Several bench-scale studies have documented that an effective corrosion control program may increase disinfectant residuals and improve water quality because fewer corrosion products are formed. So, over a three-year period, the District studied the long-term effect of corrosion inhibitors on disinfectant residuals, distribution system hydraulics, and water quality. The study proved that the use of polyphosphate-blend corrosion inhibitors is effective in maintaining residuals and will help limit water lost to labor spent on flushing. The District is in the process of implementing the program systemwide.

District Disinfectant Demand

Unlined cast-iron and steel pipes, installed between 1930 and the late 1950s, comprise about percent of the 410 miles of water distribution pipelines owned by the District. The remaining pipelines are ductile iron, polyvinyl chloride, and asbestos cement. The large number of unlined cast-iron and steel pipes promotes significant disinfectant demand in some sections of the distribution system. The demand can be divided into two major categories:

• the demand of the bulk fluid, and

• the demand of the pipe wall surface.

The disinfectant demand of the bulk fluid is exerted by reactions with organic, inorganic, and bacterial 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 disinfectant 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 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 inhibitors is that existing corrosion products on pipe wall surfaces are softened and removed shortly after adding the blend. The sloughing of these corrosion products may lead temporarily to increased disinfectant demand and increased microbial activity in the distribution system.