Corrosion Control Aids Residuals
cast-iron and steel pipes promotes significant disinfection demand in some sections of the distribution system. The demand can be divided into two major categories:
The disinfectant demand of the bulk fluid is exerted by reactions with organic, inorganic, and bacterial
corrosion products that would be easier to remove by flushing. Because of the reduction in corrosion products, we expected an additional benefit of less disinfectant demand and a smoother and hydraulically superior pipe surface.
A short-term disadvantage of polyphosphate-blend corrosion inhibition is that existing corrosion products on pipe wall surfaces are softened and removed shortly after adding the blend.
constituents of the water. Reactions with corrosion products, oxidation of dissolved iron originating from the pipe walls, biofilms that reside on the pipe surface, and the organic and inorganic constituents, that accumulate within the corrosion product-biofilm matrix cause disinfectant demand.
Loss of disinfection residual, especially in low- flow areas, increases biofilm accumulation, nitrification, and customer complaints about objectionable taste and odor, color, and particles in the water. To minimize these water quality problems and reduce consumer complaints, the District has a comprehensive flushing program that includes annual systemwide flushing, unidirectional flushing, dead-end flushing, and routine flushing of problem areas. These flushing programs use approximately 100 acre-ft (32.6 mil gal) of water per month and require three to four people to perform the flushing.
Polyphosphate-Blend Corrosion Inhibitors
Bench-scale studies done at the Montana State University-Biofilm Institute showed that polyphosphate-blend (approximately 77 percent polyphosphate and 23 percent orthophosphate) corrosion inhibitors produce corrosion products that are physically much softer and less cohesive than corrosion products formed with zinc orthophosphate-based corrosion inhibitors. Based on these studies, the District anticipated that use of this polyphosphate-blend would produce
Corroded pipes comprise about 40 percent of the District's distribution system.
The sloughing of these corrosion products may lead temporarily to increased disinfection demand and increased microbial activity in the distribution system.
Study Description and Setup
The polyphosphate-blend-addition pilot study began in November 1998, in less than 1 mil2 isolated area of the distribution system. The District expanded the study in June 1999 to about a 3.6 mi2 area. Pipes in the study area vary from 4 to 12 in. in diameter and are approximately 50 percent unlined cast iron,
25 percent cement-lined cas iron, and 25 percent asbestos-cement pipe. The study area, comprised of approximately 80 percent residential and 20 percent commercial and light industrial facilities, is representative of the rest of the distribution system.
Polyphosphate- blend solution
Y. Koby Cohen, PE is vice president of quality assurance for Suburban Water Systems and chair of the AWWA Water Quality in Transmission and Distribution System Committee. Contact him at (626)543-2551 or kcohen @suburbanwatersystems.com. Contact Calvin G. Abernathy, PhD, PE, associate professor at the University of Memphis, Department of Civil Engineering, at (901)678-3283 or cgabrnth @memphis.edu. Reach Christopher P. Hill, PE, an engineer with Malcolm Pirnie, at (614)430-2683 or chill@pirnie.com.
Opflow / January 2003
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