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In follow up to the October event where there were two brief < 1 hour periods that failed to meet disinfection CTs at the Alder Creek WTP, as noted in two previous contact reports, Mike clarified the following:
- November and December disinfection CTs have been met as a minimum chlorine residual setpoint at the Terra Fern reservoir pump header (CT compliance point) has been increased.
- Chlorine residual, pH, and temperature are taken at the suction pump header off Terra Fern reservoir after the contact time segment (around 1 mile of 20 inch pipe between chlorine injection at the WTP and the reservoir. This location is not the same as the common inlet and outlet (for filter backwash only) in Terra Fern.
- Jacobs, contractor for WTP, has repaired and placed into service a smaller 350 gpm treatment unit at Alder Creek, which will more closely match lower system demands. This will produce longer filter run times, and consequently production will more closely match demand, to shorten water age and reduce chlorine demand in Terra Fern to keep a higher residual.
- City will explore setting the low chlorine alarm at the WTP to stop production at the plant at a certain setpoint that would indicate CTs likely not met. Also, city will explore moving the low chlorine alarm/shut down from the plant to CT compliance point, after Terra Fern where a CL-17 is located.
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