Municipal wastewater plant intensifies treatment,
adding nitrification capacity without adding footprint.

Project Background & Challenges

In the heart of Alabama, where the counties of Shelby, Jefferson, and St. Clair intersect, sits the town of Leeds, birthplace of basketball legend Charles Barkley. Flowing right through town is the Little Cahaba River, which places the municipality within the Mobile River basin. This system of rivers flow into the Gulf of Mexico, where the nutrients they pick up contribute to one of the largest dead zones in the world.

But even before those nutrients hit the Gulf, effluent ammonia that might flow from a wastewater treatment plant can have a disastrous impact on the local ecosystem, and with all 194 miles of the Cahaba River flowing within the Alabama’s borders, the need for Leeds to protect this local jewel becomes clear.

Located 20 minutes’ drive due east of the state’s biggest city, Leeds’ development as a Birmingham bedroom community is dependent in part on available wastewater treatment. With its existing biofilter system approaching its design capacity, Leeds’ treatment plant showed signs it could no longer be counted on to fully nitrify. But beyond ensuring the plant could reliably meet its regulatory limits and retain dissolved oxygen in the effluent, Leeds’ plant operators were presented with a pair of challenges from the site itself. Most prominently, the site’s lack of available footprint meant any new process technologies would have to fit within its existing space. Furthermore, the site’s residential-scale, single-phase electrical supply meant that common, industrial- or municipal-scale equipment wouldn’t be able to run. With those challenges in mind, plant managers went looking for a solution.

The Nexom Answer

Given the lack of available footprint, wastewater treatment intensification became the focus for Leeds’ plant managers and town officials. In doing so, they came to Nexom’s biological treatment experts who design the BioPorts moving bed biofilm reactor (MBBR).

The idea underpinning MBBR technology is that biomass is easiest to retain when it’s fixed to a surface. Whether the intent is to remove BOD, nitrify ammonia, or even denitrify, growing biomass on a surface maximizes the biomass inventory, resulting in high-rate, low-footprint processes. BioPorts provides that surface area—and plenty of it—in the form of dynamic submerged media that is contained in a bioreactor.

While the biofilm is fixed on the media, the media itself circulates in the tank. The integrated aeration in the tank not only ensures high dissolved-oxygen levels throughout, it also moves the media within the tank, enabling a more even distribution of loading and oxygenation to the biomass.

At Leeds, Nexom’s engineers worked with plant operators to design the BioPorts technology to fit in a small strip of property at the front of the plant as a tertiary nitrification MBBR. By taking into account factors including influent wastewater characteristics, existing treatment performance, and effluent requirements, Nexom’s team was able to calculate aeration needs, source the exclusive “BioPorts 600” media, and provide installation supervision for the solution’s components at the site.

In addition, to overcome the electrical limitations, Nexom supplied Leeds with blowers equipped with variable frequency drives (VFDs). The VFDs were installed to convert single phase power to 3 phase power so an efficient blower system could be used for aeration. Also the VFDs allow for the changing flow over time and the wide range of treatment conditions that occur between the winter and summer temperatures.

Furthermore, the BioPorts system has been designed such that, once the biofilter currently in place reaches the end of its useful life, Leeds will only need to add one more BioPorts tank of the same size in front of the ones already installed and place a filter after the effluent of the new tanks in order to safely abandon the existing biofilter system and meet expanding treatment requirements for years to come.

Site Construction

Project Information

Location: Leeds, AL

Project Type: Municipal Wastewater Treatment Intensification

Completion Date: March 2020

Treatment Objectives

Design Flow:
450 m³/day (0.12 MGD)

Effluent Objectives:

  • < 1.0 mg/L Total Ammonia-Nitrogen
  • < 5 mg/L BOD
  • < 18 mg/L BOD

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Startup Period

With any biological technology, some startup period is to be expected: the biology required for treatment exists within the influent wastewater, but not in the concentration necessary for full treatment. Whereas MBBRs are designed to be able to dynamically adjust to variable levels of influent BOD, ammonia, or nitrates once started, Leeds was looking to accelerate the startup period as well.

Adding a layer of complexity to the rapid startup was the nature of the plant’s secondary effluent: having run through the existing biofilter and first-phase UV, the MBBR saw low levels of incoming solids and bacteria needed to build biomass.

As a solution, Nexom’s engineers assisted Leeds’ plant operators with bypassing a specific proportion of plant influent—approximately 10%—directly to the MBBR. The amount was carefully calculated to minimize both the MBBR startup period as well as any risk of exceeding regulatory compliance levels in the effluent. The solution was a success, and within a month, the MBBR was observed to have established the biomass to achieve necessary nitrification.

Upgraded System Performance

Since commissioning in March of 2020 and subsequent biomass establishment, the Leeds wastewater treatment plant and BioPorts MBBR has successfully produced reliably-compliant effluent, beating its 1 mg/L monthly average effluent ammonia target as well as its weekly 1.5 mg/L objective