Mixing in BNR

Mixing
Mixers in the anaerobic and anoxic stages of biological treatment trains help to ensure good contact between the biomass and waste constituents. Without proper mixing, the biomass tends to settle toward the bottom of tanks and does not come into contact with the soluble carbon, nitrogen, and phosphorus compounds in the water. With inadequate mixing, there can be a measureable difference in mixed liquor suspended solids between the top and bottom of the tank.
Improper mixing can also introduce additional DO into mixed zones above that normally transferred across the air-water interface. Both suspended solids gradients and added DO can reduce the efficiency of treatment processes requiring anaerobic or anoxic conditions. The goal in mixing is to provide adequate mixing to optimize performance while minimizing the power expended to provide that mixing. Short-circuiting or imbalances in the MLSS concentrations between different zones of a bioreactor can have a measurable effect on reliable performance.

In addition, visual observation of the liquid surface in the anaerobic and anoxic tanks can indicate the regular appearance and dissipation of surface vortices. Vortexing is undesirable as it introduces additional oxygen into the mixed tanks, thereby decreasing the amount of carbon available for denitrification. The formation of vortices can be minimized by proper mixing.
The adequacy of the mixers can be evaluated by comparing typical design ratios with those for the mixers at a treatment plant and measuring solids profiles in the mixed tanks. Selection of appropriate mixers requires input from equipment manufacturers. Before the development of computer simulations to predict velocities in mixed tanks, several standard
dimensional ratios were used for the design of traditional mixed reactors (Oldshue, 1983). The recommended liquid depth to basin width ratio for optimizing power consumption and solids suspension is 0.6 to 0.7. Liquid coverage over the impeller, at a minimum, should be 0.5 to 2 times the impeller diameter. Finally, impeller height above the basin bottom should be no more than 1.3 times the impeller diameter.
Due to the geometry of the anaerobic and anoxic basins at most plants, computational fluid dynamics (CFD) analysis can be performed on the mixed basins to gather information to help decide whether replacing the mixers would significantly improve process performance. The CFD
analysis should include the influence of solids on the flow regime.

With HPCD, such dimensional ratios DO NOT apply. HPCD is typically installed at top of the reactor. The Tornado-like flow pattern it created provide superior solids lifting capacity, and disperse the particles with the fireworks-like dispersing force.

The top of HPCD prevent vortex formation and NO oxygen will be introduced to the liquid. This provide the ideal-mixing of anoxic/anaerobic reactors.

Case Study 2
The second case study concerns a utility that was under a consent order for regular violation of effluent TN and TP permit limits. Visual observation of the mixed liquor in both the anaerobic and first anoxic tanks suggested the mixing in these tanks was not adequate for providing solids suspension. Suspended solids measurements were taken in each tank at various depths with a portable suspended solids probe to determine if there was a measurable difference in the solids concentration between the top and bottom of the tanks. The probe reached bottom at a depth of 10 ft even though the tank has a side water depth of 21.7 ft. This suggests large amounts of grit have accumulated in the anaerobic tank. The solids profile in the first anoxic tank was also measured upon arrival at the facility. After taking the measurements, the mixer in the first anoxic tank was reversed momentarily and restarted. There was not a significant difference in the solids measurements at various depths. However, visual observation also showed improved mixing from the surface after reversal and restarting of the mixer. This suggests that rags are accumulating on the impeller and causing poor mixing in the tank. Both the anaerobic and first anoxic zones would benefit from improved screening, grit removal, and replacing the existing impellers on the mixers with a ragless impeller design.
The second anoxic zone has submersed mixers. Visual observation of the second anoxic tanks indicates that the mixers are insufficient for providing adequate mixing in the second anoxic zone. Outside of the immediate vicinity of mixer, significant settling is observed. Toward the end
of the second anoxic zone, a significant scum layer has collected on the surface, indicating very poor mixing in this section. These mixers should be replaced with a more traditional, vertical platform mixer design with ragless HPCD type mixers. The utility is in the process of replacing these mixers.