Treatment Plant Operations
Bed Expansion Affects Performance
by Ed Chescattie and Kevin Anderson
The experience of Pennsylvania Department of Environmental Protection staff over 14 years conducting filter plant performance evaluations led investigators to identify a common thread: "inadequate filter bed expansion eventually contributes to poor filter performance and reduced finished water quality."
The article points out that an insufficiently expanded filter is insufficiently cleaned, and this can lead to numerous problems, specifically: "poor post-backwash recovery, sensitivity to hydraulic surges (flow changes), shortened filter run times, and eventual mudball formation. All these problems can increase the likelihood of a breakthrough of waterborne disease-causing organisms" - with the final conclusion that "proper filter bed expansion is a significant operational consideration."
The examiners report that while plant-specific goals for filter bed expansion are recommended, plants that achieve a minimum of 20% expansion of the filter media perform better - 20% expansion is a good starting point for determining optimal media expansion. Noticeable filter performance problems are likely when expansion drops below 15%.
Finally, investigators recommend that bed expansion be measured in several locations to better understand the effectiveness of the backwash - and, operators should take routine measurements for comparison with other filter performance parameters.
The full text of this article is available through Opflow Online at the AWWA website.
Synopsis of the article: Bed Expansion Affects Performance, Opflow, Vol. 29, No. 11 (November 2003), Reviewed here by permission. Copyright © 2003, American Water Works Association. Permission to reproduce this material does not represent or imply approval or endorsement by AWWA of any product or service.
