Oxidation Ditch – Orbal and VLR (simultaneous nit/denite)

Evoqua – Oxidation Ditch is a modified activated sludge biological treatment process that utilizes long solids retention times (SRTs) to remove biodegradable organics. Orbal® system with a Concentric Loop™ configuration that creates dedicated zones for specific treatment processes. These treatment zones operate in series which is essential to the simultaneous nitrification-denitrification (SND) process. VLR® Vertical Loop Reactors system are similar to oxidation ditches that have been flipped on their sides. The VertiCel® System utilizes tanks in series design and consists of an aerated-anoxic tank followed by an aerobic tank.


The simultaneous nitrification/denitrification (SND) process is the backbone. The Orbal system incorporates a unique Concentric Loop™ configuration that creates dedicated zones for specific treatment purposes. These treatment zones operate in series, which is essential to the SND process.

Series ditches allow for anoxic, anaerobic and aerobic conditions within basins to eliminate need for “selector basins” with mixers.
Eliminates need for internal recycle pumps with “Simultaneous nitrification-denitrification”.


VLR Systems utilize looped reactors in series that allows DO stratification. The tanks are similar to oxidation ditches that have been flipped on their sides. There is an upper and lower compartment, separated by a horizontal baffle. The process was adapted from Orbal® Multichannel Oxidation Ditch technology and uses the same surface mounted discs to provide mixing and deliver oxygen. Typically, two or more basins make up the system, with the first tank operating as an aerated anoxic reactor.

Video: Orbal Disc Retrofit of  Brush Aerators: Mechanical Advantages of Disc Aerators -Part 1

  • Will Disc Aerators Fit?
    • Yes, same structure and footprint
  • Compatibility with wastewater and H2S?
    • Disc: HDPE
    • Brush: Galvanized or painted carbon steel brush rotors
  • Mixing Efficiency
    • Disc: 10 HP/ MGal required
      • 4X mixing as brushes
    • Brush: 40 HP / MGal required
      • Need adjustable baffles and possible mixers
  • See video below for the mechanical advantages of Disc Aeration

Video: Orbal Disc Retrofit of  Brush Aerators: Process Advantages of Disc Aerators -Part 2

  • Less Energy for Aeration
  • Biological Nutrient Removal
  • Convert parallel of brush system to series with disc aeration
    • Parallel:
      • Both Ditches at DO 2 mg/l
      • Cannot have denitrification because of excess oxygen
    • Series:
      • One Ditch at DO 2 mg/l; following ditch is DO in denitrification mode (DO ~0 mg/l)
  • Smart BNR process control system
    • Integrated DO or ORP Control Strategy
    • Automated Wasting for optimum sludge age
    • Automated response to peak flow or storm conditions
  • See video below for the process advantages of Disc Aeration

Video : Orbal True SDN

Project Profile: St. Charles, Missouri

In 2008, HDR, Inc. recommended the replacement of brush aerators and internal “boat clarifiers” in three parallel operated oxidation ditches.  The brush aerators were experiencing problems with icing and shaft breakage, and many of the aeration “stars” were corroded and had broken off.  The boat clarifiers—originally designed as a pre-clarification step to lower the Solids Loading Rate on the existing final clarifiers—were inefficient and interfered with channel velocity, causing mixed liquor suspended solids (MLSS) to settle in the basins.  The ditches  were  retrofitted with Evoqua with 12 VLR mounted disc aerators to achieve a  7.54 MGD design flow plant, which allowed for the treatment of higher organic loading within the existing footprint. The disc aerators resulted in lower noise, provided better mixing, minimized dead zones and resulted in fewer odors resulting from good oxygenation within the basins.

The existing oxidation ditches flow pattern were changed from parallel to series to create independent environmental zones to increased the nutrient removal capacity of the plant. This allowed denitrification of recycled nitrates, as well as simultaneous nitrification/ denitrification.  This process allows some of the influent ammonia to be converted from nitrite directly to nitrogen gas, reducing energy consumption and total nitrogen discharges.  Controlling the oxidation-reduction potential of the first tank in the series also enhances the biological phosphorus removal capacity of the system. The series flow pattern increases the efficiency of treatment without increasing tank volume.  Denitrifying bacteria use the chemically combined oxygen to stabilize influent biological oxygen demand, rather than absorbing mechanically injected dissolved oxygen.  Reducing mechanical oxygen injection reduces purchased power.  The design model indicates the new treatment system is 20–25% more energy efficient than the parallel brush aerator installation.

The disc aerators provide access platforms to the drives and outboard bearings, making them easier to maintain.  Because the discs have no deterioration or corrosion, the expected lifetime of the system is 20+ years.

Fort Dodge, Iowa

Expansion of the existing VLR system to increase primary treatment capacity to 12.0 MGD and secondary treatment to 18.6 MGD. The schedule for the project was very tight. McClure Engineering designed the plant with a start-up in mid-2014. The contractor was Gridor Construction of Buffalo, Minn. The plant is operated for the city by US Water.

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