EFFECT OF MOISTURE CONTENT ON FED BATCH COMPOSTING REACTOR OF
VEGETABLE AND FRUIT WASTES
B. JOLANUN**1, S. TRIPETCHKUL2, C. CHIEMCHAISRI3, P. CHAIPRASERT2 AND S.
TOWPRAYOON*1
The Joint Graduate School of Energy and Environment, King Mongkut’s University of Technology
Thonburi, Thailand
2
School of Bioresources and Biotechnology, King Mongkut’s University of Technology Thonburi,
Thailand
3
Department of Environmental Engineering, Faculty of Engineering, Kasetsart University, Thailand
1
Abstract
Vegetable and fruit wastes mixed with sawdust were composted in a laboratory scale reactor by
controlling the waste feeding rate at 21 kg m-3day-1 and aeration rate at 10.6 l m-3 min-1. The effect of
initial moisture content on organic matter degradation and process performance of fed batch composting
were investigated. The absolute amount of removal, removal percentage, and removal rate of dry mass
obtained were substantially different among the initial moisture contents. The rapid rise of moisture
content and the lowest absolute amount of removal observed were achieved in the 50% condition. The
initial moisture content yielding the largest absolute amount of removal in both feeding and curing stage
was 30% whereas the removal percentage and rate constant of waste decomposition were highest in the
50% condition. Examined by traditional soil physics method, the moisture content at 50-55% was
suitable for satisfying the degree of free air space (65-70%) of compost during the fed batch composting.
Most degradable organic matter was mainly consumed in the feeding stage as indicated by a higher
removal rate of dry mass in all cases. It is recommended that the initial moisture content of 30% and
mode of aeration and agitation should be adopted for achieving practical fed batch composting of
vegetable and fruit wastes. The study also demonstrated that the composting kinetics of vegetable and
fruit wastes mixed with sawdust can be described by a first order model.
Keywords: Composting, fed batch, moisture content, vegetable and fruit wastes.
INTRODUCTION
A remarkable generation (0.5-2.1 ton day-1) of vegetable and fruit wastes containing high organic
content (70-80%) and moisture content (80-90%) from markets now requires an environmentally
acceptable solution for handling this waste urgently [1,2,3,4]. From environmental, economical, and
engineering points of view, composting is a suitable biological treatment process for recycling these
organic wastes in developing countries like Thailand. There are many methods using batch culture for
composting vegetable and fruit wastes such as turned windrows, vessel composting, and aerated static or
dynamic piles [3,4,5,6,7]. Recently, the composting machine using the technique of feeding substrate
into a batch culture, named fed batch composting, has been widely employed to compost meal residues in
Japanese households, hospitals, and restaurants. The difference between the former composting methods
and the later is the fed batch composting allows the new wastes to be added intermittently into the system
at a constant rate [8,9,10,11]. Therefore the fed batch composting has been developed and selected for
this study because its operation suits the intermittent generated wastes from markets. Composting likes
any biological processes that turn the organic matter into stable end products. The composting process is
influenced by many environmental factors, however the major factor is the moisture content, since the
moisture level is directly related to oxygen supply and microbial activity [4,5,7,12,13,14,15,16,17,18,19].
Vegetable and fruit wastes tend to release a large amount of leachate during the composting, causing the
mixture to compact and hinder the oxygen supply, thus careful management of initial moisture content
must be ensured for practical fed batch composting of these green wastes [3,4,6,15]. Numerous reports
have studied the effect of moisture content on batch composting of vegetable and fruit wastes but not fed
batch composting [4,15,17,18,20]. Understanding the effect of initial moisture content on a fed batch
composting system helps us to further achieve practical reactor design for fed batch composting under
Thailand’s conditions. The aim of this study was not only to investigate the process performance but also
to evaluate the effect of different initial moisture contents on the changes of physical and chemical
properties and the organic matter degradation of vegetable and fruit wastes using fed batch composting
MATERIALS AND METHOD
Vegetable and fruit wastes from Bangkok Islam-Mai market, Thailand were shredded manually to
2.5-5.0 cm in size and used as a raw material. The wastes were mixed with sawdust (by weight), which
was screened with a sieve of mesh no. 40 (0.425 mm) and no. 60 (0.250 mm), and composted under the
initial moisture contents of 30%, 35%, 40%, and 50%. Urea as nitrogen source was added to adjust the
initial C/N ratio to 30-40. Two replicates of the experiments were carried out by using eight acrylic
reactors with working volume of 4.71 l for each reactor (20 cm diameter, and 20 cm height). Each reactor
was installed with a perforated acrylic plate, which was placed on a stand at 5 cm above the base of the
reactor in order to support the loaded materials and distribute the air. Forced aeration was supplied by
using an air compressor and operated under a continuous positive pressure mode. The air was controlled
at a constant rate by using an air flow meter and was ventilated via an air vent (1 cm diameter) from the
top of the reactor.
As presented by Vallini et al. (1993), the composting of green wastes at high moisture content
(>80%) can release the large amount of leachate and cause system failure under the acidic environment.
In order to overcome this problem, the fed batch operation was divided into two stages - feeding and
curing stage. In the feeding stage, the new wastes were added daily into the reactor at a constant rate until
the moisture content of the mixture reached 70-80% then the process entered the curing stage. In the
curing stage, the mixture was continuously composted without the intermittent addition of new wastes
until it turned into compost. Fed batch composting was conducted under the waste feeding rate of 21 kg
m-3day-1 and 10.6 l m-3 min-1 aeration rate. The mixture was turned manually once a day in order to
ensure uniform decomposition.
Composite samples taken from the top, middle, and bottom of the reactor were analyzed for
moisture content, ash content, total organic carbon (TOC), total Kjeldahl nitrogen (TKN), and pH
according to standard methods [21]. The means and standard deviations for all parameters measured
were calculated. The degradation of organic matters during the fed batch composting was determined in
terms of absolute amount of removal and removal percentage of dry mass. The calculation of weight loss
in this study, especially under the feeding stage was based on two assumptions. Firstly, when
microorganisms are incubated in the presence of two or more substrates, the substrates will be degraded
in the order of their ease of degradation. Secondly, the major part of sawdust is made up of resistant
materials (lignocelluloses), which require more time for biological degradation [10, 22]. Therefore, the
weight loss during the feeding stage was calculated by subtracting the residual wet weight of the
composting materials, excluding the sawdust, from the cumulative wet weight of waste introduced into
the reactor. The percentage of dry mass loss was the ratio of the decrease of dry weight in the reactor to
the cumulative dry weight introduced into the reactor. The rate constants of waste degradation (k) were
determined in terms of the degraded TOC as a function of time, which was obtained by plotting the ln of
the ratio of the TOC at any time to the initial TOC versus time. The k value is the slope of the fitted
straight line obtained in each condition [23].
The free air space (FAS), which is the volume fraction of air (reported either on a decimal or
percentage basis) in a porous matrix of compost, was calculated according to the traditional soil physics
equation proposed by Richard et al., (2002) as follows:
FAS=1-tot.[(1-DM/w)+(DM.VS/vs)+(DM.(1-VS)/ash)]
(i)
The Equation 1 was based on the density of water (w), organic matter (vs), and ash (ash), and
knowledge of the moisture content and bulk density of composting materials. The organic fraction (VS)
was assumed to have a particle density (vs) of 1.6x103 kg m-3 and the inorganic fraction (ash) was
assigned to have a particle density (ash) of 2.5x103 kg m-3 [24,25].
RESULTS AND DISCUSSIONS
Physical and Chemical Characteristics of Compost
Temperature and moisture content
The temperature profiles under four different initial moisture contents are shown in Figure 1.
During the composting period, the ambient temperature fluctuated within a narrow rage of 27-29˚C. The
temperature of all treatments increased rapidly from Day 1 of new waste addition before dropping to 3134˚C at Day 3 of the composting. The second increase in temperature was observed from Day 3 to Day
12 before declining to the ambient temperature from day 12 to day 30. The peaks were highest in the
40% and 50% condition (37-37.5˚C) and lowest in the 30% and 35% condition (35.7-36˚C). Although
the pattern of temperature profiles was similar, the temperature profile in the 50% condition dropped
dramatically and approached the ambient temperature within Day 14 of composting.
As the new waste addition proceeded, the initial moisture content in all treatments increased
continuously until the end of the feeding stage with the moisture content ranging from 65-68%. The
feeding stage was ended at Day 4, Day 8, Day 10, and Day 14 for the treatment with the initial moisture
content of 50%, 40%, 35% and 30%, respectively. The composting then entered the curing stage (Figure
2). During the curing stage, the moisture contents of all treatments decreased gradually and the final
moisture contents observed ranged from 20-46% at Day 31, Day 45, Day 71, and Day 92, for the
treatment with the initial moisture content of 50%, 40%, 35%, and 30%, respectively.
In the feeding stage, a rapid increase in temperature in all treatments resulted from the rapid
breakdown of degradable organic matters in vegetable and fruits wastes by microbes. In addition, the
new added wastes provided substrates for microbial growth that enhanced biological activities as well as
the production of heat [26]. A drop of temperatures may be caused by an adaptation of organisms to the
new substrates and a cooling effect from the increase in moisture content, particularly under the 50%
condition [14,15,17,19,26]. Starting the moisture content at 30-35% could inhibit microbial activity as
indicated by the low temperature peaks, however the system offered a longer feeding stage of composting
by keeping the moisture content at low level. In contrast, the feeding stage was ceased early for the 40%
and 50% condition due to the rapid increase in moisture content, despite higher temperature peaks. The
higher moisture content not only affected the temperature evolution, oxygen diffusion, and porous
structure adversely, but also it shortened the length of feeding stage of fed batch composting. It should be
noted that the initial moisture content was one of the important factors that should be controlled carefully
for fed batch composting, particularly vegetable and fruit wastes. The changes of temperature and
moisture content observed were greatly influenced by different initial moisture contents. The results
revealed that the 40% and 50% conditions promoted a cooling effect detrimental to the temperature
development and the shortest feeding period for new waste addition. The feeding stage was longest in the
30% condition due to its high capacity to absorb the released leachate, indicating that the most preferable
process performance exists at 30%, under which the total amount of new wastes added intermittently is
largest.
Carbon to nitrogen ratio (C/N) and pH
The C/N ratio and pH are presented in Table 1. The initial C/N ratio for the 40% and 50%
condition fluctuated from 45-60 before increasing to above 60 from Day 16 to Day 31. Thereafter the
C/N ratios decreased gradually to 51 by Day 39 of the composting. Under the There initial moisture
content of 30% and 35%, the C/N ratios increased dramatically from Day 25 onwards to 64-78 and it was
leveled off at this level until Day 45. Then, the C/N ratio declined gradually to 40-41 by Day 83 of the
composting.
The initial pH of all treatments was 4-5. The pH increased rapidly to above 8 by Day 3 of the
composting (Table 1). After that it leveled off at pH 8-9 from Day 3 until Day 16 for the 50% condition
and Day 25-31 for the 30%, 35%, and 40% condition. The pH of the finished compost was 7.3-7.7,
which was slight alkaline.
In the early stage of the composting, a rapid increase in pH resulted from urea mineralization, in
which a large amount of NH4-N was liberated into the system and caused an increase in pH. The increase
in temperature and pH at the beginning of the feeding stage suggested that the rate of decomposition
might be accelerated by urea [27, 28]. However, urea supplementation could enhance the loss of nitrogen
through volatilization under high pH condition (>8), as indicated by an increase in the C/N ratios [4, 13].
Obviously, there was a high N loss under the 30% and 35% condition as there was a strong smell of
ammonia and a substantial increase in the C/N ratios. This phenomenon might caus an increase in the
porous structure, an absorption capacity, and the N loss through volatilization [4,13,25]. Decreasing the
final C/N ratio and pH could result in further immobilization of NH3-N by nitrification suggesting that the
chemical properties of materials had been more stabilized [12,13,16,18].
Bulk density and free air space of compost
The relationship between free air space (FAS,%) and bulk density (BD,kg m-3) covering the full
range of data is shown in Figure 3. The linear relationship between these two parameters could be
established as follow:
FAS=110.15-0.1076BD
(ii)
As the feeding stage progressed, the moisture content increased and the free air space of compost
decreased as seen in Figure 4. The results revealed that the free air space of mixtures decreased rapidly
where the moisture level increased over 55%. It is well known that the moisture content and free air
space plays an important role of the composting, especially oxygen transportation. In order to extend the
feeding period, the moisture level at 50-55% should be controlled carefully as a critical value for
satisfying the degree of free air space (65-70%) of fed batch composting of vegetable and fruit wastes.
During the curing stage, the bulk density of compost did not increase. It was expected that a breakdown
of particle size of materials would result in more compact compost. Figure 5 shows that the bulk density
of compost of all treatments continuously decreased and the finished compost had the bulk density at 300350 kg m-3. This could result from the composting materials being mostly dried out in the later stage
through substantial evaporation, causing the bulk density of compost to decrease [13, 29].
Degradation of Organic Matter
Feeding stage
During the feeding stage, the total amount of new intermittently added wastes were 400g, 800g,
1000g, and 1400g for the treatment with the initial moisture content of 50%, 40%, 35%, and 30%,
respectively. The losses of dry mass for all treatments are shown in Table 2. The study found that the
treatments of 35% and 40% condition had similar absolute amounts of dry mass removal which was
ranging between 57g and 68g. The initial moisture content of 30% yielded the largest absolute amount of
dry mass removal (92g). This could be a result of the highest potential in decomposing the organic
matters as well as receiving the intermittent addition of new wastes of this condition. In contrast, the
treatment with the initial moisture content of 50% yielded the lowest absolute amount of removal (32g)
despite it having the highest percentage and rate of dry mass removal. By evaluating the percentage and
rate of dry mass removal, it was found that the highest values were observed in the 50% condition (81%,
8g day-1) followed by the 40% condition (72%, 7g day-1), 35% condition (68%, 7g day-1), and 30%
condition (66%, 7g day-1).
Evidently, starting the moisture content of the mixture at 40%-50% was preferable for biological
degradation since there was a rapid increase in temperature and the highest percentage and rate of dry
mass removal (Figure 1, Table 2). However the feeding stage was terminated shortly after the start of the
composting (4-8days), which could be caused by the rapid attainment of moisture content of 70% (Figure
2). The high moisture content greatly reduces the oxygen diffusion and porous structure of the compost.
This was evidenced by a fast reduction of free air space of the compost when the moisture content
increased to above 55% (Figure 4). Thus the initial moisture content of 50% might not be the most
preferable condition for fed batch composting of vegetable and fruit wastes since it had lowest potential
in receiving and decomposing the intermittent addition of new wastes. Although there was a low
percentage and rate of dry mass removal in treatment 30%, the results revealed that under this condition,
the moisture might not be too low to inhibit the microbial activity in fed batch operation since the
percentage and rate of dry mass removal was only 13-18% smaller than the treatment 50%.
Curing stage
In the curing stage, the results were compared at Day 29 and at the finish of the composting
process (Table 3). At Day 29, the absolute amount of removal, removal percentage, and removal rate of
dry mass obtained were highest in treatment 50% and 40% (Table 3), suggesting that the decomposition
of organic matter was fastest as indicated by the maximum rate constant (0.0074-0.0081 day-1) [6,23].
However, it was found that at the end of the composting process, the treatment 30% presented the highest
absolute amount of removal of dry mass and had 3 times longer composting period than the treatment
50% (Table 3). The longer curing period for the 30% condition could result from the microorganisms
requiring more time to degrade the resistant materials. A comparison of the data in Table 2 and Table 3
shows the removal rate of dry mass of all treatments in the feeding stage was 4 times higher than the
curing stage. This indicated that the most easily degradable organic matters were consumed at the early
stage (feeding stage) of composting. Thus the degradation of recalcitrant parts of organic matter in
vegetable and fruit wastes, which provides abundant resistance and extensive time for microbial
degradation, would occur in the later stage (curing stage) of composting. Consequently, the longest
curing time (78days) for the treatment 30% could be due to more time being required to decompose the
abundant resistant material remaining from the feeding stage [22,30]. Table 4 shows that the total dry
mass removal was highest in the 30% condition (266g) whereas the highest percentage of total dry mass
removal was obtained in the 50% condition (103%) within the shortest composting time (31days). This
suggested that the control of initial moisture content at 30% could treat largest amount of the intermittent
addition of new wastes. However, the decomposition of organic matter under the 50% condition was
faster than the 30% condition.
CONCLUSION
The performance of a fed batch reactor was investigated by performing composting of vegetable
and fruit wastes mixed with sawdust. The composting was carried out under four different initial
moisture contents. The study demonstrated that the change in physical and chemical properties and the
degradation of organic matter were greatly influenced by the initial moisture content. Controlling the
moisture content in the feeding stage at 50-55% could accommodate larger amounts of intermittent
addition of new wastes in fed batch composting of vegetable and fruit wastes. Fed batch operation under
a high initial moisture content not only resulted in a rapid increase in moisture content in the feeding
stage, which had an adverse effect on the development of temperature and porous structure, but also
resulted in the lowest absolute amount of dry mass removal. The results of this study also suggested that
the fed batch composting of vegetable and fruit wastes would be most effective if:
1)
The initial moisture content is controlled at 30%, which is lower than a typical value for the batch
composting (40-60%), in order to maintain stable degradation and achieve the highest absolute
amount of dry mass removal.
2)
The rate of degradation of resistant materials is accelerated in the curing stage particularly under
the treatment 30% in order to reduce the length of composting process, however the degree of
aeration and agitation may be further determined under this condition.
It was found that most degradable organic matters were consumed in the feeding stage and the
composting kinetics of vegetable and fruit waste mixed with sawdust can be described by a first order
model.
ACKNOWLEDGEMENTS
This research was fully supported by Energy Planning and Policy Office (EPPO) and The Joint
Graduate School of Energy and Environment, King Mongkut’s University of Technology, Thonburi,
Thailand.
REFERENCES
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
16.
17.
18.
19.
20.
21.
22.
Macro Consultant and Fichtner (ASIA) PTE, Comparative study on appropriate methods for
solid waste disposal, Pollution Control Department, Thailand (1993).
Environmental Care Center Co., Ltd., The feasibility study report of solid waste management
system for kangkhoi local government area, Thailand (1998).
Vallini, G., A. Pera, M. Valdrighi, and F. Cecchi, Process constraints in source-collected
vegetable waste composting, Wat. Sci. Tech., 28, 229-236 (1993).
Barrington, S.F., K.EL. Moueddeb, and B. Porter, Improving small-scale composting of apple
waste, Canadian Agriculture Engineering, 39, 9-16 (1997).
Haug, R.T., The practical handbook of compost engineering, Lewis Publishers, U.S.A. (1993).
Huang, J.S., C.H. Wang, and C.G. Jih, Empirical model and kinetic behavior of thermophilic
composting of vegetable waste, Journal of Environmental Engineering, 126, 1019-1025 (2000).
Jolanun, B., S. Tripetchkul, and S. Towprayoon, Interaction between the ratio of lignocellulosic
bulking agent and aeration rate in the vegetative and fruit waste composting: A kinetic
approach, Proceedings of the International symposium composting and compost utilization, May
6-8, Columbus (OHIO), USA, 1233-1245 (2002).
Nakasaki, K., N. Akakura, K. Atsumi, and M. Takemoto, Degradation patterns of organic
material in batch and fed-batch composting operation, Waste Manage. Res., 16, 484-489 (1998).
Tripetchkul, S., S. Techamena, P. Chaiprasert, and M. Terazawa, Composting production from
food waste in a garbage automatic decomposer extinguisher (GADE) bioreactor II:
microbiological and biochemical changes during the composting process, Proceeding of
International Workshop on Sustainable Utilization of Regional Resources, 88-94, Tokyo, Japan
(1999).
Nakasaki, K. and A. Ohtaki, A simple numerical model for predicting organic matter
decomposition in a fed-batch composting operation, J. Environ. Qual., 31, 997-1003 (2002).
Jolanun, B., S. Tripetchkul C. Chiemchaisri, P. Chaiprasert, and S. Towprayoon, Semicontinuous process in source-collected green waste composting, Proceedings of the Second
Regional Conference on Energy Technology Towards a Clean Environment, February 12-15,
Phuket, Thailand, 1073-1078 (2003).
Polprasert, C., Oraganic waste recycling, 2nd ed. John Wiley & Sons, England. (1996).
Epstein, E., The science of composting, A Technomic Publishing, Pennsylvania, U.S.A. (1997).
Lau, A.K., K.V. Lo, P.H. Liao, and J.C. Yu, Aeration experiments for swine waste composting,
Bioresource Technology, 41, 145-152 (1992).
Nakasaki, K., N. Aoki, and H. Kubota, Accelerated composting of grass clippings by controlling
moisture level, Waste Management & Reseach., 12, 13-20 (1994).
Brouillette, M., L. Trepanier, J. Gallichand, and C. Beauchamp, Composting paper mill deinking
sludge with forced aeration, Canadian Agricultural Engineering, 38, 115-122 (1996).
Tiquia, S.M., N.F.Y. Tam, and I.J. Hodgkiss, Microbial activities during composting of spent
pig-manure sawdust litter at different moisture contents, Bioresource Technology, 55, 201-206
(1996).
Tiquia, S.M., N.F.Y. Tam, and I.J. Hodgkiss, Changes in chemical properties during composting
of spent pig litter at different moisture contents, Agriculture Ecosystems & Environment, 67, 7989 (1998).
Lu, S.G., T. Imai, H.F. Li, M. Ukita, M. Sekine, and T. Higuchi, Effect of enforced aeration on
in-vessel food waste composting, Environmental Technology, 22, 1177-1182 (2001).
Richard, T.L., B. Hamelers, A.V. Veeken, and T. Silva, Moisture relationship in composting
processs, Proceedings of the International symposium composting and compost utilization, May
6-8, Columbus (OHIO), USA, 233-250 (2002).
American Public Health Association (APHA), Standard methods for the examination of water
and wastewater, 18 th ed. American Public Health Association, Washington, D.C. (1992).
Tuomela, M., M. Vikman, A. Hatakka, and M. Itavaara, Biodegradation of lignin in a compost
environment: a review, Bioresource Technology, 72, 169-183 (2000).
23.
24.
25.
26.
27.
28.
29.
30.
Hamoda, M.F., H.A. Abu Qudais, and J. Newham, Evaluation of municipal solid waste
composting kinetics, Resource, Conservation and Recycling, 23, 209-223 (1998).
Rahman, S., Food Properties Handbook, CRC Press, Boca Raton, Florida, U.S.A. (1995).
Van Ginkel, G.T., P.A.C. Raats, and I.A.van Haneghem, Bulk density and porosity distributions
in a compost pile, Netherlands Journal of Agricultural Science, 47, 105-121 (1999).
Metcalt and Eddy, Wastewater Engineering: Treatment, Disposal and Reuse, 3 rd ed. McGrawHill, New York, U.S.A. (1991).
Cappaert, I., O. Verdonck, and M. De. Boodt, Composting of Hardwood Bark, Compost Sci., 16,
12-15 (1975).
Darbyshire, J.F., M.S. Davidson, G.J. Gaskin, and C.D. Campbell, Force aeration composting of
coniferous bark, Biological Wastes., 30, 275-287 (1989).
Day, M. and K. Shaw, Compost Utilization in Horticultural Cropping Systems, CRC Press LLC,
Boca Raton, Florida, U.S.A. (2001).
Eklind, Y., and H. Kirchmann, Composting and storage of organic household waste with
different litter amendments I: carbon turnover. Bioresource Technology, 74, 115-124 (2000).