WO2009062581A2

WO2009062581A2 - Two-stage method for the recovery of chemicals from waste water

Info

Publication number: WO2009062581A2
Application number: PCT/EP2008/008704
Authority: WO
Inventors: Esa-Matti Aalto; Hans-Ludwig Schubert; Volker Paasch
Original assignee: Voith Patent Gmbh
Priority date: 2007-11-12
Filing date: 2008-10-15
Publication date: 2009-05-22
Also published as: WO2009062581A3; DE102007054207A1

Abstract

The invention relates to a method for recovering chemicals from waste water that is produced during the production of fibrous material from lignocellulose raw material using sulphuric salts, the waste water being subjected to an anaerobic waste water treatment comprising at least one stage and sulphur and/or sulphuric compounds being produced.The method is characterised in that the anaerobic waste water treatment has at least one stage for the preferential production of sulphur and/or sulphuric compounds and at least one stage for the preferential production of methane and/or carbon dioxide.

Description

description

Two-stage process for the recovery of chemicals from wastewater

The invention relates to a method for the recovery of chemicals from wastewater, which is produced in the production of pulp from lignocellulosic raw material by means of sulfur compounds.

The production of pulp from lignocellulosic raw material for the production of z. As paper or cardboard takes place on a large industrial scale. In the majority of processes for the preparation of z. As CTMP, high-yield or full-cell pulp sulfur salts are used in aqueous solution, optionally with the addition of organic solvents such as alcohol in particular, for softening and / or dissolving the lignin. Mainly sulfites or sulfides are used for the digestion of wood or annual plants. The bases used are usually sodium, calcium, magnesium and / or ammonium. Depending on the digestion process, additional acids, alkalis or additives such as quinone components are added. A typical example of a digestion chemical is sodium sulfite, e.g. Used in pulp digestion, run at initial pH's of 9-10, to produce high yield pine pulp.

Both because of the costs and for reasons of environmental protection, the chemicals used, in particular the sulfur salts, if not very small amounts be used- be recovered. This is done today by first concentrating the effluent from pulp production by evaporation. Then the concentrated wastewater is burned at temperatures above 1000 ⁰ C. The organic components (carbohydrates, lignin) are decomposed during combustion, the inorganic components are collected and processed for re-use for the production of pulp. The disadvantages here are above all the considerable investment costs for the plants in which the recovery is carried out. is led. A further disadvantage is the complex and complicated process management in the treatment of the digestion chemicals. In particular, the evaporation and burning of the wastewater is energy-consuming and requires expensive equipment.

There is therefore a need to propose a process for the recovery of chemicals for the digestion of pulp from lignocellulosic raw material using sulfur salts, which causes low investment costs.

According to the invention, a method according to claim i is proposed for achieving the above object.

The method requires the treatment of waste water from pulp production by means of anaerobic microorganisms. Wastewater containing substances whose degradation consumes oxygen are called effluents with a COD load (COD = chemical oxygen demand). Such effluents are diverse, be it as commercial, municipal or industrial wastewater. In anaerobic wastewater treatment, the COD load is degraded by the action of microorganisms (bacteria), the oxygen, usually in the dark and at temperatures of 15 ⁰ C to 25 ⁰ C (psychrophilic microorganisms), from 25 ⁰ C to 45 ⁰ C (mesophilic microorganisms) or from 45 ⁰ C to 70 ⁰ C, isolated even at higher temperatures, (thermophilic microorganisms) these oxygen-consuming substances, mostly organic matter, degrade.

The microorganisms are mixed as a sludge with the wastewater to be treated, with various methods are known, from mixing reactors via fixed bed reactors to reactors that are used as sludge bed or pellet reactors z. B. after the UASB process (upflow anaerobic sludge blanket method) work, such. B. in WO 99/51 532 (Zeppelin silo and Apparatetechnik GmbH) or in DE 10 2005 050 997 (Aquatyx GmbH) described. The EGSB process (Expanded Granulated Sludge Blanket process) is also suitable for carrying out the process according to the invention. The description of the construction and method of UASB reactors disclosed in the specified documents is herewith the subject of this patent application. It is expressly noted that the invention is not limited to certain processes or reactors of anaerobic wastewater treatment is limited.

At the beginning of anaerobic wastewater treatment, wastewater and microorganisms are brought into contact with each other. At the end of the wastewater treatment, sludge, filtrate and gaseous components are formed. The sludge makes up only a relatively small part with about 10% of the COD load. The gas, usually methane and carbon dioxide, called biogas, accounts for 90% of the COD load and is used to generate energy. The filtrate is fed as purified wastewater to the receiving water.

Anaerobic wastewater treatment involves four main reaction steps. First, the organic substances that are detected as COD load are hydrolyzed. The hydrolysis products, eg. b. Oligomers or monomers are converted to acidogenesis into organic acids and / or alcohols. These organic acids or

Alcohols are converted by acetogenesis into hydrogen and acetates. Actetate and hydrogen are converted to methane as a result of methanogenesis. The leading to the production of methane reactions are preferred in the known methods of wastewater treatment. At the same time run off unwanted, competing reactions from z. B. lead to the release of carbon dioxide and / or hydrogen sulfide.

The organic substances are thus reduced by the action of anaerobic microorganisms. To reduce COD, all of the above reactions contribute. Acetate compounds in particular serve as nutrients for bacteria, both for sulfur-reducing bacteria and for methane-producing bacteria.

In the field of pulp production and processing, the use of anaerobic wastewater treatment has been described for the reduction of the COD load of wastewater from sulfur-free soda outcrops of agricultural raw materials, for the reduction of COD load of waste water from paper production and for the Degradation of the COD load of vapor condensates of Viskoseherstellung.

For the known, above-described anaerobic wastewater treatment, it is always noted that the effluents to be treated should be free of sulfur compounds, - A -

because the conversion of sulfur compounds into hydrogen sulphide is preferred over the desired conversion of carbon compounds to methane. In the presence of sulfur compounds, therefore, the yield of methane is impaired. In addition, hydrogen sulfide causes corrosion problems when using biogas.

It is the particular achievement of the inventors to have recognized the suitability of anaerobic wastewater treatment for the recovery of chemicals from pulp wastewater containing sulfur salts. The inventors propose quite different priorities from the prior art in carrying out the method according to the invention. According to the invention, it does not depend on a maximized production of biogas or on a maximized reduction of the COD load. Rather, the sulfur salts contained in the wastewater should be separated from the base (eg sodium, calcium, magnesium or ammonium) and converted into sulfur and / or sulfur compounds. Preference is given to the production of sulfur, hydrogen sulfide and / or sulfur dioxide. Sulfur floats on the filtrate, hydrogen sulfide is released as gas, sulfur dioxide is either gaseous or dissolved in water or filtrate. The recovery rate of the sulfur salts of the wastewater is with the inventive method at least 10%, preferably up to 50%, usually up to 70%.

In order to optimize the recovery of the digestion chemicals, it is proposed according to the invention that the anaerobic wastewater treatment has at least one stage for preferential production of sulfur and / or sulfur compounds and at least one stage for preferential production of methane and / or carbon dioxide.

By providing two stages designed to produce different degradation products, microorganisms may be separately conditioned for each stage, on the one hand preferably conditioned for the production of sulfur and / or sulfur compounds, and on the other hand for the production of methane and / or carbon dioxide are conditioned. Competing reactions are thus selectively separated, resulting overall in improved process efficiency and more stable operation of the wastewater treatment leads. The extensive separation of the production of sulfur and / or sulfur compounds, mainly hydrogen sulfide, from the production of biogas also allows easier processing of the released from the anaerobic microorganisms gases. The plants can, for. For example, with regard to the building materials to be used, the gases that are more or less corrosive depending on the stage are better adapted to the different stages.

Since the conversion to sulfur and / or sulfur compounds takes place preferably in preference to the production of methane and / or carbon dioxide, it has proven to be advantageous to use the at least one stage for producing sulfur and / or sulfur compounds before the at least one stage for producing methane and / or to arrange carbon dioxide. In this way it is possible to recover at least 10%, preferably up to 50%, advantageously up to 70% of the sulfur, which is in the form of sulfur salts in the wastewater, in the form of sulfur and / or sulfur compounds.

It is also advantageous if, in the anaerobic wastewater treatment, the base separated from the sulfur, which is dissolved in water, is discharged with the filtrate. Preferably, the base is oxidized to carbonate, e.g. For example, sodium, calcium, magnesium or ammonium carbonate. But it can also be converted to hydroxides; a mixture of oxidation products is possible, for example in the form that carbonates and hydroxides are present side by side.

The inventive method can be carried out in the simplest case, each with a stage for the production of sulfur and / or sulfur compounds on the one hand and methane and carbon dioxide on the other. To achieve economies of scale for the wastewater treatment reactors, multiple reactors can be operated in parallel. However, it is also possible in each case to carry out a plurality of the two anaerobic treatment stages proposed according to the invention in succession, in particular if this increases the recovery rate of the chemicals.

With the sulfur, the gaseous and / or dissolved in water sulfur compounds conditions and optionally the alkaline filtrate are recovered from wastewater chemicals available. The inventive method operates at temperatures well below 100 ⁰ C. The suitable for the activity of psychrophilic, mesophilic or thermophilic microorganisms temperatures are at most 70 ⁰ C, well below the temperatures at which the known recovery methods work.

It should be regarded as a particular advantage of the method according to the invention that evaporation of the wastewater to be treated is not required. The saving of evaporation causes a significant reduction in operating and investment costs and a much simplified process management. Saving off waste-gas combustion also reduces operating and system costs and avoids a critical and complex process step.

According to a preferred embodiment of the process according to the invention, low-molecular oxygen-consuming substances are also reacted in the at least one stage for the preferred production of sulfur and / or sulfur compounds. Monomers and / or oligomers contained in the waste water of pulp production are degraded by the anaerobic microorganisms, which are preferably conditioned on the conversion of sulfur salts to sulfur and / or sulfur compounds.

According to a preferred embodiment of the method according to the invention, higher molecular weight oxygen-consuming substances are also reacted in the at least one stage for the preferential production of methane and / or carbon dioxide. Higher molecular oxygen-consuming substances, typically lignin or lignin degradation products, but also higher molecular weight carbohydrates are degraded by the anaerobic microorganisms, which are preferably conditioned to the conversion of oxygen-consuming substances to methane and / or carbon dioxide. This special conditioning of the anaerobic microorganisms in each case to the degradation of low or high molecular weight oxygen-depleting substances overall improved degradation of the COD load of the wastewater is achieved. The production of methane and / or carbon dioxide in addition to the implementation of sulfur compounds is an advantageous option of the method according to the invention, because over conventional chemical recovery excess energy can be provided not only in the form of electricity and heat. Rather, a primary energy source is generated, which can be used and used much more diverse. It is particularly advantageous that this energy source can also be stored if necessary.

According to a preferred embodiment of the method according to the invention, the anaerobic wastewater treatment degrades at least 5%, but preferably more than 25%, particularly preferably up to 50%, of the COD load originally present in the wastewater. Although the process according to the invention gives priority to the recovery of the chemicals, it nevertheless contributes to the reduction of the COD load. The implementation of sulfur compounds, which does not lead to a corresponding methane generation, contributes to a further improved energy balance of the process.

It also proves to be advantageous if the COD load of the wastewater from the pulp production contains up to 150 g / l of COD, because then the recovery rate of the chemicals and the degradation rate of the COD are particularly high and the process can be carried out accordingly economically , Preference is given to using effluents which have a COD load of from 20 g / l to 120 g / l, in particular from 40 g / l to 100 g / l. In particular, effluents of CTMP pulp or high yield pulp produced using sulfur compounds have COD loads in the aforementioned framework.

It is to be regarded as a particular advantage of the method according to the invention that the recovered chemicals can be implemented in a preferred development with only a few, simple and energy-consuming steps in chemicals that can be used for reuse in a pulp digestion by means of sulfur salts. Depending on the composition of the waste liquor, sulfur is present as the reaction product of the anaerobic wastewater treatment. Sulfur is produced in particular when, in the context of anaerobic wastewater treatment, the sodium component of the spent liquor does not become sodium carbonate but rather essential alkaline sodium hydroxide is reacted. The sulfur can be oxidized by means of oxygen supply, for example by burning, to sulfur dioxide and then further reacted with alkaline filtrate to alkali sulfite.

Alternatively, hydrogen sulfide may form during anaerobic wastewater treatment, which may be reacted with oxygen to form sulfur dioxide and then alkaline filtrate to alkali sulfite. In addition, gaseous or dissolved in water sulfur dioxide can be reacted with alkaline filtrate to alkali sulfite. It may also be a direct reaction of the hydrogen sulphide with water and alkali, e.g. with filtrate in the course of a gas scrubbing, to alkali sulphide. This demonstrates the simplicity of process management, which provides ready-to-use digestion solution without complex process steps.

In order to improve the efficiency of the anaerobic wastewater treatment, according to a preferred embodiment of the method according to the invention, anaerobic wastewater treatment may be preceded by an aerobic and / or anaerobic pretreatment of the wastewater. Such a pretreatment, for example, a pre-acidification, proves to be advantageous if the wastewater is to prevent or minimize the formation of undesirable, methanogenesis interfering substances. The pre-acidification is an anaerobic wastewater treatment, which proceeds in the acidic pH range, and which preferably leads to the formation of acetates.

Alternatively, an aerobic pretreatment can take place, for. B. by white rot fungi act on the waste liquor. This biological pretreatment contributes to increasing the efficiency and stability of the anaerobic wastewater treatment when the chemicals in the wastewater are oxidized as much as possible before the anaerobic wastewater treatment according to the invention. Sulfur compounds are oxidized to sulfate, carbon compounds, as far as they are oxidized, are oxidized to carbonates.

As far as various embodiments of the recovery process according to the invention have been described in this patent application, these different versions can also be combined. Details of the method according to the invention will be explained with reference to the following embodiment.

A plant for the anaerobic wastewater treatment of waste water from a waste paper treatment sludge is removed containing anaerobic microorganisms. This sludge is supplied over a period of about 3 to 6 months with a wastewater, which, starting from the waste water of waste paper processing, in steps of 5 percent by volume is increasingly replaced by wastewater of an alkaline sulphite process for the production of high-yield pulp, so that the microorganisms are completely converted to the new wastewater substrate at the end of the said period.

The alkaline sulfite process is described in DE 10 2006 027006. We expressly declare that the content of this patent application is also intended to be a disclosure of this patent application. In a typical embodiment of this process, sodium sulfite is used to break down wood. The yield of the process is 85% based on the originally used, oven-dry wood pulp. So it go during the digestion about 15% of the wood substance in solution and are discharged with the wastewater. The effluent of such digestion of lignocellulosic raw material, here wood, has a COD load of 55 g / l. The pH of the waste water is between 6.5 and 7.0, making it ideal for efficient anaerobic microorganisms. The wastewater leaves the pulp washing with a temperature of about 60 ⁰ C and can thus be fed directly to the aerobic or anaerobic wastewater treatment.

The effluent from the high yield sulfite pulping contains lignin, including sulfonated lignin, carbohydrates, including cellulose and hemicelluloses, and digestion chemicals and unused chemicals. This wastewater is then sent to a first stage of anaerobic wastewater treatment. The wastewater is introduced below into a UASB reactor tower and rises in the tower, penetrating through the sludge / mesophilic microorganisms contained therein. The anaerobic microorganisms conditioned on the treatment of this effluent preferably produce sulfur and / or sulfur compounds, especially hydrogen sulfide and sulfur dioxide. Since the conversion of the sulfates to hydrogen sulfide preferably proceeds, the sulfates are first reacted. Most of the hydrogen sulfide escapes as gas, but part of it is also converted into sulfur dioxide with water, which partially escapes in gaseous form, partially remaining dissolved in water. At the same time low molecular weight oxygen-consuming substances, typically monomers or oligomers z. As lignin or carbohydrates) degraded. If the alkaline component of the wastewater is reacted proportionally or predominantly to sodium hydroxide instead of sodium carbonate, elemental sulfur also forms in the reaction of sulfur compounds.

The wastewater (the first-stage filtrate), which is largely purified from sulfur-containing compounds in the first stage of anaerobic wastewater treatment, is subsequently subjected to further wastewater treatment in a second anaerobic stage. In it, the reduction of the COD burden through the conversion to methane via the steps of hydrolysis, acidogenesis, acetogenesis and methanogenesis. In addition to the predominantly formed methane and carbon dioxide is formed. Also for this second stage of anaerobic wastewater treatment, the microorganisms are conditioned in the manner described for the first anaerobic stage.

Hydrogen sulphide, methane and carbon dioxide escape in gaseous form respectively at the upper end of the tower of the first and the second stage of anaerobic wastewater treatment. The gases that are initially bound to the sludge pellets when the wastewater rises in the tower or are stored in these are separated from the pellets by scrapers and collected. The gas released pellets lose buoyancy and sink back to the bottom of the tower where the cycle of chemical reaction begins again. The filtrate, which still contains a remainder of the COD load and the sodium carbonate and / or hydroxide, is taken from overflows at the top of the tower. The filtrate withdrawn at the overflow of the first UASB tower designed to produce sulfur compounds is introduced down into the second UASB tower to further reduce the COD load of the effluent and to increase the economic efficiency of the process by generating Further increase biogas. It should be noted that there is no separation of hydrogen sulphide and sulfur dioxide on the one hand and methane and carbon dioxide on the other. Sulfur and sulfur compounds can be removed at the top of the first UASB tower and introduced directly into the filtrate produced at the overflow of the second UASB tower. Elaborate drainage or storage systems, which must be designed for the corrosive hydrogen sulfide, can thus be almost completely avoided. Elemental sulfur floats on the first stage filtrate. It can be withdrawn there and, like hydrogen sulfide, is converted by combustion into sulfur dioxide, which can then be further converted into sodium sulfite in aqueous solution with alkaline filtrate.

The filtrate has a reduced by 50% COD load compared to the wastewater. The present in the form of sulfur salts in the wastewater sulfur was converted to 75% to hydrogen sulfide, sulfur dioxide (and subsequently using the alkali see filtrate to sodium sulfite).

For a sulfide digestion, the same process procedure (1st stage: preferred production of sulfur, 2nd stage: preferential production of methane) is selected for the treatment of the waste water.

Claims

claims

l. A process for the recovery of chemicals from waste water resulting from the production of pulp from lignocellulosic raw material using sulfur salts, the waste water being subjected to anaerobic waste water treatment to produce sulfur and / or sulfur compounds, characterized in that the anaerobic waste water treatment comprises at least one of Stage for the preferred production of sulfur and / or sulfur compounds and at least one stage for the preferred production of methane and / or carbon dioxide.

2. The method according to claim 1, characterized in that the at least one stage for the production of sulfur and / or sulfur compounds before the at least one stage for the preferred production of methane and / or carbon dioxide is arranged.

3. The method according to claim 1, characterized in that in the at least one stage for the preferred production of sulfur and / or sulfur compounds and low molecular weight oxygen-consuming substances are reacted.

4. The method according to claim 1, characterized in that in the at least one stage for the preferred production of methane and / or carbon dioxide and higher molecular weight oxygen-consuming substances are reacted.

5. The method according to claim 1, characterized in that the sulfur compounds produced in the anaerobic wastewater treatment, in particular

Sulfur, hydrogen sulfide and sulfur dioxide, to be converted to sulfur salts for the production of fibrous materials.

6. The method according to claim l, characterized in that in the anaerobic wastewater treatment produced methane and carbon dioxide is used to generate energy.

7. The method according to claim 1, characterized in that in the anaerobic wastewater treatment produced filtrate for the production of sulfur salts is used.

8. The method according to claim 7, characterized in that filtrate containing alkaline compounds, in particular carbonates, is used for the preparation of sulfur salts.

9. The method according to claim 1, characterized in that the sulfur salts contained in the wastewater to at least 10%, preferably at least 50%, preferably up to 75% by the anaerobic wastewater treatment in sulfur and

Sulfur compounds are reacted.

10. The method according to claim 1, characterized in that for anaerobic wastewater treatment psychrophilic, mesophilic or thermophilic microorganisms are used.

11. The method according to claim 1, characterized in that by the anaerobic wastewater treatment at least 5%, preferably up to 25%, advantageously up to 50% of the original existing in the wastewater chemical oxygen demand (COD) are degraded.

12. The method according to claim 1, characterized in that the COD content of the waste water of the pulp production contains up to 150 g / l COD, preferably from 20 g / l to 120 g / l, more preferably from 40 g / l to 100 g / l.

13. The method according to claim 1, characterized in that the wastewater containing sulfur salts undergoes an oxidizing pretreatment before the anaerobic wastewater treatment.