Production process for biogas production by anaerobic co-digestion

Abstract

The present invention relates to a production process for producing biogas through anaerobic co-digestion, comprising: (a) thermally hydrolyzing waste at 1.5 to 4.5 bar and 120 to 160°C to produce hydrolyzed biomass from organic solid waste, thereby producing raw biomass from which impurities have been separated, producing clean hydrolyzed biomass having at least 90% organic matter, a ratio of volatile solids to total solids of at least 0.6%, and at least 5% total solids; (b) mixing the clean hydrolyzed biomass with sludge from a wastewater treatment plant (WWTP) to produce a mixture having a solids concentration of less than 30% and conditioned to a concentration of less than 20%; and (c) undergoing a wet anaerobic digestion step in a digester at 25°C to 40°C or 50°C to 60°C for an HRT of 12 to 30 days to produce biogas and digestate.

Description

Technology Field

[0001] The object of the present invention relates to the technical field of environmental technology, and in particular, to a new production process for producing biogas by anaerobic co-digestion of the following organic wastes: organic solid waste (hereinafter OSW), understood as both organic waste originating from sludge derived from municipal wastewater (hereinafter UWW) and organic fractions of municipal solid waste (hereinafter MSW) and assimilated waste, preferably optional collections of MSW, as well as organic waste from agriculture, horticulture, hydroponics, forestry, hunting, fisheries, food and industrial organic waste manufacturing and production, etc. Background Technology

[0002] The present invention stems from the need to improve current urban waste management processes. In particular, it relates to a new process for optimizing biogas production by treating sludge originating from two types of organic waste that have been managed independently to date: OSW and UWW.

[0003] The treatment of UWW at treatment plants mainly involves the following two process lines: on the one hand, the water is made available for disposal into waterways and / or reuse for street cleaning and / or garden irrigation, and on the other hand, the solid fractions or sludge separated or generated during wastewater treatment are managed. In this case, sand or bulky waste is removed and sent to a landfill, while the sludge generated during biological treatment or removed during the sedimentation stage mostly undergoes a wet anaerobic digestion process.

[0004] For municipal solid waste and assimilated waste, general treatment consists of separating the different fractions present therein (mainly plastic, metal, and organic fractions) and then treating and recovering each fraction independently. For organic fractions, the main forms of recovery are composting or aerobic stabilization, anaerobic digestion, or heat recovery (incineration, gasification, pyrolysis, etc.).

[0005] In the case of anaerobic digestion, there are the following two major trends in waste management:

[0006] ● Dry process, when the solid concentration exceeds 20 weight%, and

[0007] ● Wet process, when the solid concentration is less than 20 weight% (generally about 10 or 12%).

[0008] Numerous inventions related to sludge treatment processes by anaerobic digestion can be found in patent literature.

[0009] Accordingly, for example, patent US3338826 relates to a process for treating wastewater by anaerobic digestion, comprising the step of applying a pressure of 2 to 4 atmospheres to the wastewater to accelerate decomposition.

[0010] Consequently, patent EP0737651 relates to a method for treating wastewater, comprising, among other steps, a step of heat pre-treating excess sludge at a temperature of 60°C or higher, a step of dewatering the excess sludge, and a step of mixing the dewatered sludge with digested sludge generated by a methane fermentation process, and using the obtained mixture as a substrate for a new methane fermentation process.

[0011] Although many solutions have been proposed to optimize the treatment process of wastewater sludge generated at wastewater treatment plants (WWTP), to date, no solution based on conditioning through a thermal hydrolysis process of another type of organic material has been found. This waste is solid waste generated from the selective collection of waste, preferably the organic portion of MSW, which, once treated, is used to improve the efficiency of the anaerobic digestion process of sludge generated or separated at the wastewater sludge treatment plant. Unlike the objective of the present invention, processes known in the art based on the joint treatment of organic waste and sludge from treatment plants by co-digestion are based on the use of raw organic waste, that is, waste that has not undergone any prior treatment after a foreign matter removal step.

[0012] As a result of the thermal hydrolysis treatment of organic solid waste, various benefits are achieved, as described in detail below. Among these, the remarkable results achieved with respect to the production and quality of the obtained biogas should be noted, which contribute to the self-supply of energy for the treatment facility and even the generation of surplus energy.

[0013] Thus, the object of the present invention is to a production process for producing biogas through anaerobic co-digestion, characterized by comprising the following:

[0014] (a) A first step of producing hydrolyzed biomass from organic solid waste. This first step comprises, in turn, the following:

[0015] i. A first sub-step comprising a thermal hydrolysis treatment of organic solid waste. This thermal hydrolysis treatment may preferably be performed by applying the waste to a pressure including 1.5 to 4.5 bar and a temperature of 120 to 160°C for a time that may be in the range of 10 to 75 minutes. Nevertheless, the pressure and time may be varied to optimize the effect of the thermal hydrolysis without generating inhibitory compounds that may affect the next step of the process.

[0016] The following benefits are obtained through this thermal hydrolysis treatment:

[0017] - Waste Sanitation;

[0018] - Partial decomposition of organic fractions without affecting foreign substances (understood as such substances are waste fractions or components that are difficult to recover in anaerobic digestion processes). This results in the following:

[0019] ● Homogenization of organic fractions,

[0020] ● The efficiency of foreign substance separation is increased, thereby achieving higher utilization of organic fractions and minimizing losses in the subsequent washing process;

[0021] - Thermal and biological stabilization of organic matter;

[0022] - Conversion of complex compounds into soluble and more easily decomposable molecules.

[0023] The result of this step of thermal hydrolysis is a product to be referred to as "raw biomass" for the purposes of this patent. Although the characteristics of the "raw biomass" may vary significantly depending on the source of the starting waste, in certain embodiments, the "raw biomass" is characterized by comprising: 70 to 75 weight percent of biodegradable organic matter; 10 to 15 weight percent of light foreign matter (materials that tend to float because they have a lower density than the suspension of organic matter when the organic matter is solubilized, understood as, e.g., textiles, plastics, plastic-cardboard containers, wood pruning waste, wood, or cork, etc.); and 10 to 15 weight percent of heavy foreign matter (materials that tend to settle because they have a higher density than the suspension of organic matter when the organic matter is solubilized, understood as, e.g., glass, plate glass, sand, stone, bone, etc.);

[0024] ii. Subsequently, a second sub-step for separating foreign substances is performed for post-processing of the raw biomass. In particular, the separation of light foreign substances and heavy foreign substances is performed.

[0025] The removal of foreign substances may be carried out using any method known in the art to perform the above separation process, for example, a trommel-type separator, a vibrating table or a depacker for the removal of light foreign substances and, in particular, a sedimentation tank, a hydrocyclone, a hydroclassifier or a sand trap for the removal of heavy foreign substances.

[0026] As a result of these sub-steps, hydrolyzed biomass, referred to for the purposes of this patent as a "substrate" or "clean hydrolyzed biomass," is obtained, which may be stored or continued in the process. This substrate is characterized by comprising a percentage of at least 90 weight%, more preferably at least 98 weight% of organic matter. Preferably, the weight ratio of volatile solids to total solids in the product is at least 0.6, more preferably at least 0.8, and the total solids content is at least 5 weight%;

[0027] (b) a second step of mixing the clean hydrolyzed biomass obtained in the preliminary step with sludge from a wastewater treatment plant (WWTP) to produce a mixture having a solid content of less than 30% by weight, generally 5% to 15% by weight. Preferably, the amount of clean hydrolyzed biomass in the mixture may be in the range of 5% to 65% by weight, while the amount of sludge may be in the range of 35% to 95% by weight. Nevertheless, the final amount of each component in the mixture is not limited and may vary depending on seasonality or other circumstances.

[0028] In this way, the claimed process ensures that constant biogas production is maintained year-round, regardless of fluctuations that may exist in sludge production (e.g., urban areas with annual population changes). In such cases, biogas production can be maintained by using a larger amount of hydrolyzed biomass to compensate for periods when less sludge is produced.

[0029] Subsequently, the obtained mixture undergoes a conditioning process to adjust the total solid content to less than 20 weight%, preferably to a percentage of 5 to 15 weight%, said amount being the amount required to perform the next step during the process;

[0030] (c) A third step of wet anaerobic digestion of the mixture obtained in the preliminary step in at least one digester to produce biogas and digestate.

[0031] Preferably, anaerobic digestion will be carried out under mesophilic conditions (temperatures of 25°C to 40°C, more preferably 35°C to 38°C) and for a hydraulic retention time (HRT) of 12 to 30 days. In other specific embodiments of the invention, anaerobic digestion may be carried out under thermophilic conditions (temperatures of 50°C to 60°C).

[0032] Consequently, the pH of the anaerobic digestion process is preferably in the range of 7 to 8.5 and 1.5 to 5 kg VS / m² 3 It is likely the organic loading rate of work.

[0033] In a specific embodiment of the present invention where the organic solid waste is an organic fraction of municipal solid waste and assimilated waste, the process may include a prior step of preparing the waste by separating bulky foreign matter (understood as a large volume fraction or component that is easily separable by size, and generally has a size greater than 80 mm) and / or metals present therein. In particular, the separation of the bulky waste may be performed using at least one trommel-type rotary separator having a mesh size including preferably 60 to 120 mm. Consequently, if ferrous metals are present, said metals may be separated by at least one magnetic separator.

[0034] Likewise, this process may preferably include an additional step of utilizing biogas obtained by combined heat and power generation (producing both heat and electricity), in a boiler for heat generation, or by undergoing a purification (improvement) process to obtain biomethane.

[0035] Additionally, the process may preferably include an additional step of utilizing the digestate for agricultural use as a biofertilizer.

[0036] For the purposes of this patent, organic solid waste is understood as both organic fractions of MSW and assimilated waste (and preferably organic waste originating from optional collections of MSW) as well as organic waste from agriculture, horticulture, hydroponics, forestry, hunting, fisheries, or the manufacture and production of food, industrial organic waste, etc.

[0037] Consequently, MSW is understood as arbitrary household waste, that is, any material or object discarded from the home as a result of household activities. Assimilated waste MSW is considered to be waste similar to the aforementioned generated in shops, industries, and institutions, such as market waste, street cleaning waste, sewage cleaning waste, etc.

[0038] In particular, organic solid waste is characterized by being biologically degradable waste and may include, without limitation, the following: fruit and vegetable waste, meat and fish waste, eggshells, seashells and dried fruit and nuts or other food waste, waste from infusions and coffee grounds, used napkins, dirty paper towels, and dirty paper and cardboard from oil or food waste, small garden waste (plants, fallen leaves or bouquets), etc.

[0039] Furthermore, for the purposes of this patent, sludge from a wastewater treatment plant (WWTP) is understood as a mixture of water and solids obtained from a facility treating domestic or municipal wastewater, or industrial wastewater and / or stormwater runoff and a mixture thereof. Accordingly, the composition may vary depending on the composition of the initial wastewater, its origin, and / or the type of treatment to be performed on the wastewater. Generally, the percentage of water in the sludge will be greater than 95 weight percent.

[0040] There are many advantages derived from the process purpose of the present invention based on the use of organic solid waste (which has undergone a thermal hydrolysis process) as a substrate for an anaerobic digestion process for digesting sludge from WWTPs. In particular, the composition of the hydrolyzed biomass as a substrate for anaerobic digestion results in the following:

[0041] (1) Utilizing biowaste (organic fractions of MSW and assimilated waste that have been hydrolyzed as previously described, or organic waste of different origin) from existing facilities that are currently lacking energy (in particular, facilities of WWTP facilities);

[0042] (2) Integrating waste management that was traditionally managed separately;

[0043] (3) Although a new facility could be built to integrate the management of different types of waste into one facility, the existing facilities of WWTP are utilized to avoid new investment;

[0044] (4) The organic loading rate (OLR) is increased by increasing the concentration of volatile solids per unit volume of the digester as well as the concentration of total solids. In a specific embodiment of the present invention, an increase in OLR of 13 to 108% was achieved. In addition to increasing the organic loading rate of the digester, a constant organic loading rate is maintained regardless of seasonality;

[0045] (5) A remarkable increase in biogas production as a result of the synergistic effect achieved through the claimed waste mixture. In certain embodiments, an increase in biogas production of 15 to 124% was achieved in relation to the production currently achieved at WWTPs with a sludge / biomass ratio of 93 / 7 to 80 / 20. As a result of this significant increase in biogas production, not only is higher production of renewable energy achieved, but it also contributes to the energy self-sufficiency of the treatment facility and even generates surplus energy;

[0046] (6) Improving the quality of the biogas to increase the percentage of CH4. In a preferred embodiment of the present invention, the biogas obtained by the claimed process may contain 64.6 to 67.4% CH4 and 35.4 to 32.6% CO2. In combination with increased production, the fact that the obtained biogas has a superior quality compared to that currently obtained using sludge treatment at WWTPs is a significant advantage in many ways. In particular, this enables obtaining a higher percentage of electricity in the combined cycle, thereby improving the percentage of self-supply. Additionally, when self-supply is achieved, the surplus biogas can be used for other purposes, for example, to obtain biomethane through a purification (improvement) process that can be sold, and to obtain additional revenue for water treatment facilities;

[0047] (7) Increase the low heating value (LHV) of the fuel by 1 to 10% due to an increase in the concentration of methane in the biogas;

[0048] (8) Reduces the need for deodorization and concentration (improvement), resulting in savings in reagents and lower operating and maintenance costs; likewise, the fact that it has higher quality can extend the service life of the combined heat and power engine utilizing it;

[0049] (9) Increase process stability in the following cases:

[0050] ● When the organic loading rate of the digester increases and the hydraulic retention time (HRT) decreases by 5 to 60%;

[0051] ● When the use of chemical reagents for process control is minimized; and

[0052] ● When the decomposition rate of organic matter in the digester increases, resulting in a 6% to 27% increase in the reduction of total solids and volatile solids compared to the current process.

[0053] Likewise, the fact that this process is based on the use of hydrolyzed biomass allows for a series of additional advantages to be obtained compared to a process based on the use of non-hydrolyzed organic waste, as described below:

[0054] ● First, higher biodegradability of organic waste is achieved, leading to greater biogas production;

[0055] ● Furthermore, the separation of foreign substances during the process is more efficient. In particular, during the general process of washing foreign substances from MSW, 30 to 40 weight percent of organic matter is lost, which hinders obtaining a high-quality substrate such as that achieved as a result of the claimed process. Thus, as a result of the hydrolysis process of the present invention, the loss of organic matter in the washing process is less than 5 weight percent, which indicates that the utilization of organic matter present in the starting waste is higher;

[0056] ● Consequently, as a result of waste sanitation, the additional addition of pathogens that could affect digester operation is prevented;

[0057] ● Finally, as a result of thermal and biological stabilization, the emission of odors associated with the decomposition of organic matter is minimized. Brief explanation of the drawing

[0058] To supplement this description, the following drawings are attached as part of the embedded content: Fig. 1 The graph shows a comparison of the theoretical production of biogas with the production of biogas obtained by the process purpose of the present invention. Specific details for implementing the invention

[0059] Specific embodiments of the present invention are described below for the purpose of demonstrating the advantages of the claimed process described above. In particular, the specific embodiments were carried out based on organic solid waste originating from selective collections of the MSW in the City of Madrid, the composition of which is shown in the following table:

[0060] Table 1. Composition of OSW used in the process

[0061]

[0062] Fruit and vegetable waste, meat and fish waste, eggshells, seashells, dried fruit and nuts or other food waste, infusions, coffee grounds, etc.

[0063] The above waste underwent the following steps:

[0064] ● A previous step of manufacturing waste by separating bulky foreign matter performed in a trommel having a mesh size of 80 mm;

[0065] ● A step of producing hydrolyzed biomass from organic solid waste obtained in a preliminary step. Consequently, the step included the following:

[0066] - First sub-stage of thermal hydrolysis treatment for a time of 20 minutes at a pressure of 4 bar and a temperature of 150℃,

[0067] - A second sub-step for removing light foreign matter using a depacker, as well as removing heavy foreign matter by a sand trap.

[0068] At the end of this stage, a “substrate” or “clean hydrolyzed biomass” having an organic content of 98 wt% was obtained. The substrate was used in the co-digestion test described below.

[0069] In particular, to perform the above test, two blanks characterized by containing only anaerobic sludge (ANS) and four samples having various aerobic sludge (AES) and clean hydrolyzed biomass ratios were prepared. Specifically, the prepared samples were as follows:

[0070] Table 2. Description of Sample, Weight %

[0071]

[0072] Table 3. Mass of each sample (g)

[0073]

[0074] The key characteristics of each sample are summarized in the following table, where the methods for measuring different parameters were as follows:

[0075] ● Total Solids (TS): APHA 2540 B

[0076] ● Volatile Solids (VS): APHA 2540 E

[0077] ● COD: APHA 5220 D

[0078] ● COD sol.: APHA 5220 D

[0079] ● VFA: APHA 2310 B

[0080] Table 4. Main features of the sample of the present invention

[0081]

[0082] Water was added to each sample until a total weight of 500 g was reached. Each sample was prepared through three repeated experiments.

[0083] The prepared samples underwent an anaerobic digestion process at a temperature of 35°C. No nutrients or buffers were added.

[0084] Next, the obtained results were analyzed, and these results are shown in the following table:

[0085] Table 5. Results of TS and VS removal

[0086]

[0087] Therefore, it was proven that the percentage of VS / TS increases significantly when hydrolyzed biomass derived from the organic fraction of MSW is used as a substrate for the digester. In particular, the percentage of VS / TS was 69% in the blank (AES), 75% in sample 1 (ANS), and 80 to 85% in samples 2 to 4, which confirms the increase in the biodegradability of the mixture when hydrolyzed biomass is fed to the digester.

[0088] Table 6. Results of BOD and VFA removal

[0089]

[0090] Previous results show that even with the same amount of VS, COD increases in the presence of hydrolyzed biomass, as occurs in VFA, confirming the higher biodegradability of the sample.

[0091] Table 7. Specific CH by removed BOD 4 Production results

[0092]

[0093] * Indicates the increase in CH4 production of each sample relative to Sample 1

[0094] The results demonstrate the synergy achieved as a result of mixing hydrolyzed biomass obtained by the process of hydrolysis of organic fractions of MSW with sludge from WWTP. In particular, the process objective of the present invention is to achieve non-linear production during biogas generation, and to achieve a 340.8% increase in specific methane production per kilogram of removed COD and a 213.8% increase per kilogram of supplied COD using 20% ​​hydrolyzed biomass supplied to the digester. Furthermore, it has been proven that the use of hydrolyzed biomass in the mixture supplied to the digester where the anaerobic digestion process takes place increases the degree of hydrolysis, thereby increasing biodegradability.

[0095] In addition to improving biogas production, an analysis of its methane (CH4) content was performed as shown in the following table:

[0096] Table 8. Percentage of methane (by volume)

[0097]

[0098] Previous results demonstrate an improvement in the quality of the obtained biogas by increasing the percentage of hydrolyzed biomass used in the digester.

[0099] Finally, to demonstrate the synergy obtained as a result of the process objective of the present invention, a comparison was performed between the production of biogas obtained by the claimed process and what would theoretically be obtained by the sum of biogas produced from sludge from a treatment plant and biogas produced by hydrolyzed biomass. The results obtained are shown in FIG. 1. Specifically, as shown in the figure, the actual production of biogas (continuous line) is 50% higher than the maximum theoretical production (discontinuous line) equivalent to the theoretical sum of the production of biogas obtained independently from both substrates (i.e., without mixing them). This is due to the improvement resulting from the addition of hydrolyzed biomass to the sludge from the treatment plant. In particular, the following aspects are improved:

[0100] ● Organic carbon composition that increases the C / N ratio (generally, sludge from treatment plants is nitrogen-rich and carbon-deficient);

[0101] ● Composition of micronutrients such as Zn, Co, Fe, K, or P.

[0102] The fact that actual biogas production in mixed digestion is 50% higher than the theoretical maximum demonstrates the synergistic effect that exists when sludge is mixed with hydrolyzed biomass.

Claims

Claim 1 A production process for producing biogas through anaerobic co-digestion, characterized by comprising: (a) a first step of producing hydrolyzed biomass from organic solid waste selected from the group consisting of organic fractions of municipal solid waste and assimilated waste, organic waste from optional collections of municipal solid waste, and any combination thereof, wherein the first step comprises, in turn, the first step comprising: i. a first sub-step comprising a thermal hydrolysis treatment of organic solid waste, wherein the thermal hydrolysis treatment is performed at a pressure comprising 1.5 to 4.5 bar and a temperature of 120 to 160°C to produce raw biomass; ii. Subsequently, a second sub-step performed for the post-treatment of raw biomass to separate foreign matter and produce clean hydrolyzed biomass, characterized in that it comprises at least 90% by weight of organic matter, at least 0.6% by weight of volatile solids to total solids, and at least 5% by weight of total solids; (b) a second step of mixing 5% by weight to 65% by weight of clean hydrolyzed biomass obtained in the prior step with 35% by weight to 95% by weight of sludge from a wastewater treatment plant (WWTP) to produce a mixture having a solids concentration of less than 30% by weight, wherein the mixture subsequently undergoes a conditioning process until a solids concentration of less than 20% by weight is achieved; (c) a third step of wet anaerobic digestion of the mixture obtained in the prior step in at least one digester to produce biogas and digestate, wherein the anaerobic digestion A third step, performed under mesophilic conditions of 25°C to 40°C or thermophilic conditions of 50°C to 60°C and for a hydraulic residence time (HRT) of 12 to 30 days. Claim 2 A process according to claim 1, wherein the process comprises an additional step of utilizing the biogas for combined heat and power generation, use in a boiler for heat generation, or for the production of biomethane by a purification or improvement process. Claim 3 A process according to claim 1 or 2, wherein the process comprises an additional step of utilizing digested material for agricultural purposes as a biofertilizer. Claim 4 A process according to claim 1 or 2, wherein the organic solid waste is an organic fraction of municipal solid waste, and the process comprises a prior step of separating bulky foreign matter understood as material having a size greater than 80 mm, and / or separating ferrous metal by at least one magnetic separator. Claim 5 A process according to paragraph 3, wherein the organic solid waste is an organic fraction of municipal solid waste, and the process comprises a prior step of separating bulky foreign matter understood as material having a size greater than 80 mm, and / or separating ferrous metal by at least one magnetic separator. Claim 6 delete

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