Kinetic analysis of forced aeration composting –I. Reaction rates and temperature

Kinetic analysis of forced aeration composting ±I. Reaction rates and temperature

The kinetics of the forced aeration composting process

operated under different aeration modes was studied using

two speci®cally designed heat insulated closed pilot-scale

reactors. Five pilot-scale composting tests using fresh mixture

of organic solid waste were performed. The modes of aeration

applied were up¯ow, down¯ow, alternate up¯ow/down¯ow,

and internal air recirculation. Temperatures at different

heights of the composting mass and air velocity were

continuously monitored. Air ¯ow was continuous or

intermittent depending on temperature. Kinetic analysis

showed that (i) temperature dependence of the reaction rates

of all different aeration mode composting tests clearly

followed the Arrhenius equation; (ii) the degradation of

organic solids could be quantitatively predicted using the ®rst

order reaction model; and (iii) extent of degradation in the

composting mass could be predicted on the basis of outlet air

temperature instead of internal temperature of the

composting mass.

Quazi H. BariAlbert KoenigTao GuiheDepartment of Civil Engineering, The University of Hong

Kong

Keywords ± Aeration mode; composting; degradation;

organic solid waste; reaction rate; temperature

Corresponding author: Albert Koenig, Department of Civil

Engineering, The University of Hong Kong, Pokfulam Road,

Hong Kong (E-mail: [email protected])

Received 21 May 1999, accepted in revised form 12 December

1999

Introduction

Composting is an important element of sustainable solid

waste management. The popularity of composting has

increased in the past decade on account of the (i) potential

recovery of organic waste material in the form of compost for

utilization in agriculture or other applications (e.g. as organic

fertilizer or soil conditioner); (ii) effective inactivation of

pathogenic bacteria present in the organic waste (Obeng &

Wright 1987); and (iii) stabilization and volume reduction of

the waste materials prior to environmentally sound ®nal

disposal in land®lls. For ef®cient application of composting

in a closed controlled system it is necessary to know the

reaction rate at which biological degradation occurs in the

organic waste, and the extent of degradation achieved. In

composting systems, the oxygen consumption rate or carbon

dioxide evolution rate are commonly used to indicate the

degradation rate (reaction rate) of organic waste (Schulze

1960; Bach et al. 1985). Generally, the effect of temperature

on the reaction rate in chemical reactions can be estimated

from the Arrhenius equation (Haug 1993). Biological

reactions exhibit a similar tendency except that the reaction

rate at very high temperatures becomes unstable and

decreases rapidly, because enzymes in biological cells

denature at such temperatures (Larry & Clifford 1980).

Measured temperatures in composting plants usually vary

according to aeration mode as well as location of measure-

ment points in the composting mass (Finstein et al. 1986;

Hogan et al. 1989; Koenig & Bari 1998). Therefore the

objective of this study was (i) to determine biological

degradation rates (reaction rates) of organic waste and their

dependence on temperature during different modes of forced

Waste Manage Res 2000: 18: 303±312

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Copyright # ISWA 2000

Waste Management & ResearchISSN 0734±242X

303

aeration composting processes; and (ii) to predict the extent

of biological degradation by applying a temperature based

reaction model.

Materials and methods

Source, type and preparation of wastesFor the composting tests, organic solid wastes from the

University of Hong Kong were used. These consisted of two

types of waste, namely food waste (from the student canteen)

and mixed of®ce waste paper. In addition, saw dust from the

University Industrial Centre was used as a bulking agent.

The few non-compostable and oversized waste components

like plastic trays, aluminium cans, etc. were manually

removed from the food waste prior to use. The mixed waste

paper was shredded to a suitable particle size (1 to 1.5 cm) by

a cutting machine. Then the presorted food waste and

shredded waste paper were mixed and homogenized in a large

concrete mixing machine. The moisture content (MC) of

the initial waste mixtures was adjusted to about 50 to 60%

for optimum consistency and porosity by adding sawdust.

The mean initial MC of the raw presorted food waste,

wastepaper, and saw dust was 71.7, 8.5 and 8.3%,

respectively. The initial waste mixtures of all tests contained

approximately 75.0, 12.5 and 12.5% by wet weight of food

waste, wastepaper and sawdust, respectively.

Pilot-scale reactorTwo specially designed heat insulated closed reactors were

used in this research. Each reactor has a total volume of

about 200 l, with a height of 2.2 m. The reactors are

insulated with a 10-cm thick layer of polyurethane on all

sides. The waste mixture is put into the reactor from the top,

which can be easily opened. The thermocouples for

measurement of temperature during the composting tests

are placed inside the reactor at different heights of the

composting mass: at 10, 30, 50, 70, 90, and 110 cm

respectively, starting from the bottom. Forced aeration is

provided by an air blower, with an option for continuous or

intermittent operation. After leaving the reactor, the hot

spent air cools down naturally and the condensate is

collected. Finally, prior to discharge to the atmosphere, the

spent air is ®ltered through a bio®lter (cross-sectional area

900 cm2), which is made up of a lower layer (20 cm) of

gravel and an upper layer (90 cm) of mature compost of

optimum (MC). Leachate is collected at the bottom of the

reactor column in an intermediate storage tank and

intermittently recycled to the top of the column. The

pipes for air recirculation are insulated with a 5-cm thick

layer of Fanyalon to prevent heat loss and minimize

condensation. Internal air recirculation can be varied

according to the air recirculation ratio (r) which is de®ned

as the ratio of recirculated air over fresh air input, in per

cent. A schematic diagram of an experimental pilot-scale

composting reactor is shown in Fig. 1.

Experimental programme and mode of aerationA number of extensive pilot-scale composting tests were

conducted with similar mixtures of organic solid wastes. The

modes of aeration studied were up¯ow, down¯ow, up¯ow/

down¯ow and internal recirculation. Each pilot-scale

composting test was performed in two stages of 20 to

30 days duration each. The ®rst stage composting tests were

conducted using fresh organic solid waste mixture, whereas

the second stage composting tests were conducted using fresh

compost produced by the corresponding ®rst stage compost-

ing tests. In this paper on kinetic analysis, only the results of

®rst stage composting tests are used; the effects of aeration

and air recirculation on ®rst and second stage composting

tests were presented elsewhere (Koenig & Bari 1998). The

aeration modes and duration of the different ®rst stage

composting tests are summarized in Table 1.

To achieve desired temperature and oxygen levels during

active composting, variation of aeration rate is considered as

one of the key process control methods. Based on previous

composting tests by Koenig & Tao (1996), using the same

pilot-scale reactor and similar organic waste, the range of air

Fig. 1. Schematic diagram of an experimental pilot scale composting reactor.

Q. H. Bari, A. Koenig, T. Guihe

304 Waste Management & Research

¯ow rate was set at 17 to 20 l min±1 (equivalent to about

0.67 l min±1 per kg initial VS) during the ®rst 2 days. This

was followed by an air ¯ow rate of 8 to 10 l min±1 according

to the following rule: (i) if the temperature at any measuring

point in the composting mass exceeded 608C, continuous

aeration; (ii) between 558C and 608C, intermittent aeration

according to a preset cycle of 5 min aeration and 5 min

pause; and (iii) for temperatures in the compost mass below

558C, intermittent aeration according to a preset cycle of

5 min aeration followed by 10 min pause.

No replicate tests were performed because of (i) the large

scale at which the experiments were carried out, with full

practical height of the composting mass as in actual

composting plants (ii) the long duration of each experiment,

and (iii) the need to test a wide range of different aeration

modes. Besides, to establish the dependence of reaction rate

on temperature in-situ, suf®cient data points were obtained

from each individual composting test.

Physico-chemical monitoring and analysesTemperatures at different heights were continuously mon-

itored by means of the thermocouples which were connected

to a datalogger. For determination of air¯ow rates, air

velocity was continuously monitored by datalogger through

an air velocity transmitter. Oxygen content in the spent air

was regularly measured by means of an oxygen meter (MSI

Elektronik GmbH type MSI 150, 5803 Hagen, Germany).

Daily leachate and condensate production as well as height

reduction of composting mass in the reactor were manually

monitored. At the beginning and end of each composting

test, the initial waste mixture (food waste and waste paper)

and/or compost was analysed with regard to the most

relevant physico-chemical parameters such as MC in

percentage of wet weight; total solids (TS); and, volatile

solids (VS), in percentage of TS. All analyses were carried

out according to standard methods for analysis of compost

(Austrian Standard 1986). In addition, the self-heating test

(LAGA 1985) was applied to determine the degree of

biological stability and remaining biodegradable volatile

solids (BVS) of the waste after composting.

Results of composting tests

Changes in physico-chemical parametersChanges in total mass, MC and VS during pilot-scale

composting tests are presented in Table 2. The range of

initial weight of waste mixture put inside the reactor for the

pilot-scale tests was 55 to 77 kg. Initial MC of the waste

mixtures varied between 53 and 60%, thus staying within the

recommended range (Golueke 1977; Hay & Kuchenrither

1990). The ®nal mean MC of the tests ranged from 31 to

53%. In all waste mixtures, VS amounted to more than 94%

of the TS; however, the VS originating from paper and

sawdust were considered less readily biodegradable on

account of their high cellulosic content. Depending on

the amount of initial waste mixture put in the reactor, the

range of VS reduction was 6 to 12 kg after different durations

of composting.

Temperature and cumulative oxygen consumptionThe mean temperature variations during the composting

tests are shown in Fig. 2. The mean temperatures were

determined as the mean of the six measuring points in the

composting mass at a given time. One day of lag period is

observed in all the temperature curves representing the time

required for the organisms to acclimate with the new

environment. The oxygen consumption rates of different

composting tests are shown in Fig. 3; they were computed on

the basis of air ¯ow rate and measured oxygen concentration

in the spent air. After 12 to 20 days, the O2 consumption

rate had decreased to a very low, relatively stable value in all

tests, indicating the completion of the ®rst stage of

composting.

Extent of degradationThe total initial mass of BVS for different pilot-scale tests

was estimated from the sum of the VS degraded in the ®rst

stage, second stage and a further self-heating test (LAGA

1985; Koenig & Bari 1998) after the second stage. The sum

of the degraded VS was increased by 10% to obtain a suitable

estimate of initial BVS. The value of 10% takes into account

the fact that the degradation process is not completed even

after reaching biological stability. In the absence of any

other reliable information, 10% was selected as a reasonable,

but conservative value. The estimated mass of initial BVS

and BVS degraded is presented in Table 3. The BVS

reduction in different tests varied from 52 to 75% for

different durations of composting. However, when the

degradation was estimated and compared for an equal

Table 1. Experimental programme of ®rst stage composting tests

Test Aeration mode Duration (days)

1 Up¯ow (Koenig & Tao 1996) 41

2 Down¯ow 24

3 Up¯ow, internal recirculation (r 5 20 to 50%) 31

4 Up¯ow, internal recirculation (r 5 100 to 200%) 29

5 Up¯ow/down¯ow, alternately every 3 to 4 days 23

Kinetic analysis of forced aeration composting ± I

Waste Management & Research 305

duration of 20 days for all the tests (calculation based on the

cumulative O2 consumption curves of Fig. 3), the BVS

degradation ranged from 47 to 59%. It could be demon-

strated that up¯ow aeration with internal air recirculation

and alternate up¯ow/down¯ow aeration with optional

addition of water accelerated the rate of composting and

thereby increased the extent of degradation when compared

to unidirectional up¯ow or down¯ow aeration. In Table 3

the ratio of kg oxygen consumption to kg BVS degradation of

different tests is shown. The ratio varied between 1.22 and

1.33, which was similar to the chemical oxygen demand

(COD) range of 1.30 to 1.35 of the waste mixture used for

this experiment. For comparison, the calculated oxygen

demand of food waste was 1.34 kg O2 kg±1 of substrate of

chemical composition C18H26O10N as proposed by Kayha-

nian & Tchobanoglous (1993).

Kinetic analysis and application

Reaction ratesIn order to describe the changes that occur in the waste

material during composting, and the extent of these changes,

it is important to know the reaction rates or degradation

rates. The reaction rates are signi®cant for designing or

optimizing a composting process because they directly affect

the processing time, size of the reactor or pile area and the

product quality. A high degradation rate usually indicates

lower capital and operational cost for a composting plant.

Finstein & Miller (1985) noted that, for any given

processing duration, the higher the rate the more stable

and easily handled the residue and this facilitates storage,

transport, and ®nal disposal with a minimal cost.

The general equation to de®ne the rate of reaction in

biological degradation processes such as composting is,

r~{dBVS

dt~kBVSa

t �1a�

k~r

BVSat

�1b�

where r 5 overall rate of reaction, in kg d±1

t5 time, in days

BVS5 biodegradable volatile solids, in kg

BVSt5 remaining biodegradable volatile solids after time t,

in kg

{ dBVSdt 5rate of change in mass of BVS (the negative sign

indicates that the quantity of BVS decreases with time due

to degradation)

a 5 exponent to de®ne reaction order

k 5 rate constant or reaction rate constant.

For waste treatment processes, zero order (a 5 0), ®rst

Table 2. Changes in total mass, moisture content (MC) and volatile solids (VS) during pilot-scale composting tests

Test Initialmass (kg)

Finalmass (kg)

Initial(%)

FinalMC (%)

InitialVS (%)

FinalVS (%)

kg VSdegraded

1 71.60 42.50 55.61 53.54 94.20 92.28 11.71

2 60.33 33.12 55.83 38.95 96.09 95.30 6.33

3 77.20 42.26 60.10 43.61 96.70 95.28 11.03

4 63.74 32.28 58.93 31.01 95.17 94.74 6.34

5 55.41 39.09 53.41 52.49 94.25 92.37 7.16

Fig. 2. Mean temperature over time for different composting tests.

kg O

2d_

1

Fig. 3. Oxygen consumption rates for different composting tests.



order (a 5 1), or second order (a 5 2) reaction rates are

commonly applied. The rate of biological reactions increases

with temperature within a limited range, suitable for

microorganisms. Above this range of temperature the

activity of enzymes, responsible for mediating the biological

reaction, decreases as a result of enzyme denaturation. The

in¯uence of temperature on the reaction rate constants can

be described by the Arrhenius equation:

d�ln k�dT

~Ea

RT2�2�

or

ln k~C{Ea

R

1

T

� ��3�

or

k~eC{EaR

1T� �~Ae{Ea

R1T� � �4�

where:

Ea 5 activation energy, kJ mol±1 (assumed to be

temperature ± independent)

R 5 ideal gas constant, 8.314 3 10±3 kJ mol±1

T 5 absolute temperature, 8KC 5 a constant

A 5 eC 5 frequency factor, also called van't Hoff ±

Arrhenius coef®cient with unit depending on order of

reaction

In this study, the reaction rate constants k8 (zero order),

in kg d±1, k' (®rst order), in d±1, and k0 (second order), in

kg±1 d±1, were estimated according to Equation 1b where

overall rate of reaction r is equal to daily O2 consumption

rates divided by a factor of 1.27 which is the mass ratio of

oxygen consumed per unit of BVS degraded (see Table 3).

Figure 4 shows the typical Arrhenius plot of ln(k) versus 1/T

for test four, yielding the activation energy, which can be

used to calculate reaction rate constants at intermediate

temperatures. Although, as shown in Table 4, the best

correlation with the actual data was found for the ®rst order

reaction model (mean R2 5 0.65), the second order reaction

model (mean R2 5 0.58) also exhibits a high correlation for

most composting tests. The results of this study thus appear

to con®rm the common application of ®rst order reaction

models to the composting process. Table 5 summarizes for

each test the activation energy Ea, the ®rst order reaction

rate constants at 25 and 508C, and the temperature

coef®cient Q10 (de®ned as kT+10/kT) as derived from the

Arrhenius plot. The mean value of the ®rst order reaction

rate constants was 0.008 d±1 at 258C and 0.040 d±1 at 508C,

while Q10 had a mean value of 1.94, which compares well

with Schulze (1960). The activation energy Ea varied from

44 to 66 kJ mole±1 with an mean of 53.47 kJ mole±1.

Comparative values of 92.05 kJ mole±1 were reported for

thermophilic bacteria at 50 to 708C (Nakasaki et al. 1985)

and of 58 kJ mole±1 for Aerobacter aerogenes at 358C(McKinley & Vestal 1984). For wastewater treatment

processes, Ea values from 8.4 to 84 kJ mole±1 have been

reported (Metcalf 1979).

It has been suggested that the reaction rate constant

decreases towards the completion of the composting process

as the remaining substrate becomes increasingly dif®cult to

Table 3. Biodegradable volatile solids (BVS) degradation and associated oxygen consumption

Test Initial BVS(kg)

DBVS(kg)

DBVS(%)

DBVS(% in 20 days)

O2 used(kg)

kg O2/kgDBVS

1 15.58 11.72 75.22 47.00 14.35 1.22

2 12.10 6.33 52.31 49.21 8.51 1.33

3 16.87 11.03 65.38 58.91 14.12 1.28

4 11.35 6.34 55.86 50.49 8.31 1.31

5 11.66 7.16 61.41 57.66 8.85 1.22

Mean ± ± ± ± ± 1.27

DBVS 5 BVS degraded.

Fig. 4. Arrhenius plot for test four where k is reaction rate constant.



degrade because of its more refractory, ligno-cellulosic

nature. The second order reaction model would take such

effect into account: if the ®rst order reaction rate constant k'is replaced by k'' BVS, indicating a decreasing reaction rate

constant proportional to the remaining BVS, then the ®rst

order reaction model (Equation A6 of Appendix 1) is

transformed into a second order reaction model (Equation

A11 of Appendix 1). Assuming the validity of such non-

uniform degradability in the composting tests of this study,

the high correlation with the actual data for the second order

model could be thus explained. It would be interesting to

investigate further whether the BVS degradation in forced

aeration composting actually follows a ®rst or second order

reaction model, or an intermediate reaction model with a

fractional exponent.

Prediction of BVS degradation from mean temperatureUsing the recorded mean temperature, the temperature-

dependent reaction rate constant according to the Arrhenius

equation (as shown in Fig. 4 for test four), and the

determined initial BVS value, the percentage BVS degrada-

tion could be calculated for each test according to the three

different reaction orders. The mathematical expressions (see

detailed derivation in Appendix 1) for predicting the

percentage BVS degradation were for,

zero order reaction rate (see Equation 5 of Appendix 1),

DBVSn

BVS0%~

100�k01zk0

2z . . . k0n�Dt

BVS0�5�

®rst order reaction rate (see Equation 10 of Appendix 1),

DBVSn

BVS0%~100{100�1{k'1Dt�

| �1{k'2Dt� . . . �1{k'nDt� �6�second order reaction rate (see Equation 15 of Appendix 1),

DBVSn

BVS0%~

�k''1zk''2zk''3z . . . k''n�BVS0Dt

1z�k''1zk''2zk''3z . . . k''n�BVS0Dt|100

�7�where:

DBVSn 5 (BVS0 ± BVSt) 5 amount of BVS degraded at

time n.Dt in kg

BVS0 5 amount of initial BVS in kg

kn 5 reaction rate constant (k8n for zero order, k'n for ®rst

order and kn2 for second order) at the mean absolute

temperature at time interval n.Dt

Dt 5 time interval, d (taken as one day in this study)

n 5 numbered sequence of time intervals, from 1 to n

Figure 5 shows an example of the predicted percentage BVS

degradation for test four using the different reaction orders.

The predicted degradation curves from the three different

reaction orders compared well with the actual BVS

degradation curve, almost independent of reaction order;

however, the ®rst order reaction model and second order

reaction model produced the best ®t with the actual data (see

Table 4. Correlation coef®cients R2 for different order reaction rate models

Test R2

zero order ®rst order second order

1 0.336 0.450 0.360

2 0.448 0.610 0.612

3 0.744 0.778 0.474

4 0.737 0.844 0.799

5 0.296 0.562 0.636

Mean 0.512 0.649 0.576

Table 5. First order reaction rates k' and temperature coef®cient Q10 according to Arrhenius equation for each test

Test A 5 eC

(a constant)Ea

kJ mole±1k' at 258Cd±1

k' at 508Cd±1

Q10

1 108.986 66.23 0.002 0.018 2.41

2 105.615 43.53 0.009 0.037 1.67

3 107.843 56.91 0.007 0.044 1.97

4 107.700 55.60 0.009 0.051 1.94

5 105.998 45.07 0.013 0.051 1.71

Mean 107.209 53.47 0.008 0.040 1.94



also Table 4). It would appear that in composting the effect

of temperature on BVS degradation is much more

pronounced than the effect of reaction order.

Prediction of BVS degradation at constant temperatureUsing ®rst order reaction rate constants from Fig. 4, the BVS

degradation at an assumed constant temperature of 308C(5 3038K) could be calculated for test four using the

following equations:

k'~e 17:73{6688:5303� �~0:01298 in d{1 �8�

and

DBVSn

BVS0%~100 1{e{k't

ÿ �~100 1{e{0:01298t

ÿ � �9�

Using similar equations as Equation 9, the BVS degradation

for assumed constant temperatures of 40, 50 and 608C for

test four was also calculated. Figure 6 shows actual and

predicted percentage BVS degradation at different assumed

constant temperatures for the ®rst order reaction model of

test four. It is clearly shown that, after an initial lag period,

for the ®rst 10 days the actual degradation rate of test four

corresponded to a temperature of about 608C while actually

the mean temperature was above 508C for that period as

shown in Fig. 2. After 10 days the actual degradation rate

became slower as temperature in the composting mass fell

towards 308C.

Prediction of BVS degradation from outlet temperatureAn attempt was made to predict BVS degradation using

outlet air temperature instead of mean temperature within

the composting mass. Figure 7 shows the correlation

between the mean temperature in the composting mass

and the outlet temperature of test four, where up¯ow

aeration plus internal air recirculation was applied. It was

found that the mean temperature was 0.90 times the outlet

temperature with a correlation coef®cient R2 5 0.757. This

relationship was used to convert the measured outlet air

temperature of test three to the mean internal temperature

which in turn served as input temperature for the ®rst order

reaction model of Equation 6. Figure 8 shows the excellent

agreement between the BVS degradation predicted from the

outlet temperature and the actual data, also when compared

to the predicted BVS degradation using the measured mean

temperature in the composting mass. Another relationship

between mean temperature in the composting mass and the

outlet air temperature of tests two and three is shown in

Fig. 9. Down¯ow and up¯ow (r 5 20 to 50%) aeration mode

were applied in tests two and three, respectively. In this case

the mean temperature was 0.92 times the outlet temperature

with a correlation coef®cient R2 5 0.851. Using this

relationship, a prediction of percentage BVS degradation

Fig. 5. Actual and predicted percentage BVS degradation for test four usingdifferent reaction orders.

Fig. 6. Actual and predicted percentage BVS degradation for test four using®rst order reaction at different constant temperatures.

Fig. 7. Relationship between mean temperature in the composting mass andoutlet temperature of test four where up¯ow aeration was applied (r 5 100 to200%).



was made for test ®ve where alternate up¯ow and down¯ow

aeration mode was applied. In Fig. 10, the predicted BVS

degradations using outlet and mean temperature of test ®ve

are compared to the actual degradation with good agreement

obtained. Hence the extent of degradation could be

predicted based on the easily measurable outlet air

temperature and using the derived relationships, instead of

using the mean temperature calculated from the temperature

measured at different points throughout the height of the

composting mass.

Engineering signi®cance

The results of this study support, con®rm, and amplify some

actual operating practices in forced aeration composting

plants. The linear relationship between outlet air tempera-

ture and the mean internal temperature of the composting

mass, independent of air¯ow direction, strongly suggests that

the temperature of the outlet air will be the most important

process control parameter for monitoring and quantitatively

evaluating the progress of overall degradation of organic

waste. Since it was shown that the reaction rates and their

dependence on temperature could be established in-situ

during the composting process, application of the simple

reaction rate model in conjunction with the Arrhenius

relationship provides an effective means for quantitative

estimation of BVS degradation. In well operated composting

plants, the ®nal drop in outlet air temperature indicates

therefore a decrease in degradation rate and the composting

process (i.e. the ®rst stage of composting in this study) should

be terminated as further aeration will not substantially

increase the extent of degradation. This operational practice

may lead to savings in required composting area and energy

consumption for aeration. However, since unidirectional

aeration modes lead to signi®cant temperature gradients

within the composting mass and hence to non-uniform

degradation rates, some of the organic waste will only be

partially degraded. In the second stage of composting this

remaining organic waste matter will then be further

degraded. If in each composting stage about 70% of the

initial biodegradable organic matter are removed as is

strongly suggested by this study, then at the end of the

second stage about 9% of the initial BVS would remain.

Incidentally, this tends to support the assumption of adding

10% to the sum of total degraded VS for an initial BVS

estimate as mentioned earlier. For the second stage of

composting, similar relationships as in ®rst stage composting

were found between outlet air temperature and mean

internal temperature of the composting mass as well as for

temperature dependence of reaction rate. In second stage

composting, as is practised in many composting plants, the

Fig. 8. Comparison of actual and predicted BVS degradation of test threewhere up¯ow aeration was applied (r 5 20 to 50%).

Fig. 9. Relationship between mean temperature in the composting mass andthe outlet temperature for up¯ow and down¯ow aeration modes of tests threeand two.

Fig. 10. Comparison of actual and predicted BVS degradation of test ®vewhere alternate up¯ow and down¯ow aeration mode was applied.



outlet air temperature will therefore also serve as the most

important process control parameter for monitoring and

quantitatively evaluating the progress of organic waste

degradation.

Conclusions

The following conclusions are drawn:

(1) Temperature dependence of the reaction rates of

different aeration mode composting tests clearly followed the

Arrhenius equation, independent of aeration mode.

(2) First order reaction rates calculated from temperature

and oxygen consumption rates produced the best ®t with

actual data followed by second order and zero reaction rates.

(3) The percentage degradation of organic solids could be

reliably predicted from the ®rst order reaction model.

(4) Using a linear relationship between outlet air

temperature and mean internal temperature of the compost-

ing mass the extent of degradation in the composting mass

could be predicted on the basis of outlet air temperature

alone.

ReferencesAustrian Standard S 2023 Analytical Methods and Quality Control for

Waste Compost (1986) Austrian Standardization Institute, Vienna.

Bach, P. D., Shoda, M. & Kubota, H. (1985) Composting reaction rate of

sewage sludge in an autothermal packed bed reactor. Journal of

Fermentation Technology 63 (3), 271±278.

Finstein, M. S. & Miller, F. C. (1985) Principles of Composting Leading to

Maximization of Decomposition Rate, Odor Control and Cost

Effectiveness. In: Gasser, J.K.R. (ed. ) Composting of Agricultural and

Other Wastes. London, UK: Elsevier Applied Science Publishers,

pp. 13±26.

Finstein, M. S., Miller, F. C. & Storm, P. F. (1986) Waste Treatment

Composting as a Controlled System. Biotechnology: A Comprehensive

Treatise in 8 volumes (eds. H.-J. Rehm and G. Reed), 8, Microbiol

Degradations (volume editor W. SchoÈrborn). VCH Verlagsgesellschaft

MbH, Weinhelm, Germany. pp. 363±398.

Golueke, C. G. (1977) Biological Reclamation of Solid Wastes. Emmaus, PA,

USA: Rodale Press.

Haug, R. T. (1993) The Practical Handbook of Composting Engineering. Boca

Raton, USA: Lewis Publishers.

Hay, J. C. & Kuchenrither, R. D. (1990) Fundamentals and application of

windrow composting. Journal of Environmental Engineering 116 (4),

746±763.

Hogan, J. A., Miller, F. C. & Finstein, M. S. (1989) Physical Modeling of the

Composting Ecosystem. Applied and Environmental Microbiology 55,

1082±1092.

Kayhanian, M. & Tchobanoglous, G. (1993) Innovative two-stage process

for the recovery of energy and compost from the organic fraction of

municipal solid waste (MSW). Water Science and Technology 27 (2),

133±143.

Koenig, A. & Bari, Q. H. (1998) Effect of Air Recirculation on Single-

Reactor and Two-Reactor Composting System. 91st Annual Meeting

and Exhibition, Air and Waste Management Association, June 14±18, San

Diego, California, USA. CD-ROM Proceedings Paper No 98-WP86

01, pp. 15.

Koenig, A. & Tao, G. H. (1996) Accelerated forced aeration composting of

solid waste. Proceedings of the Asia-Paci®c Conference on Sustainable

Energy and Environmental Technology. Singapore: World Scienti®c

Publishing, pp. 450±457.

LAGA (Laenderarbeitsgemeinschaft Abfallbeseitigung) (1985)

Memorandum M 10 Quality criteria and recommendation for

application of compost from solid waste (in German). In: Kumpf, W.

et al. (eds) Muell-Handbuch. Kennzahl 6856, Lieferung 1/85. Berlin,

Germany: Erich Schmidt-Verlag, pp. 1±37.

Larry, D. B. & Clifford, W. R. (1980) Biological Process Design for Wastewater

Treatment. Englewood, NJ, USA: Prentice Hall.

McKinley, V. L. & Vestal, J. R. (1984) Biokinetic analyses of adaptation and

succession: microbial activity in composting municipal sewage sludge.

Applied and Environmental Microbiology 47 (5), 933±941.

Metcalf, & Eddy (1979) Wastewater Engineering Treatment, Disposal and

Reuse (2nd edn). New York, USA: McGraw-Hill, p. 146.

Nakasaki, K., Soda, M. & Kubota, H. (1985) Effect of temperature on

composting of sewage sludge. Applied and Environmental Microbiology

50 (6), 1526±1530.

Obeng, L. A. & Wright, F. W. (1987) The Co-composting of Domestic Solid

and Human Wastes. UNDP Project Management Report Number 7,

World Bank Technical Paper no. 57, The World Bank, Washington,

D. C., USA.

Schulze, K. L. (1960) Rate of oxygen consumption and respiratory quotients

during the aerobic decomposition of a synthetic garbage. Compost

Science, 1, 36±40.

Appendix

A.1 Prediction of BVS degradation for zero orderreaction rateFrom Equation 1a for zero order reaction, substituting a 5 0,

{dBVS

dt~k0BVS0~k0 �Equation A1�

where: ko 5 zero order rate constant, in kg d±1

Integrating Equation A1 above it follows that,

BVSt~BVS0{k0t �Equation A2�

where BVSo5initial BVS at t50



BVSt5remaining BVS after time t

If Ko is not constant over time, Equation A2 can be solved

numerically as

BVSt~BVS0{�k01zk0

2z . . . k0n�Dt �Equation A3�

where: ko1, ko

2, ........ kon5the reaction rate constant for time

intervals Dt1, Dt2, ........ Dtn, respectively, and Dt1, Dt25

........ 5Dtn5Dt

Therefore, the percentage degradation of BVS after any time

is

BVS0{BVSt

BVS0|100~

k0t

BVS0|100 �Equation A4�

or

~�k0

1zk02z . . . k0

n�Dt

BVS0|100 �Equation A5�

where: Dt5time interval, d

n5numbered sequence of time intervals, from 1 to n

A.2 Prediction of BVS degradation for ®rst order reactionrateFrom Equation 1a for ®rst order reaction, subsituting a51,

{dBVS

dt~k'BVS1~k'BVS �Equation A6�

where: k'5®rst order rate constant, in day±1

Integrating Equation A6 it follows that

BVSt~BVS0 e{k't �Equation A7�

If K' is not constant over time, the numerical solution is

BVSt~BVS0.e{k'1Dt.ek'2Dt . . . ek'nDt �Equation A8�or

BVSt~BVS0.e{�k'1zk'2z... k'n�Dt

It can be shown that for small exponents e±k'1Dt is

approximately equal to (1±k'1Dt) or

e{k't~�1{k'1Dt��1{k'2Dt� . . . �1{k'nDt�

Therefore the percentage degradation of BVS after time t is

BVS0{BVSt

BVS0|100~100�1{e{k't� �Equation A9�

or

~100�1{k'1Dt��1{k'2Dt� . . . �1{k'nDt��Equation A10�

A.3 Prediction of BVS degradation for second orderreaction rateFrom Equation 1a for second order reaction, substituting

a52,

{dBVS

dtk''BVS2 �Equation A11�

where: k''5second order rate constant, in kg±1.day±1

Integrating Equation A12 it follows that

BVSt~BVS0

1zk''tBVS0�Equation A12�

If k'' is not constant over time, a numerical solution can be

derived as follows:

BVS1~BVS0

1zk''1BVS0Dt

BVS2~BVS1

1zk''2BVS1Dt

or

BVS2~BVS0

1z�k''1zk''2�BVS0Dt

similarly

BVSt~BVS0

1z�k''1zk''2zk''3z . . . k''n�BVS0Dt

�Equation A13�

Therefore, the percentage degradation of BVS after time t is

BVS0{BVSt

BVS0|100~

k''tBVS0

1zk''tBVS0|100 �Equation A14�

~�k''1zk''2zk''3z . . . k''n�BVS0Dt

1z�k''1zk''2zk''3z . . . k''n�BVS0Dt|100

�Equation A15�



Documents

Kinetic analysis of forced aeration composting –I. Reaction rates and temperature