compostare.pdf

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Biosystems Engineering (2002) 81 (4), 433441

doi:10.1006/bioe.2001.0047, available online at http://www.idealibrary.com on

SE}Structures and Environment

Compost Airflow Resistance

Suzelle Barrington; Denis Choini"eere; Maher Trigui; William Knight

Department of Agricultural and Biosystems Engineering, Macdonald Campus of McGill University, 21 111 Lakeshore, Ste Anne de Bellevue,Qu!eebec, Canada H9X 3V9; e-mail of corresponding author: [email protected]

(Received 13 March 1999; accepted in revised form 9 January 2002)

Four bulking agents, pine shavings, mixed (long and chopped) grass hay, chopped grass hay and long (whole)

wheat straw, were each mixed with pig slurry and tap water to obtain three moisture contents (MC) of 60, 65and 70%. Quadruplets of each treatment were placed in laboratory composting vessels with a capacity of 105 l

and a composting depth of 095 m. Using the air plenum at the bottom of each vessel, air was forced at

apparent velocities of 00002ms1 through each compost mass to measure the air static pressure drop across

the compost mass as a function of apparent air velocity. Airflow resistance values were measured for compost

depths ranging from 055 to 085 m. Following this test, all mixtures were aerated for 21 days of composting

without overturning. The static pressure measurement procedure was then repeated on all quadruplet

mixtures.

The air static pressure drop was found with respect to a packed bed under laminar flow, defined using the

particle size distribution, porosity, depth and airflow channel characteristics of the compost material.

Although MC affected the value of the airflow channel characteristics of the compost material, both the hay

and straw demonstrated similar values, while shavings demonstrated values more variable and wider valuesfor MC between 60 and 70%.

There was a significant increase in airflow resistance after 21 days of composting, which supports the

need for compost overturning to reestablish the materials structure and to restore the airflow channels or

pores. # 2002 Silsoe Research Institute. Published by Elsevier Science Ltd. All rights reserved

1. Introduction

Aeration is a key element in controlling the tempera-

ture regime and thus, the performance of any compost-

ing operation. Under aerobic conditions, microbes canrapidly degrade the available hydrocarbons releasing

large amounts of energy to elevate the compost

temperature to the thermophilic range. Aeration is

further used to maintain the temperature for several

days, between 55 and 658C. Higher temperatures will

destroy the microbes composting the waste material,

while lower temperatures will not stabilize the waste

material. Waste stabilization is the most significant

objective of composting because it reduces the incidence

of parasites, pathogens and viruses. Aerobic conditions

also reduce the amount of putrefactive odours released

from the waste during its treatment. Furthermore,

composting generally dries the waste material below

50% and eliminates the readily degradable hydrocar-

bons, thus producing a waste material unattractive to

insects. Finally, aerobic conditions allow for a rapid

treatment reducing the size and improving the efficiency

of the composting unit.

The ventilation system used in the compost plant is,

therefore, an important design element, and represents asignificant cost. Airflow resistance parameters are

required to design a composting ventilation system with

sufficient capacity without being oversized. Relating air

static pressure drop to airflow through the compost

mass allows for the efficient design of the ventilation

system whether it be passive (floor ducts) or active

(ducts and fan system).

The objectives of this research were to measure and

describe, using laboratory vessels, the static pressure

drop of air flowing through compost materials. Four

bulking agents were used as compost materials, pine

shavings, a mixture of long and chopped grass hay,

chopped grass hay and long (whole) wheat straw. These

bulking agents were each mixed with enough pig slurry

to obtain a C/N ratio of 20. Furthermore, each compost

1537-5110/02/$35.00 433 # 2002 Silsoe Research Institute. Published by

Elsevier Science Ltd. All rights reserved


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material was tested at three moisture contents, namely

60, 65 and 70%. The depth of the aerated compost

ranged between 055 and 085 m. All mixtures (four

bulking agents at three moisture levels) were tested in

quadruplets for airflow resistance. Measurements were

conducted on the fresh (2 h old) compost mixtures and

after 21 days of composting.

2. Theoretical analysis

The pressure drop P through compost material has

been expressed using an empirical equation with terms

related to the physical properties of the material Cp, its

depth H and the apparent air velocity across its face V

(Higgins et al., 1982):

P Cp Hj Vn 1

where apparent air velocity, V, is defined as thevolumetric airflow rate divided by the cross-sectional

area of the compost bed perpendicular to this airflow.

Thus, Cp is affected by the mixing regime, degradation

and ageing of the compost (Higgins et al., 1982). The

coefficient j, exponent of H, was found to be 10, for

compost with over 40% dry matter and to be more than

10 otherwise (Keener et al., 1987). The coefficient n,

exponent of V, is directly related to the porosity of the

compost material (Saint-Joly et al., 1989).

The coefficients Cp, j and n have been measured for

various compost materials (Table 1). The coefficient Cpwas found to vary between 4 and 60 for sewage sludge

and wood chip compost, while j and n ranged between

105 and 147 and between 148 and 174 (Higgins et al.,

1987; Keener et al., 1987). For cage layer and sawdust or

maize cob compost, the value for Cp ranged between

0556 and 2554, while j was 10 and n ranged between

123 and 182 (Keener et al., 1987). For cattle manure,

values of 031041 were measured for Cp Hj and 083

215 were measured for n (Saint-Joly et al., 1989). The

value of Cp Hj increased as n increased (Keener et al.,

1987; Saint-Joly et al., 1989).

Pressure drop across a layer of compost can also beexpressed according to the HagenPoiseuille equation

(Geankoplis, 1978), describing the airflow across a

packed bed under conditions of laminar flow:

Pl 72ClHmV1 e2=S2

p D2

pe3 2

where Pl is the pressure drop across the bed depth under

laminar flow in Pa, Cl is a dimensionless laminar airflow

channel or pore length correction factor applied to H,

and having a value of 2 for a packed sand bed with a

porosity of less than 05, m is the viscosity of air in Pa s, e

is the dimensionless void ratio of the bed, Sp is adimensionless particle shape correction factor, and Dp is

the average diameter of the particles in the packed or

compost bed in m. When the flow is turbulent, the

pressure drop Pt expression (Geankoplis, 1978) changes

to

Pt 3fCtHrV21 e=SpDpe

3 3

where f is a dimensionless friction factor, Ct is a

dimensionless turbulent airflow channel or pore length

correction factor and r is the density of the air in

kg m3. In packed beds, both laminar and turbulent

flow can occur (Geankoplis, 1978):

Pt 3fCtHrV21 e=SpDpe

3

72ClHmV1 e2=S2

p D2Pe

3 4

Cd discharge coefficient of the compost vessel caporifice

Cl pore length correction factor for laminar flowCp pressure drop coefficient related to the physi-

cal propertiesof the compost material

Ct pore length correction factor for turbulentflow

Dp average diameter of the compost particles, mf friction factorH depth of compost material opposing airflow,

mj; n respective empirical exponential coefficients

for H and V

L coefficient characterizing airflow channels ofthe composting material

M compost moisture content, decimal

P pressure drop across the compost, PaPc static air pressure under the vessel cap, PaPl pressure drop across the compost depth under

laminar flow, PaPp static air pressure in the plenum, PaPt pressure drop across the compost depth under

turbulent flow, PaSp particle shape correction factorV apparent velocity of the air across the face of

the compost material, m s1

Vo air velocity across the face orifice, m s1

Greek letters

r air density, kg m3

m viscosity of air, Pa se void ratio of the compost

Notation

S. BARRINGTON ET AL.434


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where the first term applies to turbulent airflow in smallpores (high Reynolds number) and the second term

applies to laminar airflow in the large pores (small

Reynolds number).

Thus, pressure drop across a depth of compost can be

expressed using either Eqns. (2) or (4), whether the flow

is laminar or mixed (laminar and turbulent). Generally,

the flow should be laminar, considering the relatively

low airflow required. Barrington et al. (1997) success-

fully obtained compost temperatures of 658C, using a

maximum value for V of 00002ms1. Similarly, the

aeration of grain beds, demonstrates laminar flow(exponent for V of 10) under aeration velocities of up

to 00 2 m s1 (ASAE, 2000). The parameter V demon-

strates an exponent of 20 only for beds of large particles

(ear maize) exposed to velocities over 01 0 m s1. For

small grains (wheat and maize) exposed to apparent

aeration velocities above 00 2 m s1, the parameter V

has an exponent between 10 and 20, indicating that

some pores are under laminar flow while others are

under turbulent flow. Furthermore, the parameters Ctand Cl vary with the type of material.

The empirical expression, Eqn (1), is similar to Eqns

(2) and (4), when considering the value of the exponent n

(Table 1). A value of 2 indicates a completely turbulent

flow, while a value between 1 and 2 indicates laminar

and turbulent flow, and a value of 1 indicates laminar

flow. As it may be difficult to separately and experi-mentally design both Cl and S

2p , Eqn (2) can be redefined

using

L Cl=S2

p 5

giving

Pl 72LHmV1 e2=D2pe

3 6

where L is a dimensionless coefficient characterizing the

airflow channels of the composting material.

3. Materials and methods

3.1. The experimental material

The bulking agents used were a mixture of 33% long

(unchopped) grass hay and 67% chopped grass hay,

100% chopped grass hay, long (unchopped) wheat straw

and pine shavings. The pig slurry used to adjust the C/N

ratio of the bulking agents was obtained from the pig

fattening house of the Macdonald Campus farm of

McGill University (Table 2). Soya beans were added to

the pine shavings for a C/N of 20. Urea was initially

tested as nitrogen supplement, but was not used as it

would increase the pH of the mixture above 9 0. The pH,

Table 1Airflow resistance coefficients for various fresh compost mixtures

Compost Compost material Pressuredrop coeff.

Exponent Reference

Bulking

agent

Ratio Wet bulk

density, kg m3Dry matter,

%

(Cp), MPa j n

Sewage sludge Wood chip 1:0 90 95 381 108 174 Higgins et al.(1982)

1:3 485 31 730 105 1611:75 680 20 1234 130 1631:11 750 17 2028 147 1471:22 850 14 6007 141 148

Cage layermanure

Sawdust 1:2 670 40 0556 100 173 Keener et al.(1993)

Maize cobs 1:2 400 40 2554 100 160Sludge Wood chips

and leaves1:5:7 325 50 1109 100 173 Keener et al.

(1993)5:1:10 515 51 1694 100 182

3:5:7 320 51 2473 100 2021:0:17 400 48 1181 100 123

Cattle manure Straw 1:33 960 133 937 } 215 Saint-Joly et al.(1987)

1:17 880 147 448 } 2011:11 510 153 24 } 169

1:7 430 172 24 } 1691:22 170 232 27 } 083

Note: The ratio is expressed in terms of bulking agent mass over wet sludge or manure mass; the coefficient and exponentsapply to Eqn (1).

COMPOST AIRFLOW RESISTANCE 435


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moisture content (MC), total nitrogen (TN), total

carbon (C) and dry particle density of the experimental

materials were measured prior to starting the tests

(Table 3). The particle size distribution for all the

experimental materials is illustrated in Fig. 1.

The three moisture contents, 60, 65 and 70%, were

tested in quadruplets for each bulking agent and one

bulking agent was tested at any one time. Tap water was

used to obtain the desired MC for all compost mixtures,

and its quantity varied among bulking agent tests, to

obtain the required moisture content. The 70% MCcorresponded to the maximum amount of humidity

capable of being held by the bulking agent while the

60% MC corresponded to the minimum MC at which

composting can be carried out effectively.

3.2. The equipment

The tests were conducted using 12 identical 105 l

cylindrical plastic vessels, 095m high and 040m in

diameter (Fig. 2). Each container was insulated with

100 mm of mineral wool for a thermal resistance valueof 25 m28C W1. An air plenum, 100 mm in height, was

created at the bottom of each container using a

supporting metal wire mesh. Before conducting the test,

the volumetric capacity of the experimental vessels was

determined as a function of depth by gradually filling

with a known volume of water.

The air plenum under the compost had two perfora-

tions, each equipped with a tube. One perforation was

used to inject air and the other was connected to a

manometer tube to measure the static air pressure.

Caulking was used around each connection for air

tightness. Air was supplied by a pressurized air tank

connected to a compressor unit. All static pressure

readings were conducted with a vertical Dwyer micro-manometer with an accuracy of 0062 Pa.

3.3. The method

One test was conducted for each of the four bulking

agents. For each test, 12 compost vessels were filled

to give three levels of moisture tested in quadruplets

(Table 2). The bulking agent was sampled and analysed

before each test. The pig slurry could not be analysed

ahead of time because of the significant loss in TN

content during the 24h required to carry out theanalytical procedures. However, its composition

changes only slightly from one test to another and an

average MC, TN and C content could be assumed.

Table 2Compost composition for the experimental tests

Item Units Trial

1 2 3 4

Bulking agent type Wheat straw Mixed hay Chopped hay Wood shavings% d.b. 74 76 77 77

Pig slurry % d.b. 26 24 15 15N complement type } } } Soya beans

% d.b. 8Total % d.b. 100 100 100 100Moisture % d.b. 63 60 60 59content 67 67 65 64

72 75 70 69C/N ratio 18 23 21 20

Note: d.b., dry weight basis.

Table 3Characteristics of the experimental materials

Material Dry matter, % Total N, g kg1 pH Ash, % C, % Density, g ml1

Wheat straw 869 (05) 979 (025) 63 (060) 90 (04) 498 050 (004)Hay 872 (11) 110 (18) 52 (010) 65 (03) 511 066 (009)Pine shavings 924 (03) 064 (04) 44 (013) 038 (013) 544 078 (010)Pig slurry 168 (09) 751 (55) 72 (016) 198 (14) 438 130 (013)Soya beans 900 (05) 68 (03) } 23 (02) 534 }

Note: All analyses are reported on a dry weight (d.w.) basis; the value for the pig slurry is typical of the six tests; the C contentwas calculated from {(100% ash (%))/183}; the values in parentheses are the standard deviations.



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A 15 kg mass (d. b.) of bulking agent was mixed with

the required amounts of pig slurry, water and amend-

ment (soybeans for the pine shavings) to obtain a C/N

ratio of 20 and a moisture content of either 60, 65 or

70%. The batches were prepared by weighing all

ingredients on a scale with an accuracy of 01kg,

dumping the ingredients in a 400 l container and

thoroughly mixing all ingredients manually. Then, the

mixture was used to fill two experimental vessels. The

procedure was repeated for all other treatments using

the same amount of bulking agent and swine liquid

manure, but with different amounts of tap water. The

depth of compost in each vessel was recorded, and the

volume, bulk density and air porosity of the fresh

compost were determined.Air pressure differentials with air velocity across the

compost was measured 12 h after filling the experi-

mental vessels. The top of the vessel was closed using an

air-tight cap with a 5 mm orifice. Air was introduced

into the plenum at different flow rates, while the air

static pressure of the plenum Pp and the top orifice

velocity Vo were measured. The pressure differential was

calculated from the air static pressure in the plenum less

that created under the cap, calculated from the orifice

air velocity, using Bernoullis principle:

Pc rV2o =2 7

where Pc is the static air pressure under the vessel cap in

Pa, r is the air density in kgm3 and Vo is the air

velocity across the face orifice in m s1. The pressure

differential across the compost mass was therefore

calculated as

P Pp rV2o =2 8

The orifice air velocity Vo was measured using a hot

wire anemometer glued to the centre of this orifice. The

airflow rate across the compost was determined from the

air velocity and the discharge coefficient Cd of the vessel

cap orifice. The value for Cd was determined before the

test, using an empty vessel. An airflow rate measured

using a flow meter (01201 lmin1) was introduced at

Fig. 1. Particle size distribution of the experimental materials , wood shavings; , long hay; , chopped hay; , longitem; , pig slurry

Fig. 2. The experimental composting vessel



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the bottom of the vessel while measuring the central

velocity of the air jet escaping the cap orifice. The

discharge coefficient Cd of this orifice for an airflow Vobelow 27 5 m s1 was found to be

Cd e01547Vo

9and for Vo above 27 5 m s

1:

Cd 0 64 10

After measuring initial airflow resistances, the mix-

tures were aerated at a rate of 2 lmin1 and composted

for 21 days. The mixtures composted adequately,

reached a temperature of 658C after 2472 h and

maintained this temperature for at least 3 days

(Barrington et al., 1997).

After 21 days of composting, the temperature of the

mixtures dropped below 358C. Using the proceduredescribed earlier, the air static pressure test was repeated

for all vessels except that for the shavings, because of

technical difficulties.

3.4. Analytical procedures

All compost materials were analysed using standard

methods (APHA et al., 1995). The moisture content was

determined by drying at 1038C for 24h. The pH was

determined with a probe on 5 g of material soaked in

50 ml of distilled water for 24 h. The total Kjeldahlnitrogen (TKN) was determined by digesting the

material with sulphuric acid at 5008C and measuring

the NH3-N content at a sample pH of 13, using an NH3

sensitive electrode. The NO3-N content was determined

by soaking a 5 g sample in 50 ml of distilled water for

24 h and measuring the level of NO3-N using an NO3

sensitive electrode. The ash content was obtained by

burning each dried sample at 5008C for 4 h. The organic

matter content was equated to the volatile portion of the

burned samples and converted to C using a factor of

183 (Castellanos & Pratt, 1981). The particle drydensity was measured by soaking in commercial grade

kerosene and measuring the volume against the weight

of the particles.

3.5. Statistical analysis

The air static pressure drop P in Pa for each test was

related to the apparent air velocity V in m s1 across the

compost by applying the procedure of linear regression

(Steel & Torrie, 1986). Averaged from the quadruplets,

the slope gave the value of Hm1 e2=D2pe3 in Eqn

(2), while the intercept represents the amount of pressure

required before any airflow can occur through the

compost. The value for L was calculated from the

known value ofm, H, Dp and e. The true value ofDp was

assumed to remain the same throughout the composting

period (Geankoplis, 1978).

The procedure for the analysis of variance with blocks

representing the moisture levels was used to determine

whether or not there was a significant difference between

the values for the coefficient L obtained before and after

composting and among the three MCs. Duncans New

Multiple Range Test was used to establish which

treatment was significantly different (Steel & Torrie,

1986).

4. Results and discussion

The air pressure differentials measured across the

compost material for all four bulking agents and the

three MCs are summarized in Tables 47. The pine

shavings compost offered the least resistance to airflow

despite its higher wet and dry bulk density. The chopped

grass hay offered less resistance to airflow than the

mixed grass hay. The long wheat straw compost offered

the most resistance to airflow, especially at a moisture

content under 67%.

The air static pressure drop for all fresh compost

generally increased with MC, air velocity across the

compost and age of the compost. For apparent air

velocities under 0002ms1, the pressure drop regres-sion equations respected the laminar flow model [Eqn

(2)] with coefficients of determination R2 ranging from

085 to 097. The slope of the regression equation

increased slightly with apparent air velocity, thus

lowering the value of R2. This increase in slope value

indicates that higher airflows were likely to change the

size and geometry of the airflow pores, or that some

pores were starting to experience turbulent air flow. All

regression equations gave a zero Y-axis intercept.

4.1. The hay compost

The air static pressure drop for the chopped and

mixed hay are summarized in Tables 4 and 5. The value

for L for the fresh low, medium and high MC compost

was 10 500, 1800 and 900, while that for the 21-day

compost was 10 000, 9000 and 11 400. As expected, the

value for L was the same for the chopped and the mixed

hay, for all MC and compost age, because both bulking

agents had the same shape and created the same pore

tortuosity.Therefore, L for fresh hay compost can be expressed

as

L 25 M16 11



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Table 4Pressure losses with aeration of the chopped hay

Moisturecontent, %

Bulk density, kg m3 Voidratio (e)

Depth ofcompost (H), m

Pressuredrop (P), Pa

Coeff. of determin-ation (R

2)

Airflow channelcoeff. (L)

Wet Dry

Fresh compost60 250 100 071 063 24 100V 084 10 50065 295 103 067 062 56 200V 093 1 80070 325 98 062 062 51 100V 094 900

Compost, 21 day old62 185 70 079 062 33 200V 088 10 00053 225 106 075 049 43 600V 086 900067 245 80 073 047 40 500V 083 11 400

Note: R2 is for linear regression describing P; the regression equation for P is valid for the apparent airflow velocity V of500015m1 s; average compost diameter Dp is 20mm.

Table 5Pressure losses with aeration of the mixed hay

Moisturecontent, %

Bulk density, kg m3

Voidratio (e)



Coeff. ofdetermination (R2)


Wet Dry

Fresh compost60 195 78 076 066 95 700V 089 10 60067 245 51 071 064 54 700V 087 1800

75 380 98 057 061 74 100V 093 900

Compost, 21 day old52 156 75 080 054 56700V 085 10 00059 174 71 076 049 77 200V 082 900067 245 81 073 047 134 200V 087 11 400

Note: R2 is for linear regression describing P; the regression equation for P is valid for the apparent airflow velocity V of50002ms1; average compost diameter Dp is 32mm.

Table 6Pressure losses with aeration of the wheat straw

Moisturecontent, %




Coeff. ofdetermination (R

2)


Wet Dry

Fresh compost63 175 65 075 085 48 200V 087 550067 205 69 071 085 30 000V 094 220072 235 70 065 085 22 600V 088 900

Compost, 21 day old64 175 63 079 075 77 500V 084 16 70069 185 57 078 075 56 500V 090 10 60075 200 50 076 075 26 100V 085 3800




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with a value for R2 of 097, and for unturned 21-day-old

compost from hay by

L 10 0001000 12

where M is the moisture content of the compost

expressed as a decimal.

For the fresh compost, the initial MC had a

significant effect on P, whereas after 21 days of

composting, the MC had dropped and no longer exerted

any significant effect (95% confidence level). Thus,

overturning the compost during its fermentation stage

is likely to reduce in pore clogging.

4.2. The whole wheat straw compost

The value of L, for the fresh low, medium and high

MC straw compost was 5500, 2200 and 900, while that

for the 21-day compost was 16 700, 10 600 and 3800. The

value of L was in the same range as that for the hay,

except that MC had a significant effect on the value ofL

after 21 days of composting. These higher values

resulted from the more extensive decomposition and

collapsing of the straw stems, as opposed to the hay

stem which remained hollow after 21 days. Thus,

the straw composted offered more airflow resistance

after 21 days of composting, as compared to the haycompost.

Therefore, L for fresh straw compost can be expressed

as

L 10 M155 13

and for unturned 21-day-old compost from straw by

L 91 600 117 000 M 14

with a value for R2 of 10 in both cases.

4.3. The pine shaving compost

The value of L, for the fresh low, medium and high

MC shavings compost was 2030, 760 and 260. The value

for L was generally lower than that for the hay and the

straw, likely because the shaving particles are more

spherical in shape than the long tubular shape of the hay

and straw. Thus, the airflow pores of the shavingcompost are more regular in geometry. As for the hay

and straw compost, the shavings compost demonstrated

a value for L decreasing with increasing MC.

Therefore, L for fresh shavings compost can be

expressed as

L 12 355 17 720 M 15

with a value for R2 of 094.

5. Conclusions

For apparent air velocities Vo under 0002ms1, the

static air pressure drop P occurring in compost of hay,

wheat and shavings can be described using a laminar

airflow equation based on porosity, particle size

distributution, depth and airflow channel characteristics

of the compost material. These equations can be solved

if the airflow channel coefficient L is known. This

coefficient corrects the effective length of the airflowpaths, based on the compost depth H, and the effective

shape of the compost particles, based on an average

particle diameter measured by sieving D2p. Although

there were some variations in the value of L with MC,

both the hay and straw demonstrated values of 1000 and

15 000, while shavings demonstrated values of 2602030

for MC of 7060%. Ageing increased the value of L for

all composts. The loss of compost structure resulted

from the blocking of pores and in the increase in the

value of L. Overturning can help reestablish the

materials structure and unclog the air pore channels.

Compost material offers a value of L much higher than

that experienced with the bed of packed granular

material, as compost contains particles of an irregular

shape, resulting in tortuous airflow channels.

Table 7Pressure losses with aeration of the fresh wood shavings compost

Moisturecontent, %




Coeff. ofdetermination (R

2)


Wet Dry

59 300 123 076 054 192V 096 203064 326 117 071 052 124V 097 76069 412 128 057 051 176V 094 260




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Acknowledgements

This project was financially supported by the Natural

Sciences and Engineering Research Council of Canada.

References

APHA; WPCF; AWWA (1990). Standard analysis for waterand waste water. American Public Health Association,American Waster Works Association and Water PollutionControl Federation, Washington, DC

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Barrington S F; Choini"eere D; Trigui M; Knight W (1997).Passive versus active aeration for composting. In: Proceed-

ings of the CSAE Technical Sessions, pp. 514523.

Canadian Society of Agricultural Engineering, Saskatoon,Saskatchewan, Canada

Castellanos J Z; Pratt P F (1981). Mineralization of manurenitrogen}correlation with laboratory indexes. Soil ScienceSociety of American Journal, 45, 354357

Geankoplis C J (1978). Transport Processes and UnitOperations. pp. 129132. Allyn and Bacon Inc. Boston,USA

Higgins A J; Chen S; Singley M E (1982). Airflow resistance insewage sludge composting aeration systems. Transactions ofthe ASAE , 24(4), 10101014, 1018

Keener H M; Hansen R C; Elwell D L (1987). Airflow throughcompost: design and cost implications. Applied Engineeringin Agriculture, 13(3), 377384

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Steel R D G; Torrie J H (1986). Principles and procedures ofStatistics, a Biometrical Approach. (2nd edn.). McGraw Hill

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