WCW24

Gold Bar Wastewater Treatment Plant (GBWWTP) operated by EPCOR in Edmonton, Alberta has a limited footprint for expansion. To meet the treatment demand for population growth and regulatory requirement in the future, EPCOR strives to improve the treatment efficiency by intensifying the existing process. One of those efforts was to improve the settling performance by improving the hydraulic condition of secondary clarifiers. The purpose of this presentation is to share some of the experiences learned from this journey with others. The GBWWTP currently has 11 biological nutrient removal (BNR) bioreactors, each followed by a rectangular secondary clarifier. Mixed liquor from each BNR bioreactor is distributed to its secondary clarifier via a mixed liquor channel (MLC) through 10 distribution ports. Each secondary train is designed for a maximum flow of 42 million liters/day (MLD) and average of 28 MLD. In 2018, a dye test was completed in selected secondary clarifiers. During the test it was noticed that dye added at the end of the bioreactor was almost immediately visible in one front corner of the secondary clarifier. It was concluded that the MLC was not evenly distributing the mixed liquor into the secondary clarifier and density currents were causing short circuiting within the clarifier. In 2019, further field testing was completed to better understand flow patterns in the clarifier. Multiple measurements were taken using a portable flowmeter at each MLC distribution port into the clarifier, at both 28 and 42 MLD. The results confirmed the short circuiting when the mixed liquor was distributed to the secondary clarifiers. The amount of mixed liquor distributed through the distribution ports of the MLC varied from -8 to 248% of the average. In 2020, baffles were designed and installed based on computer fluid dynamic (CFD) modelling of the MLC and two full-width baffles were installed in the clarifier. Access and maintainability were strong drivers in the design. Baffles were hinged or removable to allow access, materials were chosen to reduce maintenance, and spacing was designed to reduce solids build-up With the MLC baffles, modelling predicted the amount of mixed liquor distributed through each distribution port in the range of 90% to 107% of the average flow. To validate the actual effects of the baffles, additional dye testing and field measurements were performed in 2023. In general, the dye test indicated that the baffles provided a 20% improvement on overall hydraulic retention time in secondary clarifiers. The field flow measurements showed that the MLC baffles significantly improved the distribution of mixed liquor to secondary clarifiers. The amount of mixed liquor through each distribution port is about 69% to 180% of the average flow, which presents a significant improvement and provides validation of the CFD model. Although baffles are a relatively simple means of improving clarifier hydraulics, this study took a systematic approach in design and validation of in-tank and MLC baffles. Unique challenges were experienced throughout this study, especially during performance validation, and learnings from these challenges could be helpful for other utilities undergoing this journey.