Unique Study May Result in Largest UF Retrofit Project in North America

The first pilot study of its type ever conducted for full-scale implementation may result in the largest ultrafiltration retrofit yet executed in North America. More importantly, the study identified a technology to ensure that Manatee County, Florida, meets some important water quality goals for its customers.

Manatee County draws and treats raw water from Lake Manatee, which was created by an earthen dam in the 1960s. The county’s 54-mgd surface water treatment system uses conventional chemical coagulation-sedimentation-filtration treatment primarily designed to remove color from the raw surface water.

Lake Manatee is subject to seasonal water quality fluctuations which have prompted the county to focus on cyclical taste and odor issues, the increasing difficulty to comply with limits on finished water turbidity, and the elevated operation and maintenance costs required to address these issues. While the finished water currently complies with all water quality standards, the likelihood of more stringent regulatory requirements may require enhancement of the current treatment process.

Perhaps the most difficult challenge facing the county is its location in the Southern Water Use Caution Area. This is designated as an area in which water resources are or will become critical in the next 20 years, making it very difficult to obtain additional withdrawal permits. Therefore, the county set a water loss goal on any treatment upgrade to 3 percent or less of the total plant flow. Typically, reverse osmosis (RO) membrane systems lose 15-20 percent to concentrate. As such, various membrane configurations were explored, including an innovative approach to minimize treatment losses by treating the concentrate stream for blending with the finished water.

The needs of the county are challenging, with no clear-cut solution to meet water quality/regulatory requirements within the context of water loss minimization. Finding the best solution required original research.

Alternatives Investigated

The county had three primary treatment goals: reduce intermittent turbidity spikes, decrease taste and odor compounds, and curtail disinfection by-product formation, each of which must be achieved with <3 percent water loss.

As the engineer of record for the county, McKim & Creed conducted a pilot study at the Lake Manatee Water Treatment Plant to test a wide range of technologies and improvements to address the county’s needs and goals. These technologies could be used as stand-alone solutions or combined to create a hybrid fix. The concepts originally targeted for testing included:

• Filter Rehabilitation
• Increasing Filter Depth
• Biological Contactor
• Ultrafiltration (UF)
• Low Pressure Reverse Osmosis (LPRO)
• Ion Exchange
• Biological Filtration
• Ozonation

Reiss Engineering provided the pilot units. Operations and testing were performed by Reiss and Manatee County.

Shown here are (clockwise from left) ultrafiltration, strainer and UF cartridge.

Shown here are (clockwise from top left)
LPRO cartridge filter, LPRO and chemical storage.

After initial research, the team narrowed the options to seven technologies to be tested in the study:
– Ultrafiltration (UF)
– Low Pressure Reverse Osmosis (LPRO)
– LPRO concentrate treatment to minimize water loss via:

• Magnetic Ion Exchange (MIEX)
• MIEX followed by ozonation
• Coagulation-UF
• Coagulation-UF followed by ozonation
• Ozonation

These technologies were performance-tested relative to the treatment goals. In addition, these technologies were applied to the treatment of the RO concentrate stream to reduce treatment losses. The goal was to blend the treated RO concentrate back into the finished water stream while improving the overall finished water quality—a new approach to reducing water loss and making it the first study of its type conducted for full-scale implementation of a concentrate stream that would be treated and blended into the finished water.

For seven months, the team of McKim & Creed, Reiss Engineering, and Manatee County conducted the pilot study at the treatment plant. “We began the study during the dry season when the county typically experiences poorer water quality,” explains Phil Locke, PE, McKim & Creed senior project manager. “The pilot testing was then continued through the wet season. This allowed for several different technologies to be tested, with varying levels of treatment/chemical addition to see what impacts these would have with respect to treating the RO concentrate stream.” Adds Johna Jahn, EI, McKim & Creed engineer intern, “The study was designed to pilot advanced technologies in unconventional configurations.”

Results Studied

Data from the pilot study showed that while each piloted technology was able to meet at least one of the treatment goals, none of the options met all the goals. Ultrafiltration provided highly successful turbidity reduction, but none of the configurations reduced total organic carbon (TOC) while operating within the acceptable water loss conditions. LPRO resulted in successful constituent removal and operation but had unacceptable water loss. LPRO with concentrate treatment using ozone improved taste and reduced odor, but did not significantly reduce TOC. Ozonation with BAF was a viable solution for the taste and odor concerns, but did not significantly diminish turbidity or TOC.

Recommendation Made

The team then conducted a weighted analysis of the primary water quality goals with turbidity weighted heaviest at 60 percent (as it is a regulatory-related target), 20 percent for taste and odor, and 20 percent TOC. This analysis, combined with the results of the pilot study, resulted in the team’s recommendation to retrofit existing multi-media filters with ultrafiltation.

“We worked closely with Manatee County during the pilot study and also during subsequent evaluations of alternatives for implementation,” says Locke. “Due to the downturn in the economy that occurred during the course of this project, the significance of cost as a factor in the recommendation for full-scale implementation greatly increased.”

The UF retrofit offers many potential benefits:

• Reliable turbidity barrier under the most challenging conditions,
• Reduction of current chemical costs,
• Absolute protozoan barrier,
• Utilization of existing infrastructure,
• Improved water quality without additional water treatment losses,
• Lowest-cost alternative,
• UF manufacturer performance guarantee that ensures the processed water will have nephelometric turbidity units (NTU) of less than 0.10 in 95 percent of all samples.

Moving Forward

McKim & Creed is now conducting a Basis of Design Report that will include an equipment summary, process and instrumentation diagrams, constructability reviews, phasing considerations, permitting requirements, and an implementation schedule. This report will include a summary of new equipment required for the UF system, conceptual layouts of the new equipment, electrical single-line diagrams, process and instrumentation diagrams, recommendations for electrical system and back-up power supply improvements, project phasing considerations, permitting requirements, project implementation schedule, etc.

If Manatee County decides to proceed with the project, it will be the first UF system retrofit in Florida and the largest such project ever implemented in North America.