CROSS-REFERENCES TO RELATED APPLICATIONS 
       [0001]    The present application claims the priorities of European Patent Application No. 11167622.7, filed May 26, 2011; European Patent Application No. 11155310.3, filed Feb. 22, 2011; European Patent Application No. 11152947.5, filed Feb. 1, 2011; and Patent Cooperation Treaty Application No. PCT/EP2010/064948, filed on Oct. 6, 2010; all of which are incorporated herein by reference in their entirety for all purposes. 
       FIELD OF THE INVENTION 
       [0002]    The present application relates to a method and an apparatus for providing and using hydrogen-based methanol for denitrification purposes. 
       BACKGROUND OF THE INVENTION 
       [0003]    Carbon dioxide CO2 is a chemical compound comprising carbon and oxygen. Carbon dioxide is a colorless and odorless gas. At a low concentration, it is a natural component of air and occurs in living beings during cell respiration, but also during the combustion of carbonaceous substances if sufficient oxygen is present. Since the beginning of industrialization, the CO2 component in the atmosphere has significantly risen. The main causes of this are the CO2 emissions caused by humans—so-called anthropogenic CO2 emissions. The carbon dioxide in the atmosphere absorbs a part of the thermal radiation. This property makes carbon dioxide a so-called greenhouse gas (GHG) and one of the contributing causes of the global greenhouse effect. 
         [0004]    For these and other reasons, research and development is currently being performed in greatly varying directions in order to find a way to reduce the anthropogenic CO2 emissions. In particular in connection with power generation, which is frequently performed by the combustion of fossil fuels, such as coal, oil, or gas, but also with other combustion processes, for example, garbage burning, there is a great demand for reduction of the CO2 emission. Currently, approximately 30 billion tons of CO2 are discharged into the atmosphere per year by such processes. 
         [0005]    It is considered to be a problem that CO2 arises during the combustion of fossil fuels. In addition, the fossil resources, which are finite, are irrevocably consumed. Research is being performed in greatly varying directions in order to reduce the consumption of vehicles or to develop vehicles which are driven completely using regenerative power. 
         [0006]    In addition to the problems of air contamination and stress, there are also problems in connection with water contamination. Specifically, nitrogen occurs in water both molecularly as nitrogen (N2) and also in inorganic and organic compounds. Urea is the largest nitrogen source in municipal wastewater. The limiting value for drinking water is 50 mg/L of nitrate, which corresponds stoichiometrically to approximately 11 mg/L nitrate nitrogen, according to the locally applicable drinking water code. 
         [0007]    It is known that the nitrogen component in the wastewater can be reduced by denitrification methods (also referred to as denitrification or nitrogen elimination). In denitrification, nitrates are reduced to form oxygen-poor nitrogen compounds and then to elementary, gaseous nitrogen. A biologically degradable organic substrate is required for denitrification. If sufficient substrate is not present, methanol is supplied, for example. 
         [0008]    Methanol is a particularly advantageous substrate, since it is fully soluble in water and is easily biologically degradable. However, methanol has been produced up to this point from fossil raw materials, for example, from natural gas in most cases. Numerous methods and reactors for producing methanol are known. Exemplary patent applications and patents include:
   EP 0 790 226 B1;   WO 2010/037441 A1;   EP 4 483 919 A2.   
 
         [0012]    The demand exists for the provision of methanol which is CO2-neutral and cost-effective to produce. In addition, the methanol production should not compete with food production and should not require space, as in the production from biomass. 
         [0013]    The object presents itself of developing a corresponding method and a corresponding apparatus for providing methanol especially for denitrification purposes in wastewater treatment plants, which is ecologically and economically advisable. 
       SUMMARY OF THE INVENTION 
       [0014]    Therefore, a novel method chain is proposed according to the invention, which relates to the provision and consumption of methanol in a wastewater treatment plant. 
         [0015]    The method, in a preferred form, comprises the following steps: 
         [0016]    providing a gas having a carbon dioxide component (CO2) as a carbon supplier, 
         [0017]    providing a hydrogen component (H2), 
         [0018]    mixing the carbon dioxide component and the hydrogen component to form a starting material, 
         [0019]    introducing the starting material into a reactor, 
         [0020]    passing the starting material through a section of the reactor which is at least partially equipped with a catalyst in order to synthesize methanol using a synthetic-catalytic method, 
         [0021]    obtaining a liquid mixture made of a methanol component and a water component, 
         [0022]    using the methanol containing liquid in a wastewater treatment plant for denitrification purposes. 
         [0023]    The goal of the invention is also to provide a system which functions as autonomously as possible, i.e., as much as possible independently of the power grid. In particular, this relates to a method in which at least a part of the power requirement, which exists in order to provide the hydrogen component (H2) electrolytically, is generated locally in the wastewater treatment plant or in its immediate surroundings. 
         [0024]    The power is preferably generated when waste materials which occur in a sewage plant are combusted in order to operate a power generator, or in that waste materials are converted into methane-containing gas, for example, and this gas is used for the power generation. 
         [0025]    Alternatively, the hydrogen component (H2) can also be generated directly from methane-containing gas, for example. In this case, electrolysis is not necessary to provide the hydrogen component (H2). 
         [0026]    A combination of an electrolysis plant and hydrogen provision from locally existing gas is also possible. 
         [0027]    The invention is intentionally based on carbon dioxide and hydrogen as the starting materials, since the carbon dioxide is “recycled” in this way and can serve for the denitrification via the use of methanol as the carbon supplier. 
         [0028]    Hydrogen can additionally or alternatively also be generated from regenerative energy and the carbon dioxide can be obtained from exhaust gases or generated from biomass, so that the methanol, which is synthesized from these starting materials catalytically, can be considered to be CO2-neutral. In addition, this way has the advantage that, upon the synthesis of methanol, for example, it provides a methanol-water mixture, which is directly suitable for denitrification. The composition of the methanol-water mixture is ideal to be used for denitrification, and it is distinguished in that it also has advantages from an environmental-technology aspect. 
         [0029]    In contrast to previous methanol production methods, the product of the present methanol synthesis process, comprising methanol and water, can be used directly as the liquid for the denitrification. No further energy expenditure is required to distill the product of the synthesis process for denitrification. The energy expenditure for distilling, which is typically performed in order to obtain pure, highly-concentrated methanol, makes the methanol more costly. 
         [0030]    According to the invention, synthesis gas which comprises H2 and CO2 is converted efficiently and in an economically advisable manner into methanol for denitrification purposes. 
         [0031]    According to the invention, carbon dioxide is used as the starting carbon supplier for eventual denitrification. The carbon dioxide is caused to react with the hydrogen component in the presence of a catalyst, in order to convert it into a methanol-water mixture. 
         [0032]    The carbon dioxide is preferably taken from a combustion process or an oxidation process of carbon or hydrocarbons by means of CO2 separation. For example, CO2 can originate from a sewage treatment process. If the CO2 originates from a sewage treatment process, the plant according to the invention is autonomous, since neither energy nor other materials must be externally delivered or supplied depending on the design of the plant and depending on the environmental conditions. 
         [0033]    The method according to the invention for providing the denitrification liquid is controlled and the individual processes are “linked” to one another so that 
         [0034]    the total yield and the quality of the methanol are ideal for the intended purpose, 
         [0035]    and/or the (total) CO2 emission is as minimal as possible, 
         [0036]    and/or the most consistent and long-term possible plant workload is achieved, 
         [0037]    and/or the product-specific investment and operating costs are as minimal as possible. 
         [0038]    Locally provided power and/or regenerative electrical power are preferably used to provide the methanol. 
         [0039]    Using a corresponding plant, a methanol-water mixture is preferably produced as a liquid which can be stored and transported, i.e., the locally provided power and/or renewable power is chemically converted into a liquid which is relatively simple to store and transport. 
         [0040]    The production of the liquid as a mixture which can be stored and transported relatively simply can be phased down or even interrupted at any time. The processing plant parts for producing the mixture can be phased down or shut down relatively easily and rapidly. The ultimate decision is in the scope of responsibility of the operator of the plant 
         [0041]    Preferred embodiments of the invention are based on hydrogen generation with the aid of electrical energy, which is locally (i.e., on location in the area of the wastewater treatment plant) generated regeneratively as much as possible, and originates, for example, from biomass, biogas, fermentation gas, wind, water, geothermal, and/or solar power plants, for example. Hydrogen which is generated on location via electrolysis and/or from waste materials, for example, does not need to be stored or highly compressed or cryogenically liquefied and transported over long distances, but rather serves as an intermediate product, which is preferably supplied at the location of its generation immediately or soon to the above-mentioned reaction to generate methanol. 
         [0042]    The corresponding methanol-water mixture can also be generated according to the invention employing an intelligent energy mix (as described, for example, in International Patent Application WO2010069622A1 of fossil and regenerative energy. 
         [0043]    A novel method relevant to power engineering and a corresponding use are provided according to the invention in consideration of corresponding power engineering, industrial, and economic parameters, together with the requirement for careful use of all material, energetic, and economic resources. 
         [0044]    Further advantageous embodiments can be inferred from the description, the figures, and the dependent claims. 
         [0045]    Various aspects of the invention are schematically shown in the drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0046]      FIG. 1  shows a schematic diagram, which indicates the basic steps of the preferred method; 
           [0047]      FIG. 2  shows a schematic diagram which also indicates the basic steps of the preferred method according to the invention; 
           [0048]      FIG. 3  shows a lateral external view of a reactor which can be used in a method according to the invention; 
           [0049]      FIG. 4  shows a schematic view of an overall wastewater treatment method according to the invention; 
           [0050]      FIG. 5  shows a schematic view of an overall method according to the invention for using the locally existing resources, in order to make the method autonomous. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0051]      FIG. 1  shows methanol-water mixtures  108 , which are also referred to as methanol-containing liquids. 
         [0052]    The term mixture  108  is used here, since the product which is provided at the outlet  23  of a reactor  10  ( FIG. 3 ) does not consist of 100% methanol. Rather, it is a so-called physical mixture of methanol and water. 
         [0053]    The term wastewater treatment plant is used here for any type of plant which is usable for (waste) water treatment or purification. In particular, this relates to the use in a sewage treatment plant. 
         [0054]    This especially relates to a combination of methanol denitrification and the use of methanol-water mixtures  108 , the required CO2 being obtained from the fermentation gas and/or from the combustion gas. The required electrical energy being generated from a block heating power plant  602  ( FIG. 5 ) operated by fermentation gas (combination of combustion engine with generator and waste heat usage, e.g., for heating the sewage treatment basins). 
         [0055]      FIG. 1  shows a schematic block diagram of the most important building blocks/components, or method steps, of a preferred plant  100  according to the invention. This plant  100  is designed so that a method for providing the methanol liquid mixture  108  can be executed. The corresponding method is based on the following basic steps. 
         [0056]    Carbon dioxide  101  is provided as the carbon supplier. The electrical DC energy E 1  required for generating hydrogen  103  is generated here as much as possible by means of renewable energy technology and provided to the plant  100 . Solar thermal plants  300 ,  301  and photovoltaic plants  400 , which are based on solar modules, are particularly suitable as the renewable energy technology. For example, water power, wind power, or geothermal energy can also be used as regenerative energy sources. The regenerative energy sources can also be of biogenic origin (inter alia, sewage sludge or sewage gas), for example, or methane from other sources. 
         [0057]    According to  FIG. 1 , a water electrolysis  105  is performed employing the electrical DC energy E 1 , in order to generate hydrogen  103  as intermediate product. 
         [0058]    In the plant  100 , an economically and ecologically optimum combination of regenerative power supply (e.g., by the sources  300  and/or  400 ) and conventional power supply, represented here by a part of an integrated network  500 , is preferably implemented. A part or all of the electrical energy can also be locally generated (e.g., by using a locally occurring gas or locally occurring materials which may be converted). This plant  100  therefore provides using the electrical energy E 1  substantially directly in accordance with its occurrence for chemical reactions (the electrolysis reaction  105  here) and therefore chemically binding and storing it. A further component of the required energy is acquired here, for example, from the integrated network  500  and/or from local plants (e.g., a generator). This component is converted into direct current (energy) E 2 . For this purpose, a corresponding converter  501  is used, as schematically indicated in  FIG. 1 . The corresponding plant parts or components are also referred to here as (local) power supply plant  501 . 
         [0059]    The power supply of the plant  100  according to  FIG. 1  is controlled and regulated by means of an intelligent plant controller  110 . Fundamentally, the instantaneously available excess energy share E 2  is acquired from the integrated network  500 , while the other energy share (E 1  here) is acquired as much as possible from a (plant-related) solar power plant  300  and/or  400  (and/or from a wind power plant and/or from a biomass power plant and/or from a water power plant and/or from a geothermal power plant). This principle allows the operator of a plant  100  to incorporate additional technical and economic parameters in the controller of the plant  100 . These parameters are so-called input variables  11 ,  12 , etc., which are incorporated in decisions by the controller  110 . A part of the parameters can be predefined within the controller  110  in a parameter memory  111 . Another part of the parameters can come from the outside. The method according to the invention can be guided by the controller  110  in the plant  100  so that the methanol liquid  108  which is provided at the outlet meets the desired requirements with respect to the mixing ratio and/or the CO2 neutrality. 
         [0060]      FIG. 2  schematically shows a further plant  700 , which can be used in order to execute the method according to the invention. A part of this plant  700  corresponds to the plant  100  according to  FIG. 1 . Therefore, reference is made to the preceding description of the corresponding elements. 
         [0061]    High-purity hydrogen  103 , which is converted here into a methanol-water mixture  108 , is also generated in this plant  700 , as described, by a water electrolysis  105 . The energy for this purpose originates in this embodiment entirely or substantially (preferably more than 80%) from regenerative energy sources  300  and/or  400 , or from other regenerative energy sources and/or from local energy sources. 
         [0062]    An number of control or signal lines can be provided, as illustrated on the basis of the lines  112 ,  113 ,  114 , and  115  shown as examples. These lines  112 ,  113 ,  114 , and  115  control energy or mass flows of the plant  100  or  700 . 
         [0063]      FIGS. 1 and 2  show that the methanol-water mixture  108  is used for the denitrification  600 . Water (e.g., wastewater) is supplied on the inlet side (identified by IN). After the execution of the denitrification  600 , water, which now contains less nitrogen, is discharged on the outlet side (identified by OUT). Details of this denitrification method  600  are well known and are not described in detail here. 
         [0064]    So-called software-based decision processes are implemented in the plant controller  110 . A processor of the controller  110  executes control software and arrives at programmed decisions by consideration of parameters. These decisions are converted into switching or control commands, which cause the control/regulation of energy and mass flows via control or signal lines  112 ,  113 ,  114 ,  115 , for example. The method can be guided in the plant  100  by the controller  110 , so that the liquid  108  which is provided at the outlet meets the desired requirements with respect to the mixing ratio and/or the CO2 neutrality. 
         [0065]    According to the invention, carbon dioxide  101  is used as a gaseous carbon supplier  104 , as schematically indicated in  FIG. 1  and  FIG. 2 . The carbon dioxide  101  is preferably taken from a combustion process or an oxidation process via CO2 separation (e.g., a Silicon Fire flue gas purification plant). However, the carbon dioxide  101  can also be provided from a sewage treatment plant. The carbon dioxide  101  can also come from other sources. 
         [0066]    Furthermore, in the plant  700  shown in  FIG. 2 , electrical DC energy E 1  is provided (this is also true for the plant in  FIG. 4 ). The DC energy E 1  is preferably locally generated substantially regeneratively (e.g., by one of the plants  300  and/or  400  and  FIG. 2 ) and/or in another way. The DC energy E 1  is used in the plant  700  shown to perform a water  102  electrolysis, in order to generate hydrogen  103  as an intermediate product. The electrolysis plant, or the performance of such an electrolysis, is identified in  FIG. 1 ,  FIG. 2 , and  FIG. 4  by the reference sign  105 . The carbon dioxide  101  is mixed with the hydrogen  103  to form a starting material (AS). The starting material (AS) is then introduced into a reactor  10 , as shown in  FIG. 3 , for example, in order to convert the gaseous (intermediate) products  101 ,  103  into the methanol-water mixture  108 . The reaction  106  is performed in the reactor  10 . The removal or the provision of the methanol-water mixture  108  is identified in  FIG. 1  and  FIG. 2  by the reference sign  107 . 
         [0067]    The mixing of the gases to form the starting material (AS) is critical, since the right stoichiometry has to be ensured. One might employ a gas mixer for mixing the gases. In addition, it is possible to employ one or two ring feed lines sitting on top of the reactor  10  so that equal quantities of gases to form the starting material are fed via a ring feed line(s) into the individual parallel reactor sections of the reactor. It is also possible to use a buffer space positioned in front of the individual reactor sections so that the starting material (AS) is fed into the buffer space from where it is then guided into each of the individual parallel reactor sections. 
         [0068]    Water electrolysis employing direct current E 1  is capable of generating hydrogen  103  as an intermediate product. The required hydrogen  103  is produced in an electrolysis plant  105  by the electrolysis of water H2O according to the following equation: 
         [0000]      H2O−286.02 kJ=H2+0.5O2.  (Reaction 1)
 
         [0069]    The required (electrical) energy E 1  for this reaction of 286.02 kJ/mol corresponds to 143010 kJ per kg H2. 
         [0070]    The synthesis of the methanol-water mixture  108  (CH3OH+H2O) can be performed in the reactor  10  of the plant  100  according to the exothermic reaction between carbon dioxide  101  (CO2) and hydrogen  103  (H2) as follows: 
         [0000]      CO2+3H2=CH3OH+H2O−49.6 kJ (methanol-water mixture, gaseous)  (Reaction 2)
 
         [0071]    The occurring reaction heat of 49.6 kJ/mol=1550 kJ per kg methanol=0.43 kWh per kg methanol  108  is dissipated from the corresponding reactor  10 . For this purpose, the reactor  10  comprises a fluid chamber  14  (see  FIG. 3 , for example). The reactor  10  is preferably enclosed by a reactor mantle and is cooled by a fluid (preferably water). 
         [0072]    Typical synthesis conditions in the synthesis reactor  10  are approximately 50 to 80 bar and approximately 270° C. The reaction heat can be “transferred” to other plant elements and used therein, for example. 
         [0073]    The methanol-water synthesis is performed according to the invention employing a catalyst in order to keep reaction temperature, reaction pressure, and reaction time low in comparison to other methods and in order to ensure that a liquid methanol-water mixture  108 , which is suitable as a denitrification liquid, results as the reaction product. 
         [0074]      FIG. 2  indicates on the basis of the dashed arrow  112 , which originates from the controller  110 , that the controller  110  regulates the energy flow E 1 . The arrow  112  represents a control or signal line. Other possible control or signal lines  113 ,  114  are also shown. The control or signal line  113  regulates the CO2 quantity which is available for the reaction for example. For example, if less hydrogen  103  is produced, proportionally less CO2 must also be supplied. The optional control or signal line  114  can regulate the H2 quantity, for example. Such a regulation is advisable if there is a hydrogen buffer store from which a hydrogen  103  can be taken, even if no hydrogen or less hydrogen is currently being produced by electrolysis  105 . 
         [0075]    Details of a particularly preferred embodiment of a reactor  10  for synthesizing the methanol-water mixture  108  are shown in  FIG. 3 . The statements which are made on the synthesis of methanol mixture  108  in the International Patent Application PCT/EP2010/064948 may also be transferred to the synthesis of other liquid hydrocarbons. 
         [0076]    The methanol-water mixture  108  is, as already described, synthesized employing a starting material (AS) which contains CO2 gas  101  and hydrogen gas  103 . The corresponding reactor  10  comprises a reactor element or multiple reactor elements situated in parallel to one another. There is at least one gas intake  21  for the starting material (AS) on the reactor  10  and a product outlet  23 , as shown as an example in  FIG. 3 . 
         [0077]    The starting material (AS) is successively converted into a methanol-containing mixture  108  (referred to as alcoholic coolant liquid) as it passes through or is pressed through the reactor pipe(s) of the reactor  10 . On the inlet side of the reactor  10 , the methanol concentration of the reaction fluid is preferably zero and the concentration of the respective gaseous starting material (AS) is approximately 100%. In the direction of the outlet side of the reactor  10 , the corresponding concentrations shift in opposite directions until a methanol-containing mixture  108  having a predefined methanol concentration (preferably a methanol-water mixture in the ratio 1:2) is formed at the product outlet  23 . 
         [0078]    The reactor  10  preferably delivers approximately 64 mass-% (69.2 vol.-%) methanol and 36 mass-% (30.8 vol.-%) water. 
         [0079]    The reactor  10  or the elements of the reactor  10  preferably includes a catalyst for the synthesis of the methanol-water mixture  108  in all embodiments. 
         [0080]    In all embodiments, a controller of the reactor  10  is preferably used, which initially applies hot fluid to the fluid chamber  14  at the beginning during the “startup” of the reactor  10 , in order to get the synthesis reaction going. Subsequently, a cold fluid is preferably supplied, in order to dissipate reaction heat which arises during the exothermic synthesis and thus provide an isothermal environment. 
         [0081]    The fluid chamber  14  is preferably designed in all embodiments so that at least the reaction sections of the reactor  10  which are filled with the catalyst are in the isothermal environment. 
         [0082]    The reactor  10  is schematically shown in  FIG. 3 . 
         [0083]    In all embodiments of the invention, the starting material (AS) is preferably introduced preheated and/or at elevated pressure through supply lines into the reactor  10 . The pressure and the temperature are dependent on the type of the catalyst. The temperature is preferably in the range between 100 and 350° C. The pressure is typically between 10 and 150 bar. Therefore, it can also be stated that the starting material (AS) is preferably pressed through the reactor  10  with specification of an intake-side pressure between 10 and 150 bar in all embodiments. 
         [0084]    The reactor  10  is especially suitable for the synthesis of a regenerative methanol-water mixture  108  made of carbon dioxide CO2 and hydrogen H2, which is generated via the (endothermic) electrolysis of water using regenerative electrical energy E 1  according to reaction 1, as already mentioned above. 
         [0000]      H2O−286.02 kJ/mol=H2+0.5O2  (Reaction 1)
 
         [0085]    The exothermic methanol-water synthesis (reaction 2, as already mentioned above) is represented by the summation formula: 
         [0000]      CO2+3H2=CH3OH+H2O−49.6 kJ (gaseous methanol)  (Reaction 2)
 
         [0086]    It must be emphasized that other synthesis methods and other reactors  10  or plants can also be used in all embodiments, of course, and the synthesis can be operated using regenerative energy and/or using regenerative starting material (AS). The use of regenerative energy and regenerative starting materials (AS) is preferred. 
         [0087]    The use of the invention in connection with a method for methanol-water synthesis, which operates at low pressures between 10 and 150 bar (preferably at approximately 80 bar) is particularly advantageous. 
         [0088]    The principle of the invention may also be transferred to large-scale plants, but is particularly suitable for autonomous local plants for wastewater treatment. 
         [0089]    According to the invention, CO2  101  is used as the starting material and carbon supplier for the methanol-water synthesis in the reactor  10 . Steam reforming plants, fermentation plants, and firing plants are preferably used as the CO2 sources. 
         [0090]    Depending on the synthesis reaction, copper-based catalysts (e.g., CuO catalysts) or zinc oxide catalysts (e.g., ZnO catalysts) or chromium oxide-zinc oxide catalysts may be used, for example. Other known catalysts are also suitable for use in a reactor  10 . Packed bed catalysts or fluid bed catalysts are particularly suitable. The catalyst can also comprise a suitable carrier (e.g., carbon, silicate, aluminum (e.g., Al2O3) or ceramic). Instead of the mentioned “metal” catalysts, an organic catalyst can also be used 
         [0091]    In all embodiments, the catalysts preferably has a grain, bead, or particle size between 1 and 10 mm. A grain, bead, or particle size between 3 and 8 mm is particularly preferred. 
         [0092]    Further fundamental details of the method according to the invention and the corresponding plants  100 ,  700 ,  800 ,  900  are described hereafter with reference to  FIGS. 4 and 5 . A schematic diagram of a complete plant  800  is shown in  FIG. 4 . The methanol mixture  108  is used in a denitrification tank  601  here. The process  600  occurs in the denitrification tank  601 . 
         [0093]      FIG. 5  shows the principle of the fermentation gas power generation and methanol production  900  according to the invention. The Silicon Fire mobile station  603  shown generates the methanol  108 , which is used in step  600  for denitrification. A local power generator  602  (preferably a power generator  602  of a fermentation gas power generation plant) preferably delivers electrical power (power supply) to the Silicon Fire mobile station  603 . A CO2 separator  604  delivers the CO2 for the methanol synthesis in the Silicon Fire mobile station  603 . 
         [0094]    In order to remove the nitrogen from the wastewater, a denitrification employing methanol  108  as a reducing agent is preferably executed in all embodiments as the last treatment step. 
         [0095]    The reducing agent (methanol  108  here) is to be a degradable organic substance, which can be digested by bacteria and which allows the growth of new bacteria. The methanol  108  ideally meets these specifications, since it should not contain any toxic contaminants, in contrast to methanol produced from fossil fuel. 
         [0096]    Theoretically, 1.9 kg methanol  108  is necessary to remove 1 kg nitrogen from the wastewater. In reality, approximately 2.5 kg methanol  108  is preferably used per kilogram of nitrogen. 
         [0097]    In all embodiments, separate power generation is preferably used, which is ensured, for example, by a combustion plant-generator system operated by fermentation gas (see  FIG. 5 ). 
         [0098]    In addition to ecological advantages, sustained cost advantages also result through the invention.
A green process for denitrification using a methanol-containing liquid generated by a catalytic reaction of a starting material formed by mixing carbon dioxide gas with hydrogen gas. This process can advantageously be used for denitrification in waste water treatment plants and, if the hydrogen is generated from water and/or methane derived from the waste water, the process can be self-contained and conducted completely at the waste water treatment plant.