coupling a respirometer and a pycnometer, to study the biodegradability of solid organic wastes...

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Research Paper: SE—Structures and Environment

Coupling a respirometer and a pycnometer, to study thebiodegradability of solid organic wastes during composting

L. Berthea, C. Druilhea,�, C. Massianib, A. Tremiera, A. de Guardiaa

aCemagref, Livestock and Municipal Waste Management Research Unit, 17 av. de Cucille CS 64427-35044 Rennes Cedex, FrancebUniversite de Provence, Laboratory of Chemistry and Environment, 3 pl. Victor Hugo-13331 Marseille Cedex, France

a r t i c l e i n f o

Article history:

Received 5 August 2006

Accepted 31 January 2007

Available online 11 April 2007

nt matter & 2007 IAgrE.temseng.2007.01.013

thor.: celine.druilhe@cemagr

A new method, coupling a respirometer and a pycnometer, to follow the biodegradation of

solid organic wastes is presented. The pycnometer was used to observe the development of

the free air space in the sample during the respirometric experiments and verified that this

parameter remained favourable to the substrate oxygenation. To test this new method,

respirometric experiments were conducted on four solid waste samples (a mixture of

agrofood sludge and pine bark chips, a mixture of urban sludge with new and reused pine

bark chips collected before and after 21 days of composting, and a mixture of household

wastes).

The respirometer is based on a 10 l reactor filled with the sample. It was designed to

allow the control of operating conditions: temperature, moisture and aeration. During

respirometric experiments, the material temperature was constant and homogeneous.

However, large variations in the moisture content were measured between the beginning

and the end of respirometric tests. Moreover, the development of moisture variability

throughout the substrate was observed. The aeration regime was characterised as a

perfectly mixed phase considering the results of retention time-distribution studies and

the free air space values measured with the pycnometer. The pycnometer accuracy was

confirmed by low differences between experimental and theoretical free air space values

for water and spherical glass beads. Moreover, it was verified that this measurement had no

influence of respirometric responses.

Applying the proposed method to the four substrates demonstrated the interest in

mixing the material during the respirometric tests, when the oxygen uptake rate rO2 fell to

a low and stable value. Indeed, in some cases, such mixing stimulated the restart of rO2 that

confirmed the presence of residual biodegradable organic matter. The major effect of

mixing was to restore favourable moisture content throughout the substrate. The results

revealed that a small mixing impact corresponded to either a low substrate biodegrad-

ability or a high initial moisture content. The pycnometer measurements demonstrated the

influence of a low free air space. It appeared that the absence of oxygen uptake restart after

mixing, in the presence of residual organic matter, could be due to a low free air space.

These results demonstrated the need (i) to mix the samples to quantify all its

biodegradable organic matter and (ii) to control the moisture, temperature and aeration to

ensure favourable conditions of biodegradation.

& 2007 IAgrE. All rights reserved. Published by Elsevier Ltd

All rights reserved. Published by Elsevier Ltdef.fr (C. Druilhe), [email protected] (C. Massiani).

dx.doi.org/10.1016/j.biosystemseng.2007.01.013

mailto:[email protected]

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Nomenclature

C(t) concentration of methane at time t, ppm

C0 initial concentration of methane, ppm

n moles of gas in the system, mol

na0 initial moles of gas in the additional cell, mol

nr0 initial moles of gas in the reaction cell, mol

na1 intermediate moles of gas in the additional cell,

mol

nf final moles of gas in the reaction and additional

cells, mol

P system pressure, Pa

Pa0 initial pressure in the additional cell, Pa

Pa1 intermediate pressure in the additional cell, Pa

Pf final pressure in the additional and reaction cells,

Pa

Pr0 initial pressure in the reaction cell, Pa

Qe inlet airflow rate, l h�1

Qr recirculation airflow rate, l min�1

R gas constant, J K�1 mol�1

S free air space, %

Sexp experimental free air space, %

Sth theoretical free air space, %

T gas temperature, K

Ta0 initial gas temperature in the additional cell, K

Ta1 intermediate gas temperature in the additional

cell, K

Tc water-bath temperature, 1C

Tr0 initial gas temperature in the reaction cell, K

Tf final temperature in the additional and reaction

cells, K

t time in retention time distribution experiments, s

t0 beginning of respirometric tests, h

t1 time of the first mixing, h

t2 time of the second mixing, h

t3 time of the third mixing, h

tf end of the respirometric tests, h

V system volume, m3

Va volume of the additional cell including pipes, m3

Vg volume of the whole sample including solid and

voids, m3

Vr volume of the reaction cell including pipes, m3

VRTD system volume during retention time distribution

experiments, l

Vs volume of solid sample without free air space, m3

Vs,exp experimental volume of solid sample without free

air space, l

Vs,th theoretical volume of solid sample without free

air space, l

s2reduced variances

t residence time, s

m time of passage, s


1. Introduction

The production of wastes is continuously increasing [e.g.

627 Mt year�1 in France in 2002; Ademe (2006)] and efficient

ways to treat and recycle them are needed. For organic wastes,

about 450 Mt (Ademe, 2006), composting appears as one of the

most promising options since the wastes could be converted

into useful agricultural products. However, the development of

composting often encounters limitations due to poor compost

quality resulting from a lack of treatment management.

Indeed, the quality of compost depends on the method of

preparing the initial material and the management of the

reactions occurring during the process. Composting, as an

aerobic treatment process, comprises three interdependent

phenomena: mass transfer, heat transfer and biological

processes (Haug, 1993). The mass and heat transfer operations

are closely linked to the kind of process applied, whereas the

biological reactions mainly depend on both substrate charac-

teristics (such as the amount and type of biodegradable organic

matter) and environmental conditions (such as temperature,

aeration level and moisture content) (Moletta, 2006). Thus, the

development of methods to study the processes of biodegrada-

tion, and to quantify and identify the biodegradable organic

matter content of samples, appears as a key step to achieve a

well-managed composting process (the optimisation of the

initial mixture, the composting time, the airflow rate).

The biological reactions—the use of the organic matter by

microorganisms—induce the consumption of oxygen and the

production of carbon dioxide. Therefore, the methods for

monitoring these parameters, called respirometric methods,

could be considered as a direct measure of biological activity.

For this reason, respirometric measurement is accepted as

being the more suitable method to study organic matter

biodegradation (Adani et al., 2006). Nevertheless, the respiro-

metric methods that monitor only the carbon dioxide

production could induce a misunderstanding of biological

processes and an underestimation of sample biodegradability.

Indeed, carbon dioxide results from both aerobic and

anaerobic reactions and, even if composting is an aerobic

treatment, the development of local zones with anaerobic

conditions could be observed (Haug, 1993; Epstein, 1997).

Therefore, methods based on oxygen uptake rate were

preferred to estimate the biodegradability potential of solid

organic wastes at the beginning, during and at the end of

composting.

Numerous methods based on oxygen uptake have been

proposed in the past. They are divided into static and

dynamic methods on the basis that oxygen uptake measure-

ment is made in the absence (static) or in the presence

(dynamic) of continuous aeration of the biomass (Adani et al.,

2004). Static methods induce limitation of oxygen transfer

through the biomass layers and into bacterial cells that

reduce or inhibit the aerobic biological processes (Adani et al.,

2002; Paletski & Young, 1995). In this condition, sample

biodegradability may be greatly underestimated. The dy-

namic methods have the advantage of minimising the

limitation of oxygenation. In these systems, the respirometric

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B I O SYS T EM S E NG I NE E R I N G 97 (2007) 75– 88 77

reactor filled with the studied sample is continuously

supplied with oxygen to ensure its diffusion in the whole

matter and aerobic conditions.

In addition to the dependency on the aeration level, the

biological processes are also linked to the structure, tempera-

ture and moisture of samples (Haug, 1993). To take into

account the sample physical structure [moisture, granulome-

try, porosity, free air space (FAS)] and its consistency, the

respirometric test has to be performed on substrates repre-

sentative of those in the compost system. However, most

existing dynamic respirometers cannot be used as they work

with small sample quantities, at best 1.5 l (Aguilar-Juarez,

2000) but commonly 10–100 g (Gomez et al., 2006; Ianotti et al.,

1994; Paletski & Young, 1995). Four respirometric systems

have been identified to provide a reaction volume permitting

work with raw materials: (1, 2) the Dynamic Respiration Index

design with two size of reaction volume (Adani et al., 2004,

2006) and (3, 4) the respirometers developed by Tremier et al.

(2005b) and Gea et al. (2004). The systems proposed by Adani

et al. (2004, 2006) have a sample reactor of 124 or 30 l. The one

presented by Gea et al. (2004) was set up with a reaction cell of

100 l and a reaction volume of 10 l is used in the system of

Tremier et al. (2005b). Compared to the others, the last

apparatus has two main advantages. First, its aeration system

includes a recirculation of a part of the exhaust air that

ensures a more homogeneous oxygen diffusion to the

biomass than a single forced aeration method. Thus, oxygen

is not limiting to biodegradation (Tremier et al., 2005b).

Second, in this system, the smaller reaction cell and its

immersion in a water-bath help to achieve good maintenance

and homogeneity of temperature and moisture anywhere in

the substrate and throughout the tests. In these conditions,

analysis and comparison of respirometric profiles are easier.

The preliminary experiments carried out on this respirom-

eter revealed a limitation of this tool since all biodegradable

organic matter was not degraded during the main peak of

oxygen uptake rate rO2 typically observed in respirometry. The

presence of residual biodegradable organic matter was

illustrated by a second peak of rO2 (representing between

23% and 37% of the total oxygen consumed) after a manual

material turning. This mixing was realised after the first peak

when the rO2 fell to a low and stable value, usually due to the

absence of biodegradable organic matter and considered as

the endogenous respiration. There were two possible reasons

for the presence of residual biodegradable organic matter.

First, the moisture variability throughout the material,

visually observed, during respirometry resulted in local zones

of the material with limiting moisture content for biodegra-

dation. Second, the limitation of biodegradation resulted from

an insufficient sample FAS that implied oxygen diffusion

problems. Indeed, the material moisture and the FAS are

known as being limiting factors of biodegradation (Haug,

1993). The operation of mixing re-homogenised the sample

and helped to restore favourable conditions of moisture and

FAS in the whole sample. Therefore, biodegradation of the

organic matter was restarted, which generated a new peak of

oxygen uptake rate. To follow the variation in FAS during

respirometric tests by in situ measurement, a specific device

was designed and combined with the respirometer. An air

pycnometer was used rather than other methods such as

specific gravity bottle, water retention apparatus, mercury

porosimetry and nitrogen absorption, paraffin wax method,

and water pycnometers (Agnew et al., 2003). The main

advantage of the air pycnometer is the possibility of measur-

ing precisely the material FAS during respirometric tests

without modifying the sample filling the respirometric cell.

The air pycnometer provides indirect space measurements by

relating the system’s pressures and volumes using the ideal

gas law.

The objective of this paper is to present the method,

coupling a respirometer and a pycnometer, developed to

study the aerobic biodegradation of solid organic wastes. The

respirometric method was chosen and then modified to

ensure efficient control of the environmental conditions

(temperature, moisture and aeration) and to allow the

measurement of sample FAS. The control of temperature

and moisture was studied by following these parameters

during respirometric experiments. The matter oxygenation

quality was controlled by the measurement of FAS all along

the respirometric tests and was specifically characterised

with retention time-distribution (RTD) experiments. The

validity and impact of the FAS measurements were studied

before applying this method during respirometric tests.

The FAS measurements and the respirometric profiles,

obtained for four substrates [a mixture of agrofood sludge

and pine bark chips, a mixture of urban sludge with new and

reused pine bark chips collected before and after 21 days of

composting, and a mixture of household wastes (HW)], were

used to prove the advantages of coupling respirometry and

pycnometry.

2. Materials and methods

2.1. Respirometric device

The respirometric device (Fig. 1) was set up on the basis of the

apparatus developed by Tremier et al. (2005b). This comprised

a 10 l hermetically closed glass reactor with a grid floor filled

with the sample under investigation. Ambient air is fed

through the bottom along with recirculation air to provide

homogeneous aeration conditions. The temperature was

controlled by a water-bath. Humidification of the inlet air

and condensation of the exhaust gas on the sample are used

to control the matter moisture content. This system was

modified to optimise the control of operating conditions

(temperature and moisture) and to adapt it to FAS measure-

ments. A 10 l cell made of stainless steel replaced the glass

reactor. The cell was made of stainless steel to resist the

compression required for FAS measurement. The cell was

filled with the sample put on a 3 mm round mesh grid placed

70 mm above the cell bottom. The system was supplied from

the bottom via a diffuser with compressed air instead of

ambient air to minimise the entering gas characteristic

variations (temperature, moisture, composition). Moreover,

the inlet airflow rate Qe increased from 50 to 60 l h�1 and the

recirculation airflow rate Qr from 4–5 to 6 l min�1.

To increase the temperature control in terms of homo-

geneity and stability, the respirometric cell was completely

immersed in a water-bath and was supplied with preheated

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Comair counter

Serpentine (Air preheating)

Sample respirometric cell

B

A

Compressed Gas volume

FlowmeterFlowmeter

Flowmeter

Pump

ThermostatWater bath

Sample Stainless steel

Pt100

Temperature and moisture probe

Air diffuserAir moisture saturation

Analyser

Insulation

Condenser bottle

B

A

Methane injection

Methane injection

point B

point A

Fig. 1 – Respirometric device after modifications to improve environmental parameter control.


air at water-bath temperature Tc. The air was preheated by

passing through a copper serpentine placed in the water-bath

(diameter of 10 mm; thickness of 2 mm; length of 2 m). As in

the original apparatus, the matter temperature is measured

with a Pt100 temperature probe placed in the sample centre.

The moisture-controlled system was completely modified.

The inlet gas was saturated in humidity at the water-bath

temperature by passing through two glass bottles filled with

water. The bottles were immersed in the water-bath and filled

daily with water. The exhaust air was condensed out of the

cell, in specific bottles placed at ambient temperature.

Condensation of the exhaust air outside the cell limited the

possibility of localised water falling back on the sample.

The oxygen uptake rate was obtained by measuring oxygen

concentrations with a paramagnetic oxygen gas analyser

(Multor 710, Maihak, Hamburg, Germany) in the inlet and

exhaust gas.

2.2. Optimisation of the aeration conditions

The distribution of oxygen concentration in the studied

material has to be homogeneous and provided in sufficient

quantity to create favourable aeration conditions for biode-

gradation. Aeration was characterised by the hydrodynamic

of the gas phase and was influenced by the material FAS.

The gas flow distribution was studied through the RTD

measurement method (Roustan, 2003). This method consists

of determining the time passed by each molecule in the

circuit and of quantifying them in the exhaust gas. Practically,

a gas tracer was injected in the circuit and the concentration

of this particular gas was monitored in the outlet gas.

Methane was chosen as a gas tracer after having verified

that there was no significant methane production during

the respirometric experiments and it was not noxious for

microorganisms. Moreover, its hydrodynamic characteristics

were similar to those of air. The concentration of methane in

the exhaust gas was measured every 2 s using the gas

analyser (Multor 610, Maihak, Hamburg, Germany). The

injection of the gas tracer was a pulse type of 10 ml of pure

methane at point B of the respirometric circuit (Fig. 1).

The variation in methane concentration in the whole circuit

was monitored for an inlet airflow rate of 55 l h�1 and 0, 2, 4, 6

and 7 l min�1 as recirculation flow rates. The substrate

studied was composed of pine bark chips (global volume of

5 l; FAS of 64%). The tracing trials were duplicated for the

same recirculation airflow rate. The RTD curves were

obtained as the ratio between the concentration C(t) in

methane at each time t in the outlet gas and the concentra-

tion in methane in the inlet flow C0. Methane recovery rate

was estimated as the difference between the injecting

quantity of this component (10 ml) and the volume of

methane recovered in the exhaust flow.

To interpret the tracings carried out on the whole circuit,

the flow in the analytical device was characterised during

previous tracing experiments by the injection of methane at

point A (Fig. 1). This has shown that the hydrodynamic of the

analytical circuit was of a plug type that induced a delay in

the response and non deformation. The absence of methane

losses was also checked in these experiments.

The interpretations of the curves concerned: (i) the repeat-

ability of tracing; (ii) the determination of methane recovery;

(iii) the comparison of residence time t and the time of

passage m; (iv) the estimation of the reduced variances s2 and

(v) the comparison between the experimental RTD and the

RTD of a perfectly mixed phase (Roustan, 2003):

t ¼VRTD

Qe, (1)

m ¼

RtCðtÞdtRCðtÞdt

¼

PtCðtÞPCðtÞ

, (2)

s ¼

Rðt�mÞ2CðtÞ dtR

CðtÞ dt

m2¼

Pt CðtÞPCðtÞ� m

� �

m, (3)

where t is the time in s; C(t) is the methane concentration in

ppm of the exhaust gas at time t; VRTD is the system volume in

l; and Qe is the inlet airflow rate in l h�1.

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In our case, for all parameters the sums replaced the

integrals because the experimental data were registered at

regular and small time intervals (every 2 s).

2.3. Measurement of free air space

2.3.1. Pycnometer deviceAir pycnometer (Fig. 2) provides indirect FAS measurements

by relating the system’s pressures and volumes using the

ideal gas law:

P V ¼ n R T, (4)

where P is the pressure in Pa; V is the volume in m3; n is the

gas mole amount in mol; R is the gas constant in J K�1 mol�1;

and T is the gas temperature in K.

The sample is placed in one chamber while a second

chamber is supplied with compressed air to a specific

pressure. The air is allowed to equilibrate between the two

chambers and the resulting pressure is used to calculate the

air space volume in the sample (Agnew et al., 2003).

The pycnometer consisted of two stainless-steel cells (25 l

additional cell and 10 l reaction cell), a three-way control

valve, an absolute pressure transducer (Druck, 0–3.5 absolute

bar, Asnieres, France) and two Pt100 temperature probes (one

in each cell). An absolute pressure transducer was preferred

to a gauge to measure a pressure independent of atmospheric

pressure. The valve was used to connect or isolate the cells

between them and to supply the additional cell with

compressed air. To realise in situ measurement of FAS during

respirometry, the respirometric cell filled with the sample was

used as the reaction cell of the pycnometer. Thus this system

allowed to follow the variation in FAS of the sample during

the respirometric test due to biodegradation activity and

variation in physical structure of the matter.

The gas temperature was measured during all the FAS

measurements, so it was possible to correct the gas volume as

a function of the temperature. The volumes of cells and tubes

were precisely measured by filling these items with a known

volume of water.

Additional cell

Reaction cell(respirometric cell)

Sa ple

TT

Three-way control valve

P

Compressed air

Sample

Fig. 2 – Air pycnometer device: T, Pt100 probe; P, pressure

transducer.

The FAS measurement was divided into three parts. First,

after being placed at atmospheric pressure, the two cells were

connected between them and isolated from the atmosphere.

The equilibrium state was described by

Pr0ðVr � VsÞ ¼ nr0 R Tr0, (5)

Pa0 Va ¼ na0 R Ta0, (6)

where Pa0 and Pr0 are the pressures of the additional and

reaction cells in equilibrium condition in Pa; Va and Vr are the

volumes of additional and reaction cells (including the

volume of pipes) in m3; Vs is the volume of the solid sample

(not including the FAS) in m3; na0 and nr0 are the moles of gas

in the additional and reaction cells in mol; Ta0 and Tr0 are the

gas temperature in the additional and reaction cells in K.

Then, the reaction cell was isolated and the additional cell

was connected to the compressed air. The additional cell was

supplied with compressed air until the internal pressure

reached a value between 2500 and 2800 absolute pascal. This

range of pressures optimises the measurement accuracy

(Agnew et al., 2003; Oppenheimer et al., 1997). After the

pressure and temperature stabilisation (approximately

6 min), the state of the additional cell was described by the

following:

Pa1 Va ¼ na1 R Ta1, (7)

where Pa1 is the pressure of the additional cell in Pa; na1 is the

moles of gas in the additional cell in mol; and Ta1 is the gas

temperature in the additional cell in K.

Finally, the reaction cell was progressively connected to

the additional cell. After the stabilisation of pressure and

temperature (approximately 6 min), the equilibrium state was

characterised by

Pf ðVa þ Vr � VsÞ ¼ nf R Tf , (8)

where Pf is the pressure of the system in Pa; nf is the moles of

gas in the system in mol and Tf is the gas temperature in the

system in K.

The combination of these previous equations led to the

volume of solid sample Vs and then to the sample FAS S:

Vs ¼ Va

Pf �Pa1 Tf

Ta1

Pf �Pr0 Tf

Tr0

þ Vr, (9)

S ¼ 1�Vs

Vg

� �100, (10)

where S is the FAS in %; and Vg is the volume of the whole

sample (solid and void) in m3.

2.3.2. Pycnometer calibrationThe pycnometer calibration consisted of a comparison

between the theoretical and experimental FAS values for

two experiments.

The first test was on a respirometric cell loaded with 5.988

and 3.940 l of water (theoretical FAS of 0). The second test was

carried out on 0.999 l of spherical glass beads (diameter of

5 mm; theoretical FAS of 36.86%) placed in the reaction cell.

The first measurements with the pycnometer indicated a

deformation of the additional and reaction cells with an

increase in pressure. In this condition, the volume of these

cells (Vr and Va) had to be corrected as a function of the

ARTICLE IN PRESS


pressure applied in the system. This correction was made

following specific experiments conducted to determine the

deformation coefficients. These experiments consisted of

measuring the weight variation of cells filling with water

until the pressure reached 3 absolute bar. The pressure

applied covered the range of pressures used during FAS

measurement. The volume variation was identical to the

mass variation since water was assumed to be an incom-

pressible fluid in the range of pressures studied. The volume

of the cells varied linearly with pressure. The coefficients of

deformation were evaluated during both the compression

and the decompression phases.

2.3.3. Influence of free air space measurement onrespirometric responseThe pycnometer device was developed to measure the

material FAS throughout the respirometric trials. As this

measurement required the pressurisation of gas filling

the voids between solid particles, a specific experiment was

carried out to evaluate the influence of pycnometer measure-

ment on respirometric response. Indeed, the pressurisation

could stress microorganisms, cause leaching and increase

oxygen solubility. The influence on leaching could be

observed immediately after FAS measurement and at the

end of the respirometric test by quantifying the leachates.

Two respirometric cells were filled with the same sample, a

mixture of sludge and pine bark chips. The moisture of this

sample was about 53% and the dry weight ratio between pine

bark chips and sludge was equal to 1.7 for a favourable

biodegradation.

The cells were studied in respirometry in the same

operating conditions (Tc ¼ 40 1C, Qe ¼ 60 l h�1, Qr ¼ 6 l min�1).

The FAS for the sample placed in a respirometric cell

was measured with the pycnometer whereas no measure-

ment was performed on the sample filling another respiro-

metric cell. The comparison of the respirometric curves

helped to reach a conclusion about the influence of the FAS

measurement.

2.4. Validation of the new respirometric device

2.4.1. Respirometric runsIn order to study sample biodegradation, the respirometric

measurements had to depend only on the intrinsic para-

meters of the microbial activity: biodegradable organic matter

quantity and species of microorganisms. The variation in

environmental conditions (temperature, moisture and aera-

tion) must not influence the biodegradation process during

measurement. Hence, three respirometric tests were carried

out on different organic materials to study the oxygen supply,

temperature and moisture control, and the repeatability of

respirometric responses.

The first test was conducted on a mixture of agrofood

sludge and pine bark chips (test A, mixture A). The second

test was carried out on mixtures of urban sludge with new

and reused pine bark chips collected before and after 21 days

of composting (test B, mixtures B0 and B21). The last test was

realised on household wastes (HW) (test C, mixture C).

For all tests, the water-bath temperature was equal to 40 1C,

the inlet airflow rate was 60 l h�1 and the recirculation airflow

rate was 6 l min�1 (value optimised in the RTD experiments).

The material was mixed when the oxygen uptake rate fell to a

low and stable value. The material mixing operations were

repeated until no significant restart of the oxygen uptake rate

was detected.

In test A, the sludge came from an agrofood industry and

the pine bark chips were dried at 80 1C and calibrated

(lengtho5 cm). The initial moisture of this sample was equal

to 49.7% with a dry mass ratio of 1.7 between the pine bark

chips and sludge. Four respirometric cells loaded with this

mixture were started at the same time. For these experi-

ments, three material mixings were necessary before they

had no significant influence on rO2 profile. These mixings

occurred after 691, 1050 and 1194 h of respirometry (test A). At

each mixing, one of these cells was stopped.

The second experiment (test B) was realised on mixtures of

urban sludge and bulking agent collected after 0 and 21 days

of composting (test B—mixtures B0 and B21). The composting

treatment was carried out in an airtight 300 l insulated

stainless steel cylindrical reactor (Tremier et al., 2005a). The

initial moisture of the samples (B0 and B21) was about 60%. As

for test A, the material was mixed when rO2 reached a low and

constant value until no significant restart of the oxygen

uptake rate was detected. Only one mixing was necessary for

B21 (after 260 h of test) and two mixings for B0 (after 260 and

580 h of experiment).

The test C was conducted on HW collected on an industrial

site after 4 days of fermentation in a bioreactor and after

mechanical separation to keep out the inert material. The HW

were sampled immediately after the last mechanical separa-

tion device, a flip-flow Liwell sieve with a square mesh of

10 mm. The initial moisture content of this sample was about

60%. A first mixing was carried out at 676 h and a second at

1000 h of respirometry with introduction of pine bark

chips—ratio between HW and pine bark chips of 0.88 in wet

mass—in order to modify the material structure.

2.4.2. Study of the respirometric response repeatabilityThe repeatability of the respirometric responses, studied

by comparison of the four cells of test A, was validated

(i) for respirometric curves visually identical and (ii) for

variation coefficients of maximum rO2 and amount of

oxygen uptake lower than 10%. As all cells were not stopped

at the same time, the study of repeatability concerned

four cells on the first part (0–691 h), three cells on the

second part (691–1050 h) and two cells on the third part

(1050–1194 h). The quantity of oxygen uptake corresponded

to the total oxygen uptake deducted by the endogenous

oxygen consumption. It was chosen to deduct the endogen-

ous oxygen in order to compare, in terms of organic

matter quantity, these results to others obtained for

different substrates. The endogenous respiration was

estimated as the mean value of rO2 during the last 48 h of

respirometry.

2.4.3. Study of temperature stability and homogeneityTemperature stability during tests A, B and C was evaluated

by measuring the variation coefficient of the Pt100 tempera-

ture probe data. The stability, in terms of time, was efficient

when this coefficient of variation was under 7% (arbitrary

ARTICLE IN PRESS


choice). The spatial homogeneity of matter temperature was

studied for test A by the association between the Pt100

temperature probe data and the measurements from some

thermobuttons (DS1922L, Maxim, France) placed in different

points of the sample (at a mean of three sample). The

thermobuttons were placed at several heights and radii. The

thermobuttons were similar to a clock pile and measured

temperature with a precision of 0.5 1C. Homogeneity was

validated for a difference between two points at maximum

equal to 3 1C (arbitrary choice).

2.4.4. Study of the moisture stability and homogeneityThe moisture homogeneity for tests A, B and C was visually

evaluated at each mixing and at the end of respirometry but

not quantified because of technical limits. The global

moisture stability was estimated by measurement and

comparison of material moisture (obtained after drying at

80 1C until the variation of dry mass was inferior to 0.5% in

24 h) at the beginning and at the end of respirometry for the

tests A, B and C. It was considered efficient if the variation in

moisture was inferior less than 4% (arbitrary choice).

2.4.5. Variation of free air space during compostingTo identify a possible limitation of biodegradation due to the

FAS, the sample FAS was measured at the beginning and at

the end of respirometry, and before and after each mixing

during tests A, B and C. It was measured with the pycnometer

described previously.

3. Results

3.1. Optimisation of the aeration conditions

The results of RTD experiments characterised the gas phase

hydrodynamic as the methane recovery rate was at minimum

93% and the repeatability effective (coefficient of variation

CVo4% for the reduced variance s2, the mean of residence

time m, and the rate of methane recovery) (Table 1).

The results obtained (Table 1) were in concordance with

the theory since the higher the recirculation airflow rate, the

higher was the reduced variance s2 (Roustan, 2003). The

reduced variance s2 increased sharply (from 0.24 to 0.85)

Table 1 – Results of gas-phase characterisation by tracing met

Recirculation airflow rate,l min�1 (duplicatemeasurements)

0 2

Time of passage t, s 713.08 765.3

Mean of residence time m, s 730.16 790.6

CV for m, % 2.2 1.6

(t–m)/t, % �2.4 �3.3

Mean of reduced variance s2 0.24 0.74

CV for s2, % 1.7 3.5

Mean of methane recovery rate, % 99.04 96.23

CV for CH4 recovery rate, % 0.97 0.99

CV, coefficient of variation.

between 0 and 4 l min�1 of recirculation airflow rate Qr. On

and after 4 l min�1, the reduced variance s2 increased more

slowly to reach 0.90 for the recirculation airflow rate of

7 l min�1. This last value did not reflect a perfectly mixed

phase because in this case the reduced variance s2 is

theoretically equal to 1 (Roustan, 2003). Nevertheless the

experimental RTD curves and the one for a perfectly mixed

flow were very close for the value of Qr of 6 and 7 l min�1. The

curves for these two recirculation airflow rates differed from

the theoretical profile only during the first 100 s of the

experiment (Fig. 3).

Moreover, the low difference between the passage and

residence times (o 5%) proved the absence of significant dead

zones and preferential ways (Roustan, 2003).

For the recirculation flow rates of 6 and 7 l min�1, the gas

phase was considered as perfectly mixed because (i) the

reduced variance was closed to the theoretical one of a

perfectly mixed flow, (ii) of the correspondence between

experimental and theoretical RTD profiles after 100 s and (iii)

of the absence of dead zones and preferential ways. Under

these conditions, the respirometric samples were supplied

with homogeneous and sufficient oxygen flow. A recirculation

airflow rate of 6 l min�1 was retained for the respirometric

experiments.

3.2. Pycnometer validation

3.2.1. Pycnometer accuracyThe results of tests 1 and 2 showed the accuracy and

repeatability of the pycnometric measurements (Table 2).

The repeatability was validated even if the coefficients of

variation obtained for the FAS of test 1 were greater than 10%.

For this test, the null porosity of water-induced high error that

revealed a limitation of the tool. The pycnometer accuracy

was confirmed by the low differences calculated between the

experimental and theoretical values measured for the solid

volume Vs and the FAS S. Among all tests, the highest

differences between theoretical and experimental Vs were

3.3% (test 2—Table 2) and 1.2% (test 1—Table 2). In the same

way, the experimental Sexp differed from the theoretical value

Sth at most by 5.4% (test 2—Table 2). Taking into account the

errors due to instruments, volume determination and cell

hod

4 6 7

1 765.31 765.31 765.31

8 781.10 796.42 785.90

0.5 0.2 1.4

�2.1 �4.1 �2.7

0.85 0.89 0.90

2.4 1.6 3.2

93.70 96.43 94.26

0.66 0.03 1.87

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0

0.0002

0.0004

0.0006

0.0008

0.001

0.0012

0.0014

0.0016

0.0018

0.002

0 500 1000 1500 2000 2500

Time, s

Ret

entio

n tim

e di

stri

butio

n

Fig. 3 – Theoretical retention time distribution (RTD) of perfectly mixed flow and experimental RTD curves obtained at

different values of recirculation airflow rate Qr: , Qr ¼ 0 l min�1; , Qr ¼ 2 l min�1; , Qr ¼ 6 l min�1; x, Qr ¼ 4 l min�1;

n,Qr ¼ 7 l min�1; , theoretical RTD.

Table 2 – Calibration of the pycnometer

Test Vs,th, l Vs,exp, l 1�Vs;expVs;th

;% Sth, % Sexp, % 1�SexpSth

; %

1 (water) 5.988 5.960 0.5 0 0.64 —

5.956 0.5 0 0.70 —

5.935 0.9 0 1.04 —

(CV ¼ 0.23%) (CV ¼ 27.2%)

3.940 3.943 �0.1 0 �0.08 —

3.892 1.2 0 1.21 —

3.939 0.1 0 0.02 —

(CV ¼ 0.73%) CV ¼ 187.2%)

2 (spherical

glass beads)

0.999 1.008 �0.9 36.86 36.37 1.3

1.032 �3.3 36.86 34.88 5.4

1.006 �0.7 36.86 36.53 0.9

0.986 1.3 36.86 37.82 �2.6

(CV ¼ 1.87%) (CV ¼ 3.3%)

Vs,th, theoretical volume of solid; Vs,exp, experimental volume of solid; Sth, theoretical free air space; Sexp, experimental free air space; CV,

coefficient of variation.


deformation, the pycnometer could be considered as a

reliable tool to measure FAS.

3.2.2. Influence of free air space measurement onrespirometric responsesThe respirometric responses obtained from the experiments

carried out to observe the influence of FAS are illustrated on

Fig. 4. The comparison of these curves, visually and with

reproductibility criteria, did not demonstrate any significant

influence of the FAS measurement. The curves were similar

and the criteria of reproductibility were validated (CV on

maximum rO2 of 3.2%; CV on oxygen uptake of 4.2%). No

leachate was obtained after the FAS measurement or at the

end of respirometry that confirmed the absence of significant

influence of pycnometer measurement on respirometry.

3.3. Respirometer validation

3.3.1. Description and repeatability of respirometric curvesThe profiles of respirometric curves (test A on Fig. 5, test B on

Fig. 6 and test C on Fig. 7) were typical. After a short time lag,

the oxygen uptake rate increased quickly due to biodegrada-

tion of the most easily biodegradable substrate. Then,

when all of this matter has been degraded, a break in the

ARTICLE IN PRESS

0

50

100

150

200

250

300

0 20 40 60 80 100 120 140 160Time, h

r O2,

mm

ol [

O2]

kg−

1 [D

Mi]

h−1

Fig. 4 – Influence of the free air space measurement on respirometric responses rO2: , cell without free air space

measurement; �, cell with free air space measurement; DMSi, initial dry matter of sludge (rO2).

0

10

20

30

40

50

60

70

80

90

100

110

0 200 400 600 800 1000 1200 1400Time, h

r O2,

mm

ol [

O2]

kg−

1 [D

Mi]

h−1

0

5

10

15

20

25

30

35

40

45

50

Tem

pera

ture

, ˚C

t0

Mixing 1

Temperature profiles

Respirometric profiles

Mixing 2 Mixing 3

t1 t2 t3 tf

Fig. 5 – Respirometric responses rO2 of test A carried out on agrofood sludge and pine bark chips: DMi, initial dry matter

mixture; t0, start of the test; tf, end of the test; thick lines, respirometric profiles of cells 1–2–3–4; fine lines, temperature

profiles of cells 1–2–3–4 obtained with PT100 probes.


exponential growth was observed. The oxygen consumption

rate decreased regularly down to the complete disappearance

of all the biodegradable substrates. During this decrease in

oxygen uptake rate, the slowly biodegradable organic matter

was hydrolysed before being consumed by microorganisms.

These phenomena induce the consumption of all biodegrad-

able organic matter if no limitation of temperature, moisture

and aeration occurs. The operation of mixing realised during

tests A, B and C did or did not lead to a restart of oxygen

consumption. During test A, the more numerous the previous

mixing operations, the lower was the restart (Fig. 5). The

mixing of materials studied in test B had no influence (Fig. 6).

For test C, the first mixing had no influence on the oxygen

uptake rate profile whereas the second, after addition of pine

bark chips, led to a restart of oxygen uptake rate (Fig. 7). The

study of environmental conditions (temperature, moisture,

aeration, and FAS) was used to understand the presence or

absence of oxygen consumption restart after mixing.

The repeatability of respirometric responses was confirmed

with test A. The coefficients of variation obtained for

maximum rO2 and the amount of oxygen uptake were less

than 10% during the three phases studied (Table 3). Moreover,

the curves were identical (Fig. 5).

3.3.2. Stability and homogeneity of temperatureThe Pt100 probe temperature was close to the water-bath

temperature (40 1C) throughout the experiments, even if

during the peak of rO2 the temperature could be slightly

higher (Table 4, Figs. 5–7). Except during this short period, the

difference between the mean Pt100 value and water-bath

temperature value was at the maximum 2.2 1C. Moreover, the

temporal stability of temperature was confirmed by the low

ARTICLE IN PRESS

0

20

40

60

80

100

120

140

160

180

200

0 200 400 600 800 1000 1200 1400

Time, h

0

10

20

30

40

50

60

Tem

pera

ture

, ˚C

Mixing 1

Mixing 2 - Addition of bulking agent

r O2,

mm

ol [

O2]

kg−1

[DM

i] h

−1

Fig. 7 – Respirometric response of test C carried out on household wastes until the second mixing and on a mixture of

household wastes and pine bark chips from the 2nd mixing to the end: DMi, initial dry matter of household wastes; ,

respirometric profile; , temperature profile.

0102030405060708090

100110120130140150

0 100 200 300 400 500 600 700 800Time, h

0

5

10

15

20

25

30

35

40

45

50

Tem

pera

ture

, ˚C

Mixing 1 Mixing 2

r O2,

mm

ol [

O2]

kg−1

[DM

i] h

−1

Fig. 6 – Respirometric responses (rO2) of test B carried out on urban sludge and pine bark chips: DMi, initial dry matter of

mixture; , respirometric profile for B0; , temperature profile for B0; , respirometric profile for B21; ,

temperature profile for B21.

Table 3 – Repeatability of respirometric responses duringtest A (mixture of agrofood sludge and pine bark chips;initial moisture of 49 . 7%)

rO2,max , mmol[O2] kg�1[initial dry

matter] h�1

O2 uptake, mmol[O2] kg�1[initial dry

matter]

t0–t1 t1–t2 t2–t3 t0–t1 t1–t2 t2–t3

Mean 86 29 11 9751 2640 495

CV, % 0.7 3.1 8.9 1.9 5.0 9.1

rO2,max, maximum of oxygen uptake rate; O2 uptake, oxygen

consumed; CV, coefficient of variation; t0, start of the test; t1,t2,t3,

time of mixings.


variation coefficient of P100 probe data for the majority of

trials (CVo7% for all tests except the cell filled with HW).

The spatial temperature homogeneity was effective during

test A as the difference of temperature between the two

points of measurement remained under 3 1C.

In these conditions, the respirometric device seems useful

to conduct experiments at a preset temperature.

3.3.3. Evolution of free air space during respirometric runsThe objective of studying the FAS of the sample during

respirometry was to verify whether this parameter became

limiting during the biodegradation processes. The FAS

considered optimum varies among authors. It must be in

excess of 30% and 35%, respectively, for Haug (1993) and

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Table 4 – Control of temperature and moisture for tests A, B and C

Experiments Time, h Tmean, 1C CVT, % DTmax, 1C Hi, % Hf, % HfHi� 1

Test A (agro food sludge and pine bark chips)

Cell 1 0–1316 41.02 3.3 1.3 49.7 58.2 +16.9

Cell 2 0–1194 41.07 3.7 2.01 49.7 55.2 +10.9

Cell 3 0–1050 41.36 3.8 2 49.7 45.4 �8.7

Cell 4 0–691 41.62 2.9 2.6 49.7 44 �11.6

Test B (urban sludge and pine bark chips)

B0 0–713 40.5 3.6 — 60.3 66.2 +8.9

B21 0–713 39.7 5.7 — 57.5 61.9 +7.1

Test C (Household wastes)

Until the second mixing 0–1000 42.2 9.7 — 60.2 73 +21.3

Test C (Household wastes and pine bark chips)

From the second mixing to the end 1000–1220 40.3 1.7 — 62.0 65.6 +5.8

Tmean, mean of Pt100 probe measurements; CVT, coefficient of variation on Pt100 probe data; DTmax, maximum variation of temperature in the

sample; Hi, initial moisture content; Hf, final moisture content.

Table 5 – Variation in free air space during tests A, B and C

Experiment Free air space, %

Initial Mixing 1 Mixing 2 Mixing 3 Final

Before After Before After Before After

Test A (agrofood sludge and pine bark chips)

Cell 1 67 66 66 64 67 64 66 61

Cell 2 68 62 67 65 69 64 ** **

Cell 3 67 66 70 * ** ** ** **

Cell 4 69 69 ** ** ** ** ** **

Mean 68 66 68 65 68 64 66 61

CV, % 1.6 4.2 3.5 1.9 2.1 1.0 — —


B0 54 48 55 46 49 *** *** 40

B21 64 53 59 *** *** *** *** 50

Test C (only household wastes until second mixing and then adding of pine bark chips)

41 24 23 10 58 *** *** 50

* Absence of measurement for experimental reason; ** No measurement (test ended); *** No material mixing.


Keener et al. (2002) whereas Epstein (1997) proposed a range

between 30% and 35% and Madejon et al. (2002) between 32%

and 36%. According to Mohee et al. (1998), the FAS must be

between 30% and 60%. They specified that to ensure a

favourable FAS in maturation, an initial FAS of 60% is

desirable.

The measurements performed during test A showed that

the FAS varied between 61% and 70% (Table 5). During this

test, the FAS remained in excess of 30%, so it was never

limiting to the biological processes according to the literature

data. The FAS determination at the beginning and at the end

of respirometry, and also at each mixing, did not reveal a clear

profile for this test. The variation in FAS remained within the

precision of the method.

The FAS measured on test B (Table 5) showed that this

parameter decreased between each mixing. After mixing

the matter, FAS was higher than before, which confirmed

the beneficial effect of this treatment to obtain the complete

biodegradation of samples. As for test A, the FAS measured

during test B was never limiting to the biodegradation (430%).

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Table 6 – Total oxygen consumed and oxygen uptakeafter the first mixing for tests A and B

Experiment O2 uptake,mmol

[O2] kg�1[initialdry matter]

O2 uptake afterthe first

mixing, %

Test A (agrofood sludge and pine bark chips)

Cell 3 12 643 21.1

Cell 2 12 635 20.1

Cell 1 12 975 19.1


B0 4767 o 1

B21 1849 o 1


For test C, the FAS variation during respirometry differed

from the variation in tests A and B. The initial FAS of HW

(41%) was not limiting to biodegradation but was lower than

the one for the substrates of the previous tests, which

contained a bulking agent. Before the first mixing, the

FAS became limiting (24%) and remained at the same value

after mixing (23%). Therefore, the pycnometer makes it

possible to highlight the fact that material mixing had no

effect on the FAS and to adapt the experimental conditions to

the type of the wastes. For the second mixing, pine bark chips

were added to the HW in order to modify the material

structure and increase its FAS up to a more favourable value

(58%). Until the end of respirometry, the FAS of this new

mixture (HW and bulking agent) remained favourable to

biodegradation.

3.3.4. Stability and homogeneity of moistureMoisture control was difficult as its stability throughout the

trials was not effective (Table 4). Moreover, the development

of moisture variability throughout the substrate was observed

during respirometric tests.

The initial moisture for test A was equal to 49.7% and the

relative variations were from �11.6% to +16.9% between the

beginning and end of respirometry. The global moisture

content after 1194 and 1316 h of respirometry (respectively,

before the third mixing and at the end of the experiment)

was not limiting for biodegradation with values of 55.2%

and 58.2%. Nevertheless, at 691 and 1050 h (respectively,

before mixing 1 and 2), global moisture contents were very

low (44% and 45.4%) and moisture heterogeneities were

observed. Therefore, it could be assumed that local zones in

the material with a limiting moisture content for biodegrada-

tion (o45%) appeared. As the FAS was not limiting during

this test, the effect of mixing was mainly to rehumidify

zones with limiting conditions of moisture. Thus, the

biodegradation could restart, mainly after 691 and 1050 h of

respirometry.

The material moisture was more stable for test B with

variations of +8.9% and +7.1% after 713 h of respirometry. The

initial moisture was higher than the one of test A (about 60%

for mixtures B0 and B21 against 49.7% for mixture A) and the

moisture remained high with 61.9% and 66.2% at the end of

respirometry. Thus, instead of the development of moisture

heterogeneity, the material probably remained with favour-

able conditions of moisture throughout the substrate. Similar

results were obtained by Adani et al. (2001). During their

respirometric tests, the material mixings and the addition of

water implied an increase in biological activity. The authors

concluded that these operations helped to restore favourable

conditions for biodegradation.

The variation in moisture during test C was equal to +21.3%

until the second mixing at 1000 h (no moisture measurement

for the first mixing at 676 h) and equal to +5.8% during the

study of the HW and bulking agent mixture from 1000 h to the

end of respirometry. At the second mixing, the high moisture

(73%) could induce a limitation of biodegradation by reducing

the matter aeration potential. The addition of bulking agent

at this moment decreased the moisture content to 62% and

increased the FAS up to 58%, restoring favourable environ-

ment for biodegradation.

4. Discussion

The presence and absence of an rO2 peak after material

mixing could be explained by differences in waste biodegrad-

ability and/or by the establishment of limiting environmental

conditions (temperature, moisture and aeration). The study of

matter temperature proved that it could not explain the rO2

profiles after mixing because it was stable and homogeneous

throughout all tests.

During test A, the matter mixings induced peaks of rO2.

These peaks decreased with the number of mixings. There-

fore, the modifications realised on the respirometer do not

suppress the restart of oxygen uptake after matter mixing.

The oxygen consumed after the first mixing represented 20%

of the total oxygen consumed (including the endogenous

respiration) against 23–37% for the original system (Table 6).

The restarts were probably not due to an aeration problem as

the RTD experiments proved that the gas phase was perfectly

mixed. The RTD experiments were conducted on separate

trials but the conclusion remained valid in this case because

samples were similar mixtures of sludge and pine bark chips

and because their values for FAS were similar. Moreover, the

FAS of mixture A was favourable to biodegradation through-

out respirometric experiments (values between 61% and 70%).

In contrast, the variations in global moisture content ranged

between �11.6% to +16.9% and the substrate moisture was

sometimes quite limiting to biodegradation (44% and 45.4%,

respectively, before the first and second mixings). These low

moisture contents associated with the development of

moisture heterogeneity probably led to local zones with

limiting conditions of moisture. Therefore, the two first

mixings should rehumidify these zones that stimulated the

restart of biodegradation processes. After the third and last

mixings during test A (1194 h), no peak was observed under

favourable conditions of moisture, temperature and aeration.

Thus it was assumed that all biodegradable organic matter

was consumed.

The material mixings realised during test B did not cause a

restart of oxygen uptake whereas the development of

moisture variability throughout the substrate was observed.

The FAS was not limiting, between 40% and 64%, and the

ARTICLE IN PRESS


global moisture content remained favourable (57–66%). There-

fore, there are 2 possible reasons to explain this absence of

peak after mixing. The first one was that the variability in

moisture was not sufficient to create local zones with limiting

moisture. Thus, the high initial moisture of samples B0 and

B21, in comparison to the one of mixture A (60% versus 49.7%),

helped to retain favourable moisture content to biodegrada-

tion anywhere in the sample. The second reason to explain

the absence of peak, for test B, was the difference of

biodegradability between samples of tests A and B. The test

A took 1300 h against 761 h for test B because of the lower

biodegradability of samples B0 and B21 (4767 and 1849 mmol

[O2 consumed] kg�1[initial dry matter] for B0 and B21 against

at mean 12 751 mmol [O2 consumed] kg�1[initial dry matter]

for mixture A—Table 6). Indeed, the time of respirometry for

test A helped to increase the development of moisture

heterogeneity. So the effect of mixing seemed more impor-

tant for samples with high quantity of biodegradable organic

matter because they need a longer time of respirometry,

which helped the development of moisture heterogeneity and

limitation. In spite of the respirometer modifications, the

material moisture remained heterogeneous that implied the

necessity of material mixings. The influence of this operation

on the respirometric profile depends on the nature of the

tested sample.

During test C, the first mixing on the HW did not imply an

oxygen uptake restart whereas a peak was observed after the

second mixing with addition of pine bark chips. Indeed, the

absence of peak after the first mixing was due to a limitation

of matter aeration caused by a low value of FAS (24–23%) and a

high moisture content (4 60%). This was confirmed by the

restart after the addition of bulking agent, which increased

the FAS (58%) and reduced the moisture content (62%). This

restart proved the presence of residual biodegradable organic

matter and confirmed the interest to follow moisture content

and FAS during respirometry to quantify the substrate

biodegradability.

5. Conclusion

The pycnometer developed to measure the free air space (FAS)

during respirometry appeared to be a reliable tool. The FAS

obtained for water and spherical glass beads were close to

theoretical values instead of the variation in cell volume due

to the compression. It allowed in situ measurements without

perturbing biodegradation.

The respirometric apparatus was validated in terms of

temperature and aeration. The temperature was stable and

homogeneous during all trials. The measurement of the FAS

at the beginning and end of respirometry and at each mixing

allowed to demonstrated a possible limitation of biodegrada-

tion due to this parameter.

The control of moisture could not be validated as in most

tests the stability and homogeneity of this parameter was not

effective. The development, in course of respirometry, of

zones with limiting moisture content could lead to a restart of

oxygen consumption after material turning. In similar con-

ditions of FAS, the effects of mixing seemed to be more

important for matrix with a large quantity of slowly

biodegradable organic matter and with a low initial moisture.

In the case of high moisture content, a low FAS could imply an

absence of biodegradation whereas all biodegradable organic

matter was not consumed. Indeed, a material with a low FAS

was difficult to aerate, and bulking agents were necessary.

In the objective to quantify the whole biodegradable organic

matter of wastes, the application of mixings until they had no

effect on oxygen uptake rate seemed necessary, mainly to

rehumidify local zones with low or limiting moisture con-

tents, which potentially appeared in material during respiro-

metry. Moreover, the added control of the FAS at each mixing

was essential and demonstrated that the absence of oxygen

uptake restart was effectively due to the absence of biode-

gradable organic matter and not due to aeration problems.

The new device, associating respirometric measurements in

controlled environment and in situ measurements of the FAS,

was perfectly adapted to the study of the biodegradability of

organic waste of various origins and of different degrees of

evolution.

Acknowledgements

This study was conducted within the frame of a PhD project

funded by the ADEME (French Agency for Environment and

Energy Management) and the Cemagref laboratories (Agricul-

tural and Environmental Engineering Research Institute).

R E F E R E N C E S

Adani F; Confalonieri R; Tambone F (2004). Dynamic respirationindex as a descriptor of the biological stability of organicwastes. Journal of Environmental Quality, 33(5), 1866–1876

Adani F; Lozzi P; Genevini P (2001). Determination of biologicalstability by oxygen uptake on municipal solid waste andderived products. Compost Science and Utilization, 9(2),163–178

Adani F; Ubbiali C; Generini P (2006). The determination ofbiological stability of composts using the dynamic respirationindex: the results of experience after two years. WasteManagement, 26(1), 41–48

Adani F; Ubbiali C; Tambone F; Centemero M; Genevini P L (2002).Static and dynamic respirometric indexes—Italian researchand studies. The biological treatment of biodegradable was-te—Technical aspects. Conference in Brussels, Belgium

ADEME (2006). Dechets: chiffres cles [Wastes: main numbers] -French Agency for Environment and Energy Management;/http://www2.ademe.frS

Agnew J M; Leonard J J; Feddes J; Feng Y (2003). A modified airpycnometer for compost air volume and density determination.Canadian Biosystems Engineering, 45, 627–635

Aguilar-Juarez O (2000). Analysis et modelisation des reactionsbiologiques aerobies au cours de la phase d’exploitation d’uncasier d’un centre d’enfouissement technique. [Analyse andmodelling of the aerobic biological reactions during the fillingof a landfill cell.] PhD Thesis, Institut National des SciencesAppliquees de Toulouse, France

Epstein E (1997). The Science of Composting. CRC Press, BocaRaton, FL, USA

Gea T; Barrena R; Artola A; Sanchez A (2004). Monitoring thebiological activity of the composting process: oxygen uptakerate (OUR), respirometric index (RI), and respiratory quotient(RQ). Wiley Interscience

http://www2.ademe.fr

ARTICLE IN PRESS


Gomez R B; Lima F V; Ferrer A S (2006). The use of respirationindices in the composting process: a review. Waste Manage-ment & Research, 24(1), 37–47

Haug R T (1993). The Practical Handbook of CompostingEngineering. Lewis Publishers, Boca Raton, FL, USA

Ianotti D A; Grebus M E; Toth B L; Madden LV; Hoitink H A J (1994).Oxygen respirometry to assess stability and maturity ofcomposted municipal solid waste. Journal of EnvironmentalQuality, 23, 1177–1183

Keener H; Ekinci K; Elwell D; Michel F J (2002). Principles ofcomposting process optimization. International Symposiumon Composting and Compost Utilization. Colombus, OH, USA

Madejon E; Diaz M J; Lopez R; Cabrera F (2002). New approachesto establish optimum moisture content for compostablematerials. Bioresource Technology, 85(1), 73–78

Mohee R; White R K; Das K C (1998). Simulation model forcomposting cellulosic (Bagasse) substrates. Compost Scienceand Utilization, 6(2), 82–92

Moletta R (2006). Gestion des problemes environnementaux dans lesindustries agroalimentaires. [Mangement of environmental pro-blems in agrofood industries] Edition Tec&Doc, Lavoisier, France

Oppenheimer J R; Martin A G; Walker L P (1997). Measurementsof air-filled porosity in unsaturated organic matricesusing a pycnometer. Bioresource Technology, 59(2–3),241–247

Paletski W T; Young J C (1995). Stability measurement of biosolidscompost by aerobic respirometry. Compost Science andUtilization, 3(2), 16–24

Roustan M (2003). Transferts gaz–liquide dans les procedes detraitement des eaux et des effluents gazeux. [Gas–liquidtransfers in water and gas treatment processes.] EditionTec&Doc, Lavoisier, Paris

Tremier A; de Guardia A; Massiani C; Martel J L (2005a). Influenceof the airflow rate on heat and mass transfers during sewagesludge and bulking agent composting. Environmental Tech-nology, 26(10), 1137–1149

Tremier A; de Guardia A; Massiani C; Paul E; Martel J L (2005b).A respirometric method for characterising the organic com-position and biodegradation kinetics and the temperatureinfluence on the biodegradation kinetics, for a mixture ofsludge and bulking agent to be co-composted. BioresourceTechnology, 96(2), 169–180

coupling a respirometer and a pycnometer, to study the biodegradability of solid organic wastes...

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