OHA Drinking Water Services
OHA Drinking Water Services
Contact Report Details
PWS ID: OR41 00954
PWS Name: WILSONVILLE, CITY OF
 
Who Was Contacted and Phone: Mike Greene (503) 570-1542
Contact Date: 12/03/2014
Contacted By: FARRELLY, PETER (DWP)
Contact Method/Location: Office
 
Assistance Type: OTHER REGULATORY - REGULATORY ASSISTANCE
Reasons: SWTR
Chlorine
 
Details: SUMMARY: Meeting CTs after equipment failure - 9.25.14 DETAILS: Following a backpressure valve failure on September 24th Wilsonville’s second disinfectant, sodium hypochlorite, was first overdosed to a peak of 3.91 ppm, and then manually over-quenched with a dechlorinator, calcium thiosulfate, such that no chlorine residual remained. There was a portion of water (maybe as high as 100,000-200,000 gallons) that may not have achieved adequate inactivation post-filtration, depending on the hydrodynamic mixing of the clearwell. Some post-filter water had very high CT and some perhaps close to none. Approximately 400,000 gallons of water with no chlorine residual was sent to the distribution over a two and a half hour period.

Wilsonville’s pre-filter ozonation provided 21.5 log – 22.3 log Giardia inactivation, and 1.35 log Crypto. inactivation. Filtered turbidities indicate Wilsonville’s deep bed filters operated normally (filtered turbidities were below 0.045 NTU). However, the sedimentation process immediately upstream of the ozonation contact basin was upset by the slower flows while operators troubleshot the issue into the night. Settled water turbidities fluctuated and reached as high as 7.8 NTU (normally <2 NTU). Settled water turbidities exceeded 5 NTU for 2 total hours. This higher turbidity could have reduced ozone’s inactivation efficacy somewhat, but according to a literature review ozone’s efficacy is not significantly reduced by turbidity levels experienced. Only 0.5 log Giardia inactivation post-filtration is required.

[Continued onto next contact report, Part 2] ACTION NEEDED: none beyond improved communications