DE3620980A1 - Continuous multiple-stage process for treating the bleaching waste waters from wood pulp production - Google Patents

Abstract

A process for treating the waste waters (effluents) arising in the bleaching of wood pulp with chlorine and chlorine-containing reagents, in which waste waters are contained chlorinated compounds formed by chlorination of the residual lignin, can be continuously carried out, the waste water made alkaline being subjected to a thermal hydrolysis and the waste waters before this hydrolysis stage being neutralised if necessary and at the same time receiving an addition of calcium ions with precipitation of a considerable part of these chlorine compounds (AOX), and in which case the product leaving from the hydrolysis stage can be subjected to microbial degradation.

Description

The present invention relates to a continuous process Multi-stage process for clarification with chlorinated substances highly contaminated wastewater, which is bleached with Chlorine and products containing chlorine in the pulp industry attack.

Pulp is used exclusively in the Federal Republic of Germany produced by the sulfite process, in which the wood pulping with magnesium or calcium hydrogen sulfite in sulfuric acid Solution is done. Worldwide, however, the sulfate process (information with sodium hydroxide, sodium sulfide solution) a different greater importance. During the digestion process this is Lignin contained in the wood dressing (depending on the type of wood the lignin content between 20 and 30%) by fragmentation and Introduction of hydrophilic groups up to 80 to 90% removed. The The portion that remains in the unbleached pulp is called residual lignin designated. Its further removal makes a bleaching process necessary to obtain high quality pulp. Today bleaching is done with both chlorine and bleaches containing chlorine (Hypochlorite, chlorine dioxide) as well as with chlorine-free bleaches such as B. oxygen and peroxide.

According to the current state of knowledge and development, chlorine or compounds of chlorine (hypochlorite, chlorine dioxide) indispensable Components of a pulp bleach, especially the Bleaching of sulfite pulps. This is due to the following The advantages of a chlorinating bleaching stage are justified:

• 1. Chlorine is an extremely cheap and very specific acting bleach, d. H. a conventionally run Chlorine bleach has hardly any harmful effects on the polysaccharide content of the cellulose (cellulose, hemicellulose), but almost exclusively causes water or alkali solubility of residual lignin in the unbleached pulp.  
• 2. The replacement of chlorine with oxygen or peroxide is true possible within certain limits, but in the form of high investment and operating costs (with pressure bleaching with oxygen) and quality-reducing influences (degradation of cellulose and hemicelluloses in an alkaline oxidizing environment) considerable Disadvantages with it.
• 3. The removal of the last lignin residues and the precise adjustment of the viscosity of pulps for chemical processing has so far only been possible by using chlorine dioxide and / or hypochlorite. These bleaches also have a chlorinating effect, albeit to a limited extent. This advantage is offset by serious disadvantages:
  • 1. Wastewater from chlorine bleach and the subsequent alkali extraction cannot be evaporated and burned due to their high chloride content and therefore their extremely corrosive effects. Biodegradation of the organic ingredients is demonstrably only possible to an extremely small extent. Because of this situation, this waste water represents a serious burden in the form of high COD loads.
  • 2. By the action of chlorine and to a lesser extent chlorine compounds, water-soluble chlorinated organic compounds (chlorine lignin degradation products) are released, which, for. T. have toxic and mutagenic properties. Due to their inhibitory effect, these compounds are probably also significantly involved in the resistance to biodegradation.

Increasing chlorine dosing causes the following change in non-volatile chlorine lignin degradation products:

• a) Relative decrease in connections with high molecular weight (≦ λτ 10,000) in favor of those with medium to low molecular weights (500-5000).
• b) Decrease in the aromatic character of the chlorine lignin degradation product.
• c) increase in carbonyl and carboxyl content and
• d) increase in organically bound chlorine.

Depending on the chlorination conditions (temperature, Reaction time and chlorine dosage) the chlorine content of the Chlorine lignin breakdown products between 13 and 19%.

Despite the known harmfulness of chlorinated hydrocarbons are currently the wastewater containing chlorine lignin degradation products still headed into the receiving water as there has been no satisfactory so far Processes for the clarification of this waste water exist.

In the past, there has been no lack of attempts to demonstrate methods which reduce the concentration of chlorinated lignin degradation products in these C wastewater or in the E ₁ wastewater of the first alkali extraction stage, which follows chlorination in the pulp bleaching, to reduce the chemical Oxygen requirements (COD) and reduction of the harmfulness of this waste water could have been used. In a bleaching plant, several process steps are usually carried out in succession, which can include treatment of the unbleached pulp with chlorine, alkali, hypochlorite, chlorine dioxide, oxygen or peroxide.

F.M.A. Nicolle, R. Shamash, K.V. Nayak and J.A. Histed in "Lime treatment of bleachery effluent", Pulp Paper Can. 78 (9): T 210-T 214 (Sept. 1977) the addition of soluble calcium salts part of the wastewater from chlorination (C wastewater), then the filtrate is added to the chlorination following first alkaline extraction and the addition of Lime milk to separate out a precipitate, whereby by the Adding the soluble calcium salts to the C waste water accelerates is reached when the precipitation stops. A substantial reduction in color, fish toxicity and chemical oxygen requirements of up to 43.5% are the result.

The alkaline, thermal degradation of the high molecular weight organic chlorine compounds in a concentrate, which was obtained after a first ultrafiltration step of a filtrate of the first alkali stage (E ₁ ) of a bleaching plant (sulfate process), causes an incomplete cleavage of these chlorine compounds into alkali chloride and hydroxyl compound.

That suitable bacteria or fungi certain chlorinated compounds has recently been known (Van-Ba Huyuk, Hou-min Chang and Thomas W. Joyce and T. K. Kirk: dechlorination of chloro-organics by a white-rot fungus: Tappi Res. and Development Conf. 1984, 185-191).

Even with the conventional activated sludge process, some are said to be chlorinated compounds are biodegradable and not only can be absorptively eliminated.

However, wastewater from wastewater mainly does not contain biological degradable chlorinated organic ingredients, such as from works by Göttsching and Laichter (biological cleaning of wastewater from the sulfite pulp industry, Göttsching and Laichter in: Wochenblatt für Papierfabrikation, 1982, No. 14, vol. 110, pp. 485-488) emerges, and there is no known microbiological process, that to clean these C wastewater by breaking down the chlorine lignin Products could be used by microorganisms.

The methods described above have in common that they not at the same time a sufficient reduction in the chemical Oxygen demand (COD) and reduction of the AOX value of the C- Reach wastewater from the chlorination stage or on the Degradation of high molecular weight chlorine compounds, preferably from the Filtrate of the first alkali extraction stage following the chlorination obtained by alkaline hydrolysis. AOX stands for "adsorbable organic halogen compound" and is called the Part of organic halogen compounds (X = Cl, Br, J) determined, which can be adsorbed in activated carbon, the entire adsorbed amount is converted to X = Cl.

The aim of the invention is a method that enables the Values of COD and AOX of waste water from the bleaching of Pulp with chlorine or chemicals containing chlorine at the same time to reduce to an ecologically acceptable level.  

A particular aim of the invention is the drastic reduction the AOX values in the C and E waste water from the pre-bleaching. At present, the AOX values are usually between about 3 and 7 kg AOX per ton of pulp. In the upcoming amendment to the Waste Water Tax Act the sum parameter AOX is provided as an additional one Record damage unit. For discussion stands for one damage unit (40 DM) 2 kg AOX per t cellulose. Related to the current AOX values would do this for large parts of the pulp industry means a levy that threatens existence.

The above task is used in a preparation process from bleaching with chlorine and chlorine Bleaching waste water in which by chlorination of the lignin, adsorbable organic chlorine compounds (AOX) are contained, solved in that the alkaline made waste water is subjected to thermal hydrolysis, where appropriate, the wastewater neutralized before this hydrolysis stage and at the same time are mixed with calcium ions, whereby a significant part of these chlorine compounds (AOX) precipitated is, and where appropriate that from this hydrolysis stage leaking product is subject to microbial degradation becomes.

In the first stage of the process, the strongly acidic solution is formed chlorine bleach with simultaneous introduction of calcium Ions neutralize and become adsorbable organic chlorine compounds (AOX) caused by the chlorination of the lignin as its Degradation products are created by further addition of alkali precipitated and partially dechlorinated.

In the second, the hydrolysis stage, the supernatant solution this precipitation of a thermal alkaline hydrolysis for dechlorination the non-precipitated adsorbable organic Subjected to chlorine compounds and in the third stage there is a microbial degradation of the dechlorinated and chlorinated lignin degradation products as well Carbohydrate breakdown products existing compounds for further Reduction of the COD and also the AOX value carried out.  

There are several ways to do the first stage:

In the simplest variant, the chlorinated degradation products of the lignin are precipitated by adding lime milk to the unfiltered waste water previously heated to 45 ° C (concentration C _{Ca (OH)} = 2 g / l). The dosages of lime milk are shown in Table 1 (samples 1 to 3), as are the pH values of the excess wastewater after the precipitation. The values for sedimentation and the volume of the sediment given in Table 1 come about as follows:

Three 90 ml samples of unfiltered waste water, which was previously on 45 ° C was warmed with three different amounts of milk of lime of the concentration given above (samples 1 to 3). The samples are shaken at 45 ° C for 30 min Assessment of sedimentation in the standing cylinder transferred.

The sedimentation describes the time (in minutes) of falling of the flakes on which the top 10 ml of waste water are free of flakes in the upright cylinder.

The volume of the sediment is the volume of the flocculation precipitation in the lower part of the standing cylinder, measured in ml after 30 minutes and after 2 hours.

The turbidity of the excess wastewater is assessed visually.

From the excess wastewater after precipitation with milk of lime (without previous filtration) the COD and AOX values were determined, which are shown in Table 1 (samples 1 to 3).

A preferred method according to the invention provides that sodium hydroxide is added after the calcium hydroxide addition, since the pH value (8.8 or 8.6) achieved with the addition of Ca (OH) ₂ alone for optimal precipitation of calcium Chlorine lignin degradation products are too low or, when Ca (OH) _{2 is} added, sedimentation results in comparatively poor values. The amounts added (Ca (OH) ₂ and NaOH), the pH values of the supernatant after the precipitation and the values for the sedimentation, the volume of the sediment and the turbidity of the supernatant, determined according to the procedure already described, are also reproduced in Table 1 (samples 4 to 8).

In a particularly preferred method for performing the Precipitation according to the invention is the unfiltered wastewater Chlorine level passed over a limestone column (residence time approx 20 min., Run from head to foot of the column temperature 20 to 70 ° C).

The wastewater is largely neutralized and the calcium ions required for the subsequent precipitation are made available (final pH about 6.2). Precipitation in an alkaline medium is initiated from the wastewater, which is now almost neutralized and now contains Ca ⁺⁺ ions, by adding sodium hydroxide.

To assess the sedimentation, the volume of the sediment and the turbidity of the excess wastewater in which the Limestone column neutralization was followed by precipitation proceed as follows:

Six 90 ml samples of the wastewater after the column are opened 40 ° C or 60 ° C heated and 1 M NaOH in different amounts (Dosage) added and with distilled water to 100 ml replenished. These dosages are shown in Table 2. These samples are filled into standing cylinders and the sedimentation, the volume of the sediment and the turbidity of the supernatant Wastewater assessed according to the criteria already specified (Table 2).

Precipitation occurred immediately after all NaOH dosing in the form of flakes of different sizes, the sedimentation is slightly better at 60 ° C than at 40 ° C. With three different ones NaOH doses to initiate precipitation at 40 ° C (0.67 g NaOH / l waste water; 1.11 g NaOH / l waste water and 1.56 g NaOH / l wastewater) the COD and AOX values and the UV absorption of the unfiltered, excess wastewater the precipitation. The results are shown in Table 3.  

Obtain the listed AOX, COD and UV absorption reductions on the undiluted urine wastewater.

A more compact sediment is created with the additional use of flocculants; As such, various types of Rohafloc (a product from Röhm, Darmstadt) with graded ionic character were used. The dosing of the individual components was carried out as follows: 90 ml waste water after column (45 ° C.) + 2.5 ml 1 M NaOH + 6.5 ml dist. H ₂ O + 1 ml 0.1% flocculant solution. The best results are achieved with the weakly cationic Rohafloc type KF-275.

A determination of the molar mass distribution of ingredients in the wastewater from the chlorination before and after precipitation (limestone column, Ca (OH) ₂ , NaOH) showed that in the alkaline medium the high molecular weight fraction (molecular weight 100,000-1800, based on sulfonated polystyrene as calibration substance) almost is completely felled. In contrast, in the area of low molecular weight substances with a molecular weight below 1800, only a partial precipitation takes place.

If the process is carried out continuously, this will be Waste water from the reaction vessel in which the neutralization with Lime milk was carried out or discharged from the foot of the limestone column and from top to bottom through a heat exchanger for Preheating passed to 40 to 60 ° C and then under simultaneous Dosage of lime milk and / or sodium hydroxide solution in a mixing and precipitation tank transferred and the pH in the mixing and Precipitation container set to approximately 11.5.

Part of the AOX contained in the wastewater falls in the form of Calcium salts out while stirring. After this precipitation the pH is brought to about 11.5 with NaOH.

A partial one already takes place under these precipitation conditions Replacement of the organically bound chlorine by OH groups instead.

The wastewater together with the precipitated Ca chlorine lignin degradation product is placed in a sedimentation container to let the Precipitated product transferred; with a view to a subsequent one  Dechlorination is recommended before sedimentation thermal treatment at 40 to 60 ° C for 1 hour.

The second process stage, the thermal dechlorination of the AOX in an alkaline medium, in a reactor at 40 ° C to 70 ° C and a residence time of 1 to 3 hours.

The required temperature of 40 ° C to 70 ° C can be advantageously through the use of those incurred elsewhere Realize waste heat.

The pH required for thermal, alkaline dechlorination A value of approximately 11.5 was set after the precipitation, but if necessary, add more NaOH to the wastewater easily possible before entering the reactor.

Through this thermal, alkaline dechlorination, part of the AOX is converted with hydrolysis and the organically bound chlorine is split off as NaCl. A partial desulfonation of the chloroignin products containing HSO ₃ also takes place under the reaction conditions mentioned. the pH value drops by about two units due to the consumption of OH ions.

After the precipitation (neutralization with limestone column) and afterwards Thermal hydrolysis (70 ° C, 1 h) were the COD and AOX values and UV absorption for various NaOH dosages measured. The additional thermal treatment (40 ° C, 1 h) after the NaOH dosing and before sedimentation in one Experiment carried out and omitted in another. The results are shown in Table 3.

It is possible already after the first two process stages have been carried out the invention an absolute COD reduction of 72% and achieve a 88% reduction in chlorine-containing products.

A certain, albeit small part of the AOX will be implemented the first two stages of the process not degraded at all. The extent of dechlorination is not just that OH concentration, temperature and reaction time dependent, but  also on the structure of the chlorinated compounds. Among the The reaction conditions described here are, for example symmetrical chlorine compounds and chlorinated aromatics only slightly or not at all accessible to hydrolysis.

With only thermal, alkaline hydrolysis is a dechlorination of 50 to 60%, but none COD reduction achieved because the converted by hydrolysis Substances remain in the wastewater. It could also help neutralize no lime is used, otherwise pH increases with NaOH use the precipitation due to the presence of calcium ions would.

However, it is entirely within the scope of the present invention only perform the second stage if this is appropriate for a certain area of application, e.g. B. the CKW (chlorinated hydrocarbon) containing wastewater from chemical cleaning processes processing through the second stage of the process to undergo. It also corresponds to the idea of the inventor that individual sub-streams in the paper and / or pulp production accruing waste water can be "driven" in a targeted manner.

In the last process step provided according to the invention the reaction solution formed during the thermal hydrolysis microbial, biochemical degradation. This solution still contains the non-hydrolyzed chlorine compounds and hydroxyl compounds resulting therefrom by hydrolysis and Carbohydrate breakdown products, of which the latter two stand out Bacteria with a further decrease in the COD value of the "wastewater" can be broken down well. The AOX value is also determined by the biochemical Degradation reduced again.

The metabolism of bacteria is in the presence of chlorine lignin breakdown products reduced and consequently the biochemical degradation restricted or completely prevented (biochemical inhibition). One of them is the low molecular weight chlorinated substances far stronger biochemical inhibition than from the high molecular weight.  

By the previous precipitation in the invention followed by Dechlorination is due to this biochemical inhibition reduced to such an extent that biochemical degradation takes place can.

This biochemical degradation can take place in the liquid phase; in the invention, however, is the use of a fixed bed reactor (Pillar) preferred of two different operating options should offer:

• a) anaerobic operation in circulation
• b) aerobic operation in a single pass.

In addition, the circulation / passage of the reactor takes place from below above, whereby air is blown in from below during aerobic operation.

This fixed bed reactor can have a filling of four different segments Have filling materials, whereby - from bottom to top - according to the invention

• 1st segment made from reduction coke and / or blast furnace slag
• 2nd segment of fine coke from lignite
• 3rd segment made of hydroanthracite
• 4th segment of activated carbon

consists.

However, the fixed bed reactor is preferably partially filled or entirely of open-pore sintered glass instead of the mentioned filling materials. In particular, 20 to 70 parts open-pore sintered glass in the form of spheres or Raschig rings with a pore volume of 50 to 70% and a pore diameter from 60 to 500 µm with aerobic operation of the filling materials mentioned of the segments added.

In the case of anaerobic driving, only open-pore sintered glass is preferred used for the filling.  

When clarifying in the fixed bed reactor finds an effective one biochemical degradation of the hydroxyl-containing, dechlorinated lignin and carbohydrate breakdown products as well as a partial AOX reduction instead, because

• 1. multiple runs (aerobic) no further reduction of the COD and the AOX compared to a one-time run entail; further reduction with several Runs through would appear at adsorption phenomena Speak fixed bed;
• 2. A CO ₂ evolution occurring during the passage of the wastewater through the fixed bed reactor indicates biochemical metabolic processes;
• 3. UV and IR spectroscopic and HPLC investigations the substances in the discharge of the fixed bed reactor clearly Differences to the substance composition before the run of the fixed bed reactor and
• 4. the increase in the chloride ion concentration in the drain compared to the feed the degradation of the chlorinated compounds approved.

Through this final clarification of the C waste water in the invention Fixed bed reactor becomes an additional, more extensive Reduction in COD and AOX values achieved.

In the implementation of the invention, the waste water after the alkaline, thermal hydrolysis using a suitable Cooled device, optionally stored in a container and then by a pump at the bottom of the fixed bed reactor fed in, with air at the same time at the foot during aerobic operation is entered with. After the biochemical breakdown will be in the process of the fixed bed reactor measured the values for AOX and COD. The results, which are also shown in Table 3, were obtained for a C wastewater, first by transfer  was neutralized via a limestone column; from the neutralizate was the precipitation of the calcium chloro lignin products Add 1.56 NaOH / l waste water at 40 ° C, then a one hour treatment at 40 ° C before sedimentation performed and the supernatant solution after sedimentation alkaline, thermally hydrolyzed.  

Table 1

Lime milk precipitation

Table 2

Influence of the pH value depending on the NaOH dosage on the precipitation of the chlorine lignin degradation products (after neutralization using a limestone column) at 40 ° C and 60 ° C

             I. NaOH dosage at 40 ° C       II. NaOH dosing at 40 ° C and further treatment (40 ° C, 1 h)     

Claims (11)

1. A process for the treatment of waste water obtained in the pulp bleaching by means of chlorine and chlorine-containing reagents, in which chlorinated compounds formed by chlorination of the residual lignin are contained, characterized in that the alkaline waste water is subjected to thermal hydrolysis to split off the organically bound chlorine of the chlorinated ingredients are, where appropriate, this wastewater neutralized before this hydrolysis stage and at the same time calcium ions are added, whereby a substantial part of these chlorine compounds is precipitated after alkaline adjustment, and where appropriate the product emerging from this hydrolysis stage is subjected to a microbial structure. 2. The method according to claim 1, characterized in that the Hydrolysis is carried out at temperatures from 40 to 80 ° C. 3. The method according to claim 1, characterized in that this Sewage over a limestone filled column and then run with lime milk and / or sodium hydroxide solution can be made alkaline. 4. The method according to claim 1, characterized in that this Waste water can only be mixed with milk of lime. 5. The method according to claim 1, characterized in that after neutralization and addition of calcium ions to the waste water alkaline and at temperatures between 30 and 70 ° C be heated. 6. The method according to claim 1, characterized in that the Precipitation of the chlorine compounds by adding flocculants is promoted. 7. The method according to claim 1, characterized in that the precipitated Detach chlorine compounds in a separate container be left. 8. The method according to claim 1, characterized in that the microbial build-up is carried out in a fixed bed reactor. 9. The method according to claim 1, characterized in that this Fixed bed reactor at least partially made of open-pore sintered glass is constructed. 10. The method according to claim 8, characterized in that the Fixed bed reactor constructed from different packing materials is.   11. The method according to claim 8, characterized in that the Product emerging from the hydrolysis stage from bottom to top is continuously passed through the fixed bed reactor.