Performance of a conventional surface water plant using mixed-oxidants for microflocculation and final disinfection
The City of Santa Fe, NM utilizes both surface water and well water to achieve a system-wide peak production of 22 million gallons per day (MGD). In March 1998, the city incorporated MIOX mixedoxidant disinfection technology for pre-treatment and final disinfection of the water treated in an 8 MGD surface water plant. Concurrently, a filter backwash recycling system was installed as well as hydraulic improvements in the clarifiers. The original reason for considering on-site generation was the elimination of one-ton gas chlorine cylinders to comply with EPA regulations and safety concerns. Initial jar testing in the fall of 1997 indicated benefits could be achieved with the use of mixed-oxidants for microflocculation in the clarifier. One year of operation over a complete annual cycle has confirmed the beneficial effects of the process and hydraulic improvements. Alum and polymer use has been reduced forty percent. Lime addition for pH adjustment has been eliminated. At the Canyon Road Water Treatment Plant (CRWTP), fluoride addition has been reduced up to 50 percent while maintaining the same fluoride levels in the distribution system. Average turbidity through the plant has been reduced from approximately 0.6 NTU to about 0.25 NTU average and 0.1 NTU for low flows. Turbidity reduction caused by rapid stabilization of the up-flow clarifier enabled the plant to start up in the unprecedented time of only 8 hours and to operate at greater than design capacity. Total trihalomethane (TTHM) levels with gas chlorine had shown occasional excursions near the maximum contaminant level 2 (MCL) of 100 micrograms per liter (mg/L). Since operation with mixed-oxidants, TTHM concentrations in the distribution system have averaged well below the new Stage 1 EPA level of 80 mg/L, with no excursions above this limit, despite mixed-oxidant pre-oxidation in the raw water where previously there had been no treatment. Sludge generation has been reduced approximately 25 percent due to reductions in alum addition, resulting in less frequent and lower costs for sludge disposal. Filter backwash intervals have been extended approximately 50 percent. Final disinfection dose rates at the CRWTP have been reduced approximately 40 percent while maintaining a stable free available chlorine (FAC) residual to all points in the distribution system served by surface water. Similar FAC residual effects have been seen with ground water. Safety training for chlorine gas storage and handling has been eliminated. Taste and odor complaints have been dramatically reduced. Salt handling labor is anticipated to be eliminated with conversion of an existing lime silo to a bulk feed brine generation system.