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Static mixers are used in water and waste water technology for variousprocessing steps, e.g. the oxygen enrichment of drinking water. The results of a process are strongly influenced by the mixing. The increasing requirements on the water quality demand improvements in the cleaning process.
With increasing frequency, static mixers are usedfor these tasks due to the advantages of this mixing technology. Static mixers have no moving parts, resulting in a practically maintenance-free function; they offer calculable homogeneity, an energy-efficientconcept and a favourable price/performance ratio. Thanks to the closednature of the static mixer all forms of contamination during the process are excluded.
In order to ensure optimum flocking, the necessary flocculants must be mixed quickly and evenly into the entire water flow. Static mixers achieve this with their determinable and shearing conduct. Due to the high flock density, the clarification basis canbe designed smaller and as the flocculation takes place completely inthe mixer; neither expensive flocculation basins nor dynamic mixers with their switching devices are required. Local excess concentrationsof flocculants are prevented through the even distribution.
Inorder to be able to develop optimum effectiveness, the highly viscousparent solution of the auxiliary flocculant must be diluted with 10-100 times the amount of water prior to adding it to the slurry. As theparent solution usually has a very high viscosity, it does not dissolve spontaneously in water. More intensive mixing is necessary because the more evenly the polymer is spread in the diluting water the higherits activity. The mixing in of the auxiliary flocculants into the slurry prior to dehydration is an important step that has a fundamental influence on the economic efficiency of a procedure. As a result of thecomplete mixing 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.
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. Asa result, voluminous neutralisation basins can be greatly reduced or even omitted completely.
In order to achieve more favourable mixing ratios, the neutralisation agents 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.
Disinfectants or fluoride are mixed into the drinking water in small quantities at the end of the water processing, prior to feeding it 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 mostcases there is a risk of heavy lime dispersion through local over-alkalisation, something that can lead to blockages over time. This symptomcan be reduced if the concentrated soda lye (30 or 50%) is diluted toa content of 2% with softened water prior to addition. This pre-dilution is performed in a small Engineering mixer from 1.4571 or PP, thatcan 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 waterflow 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 breaksup the air into fine bubbles of 1 to 2 mm diameter. The result is a large interphase 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
Upon giving off intothe mains, drinking water must have an oxygen content of at least 5-6mg 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 enrichedand then mixed in with the main flow. For this, 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 itselfas contact device for this task. It ensures intensive contact betweenthe 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.
The bye-pass flow arrangement shown in the illustration has proven itself above all in cases where a main waterflow has to be treated whose throughput fluctuates greatly. The bye-pass flow mixer is operated under constant conditions. As a result, a high ozone utilisation is achieved even with a low overall throughput.The mixer in the main flow can also be relocated to the dwelling timebasin as is the case with the oxygen enrichment of drinking water in the reservoir. Through suitable measures an extension of the contact time between the ozone-containing gas and the water is also possible soas, in this way, to achieve an even greater ozone utilisation. This isparticularly advantageous with water with a high ozone content. Since, in the mixer, the entire inflowing water current is obligatorily brought into contact with ozone, topped dwelling time basins can be dimensioned smaller. They must only be designed for the actual reaction time and not also for the slow mixing process in the basin.
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