EFFECT OF THE NUMBER OF THE VERTICAL PIPES FOR THE PASSIVE AERATION ON THE COMPOSTING RATE

The purpose of this experimental study was to clarify the effect of the number of the vertical perforated pipes on the composting rate and the extent of organic matter degradation. To achieve this target, composting was performed on the simulated organic solid waste, blended with wood chips and inoculums using a laboratory-scale composting reactor equipped with the vertical pipes as air suppliers for the passive aeration. The dog food (DF) was used as a simulated organic waste to be composted. For the aeration purposes, two, four and six vertical perforated pipes, which top wall was heated at the initiation phase, were embedded in the reactor at Runs A, B and C, respectively. The composting was monitored by regularly measuring the bed temperature at different points during the composting process. Thermophilic temperatures, about 55°C, were observed within the bed during each of the composting runs. The final conversions of carbon were, approximately, 86 to 96%. On the basis of a simple heat balance obtained under a steady reaction proceeding, the composting rate was analyzed. The composting reaction rate increased in increasing in the number of the vertical pipes during the composting process. The reaction rate was expressed as a first order equation and the reaction rate constant was calculated.

EFFECT OF THE NUMBER OF THE VERTICAL PIPES FOR THE PASSIVE AERATION ON THE COMPOSTING RATE

Y B Sylla; T Watanabe; K J Cho and M Kuroda

Dept of Civil Eng., Gunma Univ., 1-5-1 Tenjin, Kiryu, Gunma 376-8515, JAPAN

ABSTRACT

The purpose of this experimental study was to clarify the effect of the number of the vertical perforated pipes on the composting rate and the extent of organic matter degradation To achieve this target, composting was performed on the simulated organic solid waste, blended with wood chips and inoculums using a laboratory-scale composting reactor equipped with the vertical pipes as air suppliers for the passive aeration The dog food (DF) was used as a simulated organic waste to be composted For the aeration purposes, two, four and six vertical perforated pipes, which top wall was heated at the initiation phase, were embedded in the reactor at Runs A, B and

C, respectively The composting was monitored by regularly measuring the bed temperature at different points during the composting process Thermophilic temperatures, about 55°C, were observed within the bed during each

of the composting runs The final conversions of carbon were, approximately, 86 to 96% On the basis of a simple heat balance obtained under a steady reaction proceeding, the composting rate was analyzed The composting reaction rate increased in increasing in the number of the vertical pipes during the composting process The reaction rate was expressed as a first order equation and the reaction rate constant was calculated

KEYWORDS

Composting; Passive aeration; Temperature, Vertical pipes, Composting rate

INTRODUCTION

Composting is a method of solid waste management whereby the organic component of the solid waste

is biologically decomposed under controlled conditions to a state in which it can be handled, stored/or applied to the land without adversely affecting the environment (Golueke 1977) Aeration is an important factor for controlling the process as it ensures the growth of adequate aerobic microbe populations and the development of stabilizing temperatures In any compost system, the air supplied to the compost piles has

the dual function of supplying sufficient oxygen and removing water by heat (Nancy et al., 1996)

Based on the method of aeration, composting technologies can be divided into three modes: forced aeration, natural aeration and passive aeration Forced aeration requires use of blower for air supply Natural aeration occurs simply by diffusion and convection, governed by the exposed surfaces and their

respective properties (Barrington et al., 2003 and Fernandes et al., 1994) The later method is not

recommended for wastes with high moisture content In the passive aeration, air is drawn into the perforated pipes by convective currents developed by temperature differences between ambient air and

the warm decomposing compost mass (Sartaj et al., 1997)

Passive aeration, which uses the natural convection for air feed, is a sustainable and promising method

to compost (Barrington et al., 2003 and Patni et al., 2001) Several researchers have studied such systems and widely disseminated some good results according to their own configurations (Barrington et al., 2003, McGarry et al., 1978; Mathur et al., 1988, 1990, 1991; Nancy et al., 1996, Patni et al., 1992, 1994; Sartaj

et al., 1997 and Solano et al., 2001) However, the efficiency of the passive aeration using the horizontal

pipes for air feed is limited as it is difficult to ventilate the pipes by the natural convection In order to increase the air feed by the natural convection, it is important to use the vertical pipes rather than the horizontal ones The vertical pipes enhance the natural convection by due buoyant forces, as the pipes are

heated by the enthalpy generated from the composting reaction which is an exothermic reaction Partially heated pipes greatly boost the ventilation effect by the natural convection (Shimizu and Morita, 1962) However, there have been no studies of the effect of the vertical pipes on the passive aeration and the composting rate

The objective of this study is to evaluate the effect of the number of the perforated pipes, embedded vertically in the composting bed on the composting rate

MATERIALS AND METHODS

Materials used for the experiments

The substrate used for the composting was a commercial dog food (DF) VITA-ONE softTM (Japan Pet Food Co Ltd., Tokyo, Japan), since it has shown a good reproducibility and consistency in the

degradation of organic matter during the composting process (Nakasaki et al., 1998a)

Based on the preliminary analysis, the carbon and nitrogen contents of the DF were, respectively, 42% and 4% by dry weight basis Accordingly, the carbon to nitrogen ratio was approximately 10:1 The solid particles were reduced to a uniform size of about 3 to 4 mm and were well mixed before the use The pH

of the DF and its moisture content (MC) were 5.1 and 8.4%, respectively The MC of the raw mixture was adjusted to 60% during all the composting runs The raw materials for the composting experiments consisted of the dry DF as the main substrate to be composted, wood chips as bulking agent and inoculums as seed material Table 1 lists the initial properties of the raw materials of Runs A, B and C, respectively

Table 1 Characteristics of initial raw wastes for composting

Property Run A Run B Run C

Moisture content (%) Volatile solids (g)

pH (water extract) Carbon content (g, dry weight) Dog food (g dry weight) Wood Chips (g dry weight) Inoculums (g dry weight) Water content (g)

Mixing ratio of the raw materials (-)

60 5.7 5.1

191

455

199

125

351 4:2:1

60 6.6 5.3

185

441

166

293

551 3:2:1

60 10.4 5.2

186

443

181

252

393 3:1:1

Experimental apparatus and procedure

The schematic diagram of the experimental system is shown in Fig 1 The reactor was rectangular and was 300 mm high, 200 mm length and 200 mm wide To supply air in the composting mass, the perforated pipes were embedded in the bed vertically The number of the pipes were two, four and six for Runs A, B and C, respectively The distance between the different pipes were 20, 10 and 6 cm for Runs A,

B and C, respectively

In order to reduce the lag period of the initiation of the composting reaction, the upper wall of the pipes was heated from the room temperature to as high as 55°C by means of an electric heater It should

be noted that the heating time depends on how fast the bed temperature increases to the set level Once the bed temperature reaches about 50 to 55°C due to the occurrence of the active biodegradation reaction

to hold a self-heating condition, which is the exothermic reaction, the applied electric current is switched

off The bed is aerated through the convective forces, created by temperature differences between the composting material and the ambient air

The thermocouples (Type-K) were inserted in the packed bed at different depths: bottom, middle and top The bed temperature is the average value of the three measured points The composting was monitored by a continuous measurement of the bed temperature variation of each of the composting runs The exhaust gas was recuperated into a gasholder to estimate the gas volume evolution rate throughout the process

Fig 1 Schematic diagram of the composting reactor

Analytical methods

The concentration of CO2 was measured using an online infrared analyzer (Shimadzu, Kyoto, Japan) The temperatures were monitored by means of thermocouples (Type-K), which were connected to a data recorder The exhaust gas volume was regularly measured by means of a high-grade syringe and a gas pack (Model, I94, Mitsuba, Tokyo) The MC of the sample was determined from the weight loss after drying in an oven at 105°C for 24 hours The carbon dioxide production rate was computed on the basis

of total gas volume evolution rate and the fraction molar of the measured value of CO2

RESULTS AND DISCUSSION

Temperature profiles

The bed temperature profiles of each of the composting runs are shown in Fig 2 At the beginning of the composting, after the raw materials were initially put into the reactor, particularly when the upper wall

of the perforated pipes was heated, a rapid increase in temperature was observed in all of the three composting runs

In Run A, a maximum temperature of about 54.9°C was reached inside the bed within almost 37 hours, which corresponds to the higher degree of the bacterial activity Temperature remained between 40 to 45°C within 450 h, followed by a gradual drop to ambient level The temperature rising to the thermophilic level confirmed that the passive aeration with the vertical pipes was efficient for Run A

At the outset of the process in Run B, a quick increase in the temperature of up to 57.9°C was observed

in the reactor within the first 24 hrs Temperature ranged between 45 to 49°C within 400 hrs thereafter,

Fresh Air by the natural convection

7

4

6 8

6

7

5

3

8

1

2

1 Electric heater

2 Temperature recorder

3 Reactor

4 Thermocouple

5 CO 2 meter

6 Gas holder

7 Composting mass

8 Perforated pipes

decreased to ambient level It is apparent that thermophilic temperatures were predominant in Run B, suggesting the effectiveness of the aeration

In Run C, a peak temperature of 65°C was found in the bed within the first 10 hours of the composting This supports that the six perforated pipes exerted a great effect on the aeration rate, which caused a fast rise in the bed temperature at earlier stage of Run C, than for Runs A and B In Run C, thermophilic temperature ranged between 48 to 53°C throughout the whole process

The higher bed temperature profiles observed in Run C followed by Run B, could be likely due to the larger number of the vertical ventilation pipes, which provided more oxygen into the composting mass for

better aeration since according to Fernandes et al (1994), a limited aeration can be improved by

increasing the number of the pipes in the pile’s bottom zone Furthermore, the distance between the pipes seems to facilitate the air penetration into the composting pile For illustration, the distance between the pipes as mentioned earlier of Run C was 6 cm, compared to 20 and 10 cm for runs A and B, respectively With regards to these findings, it could be said that the composting process fundamentally appears effective in the three cases since after the initiation period, a great thermophilic ranges temperatures were kept at the steady state in each of the composting runs Several researchers have found similar

thermophilic range (Barrington et al., 2003, Fernandes et al., 1994, Mathur et al., 1990, Nancy et al.,

1997, Sartaj et al., 1997 and Solano et al., 2001)

Fig 2 Mean bed temperature over time for different composting runs

20 40 60

Time (h)

Run A Run B Run C

Conversion of carbon of the three composting runs

As the initial and final carbon contents in the composting raw materials were assessed on the solid basis before and after each of the composting runs, the final conversion of carbon, ζ could be defined by

Eq (1) The final conversion of carbon, estimated on the solid basis after the composting were, respectively, 86, 90 and 96% for Runs A, B and C Comparative values of about 80 to 90% were, found

by other researchers when forced aerations modes were used (Nakasaki et al., 1998a and 1998b)

o C

f C o

=

where ζ, Co and Cf are , respectively, the conversion of carbon, the initial carbon content in the raw

mixture and the carbon remaining after the composting Nakasaki et al (1998a) reported that the

conversion of carbon during the composting process could be calculated based on the ratio of carbon loss

as CO2 to the carbon contained in the DF alone In our investigation, the concentrations of CO2 from about 0 to 19% in the exhaust gas were detected during each of the experimental runs However, it was

unfortunately difficult to accurately determine the amount of CO2 evolved and its evolution rate, because the airflow rate was not explicitly measured in the passive aeration

Effect of the number of the vertical pipes on the composting rate

The bed temperature difference under a steady state condition of each run (see Fig 2) should correspond to the difference between the composting rates In addition, the different number of the vertical pipes, which provided varied composting aeration rates among the three runs, may cause the difference between the temperature profiles Therefore, the composting rate may be estimated on the basis

of a simple heat balance The variation of the bed temperature under the steady state condition may be expressed as follows:

0 dt hA T b T

d HM dt

dT

V

p

where the left side term of Eq (2) represents the heat accumulation of the bed [kcal･hr-1] and the right side terms are, the heat generation by the composting reaction and the heat loss from the reactor walls [kcal･hr-1], respectively The effect of the sensible heats of the influent air and exhaust gas and latent heat

by condensation or evaporation on the bed temperature were assumed to be negligible because their flow and moisture change in the bed were limited

When the bed temperature is held at a certain steady state, the heat loss from the reactor will be compensated with the heat generated by the exothermic reaction occurring therein Hence, at the steady state, the time rate of the temperature change in the left side of Eq (2) is equal to zero This allows the accumulation term to be set equal to zero, thereby simplifying Eq (2)

0

T b

T M

hA

dt

d

−

∆Η

=

ζ

The overall heat transfer coefficient h related to the heat loss from the reactor wall was estimated to be approximately 12.7 [kcal･m-2･hr-1･°C-1] by the preliminary experiment It was difficult to determine the real enthalpy of the composting reaction, ∆Η, due to a lack of data Therefore, the nutritional energy of the DF about 3500 kcal/kg-DF was assumed as the enthalpy of the reaction Table 2 summarizes the estimated values of the composting rate from Eq (3) by using the experimental results of each of the composting runs

Table 2 Estimated composting rate at the steady state Run

(-)

Pipes number (-)

Tb

(°C)

T*

(°C)

Mo (kg DF) (dry weight) d dtζ

(hr-1)

A

B

C

2

4

6

45.9 48.7 51.9

29.0 29.0 31.2

0.5 0.4 0.4

0.01 0.02 0.03

In order to determine the effect of the number of the ventilation pipes on the composting rate, an attempt was made to correlate the composting rate with the number of the perforated pipes for air supply

in the bed Fig 3 shows the effect of the number of the pipes on the composting rate, which was determined through Eq (3) It was clearly found that the number of the vertical pipes greatly affects the composting rate However, the number of the perforated pipes which should be vertically placed in the composting pile should be chosen as a function of the reactor dimensions to avoid over aeration of the composting mass, which usually leads to a high bed temperature increase of over 70°C At such a high temperature, bacterial activity is greatly inhibited, which reduce the composting rate

Fig 3 Relationship between the composting rate and the number of perforated pipes

0 0.01 0.02 0.03 0.04

Number of pipe (-)

-1 )

Haug (1993) reported that the organic matter decomposition during the composting could be modeled

as a first order reaction The rate equation can be expressed as follows:

ζ

ζ

k

dt

d

−

The reaction rate constant (k) for each of the composting runs was calculated by using the

experimental results and were found to be approximately 0.004, 0.006 and 0.009 hr-1 for Runs A, B and C, respectively

CONCLUSIONS

The effect of the number of the vertical pipes for air supply in the composting pile on the composting rate was investigated by measuring the bed temperature variations and the carbon content during the composting process From the obtained results, the following conclusions can be drawn:

1) The perforated pipes placed vertically in the composting bed were effective for the passive aeration

by the natural convection

2) The composting rate was increased in increasing in the number of the perforated pipes placed vertically in the composting bed within certain limits for air delivery in the composting mass

3) The composting rate was analyzed based on the heat balance under a steady state condition

4) The initial heating of the top wall of the vertical pipes was effective on shorting the lag period, often observed in the composting operations

5) The effectiveness of placing the perforated pipes vertically in the composting pile could be also corroborated by the conversions of carbon at the final stage of composting process, which reached approximately 86, 90 and 96%

NOMENCLATURE

A = cross sectional area of the composting reactor [m2]

C o = initial amount of carbon in the DF [kg]

C f = final carbon content after the composting process [kg]

=

p

C heat specific coefficient [kcal･kg-1･°C-1]

=

h overall heat transfer coefficient [kcal･m-2･hr-1･°C-1]

=

∆Η enthalpy of the reaction [kcal･kg-1]

k = reaction rate constant [hr-1]

M 0 = initial mass of the DF [kg]

T b = mean bed temperature [°C]

T *= room temperature [°C]

t = composting time [hr]

V = reactor volume [m3]

=

ζ conversion of carbon [-]

=

ρ bed density [kg･m-3]

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