The right mix

In process water treatment, the chemical additives used - for example, lime, activated carbon, flocculating agents and so on - are best added to the water stream in the form of a solution or slurry.

This requires a premixing stage, the equipment for which can vary with the duty. For example, most additives can typically be made up in smaller tanks that are well suited to portable or fixed-mount type mixers. On the other hand, lime and activated carbon slurries are normally made up in vessel sizes that need the larger drives provided by top-entry mixers.

This form of mixing is essentially a batch operation, but where coagulants and other chemicals are continuously added to the water stream, flash mixing or rapid mixing basins may be called for. Here the vessel sizes only need to provide a very short retention time.

The mixing requirement here is to provide sufficient blending within the retention time to ensure that liquid, gaseous or solid chemicals are dispersed instantaneously into the stream. As a result, the power input, known as the G Factor, for the high efficiency impellers required is not the only issue to be considered. The hydraulic flow characteristics of flow and shear must be also taken into account.

In the flocculation process, on the other hand, the essential purpose of the mixing operation is to provide hold-up time to permit reaction of the flocculating agents added into the rapid mix. The contents of the basin thus need very gentle blending in order to promote contact and coagulation of the suspended solids so as to produce particles of a size large enough to be removed by DAF (dissolved air flotation), sedimentation or filtration systems.

Chlorine, added either in gas or liquid form following the removal of solids via sedimentation or filtration, can be easily dispersed using a small portable or top-entry mixer. An alternative approach would be to employ a static in-line mixer. This is a very inexpensive method for producing chlorine uniformity in the water stream at typical flow rates.

Whilst good blending is of major importance, high fluid shear levels do tend to break down either the flocculation particles or the floc. This makes it necessary to find a mixer that gives high levels of mixing efficiency at low speeds so as to create low shear and high flow. For example, the Lightnin EW Series from SPX Process Equipment was developed to meet the demands of high viscosity blending, process reactor operations, high-speed flash mixing and low-speed sludge thickening.

Suitable for both open and closed tanks, this type of mixer is highly flexible in that its wide range of output speeds provides a variety of torque options to meet all mixing requirements. Fitted with a Lightnin A510 axial flow impeller, it can provide the same flow and process results at a lower power than other axial flow impellers and at lower capital and operating costs.

Moving on now to wastewater treatment, the many process stages requiring mixing can be covered by the use of fixed top-entry and floating mixers. In anoxic and anaerobic zones, mixers have to maintain sufficient flow to prevent the biomass from settling and eliminate short circuiting to maximise the effectiveness of the system. This calls for high efficiency impellers with low shear and turbulence.

Sludge digesters also need mixers that have been selected for the specific application. Those having a low power per unit volume will ensure that the digester contents are fully mixed and temperature gradients and gas pockets are eliminated. The mixing process will produce greater quantities of gas and a sludge content that is far easier to handle. A major advantage of this approach is that the retention time in completely mixed digesters can be reduced to between 7 and 10 days, compared with 30 days for non-mixed units.

Where equalisation basins are concerned, floating mixers contribute to the dampening of pH fluctuations, temperature changes and concentration gradients, thereby allowing downstream processes to be designed for average rather than peak load conditions.

The process of oxygen transfer - aeration - is dependent on mixing efficiency for its success and can be carried out using either surface or submerged turbine aerators. However, these use opposite approaches to the same mass transfer problem. As in any gas/liquid application, mass transfer is dependent on bringing the gas and liquid phases into intimate contact. The transfer of oxygen from the gas phase to the liquid phase takes place, until equilibrium is reached, across the interface between the two phases and the greater the area between the phases, the greater the mass transfer rate.

Submerged turbine (and diffused air) devices - such as the Lightnin 10 and 500 Series mixers when used as mechanical aerators - create the contact between gas and liquid by dispersing air bubbles in the wastewater. Surface aerators do the opposite - the wastewater is the transported or dispersed phase that is brought into contact with the 'continuous' air phase.

The low-speed surface aerator is highly efficient at transferring oxygen in terms of total energy consumption. In a purpose designed basin, the high fluid volume produced by low-speed aerators provides both the mass transfer required for biological activity and the mixing and solids suspension for effective BOD reduction.

This approach also provides the opportunity for varying the oxygen transfer rate and power usage of individual units while maintaining the required mixing performance. Short term fluctuations in oxygen demand, resulting from waste strength or waste flow, can often be accommodated by using 2-speed motors or variable speed drives.

Surface aerators are now being widely used in deep basins in 'enriched air' treatment plants. In these covered basins, the surface aerator promotes optimum use of the added 'pure oxygen' without the need for expensive recirculation blowers to handle the corrosive wet oxygen gas stream.

In contrast, the mixing provided by submerged turbine aerators in deep basins can be far superior to that of surface aerators. The ability to maintain biological solids in suspension enables the use of deeper basins, making it possible to reduce land requirements while still achieving the desired wastewater retention time. This advantage can even offset the benefits of decreased power offered by surface aerators.

Top-entry mixers used as submerged turbine aerators offer the flexibility to tailor the total oxygen transferred to match changes in the incoming load. This is achieved by controlling the air flow, mixer speed or both.

The mixers can also be used simply as mixers without an air flow to provide the ideal conditions for anoxic zone treatment. This enables plants operating for alternate periods of gassed and un-gassed operation to increase overall efficiency of the treatment process.

For those wastewater treatment plants that have a very high pumping capacity and the capabilities for handling high loadings on a small site, deep tanks are of prime importance. Here draft tube aerators provide an ideal solution. By using a high efficiency axial flow impeller, air introduced through sparges close to the liquid surface can be driven down to the bottom of the tank.

The flow-controlling impeller, in conjunction with a draft tube arrangement, maintains turbulence and high velocities through the draft tube for optimum contact in the oxygen mass transfer operation. In addition, the high flow generated by the impeller maintains the biomass in suspension and promotes intimate contact between the waste, biomass and oxygen.

David Bannister is a product manager with SPX Process Equipment.