Achieving the right mix is not just about personal choice

Mixing in the water and wastewater industry is integral to treatment, allowing key subsequent processes such as aeration, digestion and disinfection to be more efficient and effective.

The four main duties of mixing are:
  • Suspension, which uses a combination of bottom shear stress - to get off-bottom mixing - and bulk flow velocity for homogeneous distribution of particles, also drawing down, from the surface, solids with a density lower than that of the liquid
  • Blending, the mixing of miscible fluids - usually either batch blending or blending in a through-flow tank
  • Circulation, which relates to the transport of matter or heat, while the purpose of destratification is to prevent the separation of liquids of differing densities
  • Destratification
As bulk flow - where the entire tank volume is in motion and therefore part of the mixing - is needed to deliver these four mixing duties, the mixing system must deliver the necessary bulk flow which in turn governs mixing efficiency.

Two of the most common methods are the use of submersible mixers and jet-mixers. On the submersible solution the bulk flow is driven by the mixer's propeller while on the jet-mixer the bulk flow is driven by an external source, usually a pump.

The bulk flow is powered by the thrust (momentum flux) from either the submersible or jet-mixing unit. Via entrainment of the surrounding liquid into the jet the total flow increases to ideally include the entire tank volume.

Many applications can, in theory, be adequately served by either method, so it often comes down to the specifier's personal choice as to which system is installed.

However, there are frequently substantial benefits achievable in terms of performance, efficiency and potential cost savings by adopting a flexible, open-minded approach.

The use of a submersible mixer is the longer established solution, offering generally consistent performance alongside high efficiency, typically using around two-thirds less power than externally driven jet mixers. Submersible mixers deliver highly efficient thrust transfer, and offer great flexibility in terms of positioning for optimum mixing performance.

In some instances, the submersible mixer can be incorrectly specified and may not have sufficient thrust to generate the requisite level of bulk flow for fulfilment of the necessary mixing duties. Also, submersible mixers can be fitted with a jet-ring to increase the thrust generated - care should be taken here that the media is not too bound with rag as this can cause blockages when jet-rings are present.

The servicing of submersible mixers also needs to be considered in the provision of adequately rated lifting equipment to suit the local health and safety regulations.

There are now several methods of retrieval available to the market such as the blue rope system or the removable lifting system.

The use of an externally mounted pump to drive a jet-mixing system has been developed as an alternative to the submersible mixer. The main purpose is to reduce manual handling as the pump is accessible without the need to have to enter the main body of the tank. Due to the nature of the mixing system there are also no moving parts within the tank.

With the leading systems on the market, primary flow is created through the discharge nozzle, which induces secondary flow for increased thrust. However, power consumption will be greater due to the need to draw the media out of the tank to pump it back in via the pipework arrangement.

Installing a system that does not deliver sufficient bulk flow will have major ramifications - full re-suspension of solids will not be achieved. This will lead to sedimentation and deposition within the tank and hence a reduction of its working volume. The implications of this are reduced storage time and the process issues this leads to.

Over-specifying will result in excessive use of power for the given mixing scenario and hence inefficiency from excessive power use and increased wear. In order to achieve the correct mixing system the supplier should tailor their offer to the customers' requirements in terms of the correct system for the application. Within the system appraisal the client should be aware of all the products available to them to deliver the optimal whole life costs - including installation, maintenance and running costs; these should be considered rather than just the initial purchase cost.

As mentioned, there are major differences in power consumption between the two system types, while it will also be necessary to accurately calculate the correct size of mixer. For this, data on tank geometry, liquid rheology and mixing duties - bulk flow velocity, shear stress, yield stress and mixing time - are used to calculate the required mixer thrust.

A new ISO Mixer Testing Standard (ISO 21630:2007) provides internationally recognised criteria for evaluating mixer performance, which is expressed in terms of produced thrust, consumed power and the ratio between the two. Any mixer supplier should be asked to provide these figures to allow customers to accurately compare the performances of different mixers.

Other considerations should be borne in mind, such as rag levels, for instance. This will lead to the correct mixer selection for the application. Also, the system will only perform as well as it is maintained and the cost of this maintenance, including spares cost and lost process time due to long delivery times must also be accounted for.

Tony Price is market manager - sludge & wastewater at ITT Water & Wastewater.
T: 0115 940 0111.

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