This invention relates to a machine dishwashing process for cleaning tableware or other surfaces soiled with food remains. More particularly, the object of the invention is to improve the continuous or discontinuous process of machine dishwashing which may be carried out both in single-tank and in multi-tank machines.
In conventional dishwashing processes, food-soiled tableware, including, for example, trays, cutlery and glasses smeared with lipstick residues, is sprayed or squirted with water in a prewash zone or prewash cycle, i.e. before the actual washing process. This water is cold or preheated fresh water, cooling water from a vapor condensation process, but generally overflow water of a wash liquor which is generally sprayed under low pressure through relatively large spray arm openings on to the tableware. After this prewash zone or prewash cycle, the tableware enters the actual wash zone or wash cycle where it is sprayed with wash liquor.
The wash liquor usually consists of water with a temperature of about 50°C to 65°C to which a cleaner is added. In order to remove grease, starch, dyes and protein with sufficient reliability, detergents in powder or liquid form, for example, are added to the water, so that about 0.1 to 0.3% by weight of active washing substances are present in the cleaning solution. The resulting wash liquor is usually sprayed on to the articles to be cleaned through differently arranged nozzles. The soil on the tableware is thus removed or at least swollen or softened and partially dissolved. In a following clear-wash zone or clear-wash cycle, the soaked or softened soil and food remains are removed with water. At the same time, the wash liquor is rinsed off. In the following rinse cycle, the rinse water usually consists of fresh water and a rinse agent, which generally ensures that only a very thin film of water is left on the tableware and can drain off or evaporate in following drying zones.
The requirements which institutional and domestic dishwashing machine are expected to satisfy are different. Depending on the particular type and the application involved, institutional dishwashing machines usually comprise several tanks arranged in succession, from which rinse and wash liquor is sprayed onto the tableware to be cleaned as it passes through the machine. The tanks are generally arranged on the cascade principle, the rinse and wash liquor passing successively through the tanks from the outlet end to the inlet end of the tableware. Fresh water is generally fed into the machines at the outlet end. The required amount of detergent is dosed into at least one wash tank also known as the dosing tank. The dosing of detergent normally takes place automatically depending on the conductivity or the pH of the wash liquor or optionally by means of a dosing pump controlled by a timer or clock. Several components may also be separately dosed. For example a basic lye solution based on an aqueous alkali metal hydroxide solution may be introduced first. If required, one or more additives may be added to this basic solution. Normally, the dosage of these additives is either proportional to the addition of the basic lye or is controlled by a timer. If desired, the additives may be dosed according to the advance cycle of the chain carrying the tableware to be washed through the machine. In addition, additives can also be dosed or the additive concentration increased by determining the amount of additive in the basic lye by sensory detection of a tracer present in the additive.
As explained in EP 282 214, a problem which often occurred in the past was that, in regular conventional cleaning, periodic thorough cleaning steps could not be completely dispensed with, even when relatively large amounts of detergents were used. The purpose of this thorough cleaning is to remove coatings of, in particular, water-insoluble starch which have built up over time, but which are not immediately visible to the eye. During this thorough cleaning, the articles, preferably the tableware, are occasionally treated with wash liquor containing several times the usual concentration of active substances. This causes considerable pollution of the wastewater. As already stated, EP 282 214 tackled this problem. An attempt was made by improving cleaning performance to reduce labor, the consumption of water and chemicals and hence environmental pollution and, at the same time, to achieve satisfactory cleaning. EP 282 214 attempts to solve the problem by spraying a highly concentrated cleaning formulation onto the soiled articles, followed by a contact phase, after which the cleaning solution and soil are removed in one or more following steps.
As explained, EP 282 214 mentions reducing pollution of the wastewater as an essential aim of the invention. However, on closer examination of the process as a whole, it becomes clear that an essential aspect of the pollution of wastewater was not taken into account. In the treatment of wastewater, it is usually imperative for the wastewater to have a certain pH before or during treatment in corresponding treatment plants. This pH is generally close to the neutral point of water according to the nature of the treatment. Depending on legislation, there are even country-specific wastewater regulations which stipulate a particular pH for wastewater. All this means is that more chemicals have to added to neutralize strongly acidic or strongly alkaline wastewater before further treatment. In other words, whilst the chemicals saved in the cleaning process according to EP 282 214 no longer pollute the wastewater, more chemicals still have to be used at a later stage to neutralize the wastewater flowing off before the water treatment. EP 282 214 takes no account at all of this problem. On the contrary, throughout the disclosure of EP 282 214 only alkaline detergents are mentioned and recommended. In column 3, line 15 onwards, the cleaning agents to be used are explicitly discussed. In particular, from column 3, line 24, it is disclosed that, for example, alternately spraying with high alkalinity and then with lower alkalinity could have advantages. It is then explained that a further advantage of the system is that chemicals which are usually incompatible, for example oxidizing agents, such as hypochlorite, with water treatment substances, such as NTA for example, can be used alongside one another in the process. Further down in column 3, line 42 onwards, it is stated that various substances are known to improve penetration and to help soften and remove the soil. In addition, EP 282 214 A1 states that a basic formulation will typically contain an alkalinity source and a sequestrant source. On the bases of formulations such as these, it is possible to cater for special problems such as, for example, tea stains or other residues which may be found on tableware. After reading these passages from EP 282 214, and also after reading the following Example in column 4, line 3 onwards, the expert would inevitably conclude that it is normal to carry out processes of this kind with alkaline compositions - all the more so as it used to be the standard procedure. As explained, however, the problem arises in practice that the wastewaters produced have a high pH because of the high levels of alkalinity attributable - depending on the process - to large quantities of alkaline cleaning solutions and/or to high concentrations of alkaline detersive substances. Accordingly, it is also necessary (and indeed often even stipulated in wastewater regulations), particularly when following the teaching of EP 282 214, for the alkalinity used for the cleaning process to be neutralized with acids to a certain pH either before or during treatment of the wastewater.
WO 02/31095 A1 describes a detergent composition and a method for warewashing in which an acidic rinse is used in a mechanical dishwashing process to obtain a better soil release effect. According to WO 02/31095 A1 an acidic component is dosed into the post-wash rinse zone or the post-wash rinse step. Example 1 of the WO 02/31095 A1 describes a pre-treatment of ceramic dishes with an acidic rinse product. After this process the dishes were rinsed and soiled. A disadvantage of this process is that a pre-treatment of the ceramic dishes is necessary which makes the whole cleaning process more complex.
Accordingly, the problem addressed by the present invention was to provide a process which, firstly, would solve the cleaning problem posed by EP 282 214 without the need for thorough cleaning. Secondly and as a limitation with respect to EP 282 214, the problem addressed by the invention was to minimize the time and effort involved in neutralization before or during treatment in wastewater treatment plants. At the same time, care would be taken where possible to ensure that applicational requirements with regard to equipment and other factors would be satisfied.
The present invention relates to a process for continuous or discontinuous machine dishwashing, in which the tableware is treated at least in one process step with an acidic cleaning solution and, in another process step, with an alkaline cleaning solution, comprising the steps of
In the process according to the invention, the acidic cleaning solution mentioned contains one or more acids selected from the mineral and/or organic acids. In a particularly preferred embodiment, at least one acid is present which is selected from sulfuric acid, nitric acid, phosphoric acid, formic acid, acetic acid, propanoic acid, glycolic acid, citric acid, maleic acid, lactic acid, gluconic acid, alkanesulfonic acid, amidosulfonic acid, succinic acid, glutaric acid, adipic acid, phosphonic acids, polyacrylic acids or mixtures thereof and, in a most particularly embodiment, from formic acid, glycolic acid, gluconic acid, amidosulfonic acid or the alkanesulfonic acids, more particularly methanesulfonic acid or mixtures thereof.
The aqueous acidic cleaning solution to be sprayed on in accordance with the invention preferably contains as further components a corrosion inhibitor and/or a typical complexing agent which, in a particularly preferred embodiment, is selected from the phosphonic acids, more particularly dioctyl phosphonic acid.
In another preferred embodiment of the process according to the present invention, the acidic cleaning solution contains between 0.01 and 10% by weight of one or more detersive substances, based on the cleaning solution. In a particularly preferred embodiment, the acidic cleaning solution mentioned contains less than 0.9% by weight, more particularly less than 0.8% by weight and, in a most particularly preferred embodiment, less than 0.5% by weight of one or more detersive substances, based on the cleaning solution.
The exact conditions also depend on the more precise practical circumstances and the performance requirements the process is expected to meet. For example, if there is a risk of corrosion where the acid concentration in the acidic cleaning solution is too high through plant-specific circumstances in practice, then it is advisable to use weakly acidic cleaning solutions with an active acid content of less than 0.9% by weight, preferably less than 0.8% by weight and more particularly less than 0.5% by weight. Depending on machine parameters, weakly acidic cleaning solutions may also have the advantage that the consumption of the lye baths into which the possibly acidic solution is carried over is not too high.
Apart from the performance-related requirements, when the process is considered as a whole, this procedure also leads to a reduction in pollution of the wastewater, in contrast to EP 282 214, because of the partial neutralization. As a result, less effort is involved before further treatment of the wastewater.
In another preferred embodiment of the process according to the invention, the acidic aqueous cleaning solution is allowed to act on the tableware and the soil for a certain contact time (during which spraying stops), the contact time preferably being 2 to 100 seconds, more preferably 5 to 100 seconds and most preferably 8 to 25 seconds. In another preferred embodiment, the tableware is not intentionally sprayed during the contact time.
In another preferred embodiment, the acidic aqueous cleaning solution is sprayed onto the tableware as a fine, gentle mist-like liquid spray.
In another preferred embodiment of the process according to the invention, the acidic cleaning solution is applied to the tableware as foam.
It is pointed out at this juncture that other forms of application may also be considered and that the success of the method does not depend on the form of application alone. For example, the cleaning solution could also be applied to the tableware in droplet form or by the Ecolab process known commercially as thin film cleaning (TFC).
Depending on the form of application, appropriate nozzles or alternative aids are preferably used.
Another preferred embodiment of the process according to the invention is characterized in that the treatment with the acidic cleaning solution is preceded by an alkaline treatment of the tableware of which the pH is a particularly preferred embodiment is above 10. In another preferred embodiment, the treatment with the acidic cleaning solution is followed by an alkaline treatment of the tableware. In a most particularly preferred embodiment, however, the tableware is subjected in all to at least three cleaning steps, the treatment with the acidic cleaning solution being preceded and followed by an alkaline treatment of the tableware.
It should be further explained that, where least two alkaline steps occur in the process according to the invention, the alkaline step taking place earlier in the process sequence - in a particularly preferred embodiment - has a lower pH than the alkaline step taking place later in the process sequence. This is advantageous in order to minimize consumption in an acidic step taking place between the two alkaline steps mentioned.
In a preferred embodiment of the alkaline treatment mentioned in the process according to the invention, the tableware comes into contact with one or more aqueous cleaning solutions containing between 0.1 and 4% by weight of an alkali carrier, preferably a hydroxide selected from sodium hydroxide, potassium hydroxide or mixtures thereof.
Other preferred alkali carriers - in addition to or instead of hydroxides selected from sodium and potassium hydroxide - include, for example, alkali metal silicates, ethanolamines, such as triethanolamine, diethanolamine and monoethanolamine, and also alkali metal carbonates of an alkali carrier, preferably a hydroxide selected from sodium or potassium hydroxide. Other alkali carriers may of course also be used because it is principally a matter of increasing the pH.
It is also described an institutional dishwashing machine comprising several tanks which are arranged adjacent one another in known manner on the cascade principle and from which rinse or wash liquor is sprayed onto the tableware and then drains back into the tanks, the tanks intended for one or more acidic cleaning steps in the operation of the process according to the invention being made of acid-resistant material and/or lined with acid-resistant material.
In addition, it is described a single-tank washing machine, for example a domestic dishwasher, which can be used in a process according to the invention and which is made of and/or lined with acid-resistant material, more particularly in the places where it comes in to contact with acidic cleaning solutions.
The process of the invention has the further advantage that mineralic deposits on glass and dishes, like lime deposits, which are soluble in acidic solution are removed.
For each test, 10 new, dry dinner plates were soiled with starch at room temperature by a standardized test method. To this end, a ca. 6% aqueous composition containing corn starch was cooled to 75°C after boiling and was applied to each plate with a brush in a quantity of ca. 4 ml. The plates thus treated were left to stand for at least 3 hours and then dried at ca. 100°C for 16 hours. After the plates had cooled, comparison tests were carried out in a Krefft® single-tank dishwashing machine according to the following scheme:
The cleaning solutions and spray-on solutions were prepared with softened water. The cleaning temperature in the Krefft® single-tank dishwashing machine was 60°C.
6 tests were first carried out to the described scheme, the composition of cleaning solutions a) and d) being maintained and only the spray-on solution being varied from test to test.
Cleaning was evaluated by scoring on a scale of 1 (= no visible sign of cleaning) to 10 (= complete removal of soil).
The results of the 6 tests are se out in Table 1.
As can be seen from Table 1, by far the best cleaning results were achieved in tests 3 to 5 where a solution of methanesulfonic acid was used as the spray-on solution. Accordingly, the cleaning principle in this very advantageous case consisted in cleaning first with alkaline solution, then with acidic solution and, after that, with alkaline solution again.
Further investigations showed that similar results could also be obtained with other acids. Particularly emphasis is placed in this regard on the organic acids: formic acid, glycolic acid, gluconic acid, amidosulfonic acid or other alkanesulfonic acids, more particularly with an alkane chain of 1 to 4 C atoms.
As can be seen from this simple test arrangement, it is possible by alternating the pH to achieve excellent cleaning results in practice, even with acid concentrations of less than 0.5% by weight of active acid. It is also clear that, by using lower acid concentrations, there is a reduction in the undesirable partial neutralization in the cleaning process, for example because of residues of cleaning solution adhering to the tableware which are treated in the next step with a cleaning solution of opposite pH.
The following test was made in a conveyor dishwasher machine Meiko GSM®. The tests were carried out with soiled dishes from a cantine. The conditions in the Meiko GSM dishwashing machine were as follows: As cleaner Perclin® Intensiv Flüssig (1,5 g/l; 3,2 mS/cm) was used. In the machine softened water in an amount of 500 l/h was used. The acid content was 1,4 to 1,5% phosphoric acid. The machine had a built in spraying system with a spraying amount of 36 l/h and a pressure of 4 bar.
Table 2 shows the cleaning results dependant from the different pH-values in the different bars.
In the further tests, a dripping plate was installed in the bars with a spraying system to prevent that acid is running into the alkaline wash tanks. Furthermore, the concentration of the acid was reduced to 1 % phosphoric acids. All other parameters were not changed. The results are shown in table 3.
From table 2 and 3 can be seen that a very good cleaning result is only achieved if the pH-value of the first step is at minimum 10 or higher. The cleaning performance with the acid step is always higher than without the acid step. This can be seen at the beginning and at the end of the test. Furthermore, it can be seen that a higher concentration of the phosphoric acid causes a better cleaning performance.
To find the best pH-relation from the first and the third tank for the Meiko GSM, a test with two Krefft® single-tank dishwasher was performed. In the first machine a pH was adjusted with NaOH, a second pH was adjusted in the second Krefft® dishwashing machine. The water was softened with pH of 9,8 and the temperature was 60° C in both machines. The first step was one minute cleaning followed by a spraying step with a 1 % phosphoric acid and a contact time of 30 seconds and an additional cleaning step of two minutes. Results are shown in the following table 4.
From the table can be seen that it seems to be the best to start with a pH-value of about 11 and the acid step and follow then with an alkaline step which is higher in alkalinity than the first one.