AWWA MTC69728

The City of Ft. Lauderdale Peele-Dixie Water Treatment Plant is a 12 MGD (1,893 M³/day) facility with four membrane system trains producing 3 MGD (473 M³/day) capacity per train. This plant required a significant review of the optimal dosing of sulfuric acid in lowering feed pH and the use of antiscalant to control the precipitation of hardness while maintaining sufficient levels of alkalinity to meet permeate quality goals and chemical costs. The plant used a low TDS well water supply from the Biscayne Aquifer ranging from 350 to 500 ppm, feed iron up to 2 ppm, total hardness up to 270 ppm as CaCO₃, alkalinity before acidification up 270 ppm as CaCO₃, TOC as high as 11 ppm, and color as high as 90 Color Units. The permeate quality goals of this potable water treatment facility were very stringent and required extensive pilot testing to develop an optimal spiral wound membrane design configuration using both RO (reverse osmosis) and NF (nanofilter) membranes. A minimum level of permeate alkalinity was desired to make the finished water less corrosive in the city's distribution piping. The system configuration by train is a 53 pressure vessel 1st stage by 24 pressure vessel 2nd stage pyramidal two-stage array with seven element long pressure vessels. The design incorporated the use of seven ultra-low pressure reverse osmosis (RO) membrane elements per pressure vessel in the 1st stage. This is followed by a 2nd stage containing 4 ultra-low pressure RO membrane elements and then 3 high-flow/low-rejection nanofiltration (NF) membrane elements in the same pressure vessel. This unique design, referred to as a Double Hybrid RO/NF design, produced the client's required stringent permeate water quality at a very low operating pressure of about 100 psi. Pilot testing was required to develop and confirm a design basis and rejection criteria for each type of RO and NF membrane employed. The position of each RO and NF membrane in the system has an impact on the specific water quality that the train can produce. Includes tables, figures.