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A sewage treatment plant in Oslo has tested the "Kaldnes process" and achieved excellent results. Since a dry-mounted mixer was replaced with a Flygt mixer, denitrification in the anoxic reactors was increased from 30% to over 80% and energy costs were reduced.
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The design of the plastic element, with its uneven, corrugated surface provides an extensive area for bacterial growth. The bacteria adhere firmly to it, while, at the same time, obtaining a good contact with the waste water.
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For a long time Kaldnes Miljøteknik have been operating an experimental facility at the Bekkelaget treatment plant in Oslo. Full-scale tests have been carried out with the Kaldnes process, an especially compact form of biological sewage treatment. The Kaldnes process is, therefore, ideal for increasing capacity at plants which are compact and lack space. The process uses small, 10 mm plastic elements in such large amounts that they fill two thirds of the contact basin. The design of the element, with its uneven, corrugated surface provides an extensive area for bacterial growth. The bacteria adhere firmly to it, while, at the same time, obtaining a good contact with the waste water. Because the plastic material is somewhat lighter than water the elements float, but only just.
Currents in the water draw the elements down to fill the whole tank volume. Consequently, mixing is essential if the bacteria are to carry out their cleansing task. In aeration zones, mixing is ensured by the air rising from bottom diffusers. In anoxic zones mechanical propeller mixers have to be installed.
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Disapointing beginning
The design of the plastic element, with its uneven surface provides an extensive area for bacterial growth. The bacteria adhere firmly to it, while, at the same time, obtaining a good contact with the waste water.
The first results, which were achieved during the spring of 1992, were rather a disappointment: 25 to 30% nitrogen reduction after passage through two tanks in series. It was, however, obvious where the fault lay. The dry-mounted, mixer units, with 4 kW motors and double, 900 mm propellers, were not effective enough; layer on layer of plastic elements were floating on a more or less still surface. An underwater camera confirmed that the mixing effect was insufficient. The supplier of the mixers explained that a significantly more powerful drive unit, of 15 kW, would be necessary to achieve satisfactory mixing. Such a solution was, however, completely out of the question. A four-fold increase in energy costs would completely ruin the project.
Enter Flygt
During April the question of mixing was discussed with Flygt's experts who presented their unconventional mixing concept. The use of submersible units enables each mixer to be placed optimally. This usually ensures a great improvement in the mixing effect. After complex discussions on the exact mixing requirements for this process, it was decided that the potential for using a submersible mixer was good. This assessment was made from the prevailing circumstances and the advantages which were expected to be achieved through the better placement of mixers which submersibility provided. A mixer with a 1.8 m propeller was suggested. (In such cases, where new processes are being tested, information on the exact circumstances is always lacking because there isn't any earlier experience of the extreme conditions which prevail.) In the end, it was decided to install a Flygt mixer in one of the two anoxic zones. If its effects did not meet requirements, then it would be returned and its purchase price would be refunded. It was stipulated that a deviation of no more than 10% from the average element density should be permitted among 10 density measurements taken from random positions in the tank, but excluding extreme points such as corners and the surface. The measurements were to be taken a month after the installation.
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Right:
The use of submersible units enables each mixer to be placed optimally. This usually ensures a great improvement in the mixing effect.
The result
This type of application is not really suitable for dry-mounted mixers. Their installed capacity cannot be used optimally and they must, therefore, be run inefficiently. If, on the other hand, mixers can be placed optimally (which is usually possible with submersible mixers) then their capacity can be fully exploited and complete mixing, at a significantly lower power consumption, will be achieved. Precisely this effect was demonstrated at the Bekkelaget treatment plant; with the Flygt mixer, operational costs sank thanks to the smaller motor, and the mixing effect was improved. In addition, nitrogen reduction rose from 25% - 30% to 70% - 80%. The measurements of the spread of the elements proved, in practice, quite difficult to carry out. They also indicated that a relatively good distribution of elements had been achieved, but that closer to the bottom there were signs of gaps between groups of elements and that at the surface there was a lack of movement. The deviation of 10% was probably over-stepped. An increase in motor capacity was discussed, but the process results and a tracer element analysis indicated that only minor advantages could be expected. It was therefore decided to stick with the originally selected capacity. Following this the other dry mounted mixer was replaced by a Flygt mixer.
The Kaldnes process is now being discussed in connection with many projects throughout Europe, for example in Germany and France.
Lillehammer, in Norway, is equipping itself for the Winter Olympics, which will make the town world famous. The Kaldnes process has been chosen for the extension of Lillehammer's sewage treatment plant and this is a direct result of the experiment at Bekkelaget. It's a 70,000 P.E. plant where 10 Flygt units are required for the mixing in the denitrification process.
There is no doubt that the Kaldnes process has come at the right time. There is currently intense interest in nitrogen reduction. In Norway alone it's expected that between 20 to 30 new plants will be required to comply with the agreements reached on nitrogen emission at the Convention on the North Sea.
The Kaldnes process
The process is based on bacterial growth on small plastic elements, which have a density close to that of water and which therefore can be suspended in waste water. The plastic elements can have different forms but the design which is most used currently consists of small sections of pipe, with an internal cross and a porous, uneven, corrugated surface. The length and diameter of the element is approximately 1 cm. This form provides a large surface area which is partially shielded against the circulation erosion which occurs in the treatment tanks; an earlier version of the element had a smooth surface, but it was found that, because of the erosion, practically no bacterial growth could occur on the external surfaces. When the corrugated external surface was introduced, measurements revealed that the rate of nitrification increased by 25% thanks to the fact that the bacteria could establish themselves in the sheltered "valleys" on the outside of the elements.
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The Kaldnes process has been chosen for the extension of Lillehammer's sewage treatment plant and this is a direct result of the experiment at Bekkelaget. It's a 70,000 P.E. plant where 10 Flygt units are required for the mixing in he denitrification process.
The elements may take up between 50% to 70% of the tank volume, depending on the process being carried out. In a nitrification process their volume should be about 70%. This provides a total surface on the elements of about 450 m3 per m3. The process requires effective mixing in the entire basin. If several different processes are involved there will be a row of tanks in series. In order to prevent the plastic elements from being carried over with the liquid into the next tank, all of the tanks need to be equipped with self-cleaning, screening systems with openings of 5 mm. As mentioned earlier, the element design influences the speed of denitrification and so, to a large extent, does the oxygen concentration. In the aerobic reactors the correlation between the oxygen concentration and speed of denitrification is, more or less, linear, with a doubling of the denitrification speed when the oxygen concentration increases from 4 mg oxygen per liter to 6 mg oxygen per liter. Because the Kaldnes process aims to minimize space and equipment, it's preferable to operate it with as high an oxygen concentration as possible - consistent with a reasonable cost level. The anoxic reactors are operated with mechanical mixers at oxygen levels close to zero.
The dry-mounted, mixer units, with 4 kW motors and double, 900 mm propellers, were not effective enough; layer on layer of plastic elements were floating on a more or less still surface. 2. The use of submersible units enables each mixer to be placed optimally. This usually ensures a great improvement in the mixing effect.
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The extension concept when using the
Kaldnes bio-film-process.
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