If a plant has step feed functionality, however, the solids-holdup process can be implemented, giving plant operators a major advantage in handling peak wet weather flows by increasing the plant’s wet weather treatment capacity. When a wet weather event occurs, the secondary influent is conveyed to a downstream aeration tank feed point, while the return activated sludge (RAS) is continuously fed to the aeration tank influent. The process is like contact stabilization, where the mixed liquor in the downstream zone contacts and adsorbs pollutants, is subsequently settled in the secondary clarifiers, and finally, is stabilized/ biodegraded in the upstream zones with the RAS. The wet weather benefit of the solidsholdup process is that the aeration tank effluent MLSS is lowered by dilution, thereby lowering the secondary clarifier SLR, while at the same time, RAS solids are safely stored in upstream zones, protecting them from wet weather flow. As depicted in the third diagram of Figure
1, the same three-fold increase in secondary influent flow with the solids-holdup mode in effect shows that the SLR is maintained at 20 lbs./d/ft2. For this wet weather operating mode, the secondary influent is conveyed to the last onequarter of the aeration tank (Zone 4), causing the MLSS to drop to 1,000 mg/L, keeping the SLR at a manageable 20 lbs/d/ft2 and thus preventing solids loss in the effluent. The MLSS in the upstream aeration tank zones also increase, thereby storing and protecting the RAS from wet weather flow. With the contact zone sized to provide at least 20 minutes of contact time, the process can provide very good carbonaceous material adsorption and subsequent removal to meet secondary treatment standards for shortterm wet weather events. Lastly, the process provides fast recovery time because the aeration tank solids inventory is safely stored away in upstream zones, making it ready for use after a wet weather event has passed.
Conclusion The step feed solids-holdup process may be a cost-effective approach that utilities can use to help improve their resiliency, mitigate SSOs, and ensure long–term sustainability.
References • F WEA Utility Council 2020 and 2021 annual reports
About the Coauthor Kevin Frank, P.E., is a wastewater process engineer with AECOM Technical Services Inc. and focuses on process simulation modeling, nutrient removal, and the design of engineered oxygen transfer systems. He has worked on projects across the United States and internationally. S
Drop Savers Contest Prizes Shipped to Winners The "Drop Savers” Water Conservation Poster Contest, sponsored every year by the Florida Section of the American Water Works Association, encourages students in Florida schools, from Kindergarten to 12th grade, to create posters depicting a water conservation idea on how to be “drop savers” for water. The 15 contest winners receive an array of prizes, including Amazon gift cards, plaques and calendars displaying their posters, water conservation kits, and certificates. Melissa Velez, with Black & Veatch in Coral Springs, who coordinates the contest, enlisted help from her two boys to mail the prizes to the winners. They were “drop savers” themselves, keeping the boxes on the dolly and in the cart, so they could be safely shipped. To see the 2021 poster winners, go to page 42. S Martin maneuvers some boxes on a dolly.
The boxes of prizes ready to be mailed.
Melissa with hers sons Benjamin (left) and Martin (right).
Jack in the box? No, Benjamin in the box!
Martin shows his muscles.
Florida Water Resources Journal • August 2021
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