ES2385167A1 - Method for anaerobic digestion of solid urban waste in temperature phases - Google Patents

Abstract

The invention relates to a method for anaerobic digestion of municipal solid waste in temperature phases. The invention consists of a method for anaerobic degradation in temperature phases (thermophilic-mesophilic sequential) of OF-MSW (organic fraction of municipal solid waste), by means of which it is possible to increase the stability of the process and the per-day capacity for treating organic waste, achieving greater efficiency in the production of biogas related to the amount of organic matter supplied to the system or consumed by same. The process consists in subjecting the waste to thermophilic pre-treatment for a certain length of time in order to efficiently achieve high hydrolysis and solubilization of organic waste. The second stage of the process consists in subjecting the waste pre-hydrolyzed during the thermophilic phase to mesophilic degradation, such that the solubilized matter is consumed during this second phase.

Description

ANAEROBIA DIGESTION PROCEDURE IN TEMPERATURE PHASES OF URBAN SOLID WASTE.

SECTOR OF THE TECHNIQUE.

The present invention is part of the technical sector of urban solid waste treatment processes, specifically in relation to biological treatments of organic solid waste through anaerobic digestion technology

or biomethanization.

STATE OF THE TECHNIQUE.

The increase in the production of Urban Solid Waste (MSW) in recent years requires the adoption of effective management measures in order to minimize its impact on the environment. In this regard, the National Integrated Waste Plan for the period 2008-2015 (pNIR 2008-2015), approved by the Council of Ministers on February 26, 2009, aims to promote an appropriate waste management policy, reducing their generation and promoting their correct treatment: prevention, reuse, recycling, recovery and disposal. It also pursues the involvement of all public administrations, consumers and users, in order to assume their respective quotas of responsibility, promoting the creation of infrastructures that guarantee this correct treatment and management of waste in the places closest to its generation. In addition, it incorporates the Strategy for the Reduction of

25 Discharge of Biodegradable Waste in order to reduce its impact on the environment. In this sense, the opportunity to treat biodegradable municipal waste is considered, due to the high percentage of organic matter presented by biological techniques, among which is anaerobic digestion or biomethanization.

Biomethanization is a technology for the treatment of organic waste that enables energy recovery because, as a result of the global process, a biogas with high methane content is obtained. Additionally, the anaerobic process generates a biologically stabilized residue, with good characteristics such as

5 soil improver and that can be used with agricultural fields.

The anaerobic digestion process of organic matter allows us to consider up to four successive stages (Gujer et al. (1983) Conversion processes in anaerobic digestion. Water Science and Technology, 15 (8-9), 127-67; Breure, (1986 ) Hydrolirys and acidogenesis firmentation ofprotein and carbof? Ydrates in anaerobic waste water treatments. Off setduikkerij.

10 Kanters B. v., Alblasserdam.):

• Hydrolysis: it is the first necessary step for anaerobic degradation of complex organic substrates, since organic matter cannot be used directly by microorganisms unless it is hydrolyzed into soluble compounds, which can cross the cell membrane. Hydrolysis

15 of these organic particles is carried out by extracellular enzymes excreted by fermentative bacteria.

• Acidogenesis: soluble organic molecules are fermented by watts

microorganisms forming compounds that can be used

directly by methanogenic bacteria (acetic, formic, hydrogen) and

20 other very small organic compounds (lactic, ethanol, propionic, butyric).

• Acetogenesis: the reduced organic compounds, formed in the previous stage, have to be oxidized by acetogenic bacteria to substrates that can be used by methanogenic archaea.

25 • Methanogenesis: methanogenic archaea are responsible for the formation of methane from monocarbonated substrates or with two carbon atoms linked by a covalent bond, giving name to the general process of biomethanization. Methanogenic organisms are classified within the Archaea domain, and morphologically, they can be short and long bacilli. All 30 methanogenic archaeas involved in the process have several

special coenzymes, being coenzyme M, which participates in the final step of methane formation.

Temperature is a fundamental variable in the biomethanization process. Classically, single-stage processes in the mesophilic range have been used (30-3 CT) AND

5 thermophilic processes (50-57 ° C), each presenting advantages and disadvantages. Recently, in order to combine the advantages of both operating ranges, Anaerobic Digestion in Temperature Phases (DAFT) has begun to be used, especially for the treatment of WWTP sludge.

The invention detailed in this document focuses on the application of the process in temperature phases applied to the biomethane technology of the industrial FORSU, without involving the separation of microbiological stages.

Currently, studies have been carried out on the process in phases of temperature with different organic compounds, but there are no previous studies on this technology with this residue due to the difficulty in handling it. The main

15 operating variables in this process are the degradation time in thermophilic and the organic loading rate fed to the system.

So far the biomethanization systems of organic waste with high solids content have developed in a single temperature phase, usually in the mesophilic range (25-40 ° C) and occasionally in the thermophilic range

20 (45 ° C-60 ° C). Some authors (Cecchi et al. Cecchi F., Pavan P., Mata-Álvarez J, Bassetti A., Cozzolino C. (1991) Anaerobic digestion rif municipal solid waste: thermophilic vs. mesophilic performance at high solids. Waste Management and Research 9, 305-315; ShuGuang et al; Shu-Guang Lu, Tsuyoshi Imai, Masao Ukita And Masahiko Sekine. (2007). Start-up performances rif dry anaerobic mesophilic and thermophilic digestions rif organic solid

25 wastes Joumal of Environmental Sciences Volume 19, Issue 4, 416-420 and Hedge and Pullammanappallil, (2007) Comparison rifthermophilic and mesophilic one-stage, batch, high-solids anaerobic digestion. Environmental Technology, 28 (4), 361-369), among others have dedicated studies to compare the advantages of one range against another.

The systems in temperature phases developed so far treated residues that have low solids content (3-10%), called wet degradation, such as whey or active sludge from WWTP. The works carried out with these wastes show that the thermophilic phase not only accelerates the

5 limiting stage of anaerobic digestion, but also achieves sterilization of the residue of pathogenic organisms (Aitken MD, Sobsey MD, Van Abel NA, Blauth KE, Singleton DR, Crunk PL, Nichols C, Walters GW, Schneider M. (2007 Inactivation if Eschen · chia coli 0157: H7 during thermophilic anaerobic digestion if manure from dairy cattle. Water Research 41 (8): 1659; Viau and Peccia, (2009) SUt7Jf! And if wastewater indicators and

10 human pathogen genomes in biosolids produced lry class A and class B stabilization treatments. Applied and Environmental Microbiology 75 (1), 164-174; Riau et al., (2009); Riau et al., (2010), which allows the subsequent use of the process effluent as an agronomic application.

Also the system in phases of temperature can also improve the

15 sludge dehydration (Bivins and Novak, (2001). Changes in dewateringproperties between the thermophilic and mesophilic stages in temperature-phased anaerobic digestion {Ystems. Water Environment Research 73 (4), 444-449; Zhou and Mavinic, (2003) Pollution reduction at wastewater treatmentfacilities through thermophilic sludge digestion. Water Science & Technology Vol 48 No 3 pp 57-63.).

20 There are also publications in which the temperature phases involve a separation of microbiological stages, so that they are operated in thermophilic in the hydrolytic-acidogenic reactor and in mesophilic in the methanogenic reactor (Schmit KH, Ellis TG (2001). "Comparison of temperature-phased and two-phase anaerobic co-digestion of primary sludge and municipal solid waste ". Water Environment

25 Research 73 (3), 314-321). There are also processes in which anaerobic digestion of active sludges from WWTP in acidogenic thermophilic reactors and the passage of the effluent to methanogenic mesophilic reactors is proposed (Demirer, G., Othman, M. (2008)

Two-Phase Thermophilic Acidification and Mesophilic Methanogenesis Anaerobic Digestion if Waste-Activated Sludge. Environmental Engineering Science. Volume 25, Number 9. 30 1291-1300). However, the fact of separating stages can pose problems of inhibition in the global system, especially working at industrial scales.

This separation of stages is also included in other procedures, such as the one that appears in ES2199022Al, where an anaerobic fermentation procedure is described, with separation of said microbiological stages, operating in the mesophilic range, and in co-digestion of urban solid waste and urban sludge.

5 In summary, despite the fact that in the state of the art there are descriptions of reactors in temperature phases for the treatment of organic waste, few of them are about residues with high solids content (20-30%) and difficult to treat such as FORSU, the residue that has been used in the present invention. The difficulty of working with this type of waste, as well as the difficulty in

10 the characterization and monitoring of the anaerobic process have prevented the development of these types of studies. On the other hand, the non-separation of microbiological stages in the different temperature phases increases the stability of the system, making a difference to consider compared to current technologies.

EXPLANATION OF THE INVENTION

The invention consists of a process for anaerobic degradation in temperature phases (thermophilic-mesophilic sequential) of the FORSU through which

20 it is possible to increase the stability of the process and the treatment capacity of the organic waste per day, presenting a greater efficiency in the production of biogas referred to the amount of organic matter fed to the system or consumed by the phylphysm. The process consists in subjecting the residue to a thermophilic pretreatment

25 for a certain time to achieve high hydrolysis and solubilization of the organic residue efficiently. The second stage of the process consists in subjecting the prehydrolyzed residue during the thermophilic phase to mesophilic degradation, so that the solubilized matter is consumed during this second phase.

It should be noted that the solids content of the waste to be treated in the process is between 20-30%, so the procedure takes place under conditions that are called "dry" degradation. The procedure detailed is done without co-digestion with another residue, only the FORSU is degraded as an organic residue.

Also, in the proposed procedure there is no separation of

5 microbiological stages, but in both phases, both the thermophilic and mesophilic all the stages indicated above take place. This means eliminating or reducing inhibition problems in the global system, since the second reactor, where the mesophilic phase takes place, assumes the possible imbalances caused by the thermophilic phase.

10 When the organic solid waste, such as FORSU, is subjected to the process indicated above, the following advantages are obtained from the point of view of anaerobic reactor design and process operation:

a) There is a significant solubilization of organic matter in the reaction medium during the first phase, the stage in thermophilic range.

15 b) Through the solubilization of organic matter that is achieved through the application of pretreatment, the overall speed of the process is achieved compared to single-stage systems.

c) This increase in the overall process speed has several advantages associated with it:

1. The currently operating industrial biomethane plants could

20 work with higher organic loading speeds fed to the system, which entails economic benefits by increasing the treatment and / or management capacity of the facilities.

11. There is a significant improvement in the net production of methane and the speed of biomethanization. When the FORSU is submitted to the process

25 DAFTs have achieved, through laboratory tests, higher methane productions and higher productivities referred to organic matter consumed and fed to the system than single-stage systems with a similar duration of time.

111. The increase in methane generation entails benefits from the economic point of view in the sense that this energy vector, after being subjected to cogeneration to produce electricity, could be used to meet the demand of the facility itself and could even

5 energy surpluses be sold to large electricity companies.

BRIEF DESCRIPTION OF THE FIGURE.

Figure 1.-Flowchart and scheme of the units involved in the DAFT 10 (thermophilic-mesophilic) process of the industrial FORSU. The following elements are displayed:

• Thermophilic Reactor: Reaction site where the thermophilic treatment of the residue is carried out. This reactor is fed with the industrial FORSU. It is a semi-continuous complete mix type reactor, equipped with stirring by blades and with jacket heating

15 external.

• Mesophilic reactor: Reaction site to carry out the mesophilic treatment of the effluent from the thermophilic reactor. It is a semi-continuous complete mix type reactor, equipped with stirring by blades and with external jacket heating.

20 • Continuous green line: Represents the collection of biogas generated during the DAFT process in both reactors individually, thermophilic and mesophilic.

• Red dashed line: Indicates the treatment of FORSU in the DAFT process.

25 Figure 2. Comparison between the percentages of COD elimination in single-stage systems and in temperature phases.

Figure 3. Comparison between the percentages of COD removal in single-stage systems and in temperature phases. 30

DESCRIPTION OF AN EXAMPLE OF EMBODIMENT OF THE INVENTION.

To study the dry anaerobic degradation of the FORSU in temperature phases, two agitated tank reactors have been used, at laboratory scale 5 operating in a semi-continuous feeding regime. Each reactor has a system for collecting biogas, for stirring the medium and for its thermostatization. Thermophilic operating conditions (55-57 ° C) have been imposed in the first of the reactors and in the other mesophilic (35-37 ° C). In order to consider an integral process, a representative scheme of the same has been developed 10 (Figure 1) in which it has been considered that the units of the DAFT system must have different volumes depending on the Solid Retention Times (TRS) used . As can be seen in this scheme, FORSU is fed to the thermophilic reactor and all the effluent from this unit constitutes the mesophilic reactor feed. Biogas production is collected individually from each of

15 reactors, thermophilic and mesophilic.

Each of the elements that make up the process represented in Figure 1, as well as the flow diagram of the waste and the products generated are detailed below.

• Thermophilic Reactor: Reaction site where the

20 thermophilic treatment of the residue. This reactor is fed with the industrial FORSU. It is a semi-continuous complete mix type reactor, equipped with stirring by blades and with external jacket heating.

• Mesophilic reactor: Reaction site to carry out the treatment

Mesophilic 25 effluent from the thermophilic reactor. It is a semi-continuous complete mix type reactor, equipped with stirring by blades and with external jacket heating.

• Continuous green line: Represents the collection of generated biogas

during the DAFT process in both reactors individually, thermophilic and mesophilic.

• Red dashed line: Indicates the treatment of FORSU in the DAFT process.

An operative example is shown below, detailing how to carry out the DAFT process with industrial FORSU from an industrial tombstone (15 mm of passing light).

OPERATING CONDITIONS

Operating regime

Semi-continuous Semi-continuous

% Total Solids

20-25% 20-25%

Operating temperature

Ermophilic-Mesophilic T Thermophilic-Month or philic

TRS in thermophilic (days)

4 3

TRS in mesophilic (days)

10 6

Work pressure

Atmospheric pressure Atmospheric pressure

Stirring (rpm)

25 25

10 Some of the main results obtained in laboratory tests are shown below. Fundamentally it is intended to compare the process in phases of temperature presented in this patent against the processes in phases, referring to:

a) Percentage of elimination of dissolved organic carbon (COD) (Figure 2) 15 b) Percentage of elimination of chemical demand for soluble oxygen (COD) (Figure 3) c) Biogas productivity (Table 1)

The nomenclatures used in each of the trials are:

Nomenclature

Test

T15

Thermophilic single-stage TRS 15 days

Tl0

Thermophilic single-stage TRS 10 days

T5

Ermophilic single-stage TRS T 5 days

M20

Mesophilic single-stage TRS 20 days

M15

Mesophilic single-stage TRS 15 days

FT 4:10

Temperature phases: Thermophilic TRS 4 days and Mesophilic TRS 10 days

FT 3: 6

Temperature phases: Thermophilic TRS 3 days and Mesophilic TRS 6 days

Table 1.-The following table shows a comparison of gas productivity

in the different conditions, in phases of temperature and their corresponding times

In a single phase.

Methane Productivity

Methane (L / Lreacto. / Day)

LCH4 / g CODalim LCH4 / g CODcons L CH4 / g DQOSalim LCH4 / g CODscons

T15d

1.40 30.70 59.62 - -

T10d

1.95 35.73 65.94 8.79 25.61

T5d

4.20 38.92 77.02 13.18 25.77

M20d

1.03 35.56 51.62 9.24 15.01

M15d

1.07 29.75 51.73 10.07 15.15

FT 4:10

2.17 56.29 81.73 19.06 26.60

FT 3: 6

2.45 41.00 76.80 13.01 19.85

Claims (1)

1) Procedure for the degradation of the Organic Fraction of Urban Solid Waste in stirred tank type reactors characterized in that 5 semi-continuous regime takes place, in two phases of differentiated temperatures: -A first phase of pretreatment in thermophilic regime. -A second phase of mesophilic treatment of the residue prehydrolyzed and solubilized from the previous pretreatment. 10 2) Procedure for the degradation of the Organic Fraction of Urban Solid Waste according to claim 1, characterized in that all microbiological stages occur both in thermophilic and mesophilic regime, without separation of microbiological phases. 3 3) Procedure for the degradation of the Organic Fraction of Urban Solid Waste according to claims 1 and 2, characterized in that the process is carried out without co-digestion with any other type of residue. 4) Procedure for the degradation of the Organic Fraction of Urban Solid Waste according to claim 3, characterized in that it is carried out without co-digestion with sewage sludge. 5) Procedure for the degradation of the Organic Fraction of Urban Solid Waste according to claims 1 to 3, characterized in that the solid content of the waste is between 20-30%.