Water utilities are increasingly concerned with a distribution system's effect on
water quality. A water distribution system has come to be viewed as a large
reactor vessel that degrades water as it travels through the system. These
reactions occur within the bulk phase of the water and also occur at the interface
between 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 demand
exerted by the pipe wall. Transport between these two phases has been
described with a variety of advective mass transfer processes, which utilize
dimensionless flow parameters such as the Reynolds, Schmidt and Sherwood
numbers (1,2,3).
Older distribution systems frequently have a predominance of unlined cast iron
pipe. These pipes can experience higher rates of disinfectant decay than other
classes of pipe such as polyvinyl chloride (PVC) and ductile iron. This is
particularly evident in dead end lines or areas of distribution systems with
historically low flow patterns. Using the Norfolk Naval Base as a test site, field
data was collected from low flow areas of the water distribution system from 1999
through 2002. For unlined cast iron distribution grids, field data exhibited
significantly higher disinfectant decay rates than could be explained with
conventional modeling.
Based on this data, a new disinfectant decay model was developed with
additional decay sinks/sources applied to low flow and stagnant pipe sections.
This effort identified three other decay sinks that can be incorporated in a decay
model. These sinks were identified as: diffusion; iron release from cast
iron pipe walls; and, microbial detachment events from pipe wall biofilms.
These microbial detachment events (wall shed) were found to be the most
dominant decay sink for cast iron pipe in stagnant flow conditions. Microbial
detachment events were indirectly measured by collecting and analyzing
heterotrophic plate count samples using the R2A agar method. Includes 15 references, tables, figures.
| Edition : | Vol. - No. |
| File Size : | 1
file
, 500 KB |
| Note : | This product is unavailable in Ukraine, Russia, Belarus |
| Number of Pages : | 17 |
| Published : | 10/01/2003 |
