Static mixers in drinking water and waste water processing

Static mixers are used in water and waste water technology for various procedural steps, such as for example for the oxygen enrichment of drinking water. The result of a procedure or of a process is influenced to a fundamental extent by the mixing. The increasing requirements on the water quality require improvements in the cleaning process. Increasingly frequently, mixers are used for the mixing tasks that allow many advantages through the use of this type of mixing technology e.g.

• No moving parts, practically maintenance-free function
• Calculable homogeneity
• Energy-efficient concept
• Closed nature of the static mixer through atmospheric conditions excludes all forms of contamination during the process.
• Favourable price/performance ratio
• Mixer elements can be designed removable

Application examples from practical reality during water processing

Mixing in of flocculants
In order to ensure optimum flocking, the necessary flocculents must be mixed quickly and evenly into the entire water flow. These requirements are ideally fulfilled by the static mixer thanks to its determinable and even shearing conduct. Thanks to the high flock density, the clarification basis can be designed smaller. As the flocculation takes place completely in the mixer, neither expensive flocculation basins nor dynamic mixers with their switching devices are required. Local excess concentrations of flocculants are prevented through the even distribution, as a result of which costs are saved.

In-line thinning of auxiliary flocculants
In order to be able to develop optimum effectiveness, the viscous parent solution of the auxiliary flocculant must be diluted with 10-100 times the amount of water prior to adding to the slurry. As the parent solution usually has a very high viscosity, it likewise does not dissolve spontaneously in water. More intensive mixing up is therefore necessary because the more evenly the polymer is spread in the diluting water, something that is ensured by the use of a static mixer, the higher its activity. The result is reduced consumption of auxiliary flocculants and thus a considerable reduction in costs.

Mixing in of auxiliary flocculants into the slurry prior to dehydration
The mixing in of the auxiliary flocculants into the slurry prior to dehydration is an important procedural step that has a fundamental influence on the economic efficiency of a procedure.
As a result of the complete and simultaneously caring mixing up in the P-E mixer, the consumption of flocculants can be reduced and/or the dry substance content of the dehydrated slurry can be increased. Static mixers are used successfully in all classical dehydration methods.

pH value regulation
As a rule, neutralisation reactions run off completely within a short time, provided the transport of the reaction partners within the liquid flow is quick enough. With the in-line neutralisation, this task is assumed by the static mixer and enables the performance of the neutralisation reaction directly in the flow pipe. As a result, voluminous neutralisation basins can be greatly reduced or even omitted completely.
This space and energy economising solution is increasingly used in water processing installations. Not only basins can be saved, reduced energy costs and savings in maintenance work of dynamic stirring organs are a relief on operating costs.
In order to achieve more favourable mixing ratios, the neutralisation agents (acids, lye, milk of lime) are pre-diluted directly in line in a further small, static mixer prior to addition.
With static mixers, representative measured value recording is also ensured, an important precondition for regulation functioning reliably at all.

Mixing in disinfectants
Disinfectants or fluoride are mixed into the drinking water in small quantities at the end of the water processing, prior to feeding into the mains. In order to achieve the desired effect, these must be distributed evenly and, with chlorine-containing agents, as quickly as possible into the entire water flow.

Deacidification with soda lye
If water is deacified through the addition of soda lye then in most cases there is a risk of heavy lime dispersion through local over alkalisation, something that can lead to blockages over time. This symptom can be reduced if the concentrated soda lye (30 or 50%) is diluted to a content of 2% with softened water prior to addition. This pre-dilution is performed in a small U & A Process Engineering mixer from 1.4571 or PP, that can, for example, be integrated directly on the dosing point of the main mixer.

Physical deacidification with air
Aggressive water can be physically deacified through contact with air. In the static mixer the water flow is brought into intensive contact with as much air as possible at as low an over pressure as possible. As a rule, the water is contacted with three to four times the amount of air. The static mixer breaks up the air into fine bubbles of 1 to 2 mm diameter. The result is a large interphase of several thousand m2/m3 that is important for the mass transfer. The equilibrium adjusts itself more or less completely. With static mixers, a degree of reduction of the free carbonic acid by the factor 2.5 is achieved.

Oxygen enrichment of drinking water through bye-pass flow gasification
Upon giving off into the mains, drinking water must have an oxygen content of at least 5-6 mg O2/l. Consequently the processed water must frequently be treated with pure oxygen.
A small amount of oxygen must be completely dissolved. For this it is advantageous for just one partial flow to be enriched and then mixed in with the main flow. For this, U & A mixers from high-grade steels and synthetic materials are used (see the following illustrations).
In-line ozonisation in the bye-pass flow
The generation of ozone is expensive, consequently as complete an absorption as possible is striven for.
In practice the P-E mixer has proven itself as contact device for this task. It ensures intensive contact between the water and the ozone-containing gas. Given the correct design, a degree of utilisation of 90 to 99% of the physically possible value is achieved with this device.

Illustration: Static mixer with bye-pass flow mixer

U+ A Process Engineering GmbH
Auf der Engelhardt 29
D-51674 Wiehl / Germany