PLANT NAME

City of Dunedin Reverse Osmosis Water Treatment Facility

Address

Box 1401 Country Road 1

Dunedin, Florida 34698

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Water entering the plant is called raw water. Upon entry, potassium permanganate is injected to convert soluble ferrous iron to insoluble ferric iron so it can be filtered out. Another benefit to potassium permanganate injection is the oxidation of hydrogen sulfide (which gives the rotten egg odor). This oxidation process is vital to the plant operation in that it filters out iron and sulfur bacteria which would otherwise deposit on the membrane surface and cause fouling. It also enables the City to "BLEND" filtered water back with the reverse osmosis (R/O) product water to naturally stabilize the water. This process will be discussed further during the stabilization process.

The water then enters the greensand pressure filters utilizing the water pressure provided by the supply wells. The filters are constructed as horizontal cylinders with layers of filtering material. The top layer is anthracite. It filters out the precipitated insoluble iron produced by the injection of the potassium permanganate. Manganese greensand is the middle layer. This layer absorbs any residual potassium permanganate left over in the water. It also filters out and converts any iron still left in the water at this point. The final layers are multi-sized gravel to support the upper layers and provides filtering for any particles which may have passed through the upper layers. All of the water entering the plant passes through this iron removal process.

Sulfuric acid is added to lower the pH of the water to prevent calcium carbonate scaling on the R/O membranes. Antiscalant (polyacrylic acid) is added to prevent calcium sulfate and barium sulfate scaling on the membrane. Any scaling on the membrane surface would prevent the passage of the water through the membrane. As scaling occurs, the pressure it takes to force the water through the membrane increases, which in turn increases the power requirement.

After the injection of chemicals to prevent scaling on the membranes, the water passes through a series of 5 micron polypropylene cartridge filters. These remove any silt, turbidity, or debris larger than 5 micron which could cause irreparable damage to the membrane surface. Bacteria or other organic matter can also be filtered out during this process. The micron filters cannot be cleaned and/or reused. They are changed out as the pressure loss across them dictates or the water quality from them deteriorates to a point that possible damage to the R/O membranes could occur. This step is the last and most important in the pre-treatment process the membranes. The water is then termed pre-treated feedwater. All treatment done to this point is to protect the RO membranes from any scaling or fouling from natural constituents in the raw water.

REVERSE OSMOSIS SCHEMATIC

The flow to the R/O units is regulated with a flow control valve based on the amount of product water to be produced by the unit. Feed pressures to the units are expected to rise as the membranes age. Initial feed pressures were 100 psi with a five year projected pressure of 120 psi. This pressure is also affected by the degree of salinity of the feedwater. As the raw water quality decreases, or a brackish water supply is further developed, more pressure will be needed to produce the same quantity of water.

The R/O membrane is a basic separation process. When the feedwater enters the membrane it is separated in "Product" and "Concentrate" water. The concentrate is 17% of the feed flow into the unit and is highly concentrated with the ions from the feedwater. The concentrate is then disposed of through a separate pipeline to the City's wastewater treatment plant where it is treated and becomes part of the effluent utilized by the Reclaimed Water System. The R/O membranes pass 83% of the feedwater through the membrane as it removes calcium and magnesium ions in the water. This water is called the "Product" water. It is then piped to the next step of treatment.

CO3 « HCO3 « H2CO3 « CO 2 + H20

A grant from the Department of Health and Rehabilitative Services was obtained by the City to pay for a fluoride chemical feed system. Hydrofluosilicic acid is added to the water as a means to reduce dental caries. The dosage rate is 0.8 parts per million as fluoride. With the possibility of more stringent regulations on total trihalomethanes being imposed by EPA, ammonia facilities were incorporated into the design of the plant. This provision for disinfection with chloramines is an alternative to major treatment changes should the TTHM regulations become more stringent.

OTHER LINKS AND REFERENCES

• Welcome to City of Dunedin, Florida
• Dunedin Water Division
• Dunedin Reclaimed Water Division
• Dunedin Wastewater Division
• City of Dunedin 1997 Report on Quality and Quantity of Water
• To view History, Current Status, Future, Plant Operation, and Operating Specifications (OS), click here The OS has information on the Micron Filtration, Polyamide Spiral Wound Thin Film Composite Membranes, and Hydranautics PVDI Softenimg Membrane.
• Petition for Reclaimed Water Service
• Decision Tree for Extension of Reclaimed Water Service