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Why is Mixing Needed in Anoxic and Anaerobic Zones?

Biological Nutrient Removal (BNR) processes can be both effective and economical. Proper mixing within anoxic and anaerobic zones can help ensure lower effluent nutrients and improved treatment efficiency. 

Nutrients such as nitrogen and phosphorus are the primary cause of eutrophication in surface waters resulting in algal blooms, low dissolved oxygen, and fish kills. Efforts to reduce nutrient impairment have brought about more stringent effluent limits for wastewater treatment plants, often necessitating BNR systems to ensure compliance.

BNR is carried out through the use of microorganism selection and controlled environmental conditions within the treatment plant, characterized by the arrangement of unaerated (anoxic/anaerobic) zones upstream and/or downstream of aeration zones. Mixed liquor recycle and sludge return streams are arranged to make best use of the organic content and activated sludge in the system. With these processes in place, eutrophication is avoided in the recipient water system.

Below are characteristics of the different zone types.

  • Anaerobic Zone.  In this zone no readily-utilized forms of oxygen (such as free oxygen, dissolved oxygen, and nitrites/nitrates) are present.
  • Anoxic Zone. In this zoneno dissolved oxygen is present, but “chemically bound” oxygen, including nitrites and nitrates which microbes use for metabolic processes, is typically present.

It follows that avoiding significant oxygen transfer when mixing in anoxic and anaerobic zones is a critical requirement.

The removal of nitrogen and phosphorous require different approaches, as below.

  • For total nitrogen removal, the process must have an aerobic zone for nitrification and an anoxic zone for denitrification with nitrate-rich mixed liquor return.  A different arrangement of aerobic and anoxic zones is used in the so-called step-feed process.
  • For total phosphorus removal, particulate phosphorus is removed through solids removal, and soluble phosphorus is removed by microbial uptake through phosphorus-accumulating organisms. The process must have an anaerobic zone free of dissolved oxygen and nitrate for phosphorus release and microbe selection, and an aerobic zone for phosphorus uptake.

While aeration (and the mixing that comes with it) accounts for the majority of energy consumption in most wastewater treatment processes, securing adequate conditions in anoxic and anaerobic zones also requires energy for pumping into and out of these zones and mixing within them.  Pumping for BNR applications basically entails finding a reliable technology for transporting relatively large flows at low head for a set of well-defined duty points. The number of Xylem Flygt pumps (from the 3085 centrifugal N-pump to the  4680 horizontal axial flow propeller pump, and to the  7125 vertical axial flow propeller pump) used in these types of applications can be counted in the tens of thousands (3085 is a lower and 4680 and 7125 are upper ends of ranges). 

Submersible Mixing

Flygt 4400 and 4600 series submersible mixers in anoxic and anaerobic zones can be counted in even larger numbers. The Xylem Flygt 4320 mixer introduced in 2015 has an integrated variable frequency drive, allowing operators to fully adjust mixing speed and thrust to meet varying application demands.

But what does the mixing duty actually consist of in BNR applications?

The objective of mixing in anoxic and anaerobic zones is to maintain optimum conditions for nutrient removal. Maintaining anoxic/anaerobic zones requires mixing of the basin contents and influent streams while avoiding the introduction of free oxygen. The sludge must be kept in suspension in order to utilize the designed tank volume.

Optimum mixing must provide:

  • Off-bottom Suspension. Solids bulking on the bottom reduce volume and may result in undesired anaerobic effects.
  • Sufficient Solids Distribution. Uneven, insufficient bacterial contact with liquid throughout the entire tank is in essence, a loss of process volume.

It is often acceptable for the few top centimeters of liquid in the zone not to be penetrated by sludge clouds, since sludge exposure to atmospheric oxygen conflicts with the function of the bacteria. But atmospheric impediment of denitrification (anoxic service), for instance, is unlikely to occur unless high power is spent on surface motion. 


Multiple streams entering the tank must be thoroughly mixed together. This volumetric distribution of incoming fluid is referred to as blending. A pre-mix contact chamber for several streams is often a good choice for maintaining proper blending. If the tank is a “complete mix” design, incoming streams should still also blend with the bulk volume.

Prevention of bypass currents and stagnant zone formation is required for the process volume to remain intact. Bypass current prevention is often achieved as a side effect of accomplishing the distribution and suspension of solids. Adequate placement of tank outlets versus inlets is also critical. When inflow momentum is strong, it may be harnessed for mixing the tank content using a well-designed arrangement.

However, because the inflow will always seek the simplest way to the outlet, it’s important during system design to give careful consideration to the system’s blending mixing duty. 

Intermittent Mixing

Tanks mixed intermittently (and too seldom) risk allowing currents through unmixed. This can often be revealed during tracer testing. If the duration of the idle mixer phase is too long, a tank undergoing a tracer test with the tracer injected just before a mixing cycle will show a completely different residence time distribution (RTD) than if the tracer is injected just after a mixing cycle. The latter test will reveal bypassing, whereas the former is likely to produce a much less clear indication of the issue. 

Foaming Considerations

A frequent requirement for anoxic zone mixers is foam prevention or removal. Foam layers sometimes form on the surface in an activated sludge system. Reasons for this have been sought among upstream process specifics; increased sludge age as a denitrification process step is introduced; poor mixing; lack of organics or energy for the bacteria; and other areas.

It has even been suggested that mechanical equipment disrupts sludge flocs, leading to a different suspended behavior and different sludge agglomeration. However, intense scrutiny has not confirmed this. One of the few established facts is that filament-forming bacteria are favored by a high sludge age, with consequences for foaming.

Today, foam is often simply accepted as a consequence of the process configuration and wastewater composition at hand. Operators sometimes focus on letting the foam proceed onward in the process line, and ask that surface flow patterns promote this transfer. 

The Right Mixing Technology

Effective mixing in anaerobic and anoxic zones is often accomplished with submersibles, low-speed submersibles or impeller mixers such as top-entry agitators.  The right mixing technology works within the plant’s BNR system design to maintain optimum conditions for nutrient removal.  In addition to effective and efficient mixing, the right mixer should also provide low maintenance and low operating costs ultimately resulting in a low life cycle cost.

by Treatment