The use of submersible mixers in STPs

When planning a new STP or upgrading an existing one, both investment and operating costs need to be taken into account. For this reason, it is particularly important to install equipment which uses minimal energy.

Mixers, whose motors run in the open air, often cause a high noise level. Submersible mixers, on the other hand, operate noise-free below the wastewater surface. If maintenance or repair work is required, it is not necessary to empty the tank. Special installation systems allow the mixing units to be raised or lowered even when the tank is full.

Selecting the right mixer

The job of selecting the correct mixer should not be underestimated. In contrast to pump selection where duty point and efficiency can be calculated accurately, the mixer selection process uses parameters which are based to a large extent on practical experience. Errors in mixer selection or mixer location, as well as incorrect application of scaling-up rules can mean that the required result is not achieved, or the mixing unit selected operates using too much energy.

ABS Pumps has developed a computer assisted procedure for the selection of submersible mixers in STPs. Using this software it is possible to determine quickly and reliably the most economical mixer models, best suited for the different shapes of tank. This selection process, which is based on operating experience and results from a number of practical installations, allows optimal selection of the submersible mixing unit for a particular task.

Blending and suspension

This method of selection is used where the two most important tasks of a mixing unit, i.e. homogenisation and suspension, must be achieved.

The type of propeller influences mixing results. Simply supplying data on the volumes to be mixed in conjunction with the basic task is generally of little use since ABS engineers cannot derive any information on the expected mixing result. For example, it is possible that a propeller shape that is optimal from a hydraulic point of view and capable of mixing large wastewater volumes produces poorer results than a less ideally shaped propeller, or a propeller which causes some turbulence in the flow.

When mixing the contents of a tank using a horizontal propeller mixer, it is generally found that a homogeneous flow shape is not achieved. Together with the basic flow formation, a flow pattern showing various degrees of turbulence is obtained. Floor velocity values often specified for mixing in activated sludge tanks cannot be reliably measured using conventional measurement technology since the flow at a measuring point continuously changes direction. Typical applications at an STP are:

• the homogenisation of sludge to make it easier to pump;
• mixing of the biomass in an activated sludge tank.

Flow velocities in closed channels

In closed channel systems where horizontal flow velocities must be achieved together with simultaneous mixing or circulation, a calculation method is used which is based mainly on the impulse theory.

To prevent depositions and intensive mixing of the sludge flocculent with the fixed and soluble materials in the sewage in activated sludge tanks, it is necessary to maintain flow velocities close to the tank floor in the region of 0.15-0.30m/sec. In contrast to the case of pure mixing in a tank, conventional velocity measuring methods provide relatively good average velocity figures. The following parameters must be considered in the calculations:

• tank geometry;
• the surface conditions of the tank;
• resistance factors for the different types of structures used to cause a change of flow direction, e.g. baffle walls;
• friction losses due to structures in the tank such as aeration nozzles and surface aerators;
• the physical characteristics of the sewage to be mixed; and
• operating experience on existing installations.

Positioning the mixers

In addition to the selection of the mixers themselves, the correct positioning of the units is also important. Depending on the location of the mixers — especially in the case of tanks designed solely for mixing – different flow patterns may be formed which give different results.

Fast or medium running mixers, because of their small propeller diameter, are relatively insensitive to the turbulent flow pattern produced in tanks. Larger propeller units, on the other hand, can quite quickly experience uneven running just caused by the normal type of turbulence present in a tank.

Mixers should never be located directly in the curve of the channel, as a change in flow direction causes turbulence and vortex formation in this area. This turbulence, if it reaches the working area of the propeller, will cause unnecessary vibrations which could reduce the operating life of the unit. Practical experience has shown that the minimum distance behind the channel curve is approximately three times the propeller diameter. The best position is in the first third of the straight section of the channel.

If this tank is aerated and if the mixers continue running during aeration, then they must be located as far as possible from the aeration system. If the direct aeration zone extends into the propeller area, this may also result in vibrations which can reduce the life of the unit. Practical experience has shown that the minimum distance from the aeration system should be two to three times the propeller diameter.

Horizontal/vertical mixing

With horizontal mixing it is possible to achieve the required energy input in a large tank by installing a number of mixers one after the other in different locations. That also applies to a tank designed for pure mixing.

In the case of small to medium tanks, vertical mixing is one of the best solutions because of the need to produce a stable flow pattern. Conventional long-shaft mixers have traditionally been the norm, but the introduction of floor mixing units a few years ago filled a gap in the market.