SINK OR SWIM: A COMPARISON OF SUBMERSIBLE AND SURFACE MIXERS FOR ANOXIC AND ANAEROBIC TANK MIXING With increasing requirements for enhanced nutrient removal, many wastewater plants are adding anoxic and/or anaerobic tanks in their secondary treatment systems. There are multiple options for enhanced nutrient removal including, but not limited to, the A2O (Anaerobic, Anoxic, Aerobic) process, the UCT or MUCT (Modified University of Cape Town) process and the Bardenpho process. All involve the use of anaerobic and/or anoxic tank configurations in addition to the aerobic reactor with influent, recycle and RAS (Return Activated Sludge) at various locations in the reactors. In these processes, mixers are needed to keep the phosphate absorbing organisms (PAOs) or denitrifiers in contact with the incoming wastewater to prevent settling of the biomass and to prevent accumulation of fats, oils and grease (FOG) on the surface in a scum layer. While performing this function, the mixer must avoid adding oxygen, since the presence of oxygen will cause PAOs to switch to metabolizing stored phosphates instead of taking up soluble, readily degradable BOD. Likewise, in anoxic tanks, oxygen will cause aerobic organisms to gain advantage due to their faster growth, and nitrates will not be utilized. There are two prevalent mixer designs to consider: submersible and surface mixers. Each has its own features. Submersibles: worth the hassle? Submersible mixers are currently the more prevalent of the two designs. They can be mounted on a wall bracket with a hoist to be raised for service. Submersibles are typically low-speed mixers with an integral gearbox and motor assembly. Aqua-Lator® Direct Drive Mixers are an ideal solution for anaerobic and anoxic tank mixing in wastewater treatment applications. Submersibles are available with either standard or large impeller blades. The small-blade impeller is typically lower in cost. The large blade impeller is more efficient, and comes at a higher cost. When evaluating these two submersible options, it’s important to understand energy costs. Often, the energy savings of a large impeller mixer may not actually offset its higher lifetime cost. Submersible mixers can be set up to create either a clockwise or counter-clockwise liquid movement in the tank. This can require more than one mixer in many tank configurations. Care must also be taken to prevent short-circuiting the mixed liquor. This can occur if a weir inlet and outlet are used, as is often the case when an existing aeration basin is repurposed for mixing. COMPARISON OF MIXER TYPES Motor Cooling Required Accessories Mixing Efficiency Wall Direct Bracket Direct Drive Floating Surface mooring cables Moisture Thermal None Moderate Easier Moderate Less frequent Medium Medium Tank floor or guide posts Moisture Thermal More frequent Compact Submersible

A 3HP Aqua-Lator® Direct Drive Mixer installed in a 45 ft. diameter. by 9 ft. deep tank shows that 0.5 m/s are prevalent throughout the tank, far exceeding the 0.2 m/s velocity typically recommended to keep activated sludge in suspension. In addition to configuration concerns, submersibles have maintenance needs. Their low-speed operation requires gear reduction, which means additional parts that can fail. They’re also vulnerable to both leaks and overheating, and typically require moisture and thermal sensors to protect them. If the area around a submersible becomes covered with rags or stringy...