Conference Agenda

The Portland Bureau of Environmental Services (BES) Columbia Boulevard Wastewater Treatment Plant (CBWT) practices CEPT during wet weather events that increases settling performance in the plant’s wet weather clarifiers (WWCLs). Wet-weather flows dilute the alkalinity of the plant influent such that ferric chloride (FeCl₃) addition results in a decreased effluent pH. During the summer of 2020, the NPDES permit was updated to require continuous monitoring of pH limits between a daily minimum of 6.0 and a daily maximum of 9.0. With continuous monitoring in lieu of grab samples, BES has observed pH violations when CEPT is practiced which is believed to be due to the coagulant, ferric chloride. The objective of this project is to determine an alternative CEPT chemical or the addition of post-coagulation pH adjustment to prevent future permit violations for pH.

An investigation occurred to trial different polyaluminum chloride (PACl) based coagulants to limit pH decrease. The investigation included vendor outreach for coagulant candidates, jar testing, full-scale pilot testing during both dry and wet weather events trialing multiple coagulants, as well testing the use of post-coagulant chemical addition for pH adjustment and buffering. This presentation will present and reflect upon the results showing that PACl coagulants are effective, but products differ significantly in cost effectiveness. It will also discuss the challenges during the project including pandemic associated variables such as unstable costs, unreliable chemical availability, and labor shortages.

A potential secondary use of the CEPT system is to pre-treat wastewater to reduce loading to the secondary treatment system. BES is also testing the performance of dry weather clarifier (DWCL) CEPT to determine the effectiveness of aeration basin loading to provide a solution for growth projections with limited space for expansion. Results from a 2022 full-scale pilot and lessons learned will be presented.

Attendees will gain an understanding of the technical engineering involved in evaluating alternatives to prevent the pH excursions, as well as the creative problem-solving and collaborative effort between all parties that led to an updated CEPT process that benefited both the wet and dry weather treatment processes.

After five years of related construction activity, King County’s Georgetown Wet Weather Treatment Station (GWWTS) began treating combined sewage in December 2022 that would have otherwise been released into the Duwamish River in Seattle. The station was designed to control combined sewer overflows at the South Michigan and Brandon Street outfalls by limiting untreated discharges to an average of no more than one per year, per outfall. The GWWTS has a capacity for a peak inflow of 133 mgd. The station includes a 1.1 MG equalization basin and two 35-mgd treatment trains that use ballasted sedimentation and ultraviolet disinfection to meet discharge permit requirements. Additionally, the GWWTS includes a regulator, screening and handling, influent pump station, chemical storage and distribution, odor control and solids storage systems. During the wet weather season, the station is ready to quickly startup and treat intermittent and variable events on short notice. Startup and shutdown sequences between events consist of a recycle stream, solids discharge, water management, and tank flushing. During dry weather, the station can recycle treated water to allow for operator training, maintenance, and inspection.
The presentation will review the station’s features and discuss operational testing that was performed before commissioning. After testing individual systems in isolation, additional testing was conducted to ensure full integration and that the station operated as intended. During the first part of operational testing, the station was hydraulically tested using City water. In the second part, dry weather flow from the conveyance system was mixed with potable water to simulate the combined sewage the station would treat during a wet weather event. Rather than being discharged, treated water was recirculated, but otherwise the station operated as it would during a wet weather event. With coordination and support from the manufacturers, the performance of the ballasted sedimentation and UV disinfection systems was tested by sampling influent and effluent flows, with a third-party lab analyzing the samples.