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Highlights 2014 F & S International Edition No. 15/2015 27 1. Introduction The application of the two methods, otation and membrane filtration, for the treatment of liquids with suspended or emulsified substances is not novel. What is new, however, is the skilful combination of the two methods in one container so that they complement each other perfectly (Fig.1 and 2). Conventional Dissolved Air Flotation (DAF) was replaced through an Induced Gas Flotation (IGF) with rotating ceramic disks generating microbubbles in the range of DAF. The energy require- ments necessary for the flotation could thereby be clearly reduced (up to 90%). The optimised arrangement of the ceram- ic disks used for the flotation and the ceramic membranes used for the vacuum ltration prevents that the substances dis- charged by flotation hinder the filtration. The fouling of the ceramic membranes is thereby clearly reduced. The process can be implemented in technically interesting sizes in a container construction method. The combination of both methods in one container goes hand in hand with simpli- ed plant engineering and considerable space savings. Through the flotation, ne particles, emulsified drops and colloid organic sub- stances are continuously transported to the water surface of the tank together with the gas bubbles. The ceramic membranes are arranged in an area in which the content in colloid substances is clearly reduced on account of the flotation. The membrane stage can thereby be operated for a longer time span with a low pressure difference without cleaning. Every now and then, an effective backwashing is possible. The „akvoFloat“- technology was exam- ined and tested for water treatment with a pilot plant under realistic conditions with the water from a ship channel in Berlin over several months. In the course of this, the effect described on top was demon- strated. Without switched on flotation, a clearly reinforced fouling of the ceram- ic membranes was observed. It can be expected that the data determined with this test series are also achieved with bigger plants concerning the precipitation and the specific throughput performance for the examined application. 2. Description of the pilot plant The pilot plant of akvola Technologies has a capacity of at least 12m 3 /d. Key element is a 300 l tank (B2), in which occulation, otation and filtration take place. Fig. 1 shows the plant flow scheme. After passing through a 300 μm sieve for the removal of coarse sand and other big particles, the raw water arrives in the feed tank B1. Before tank B2, the floccu- lating agent FeCl3 (Iron(III)chloride) is added turbulently inline, in the subsequent occulation track, the flocs grow further under addition of a flocculation aid (poly- acrylamide) for stabilisation. The floccula- tion time amounts to less than 10 minutes. The ocs and suspended matter come in Flotation-filtration-process with ceramic membranes for water treatment J. Ludwig*, M. Beery*, L. León*, S. Ripperger ** The process developed by akvola Technologies, Berlin, for water treatment that is offered under the name of “akvoFloat” includes a combination of a flotation where fine gas bubbles are generated by means of rotating porous ceramic disks and a microfiltration or ultrafiltration with submerged ceramic membranes. Both methods are arranged in one single open container. With the akvoFloat pilot plant, which was designed for a throughput of 12 to 20 m³/d, pilot testing was and is taking place. The results of an application for the treatment of surface water are presented in this paper. Other possible applications of the process combination are described. *Dipl.-Ing. Johanna Ludwig Dr.-Ing. Matan Beery Dipl.-Ing. Lucas León akvola Technologies Fasanenstr. 1 10623 Berlin Tel. +49 30 314 75656 Fax +49 30 314 26915 Mob. +49 163 3617076 [email protected] ** Prof. Dr.-Ing. Siegfried Ripperger Institut für Mechanische Verfahrenstechnik, TU Kaiserslautern Fig. 1: Plant flow diagram of the pilot plant Flotation unit Filtration Membrane Fig. 2: Image of the pilot plant (viewed from the side)

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Highlights 2014

F & S International Edition No. 15/2015 27

1. Introduction

The application of the two methods, fl otation and membrane fi ltration, for the treatment of liquids with suspended or emulsifi ed substances is not novel. What is new, however, is the skilful combination of the two methods in one container so that they complement each other perfectly (Fig.1 and 2). Conventional Dissolved Air Flotation (DAF) was replaced through an Induced Gas Flotation (IGF) with rotating ceramic disks generating microbubbles in the range of DAF. The energy require-ments necessary for the fl otation could thereby be clearly reduced (up to 90%). The optimised arrangement of the ceram-ic disks used for the fl otation and the ceramic membranes used for the vacuum fi ltration prevents that the substances dis-charged by fl otation hinder the fi ltration. The fouling of the ceramic membranes is thereby clearly reduced. The process can be implemented in technically interesting sizes in a container construction method. The combination of both methods in one container goes hand in hand with simpli-fi ed plant engineering and considerable space savings.

Through the fl otation, fi ne particles, emulsifi ed drops and colloid organic sub-stances are continuously transported to the water surface of the tank together with the gas bubbles. The ceramic membranes are arranged in an area in which the content in colloid substances is clearly reduced on account of the fl otation. The membrane stage can thereby be operated for a longer

time span with a low pressure difference without cleaning. Every now and then, an effective backwashing is possible.

The „akvoFloat“- technology was exam-ined and tested for water treatment with a pilot plant under realistic conditions with the water from a ship channel in Berlin over several months. In the course of this, the effect described on top was demon-strated. Without switched on fl otation, a clearly reinforced fouling of the ceram-ic membranes was observed. It can be expected that the data determined with this test series are also achieved with bigger plants concerning the precipitation and the specifi c throughput performance for the examined application.

2. Description of the pilot plant

The pilot plant of akvola Technologies has a capacity of at least 12m 3/d. Key element is a 300 l tank (B2), in which fl occulation, fl otation and fi ltration take place. Fig. 1 shows the plant fl ow scheme.

After passing through a 300 μm sieve for the removal of coarse sand and other big particles, the raw water arrives in the feed tank B1. Before tank B2, the fl occu-lating agent FeCl3 (Iron(III)chloride) is added turbulently inline, in the subsequent fl occulation track, the fl ocs grow further under addition of a fl occulation aid (poly-acrylamide) for stabilisation. The fl occula-tion time amounts to less than 10 minutes. The fl ocs and suspended matter come in

Flotation-fi ltration-process with ceramic membranes for water treatmentJ. Ludwig*, M. Beery*, L. León*, S. Ripperger**

The process developed by akvola Technologies, Berlin, for water treatment that is offered under the name of

“akvoFloat” includes a combination of a fl otation where fi ne gas bubbles are generated by means of rotating porous

ceramic disks and a microfi ltration or ultrafi ltration with submerged ceramic membranes. Both methods are arranged in

one single open container. With the akvoFloat pilot plant, which was designed for a throughput of 12 to 20 m³/d, pilot

testing was and is taking place. The results of an application for the treatment of surface water are presented in this

paper. Other possible applications of the process combination are described.

* Dipl.-Ing. Johanna LudwigDr.-Ing. Matan BeeryDipl.-Ing. Lucas León

akvola TechnologiesFasanenstr. 110623 BerlinTel. +49 30 314 75656Fax +49 30 314 26915Mob. +49 163 [email protected]** Prof. Dr.-Ing. Siegfried RippergerInstitut für Mechanische Verfahrenstechnik, TU Kaiserslautern

Fig. 1: Plant fl ow diagram of the pilot plant

Flotation unit

Filtration Membrane

Fig. 2: Image of the pilot plant (viewed from the side)

28 F & S International Edition No. 15/2015

Highlights 2014

contact with the induced micro-bubbles in the fl otation zone and rise together to the water surface, where a fl otate layer forms. The bubbles are generated through ceramic diffusers with an admission pressure of the air of 2 bar and a fl ow rate of 50 Nl/m3 in the water. The average bubble diameter results at approx. 50μm. The fl otate layer is continuously hydraulically removed from the tank. Below this, there are submerged plate membranes from aluminium oxide with a pore size of 0,2μm. With a frequency-controlled gear pump (P2), the permeate is drawn off from the tank at constant permeate fl ow via the membrane surface and is collected in the permeate tank B3.

For the backwashing of the membrane the rotation direction of the pump P2 is reversed. The transmembrane pressure (TMP) will be continuously measured with a pressure transducer while the volume fl ows are measured with a rotameter and/or magneti-cally induced fl ow meter. Pump P3 is used for the draining of the tank and for the removal of the residues after a backwashing. The maximum fi ltration-TMP amounts to 0,5 bar, when exceeded, a backwashing is automatically initiated. The backwash effective-ness can be increased via an air scouring system. A backwash lasts 2 minutes as a rule, at a pressure of 2.1 bar and a volume fl ow of 1.2 m3/h.

The specifi c permeate stream is standardised with the following equation concerning the temperature, in order to guarantee compa-rability between the tests and with other applications /1/.

The temperature corrected specifi c fi ltrate stream (in l/m 2h) is determined according to the above equation with the actually measured specifi c fi ltrate stream (l/m2h) and the water temperature (T in °C).

3. Tests with the pilot plant at the

Landwehrkanal in Berlin

In summer, 2013, tests were carried out with the pilot plant described above at the Berliner Landwehrkanal. Here, in particular the fouling behaviour and the cleanability of the membranes were examined. Fouling is caused through the forming fi lter cakes on the membrane plates of the fi ltration stage. Particles of a size smaller than the membrane pores, deposit on or in the pores at the beginning of the fi ltration. Bigger ones cannot penetrate into the pores and, rather, they deposit on the surface. With a nearly steady permeate stream, both lead to an increase of the trans-membrane pressure (TMP). The objective was to keep the TMP increase as low as possible. The test planning is based on a Directive of the American Membrane Technology Association (AMTA) /2/.

3.1 Water parameters of the raw water

The water of the Landwehrkanal was characterized for the period of the long test duration by strong fl uctuations in the total carbon (TOC, Total Organic Carbon) and in the turbidity. The low fl ow velocity in the duct (ca. 10cm/s) and the low duct depth of 2 m contribute to summery algae blooms. In addition, the duct is used with strong rain as a relief for the municipal sewage network. Nearby the water extraction point, there was a well-used sluice, which signifi cantly infl uenced the turbidity of the raw water. During the test period, the turbidity value was > 10 NTU. Fig. 3 respectively shows the averaged over one month values of temper-ature, pH value and TOC.

3.2 Infl uence of fl otation

At the beginning of the test series, an operation without fl ota-tion and merely with the fi ltration was set, so as to demonstrate the positive effect of the combination of both process steps. The TMP- development without fl otation at a fl ux of 112 l/m 2h is shown in green in Fig. 4. The fi ltration started at a TMP of -0.15 bar (vacuum pressure because of the out-in operation of the mem-branes). Without fl otation, the maximum TMP of -0.5 bar was already achieved after four hours. Within two hours a TMP of almost -0.3 bar was achieved that subsequently rapidly dropped. Subsequent backwashings showed no effect and the TMP could not be improved any more. This so-called irreversible fouling could be removed through a chemical cleaning of the membrane.

Fig.4: Time profi le of the transmembrane pressure difference when the plant is operating without (green) and with activated (blue) fl otation

Fig. 3: Values for temperature, pH and the content of „Total Organic Carbon (TOC)“, averaged over one month respectively

Temperature

Tem

pera

ture

[°C

]

Acquired data averaged per month

Tra

nsm

em

bra

ne p

ressure

[b

ar]

Filtration time [h:mm]

Fig.5: Process parameters and TOC removal during operation without (green) and with (blue) fl otation

Without fl otation With fl otation

Avg. TMP decline TOC removal Total recovery

Highlights 2014

F & S International Edition No. 15/2015 29

Under application of the fl otation, the temporal TMP devel-opment was substantially improved. This fact in itself is known in the literature, e.g. /3/. Then, in the test series, in particular the backwash strategy was examined and a two hour-cycle turned out as the best. As can be seen in Fig. 4 in blue, the TMP after two hours, at the same initial value, amounted to only -0.24 bar and the maximum-TMP was not achieved during the test period of 9 hours. The introduction of a periodical backwashing and the fl otation had a further positive effect. The plant could be operated with an approximately constant TMP of -0.28 bar and a steady fi ltrate stream. Periodical backwash in connection with the fl ota-tion prevent the formation of an irreversible blockage on and in the membrane and permit a quasi-continuous fi ltration operation.

The turbidity of the feed was always reduced by 95 %, some-thing that led to fi ltrate-values of 0.3-3 NTU. Fig. 5 summarises the positive effects of the fl otation on the total process. For, besides a more advantageous TMP development, the fi ltrate quality was also better, because small particles and organic matter were already removed before the fi ltration. That’s why the average TMP increase amounted to only 0,009 bar/h, in contrast to 0.022 bar/h. The TOC removal (Total Organic Carbon) increased from 32% to 40%. The major part was removed through the fl otate layer (see Fig. 6). In addition, the total water yield could be increased.

The air scouring of the membrane during a backwash was activated now and then in the form of an impulse to support the removal of the deposits on the membrane and to increase the back-wash effectiveness.

3.3 Chemical cleaning

After a certain fi ltration time, a chemical cleaning must be car-ried out since, in spite of regular backwashings, fouling appears during long operating times. The period of time until a chem-ical cleaning is necessary must be determined experimentally. Nevertheless, it should normally not amount to less than to less than 24 hours. For the chemical cleaning, two execution manners were developed: a) Chemical Enhanced Backwash (CEB) and b) Chemical Cleaning in Place (CIP).

Often a combination of both methods is carried out /4/, /5/. As chemicals either acid, bases or oxidizing agents can be used. Here a base, e.g. sodium hydroxide, removes primarily organic fouling and an acid, e.g. citric acid, inorganic (for example, iron deposits as a result of the fl occulant).

Fig. 7 shows the TMP course of 26 hours of fi ltration with a fl ux of 110-120 l/m 2h. Within this period, the modes of action of

different chemical cleanings were examined. After six hours of fi l-tration (Label 1), a combined CEB/CIP was carried out. Citric acid (2100 ppm) was initially introduced with a CEB, afterwards, a CIP with the same concentration took place (exposure time 120 min-utes). During the subsequent fi ltration, a strong TMP deterioration occurred, since the inorganic components were partially dissolved and therefore could penetrate into the membrane. Nevertheless, the previous starting-TMP could almost still be achieved in the sub-sequent backwashing. After another ten hours (Label 2), the fol-lowing were carried out: a CEB with Sodium hypochlorite (1000

Fig.6: Sludge layer in the tank during operation with fl otation

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Fig.7: Time profi le of the transmembrane pressure difference during operation of the plant with fl otation and chemical cleaning of the membrane

Tra

nsm

em

bra

ne p

ressu

re [

bar]

Filtration time [hh:mm]

30 F & S International Edition No. 15/2015

Highlights 2014

ppm free Cl), 30 minutes of CIP with citric acid. The subsequent TMP was good, both with regard to the initial value as well as with regard to the development. Seven fi ltration hours later (Label 3) a 60-minute CIP took place with citric acid, which included an intensive intermixing of the solution with the air scouring equipment. The effect was similarly good as with the second cleaning.

3.4 Summary of the test resultsThe operation of the pilot plant has

shown the high effectiveness of the fl o-tation as a preliminary process of the fi ltration in the respect that less fouling and a better yield could be achieved on the basis of reduced irreversible fouling. A periodical backwash (every 2 hours) and a fl ux of 110-120 l/m2h were optimum with the given conditions. In addition, a cleaning consisting of an alkaline CEB and an acid CIP has shown to be effective and has caused a complete cleaning of the membrane.

The test phase has shown that the akvoFloat- technology is suitable for the treatment of organically polluted surface water and that more than 60% of the organic matter is removed. The turbidity is removed almost completely.

4. Potential application areas of the “akvoFloat” technology

The application areas of the “akvo-Float” technology are varied and not lim-ited to the treatment of surface water. Hereinafter, fi ve potential application areas are described in more detail.

4.1 Pre-treatment of raw water for drinking water production by means of reverse osmosis

Nowadays, reverse osmosis is the preferential method for drinking water production from brackish water and sea water. In many countries of the earth, such plants are being operated and their number will continue to rise. The “akvoFloat” technology was originally designed for pre-treatment of sea water desalination via reverse osmosis (RO). This pretreatment of the raw water is necessary to avoid deposits of fi nest particles, colloids and microorganisms on the RO membranes and to guarantee a service life of the mem-branes of several years. The suppliers of reverse osmosis membranes substantiate their requirements for water pretreatment on the basis of empirical values that are to be kept to for the operation of the mem-branes. Also, the warranty in connection with the RO membranes is subject to the fulfi lment of the requirements.

Because the raw water quality can strongly differ in different locations and is also subject to strong temporal variations,

treatment processes are in demand that do justice to the highly fl uctuating inlet conditions. Hence, multistage processes are often implemented. The combination of the fl otation and microfi ltration within an akvoFloat plant is a multistage pro-cess in an especially compact construction method, suitable for application. The test results on the Landwehrkanal in Berlin show that the implemented treatment pro-cess with fl uctuating inlet conditions and with a high organic pollution level, as may occur, e.g., climate-induced with an algal bloom in public waters, guarantees a treat-ment of the raw water that is suffi cient for RO applications. Hence, it is suitable for the pretreatment of sea and brackish water for drinking water production by means of reverse osmosis.

4.2 Pre-treatment of raw water for pure and ultrapure water production by means of reverse osmosis

The steadily rising demands on quality for products have the consequence that in the industry the demands on qual-ity of process water often go beyond the requirements for drinking water. The supply of such pure and ultrapure waters is the main application area of reverse osmosis in the industrial countries that have suffi cient drinking water resources. Accordingly, reverse osmosis is increas-ingly used to meet the industrial demand for pure and/or ultrapure water. One gen-erally calls processed water pure water. The designation ultrapure water is used if, by means of treatment processes, foreign matters was extensively removed from the water. The foreign matter includes, among other things, minerals, dissolved organic matter, colloidal contaminants as well as micro-organisms and particles. Ultrapure water is required among other things in the pharmaceutical industry, the electronic industry and in power stations as boiler feed water. The softening of drinking water for households and municipalities is still more of a niche application. The application of reverse osmosis is necessary if the content in salts and further dissolved solids in the raw water exceeds the value allowed for drinking water or if pure and/or ultrapure water with a low content in dissolved solids is required. For the prepa-ration of ultrapure water, additional pro-cesses are usually still applied upstream of the reverse osmosis, such as for example ion exchangers for the separation of the remaining ions, an ultrafi ltration for the further reduction of the colloidal sub-stances content and a microfi ltration on the “Point of use” for the guarantee of a very low particle quantity. In this process chain, application possibilities arise in the industrial sector for akvoFloat plants for

the pretreatment of raw water for the pure and ultrapure water preparation by means of reverse osmosis.

4.3 Treatment of process water and waste water for reuse

The industry uses a large part of the available water resources among other things as cooling water as well as for sol-vents and as cleaning agent. In general the trend towards water recycling exists with the objective to reduce the environmental impacts and to increase the cost effi cien-cy. Depending on local regulations, fresh water fees and waste water fees can be reduced. In the case of membrane pro-cesses, an increased water quality often also means improved product quality. The separated substances can also often be pur-posefully supplied again to a utilisation. Hence, many industrial fi rms have dras-tically reduced their fresh water demand during the last years by a recycling of the process waters. These include, for exam-ple, the paper mills, the breweries and food processing operations. Also in met-alworking and machining and in chemical operations, water is used more and more often in closed loop plants. Closed water loop plants reduce the water consumption, allow the recycling of valuable materials, minimise the waste water volume and also reduce the energy requirements in connec-tion with the recirculation of hot waters. In numerous cases, the used water is treated with membrane processes.

For akvoFloat plants, there are good application possibilities in the fi eld of process and wastewater treatment when favourable operating conditions arise for a fl otation, i.e. if hydrophobic substances, such as for example oils and fats, soot, coal should be separated. Such application areas can be found in the food industry, chemistry, metal processing and environ-mental engineering. In connection with the addition of various substances e.g. of powdered activated carbon for the adsorp-tion of organic compounds dissolved in trace amounts ozone for oxidation of com-pounds as well as coagulants and fl occu-lants, the functionality of the plant can be enhanced without big expenditure.

Often, a suffi cient water quality for a new reuse can be achieved with microfi l-tration or ultrafi ltration, like is implement-ed with an akvoFloat plant. If a low con-tent of dissolved substances is required, the effl uent water quality mostly allows that a reverse osmosis can be added down-stream without further treatment steps. A safe disinfection of the effl uent water can be achieved through a downstream UV irradiation. This has an especially good effect after the membrane fi ltration, as, through the microfi ltration, all compo-nents that cause turbidity were removed.

Highlights 2014

F & S International Edition No. 15/2015

4.4 Processing of oil-containing waters

In the oil and gas industry as well as in production engineering and environmen-tal engineering, fl otation is used for the treatment of oily waters. Here, oil or other water-immiscible hydrocarbons are emul-sifi ed in the water and form an oil-in-water emulsion (O/W-Emulsion). In connection with such waters, there are good chances for an application of akvoFloat plants. Here, it must be distinguished between stabilised and not stabilised emulsions. With a stabilised emulsion natural or arti-fi cially made and added interface-active compounds (surfactants) are adsorbed on the droplets, owing to which the deposi-tion on phase interfaces (solid surfaces, further oil droplets and air bubbles) is hin-dered extensively. Accordingly, the phase separation through fl otation is thereby also more complicated. Many additives which are added to water, for other purposes in practice, have in addition also an emulsi-fying effect. These include, e.g., cleaning agents, corrosion inhibitors, lustres and biocides.

Through the addition of a demulsifi er (also called emulsion splitting agent or emulsion separating agent) adsorbed inter-face-active compounds can be displaced or infl uenced so that their stabilising effect on the emulsion is voided. The emulsion is thereby destabilised and a separation of the oily phase through fl otation is possible. In this manner e.g. the akvoFloat plants can be applied on oily washing waters of the food industry and production engi-neering.

Emulsions with the fi nest droplets may also be stabilized by electrical surface charges. In this case, a destabilization of the emulsion can also be achieved through a change of the pH value through addition of an acid or alkali.

Demulsifi ers are often added in the infl uent stream in combination with fl oc-culants so that macrofl ocs of fi ne oil droplets form, which are well fl oatable. Here, it must also be considered that over-dosing of demulsifi ers can foster renewed emulsifying.

Oily waters occur among others as a so-called Produced Water in oil extraction and gas extraction, as washing waters in connection of processes of surface treatment (coating, spraying, galvanisation etc.), as cleaning solutions during parts and equipment cleaning and as fl ushing waters.

4.5 Separation of biological matter from waters

The tests for water treatment with an akvoFloat plant at a canal in Berlin have shown that fl occulated organic matter with a high content in micro-organisms (e.g. algae, yeasts and bacteria) can be sep-arated effectively through fl otation and the downstream membrane fi ltration. H. Bennoit and C. Schuster /6/ describe the advantageous use of a degassing fl otation for separation and concentration of excess sludge from an aerobic biological waste-water treatment plant in a paper with the title „Fortschritte des Flotationsverfahrens in der Abwassertechnik und Schlamm-behandlung“. In connection with the fl ota-tion, cost savings are indicated that could be even more signifi cant with the use of an akvoFloat plant.

Application possibilities arise in con-nection with the aerobic biological waste water treatment from this knowledge and the present experiences which are not yet being used nowadays. In the course of this, combinations with the membrane bioreactors that are increasingly being used in the waste water technology are also conceivable.

5. Outlook

The akvoFloat technology is currently being tested for the pretreatment of sea water for the reverse osmosis and for the oil -water separation with Produced Water. Results for this purpose will be published shortly.

Bibliography

/1/ Allgeier S., Alspach B., Vickers J. (2005): Membrane

Filtration Guidance Manual. US Environmental Protection

Agency

/2/ AMTA Guideline; American Membrane Technology

Association (2011): Pilot Testing for Membrane Plants,

November 2011, Florida USA.

/3/ Dramas L., Croue J.-P. (2012): Algal Organic Matter

(AOM) Fouling of Ceramic Membranes”, Expert Workshop

Red Tides and HABs: Impact on Desalination Plants,

MEDRC, Oman.

/4/ Edzwald James K., American Water Works Association

(2010), “Water Quality & Treatment: A Handbook on

Drinking Water”, McGraw Hill, Chapter 11, p. 793-793.

/5/ Raúl Pérez-Gálveza, Emilia M. Guadix, Jean-Pascal

Bergé, Antonio Guadix: “Operation and cleaning of ceramic

membranes for the fi ltration of fi sh press liquor”, Journal

of Membrane Science, November 2011, Vol. 384 (1-2),

pp 142-148.

/6/ H. Bennoit und C. Schuster: Fortschritte des Flota tions-

verfahrens in der Abwassertechnik und Schlammbe-

handlung. Ripperger, Siegfried. „Der ProcessNet Technical

Committee –„Mechanische Flüssigkeitsabtrennung (MFA)”„

Chemie Ingenieur Technik 79.11 (2007): 1748-1752.

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