Contribution to the study of utilization of ozone in laundry technology Ing. Jan Kostkan

Utilization of ozone in laundry technology is a relatively old idea. However, the way from an idea to operational implementation is complicated and sometimes very long. Ozone is an example of this. Chemistry of ozone aqueous solution is not by far so simple as it could seem at the first sight. Acting of ozone and its effect upon properties of a system of associative colloids is even more complex. Knowledge and understanding of this effect is a prerequisite of its utilization in laundry technology.

Properties and some reactions of ozone

Ozone is in gaseous state under room temperature. Concentrations between 0.2 and 0,5 ppm are detectable by olfactory sense. The molecule of ozone is non-linear, three-atom, consisting of oxygen atoms. Ozone does not show paramagnetic properties. Its structure is determined with two oxygen – oxygen links. The length of the link is 1.278 Ǻ and average valence angle is 116.49° (1, 2 ). Arrangement of the ozone molecule is usually illustrated with the following resonance structures:

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Oxygen atoms in the resonance structures I and II bear full octet of electrons while the valence spheres of the oxygen atoms in the structures III and IV represent a sextet of electrons only. A deficit of electrons in these resonance structures results in creating a partial positive charge in the molecule, which allows electrophilic attack of links rich in electrons ( 2 ).     The partial positive charge of the structure III and IV polarizes the molecule strongly. These resonance structures essentially represent a 1,3 two-pole compound that is probably a cause of ability of the ozone molecule to participate in addition reactions. (3 ) The indicated types of reactions can be observed in both gaseous state and aqueous solution in case of ozone.       Solubility of ozone in water in dependence upon temperature that is most frequently mentioned in literature is depicted in Table No. 1:

Teplota °C

Rozpustnost g/litr

0

1,09

10

0,78

20

0,57

30

0,40

40

0,27

50

0,19

60

0,14

Table No. 1


Water saturation with ozone is influenced by a number of factors. Besides already mentioned temperature, this is also a size of surface of the inter-phase interface, gas pressure, but also compounds contained in water saturated with ozone. Henry constant for distilled water reaches the value of 3.76 . 103 at 20°C ( 4 ). Ozone is instable in water and is subject to own decomposition.          A number of very detailed studies on this topic can be found in literature ( 5). Kinetic data of reactions of ozone dissolved in water together with both non-dissociable ( 6 ) and dissociable compounds ( 7 ) was published.         However, all kinetic data are associated with own stability of ozone very closely. For that reason, it is purposeful to explain considerations of various authors with respect to the mechanism of its decomposition in water.           Stability of ozone was studied in alkaline ( 8 - 15), neutral (16 – 21 ) and acidic (22 - 25) solutions. Results of these studies indicate that its stability in solution reduces in dependence upon concentration of hydroxide ions ( 8 - 14). The rate of decomposition in alkaline environment slows down in presence of carbonates and/or aliphatic alcohols (14 -15). Radical mechanism initiated by OH- ions is attributed to decomposition of ozone in alkaline environment.          Weiss ( 8 ) studied decomposition of ozone in aqueous solution of potassium hydroxide. He found out that the rate of decomposition is proportional to hydroxide concentration. On the basis of this observation, he proposed a mechanism of ozone decomposition that is initiated just by OH- ion:

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Gorbenko - Germanov and Kozlova ( 12 ) investigated decomposition of ozone in a solution of potassium hydroxide with 8M concentration using electrone resonance and absorption spectroscopy. On the basis of comparison of the results of spectral measurements of ozone decomposition, they reached the conclusion that the ozonyl radical and peroxyl radical are intermediate products of ozone decomposition.   For that reason, they proposed a singlet transfer of electron from OH- anion on ozone with creating ozonide radical as an initialisation step of ozone decomposition. .

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The ozonyl radical can consequently react with water or another substance contained in the solution according to the authors mentioned above.          Staehelln and  Hoigné ( 13 ), just like Forni ( 14 ) with his colleagues, studied ozone decomposition in solutions with range of pH values from 5 to 13. They confirmed the mechanism of initiation of decomposition proposed by Weiss. However, they proposed a mechanism somehow different for decomposition of peroxide radicals:

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But the mentioned authors viewed the mechanisms of initiation of decomposition of ozone from a quite different angle of view. According to this approach, decomposition is initiated with OH- anion, however, hydroperoxyl anion and oxygen are reaction products. Reaction of hydroperoxyl anion with ozone leads to formation of radicals from the initial components.

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It follows from the considerations indicated that hydroxyl and hydroperoxyl radicals form parts of the chain of ozone decomposition.

Hydroxyperoxyl radicals form clusters with water. Molecularly dynamic study of hydryperoxyl water clusters was published by Iyengar in 2005 ( 26 ).

E.g. Hoigné studied the role of hydroxyl radicals as oxidation intermediate products in a detailed way in 1975 ( 27 ).

Ozone decomposition was also studied in neutral and acidic solutions as already mentioned. The rate of ozone decomposition reduces considerably with dropping pH value of solution, especially between 7 and 4. Below pH value of 4, decomposition is already very slow and the rate is essentially constant with continuing drop of pH value (22 - 23).

On the other hand, it follows from literature that the rate of ozone decomposition is also influenced by a character of acid used for achieving of the corresponding pH value of the solution. Pan with his colleagues (23) carried out comparison of the rate in the solutions of sulphuric, nitric and acetic acids.

According to their finding, decomposition of ozone in sulphuric and nitric acids is twice as high as in case of a solution of acetic acid, i.e. within the range of pH values between 3.5 and 5.0. Reduction of the rate of ozone decomposition in environment containing acetic acid was published by a number of other authors (19,24 ). For that reason, acetic acid is generally considered to be a retarder of ozone decomposition.

On the contrary, ozone decomposition is catalysed with metals ( 23 – 25). The catalytic effect of metals appears in case of their concentration exceeding 3 ppm. Power of the catalytic effect of metals can be illustrated with the series ( 23):

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In case of concentration of metals below 0.5 ppm, the rate of ozone decomposition is increased only by cobalt.         However, considerable reduction of the rate of ozone decomposition even in the solutions containing metals when compared with sulphuric and nitric acids was observed again in presence of acetic acid. Hill (25) found out that decomposition of ozone is slower in acetic acid when compared with perchloric acid in presence of cobalt.              The results mentioned above correlate with the reaction mechanism published by Walling and El–Taliawi (28):

 

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Acetic acid causes termination of hydroxyl radical and thus reduces the rate of ozone decomposition. It results from the above mentioned that hydroxyl radical is a part of the ozone decomposition chain in neutral and/or acidic solutions.    


Disintegration half-life of the free hydroxyl radical is indicated in microseconds, for that reason, its concentration does not exceed 10 -12 M, ( 29 ), but in spite of this fact, it reacts with a number of compounds.       The reaction of unsaturated hydrocarbons with hydroxyl radicals and oxygen with production of hydroxyalkylperoxyl radicals can be mentioned as an example (30 ). According to literature ( 31), these radicals dimerize and provide tetraoxide intermediers. Decomposition of intermediers may take place in various ways. Fragmentation of hydrocarbons with production of α - hydroxyalkyl radicals, aldehydes or ketones is significant. α – hydroxyalkylperoxyl radical is produced by reaction of α - hydroxyalkyl radical with oxygen; this radical then decomposes in ketone or aldehyde with production of hydroperoxyl radical (30, 31). The process is illustrated with the following diagram:

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alken hydroxyalkyl radikál hydroxyalkylperoxyl radikál α – hydroxyalkyl radikál aldehyd keton

 

α – hydroxyalkyl radikál α – hydroxyalkyl peroxyl radikál aldehyd keton hydroperoxylový radikál

 

The hydroxyl radical react also with unsaturated hydrocarbons with production of hydrocarbon radical that provides the corresponding peroxyl radical with oxygen. This radical is transformed in unsaturated hydrocarbon and hydroxyperoxyl radical is released:

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nasycený uhlovodík
radikál uhlovodíku
peroxyl radikál uhlovodíku
nenasycený uhlovodík
hydroperoxylový radikál

 

If the hydrocarbon molecule is substituted e.g. with chlorine, nitrogen, sulphur or phosphorus, elimination of this substituent takes place during oxidation and the substituent is transformed in a ion compound such as chloride, nitrate, sulphate or orthophosphate ( 30, 31).       Not only hydroxyl radical, but also directly ozone provides a number of reactions with organic compounds. The reaction of ozone with unsaturated hydrocarbon belongs to the best-known ones. This reaction has been known for almost 150 years. It is possible to illustrate quite well very interesting development in opinions of its mechanism on an example of this reaction. Harries described the mechanism of this reaction in 1905 ( 32 ); he considered addition of ozone to double link of hydrocarbon with formation of ozonide as the first step:

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ozonid

 

The structure of the ozonide molecule proposed by Harries was not satisfactory for all experimental results as formation of glycol was never demonstrated at reduction of ozonide and this results logically from the essence of the model of ozonide structure. The indicated discrepancy was analysed by Staudinger ( 33 ) by formulating a new structural formula where the adjacent carbon atoms between which was the double link are linked with an oxygen atom:

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Staudinger regarded instable compound malozonide as a primary product of action of ozone on unsaturated hydrocarbon:

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Malozonide

According to Staudinger, ozonide is essentially peroxidic acetyl. Six years later, Rieche with his colleagues confirmed correctness of the Staudinger’s idea on the ozonide structure experimentally. ( 34 )       Criegee brought another view on the mechanisms of ozonolysis of olefins in 1951. (35) According to this author, the breakage of the link between the olefin carbons is preceded by addition of olefin dipole. A duplicitous ion is created in this way that is decomposed in intermediers and carbonyl compound. The intermedier provides ozonide by reaction of the carbonyl compound in an aprotic solvent. Dimerization of intermedier with formation of diperoxides and/or polymers of peroxides represents another way ( 36 ). Presence of teterasubstitued initial olefin and also non-polar solvent representing reaction environment is a prerequisite of formation of these compounds. ( 36,37 )

 

Mechanism according to Criegee

 

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Duplicitous ion

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Carbonyl compound Intermedier

 

The intermedier is transformed in hydroxyalkylhydroperoxide in protic environment such as carboxyl acid, water or alcohol.    The principles of the Criegee’s mechanism of ozonolysis are recognized till the present.

Utilization of ozone in laundry technology as already mentioned in the preface is a relatively very old idea. The references mentioned above indicate with what problems and for how long different authors studied the mechanisms of decomposition and effects of ozone. They show among others that the reaction coordinate of ozone is in principle represented by parallel reactions. Besides radical ozone decomposition, direct attack of ozone on a molecule of an organic compound may take place simultaneously.


Ozone in laundry industry

 

Complexity of ozone chemistry can be regarded as the reason of the fact that ozone has not found more significant application in laundry industry so far. On the other side, it is true that e.g. Purotek company deals with washing in ozone baths, however, more detailed technical- technological information is not provided. The same applies to other companies. Canadian Azcozon Industrie Limited offers generators for production of ozone, adsorption equipment and other systems. They mention several hotel laundries utilizing ozone in laundry technology in their promotional documents likewise REM Company Inc. from USA or Lenntech Water - & Luchtbeh. Holding  b. v. from the Netherlands.     Disinfection effects of ozone water are mentioned on the first position in description of benefits given by the mentioned companies and reduction of energetic demand for washing, costs for detergents and possibility of water recirculation follow then.      However, the essence of effects of ozone in a system of associative colloid has been found neither in company nor worldwide literature available.      However, desorption effects of aqueous ozone solution are mentioned in other associations. Effect of ozone on desorption of detergent from the surface of coal dust was published ( 38 ). Changes of wetting angle of polypropylene resulting from effects of ozone were studied ( 39 ).The change of wetting angle is explained by attack of ozone against polypropylene with formation of hydroxycarboxyl derivates in this study.       According to already mentioned company documents, ozone or ozone water is utilized in laundries for washing, bleaching and disinfection of laundry in laundries. It is obvious from this information at the first sight that mixing of three quite different technological categories takes place.  Washing is a process the moving force of which is a change of entropy of a system. Bleaching is based on a chemical reaction the result of which is elimination of chromophore by its oxidation. The biocidal properties of the bath are a resultant of a number of factors that are represented by resistance of microorganisms, concentration of ozone in environment, exposure period, pH value of solution and its temperature etc. to say the least. 


On the other hand, indisputable benefit of also this technological concept of ozone in connection with laundry care can be seen in the fact that it delimits the framework of the complete issue on a general level.        Application of ozone as a bleaching agent brings a number of problems when compared with chlorine compounds in a higher oxidation degree or peroxides. Relatively high concentration of ozone that is associated with hygienic and safety risks and also with costs for ozone production or ozonation is technically the most serious obstacle to achieving of the required bleaching effect. Corrosive effects of both ozone and its aqueous solution also cannot be omitted.  Bleaching with ozone in the conditions of industrial laundries does not represent any benefits when compared with standard procedures.      Utilization of ozone for laundry disinfection and technological equipment of laundry was hindered by legislative barrier following from the Decree No. 195/2005 in the Czech Republic as ozone does not belong to the approved disinfection means. However, the effort for overcoming this barrier started to be successful thanks to Aplikace ozonových technologií (Application of Ozone Technologies), s.r.o. (ltd.).           Various technological concepts of ozone as a component of washing bath is mentioned also in patent literature. Very quick development of ozone washing technology is clearly apparent in American patent documents. Some effects of ozone on the washing process were patented in 1996 (40). Benefits brought by utilization of ozone in laundry technology are described in this patent. A complex technological system for utilization of ozone for laundry treatment was patented in 1998 (41). One year later, technological water recycling that is based just on utilization of ozone was patented (42). Practical solution of the relationship between character of laundry soiling and concentration of ozone in operational bath was patented in 2002 ( 43 ).        Ozone is viewed as a bleaching and oxidation component of the washing bath in the cited documents. However, potential possibilities of aqueous solution of ozone that are indicated with electrophilic nature of the molecule remain aside.          The description of the mechanism of formation of clusters in aqueous solution is very inspiring (44). According to the authors, the ozone cluster is a result of interactions of delocalised links of molecules of ozone and water:

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interakce ozon – voda
klastr

 

The indicated mechanism of formation of ozone clusters has shown that use of ozone as a component of washing bath need not to be based only on its oxidation and disinfection properties, but that its electrophilic nature offers broader, more complex utilization based on synergic acting with other washing bath components.      Effect of ozone dissolved in water on the level of aggregation of surfactant was described in 2006 (45).

Starting points and possibilities of utilization of ozone


However, it was necessary to overcome a number of problems for practical implementation of the ozone washing technology represented by an associative colloid.        The basic considerations of problem solving were based on thermodynamic ideas of solvation to which all components of the operational bath are subjected. Solvation is in its principle an electric phenomenon that can be explained very schematically as orientation of water dipoles caused by polar molecules and/or their parts in energetically more advantageous arrangement. This re-arrangement is associated with a certain amount of performed work (Δ H) that represents a loss of internal energy. Stabilization of the system takes place thanks to the loss of its energy. It follows from this fact that also formation of ozone clusters leads to system stabilization.        It can also be deducted that solvent polarity may affect free enthalpy (Δ G) of the chemical reaction, namely according to differences between initial and end substances from the viewpoint of polarity.          A difference of Gibbs free enthalpy that can be expressed with the following formula is a driving force of the chemical reaction:Δ G = Δ H - T Δ S

However, the difference of Gibbs free enthalpy is also affected by the change of arrangement of the system (Δ S). The change of arrangement, i.e. entropy (Δ S) represents e.g. formation of one compound by mutual reaction of two before different substances.          It is apparent from the formula that the changes of arrangement resulting from polarity of dissolved substances and water dipoles can act also completely reversely from the viewpoint of reaction progress as an improvement of arrangement of the molecules of the system can be a result from solvation of polar substances, which means a reduction of the driving force of the reaction, to be said otherwise.      It is suitable to remind that just a change of entropy is a driving force of formation of associates, i.e. micelles of amphiphilic compounds. On the other side, it is also a driving force of the washing process.     Pilot studies were carried out with financial support of the Ministry of Industry and Commerce within the framework of Impuls program project support. The Retre s.r.o. (ltd.) laundry in Třeboň together with the companies DonGemini s.r.o. and Aplikace ozonových technologií (Application of Ozone Technologies), s.r.o. participated in the study. The study was successful not only in determining some missing technical-technological data with respect to ozone application, but elaboration of technology of stabilization of the effects of ozone water from the viewpoint of both efficiency of operational bath on washing and disinfection was a break through step. It created at least basic framework idea of technological arrangement of ozone solution in the laundry technology the operational bath represents a system of associative colloids.     The basic problem, as already was said before, consisted in stabilization of effects of water saturated with ozone. The efficiency of the stabilization was monitored on the basis of both bactericidal properties and simultaneously also washing efficiency. The values of washing efficiency determined according to Retre s. r.o Třeboň PNY laundry in dependence upon the period of standstill of stabilized ozone water at 20° C are listed in Table No. 2 for illustration. Washing was carried out at temperature of 40° C.

doba stání upravené vody ( hodin ) účinnost praní
0 97,6%
12 91,8%
24 87,1%
36 84,1%
48 82,9%

Table No. 2



To provide a better clearness, the dependence of washing efficiency on standstill period is depicted in the diagram below:

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The intensity of biological contamination of laundry was also determined on the basis of PNY of the above mentioned laundry plant. Mould or yeast was not detected even in a single case. Bacterial contamination was registered only. The results are illustrated in the Figure No. 1 below:

ImageImageImageImage

čas: 0 hod 24 hod 48 hod standard

obrázek č.1

The results indicate a sufficient degree of stabilization of effects of water saturated with ozone, which guaranteed always the same and mainly reproducible conditions for next research. For that reason, attention was focused on formulations of operational baths suitable for industrial utilization of water stabilized with ozone. Operational baths qualifying for hope for successful implementation of operational application were drafted and verified in laboratory conditions within the solution of the project already mentioned above with financial support of the Ministry of Industry and Commerce of the Czech Republic.  
Operational tests of ozone technology
The purpose of the operational tests carried out in the Retre s.r.o. laundry was to verify the following assumptions:
1. Effect of saturated water stabilized with ozone at a lower temperature of operational baths will be the same or comparable with effect of washing in standard baths at washing temperature of 85°C. 2.  Microbiological contamination of laundry will be reduced as a result of effect of        saturated water stabilized with ozone. 
Verification of the assumptions was carried out using heavily soiled overall in Primus washing machine with capacity of 22 kg of laundry.         Efficiency of washing was monitored by comparing the level of cleanness after washing using the existing technology with the technology based on a development Gemini system and saturated water stabilized with ozone at bath temperature of 30°C at the first approach. The figures No. 2 and 3 show the intensity and character of soiling of the laundry used for testing. Efficiency of washing with water stabilized with ozone is documented by the Figures No. 4 – 6.

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obrázek č. 2

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obrázek č. 3

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obrázek č. 4

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obrázek č.5

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obrázek č. 6

The tests also proved that virtually comparable washing result was achieved by the effect of saturated water stabilized with ozone at bath temperature of 30°C like when using temperature of 85 °C. But the level of microbiological contamination of laundry proved to be quite incomparable, which is illustrated in the Figures No. 7 and 8 below:

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¨ Obvyklé praní při teplotě 85°C Ozonová technologie, praní při 30°C

obrázek číslo 7 obrázek číslo 8

On the basis of the results achieved and experience obtained, operational tests of the effects of water saturated and stabilized with ozone were started using a continual washing machine in Retre s.r.o. Třeboň laundry. Besides already mentioned effect on washing efficiency at lower temperatures and bactericidal and fungicidal effects, also other effects were monitored, above all the effects on amortization of fabric, consumption of washing agents, corrosive effects on equipment and ozone emissions in working environment. The tests were carried out for the period of 6 months. The Figure No. 9 shows the generator of water saturated with ozone for feeding the continual washing machine for illustration.

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obrázek č. 9

Results of the tests of ozone technology using continual washing machine

Firstly, it is necessary to say that utilization of water stabilized with ozone for washing variable range of laundry with different character of contamination in a continual washing machine in the conditions of Retre s.r.o. laundry confirmed two basic banefits known already before.

1. Bactericidal and fungicidal properties of operational baths under low temperatures.
2. Substantial reduction of temperature of operational bath for main washing.
The effect of temperature of the main operational bath on washing efficiency is indicated in the Table No. 3 in order to illustrate energy savings.

Voda sycená ozonem
Teplota pracovní lázně na hlavní praní Účinnost praní
33°C 75,7 %
43°C 84,1 %
57°C 92,0 %
Voda bez ozonu
Teplota pracovní lázně na hlavní praní Účinnost praní
77°C 73,5

Table No. 3
Temperature is indicated as an average value in the table. Deviation from this value reached up to ± 2 °C. The Figure No. 1 illustrates graphically the dependence described in the table:                                 Dependence of washing efficiency on temperature when using water saturated with ozoneSvislá osa: Efficiency                    Vodorovná osa: Temperature ºC

 

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It results from the diagram No. 2 that the dependence of washing efficiency is virtually linear in the temperature range between 30°C and 80°C and can be expressed by the formula: 

ψ   =   0,6718t  +  54,151
where ψ is efficiency of washing of a standard soiled cloth and t is temperature in degrees of Centigrade.                        From the operational-economical point of view, temperature of the main operational bath of approximately 45°C seems to be advantageous.         Emissions of ozone exceeding the specified limits were not detected during testing. Consequences of ozone corrosion on technological equipment even on the press membranes also were not found.          For illustration, the Figure No. 10 shows laundry washed with using ozone technology.

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obrázek č. 10

 

Figure No. 10
Assessment of a sample of a standard cloth after 50 washing cycles using ozone technology in the continual washing machine of Retre s.r.o. laundry was carried out according to both laundry PNY and using methodology of TZÚ Brno.        The standard cloth used for testing was supplied by TZÚ Brno. Washing operational baths were represented by testing and development samples of the preparations of the Gemini series and were not changed during testing. Temperature of main washing operational bath ranged between 35°C and 45°C. Ozone was supplied from the generator supplied by Canadian Azcozon company. The results of examination of a sample of a standard cloth after 50 washing cycles based on laundry PNY are summarized in the Table No. 4..

Ukazatel hodnota poznámka
Anorganická inkrustace 0,2 %
Organická inkrustace 0,3 %
Vliv na bělost 104 % Vyloučen vliv OZP
Výluh ze vzorku, hodnota pH 7,5 Napájecí voda 8,5
Výluh ze vzorku, vodivost 0,28 mS/cm Napájecí voda 0,20 mS/cm
Table No. 4
Effect of operational baths on cloth after 50 washing cycles using ozone technology analysed using methodology of TZÚ Brno is illustrated in the Figure No. 11. Chemical tests in order to analyse damage of fibres (aldehydic and karboxyl groups) were comparable with the standard. Repeated tests of microbiologic contamination were negative.

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Composition of waste water reflects a substantial reduction of the quantity of washing agents, which means not only savings, but also considerable reduction of waste water burden. Example of the result of analysis of a sample of such water, namely both directly from main washing (sample No. 16876) and from rinsing bath (sample No. 16877) is illustrated in the Figure No. 12.

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Figure No. 12

It showed during testing that washing efficiency even using the ozone technology is affected by the contents of residues of detergents in laundry. If contents of inorganic incrustation in laundry exceed 1.3 %, it proved to be advantageous to remove residues from laundry at first before washing using the ozone technology.         Practical results of operational tests using both batch and continual washing machines prove that use of ozone in laundry technology reduces energetic demandness of the process, ensures required efficiency, brings detergent savings, which virtually allows potential water recirculation and ensures microbiological harmlessness of laundry. Ozone is indisputably very interesting component of bath that increases washing process efficiency considerably and reduces environmental impact of the plant under certain conditions.         Studies and operational tests represent not only a solid base, but also a flip bucket for overcoming obsolete ideas of washing technology that are economically interesting for the suppliers of washing agents, however, they cause immense damage of environment and human health in the final effect according to opposition proceedings of the Ministry of Industry and Commerce of the Czech Republic. But the complete society usually bears costs for elimination of these consequent damages.    



Only a modern concept and deep knowledge of issues associated with washing technology can start the way of development of the industry in harmonic relationship with nature. Utilization of water saturated with ozone with modern types of amphiphilic compounds is an illustration and, at the same time, an example of possible economic development of the industry on the basis of technologies originating from the the conditions of sustainable development.


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