GB2422825A

GB2422825A - Method for the accelerated composting of mixed organic waste

Info

Publication number: GB2422825A
Application number: GB0604135A
Authority: GB
Inventors: Anne Tremier; Jean-Luc Martel; Amaury De Guardia
Original assignee: Suez Environnement SAS; Centre National du Machinisme Agricole du Genie Rural des Eaux et des Forets CEMAGREF
Current assignee: Centre National du Machinisme Agricole du Genie Rural des Eaux et des Forets CEMAGREF; Vigie Groupe SAS
Priority date: 2003-09-03
Filing date: 2004-08-20
Publication date: 2006-08-09
Anticipated expiration: 2024-08-20
Also published as: WO2005023730A1; GB2422825A8; GB2422825B8; GB0604135D0; FR2859204B1; GB2422825B; FR2859204A1

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

METHOD FOR THE ACCELERATED COMPOSTING

OF MIXED ORGANIC WASTE

The present invention relates to a method for the accelerated composting of mixed organic waste comprising the utilisation of a respirometric method to determine the main conditions of accelerated fermentation of the waste that has to be composted.

Respirometric measurement is a generally used method of quantifying the biological activity. Various methods have been proposed in the past in order: 1) to form tools for quantifying the ultimate biodegradability of an organic substrate (for example for studying biodegradable plastics): A list of the standard methods connected with this first objective was given by Pagga (1997) "Testing biodegradability with standardized methods" Chemosphere 35(12):2953-2972 The author recommends the following measures in particular for the determination of the ultimate biodegradability of the solid materials: Measurement of the evolution of the production of carbon dioxide in an aqueous medium inoculated with microorganisms (Sturm test): this method is described in five ASTM standards, one JIS standard, one EC standard and three OECD standards (Feuilloley, Lablée et al. 2000) "Mise au point d'un système automatisé de mesure de Ia biodégradabilite ultime des matériaux" ["Summary of an automated system for measuring the ultimate biodegradability of materials"] Ingénieries 23: 61-70.

Measurement of the quantity of carbon dioxide produced over a period of 45 days at the time of the incorporation of a polymer in a mature compost, in an aerated closed medium kept at 60 C. ((Pagga, Beimborn et al. (1995) ; ISO 14855) "Determination of the aerobic biodegradability of polymeric material in a laboratory-controlled composting test". Chemosphere 31(11-12); 4475-4487.

2) to determine the stabilisation level of a biodegradable organic substrate (for example, stabilisation level of a compost). This determination of the level of biological stabilisation of an organic substrate was also the subject matter of numerous applications of respirometric measurements by measuring the uptake of oxygen: lannotti et a!. (lanotti-Frost, Toth et al. 1992) "Compost stability" Biocycle (November 62-66) (lanotti, Grebus et al. 1994) "Oxygene respirometry to assess stability and maturity of composted municipal solid waste" Journal of Environmental Quality 23; 1177-1183, propose a measurement of the stability of the compost 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 roughly g of dry material) having 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 tested in this manner is calculated as a function of the residual oxygen content after one hour of measurement: if this is close to the oxygen saturation rate in air, the product is considered to be biologically stable.

Stentiford et a!. (Lasaridi and Stentiford 1998) "A simple respirometric technique for assessing compost stability" Water Research 32 (12); 37173723; (Stentiford 2002) "The specific oxygen uptake rate (SOUR)". The biological treatment of biodegradable waste - Technical aspects, Brussels - developed the same type of method as lannotti et a!. but by applying it to a finely crushed sample of compost in suspension kept at 30 C.

The measurement of the uptake of oxygen is performed by a dissolved oxygen sensor.

The reaction enclosure is reoxygenated in sequence. The measurement is carried out over a period of 20 to 60 hours depending on the substrates. The two parameters for calculating the stability are the maximum uptake kinetic determined during the measurement (SOUR) and the total amount of oxygen taken up over the first 20 hours of measurement (0D20). The two parameters vary in an identical manner as a function of the age of the substrate. This evolution is also comparable with that obtained by the method developed by lannotti et a!.. However, the values of the maximum oxygen uptake kinetics are lower in the case of a solid substrate than in the case of a solid substrate crushed in aqueous suspension In this second case, the limitations caused by the transfer of oxygen are minimised and Stentiford et a!. prefer this method to study the maximum biodegradability potential of the substrate.

Other stability indices based on respirometric methods have again been developed: AT4 (Binner and Zach 1999) "Laboratory tests describing the biological reactivity of pretreated residual wastes". Organic Recovery and Biological Treatment - ORB IT 99, Weimar Rhombos - Verlag, uses a gasometric method combined with an electrolytic system. Applied to roughly 30 g of compost sample, the moisture content of which is adjusted between 40 and 50 %, this method proposes evaluating the stability of the material as a function of the cumulative quantity of oxygen taken up at the end of 96 hours of measurement. The reaction medium is set at 20 C.

The dynamic respiration index (DRI) (Adani, Scatigna et al. 200) "Biostabilization of mechainically 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 uptake of oxygen between the air inlet and the air outlet in a continuously aerated system. The DRI value considered to calculate the stability of the organic substrate is the average of 12 values, measured every two hours when maximum activity is achieved.

3) to model the degradation kinetic of a biodegradable organic substrate: equivalent methods have also been used to study the evolution of the biological degradation kinetics of the organic solid substrates: Lasaridi et a!. (Lasaridi, Papadimitr,ou et al. 1996) "Development and demonstration of a thermogradient respirometer". Compost Science and Utilization 4(3): 53-6 1 have developed a respirometric method for studying the evolution of oxygen uptake kinetic during composting as a function of the temperature. The method used is a gasometric method at constant volume. 75 to 100 g of a compost sample, in which the moisture content is adjusted to 60% of the maximum water retention capacity of the material, are placed in a 350 ml flask. A series of flasks is placed on a heating bench, establishing a temperature gradient between the flasks. A measurement of roughly 30 to 60 minutes allows the oxygen uptake kinetic to be calculated for a range of temperatures. This test allows the evolution of biodegradation to be studied during composting and according to the prescribed temperature.

A respirometric method was also used by Aguilar et a!. (Aguilar-Juarez 2000) "Analyse et modélisation des reactions biologiques aérobies au cours de Ia phase d'exploitation d'un casier d'un centre d'enfourissement technique" [Analysis and modelling of aerobic biological reactions during the operating phase of a strip of an authorised landfill site]. Genie des procédés. Toulouse, Institut National des Sciences appliqués 233 SO as to model the aerobic biological degradation kinetics of domestic waste during the operating phase of a strip in an authorised landfill site. In a hermetically sealed enclosure, a volume of waste of roughly 1.8 I is continuously aerated. The oxygen uptake is continuously measured by analysing the oxygen content in the inlet gas and outlet gas of the respirometric cell. The measurements are carried out on waste comparable with that treated at the authorised landfill site. The test is continued until a very weak, constant uptake is observed, It was possible for the authors to simulate the kinetic curves obtained thanks to a model including the effects of the oxygen content, the temperature and the composition of the waste on the biological kinetics.

This prior art of the methods for measuring the biodegradability of solid substrate shows that most of these methods do not respect the physical matrix of the substrate, thus separating biodegradability of a material from the physical conditions of biological degradation of this material. Moreover, the result of the measurement is often used directly without linking it with the biological phenomena that are responsible for the biodegradation of the organic material. The consequence of this is that it is not possible to transfer the thresholds of values defined for a particular type of substrate. Finally, none of these methods proposes qualifying the initial characteristics of an organic solid substrate to be biologically treated: quantification and qualification of the biodegradable material, determination of the degradation kinetics and of the influence of environmental parameters such as temperature or humidity on these kinetics.

To resolve the above-mentioned problem, the present invention proposes using a respirometric method which respects the nature of the solid matrix of the substrate, combined with data processing of the measurement result. This new technique has been developed so as to allow an operator a composting facility: To calculate the capacity of waste to be treated by this method as a function of identified biodegradable material (MH(O) and MB(O)).

To direct the choice of waste mixtures to optimise the quantity and the quality of biodegradable material (MH(O) and MB(O)) without undertaking tests on large volumes of waste.

To calculate the degradation kinetics (pm, Kh) of this waste and consequently to optimise its treatment time on the composting facility.

To optimise the mixing conditions (humidity) then the management conditions of the method (fine control of temperatures) as a function of the influence of these parameters on the biodegradation kinetics.

Consequently, the present invention relates to a method for the accelerated composting of mixed organic waste 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 preliminary step consisting in characterising and optimising the biodegradability and the degradation kinetic of the initial mixture comprising different types of waste to be composted, this preliminary step consisting of the rapid measurement of the uptake of oxygen (respirometric measurement) by a solid organic biological medium and the processing of the result of said respirometric measurement by means of modelling.

With reference to the single figure of the attached drawings, an exemplified embodiment of the respirometric measurement used during the preliminary stage of the method which is the subject matter of the invention is described in the first instance. In this example, the measurement conditions are as follows: The organic waste tested is prepared so as to obtain a porous, solid structure having a quality, presentation and fermentability that are comparable with the structure treated in industrial composting. Here an attempt is made to achieve a real biodegradation kinetic, which is not accelerated, thanks to crushing, for

example.

The measured oxygen uptake kinetic must only depend on intrinsic parameters of the microbiological activity (quantity of microorganisms, quantity of biodegradable substrate, etc.). Therefore the kinetic must not be limited by low availability of oxygen for the microorganisms. Consequently the oxygen distribution must be sufficient and homogenous in the entire respirometric enclosure.

Similarly, for a given measurement, the oxygen uptake kinetic must not be capable of varying as a function of the temperature or of the moisture content of the mixture tested. The temperature and the moisture content therefore have to be kept constant during measurement.

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

An example of a respirometric system applied to the study of solid organic waste treated during composting is shown on the single figure.

In this exemplified embodiment, on this figure it can be seen that: The respirometric monitoring system is formed by a hermetically sealed enclosure 1. In accordance with the present invention, this respirometric enclosure has a capacity in the order of 6 to 10 litres (which can contain 4 to 6 kg of samples) which represents a capacity that is completely unusual for laboratory equipment, but which is the only way of testing a granulometry sample which is sufficiently comparable with that of the raw product, with these characteristics as a whole increasing the reliability of the measurement. This enclosure 1 contains the mixture to be composted 3, placed on a grate 2 positioned above the bottom of the enclosure. The mixture to be composted is continuously supplied with oxygen via an ambient air inlet 4. After flowing into the mixture, the air leaves the enclosure again and its oxygen content is continuously analysed at 5.

So that it is placed in conditions that are not constraining from the hydrodynamic point of view (no dead volume, no short circuit nor preferential paths of the gas flow), the volume of air is recirculated in the circuit 6 in a very fast manner to produce perfectly agitated flow behaviour in the enclosure 1. Consequently, the system can be used in an open gas circuit with recirculation or just in a closed gas circuit.

A thermostatically controlled bath is used to keep the temperature of the mixture to be composted 3 constant during the monitoring process. This temperature is continuously controlled by a probe 8.

Finally, the inlet air is humidified at 9 and the water vapour contained in the gas stream is condensed at 7 so that it is possible to keep the moisture content of the mixture as constant as possible.

The uptake of oxygen is obtained by measuring the oxygen content in the inlet gas and the outlet gas of the respirometric cell thanks to the analyser 5.

As mentioned above, the said preliminary step of the method according to the invention then comprises the processing of the result of the respirometric measurement by means of modelling.

The simulation model of the aerobic biodegradation kinetic of the organic composting material is based on the ASM models developed within the framework of the study of the biological treatment of wastewater (Henze, Gujer et al. 200) "Activated Sludge Models ASM1, ASM2, ASM2D and ASM3". London IWA Publishing. This model expresses the growth and decay of the microorganisms, the uptake of the organic substrate and the uptake of oxygen connected with this growth.

t)t(t) (1) M1f(t)/ d(MFJ) - K /X(t) x(t) (2) dt -- MI KM,f MH(t)/ Ks+MB(t) X(t)+Kh %() KMH (3) rOz(t)= 1) K MB(tjX(t>1< i-j).X(t) (4) In this case a global population of microorganisms X is considered, the activity of which is located in the aqueous phase of the solid substrate. The growth in these microorganisms is developed on the basis of a Monod kinetic.

The organic material of the mixture to be composted is modelled according to several fractions: a microbiological population X a directly assailable biodegradable fraction MB a biodegradable fraction after hydrolysis MH a non-biodegradable or inert fraction Ml For their growth, the microorganisms use the immediately assimilable substrate MB. This substrate is in part initially present in the composting mixture. On the other hand, MB is formed during the biological reaction via hydrolysis of the slowlybiociegradable organic fraction MH. The hydrolysis kinetic is limited by the relationship between the quantity of substrate to be hydrolysed and the quantity of biomass The biodegradable organic material (MH + MB) allows the formation of microorganisms according to the coefficient of performance Y. The quantity (1-Y) not converted into biomass is oxidised in the presence of oxygen to provide energy.

Finally, the microorganisms die. The dead biomass is partially oxidised according to a fraction (1-f). This oxidation forms the endogenic respiration phenomenon. The fraction f of non-oxidised dead biomass makes up inert organic material.

The oxygen uptake kinetic (r02) therefore depends on the uptake kinetic of the organic substrate by the microorganisms and the decay kinetic of the microorganisms (endogenic respiration).

This model numbers 10 parameters, for which the determination forms the characterisation of the biodegradability (quantification, qualification and kinetic) of the solid waste that is tested: Parameters describing the initial composition of the organic material: X(0), MB(0) and MH(0) the initial content of microorganisms, immediately biodegradable material and slowly biodegradable material.

Kinetic parameters: pm, KB, b, Kh, KMH respectively specific growth kinetic of the organisms, MB substrate demi-saturation constant, decay kinetic of the microorganisms, hydrolysis kinetic and MH substrate demisaturation constant.

Performance parameters: Y and f, respectively the conversion efficiency coefficient of the microorganism substrate and conversion coefficient of the dead biomass comprising inert organic material.

- 10 - The initial values of these parameters are determined by graphic processing of the result of the respirometric measurement (pm, Kh, X(0), MB(0) and MH(0)) or fixed by default (Y, f, b, Kb, KMH).

The simulation via the initial parameters obtained as described above only enables an approximate image of the experimental curve to be obtained. In order better to adjust the real values of the parameters, an optimisation method by minimising the sum of the squares of the errors has been deployed. This method enables the initially given parameters to be varied in a range of values defined by the user, in order to determine the necessary set of parameters so that the sum of the squares of the errors between the experimental values and the simulated values is minimal. Optimisation is performed until the average error (E) on the simulation is less than or equal to 30 % x-x s r x : in which x is the simulated value of r02 for a given date, Xr is the real value of r02 for this same date.

As mentioned above, this preliminary step of the method which is the subject matter of the invention allows the formulation of the mixture of solid or pasty organic substrates to be treated to be characterised and optimised and the management conditions of the fermentation phase of this treatment to be predicted (minimum residence time, optimal and limiting temperature and humidity conditions for the method, minimum amount of oxygen to be supplied).

Exemplified embodiments of these uses will be given below: 1) Characterisation of solid organic substrates to be treated by a composting method and optimal formulation of the initial mixture: - 11 - Analysis at constant temperature and humidity (i.e. the respirometric measurement and the digital processing of the results of this measurement by the biological model as described above) of the treated substrates or substrates to be treated on the composting facility and establishment of a data base of characteristics X(0), MH(0), MB(0) of each substrate. The accuracy on X(0) will be 50%. The accuracy on MH(0) and (MB(0) will be 30%.

Forecasting of accelerated compostability Ca of a substrate or mixture of substrates by using the data base.

The fraction of biodegradable material during the said fermentation phase of the treatment - or accelerated compostability - will be: Ca= _________________ to tale in which w, is the mass concentration of each constituent i and DCOtotai is the chemical oxygen demand of the mixture under consideration.

Evaluation of the industrial feasibility of composting treatment for a substrate or mixture of substrates. The setting of the biological reaction processes by compositing resulting in sufficient, durable heating of the reaction mass will be observed for a minimum compostablity level Ca of the initial mixture of between 5 and 10 %.

2) Laying down optimal humidity and temperature conditions for the compositing of the adopted formulation: Analysis of the chosen mixture to be composted on a respirometric bench according to the method described above with reference to the figure, at three temperatures and three different moisture contents (using 9 bench cells). Scrutiny of the results and the analysis enables the optimal level of water content of the initial mixture - 12 - and the temperature range for which the microbial activity is the strongest to be determined.

3) Determination of minimum oxygen supplies and of the minimum residence time in the accelerated fermentation reactor: Analysis of the chosen substrate mixture used for the optimal moisture content on the respirometric bench according to the above-described method and with two repetitions but varying the temperature and without controlling the humidity so as to reproduce the thermal conditions of the composting method that is used.

The minimum residence time may be calculated as being the time from which an oxygen uptake kinetic of between 1 and 4 mmcl of 02/h/kg MS is observed with a variation 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 oxygen uptake kinetics of t=0 at t= minimum residence time (Jr02(t).dt). The quantity of oxygen to be supplied to the industrial process will be calculated as follows: Q02 = minimum Q02 +20 %.

A method comparable with the method described above may be developed and applied to determine the aerated maturation compostability of a fine substrate after accelerated fermentation and screening and to lay down the application conditions of this new phase of the composing method corresponding to aerated maturation (temperature, humidity, residence time and minimum oxygen supply).

To sum up, the utilisation by the operator of the preliminary step of the process according to the invention, which is 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 selection of a mixture formulation with other substrates of known accelerated compostability so as to achieve the desired level of accelerated compostability.

- 13 - B. Study of the optimal conditions of humidity and temperature for the fermentation of the chosen mixture.

C. Determination of the minimum residence time and of the minimum quantities of oxygen to be supplied to the chosen mixture.

The method according to the invention may allow, using a protocol similar to that method above, the aerated maturation phase of a composted product to be parameterised.

Claims

- 14 - Claims 1. A method for the accelerated composting of mixed organic

waste, characterjsed 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 preliminarily step for charactering and optimising the biodegradability and the degradation kinetic of the initial mixture comprising different types of waste to be composted, this preliminary step consisting of a rapid measurement of the uptake of oxygen (respirometric measurement) by a solid organic biological medium and the processing of the result of the said respirometric measurement by means of modelling, the said measurement comprises: - the preparation of the organic waste so as to obtain a solid porous structure comparable with that treated in industrial composting; - the homogenous distribution of oxygen within the mass of waste placed in a respirometric enclosure; - maintaining firstly the temperature and secondly the moisture content of the sample mass at constant values during the measurement and the measurement of the oxygen uptake of the waste over a period in the order of 10 to 20 days.
2. A method according to Claim 1, characterised in that the respirometric measurement is carried out using a respirometer having a capacity in the order of 6 to 10 litres.
3. A method according to Claim 1, characterised in that the processing of the said respirometrjc measurement by modelling is based on a model expressing the growth and the decay of the microorganisms in the waste, the uptake of the organic substrate and the oxygen uptake connected with the said growth.
4. A method according to Claim 3, - 15 - characterised in that the organic material of the mixture to be composted is modelled according to several fractions: a microbiological population an immediately assimilable biodegradable fraction a fraction that is biodegradable after hydrolysis a non-biodegradable or inert fraction
5. A method according to one of Claims 3 or 4, characterised in that the said model comprises ten parameters, whose the determination forms the characterisation of the biodegradability (quantification, qualification and kinetic) of the solid waste tested: parameters describing the initial composition of the organic material: respectively the initial content of microorganisms, immediately biodegradable material and slowly biodegradable material; kinetic parameters: specific growth kinetic of the microorganisms, semi-saturation constant of immediately biodegradable substrate, decay kinetic of the microorganisms, hydrolysis kinetic and semi-saturation constant of slowly biodegradable material, performance parameters: conversion efficiency coefficient of the microorganism substrate and conversion coefficient of the dead biomass of inert organic material; it being possible to determine the initial values of these parameters by graphics of the result of the respirometric measurement.
6. A method according to Claim 5, characterised in that the real values of the said parameters are optimised by minimising the sum of the squares of the errors allowing the initial values of the parameters to be varied in a value range defined by the user.
7. A method according to any one of the preceding Claims, - 16 characterjsed in that it is used to formalise within the accelerated composting process, the said step involving the characterisation and optimisation of the formulation of a solid or pasty organic mixture of substrates to be treated as well as the prediction of the conditions for the management of the fermentation phase of the said treatment, this method comprising the following steps: A) characterisation of the solid organic substrates to be treated by a composting method and optimal formulation of the initial mixture, comprising: - a respirometric measurement of the substrates and numerical processing of the results of this measurement by a biological model, carried out on the composting facility, at constant temperature and humidity and establishment of a data base respectively for the initial microorganism content, the immediately biodegradable material and the slowly biodegradable material of each substrate; - forecasting of accelerated composting of a substrate or of a mixture of substrates by using the said data base and - evaluation of the industrial feasibility of composting treatment of a substrate or a mixture of substrates; B) laying down optimal humidity and temperature conditions for the composting of the substrate or of the substrate mixture of substrates having the chosen formulation and; C) determination of the minimum oxygen supplies and the minimum residence time in the fermentation reactor.

- 17 - Key to the Figure: pompe pump débimétre flowmeter compteur volumique volumeter analyseur 02 02 analyser air ambiant ambient air condenseur a eau water condenser bain thermostate thermostatically controlled bath entrée air ambiant ambient air inlet recirculation recirculation sortie air réacteur air outlet reactor