Patent Description:
Solutions of peroxyacetic acid (also known as peracetic acid and indicated in the sector with the abbreviation PAA) have been used since the early <NUM> to disinfect wastewater from biological purifiers. The advantages of using PAA are:.

The PAA solution commonly used for this application is a chemical equilibrium solution containing, by weight:.

This solution is added to previously purified water in amounts generally comprised between <NUM> and <NUM> grams of solution per cubic meter of water to be treated; the water added with the solution remains in a contact tank for about thirty minutes and is then released into the environment.

During the stay in the contact tank, PAA reacts with the bacteria and impurities, and its amount is reduced by over <NUM>%; hydrogen peroxide is inherently more stable and is reduced by about <NUM>%. Some studies in the sector have shown that, at <NUM>, hydrogen peroxide has a half-life of <NUM> minutes, while PAA has a half-life of less than <NUM> minutes.

Using, for example, <NUM> of PAA solution per cubic meter of water to be treated, and assuming during treatment a PAA conversion > <NUM>% and an average H<NUM>O<NUM> conversion of about <NUM>%, in the effluent there will be:.

During the thirty years since the introduction of the use of PAA in this application, the toxicity of water purified and disinfected with this compound has been constantly evaluated following the official methods established by current regulations (see for example the "OECD Guidelines for the Testing of Chemicals", and the ISO <NUM> standard), which include ecotoxicity tests on Daphnia magna and luminescent bacteria, without ever finding any traces.

The Biocides Directive <NUM>/<NUM>/EC and its subsequent updates have introduced many limitations on the production and use of biocides.

According to the study "<NPL>), PAA would be particularly toxic to embryos of zebrafish (Danio rerio), a small freshwater fish native to Asia. The toxicity detected in the study is due to the sum of the amounts of PAA and hydrogen peroxide still present at the exit from the contact tank. Studies on zebrafish are particularly important for several reasons: their embryos are nearly transparent, which allows researchers to easily examine the internal structures development; it has a genetic structure similar to that of humans, sharing <NUM>% of genes; <NUM>% of genes known to be associated with human diseases have a zebrafish counterpart; finally, the zebrafish genome has been completely sequenced at a very high quality, which allows scientists to create mutations in more than <NUM>,<NUM> genes to study their function.

The importance of studies carried out on zebrafish was recently confirmed by a Risk Assessment Committee (RAC) of the European Chemicals Agency (ECHA) which issued an opinion, dated <NUM> June <NUM>, regarding the chronic toxicity of PAA. The opinion is available at the link: https://echa. eu/it/registry-of-clh-intentions-until-outcome/- /dislist/details/0b0236e184e0eae4.

On page <NUM> of the document (English text) it is stated that "The chronic toxicity study on fish (Danio rerio) according to OECD TG <NUM> indicates that fish are the most sensitive organism in case of chronic exposure"; towards the end of the same opinion page, it is stated that "In this case, the most stringent classification and M-factor is based on the results of the chronic toxicity value with fish (Danio rerio). Therefore, RAC considers that PAA warrants chronic classification as: Aquatic Chronic <NUM> based on NOEC = <NUM>/L for Danio rerio. As this chronic toxicity value falls within the <NUM> < NOEC ≤ <NUM>/L range, the chronic M-factor is <NUM>.

This ECHA opinion therefore recognizes the general validity of the toxicity results obtained in zebrafish, and the fact that these results can be used as indicators of toxicity for all species.

Based on Scheerbaum's work, it can be deduced that the concentration value of PAA, in the waters downstream of a disinfection treatment, for which there are no longer toxic effects on zebrafish, is <NUM>×<NUM>-<NUM> mg/l (data also included in the "<NPL>"); this maximum admissible value is indicated in the sector as PNEC (from Predicted No Effect Concentration).

Following studies on Daphnia magna reported in the "<NPL>", toxic effects have been identified also for H<NUM>O<NUM>; in this case the PNEC value is <NUM>×<NUM>-<NUM> mg/<NUM>. Further relevant documents are <CIT>, <CIT>, <CIT>.

Following these studies, the competent bodies have expressed reservations on the possibility of using the PAA solutions currently on the market for water disinfection.

The object of the present invention is to provide a novel aqueous solution formulation based on peroxyacetic for disinfecting wastewater from biological purifiers, which has a disinfection efficacy comparable to the solutions currently in use but reduced or no toxicity, as well as providing the method for using the aqueous solution in the disinfection of said waters.

These objects are achieved according to the present invention, which in a first aspect thereof relates to a solution containing peroxyacetic acid and hydrogen peroxide in different ratios compared to those of the known solutions comprising, by weight:.

provided that the percentage by weight of peroxyacetic acid calculated on the sum of peroxyacetic acid and hydrogen peroxide weights is at least equal to:.

In the second aspect thereof, the invention relates to a method for the disinfection of wastewater from biological purifiers through the use of the solution described above, which consists in dosing a predetermined amount of solution in the water to be treated, leaving the water thus added in a treatment/containment tank for a time between <NUM> and <NUM> minutes, preferably between <NUM> and <NUM> minutes, and then unloading the tank.

In the following description, all percentages are to be intended by weight unless otherwise indicated.

The invention is based on the observation by the inventors that the bactericidal effect of these solutions essentially depends only on PAA, while the toxicity of the wastewater downstream of the treatment also, if not above all, depends on hydrogen peroxide. The solutions of the invention are used in amounts such as to keep the amount of PAA essentially constant per cubic meter of water used in the known methods.

In the field of environmental studies, the PEC/PNEC ratio is used as a parameter for evaluating the effects of chemical compounds, where PEC stands for "Predicted Environmental Concentration" (see for example the report by the <NPL>, available online at the link: https://www. no/globalassets/project/erms/reports/erms-report-no-10_pec_pnec-to-risk-ssd_tno. In order for PAA solutions to produce, downstream of the treatment, nontoxic concentrations of PAA and H<NUM>O<NUM> species, the condition PEC/PNEC < <NUM> should be satisfied, where the PEC and PNEC values considered are the cumulative values for the two species indicated above.

The inventors observed that the solutions of the invention are able to obtain the desired disinfection effect, while producing concentrations of PAA and H<NUM>O<NUM> in the water downstream of the treatment such as to satisfy the condition PEC/PNEC < <NUM>.

Preferred solutions for the purposes of the present invention comprise, by weight:.

Even more preferably, the solutions of the invention comprise, by weight:.

In particular, the preferred composition of the invention comprises, by weight:.

The peroxide stabilizers which can be used in the invention are those known for the purpose in the chemistry field and they can be, for example, a mixture of dipicolinic acid and hydroxy-ethylene-diphosphonic acid in an amount lower than <NUM>% by weight in the solution, in the presence of a mineral acid in an amount lower than <NUM>% by weight in the solution.

In the preparation of the solutions of the invention, it is necessary to operate in such a way as to avoid entering the explosive range of the mixture; for this purpose, hydrogen peroxide is added to acetic acid, and the mixture thus obtained is then added to PAA.

Table <NUM> shows, as an example, the amounts of standard solution of the prior art and of a solution of the present invention necessary to add <NUM> of peroxyacetic acid to a cubic meter of wastewater from a biological purifier. The standard solution contains <NUM>% of peroxyacetic acid, <NUM>% of hydrogen peroxide and <NUM>% of acetic acid (balance with water and known peroxide stabilizers); the solution of the invention contains <NUM>% of peroxyacetic acid, <NUM>% of hydrogen peroxide and <NUM>% of acetic acid (balance with water and known peroxide stabilizers). The table shows the amounts of PAA, hydrogen peroxide and acetic acid which are added to the water through these solutions. The percentage values of the PAA/(PAA+H<NUM>O<NUM>) weight ratio for the two solutions are also reported.

Table <NUM> shows instead the estimated amounts of PAA, H<NUM>O<NUM> and AcOH which are released together with the treated solution, assuming a PAA conversion during the treatment equal to <NUM>% and an average conversion of H<NUM>O<NUM> equal to <NUM>%, and considering a treatment time of <NUM> minutes and a dilution factor of <NUM>; the dilution factor is determined (with a conservative estimate) by the flow of treated wastewater in the watercourse or reservoir (e.g. river) into which it is finally unloaded.

The last lines of the table also show the values of the PEC/PNEC ratio, for the individual PAA and H<NUM>O<NUM> species and for the total, obtained with the two solutions.

The amounts of AcOH leaving the treatment are higher than the initial ones due the fraction deriving from PAA decomposition.

As can be seen from the data in Table <NUM>, using the solutions of the invention with the same PAA added, and therefore the same bactericidal effect, the PEC/PNEC TOT value is much higher than <NUM> in the case of the known solution, and lower than <NUM> in the case of the invention, prove of the absence of toxic effects of the latter.

In its second aspect, the invention relates to the method for using the solutions described above for the disinfection of wastewater from biological purifiers. The method consists in dosing a desired amount of solution in the water to be treated; typically this amount is between <NUM> and <NUM> of solution of the invention per cubic meter of water. The treatment consists in leaving the water thus added in a treatment tank for between <NUM> and <NUM> minutes, and then unloading the tank; these treatment times are generally obtained with tanks with a "labyrinth" system to guarantee the desired contact time between the wastewater and the PAA.

The invention will be further illustrated by the following examples.

This example refers to the verification of the biocidal properties of a solution of the invention and, for comparison, of a standard solution of the prior art.

The solutions tested are the same as reported in Table <NUM> above. To carry out the tests, the solutions were diluted to a concentration of <NUM>% by weight with hard water and the test microorganisms were added separately to each container.

Suspensions of test microorganisms were standardized by dilution using a tryptone saline solution (previously sterilized at <NUM> for <NUM> minutes) and bringing the final concentration of the bacterial inoculum into contact with the product, as follows:.

A test with the two solutions was carried out for each of the bacterial cultures indicated above and for each of the predefined contact times, shown below, between solutions and said cultures.

A culture control sample was collected immediately and represents the concentration present at the start of the test, i.e. time zero. At time intervals of <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM> and <NUM> minutes, a <NUM> aliquot was homogenized by shaking and transferred to a filtration apparatus equipped with a <NUM> pore membrane and immediately filtered. <NUM> of rinsing liquid were then transferred into the filtration apparatus and immediately filtered. The filtration procedure was completed with a filtration with <NUM> of sterile water. After filtration, the membrane was laid on the surface of a Petri dish containing Agar. The plates were incubated at <NUM> ± <NUM> for <NUM> days. After incubation, using the calculated concentrations of CFU/<NUM> present at the start of the test and CFU/<NUM> concentration for each microorganism after the test intervals, the microorganisms decrease percentage were calculated by comparing the initial microbial concentrations with the final microbial concentrations; the decrease percentage was calculated with the formula (X - Y)/X × <NUM>, where X = number of CFU/<NUM> in the control immediately after inoculation and Y = number of CFU/<NUM> after the contact time.

The tests results are shown in Tables <NUM>, <NUM> and <NUM> for total Coliforms, Escherichia coli and Streptococcus faecalis, respectively.

This example refers to the verification of the toxicity properties of solutions containing different concentrations of hydrogen peroxide.

Two solutions were prepared, one of the invention and one for comparison. The solution of the invention had essentially the same composition as the solution used for the test of Table <NUM>, i.e., <NUM>% peroxyacetic acid, <NUM>% hydrogen peroxide and <NUM>% acetic acid, balance to <NUM>% with water and known peroxide stabilizers. To highlight the effects of hydrogen peroxide, the comparison solution had a high concentration of this compound; the composition of this solution was <NUM>% peroxyacetic acid, <NUM>% hydrogen peroxide and <NUM>% acetic acid, balance to <NUM>% with water and known peroxide stabilizers.

The two solutions were dosed in water in amounts of <NUM>/l and <NUM>/l, respectively, to have the same concentration of peroxyacetic acid, i.e., <NUM>/l, in the resulting solution.

The two solutions thus obtained were used for toxicity tests on embryos of zebrafish (Danio rerio), carried out according to the OECD <NUM>:<NUM> standard, over a time span of <NUM> hours. The results of the two tests were a mortality of zebrafish embryos equal to <NUM>% in the case of the comparison solution, and equal to <NUM>% in the case of the solution of the invention.

As can be seen from the data in Tables <NUM>-<NUM>, the solutions of the invention have bactericidal properties essentially comparable to those of the prior art solution, but with a lower or distinctly lower H<NUM>O<NUM> content; this confirms that H<NUM>O<NUM> has essentially no influence on the bacterial load abatement properties, while its decrease according to the present invention allows to reduce the toxicity of the waters released downstream of the antibacterial treatment, as evidenced by the PEC/PNEC value data shown in Table <NUM>.

Claim 1:
Aqueous solution comprising, by weight:
- from <NUM> to <NUM>% of peroxyacetic acid;
- from <NUM> to <NUM>% of hydrogen peroxide;
- from <NUM> to <NUM>,<NUM>% of acetic acid;
- water balance to <NUM>%,
provided that the amount of peroxyacetic acid is at least equal to <NUM>% with respect to the sum of the weights of peroxyacetic acid and hydrogen peroxide.