Project Background & Challenges

The town of Citronelle, Alabama consists of approximately 5,000 people located about an hour from the Gulf Coast within the boundaries of Mobile County. Geographically unique, Citronelle is built atop a giant salt-cored dome that is still rising steadily, within the eastern Interior Salt Basin in southwest Alabama.

With over 2,400 kms (1,500 miles) of inland waterways, Alabama has one of the highest percentages of water among all US states. With this knowledge in mind, it is in Alabama communities like Citronelle, who have the potential to experience both the far-reaching effects of phosphorus and where changes made to reduce discharged phosphorus could see the greatest long-term impact.

With phosphorus-fed algae blooms plaguing many North American bodies of water, and in response to environmental concerns, regulators are pressuring wastewater treatment plants (WWTP) to meet ever decreasing phosphorus limits. These limits vary, but are usually tied to Total Maximum Daily Loads (TMDL) established for bodies of water. In regions where permits have addressed phosphorus pollution, discharge limits traditionally range from 1 mg/L down to 0.1 mg/L total phosphorus (TP). However, as populations shift around waters with established TMDLs, targeted treatment levels for phosphorus are driven lower.

A 0.1 mg/L limit has historically been considered an ultra-low target. But in Citronelle, they faced an even steeper challenge from regulators with a new ultra-low phosphorus target of <0.02 mg/L, which is among the lowest requirements in North America.

The Nexom Answer

The permit issued to Citronelle, Alabama is currently among the most stringent requirements being met by reactive filtration to date, at <0.022 mg/L. There, the wastewater treatment plant discharges to Puppy Creek, contributing a very significant percentage of dry weather flow to the creek.

In collaboration with Speaks & Associates Consulting Engineers, Inc., Nexom created an upgraded system for Citronelle by adding Blue PRO® reactive filtration for enhanced treatment efficiency and cost minimization. The reactive filtration phosphorus compliance upgrade was sized for a peak daily flow of 0.76 MGD, and its implementation has allowed the WWTP to meet its discharge permit cost effectively.

The project was contracted in late 2014, equipment was delivered to the site during late summer of 2015, assembled in the fall, and construction was completed in December 2015. The system was commissioned, and all checkouts were completed before year-end in preparation for performance testing.

Site Construction

Upgraded System Performance

Conclusion

Citronelle’s WWTP meets its treatment goals operating with manual dose setpoint control and no online phosphorus feedback to the control system, since no online analyzers could reliably read total phosphorus below the required control line when the plant was designed. The facility also uses low molar dosing ratios ranging from 2 to 4 Fe:P, despite its ultra-low limits and use of dose setpoint control.

Citronelle tells a story. It shows reactive filtration as being among the most effective treatment solutions for meeting low µg/L phosphorus targets, in a manner that is both efficient and relatively simple to maintain. As technology evolves further, the future suggests that online analysis may lead to further chemical optimization under certain site design considerations. Until then, the results from Citronelle, Alabama, indicate an effective and sustainable solution for maintaining its permit of <0.022 mg/L TP.

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