FI95235B - Method for removal of AOX, COD, color, nitrogen and phosphorus from forest industry effluents - Google Patents

Description

95235

Process for the removal of AOX, COD, color, nitrogen and phosphorus from forest & industrial effluents

The present invention relates to a method for the treatment of waste water according to the preamble of claim 1.

According to such a method, the compounds dissolved and suspended in the wastewater are separated by enzymatic oxidation and precipitation.

Many wastewaters, especially those from the forest and mining industries, contain soluble phenolic compounds that are harmful and degrade very slowly under natural conditions. Wastewater from the forest industry includes e.g. carbohydrates and organic acids that can be removed by conventional biological purification methods. In addition to readily biodegradable compounds, wastewater from the forest industry, and in particular from pulp mills, contains a wide variety of phenolic compounds that cannot be removed by current biodegradation processes.

When chlorine is used for pulp bleaching, the pulp mill effluents also contain organochlorine compounds. Activated sludge plants are able to reduce the amount of these (AOX, adsorbable organic halogens) by 50___60% at best.

Physical and chemical methods provide • · ·· good decolorization, but these methods eliminate its relatively poor biological oxygen demand (BOD). The use of chemical methods is usually limited by the high cost of precipitating chemicals such as aluminum sulfate and iron trivalent salts. Iron divalent salts, which are abundant e.g. in the effluent of the chemical industry, no precipitation is caused instead.

Wastewater from the forest industry is low in nitrogen and phosphorus for the optimal operation of a conventional activated sludge plant, which is why these have to be added to the waste water to ensure a good BOD reduction. However, as a result, without separate measures, the nitrogen and phosphorus concentrations in wastewater will increase, i.e. the nutrient load in discharge waters will increase.

Municipal wastewater treatment plants, which are rich in nitrogen and phosphorus, have used chemical direct precipitation, post-precipitation after biological treatment or precipitation of phosphorus in the climatic phase of the biological treatment plant, the latter of which is called co-precipitation (H.), to reduce the nutrient load. Nutrient removal from wastewater, Chemical Industry 26 (1969): 9, 713-724).

In co-precipitation, ferrous sulfate is added to the biological step to chemically bind phosphorus. Divalent iron changes to trivalent in the aeration tank when the pH is eg 6 ... 8 and the redox potential is over 350 mv (Määttä).

Since the precipitation of soluble phosphorus and other compounds depends on the amount of ferric compounds, the Fe3 + / Fe2 + ratio should be as high as possible. Equilibrium is dependent on both pH and redox potential. At high pH and redox potential, the equilibrium is on the trivalent iron side, as seen in Figure 1 (Hem, J.D., J. Amer. Water Works M. 'ASS. 53 (1961) 211).

1 · ·

The addition of ferrous sulphate works in municipal wastewater treatment plants, but not in connection with the treatment of wastewater from a pulp mill. Color and nitrogen reduction is poor in forest industry wastewater and the decrease in phosphorus content is relatively small (Jokinen, S. Hyytiä, H., Väänänen, P. and Kukkonen, K., Chemical wastewater treatment in the forest industry,

Paper and Wood 70 (1988): 7610-613).

The poor precipitation is apparently due to the fact that when ferrous sulphate is added to either treated or untreated pulp mill effluent, the redox potential remains so low that the Ferri / Ferro equilibrium is on the divalent iron side and no massive precipitation occurs.

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Figure 2 shows the pH dependence of a wastewater treatment in a forest industry (Jokinen, S., Chemical wastewater treatment in the forest industry, part 1 69 (1987): 7 585-590). It can be seen from the figure that trivalent iron precipitates best in the pH range of 3.8 ... 4.4.

It is known that phenol oxidases such as laccase (EC

1.10.3.2), catalyze the oxidation of lignins and phenolic compounds in general in a reaction in which oxygen in the air acts as a reducing substrate. The reaction enzymatically forms a phenoxy radical which is further disproportionated in the non-enzymatic reaction. The end products of the reaction are mainly quinone, dimeric and polymeric products. Ligninases and other peroxidases, which require peroxides instead of molecular oxygen, work similarly.

Your lacquer also catalyzes the oxidation of non-phenolic compounds. For example, potassium ferrocyanide is a good substrate for laccase.

Laccase and oxygen in the air cause oxidation and polymerization of phenolic compounds in forest industry effluents, such as bark water, mechanical pulp production and bleaching plants (Forss, K., Jokinen, K., Savolainen, M. and Williamson, H., Utilization of enzymes for effluent treat - • ment in the pulp and paper industry, Paper and Wood 71 (1989): "10, 1108-1112). Polymerized compounds generally do not precipitate from solution without chemicals. However, precipitation is effected, for example, with aluminum sulphate. Ferrosulphate does not precipitate.

The object of the present invention is to eliminate the drawbacks associated with the prior art and to provide a completely new method for the treatment of waste water, in particular industrial waste water.

.. Our invention is based on the surprising finding that the addition of an enzyme that catalyzes redox reactions, i.e., 4,952,35 oxidoreductase, to wastewater containing - or co-introduced with - the enzyme - oxidizing substrate, reducing substrate, and precipitating chemical is provided to provide efficient effluent treatment. flocculation of compounds, whereby e.g. color, suspended material, COD, AOX, phosphorus and nitrogen.

More specifically, the method according to the invention is mainly characterized by what is set forth in the characterizing part of claim 1.

In the context of the present invention, the term "oxidoreductase" generally refers to enzymes which catalyze redox reactions, in particular oxidases and peroxidases.

In the Redox reaction, the reducing agent, in turn, is referred to in this application as a "reducing substrate", which usually consists of an oxygen-containing substance such as air, oxygen gas, ozone or hydrogen peroxide. Similarly, the oxidizing agent in wastewater is referred to as an "oxidizing substrate."

In the oxidation of wastewater catalyzed by oxidoreductase alone or by the addition of a precipitating chemical alone, no mass. . . vista precipitation happen. We do not yet fully understand all the causes of this phenomenon, but it seems likely that an oxidoreductase such as laccase catalyzes the oxidation of an oxidizable substrate, usually phenolic compounds, while the redox potential of the solution rises so high that divalent iron becomes trivalent at low pH, i.e. in the optimum pH range for iron precipitation (3.8 ... 4.4). The effect of laccase on the redox potential is seen in Figure 3, which shows the changes in redox potential as a function of time with the addition of laccase and / or ferrous sulphate to the pulp mill effluent.

It can be seen from the figure that in the presence of a reducing substrate of the redox reaction alone, such as molecular oxygen, ferro-95235 iron does not become trivalent in the pulp mill effluent. Addition of laccase, on the other hand, causes a rapid increase in redox potential.

It should be noted that the relative order of addition of the precipitating chemical and the oxidoreductase is essential for the performance of the method. As stated above, in the prior art [Paper and Wood 71 (1989): 10, 1108-1112], a settling precipitate was not obtained by adding ferrous sulfate only after enzyme addition. According to the invention, wherein at least a part of the precipitating chemical is added before, or preferably simultaneously with the enzyme, the dissolved or suspended substances are effectively precipitated.

The method according to the invention can be applied to the treatment of various wastewater from the forest industry, such as the effluent of a pulp mill, the effluent from the production of mechanical pulp or the effluent from peeling. As can be seen from the working examples below, the effluent may be biologically purified, e.g. by the activated sludge method (Examples 1-4) or anaerobic method, respectively, but may also be untreated (Examples 5 and 6).

The method can also be used to treat wastewaters in which the compounds or particles to be doped have already been oxidized or do not initially oxidize enzymatically. By adding an oxidizing enzyme substrate to such water - in the case of phenol oxidases - phenolic compounds, such as tannins preferably contained in the shell - before enzymatic oxidation, the formation of joy is achieved. As the oxidizing substrate, in addition to tannin, lignosulfonate, pulp bleaching water, peel effluent, effluent from mechanical pulp production, hydroquinone, guaiacol or another phenolic compound or potassium ferrocyanide or other enzymatically oxidizing substrate can be used. 1 · «6 95235

According to an alternative embodiment of the invention, the ferrous effluent of the chemical industry is purified by adding to it effluent which contains an oxidizable substrate in the solution phase, in particular phenolic compounds oxidized by the action of an enzyme. In this embodiment, no separate precipitating chemical needs to be added at all, but soluble iron acts as a precipitant and precipitates with phenolic compounds after enzymatic oxidation. If necessary, first raise the pH of the effluent by adding alkali to the level required for the enzymatic reaction (see below).

In the process according to the invention, the necessary enzymatic oxidation is carried out, e.g. with one or more isolated oxidoreductases. Suitable enzymes include various 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 introduced into wastewater as microorganisms producing said enzymes, for example: Polyporus millet, ghanerochaete Chrysosporium. Trametes veraicolor and other strains of white rot fungi. According to a third alternative embodiment, the enzyme (s) are added as growth solutions containing them, in which said microorganisms or suitable plants or bacteria have been grown.

The amount of enzyme added to the effluent varies with the amount of compounds dissolved in the effluent and is typically less than 5 U / ml (1 U 1 change in absorbance per minute at 470 nm, as oxidizing substrate in guaiacol citrate-phosphate buffer, pH 4.5), preferably less than 0.5 U / ml or even less than 0.05 U / ml.

'· The amount of precipitating chemical required to effect precipitation • · 1 varies depending on the dissolved and suspended compounds. In direct precipitation with trivalent iron, ferric sulphate (Fe2 (804) 3) has been sufficiently represented at 850 ... 1000 mg / l with a COD content of 600 mg / l in the effluent. In this case, COD can be reduced by 72 ... 88% (Jokinen, S., 7 95235

Hyytiä, H. et al.). The process according to the invention requires clearly less ferrous sulphate (FeSO 4 * 7H 2 O) to obtain a good precipitation result. Preferably at most about 1000 mg, particularly preferably less than about 400 mg and most preferably even less than about 200 mg of ferrous sulphate / l with a COD of 500 mg / l are added.

Instead of ferrous sulphate, other suitable precipitating chemicals known per se, such as ferric or aluminum sulphate, can be used as the precipitating chemical. The precipitating chemical can also be added in the form of an aqueous solution containing ferrous, ferric or aluminum ions, such as ferrous wastewater from the chemical industry. Because the pH of such effluents is often quite low (even <1), they are particularly well suited for neutralizing and enzymatically oxidizing the alkaline effluent from the pulp bleaching process to achieve the desired precipitation.

Some of the precipitating chemical can be added to the wastewater already in connection with a possible pretreatment, such as in the case of co-precipitation in the equalization tank or aeration section of the activated sludge plant. The enzyme and additional precipitation chemical are then added to the clarified effluent of the activated sludge plant, whereupon precipitation occurs after oxidation.

In addition to the precipitating chemical, a flocculation enhancing chemical known per se can be used in the process, in which different cationic polymers, anionic polymers and polyelectrolytes can be used depending on the composition of the effluent. The flocculant enhancing chemical is added before or after.

In the process according to the invention, the pH is 2.5 to 7. Particularly preferably, the pH at the beginning of the oxidation is 4.5 to 5.5, from which it typically falls to 3.3 to 4 in the oxidation. The temperature is 15 to 60 ° C, preferably the oxidation is carried out at 40 ° C or at the typical operating temperature of the activated sludge plant.

Air or pure oxygen (aeration) is usually used as the oxidizing agent required in the precipitation step, i.e. the reducing substrate. However, ozone and hydrogen peroxide may also be considered.

The invention has considerable advantages over conventional chemical precipitation. Thus, the most costly waste chemical generated in the chemical industry can be used as the precipitating chemical. The dosage of the chemical used is low and the reduction of AOX, COD, color, phosphorus, nitrogen and suspended matter in particular is very good.

The following are examples that illustrate the method of the invention.

Of the accompanying drawings, Figure 1 shows the presence of different forms of ferric and ferro in wastewater, Figure 2 shows the determination of the optimum pH in ferrous sulphate precipitation of mechanical pulp and paper mill effluent and Figure 3 shows changes in redox potential as a function of laccase and / or ferrous sulphate. the waste water.

Example 1

Table 1 shows the analytical values of the pulp mill's total water before and after the aerobic biological treatment performed on a laboratory scale.

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Table 1. Analytical values of pulp mill wastewater before and after biological treatment in the laboratory.

before after COD, g / l 0.80 0.52 AOX, rng / 1 14.2 14.1 The sample after biological purification has not been filtered here, which is why e.g. the AOX value is relatively high.

Laccase 1 U / ml and ferrous sulphate 200, 400 and 600 mg / l are added to the effluent treated by the pulp mill activated sludge method. Air or pure oxygen is introduced into the wastewater for 90 minutes, after which the wastewater is clarified in a 1 liter funnel.

Table 2 shows the effect of laccase treatment and addition of ferrous sulphate on the analytical values of the pulp mill's biologically treated wastewater. The samples are clarified effluent and have not been filtered prior to analysis.

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Table 2. Laccase oxidation and simultaneous ferrous sulphate precipitation of pulp mill effluent.

FeSO417 H2O mg / 1600 0 600 400 200

Laccase, U / ml 0 1 1 1 l COD, g / l 0.50 0.49 0.16 0.16 0.14 AOX, mg / l 13.5 12.7 4.3 4.2 4.4 P, mg / l 1.48 2.55 0.07 0.08 0.09 N, mg / l 1.0 1.0 0.6 0.6 0.7 0.7

Fe, mg / 1124 - 49 14 12

As can be seen from Table 2, the mere addition of ferrous sulfate to wastewater and oxidation with oxygen does not substantially reduce COD and AOX values. Oxidation with laccase without the addition of ferrous sulphate reduces the AOX content by about 10%, indicating that laccase is a dechlorinating enzyme.

It can be further seen from Table 2 that the simultaneous addition of laccase and ferrous sulphate to the effluent reduces COD and AOX values by up to 70% as a result of oxidation. The decrease in phosphorus content is even more than 95% compared to the addition of ferrous sulphate alone. The results further show that the added iron also precipitates efficiently.

Example 2

Laccase 0.25 and 0.37 U / ml and ferrous sulphate 500 mg / l are added to the effluent treated by the pulp mill activated sludge method. The oxidation and precipitation are carried out otherwise as in Example 1.

Table 3 shows the results of enzymatic oxidation and precipitation.

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Table 3. Laccase oxidation of pulp mill wastewater, Ferro-sulphate dosage 500 mg / l.

Lacquer, U / ml 0 0.25 0.37

Precipitate,% (v / v) 0 88 AOX, mg / l 13.2 4.4 4.4 COD, g / l 0.50 0.14 0.15 P, mg / l 1.44 0.10 0 , 15 N, mg / l 1.4 0.8 0.7

The results in Table 3 show that AOX is reduced by 67% and COD by 72% compared to an experiment in which ferrous sulfate alone is added to water.

The phosphorus content decreases by 93% at best.

The amount of enzyme at 0.25 U / ml gives a precipitate of 8.2% (v / v) and 0.37 U / ml 8%.

The pH in the blank test (clarifier) after 18 hours is 4.4.0.25 U / ml in test 3.4 and 0.37 U / ml in test 3.3.

Example 3

The efficiency and speed of lacquer treatment can be improved by adding phenolic compounds to the wastewater. Thus, by adding the shell press water generated during peeling, which contains e.g. rich in laccase-oxidative tannins, small amounts in pulp mill effluent raise the redox potential in laccase oxidation clearly higher than without the addition of bark water. The precipitation following oxidation then takes place faster and more efficiently.

The effluent treated by the activated sludge method is supplemented with shell water, laccase and ferrous sulphate. After a short (60 ... 90 min) oxidation, precipitation of dissolved compounds and clarification of the effluent takes place. Table 4 shows the redox potentials of the wastewater after oxidation and the decrease in absorbance measured at 280 nm. The ab sorbence value describes the concentrations of wastewater color and COD.

Table 4. Effect of peel water addition on pulp mill effluent redox potential in laccase oxidation with ferrous sulfate. Ferrous sulphate added 400 mg / l and laccase 0.5 U / ml.

Shell water Eh, mv pH abs 280 nm Fe, ml / 1000 ml acid tr. hap.jälk. decrease,% mg / 1 j wastewater 0 ml 560 4.0 75 9 20 ml 600 3.6 80 7 40 ml 630 3.5 80 6 ·· As can be seen from Table 4, the redox potential of wastewater increases due to the addition of shell water and at the same time soluble phenolic etc. The amount of color-causing compounds decreases. Adding peel water also reduces the amount of iron left in the solution.

t

Example 4

Ferrous wastewater from the chemical industry can also be used for precipitation. 10, 15 and 20% (v / v) of ferrous wastewater is added to the effluent taken from the overflow of the pulp mill activated sludge plant, the analysis results of which are shown in Table 5.

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Table 5. Analysis results of ferrous sulfate-containing wastewater from the chemical industry.

Sulfuric acid 10.5 g / l

Fe 2.37 g / l pH 0.6 The pH is adjusted to 5.0 ... 5.5 before adding the laccase.

In the oxidation of wastewater and ferrous wastewater from a pulp mill, the redox potential increases from 300 to 350 millivolts to 510 to 610 millivolts, whereupon precipitation occurs. After precipitation, the effluent is light and clear in color. Table 6 shows the 280 nm decrease in absorbance and 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 at the beginning of the acid trace acid trace (0.1 N NaOH) pulp, wastewater initial 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 in Table 6, it can be seen that the absorbance of the wastewater: at 280 nm decreases by 74-83%, i.e. soluble compounds precipitate efficiently.

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Example 5

Ferrous sulphate and laccase are added to the effluent from peeling. After aeration, precipitation occurs, whereby the water turns light and clear in color. When ferrous sulfate alone is added, the wastewater becomes black and cloudy and no flocculation occurs. Table 7 shows the results of enzymatic oxidation and ferrous sulfate precipitation of shell water.

Table 7. Enzymatic flocculation of peel water with ferrous sulfate. Laccase added 1 U / ml.

Ferrosul- Eh, mv pH abs 280 nm COD BOD P phate end end 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 0.84 0.03 800 650 2.7 8.1 2.18 0.93 0.28

Example 6

The total wastewater of the pulp mill, which is taken from the so-called from the expansion tank before the aeration section of the activated sludge plant, add laccase 0.5 U / ml and ferrous sulphate. Oxygen is introduced into the mixture for 60-90 minutes, followed by flocculation. Table 8 shows the results of the analysis of the light and clear effluents formed during flocculation.

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Table 8. Wastewater analysis results.

Ferrosul- Eh, mV pH AOX, COD, BOD, N, phate, end end mg / 1 mg / 1 g / 1 mg / 1 mg / 1600 no varnish. 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 method gives good reductions in COD and AOX when the wastewater is treated before activated sludge treatment. Chemical precipitation is not known to remove compounds that cause biological oxygen demand very well, as can be seen from the BOD values.

Example 7

Ferrous sulphate 500 mg / l and horseradish peroxidase (EC.1.11.1.7) and hydrogen peroxide (6 pmol / l) are added to the treated effluent of the pulp mill activated sludge plant as a reducing substrate. After mixing (15 ... 60 min), polymer (Fennopol N 300) is added to the wastewater and the water is clarified.

In the comparative experiment, no enzyme is added to the wastewater. The addition of hydrogen peroxide to the wastewater increases the redox potential of the water, which also causes precipitation. However, the addition of peroxidase improves the precipitation of soluble compounds; from, which is reflected, for example, by a larger decrease in absorbance at 280 nm. In the comparative experiment, the absorbance of the clarified wastewater was 5.1 when it was 4.2 when peroxidase was added. The absorbance of the original untreated wastewater was 5.5.

Claims (10)

A process for removing at least AOX, COD, carbon, nitrogen and phosphorus from the wastewater of the forest industry, wherein dissolved and suspended compounds are separated from the wastewater by enzymatic oxidation and precipitation, characterized in part of the ferrous sulfate son is used as a precipitating chemical and at least part of the oxidoreductase substrate is reduced. 2. A process according to claim 1, characterized in that a phenol oxidase, such as lacquer or tyrosinase, or a peroxidase such as lignin peroxidase, manganese peroxidase or horseradish peroxidase is used as oxidoreductase. 3. A method according to claim 1, characterized in that, during the wastewater treatment, a culture solution containing an oxidoreductase and in which plants, fungi or bacteria have been grown is used. 4. A process according to claim 1, characterized in that the oxidoreductase is supplied to the wastewater i. form of a microorganism producing the same, which microorganism may be Polvporus hirsutus. Phanerochaete chrysosporium. Trametes versicolor, or another carrot fungus strain. Process according to any of the preceding claims, characterized in that the oxidoreductase is supplied to the waste water at a temperature of 15 ... 65 ° C, preferably at 20 ... 50 ° C, and a pH of 2. 5 ... 7.0, preferably 3.5 ... 5.5. Process according to any one of claims 1-5, wherein the wastewater is subjected to a separate pretreatment, characterized in that at least part of the ferrous sulphate is fed to the wastewater at the pre-treatment stage. Process according to any of the preceding claims, characterized in that FeSO 4 7 H 2 O is supplied in an amount of not more than about 1000 mg, preferably below about 400 mg, in particular not more than about 200 mg per 500 mg of COD in the wastewater. Process according to any of the preceding claims, characterized in that the wastewater from the forest industry obtained from the biological pretreatment is simultaneously added to varnish and ferrous sulfate under aeration. 9. A process according to any of the preceding claims, characterized in that a chemical is used to intensify flocculation in addition to the precipitation chemical. solution. Process according to any of the preceding claims, characterized in that as the oxidoreductase substrate which is reduced, one uses air, pure oxygen, ozone or H2 O2. • ·