WO1991013036A1

WO1991013036A1 - Process for purification of waste water

Info

Publication number: WO1991013036A1
Application number: PCT/FI1991/000060
Authority: WO
Inventors: Martti Savolainen; Kirsti Jokinen
Original assignee: Oy Keskuslaboratorio - Centrallaboratorium Ab
Priority date: 1990-02-27
Filing date: 1991-02-27
Publication date: 1991-09-05
Also published as: FI900994A; FI95235C; AU7335891A; FI900994A0; FI95235B

Abstract

The invention concerns a process for the purification of waste waters. According to the process the compounds dissolved or suspended in the waste water are separated by means of enzymatic oxidation and precipitation. According to the invention, there is added to the waste water at least one oxidoreductase, one oxidising agent, if necessary, at least one precipitation chemical and, if necessary, one substrate which is oxidised in the presence of the oxidoreductase such that at least a part of the precipitant, of the oxidising agent and of the enzyme substrate is added to the waste water no later than simultaneously with the oxidoreductase. With the help of the process it is possible to reduce the AOX, COD, phosphorus and nitrogen contents of pulp and paper industry waste waters.

Description

Process for purification of waste water

The present invention concerns a process in accordance with the preamble of claim 1 for purification of waste water.

According to such a process, the compounds dissolved and suspended in the waste water are separated by means of enzymati oxidation and precipitation.

Many waste waters and, in particular, the waste waters stemming from the pulp and paper industry and the mining industry contai soluble phenolic compounds, which are harmful and which decompose very slowly in nature. The pulp and paper industry waste waters contain, e.g., carbohydrates and organic acids, which can be removed by normal biological purification methods. In addition to compounds which easily decompose biologically, the waste waters of the forest industry and, in particular, of the pulping industry contain several different kinds of phenoli compounds which cannot be removed by means of modern biological purification processes.

When chlorine is used for bleaching of pulp, the paper and pulp industry waste waters also contain chlorinated organic compounds. Activated sludge plants are capable of reducing the amount of these compounds (AOX, adsorbable organic halogens) by 50 to 60 per cent, at the most.

By physical and chemical methods, a good colour reduction is attained, but these methods rather poorly reduce the biological oxygen demand (BOD). Generally, the use of chemical methods is limited by the high cost of precipitating chemicals, such as aluminium sulphate and trivalent iron salts. Iron(II)salts which are abundant in the waste waters of, i.a. , the chemical industry, cannot attain precipitation.

As far as the optimal operation of a conventional activated sludge plant is concerned, the waste waters of the forest industry contain very little nitrogen and phosphorus. These elements have to be added to assure a reasonable reduction of the BOD. This results, however, in increasing concentrations of nitrogen and phosphorus in the treated waste waters, if no separate measures are taken to decrease the amounts of said elements.

In sewage treatment plants, in which the waste water that is to be treated contains a lot of nitrogen and phosphorus, the nutrient load has been reduced by the application of direct chemical precipitation or by carrying out postprecipitation after the biological treatment plant. Furthermore, phosphorus has been precipitated in the aeration phase of the biological purification plant by an operation known as simultaneous precipitation (Maatta, R. , Ravinteiden poisto jatevesista, Kemian Teollisuus 26 (1969): 9, 713-724). During simultaneous precipitation ferrous sulphate is added to the biological phase for chemically binding the phosphorous. The bivalent iron ion is then converted to the trivalent ion in the aeration basin at a pH value of, e.g, between 6 and 8, and at a redox potential exceeding 350 mV (Maatta). How much soluble phosphorus and other compounds are precipitated depends on the amount of ferric compounds, the value of the ratio Fe(III)/Fe(II) should be kept high as possible. The equilibrium is dependent on the pH as well as on the redox potential. High pH values and high redox potentials shift the equilibrium towards the trivalent ion, as shown in figure 1 (Hem, J.D. , J. Amer. Water Works Ass. 53 (1961) 211).

Ferrous sulphate addition provides precipitation in sewage treatment plants but it is not applicable to purification of waste waters of a pulp mill. For waste waters stemming from the forest industry, colour and nitrogen reduction achieved is poor and the decrease in phosphorous concentration is also relatively small (Jokinen, S., Hyytia, H. , Vaananen, P. and Kukkonen. , Kemiallinen jatevedenpuhdistus metsateollisuudessa, Paperi ja PUU 70 (1988): 7, 610-613).

The poor precipitation effect obviously arises from the fact that when ferrous sulphate is added to purified or unpurified waste water from a pulp mill, the redox potential remains so low that the equilibrium between the ferric and the ferrous species lies on the ferrous species and no massive precipitation takes place.

Figure 2 depicts the purification result as a function of the pH value for the waste water of a certain pulp and paper mill (Jokinen, S., Kemiallinen jateveden puhdistus metsateollisuudessa, osa 1 69 (1987): 7 585-590). The figure shows that best precipitation results using trivalent iron are achieved in the pH range from 3.8 to 4.

It is known in the art that phenol oxidase enzymes, such as laccase (EC 1.10.3.2), catalyse the oxidation of lignins and generally of phenolic compounds in a redox reaction in which oxygen constitutes the substrate which is reduced (oxidising agent) . In the reaction there is enzymatically formed a phenoxy radical, which is further disproportionated in a non-enzymatic reaction. The final reaction products comprise mainly quinone, as well as dimeric and polymeric compounds. Ligninase enzymes and other peroxidase enzymes which utilize peroxides instead of molecular oxygen work in a similar way.

Laccase also catalyses the oxidation of compounds other than the phenolic ones. Thus, potassium ferrocyanide is a good substrate for laccase.

The phenolic compounds contained in forest industry waste waters such as debarking waste waters, waste waters from mechanical pulping and bleaching plants, can be oxidised and polymerised by using laccase in the presence of air (Forss, K. , Jokinen, . , Savolainen, M. and Williamson, H. , Utilization of enzymes for effluent treatment in the pulp and paper industry. Paperi ja Puu 71 (1989): 10, 1108-1112). The polymerised compounds do not generally precipitate from the solution without chemicals. Precipitation can, however, be achieved by, e.g., aluminium sulphate. Ferrous sulphate does not produce any settling precipitate.

It is an object of the present invention to eliminate the drawbacks of the prior art and to provide an entirely novel process for purification of waste waters, in particular industrial waste waters.

Our invention is based on the surprising observation that it is possible to achieve good flocculation of soluble or suspended compounds in waste water by adding an enzyme that catalyses redox reactions, i.e. an oxidoreductase, to waste water which contains - or to which is added simultaneously with the enzyme - a substrate which is oxidised, a substrate which is reduced and a precipitant. I.a. the colour, the suspended material, the COD, the AOX, the phosphorus and the nitrogen of the waste water, are efficiently removed.

In particular, the invention is mainly characterised by what is stated in the characterising part of claim 1.

Within the scope of the present invention, the term "oxidoreductase" is used generally to designate all kinds of enzymes which catalyse redox reactions, for instance oxidases and peroxidases.

Within the scope of the present invention, the material which is reduced during the redox-reaction is called "the oxidising agent". It usually comprises an oxygenous material, such as air, gaseous oxygen, ozone or hydrogen peroxide. Similarly, the material of the waste water which is oxidised is called "the enzyme substrate".

During normal oxidation of waste waters using only an oxidoreductase or by adding only a precipitation chemical, no massive precipitation takes place. We do not yet completely understand all reasons that lie behind this phenomenon, but it seems likely that when the oxidoreductase, such as laccase, catalyses the oxidation of the enzyme substrate, normally the phenolic compounds, the redox potential of the solution rises so much that the bivalent iron is converted into trivalent iron even at low pH values, i.e. at an optimal pH range (3.8 to 4.4) as far as trivalent iron precipitation is concerned. The influence of laccase on the redox potential is shown in figure 3, which depicts how the redox potential varies as a function of the laccase and/or ferrous sulphate added to the waste water of a pulp mill.

The figure shows that in the presence of only the oxidising substrate (oxidising agent) of the redox reaction, such as molecular oxygen, ferrous sulphate is not converted into trivalent iron in the waste water of a pulp mill. The addition of laccase provides, instead, a rapid increase of the redox potential.

It should be noticed that the relative order in which the precipitation chemical and the oxidoreductase are added is of essential importance as far as the workability of the process is concerned. As mentioned above, in the prior art (Paperi ja Puu (1989): 10, 1108-1112) no settling precipitate is obtained by adding ferrous sulphate after the addition of the enzyme. According to the invention, which comprises the addition of at least a part of the precipitation chemical before or preferably simultaneously with the enzyme, the dissolved and suspended materials are efficiently precipitated.

The process according to the invention can be applied to purification of different kinds of waste waters from the forest industry, such as the waste water of a pulp mill, the waste water from the manufacture of mechanical pulp or the waste water from debarking. As the following working examples will show, the waste water can be pretreated by biological methods, such as by the activated sludge process (examples 1 to 4) or by a corresponding anaerobic process, but it can also be untreated (examples 5 and 6).

The process can also be used for purification of waste waters in which the compounds or particles that are to be precipitated have already been oxidised or even inherently cannot be enzymatically oxidised. By adding to such waters an enzyme substrate which can be oxidised - in the case of phenol oxidases, phenolic compounds, such as preferably tannins from the. bark - before the enzymatic oxidation, the formation of a flocculent is achieved. Alternatively, the said substrate can be comprised of lignosulphonates, bleaching water from pulping, waste water from debarking, waste water from the preparation of mechanical pulp, hydroquinones, guaiacols or other phenolic compounds or potassium ferrocyanide or any other substrate which can be enzymatically oxidised.

According to an alternative embodiment of the invention, iron- containing waste water of the chemical industry is purified by adding waste water containing dissolved enzyme substrate(s) , in particular phenolic compounds which are oxidised under the influence of the enzyme. In this embodiment, no separate precipitation chemical needs to be added at all, but the dissolved iron works as precipitant and is precipitated together with the phenolic compounds after the enzymatic oxidation. If necessary, the pH value of the waste water is first raised to the level required in the enzymatic reaction by adding a base (c . below) .

According to the process of the invention the necessary enzymatic oxidation is carried out by using, e.g. , one or several isolated oxidoreductase(s) . The suitable enzymes are selected from the group consisting of different phenol oxidases, such as laccase and tyrosinase, and peroxidases, such as lignin peroxidase, manganese peroxidase and horseradish peroxidase (EC 1.11.1.7). The enzyme can also be added to the waster water in the form of microorganisms producing the above enzymes. The following examples may be mentioned: Polyporus hirsutus. Phanerochaete chrvsosporium. Trametes versicolor and other white rot fungi strains. According to a third alternative embodiment the enzyme(s) is (are) added in the form of fermentation broths containing them and used for fermenting said microorganisms or suitable plants and bacteria.

The amount of enzyme added to the waste water varies depending on the amount of dissolved compounds contained in the waste water and it typically lies below 5 U/ml (1 ϋ = the change of absorbance per minute at the wave length 470 nm, the reducing substrate being guaiacol in citrate-phosphate buffer at pH 4.5), Preferably the added amount is less than 0.5 or even less than 0.05 U/ml.

The amount of precipitant needed for achieving precipitation varies depending on the amount of dissolved and suspended compounds. The amount of ferric sulphate (Fe₂(S0₄)₃) needed in direct precipitation with trivalent iron has been reported as being 850 to 1000 mg/1 at a COD of 600 mg/1. In this way, the COD can be decreased by 72 to 88 per cent (Jokinen, S., Hyytia, H. , et al.). According to the present invention considerably less ferrous sulphate (FeS0₄ 7H₂0) is needed for providing efficient precipitation. It is preferred to add at the most about 1000 mg, in particular less than 400 mg and preferably even less that about 200 mg ferrous sulphate for each litre of waste water having a COD of 500 mg/1.

Instead of ferrous sulphate other suitable precipitants may be used such as ferric or aluminium sulphate. The precipitant can also be added in the form of an aqueous solution containing ferrous or ferric or aluminium ions, such as iron-containing waste water from the chemical industry. Since the pH of such waste waters is also rather low (even below 1) , they are particularly suitable for neutralisation and enzymatic oxidation of waste water from the alkaline stage of pulp bleaching, leading to the desired precipitation.

A part of the precipitant can be added to the waste water already in connection with any pretreatment, such as in the case of simultaneous precipitation to the equalization basin or to the aeration basin of an activated sludge plant. The enzyme and the additional precipitant are thereafter added to the clarified waste water of the activated sludge plant, precipitation taking place after oxidation.

In addition to the precipitant some known chemical for improving flocculation can be used in the process. Depending on the composition of the waste water, different kinds of cationic polymers, anionic polymers and polyelectrolytes can be employed. The flocculation improving chemical is added before or after the addition of the oxidising agent.

The pH value of the process according to the invention is kept between 2.5 and 7. In particular, the pH is in the beginning of the oxidation between 4.5 and 5.5 from which value it decreases during oxidation typically to between 3.3 and 4. The temperature is from 15 to 60°C, preferably the oxidation is conducted at 40°C or at the normal operational temperature of an activated sludge plant.

The oxidising agent, used in the precipitation phase usually comprises air or pure oxygen (aeration) . Also ozone or hydrogen peroxide may, however, be used.

The invention provides considerable benefits in comparison to conventional chemical precipitation. Thus, the precipitant can comprise waste water from the chemical industry which normally is a waste chemical that only incurs costs. The applied chemical dosage is small and, in particular, the reduction of AOX, COD, colour, phosphorus, nitrogen and suspended material is very good.

In the following, some working examples are described which illustrate the process according to the invention. However, it should be understood that these examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modification within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.

Of the drawings enclosed

figure 1 depicts the occurrence of different ferric and ferrous species in waste water figure 2 shows the determination of the optimal pH value during ferric sulphate precipitation of waste water from a plant for preparing mechanical pulp and paper and figure 3 shows the changes in the redox potential as a function of time during addition of laccase and/or ferrous sulphate to waste water from a pulp mill.

Example 1

Table 1 indicates the analysis of the total waste water from a pulp mill before and after aerobic biological purification on laboratory scale.

Table 1. Analysis of waste water from a pulp mill before and after biological purification on laboratory scale.

before after

COD, g/1 0.80 0.52

AOX, mg/1 14.2 14.1

The sample was not filtered after the biological purification, the AOX value being relatively high of this reason.

To pulp mill waste water purified by the activated sludge process there is added 1 U/ml laccase and 200, 400 and 600 mg/1 ferrous sulphate. Air or pure oxygen is fed to the waste water for 90 min. after which period of time the waste waters are clarified in a 1 litre funnel.

Table 2 shows the influence of laccase treatment and ferrous sulphate addition on the analysis of the pulp mill biologically purified waste water. The samples comprise clarified waste water and they were not filtered before the analysis. Table 2. Simultaneous laccase oxidation and ferrous sulphate precipitation of pulp mill waste water

FeS0₄-7 H₂0 mg/1 600 0 600 400 200

Laccase, U/ml 0 1 1 1 1

COD, g/1 0.50 0.49 0.16 0.16 0.14

AOX, mg/1 13.5 12.7 4.3 4.2 4.4

P, mg/1 1.48 2.55 0.07 0.08 0.09

N, mg/1 1.0 1.6 0.6 0.7 0.7

Fe, mg/1 124 49 14 12

As shown in table 2, the mere addition of ferrous sulphate to the waste water followed by oxidation with oxygen does not essentially decrease the COD and AOX values. Oxidation with laccase without ferrous sulphate addition reduces the AOX content by ca 10 per cent, which shows that laccase is a dechlorinating enzyme.

Table 2 indicates further that by adding simultaneously laccase and ferrous sulphate to the waste water, it is possible to reduce the COD and AOX values by means of oxidation by up to 70 per cent. There is even a 95 per cent drop in the phosphorus content in comparison to the addition of ferrous sulphate only. The results further show that the added iron is also efficiently precipitated.

Example 2

To pulp mill waste water purified by means of the activated sludge process there is added 0.25 and 0.37 U/ml laccase and 500 mg/1 ferrous sulphate. Otherwise, the oxidation and precipitation is conducted as indicated in Example 1.

Table 3 gives the results of the enzymatic oxidation and precipitation. Table 3. Laccase oxidation of pulp mill waste water, ferrous sulphate dosage 500 mg/1

Laccase, U,ml 0 0.25 0.37

Precipitate, % (v/v) 0 8 8

AOX, mg/1 13.0 4.4 4.4

COD, g/1 0.50 0.14 0.15

P, mg/1 1.44 0.10 0.15

N, mg/1 1.4 0.8 0.7

The results of table 3 show that the AOX is decreased by 67 per cent and the COD by 72 per cent in comparison to a test, wherein only ferrous sulphate is added to the water.

The phosphorus content is decreased by 93 per cent at the most.

The enzyme amount 0.25 U/ml gives a 8.2 per cent precipitation (vol./vol.) and 0.37 U/ml 8 per cent.

After 18 hours, the reference test pH (of the clarified water) is 4.4, in the 0.25 U/ml test the pH is 3.4 and in the 0.37 U/ml test 3.3.

Example 3

The laccase treatment efficiency and speed can be improved by adding phenolic compounds to the waste water. Thus, by adding to pulp mill waste water small amounts of bark press water of a debarking process containing, i.a., plenty of tannins which are oxidised by laccase, the redox potential is increased during laccase oxidation distinctly to a higher level than without debarking water addition. The precipitation following the oxidation takes place at a higher rate and more efficiently.

To waste water purified by the activated sludge process there is added bark press water, laccase and ferrous sulphate. After a short oxidation (60 to 90 min), the dissolved compounds are precipitated and the waste water is clarified. Table 4 indicates the redox potentials of the waste waters after oxidation and the decrease of absorbance measured at the wave length 280 nm. The absorbance value reflects the amounts of colour and COD of the waste water.

Table 4. The influence of debarking water addition to the redox potential of pulp mill waste water during laccase oxidation in the presence of ferrous sulphate. The added amounts of ferrous sulphate: 400 mg/1 and the added amounts of laccase: 0.5 U/ml

Debarking Eh, mV pH abs. 280 nm Fe water ml/1000 ml after ox. after ox. decrease, % mg/1 waste water

0 ml 560 4.0 75 9

20 ml 600 3.6 80 7

40 ml 630 3.5 80 6

As shown in table 4, the redox potential of the waste water is increased after the addition of debarking water and at the same time the amount of dissolved phenolic compounds and other compounds colouring the water is decreased. The addition of debarking water also reduced the amount of iron retained in the solution.

Example 4

Iron-containing waste water of the chemical industry can also be used for precipitation. To effluent waste water of an activated sludge plant of a pulp mill, 10, 15 and 20 per cent (vol./vol.), respectively, of ferrous waste water is added. The analysis of the waste water is shown in table 5. Table 5. Analysis of waste water from the chemical industry containing ferrous sulphate.

Sulphuric acid 10.5 g/1 Fe 2.37 g/1 pH 0.6

The pH is adjusted to between 5.0 and 5.5 before the addition of laccase.

The redox potential of the pulp mill waste water and the iron- containing waste water rises during oxidation from between 300 and 350 mV to between 510 and 610 mV. Clarification of the solution takes place subsequently. After precipitation, the colour of the waste water is light and the waste water is clear. Table 6 contains the results obtained for the decrease in absorbance at 280 nm and the redox potentials before and after oxidation.

Table 6. Changes in absorbance and redox potentials

Ferrous waste Eh, mV pH abs. 280 nm water, ml/1000 ml init. after oxid. after oxid. pulping waste water

original waste water - - - 5.8

100 ml 350 510 5.3 1.5

150 ml 300 500 5.1 1.0

200 ml 325 610 4.0 1.2

From the results of table 6 it appears that the waste water absorbance at the wave length 280 nm decreases with 74 to 83 per cent, i.e. the dissolved compounds are efficiently precipitated. Example 5

Ferrous sulphate and laccase are added to waste water formed during debarking. After aeration precipitation takes place, the colour of the water turning light and clear. After the addition of ferrous sulphate the waste water becomes black and turbid and no flocculation is achieved. Table 7 contains the results of enzymatic oxidation and ferrous sulphate precipitation of the debarking water.

Table 7. Enzymatic flocculation of debarking water with ferrous sulphate. The added amount of laccase: 1 U/ml.

Ferrous Eh, mV pH abs. 280 nm COD BOD P sulphate at the end at the end at the end g/1 g/1 mg/1 mg/1

0 400 4.1 14.0 3.02 1.32 5.3

400 655 3.2 6.0 1.93 0.75 0.04

600 660 2.9 6.1 2.03 0.85 0.03

800 650 2.7 8.1 2.18 0.93 0.28

Example 6

Laccase (0.5 U/ml) and ferrous sulphate is added to the total waste water from a pulp mill taken from an equalization basin before the aeration basin of an activated sludge plant. Oxygen is conducted through the mixture during a period of 60 to 90 min. , after which period of time flocculation takes place. Table 8 indicates the analysis of the light and clear waste water formed after flocculation. Table 8. Analysis of waste water

Ferrous Eh, mV pH AOX COD BOD N sulphate at the end at the end mg/1 mg/1 mg/1 mg/1 mg/1

600 no lace. 220 4.4 25.7 810 115 2.50

400 + 0.5 U/ml 600 3.3 10.1 430 110 0.34

600 + 0.5 U/ml 550 3.3 9.9 390 120 0.15 800 + 0.5 U/ml 555 3.3 9.7 410 120 0.29

The process provides good reductions of the COD and the AOX when the waste water is pretreated before the activated sludge treatment. Chemical precipitation is not very efficient when it comes to removing the compounds causing biological oxygen demand, as evidenced by the BOD values.

Example 7

Ferrous sulphate, 500 mg/1, and horseradish peroxidase (EG.1.11.1.7) and hydrogen peroxide (6 umol/1) as oxidising substrate are added to waste water purified in the activated sludge plant of a pulp mill. After mixing (15 to 60 min.) a polymer (Fennopol N 300) is added and the water is clarified.

No enzyme is added in the control test. The addition of the hydrogen peroxide increases the redox potential of the water and precipitation is attained. The addition of peroxidase improves the precipitation of the dissolved compounds as indicated by the greater decrease in absorbance at 280 nm. The absorbance of the clarified waste water was 5.1 in the control test whereas it amounted to 4.2 upon addition of peroxidase. The absorbance of the original untreated waste water amounted to 5.5.

Claims

Claims :

1. Process for the purification of waste waters, according to which process the dissolved and suspended compounds of the waste water are separated by means of enzymatic oxidation and precipitation, c h a r a c t e r i s e d by adding to the waste water

- at least one oxidoreductase enzyme,

- at least one oxidising agent, - if necessary, at least one precipitant, and

- if necessary, enzyme substrate which is oxidised in the presence of the oxidoreductase, with the proviso that at least part of the precipitant, at least part of the oxidising agent, and at least part of the enzyme substrate are added to the waste water before or no later than at the same time as the oxidoreductase.

2. The process as claimed in claim 1, c h a r a c t e r i s e d by adding - the oxidoreductase,

- at least a part of the precipitant and

- at least a part of the oxidising agent at the same time to the waste water.

3. The process as claimed in claim 1 or 2, c h a r a c t e r i s e d in that the oxidoreductase is selected from the group consisting of phenol oxidases, such as laccase or tyrosinase, and peroxidases, such as lignin peroxidase, potassium peroxidase or horseradish peroxidase.

4. The process as claimed in claim 1 or 2, c h a r a c t e r i s e d by adding the oxidoreductase to the waste water in the form of an oxidoreductase-containing fermentation broth in which plants, fungi or bacteria have.been cultivated.

5. The process as claimed in claim 1 or 2, c h a r a c t e r i s e d by adding the oxidoreductase to the waste water in the form of a microorganism producing the same. said microorganism being selected from the group consisting of Polvporus hirsutus. Phanerochaete chrvsospoT-iirm_r Trametes versicolor and equivalent white rot fungi strains.

6. The process as claimed in any of the previous claims, c h a r a c t e r i s e d by adding the oxidoreductase to the waste water at a temperature of from 15 to 65°C, preferably at a temperature of from 20 to 50°C and at a pH value of from 2.5 to 7.0 preferably at a pH value of from 3.5 to 5.5.

7. The process as claimed in any of claims 1 to 6, in which the waste water is subjected to a separate pretreatment, c h a r a c t e r i s e d by adding at least a part of the precipitant during the pretreatment of waste water.

8. The process as claimed in any of the previous claims, c h a r a c t e r i s e d in that the precipitant is selected from the group consisting of ferrous salt, ferric salts and iron-containing water.

9. The process as claimed in claim 8, c h a r a c t e r i s e d by adding a maximum of 1000 mg, preferably less that 400 mg and in particular at the most

200 mg of FeS0₄"7 H₂0 for each 500 mg of COD of the waste water.

10. The process as claimed in any of the previous claims, c h a r a c t e r i s e d by adding while aerating simultaneously laccase and ferrous sulphate to waste water obtained from a biological pretreatment.

11. The process as claimed in any of the previous claims, c h a r a c t e r i s e d by adding to the waste water a substrate which is oxidised in the presence of the oxidoreductase.

12. The process as claimed in claim 11, c h a r a c t e r i s e d in that said substrate which is oxidised is selected from the group consisting of tannin, lignosulphate, pulp bleaching water, debarking waste water. waste water from the preparation of mechanical pulp, hydroquinone, guaiacol and other phenolic compounds, potassium ferrocyanide and equivalent substrates which can be enzymatically oxidised.

13. The process as claimed in any of claims 1 to 12, c h a r a c t e r i s e d in that a flocculation improving chemical is used in addition to the precipitant.

14. The process as claimed in any of the previous claims, c h a r a c t e r i s e d in that the oxidising agent is selected from the group consisting of air, oxygen, ozone and hydrogen peroxide.