AWWA DSS58709

AWWA DSS58709 Mass Transfer Reactions and Decay Sinks for Disinfectants in Water Distribution Systems
Conference Proceeding by American Water Works Association, 10/01/2003

Jones, Jeremy B.

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Water utilities are increasingly concerned with a distribution system's effect onwater quality. A water distribution system has come to be viewed as a largereactor vessel that degrades water as it travels through the system. Thesereactions occur within the bulk phase of the water and also occur at the interfacebetween the water and the pipe wall. To understand and predict these reactions,water quality modeling has been used extensively to study disinfection decay.Current models typically divide this decay into two distinct phases that include:decay occurring in the bulk phase of water; and, decay attributed to a demandexerted by the pipe wall. Transport between these two phases has beendescribed with a variety of advective mass transfer processes, which utilizedimensionless flow parameters such as the Reynolds, Schmidt and Sherwoodnumbers (1,2,3).Older distribution systems frequently have a predominance of unlined cast ironpipe. These pipes can experience higher rates of disinfectant decay than otherclasses of pipe such as polyvinyl chloride (PVC) and ductile iron. This isparticularly evident in dead end lines or areas of distribution systems withhistorically low flow patterns. Using the Norfolk Naval Base as a test site, fielddata was collected from low flow areas of the water distribution system from 1999through 2002. For unlined cast iron distribution grids, field data exhibitedsignificantly higher disinfectant decay rates than could be explained withconventional modeling.Based on this data, a new disinfectant decay model was developed withadditional decay sinks/sources applied to low flow and stagnant pipe sections.This effort identified three other decay sinks that can be incorporated in a decaymodel. These sinks were identified as: diffusion; iron release from castiron pipe walls; and, microbial detachment events from pipe wall biofilms.These microbial detachment events (wall shed) were found to be the mostdominant decay sink for cast iron pipe in stagnant flow conditions. Microbialdetachment events were indirectly measured by collecting and analyzingheterotrophic plate count samples using the R2A agar method. Includes 15 references, tables, figures.