Nanofiltration As a Post-Treatment at a Conventional Water Treatment Plant

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Introduction

The Clifton Water District (Colorado) covers roughly 20 square miles stretching east of Grand Junction to Palisade, and extending south from I-70 to the Colorado River. The District has approximately 7,400 service connections serving a population of 30,000. Utilizing the Colorado River as its primary source, the District's Charles A. Strain Water Treatment Plant has a capacity of 12 million gallons per day (mgd). Conventional water treatment processes are employed including presedimentation, coagulation, flocculation, sedimentation and sand filtration. Chemicals used in the treatment process include aluminum sulfate, cationic polymer, fluoride, chlorine, and occasionally powdered activated carbon.

The Colorado River has tremendous seasonal fluctuations in total dissolved solids (TDS), minerals and organics. Calcium, magnesium, sodium, sulfate and chloride account for most of the river's TDS content. During the fall and winter months, TDS concentrations can reach 1000 milligrams per liter (mg/L), whereas during the spring and summer months the TDS concentrations are as low as 200 mg/L. The high TDS concentrations and extreme fluctuations have created periodic customer complaints concerning the aesthetics of Clifton's drinking water.

These complaints prompted the Clifton Water District Board of Directors as early as 1987 to begin addressing aesthetic water quality issues. In 1990, the Clifton Water District began periodically blending its treated water with lower TDS water from the City of Grand Junction in an attempt to reduce the TDS concentrations. In 1992, the Board of Directors requested that Manager Raymond Schuster prepare a 20-year planning report to identify the District's strategy for meeting future water demands and options for the District to reduce and stabilize TDS levels. The report identified four alternative methods of reducing TDS in the District's water supply. The options were ion exchange, lime/soda ash softening, development of an alternative water supply and membrane technology.

Evaluation of Options

Ion Exchange: It was determined that an ion exchange facility would be too costly to operate due to the acid and caustic required to regenerate the media. In addition, the regeneration process would produce an undesirable discharge, creating a safety and disposal problem. Furthermore, ion exchange technology would not provide a filtration benefit. This technology would not remove the wide range of minerals desired by the District and would not provide a long-term solution to reducing the TDS concentrations.

Lime/Soda Ash Softening: Lime/soda ash softening is more economical than ion exchange. However, the process does not provide a filtration benefit and produces a large volume of sludge that is costly and difficult to dispose of. In addition, the lime/soda ash softening process is only effective in removing the calcium and magnesium concentrations, thus leaving high concentrations of sulfates, chlorides and sodium which are not acceptable.

Alternative Water Sources: The primary drawback in developing alternative water sources relates to the geographical location of the Clifton Water District in respect to available mountain water. The security of water rights and availability of the mountain water sources in addition to the extended transportation distances (20 to 40 miles) create an extremely costly endeavor. The District continues to negotiate with other water providers to supply low TDS water to the District. Financial issues are the primary concern in these negotiations. In addition, the District is also concerned about relying on other utilities with respect to their current treatment methods and ability to comply with future water quality issues including disinfection by-products and microbial contaminants.

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