner Lee County Utilities , Ft. Myers, Florida
Designer Jacobs Engineering Group, Dallas
Water resources engineering Johnson Engineering Inc., Ft. Myers
Electrical engineering RKS Consulting Engineers, Ft. Myers
Construction manager at-risk Garney Construction, Kansas City, Missouri
© ASCE, Civil Engineering, May 2019