CItronelle, Alabama

Wastewater treatment plant upgrade doesn’t just beat an ultra-low Phosphorus limit, it beats North America’s lowest municipal Phosphorus limit.

Project Information

Location: Citronelle, Alabama
Project Type: Municipal Wastewater Treatment
Completion Date: April 2016

Treatment Objectives

Design Flow:
1,500 m³/day (0.40 MGD) Average
2,900 m³/day (0.76 MGD) Peak
Effluent Quality:
0.022 mg/L Total Phosphorus

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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

The performance test was a condition of process warranty and was conducted in February and March of 2016. Filter effluent during the performance test averaged <0.013 mg/L TP in certified lab-analyzed 24-hour composite samples; values were corroborated by split samples analyzed in the field averaging 0.006 mg/L TP. The performance test’s successful completion signaled project completion.

The Citronelle WWTP must comply with a monthly average discharge limit of 0.022 mg/L TP from April through October.

Its first ultra-low-level compliance season was completed in 2016. Results from the site’s Discharge Monitoring Reports (DMRs) exhibit compliance for all months except August, when the site experienced an Act of God: a lightning strike damaged the plant controls. Despite this, the plant averaged just over 0.1 mg/L TP running many systems in manual settings. Replacement control system components were installed, and the plant was back within full compliance in under two weeks.


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.

Other case studies you may be interested in:

Bloomer, WI

Like many municipalities across North America facing environmental concerns over phosphorus—Bloomer was given a new Total Phosphorus (TP) limit that its existing wastewater treatment was unable to achieve.

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Burrillville, RI: Blue PRO

Northeastern municipal wastewater treatment plant upgraded with Blue PRO reactive filtration for ultra-low phosphorus and metals removal.

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