WO2005023730A1

WO2005023730A1 - Method for the accelerated composting of mixed organic waste

Info

Publication number: WO2005023730A1
Application number: PCT/FR2004/002176
Authority: WO
Inventors: Anne Tremier; Amaury De Guardia; Jean-Luc Martel
Original assignee: Suez Environnement; Centre National Du Machinisme Agricole, Du Genie Rural Des Eaux Et Des Forets (Cemagref)
Priority date: 2003-09-03
Filing date: 2004-08-20
Publication date: 2005-03-17
Also published as: GB2422825A; GB2422825B8; FR2859204B1; GB2422825A8; GB0604135D0; FR2859204A1; GB2422825B

Abstract

The invention relates to a method for the accelerated composting of mixed organic waste. The invention is characterised in that the conditions relating to the formulation, establishment and control of the phase involving the initial fermentation of the waste to be composted are determined using a pre-step consisting in characterising and optimising the biodegradability and the degradation kinetic of the initial mixture comprising different types of waste to be composted. More specifically, the aforementioned pre-step comprises (i) the rapid measurement of the uptake of oxygen (respirometric measurement) by a solid organic biological medium and (ii) the processing of the result of said respirometric measurement by means of modelling.

Description

ACCELERATED COMPOSTING PROCESS OF ORGANIC WASTE MIXTURE

The present invention relates to an accelerated composting process for mixed organic waste comprising the implementation of a respirometric method in order to determine the main conditions for accelerated fermentation of the mixture to be composted.

Respirometric measurement is a commonly used method of quantifying biological activity. Various techniques have been proposed in the past in order to:

1 °) constitute tools for quantifying the ultimate biodegradability of an organic substrate (for example for the study of biodegradable plastics): An inventory of standard methods linked to this first objective was given by Pagga (1997) "Testing biodegradability with standardized methods »Chemosphere 35 (12): 2953-2972. The author recommends, in particular for the determination of the ultimate biodegradability of solid materials, the following measures: • Measurement of the evolution of carbon dioxide production in a aqueous medium inoculated with microorganisms (Sturm test): this technique is described through five ASTM standards, one JIS standard, one EC standard and three OECD standards (Feuilloley, Labiée et al. 2000) "Development of an automated system for measuring the biodegradability of materials ”Engineering 23: 61-70. • Measurement of the quantity of carbon dioxide produced during a period of 45 days during the incorporation of a polymer in a ripe compost, in a ventilated closed environment and maintained at 60 ° C. ((Pagga, Beimborn et al. (1995); ISO 14855) "Determination of the aerobic biodegradability of polymeric material in a laboratory conrrolled composting test". Chemosphere 31 (11-12); 4475-4487.

2) determine the level of stabilization of a biodegradable organic substrate (for example level of stabilization of a compost). This determination of the level of biological stabilization of an organic substrate has also been the subject of

SHEET BE REPLACEMENT (fiÈ € LE 26} numerous applications of respirometric methods by measuring oxygen consumption:

lannotti and ai. (Ianotti-Frost, Toth et al. 1992) "Compost stability" Biocycle (November 62-66) (Ianotti, Grebus et al. 1994) "Oxygen respirometry to asses stability and maturity of composted municipal solid waste" Journal of Environment Quality 23 ; 1177-1183, propose a measurement of the stability of composts by a respirometric test based on the measurement of gaseous oxygen in a closed enclosure using a polarographic probe. The test is carried out on compost samples (equivalent to approximately 60 g of dry matter) with a minimum humidity of 50%, placed in hermetically sealed 500 ml Erlenmeyer flasks, at a temperature of 37 ° C. After 16 hours of continuous aeration, the disappearance of oxygen in the enclosure is measured for one hour. The stability of the compost thus tested is estimated as a function of the residual oxygen rate after one hour of measurement: if it is close to the oxygen saturation rate in the air, the product is considered to be biologically stable.

Stentiford et al. (Lasaridi and Stentiford 1998) "A simple respirometric technique for assessing compost stability" Water Research 32 (12); 3717-3723; (Stentiford 2002) "The specifies oxygen upta e rate (SOUR)". The biological treatment of biodegradable waste - Technical aspects, Brussels, -have developed the same type of method as lannotti et al. but by applying it to a finely ground compost sample suspended in an aqueous medium maintained at 30 ° C. The measurement of oxygen consumption is carried out by a dissolved oxygen sensor. The reaction chamber is reoxygenated in a sequenced manner. The measurement is carried out over a period of 20 to 60 hours depending on the substrates. The two stability estimation parameters are the maximum consumption kinetics determined during the measurement (SOUR) and the cumulative oxygen consumed during the first 20 hours of measurement (OD ₂₀ ). The two parameters vary identically depending on the age of the substrate. This development is also comparable to that obtained by the method developed by lannotti et al. However, the values of the maximum kinetics of oxygen consumption are lower in the case of a solid substrate than in the case of a solid substrate ground in aqueous suspension. In this second case, the limitations due to oxygen transfer are minimized and Stentiford et al. prefer this method in order to study the maximum biodegradability potential of the substrate.

Other stability indices based on respirometric methods have also been developed:

• AT4 (Binner and Zach 1999) "Laboratory tests describing the biological reactivity of pretreated residual wastes". Organic Recovery and Biological Treatement - ORBIT 99, Weimar Rhombos - Verlag, uses a gasometric method coupled with an electrolytic system. Applied to approximately 30 g of compost sample whose humidity is adjusted between 40 and 50%, this method proposes to assess the stability of the material as a function of the cumulative amount of oxygen consumed after 96 hours of measurement. The reaction medium is thermostatically controlled at 20 ° C.

• The dynamic respiration index (DRI) (Adani, Scatigna et al. 2000) "Biostabilization of mechanically separated municipal solid waste fraction" Waste Management and Research 18: 471-477. (Adani, Ubbiali et al. 2002) "Static and dynamic respirometric indexes - Italian Research and studies". The biological treatment of biodegradable waste - Technical aspects, Brussels, is based on a measurement of the oxygen consumption between the entry and exit of air in a continuously aerated system. The DRI value considered to estimate the stability of the organic substrate is the average of 12 values, measured every two hours when the maximum activity is reached.

3) modeling the degradation kinetics of a biodegradable organic substrate: equivalent methods have also been used to study the evolution of the biological degradation kinetics of organic solid substrates: • Lasaridi et al. (Lasaridi, Papadimitriou et al. 1996) "Development and demonstration of a thermogradient respirometer". Compost Science and Utilization 4 (3): 53-61 have developed a respirometric method to study the evolution of the kinetics of oxygen consumption during the composting and depending on the temperature. The method used is a constant volume gas technique. 75 to 100 g of compost sample, the humidity of which is adjusted to 60% of the maximum water retention capacity of the material, are placed in a 250 ml bottle. A series of bottles is placed on a heating bench, establishing a temperature gradient between the bottles. - A measurement of approximately 30 to 60 minutes makes it possible to estimate the kinetics of oxygen consumption of a type of sample for a panel of temperatures. This test makes it possible to study the evolution of biodegradation during composting and according to the imposed temperature. • A respirometric method has also been implemented by Aguilar et al. (Aguilar- Juarez 2000) "Analysis and modeling of aerobic biological reactions during the operating phase of a locker in a technical landfill center". Process Engineering. Toulouse, National Institute of Applied Sciences 233 in order to model the aerobic biological degradation kinetics of household waste during the operating phase of a Technical Landfill locker. In a hermetically sealed enclosure, a waste volume of approximately 1.8 L is continuously aerated. Oxygen consumption is continuously measured by analyzing the oxygen content in the gas entering and leaving the respirometric cell. The measurements are carried out on a waste comparable to that treated in CET. The test is extended until very low and constant consumption is observed. The kinetic curves obtained could be simulated by the authors using a model including the effects on biological kinetics of the oxygen content, the temperature and the composition of the waste.

This prior state of the methods for measuring the biodegradability of a solid substrate suggests that most of these methods do not respect the physical matrix of the substrate, thus dissociating the biodegradability of a material from the physical conditions of biological degradation of this material. Furthermore, the measurement result is often used directly without relating it to biological phenomena responsible for the biodegradation of organic matter. This has the consequence of making the thresholds of values defined for a particular type of transfer difficult to transfer. substrate. Finally, none of these methods proposes to qualify the initial characteristics of an organic solid substrate to be treated biologically: quantification and qualification of the biodegradable material, determination of the degradation kinetics and of the influence on these kinetics of environmental parameters such as the temperature. or humidity.

In order to solve the problem mentioned above, the present invention proposes to implement a respirometric method that respects the nature of the solid matrix of the substrate, combined with computer processing of the measurement result. This new technique has been developed to allow the operator of a composting platform:

• Estimate the capacity of a waste to be treated by this process according to the quantity of biodegradable material identified (MH (0) and MB (0)).

• To guide the choice of waste mixtures to optimize the quantity and quality of biodegradable material (MH (0) and MB (0)) without undertaking tests on large volumes of waste.

• Estimate the degradation kinetics (μm, Kh) of this waste and therefore optimize their treatment time on the composting platform.

• Optimizing the mixing conditions (humidity) and then process management (temperature control) according to the influence of these parameters on the biodegradation kinetics.

Consequently, the present invention relates to an accelerated composting process for mixed organic waste, characterized in that the conditions for formulation, establishment and control of the initial fermentation phase of the waste to be composted are determined by a pre-stage of characterization and optimization of the biodegradability and the kinetics of degradation of the initial mixture consisting of the different wastes to be composted, this pre-step consisting in a rapid measurement of the oxygen consumption (measurement respirometric) by a solid organic biological medium and in a modeling treatment, of the result of said respirometric measurement.

Referring to the single figure of the appended drawing, a description will first be given of an embodiment of the respirometric measurement implemented during the pre-step of the method which is the subject of the invention. In this example, the measurement conditions are as follows:

• The organic waste tested is prepared so as to obtain a solid porous structure of quality, presentation and fermentability comparable to that treated in industrial composting. We are seeking here to know a real biodegradation kinetics and not accelerated by grinding for example.

• The kinetics of oxygen consumption measured should only depend on intrinsic parameters of microbiological activity (amount of microorganisms, amount of biodegradable substrate, etc.). The kinetics should therefore not be limited by a low availability of oxygen for microorganisms. The oxygen distribution must therefore be sufficient and homogeneous throughout the respiratory chamber.

• Similarly, for a given measurement, the kinetics of oxygen consumption must not be liable to vary depending on the temperature or humidity of the mixture tested. The temperature and humidity must therefore be kept constant during the measurement.

• The respirometric measurement lasts 10 to 20 days depending on the waste tested.

In the single figure, a schematic example of a respirometric system applied to the study of solid organic waste treated in composting has been schematized.

In this figure we see that, in this exemplary embodiment,

The respiratory monitoring system consists of a hermetically sealed enclosure 1. According to the present invention, this respirometric enclosure has a capacity of the order of 6 to 10 liters (which can contain from 4 to 6 kg of samples), which represents a capacity which is quite unusual for a laboratory apparatus, but which is the only means of testing a sample of sufficiently large particle size comparable to that of the gross product, all of these characteristics significantly increasing the reliability of the measurement. This enclosure 1 contains the composting mixture 3, placed on a grid 2 placed above the bottom of the enclosure. The composting mixture is continuously supplied with oxygen via an ambient air inlet 4. After having flowed through the mixture, the air leaves the enclosure and its oxygen content is continuously analyzed at 5.

In order to be placed in non-binding conditions from the hydrodynamic point of view (no dead volume, no short-circuit or preferential paths of the gas flow), the volume of air is recirculated according to circuit 6 very quickly to produce a perfectly agitated type flow in enclosure 1. Consequently, the system can be used in open gas circuit with recirculation or in closed gas circuit only.

A thermostatic bath is used in order to keep the temperature of the compost mixture 3 constant during monitoring. This temperature is continuously monitored by a probe 8.

Finally, the incoming air is humidified in 9 and the water vapor contained in the outgoing gas flow is condensed in 7 in order to keep the humidity of the mixture as constant as possible.

Oxygen consumption is obtained by measuring the oxygen contents in the gas entering and leaving the respirometric cell using the analyzer 5.

As mentioned above, said pre-step of the method according to the invention then includes a processing of the result of the respirometric measurement by modeling.

The aerobic biodegradation kinetics simulation model of organic matter in composting is based on the ASM models developed in the framework of the study of biological wastewater treatment (Henze, Gujer et al. 2000) "Activated Sludge Models ASM1, ASM2, ASM2D and ASM3". London IWA Publishing. This model expresses the growth and decline of microorganisms, the consumption of organic substrate and the consumption of oxygen linked to this growth.

We consider here a global population of microorganisms X whose activity is localized in the aqueous phase of the solid substrate. The growth of these microorganisms is developed on the basis of Monod kinetics.

The organic matter of the mixture to be composted is modeled according to several fractions: • A microbiological population X • A biodegradable action directly assimilable MB • A biodegradable fraction after hydrolysis MH • A non-biodegradable or inert action MI Microorganisms use for their growth the directly assimilable substrate MB. This substrate is partly initially present in the mixture to be composted. On the other hand, MB is formed during the biological reaction via the hydrolysis of the slowly biodegradable organic action MH. The kinetics of hydrolysis is limited by the ratio between the amount of substrate to be hydrolyzed and the amount of biomass. Biodegradable organic matter (MH + MB) makes it possible to form microorganisms according to the yield coefficient Y. The quantity (1-Y) not converted into biomass is oxidized in the presence of oxygen to provide energy.

Finally, microorganisms die. The dead biomass is partly oxidized according to a fraction (1-f). This oxidation forms the phenomenon of endogenous respiration. The fraction f of non-oxidized dead biomass constitutes inert organic matter.

The kinetics of oxygen consumption (rO ₂ ) therefore depend on the kinetics of consumption of the organic substrate by microorganisms and the kinetics of decline of microorganisms (endogenous respiration).

This model has 10 parameters, the determination of which constitutes the characterization of the biodegradability (quantification, qualification and kinetics) of the solid waste tested: • Parameters describing the initial composition of the organic matter: X (0) _> MB (0) and MH ( 0) the initial contents of microorganisms, directly biodegradable matter and slowly biodegradable matter. • Kinetic parameters: μm, KB, b, Kh, K _MH respectively kinetics of specific growth of microorganisms, semi-saturation constant on MB substrate, kinetics of decline of micro-organisms, kinetics of hydrolysis and constant known as semi-saturation in MH substrate. • Yield parameters: Y and f, respectively the coefficient of conversion efficiency of the substrate into micro-organisms and the coefficient of conversion of dead biomass into inert organic matter.

The initial values of these parameters are determined by graphical processing of the result of the respirometric measurement (μm, Kh, X (0), MB (0) and MH (0)) or fixed by default (Y, f, b, Kb, K _MH )

The simulation via the initial parameters obtained as described above only allows to obtain an approximate image of the experimental curve. In order to better calibrate the real values of the parameters, an optimization method by minimization of the sum of squares of errors has been implemented. This method makes it possible to vary the parameters initially given in a range of values defined by the user, this in order to determine the set of parameters necessary so that the sum of the squares of the errors between the experimental values and the simulated values is minimal. Optimization is continued until the average error (E) on the simulation is less than or equal to 30%:

where x _s . is the simulated value of rO ₂ for a given date, x _rj is the real value of rO ₂ for this same date.

As mentioned above, this pre-step of the process which is the subject of the invention makes it possible to characterize and optimize the formulation of the mixture of solid or pasty organic substrates to be treated and to predict the conditions for managing the fermentation phase of this treatment (minimum residence time, optimal and limiting temperature and humidity conditions for the process, minimum quantity of oxygen to be supplied).

Examples of the implementation of these implementations will be given below.

1) Characterization of solid organic substrates to be treated by a composting process and optimal formulation of the initial mixture:

• Analysis at constant temperature and humidity (ie the respirometric measurement and the digital processing of the results of this measurement by the biological model as described above) of the substrates treated or to be treated on the composting platform and constitution of a database of characteristics X (0), MH (0), MB (0) of each substrate. The precision on X (0) will be ± 50%. The accuracy on MH (0) and MB (0) will be ± 30%. • Prediction of Accelerated Compostability C _a of a substrate or a mixture of substrates by using the database. The fraction of biodegradable material during the so-called fermentation phase

treatment - or accelerated compostability -

mass fraction of each constituent i and COD _{to cover} the total chemical oxygen demand of the mixture considered. • Assessment of the industrial feasibility of treatment by composting a substrate or mixture of substrates. The establishment of the biological reaction processes of the composting treatment leading to sufficient and lasting heating of the reaction mass will be observed for a minimum level of accelerated compostability C _a of the initial mixture of between 5 and 10%.

2) Establishment of optimal humidity and temperature conditions for the composting of the selected formulation:

Analysis of the compost mixture retained on a respirometric bench according to the process described above with reference to the figure, at three different temperatures and three humidity levels (using 9 cells of the bench). Examination of the results of the analysis makes it possible to determine the optimal level of water content of the initial mixture and the temperature range for which the microbial activity is the strongest.

3 / Determination of the minimum oxygen supplies and the minimum residence time in the accelerated fermentation reactor:

Analysis of the mixture of substrates retained implemented at the optimal humidity level on the respirometric bench according to the method described above and with two repetitions but by varying the temperature and without controlling the humidity so as to reproduce the thermal conditions of the composting process used.

The minimum residence time can be estimated as being the time from which a kinetics of oxygen consumption of between 1 and 4 mmol of O 2 / h / kg DM initial is observed with a variation of less than 10% for 24 hours. The residence time to be applied to the industrial process will be equal to the minimum residence time + 20%. The minimum quantity of oxygen to be supplied to the substrate is calculated by integrating the kinetics of oxygen consumption from t = 0 to t = minimum residence time (

The quantity of oxygen to be supplied to the industrial process will be calculated as follows: Qo ₂ ⁼ minimum Q o ₂ + 20%.

A method comparable to that described above can be developed and applied to determine the compostability in ventilated maturation of a fine substrate after accelerated fermentation and screening and fix the conditions for application of this new phase of the composting process corresponding to maturation. ventilated (temperature, humidity, residence time and minimum oxygen supply)

In summary, the implementation by the operator of the pre-step of the method according to the invention, described above, corresponds, for the treatment of a new substrate to the following industrial protocol:

A / Calculation of the accelerated compostability of this substrate and choice of a formulation of mixture with other substrates of known accelerated compostabilities in order to reach the required level of accelerated compostability

B / Search for optimal humidity and temperature conditions for the fermentation of the selected mixture

C / Determination of the minimum residence time and the minimum quantities of oxygen to be supplied to the retained mixture

The method according to the invention can make it possible, using a protocol similar to that mentioned above, to configure the ventilated maturation phase of a composted product.

Claims

1) Process for accelerated composting of organic waste in mixture, characterized in that the conditions for formulation, establishment and control of the initial fermentation phase of the waste to be composted are determined by a pre-stage of characterization and optimization of the biodegradability and the kinetics of degradation of the initial mixture consisting of the different wastes to be composted, this pre-step consisting in a rapid measurement of the oxygen consumption (respirometric measurement) by a solid organic biological medium and in a treatment, by modeling , of the result of said respirometric measurement, which measurement comprises:

- preparation of organic waste so as to obtain a porous solid structure comparable to that treated in industrial composting;

- a homogeneous distribution of oxygen within the mass of waste placed in a respirometric enclosure;

- maintaining at constant values during the measurement on the one hand of the temperature and on the other hand of the humidity level of the mass of waste and

- a measurement of the oxygen consumption of the waste for a period of the order of 10 to 20 days.

2) Method according to claim 1, characterized in that the respirometric measurement is carried out using a respirometer having a capacity of the order of 6 to 10 liters.

3) Method according to claim 1 characterized in that the processing by modeling of said respirometric measurement is based on a model expressing the growth and decline of microorganisms contained in waste, consumption of organic substrate and oxygen consumption related to said growth.

4) Method according to claim 3 characterized in that the organic matter of the mixture to be composted is modeled according to several fractions: • A microbiological population • A biodegradable fraction directly assimilable • A biodegradable fraction after hydrolysis • A non-biodegradable or inert fraction

5) Method according to one of claims 3 or 4 characterized in that said model comprises ten parameters whose determination constitutes the characterization of the biodegradability (quantification, qualification and kinetics) of the solid waste tested: • Parameters describing the initial composition of organic matter: respectively the initial contents of microorganisms, directly biodegradable matter and slowly biodegradable matter; • Kinetic parameters: specific growth kinetics of microorganisms, semi-saturation constant in directly biodegradable material substrate, kinetics of microorganism decline, kinetics of hydrolysis and semi-saturation constant in slowly biodegradable material substrate, • Des yield parameters: coefficient of conversion efficiency of the substrate into micro-organisms and coefficient of conversion of the dead biomass into inert organic matter;

the initial values of these parameters can be determined by graphical processing of the result of the respirometric measurement. 6) Method according to claim 5 characterized in that the real values of said parameters are optimized by minimizing the sum of the squares of the errors making it possible to vary the initial values of the parameters in a value interval defined by the user. 7) Method according to any one of the preceding claims, characterized in that it is implemented to formalize within the accelerated composting process said characterization and optimization step of the formulation of a mixture of solid organic substrates or pasty to be treated as well as the prediction of the conditions for managing the fermentation phase of said treatment, this process comprising the following steps:

A) Characterization of the solid organic substrates to be treated by a composting process and optimal formulation of the initial mixture comprising:

- a respirometric measurement of the substrates and a digital processing of the results of this measurement by a biological model, carried out on the composting platform, at constant temperature and humidity and constitution of a database respectively of the initial contents of microorganisms, of the directly biodegradable material and slowly biodegradable material from each substrate;

a forecast of the accelerated compostability of a substrate or of a mixture of substrates by using said database, and

- an evaluation of the industrial feasibility of treatment by composting a substrate or a mixture of substrates;

B) Establishment of optimal humidity and temperature conditions for composting the substrate or the mixture of substrates having the selected formulation and;

C) Determination of minimum oxygen supplies and minimum residence time in the fermentation reactor.