Bioresource Technology 100 (2009) 1122–1129
Contents lists available at ScienceDirect
Bioresource Technology
journal homepage: www.elsevier.com/locate/biortech
Characteristics and biogas production potential of municipal solid wastes
pretreated with a rotary drum reactor
Baoning Zhu a,b, Petros Gikas b,c, Ruihong Zhang b,*, James Lord d, Bryan Jenkins b, Xiujin Li a
a
Department of Environmental Science and Technology, Beijing University of Chemical Technology, Beijing 100029, China
Department of Biological and Agricultural Engineering, University of California, One Shields Avenue, Davis, CA 95616, USA
c
Greek Ministry of Environmental Planning and Public Works, General Secretariat of Public Works, Special Service of Public Works for Greater Athens Sewerage and
Sewage Treatment, Athens 11474, Greece
d
LandFirst Consultants Inc., Los Gatos, CA 95030, USA
b
a r t i c l e
i n f o
Article history:
Received 25 March 2008
Received in revised form 1 August 2008
Accepted 7 August 2008
Available online 11 October 2008
Keywords:
Anaerobic digestion
Biogas
Methane
Municipal solid waste
Rotary drum reactor
a b s t r a c t
This study was conducted to determine the characteristics and biogas production potential of organic
materials separated from municipal solid wastes using a rotary drum reactor (RDR) process. Four different types of wastes were first pretreated with a commercial RDR system at different retention times (1, 2
and 3 d) and the organic fractions were tested with batch anaerobic digesters with 2.6 g VS L1 initial
loading. The four types of waste were: municipal solid waste (MSW), a mixture of MSW and paper waste,
a mixture of MSW and biosolids, and a mixture of paper and biosolids. After 20 d of thermophilic digestion (50 ± 1 °C), it was found that the biogas yields of the above materials were in the range of 457–
557 mL g VS1 and the biogas contained 57.3–60.6% methane. The total solid and volatile solid reductions
ranged from 50.2% to 65.0% and 51.8% to 66.8%, respectively. For each material, the change of retention
time in the RDR from 1 to 3 d did not show significant (a = 0.05) influence on the biogas yields of the
recovered organic materials. Further studies are needed to determine the minimum retention time
requirements in the RDR system to achieve effective separation of organic from inorganic materials
and produce suitable feedstock for anaerobic digesters.
Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction
In 2005, 42 million tons of municipal solid wastes (MSW) were
landfilled in California, about 64% (by wet weight) of which is of
biological origin (California Integrated Waste Management Board,
2007). The organic fraction of MSW is naturally degraded over time
in landfills producing various gases including greenhouse gases
(Augenstein, 1992). Even though many landfills have installed
landfill gas recovery systems, the whole waste management industry is still facing challenges such as limited land availability, odor
and ground water pollution from leachate (Lewis et al., 2003). To
reduce the amount of wastes destined for landfills, the organics
from MSW can be separated and treated through conversion technologies for volume reduction and generation of valuable byproducts, such as biogas energy and compost. Composting has been a
more widely applied process for treating municipal organic solids
compared to anaerobic digestion. However, in the US, the economical value of the MSW-derived compost is usually low, often less
than 13 US$ m3, and many composting plants offer the compost
* Corresponding author. Tel.: +1 5307549530; fax: +1 5307522640.
E-mail address: [email protected] (R. Zhang).
0960-8524/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.biortech.2008.08.024
for free. The increasing energy prices make the anaerobic digestion
appear to be a more attractive process for achieving both waste
reduction and energy recovery.
Usually it is recommended to remove a certain amount of inorganic fractions (e.g., metals and glass) in raw MSW prior to anaerobic digestion for more efficient operation of anaerobic digesters
and prevention of equipment failures involved in material handling. Some pre-treatment methods may also be applied to increase the digestibility of the organic solids and increase the
efficiencies of anaerobic digesters (Bernal et al., 1992). In the study
reported by Capela et al. (1999), a pre-composting stage was used
for the pulp mill sludge to obtain a slight degradation of organics to
prevent fast acidification during anaerobic digestion. After a 49 d
experiment, the pretreated sludge had higher volatile solids (VS)
reduction (50%) than the untreated sludge which had 34% VS
reduction. Previous studies have also shown that a rotary drum
reactor (RDR) process provided an effective means for separating
the organics from MSW using a combination of mechanical forces
and biological reactions (Hayes, 2004; Spencer, 2006).
Thermophilic conditions have certain advantages over mesophilic conditions for digesting the organic wastes separated from
MSW, such as faster degradation rate, higher biogas production
rate, lower effluent viscosity, and higher pathogen–destruction
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B. Zhu et al. / Bioresource Technology 100 (2009) 1122–1129
(Cooney and Wise, 1975). Maly and Fadrus (1971) conducted batch
digestion tests of activated sludge from a municipal wastewater
treatment plant at 20, 30 and 50 oC. After 140 d digestion, the total
biogas production and solid reduction of the sludge at different
temperatures were almost identical, but the higher temperature
of 50 oC resulted in higher organic degradation rate than the low
temperatures. Cecchi et al. (1990, 1991) compared the biogas production of the organic solid waste at mesophilic (37 oC) and thermophilic (55 oC) conditions using a 3 m3 continuous stirred-tank
reactor (CSTR). The organic waste was separated from household
MSW and contained TS and VS of 76% and 34%, respectively. The
mesophilic experiment was conducted under the organic loading
rate (OLR) of 7.5 g VS L1 d1 and hydraulic retention time (HRT)
of 14.7 d and the biogas yield achieved was 200 mL g VS1 and
VS reduction was 23%. The thermophilic experiment was conducted at OLR of 6.9 g VS L1 d1 and HRT of 11.7 d and the biogas
yield was 410 mL g VS1 biogas yield and 43% VS reduction. But
when the OLR was increased to 9.2 g VS L1 d1, the thermophilic
system was found to be overloaded due to noticeable reduction
in biogas yield (290 ml g VS1) and VS reduction (34%). A number
of studies have recently been reported in the literature on the
anaerobic digestion of food processing wastes and grocery organic
wastes (Cho et al., 1995; Mata-Alvarez et al., 1992, 2000; Viswanath et al., 1992; Rao et al., 2000), with the reported biogas yield
being in the range of 661–796 mL g VS1 and the methane content
of 52–70%. Zhang et al. (2007) reported the thermophilic digestion
of food wastes separated from restaurant wastes. The average biogas yield and VS reduction in the food waste after 28 d of digestion
were determined to be 600 mL g VS1 and 81%, respectively. About
80% of the biogas was produced in the first 10 d of digestion with
the biogas yield being 480 mL g VS1 and methane content of biogas being 73%.
The present study was carried out to evaluate the anaerobic
digestibility and biogas production potential of the organic materials separated from MSW using the RDR process. The specific objectives of this study were to characterize the organic materials
separated from different types of solid waste available in municipalities by a RDR process operated at different retention times, to
determine their anaerobic digestibility and biogas production potential, and to assess their suitability for use as feedstock for anaerobic digestion systems.
2. Methods
2.1. Description of the rotary drum reactor (RDR) process
The RDR used for this study was at a commercial MSW treatment facility in Arizona, US, where the MSW collected from local
community and biosolids generated from the city wastewater
treatment plant were co-composted. The RDR process used in this
study comprised a rotary drum reactor of 3 m diameter and 38 m
length followed by a trommel screen with 31.8 mm openings
(Fig. 1). It was used to process 20–30 t d1 of MSW, cardboard
and paper waste, and biosolids with an average retention time of
3 d. Air was blown into the drum to enhance the aerobic microbial
activities. The drum was foam insulated to keep the temperature at
45–68 °C. The material discharged from the rotary drum was
passed through the trommel screen with the finer size fraction
being collected through the screen and the coarser size fraction
being collected on the screen. The finer fraction contains mainly
biodegradable organic materials and was sent to windrow composting and the coarse fraction contains mainly non-biodegradable
organic (e.g., plastics) and inorganic (e.g., metals, glass) materials
and was sent to a landfill. The fines account for 50–55% of the original weight of material and have a moisture content of 55–60%.
Gas exhaust
Air blow
Rotary Drum Reactor
Sampling
port 1
Sampling
port 2
Discharge
Raw solid waste
Trommel
screen
Organic
material
Inorganic
material
Fig. 1. Schematic of the rotary drum reactor process for the pre-treatment of solid
wastes.
2.2. Experimental setup, sample collection and analysis
The experiments for this study were designed to examine the
characteristics of the organic materials (fines) produced by RDR,
as affected by different types of waste and retention times in the
RDR. To accomplish this, the RDR was operated for a week from
February 26 to March 4, 2007, with four different waste types:
MSW, mixture of MSW and cardboard and paper waste, mixture
of MSW and biosolids, and mixture of cardboard and paper waste
and biosolids. Cardboard and paper waste was here referred to as
simply paper. For each type of waste, three retention times (1, 2
and 3 d) were evaluated. The weight and composition of the four
waste types tested are shown in Table 1.
According to the regular RDR operational protocols, the desired
moisture content of the material entering the drum should be
about 55%, wet basis. Therefore, water was added to MSW and to
the mixture of MSW and paper to bring the moisture content to
this value. No water was needed for the mixtures containing biosolids because the moisture content of biosolids was already high,
about 82%. The average speed by which the waste traveled inside
the drum was controlled by the loading and unloading rates. This
speed was kept constant at approximately 1/3 of the drum length
each day, or 0.53 m h1. One waste type was introduced into the
drum each day, with the simultaneous unloading of pretreated
material from the exit of the drum. Thus one third of the drum
was occupied by fresh waste everyday and there was no back-mixing observed before wastes were discharged from the other end of
the drum. Samples were collected every day from the drum exit
and two sampling ports located at 1/3 and 2/3 drum length from
the entrance. A total of 12 samples were collected from the drum,
corresponding to the above mentioned four different waste types
with three retention times in the drum for each waste type. All
the samples collected from the rotary drum were screened on-site
using a trommel screen of 31.8 mm openings to remove the large
size fraction (mainly non-biodegradable materials). The recovery
of organic materials (fines) from 3-d RDR treated solid wastes
was calculated (Table 2). The screened samples were packed on
Table 1
Quantities of wastes and water used in conducting RDR tests for organic material
recovery
Waste type
MSW
MSW and paper
MSW and biosolids
Paper and biosolids
Amount
MSW (kg)
Paper (kg)
Biosolids (kg)
Water (kg)
9400
6800
15,000
–
–
450
–
6500
–
–
10,500
7800
3600
3200
–
–
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B. Zhu et al. / Bioresource Technology 100 (2009) 1122–1129
Table 2
Characteristics of organic materials recovered from municipal solid wastes using the RDR process with different retention times (characteristics of food waste are included for
comparison)
Waste type
Retention time in RDR
(d)
Organic recovery after
trommel screen* (% wet
weight)
Characteristics of the organics recovered from RDR process
Moisture content
(%)
VS/TS
(%)
Total C (% dry
weight)
Total N (% dry
weight)
C/N
pH
MSW
1
2
3
–
–
37
59.6
50.7
49.8
75.7
73.9
71.9
41.5
42.1
41.2
1.3
1.6
1.6
30.8
27.0
26.2
6.0
7.5
5.4
MSW and Paper
1
2
3
–
–
41
57.5
50.9
51.6
72.4
69.2
75.3
41.2
38.8
40.3
1.2
1.1
1.3
33.8
36.6
30.0
7.3
7.1
6.1
MSW and
Biosolids
1
2
3
–
–
57
57.2
56.6
54.0
70.8
73.1
76.0
38.2
43.4
41.4
1.8
1.9
1.5
21.8
22.5
26.7
7.5
7.3
6.2
Paper and
Biosolids
1
2
3
–
–
68
52.5
56.0
57.4
75.0
76.9
80.0
38.9
40.7
43.2
1.8
1.7
1.8
21.1
24.6
23.8
7.5
6.2
5.9
Food Waste
–
–
74.4
96.4
50.0
6.9
7.2
4.2
*
Organic recovery for 1 and 2 d retention time in the RDR were not determined.
ice and shipped overnight to the Bioenvironmental Engineering Research Laboratory at the University of California, Davis (UC, Davis).
The samples were then put into a freezer for storage at 20 °C until
analyses. Prior to the anaerobic digestion tests, samples were manually screened using a stainless steel screen with 6.4 mm openings
to remove the large glass and metal particles in order to fit the
samples into the anaerobic reactors for biogas and methane yield
measurements.
Two sets of analyses were performed on the collected samples.
The samples collected from 3-d retention time trials were analyzed in greater detail than the samples collected with 1-d and
2-d retention time trials because the 3-d retention time is the current standard for the RDR plant used for this study. The analyses
performed for different samples are shown in Tables 2–4. All the
elemental analyses and the measurements of the various carbohydrates, lignin, NH4–N and NO3–N concentrations were performed
by the DANR laboratory of UC Davis based on the standard laboratory protocols (ANR analytical lab, 2007). The rest of the analyses
were carried out at the Bioenvironmental Engineering Research
Laboratory of UC Davis, using standard methods (APHA, AWWA
and WEF, 1998; Leege and Thompson, 1997). The experimental
procedures for determining the biogas and methane production
potential using batch anaerobic digestion tests are described
below.
2.3. Anaerobic digestion tests
The biodegradability and biogas production potential of the
waste samples collected from the RDR process were determined
using batch anaerobic digestion tests as described in Anaerobic
Lab Work (2000). The digestion was carried out in 1 L glass bottles
(KIMAXÒ No. 14397, USA) at 50 ± 1 °C. Digestion tests were performed in duplicate on the 12 waste samples along with a food
waste sample which was used for comparison. The food waste
was collected from a student cafeteria on the campus of University
of California, Davis, and consisted predominantly of leftover food.
The inoculum sludge used in the digestion tests was collected
from thermophilic anaerobic digesters at a municipal wastewater
treatment plant in Oakland, California. The TS, VS, VS to TS ratio
(VS/TS) and pH of the anaerobic sludge were measured to be
2.47%, 1.48%, 0.60, and 6.74, respectively.
Each batch reactor was initially loaded with 150 mL inoculum
sludge, waste sample containing 2.65 g VS, and 342 mL distilled
water to achieve a food to microorganism ratio (F/M) of 1.2 and
Table 3
Compositions of organic materials recovered from different types of municipal solid
wastes via RDR process at 3 d retention time
Parameters
Unit
MSW
MSW
and
paper
MSW
and
biosolids
Paper
and
biosolids
Cellulose
Hemi-cellulose
Lignin
Starch
Glucose-total
Fructose
Sucrose
Total nonstructural
carbohydrates (TNC)
Total kjeldahl nitrogen (TKN)
Ammonia nitrogen (NH4–N)
Nitrate nitrogen (NO3–N)
%
%
%
%
%
%
%
%
37.4
9.1
10.5
0.70
0.80
<0.2
<0.2
0.80
37.8
9.05
10.25
0.50
0.55
<0.2
<0.2
0.60
37.2
9.4
9.2
0.60
0.70
<0.2
<0.2
0.70
38.5
8.5
9.7
<0.5
<0.5
<0.2
<0.2
<0.5
%
ppm
ppm
1.35
240
<10
1.12
165
10
1.35
305
15
1.63
410
10
The results listed in this table are based on dry weight.
Table 4
Element concentrations in organic materials recovered from different types of
municipal solid wastes via RDR process at 3 d retention time
Parameters
Unit
MSW
MSW and
paper
MSW and
biosolids
Paper and
biosolids
Phosphorus (P)
Potassium (K)
Calcium (Ca)
Magnesium (Mg)
Chloride (Cl)
Sodium (Na)
Sulfur (S)
Iron (Fe)
Aluminum (Al)
Silicon (Si)
Zink (Zn)
Manganese (Mn)
Copper (Cu)
Cadmium (Cd)
Chromium (Cr)
Cobalt (Co)
Lead (Pb)
Molybdenum (Mo)
Nickel (Ni)
Selenium (Se)
Arsenic (As)
%
%
%
%
%
%
%
%
%
%
ppm
ppm
ppm
ppm
ppm
ppm
ppm
ppm
ppm
ppm
ppm
0.33
0.30
2.04
0.17
0.34
0.38
0.35
0.83
1.16
4.00
444
213
152
0.8
13.7
2.1
68.2
3.0
25
0.8
1.8
0.31
0.34
2.33
0.16
0.38
0.42
0.38
0.87
1.26
4.12
524
256
201
1.1
15.7
2.1
98.5
3.2
28
0.6
2.0
0.35
0.47
2.85
0.20
0.47
0.51
0.55
0.95
1.28
2.77
478
243
164
1.3
15.8
2.2
98.1
3.1
29
0.8
1.7
0.40
0.41
2.41
0.18
0.40
0.45
0.43
0.91
1.15
2.14
410
214
124
1.1
14.1
2.0
61.0
2.8
27
1.0
1.9
The results listed in this table are based on dry weight.
B. Zhu et al. / Bioresource Technology 100 (2009) 1122–1129
a working volume of 500 mL. The F/M was calculated by dividing
the feedstock VS by the inoculum VS. Each reactor was tightly
closed with a rubber septa and a screw cap. To assure anaerobic
conditions, the head space of each reactor was purged with helium
gas for five minutes. Two blank reactors were used to correct for
the biogas produced from the inoculum. Each blank reactor contained 150 mL anaerobic sludge and 340 mL distilled water.
1125
Statistical analyses were performed using Statistics Analysis
System software (SAS, version 9.1) to determine if there were significant differences in terms of cumulative biogas yield and biogas
yield potential for different retention times used in the RDR (1, 2 or
3 d). The significance test was based on Tukey’s studentized range
test and least significant difference (a = 0.05).
3. Results and discussion
2.4. Biogas production and composition measurement
3.1. Characteristics of the organic materials produced from RDR
Biogas production was estimated daily by measuring the pressure in the head space of each reactor and then converting to volume by application of the ideal gas law. Pressure was measured
using a membrane pressure gauge (Model 3150, WAL Mess-und
Regelsysteme GmbH, Germany). After the pressure measurement,
the biogas in the head space was released under water. Then the
pressure in the head space was measured again as an initial condition for the next-day measurement. Daily pressure differences
were converted into biogas volume as:
V biogas ¼
ðP2 P1 Þ T a
Vr
Pa T r
where
Vbiogas = volume of daily biogas production (mL);
P1 = post-release headspace pressure of the previous day (kPa);
P2 = headspace pressure before biogas release (kPa);
Pa = ambient pressure (kPa);
Ta = ambient temperature (K);
Tr = temperature of the reactor (K);
Vr = headspace volume (mL).
Daily biogas yield was determined by dividing the volume of biogas produced each day by the initial VS loaded into the reactor.
Cumulative biogas yield was calculated by summing the daily biogas yields at the end of each test.
The biogas composition (H2, CH4 and CO2) of each reactor was
measured daily using a gas chromatograph (GC) (AgilentÒ
GC6890 N) equipped with a thermal conductivity detector (TCD)
and packed column (AlltechÒ C-9000) of length 3.05 m, outside
diameter 3.18 mm, internal diameter 2.16 mm and with 80/100
mesh carbosphere. The carrier gas was argon at a flow rate of
30.8 mL min1. The injector, oven and detector temperatures were
120, 100 and 120 °C, respectively.
2.5. Kinetic and Statistical analyses
A kinetic model modified from the Gompertz growth equation
(Schnute, 1981; Zwietering et al., 1990) was proposed to determine
the biogas yield potential as follows:
Rm e
BG ¼ BGP exp exp
ðk tÞ þ 1
BGP
ð1Þ
where
BG = cumulative biogas yield (mL g VS1),
t = digestion time (d),
BGP = biogas yield potential (mL g VS1),
Rm = maximal daily biogas yield (mL g VS1 d),
k = bacteria growth lag time (d),
e = mathematical constant (2.718).
BGP, Rm, and k were determined using single Levenberg–Marquardt iteration by Origin 7.5 (Originlab Corporation, Northampton,
MA 01060).
The moisture content, VS/TS, TC and TN, C/N and pH of all the
samples analyzed are shown in Table 2. The moisture content of
the samples varied from 49.8% to 59.6%. Generally, longer retention
time in the drum resulted in lower moisture content in the organic
material, most likely due to the moisture loss under the forced aeration. In comparison, the food waste had a moisture content of
74.4 ± 1.8%.
The VS/TS of all the RDR pretreated waste samples were in the
range of 69.2–80.0%. Theoretically, longer retention time in the
drum could result in lower VS/TS because it was expected that biodegradable organics are removed from the waste due to the biological oxidation inside the rotary drum. However, measurements did
not support this prediction. For each type of waste, VS/TS changed
less than five percentage points with extension in retention time,
indicating that the loss of volatile matter during pre-treatment
was small. Organic materials separated from the mixture of paper
and biosolids resulted in the highest VS/TS, which may be attributed to the fact that biosolids and paper waste contained more volatile matter compared to MSW. Regarding all the RDR treated
wastes, the VS/TS values were lower than that of food waste
(96.4%), as expected. Similar to the VS/TS, the TC values of the samples were fairly consistent (40.3–43.4%). The TC of food waste was
approximately 20% higher than that of the RDR waste samples.
For a particular waste type pretreated in the RDR for different
retention times, the TN contents of the recovered organics were
similar. The organic material produced from the wastes containing
biosolids had slightly higher TN (1.5–1.9%) and lower C/N (21.1–
26.7) than the organic material from the wastes containing no
biosolids (1.1–1.6% TN and 26.2–36.6 C/N). The C/N values of all
the organic materials recovered were considered to be appropriate
for anaerobic digestion, as they were in the optimum range of 20–
30 reported by McCarty (1964). In comparison, the food waste had
TN of 6.9% and C/N of 7.2.
The pH of the organic materials recovered from 1-d retention
time in RDR was 6.0–7.5, with an average of 7.1. The pH of the
materials recovered from 3-d retention time in RDR was 5.4–6.2,
with an average of 5.9. As a general observation from Table 2,
the pH decreased in most cases along with the retention time in
the RDR. In comparison, the pH of the food waste was 4.2.
Compositions and elemental analyses of the organic materials
recovered from four different waste types with 3-d retention time
in RDR are shown in Tables 3 and 4. The compositions and elements of the four different wastes were also similar except that
the wastes containing biosolids showed higher total kjeldahl nitrogen (TKN) and ammonia–nitrogen (NH4–N).
3.2. Biogas yield and biogas yield potential
The biogas production potential is presented in terms of biogas
yield, methane content and methane yield. The results of daily and
cumulative biogas yield are shown in Figs. 2–6. Biogas production
after the 15th day of digestion was negligible, and most of the biogas was produced during the first 10 d of digestion. Regarding all
the RDR treated wastes, the peak value of daily biogas yield usually
occurred on the 4th day of digestion.
700
2d
3d
140
Daily Biogas Yield
Cumulative Biogas Yield
600
120
500
100
400
80
300
60
200
40
100
20
0
0
1
1d
120
400
80
300
60
120
500
100
400
80
300
60
200
40
100
20
2 3 4 5 6
200
40
100
20
0
7 8 9 10 11 12 13 14 15 16 17 18 19 20
0
160
Daily Biogas Yield (mL gVS-1 d-1)
Daily Biogas Yield (mL gVS-1 d-1)
Daily Biogas Yield
Cumulative Biogas Yield
600
1
500
100
Fig. 5. Daily and cumulative biogas yields of RDR pretreated mixture of paper and
biosolids.
Cumulative Biogas Yield (mL gVS-1)
700
0
600
Digestion Time (d)
160
140
Daily Biogas Yield
Cumulative Biogas Yield
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
Fig. 2. Daily and cumulative biogas yields of RDR pretreated MSW.
3d
3d
140
Digestion Time (d)
2d
2d
0
2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
1d
700
160
140
700
Daily Biogas Yield
Cumulative Biogas Yield
600
120
500
100
400
80
300
60
200
40
100
20
0
0
Cumulative Biogas Yield (mL gVS-1)
1d
Daily Biogas Yield (mL gVS-1 d-1)
Daily Biogas Yield (mL gVS-1 d-1)
160
Cumulative Biogas Yield (mL gVS-1)
B. Zhu et al. / Bioresource Technology 100 (2009) 1122–1129
Cumulative Biogas Yield (mL gVS-1)
1126
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
Digestion Time (d)
Digestion Time (d)
700
Daily Biogas Yield (mL gVS-1 d-1)
160
1d
140
2d
3d
Daily Biogas Yield
Cumulative Biogas Yield
600
120
500
100
400
80
300
60
200
40
100
20
0
0
Cumulative Biogas Yield (mL gVS-1)
Fig. 3. Daily and cumulative biogas yields of RDR pretreated mixture of MSW and
paper.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
Digestion Time (d)
Fig. 4. Daily and cumulative biogas yields of RDR pretreated mixture of MSW and
biosolids.
For the MSW samples with different retention times in the rotary drum, the daily and cumulative biogas yields are presented
in Fig. 2. The daily biogas yields appeared to decrease slightly with
the increase of retention time in the drum, probably due to the aerobic degradation of organics occurring in the drum. The peak values were calculated to be 131, 121 and 97 mL g VS1 d1, for 1, 2
and 3 d retention times in the RDR, respectively. The cumulative
Fig. 6. Daily and cumulative biogas yields of food waste.
biogas yields were calculated to be 521, 502 and 466 mL g VS1,
respectively.
Similar trends were found for the biogas yields of RDR treated
MSW and paper mixtures (Fig. 3). The peak values of daily biogas
yield were 124, 116 and 100 mL gVS1 day1, for 1, 2 and 3 d retention times. The cumulative biogas yields did not vary significantly
(a = 0.05) with the retention time in RDR, as they were determined
to be 485, 526 and 516 mL g VS1, respectively.
As shown in Fig. 4, the daily biogas yields of the waste samples
produced from mixtures of MSW and biosolids showed minor differences. The peak values for 1, 2 and 3 d retention times in the
drum were 119, 126 and 110 mL g VS1 day1, respectively, on
the 4th day of digestion. The cumulative biogas yields were 557,
534 and 492 mL g VS1, respectively. Compared with the other
three types of RDR treated wastes, the cumulative biogas yields
for the MSW and biosolids mixtures were clearly higher. The combination of MSW with biosolids appears to be more favorable for
biogas production compared to the other combinations, possibly
because of comparatively higher nitrogen content.
The samples produced from the mixture of paper and biosolids
had the lowest biogas yields (Fig. 5) among all the samples evaluated. The peak values of daily biogas yield were 98, 125 and
111 mL gVS1 day1, respectively, while the cumulative biogas
yields were 457, 507 and 504 mL g VS1, respectively. This may
be attributed to the fact that cellulose contained in paper is more
difficult to digest than other organics contained in MSW (Müller
et al, 2004).
1127
B. Zhu et al. / Bioresource Technology 100 (2009) 1122–1129
As a comparison, the daily and cumulative biogas yields of the
food waste are shown in Fig. 6. The main difference between food
waste and RDR treated solid wastes lay in the daily biogas yield
and the duration of the anaerobic digestion process. The biogas
production duration of food waste was prolonged, with lower daily
biogas yields (peak value: 90 mL g VS1 day1) compared to the
four RDR treated wastes. However, the cumulative biogas yield of
the food waste (609 mL g VS1) was significantly (a = 0.05) higher
than the cumulative biogas yields obtained from the RDR treated
wastes. Higher cumulative biogas yield with lower daily biogas
yield was expected for food waste, as food waste contained more
protein compared to the organic materials in MSW and paper.
According to Bushwell’s formula (Symons and Bushwell, 1933),
proteins have higher biogas production potential but lower degradation rates compared to carbohydrates.
The nonlinear regression on the cumulative biogas yield and
digestion time (Table 5) shows that the Gompertz growth equation
fitted the experimental results well. With the RDR retention time
extended from 1 to 3 d, the biogas yield potential of the pretreated
MSW decreased from 522 to 482 mL g VS1 and the maximum daily biogas yield decreased from 97 to 72 mL g VS1 d. The results of
statistical analyses also showed significant (a = 0.05) differences in
these parameters. These facts indicated that the longer retention
time in RDR could lead to the higher loss of biodegradable compounds in the MSW. Similar trends were shown in the results for
the mixture of MSW and biosolids, with the biogas yield potential
decreasing from 562 to 532 mL g VS1 and maximum daily biogas
yield (Rm) from 99 to 83 mL g VS1 d1. Therefore 1-d retention
time in the RDR is preferred for obtaining good biogas yields and
production rates. For the mixture of MSW and paper, 2-d retention
time resulted in slightly higher biogas yield potential of
532 mL g VS1, maximum daily yield of 93 mL g VS1 d1, as compared to 1- and 3-d retention times. For the mixture of paper and
biosolids, the biogas yield potential increased from 457 to
510 mL g VS1 when the retention time in the RDR varied from 1
to 3 d and the maximum daily biogas yield (Rm) increased from
73 to 86 mL g VS1 d1. The reason may be that the cellulosic compounds in the paper need longer retention time in the RDR to become more digestible. Compared with RDR pretreated solid
wastes, food waste showed higher biogas yield potential
(613 mL g VS1).
Based on the results of Tukey’s significance test (Table 6), for
each type of RDR treated waste, no significant difference
(P < 0.05) was found among the experimental biogas yields for
the samples collected under different RDR retention times (1, 2
and 3 d). Since a shorter retention time in the RDR translates to a
smaller size of rotary drum and better economics, 1-d retention
time in the rotary drum is recommended for providing the pretreatment of MSW prior to anaerobic digestion. Further research
needs to be carried out in order to determine if the retention time
in the drum could be further reduced (i.e., whether simple separation might be equally beneficial). This study did not quantify the
separation or recovery efficiencies of organic materials separated
from the wastes which were treated by RDR with different retention times. The effect of the retention time in RDR on organic
recovery needs to be investigated in the future research. A final
Table 5
Kinetic parameters of biogas yield estimated using modified Gompertz equation
Waste type
Retention time in
RDR (d)
Biogas yield potential* (BGP)
(mL g VS1)
Maximal daily biogas yield (Rm)
(mL g VS1 d)
Bacteria growth lag time
(k) (d)
Correlation
coefficient R2
MSW
1
2
3
1
2
3
1
2
3
1
2
3
–
522a
509a,b
482b
487c
532c
516c
562d
550d,e
532e
457f
510g
510g
613
97
87
72
94
93
84
99
93
83
73
83
86
80
1.2
1.5
1.7
1.3
1.7
1.8
1.2
1.3
1.7
1.1
1.3
1.7
0.9
0.983
0.993
0.995
0.981
0.996
0.998
0.982
0.985
0.996
0.980
0.989
0.996
0.989
MSW and paper
MSW and
biosolids
Paper and
biosolids
Food waste
*
Values within the same type of waste followed by different letters are significantly different at P < 0.05.
Table 6
TS and VS reduction and biogas and methane yields of organic materials recovered from different types of municipal solid wastes via RDR process
Waste type
Retention time in RDR
(d)
Feedstock TS reduction
(%)
Feedstock VS reduction
(%)
Biogas yield*
(mL g VS1)
Methane yield
(mL g VS1)
Methane content of
biogas (%)
MSW
1
2
3
52.3
60.1
65.0
59.0
63.9
66.8
520a,*
502a,b
466b
298
300
282
57.3
59.9
60.6
MSW and paper
1
2
3
48.4
56.5
58.8
53.8
57.2
62.6
485c
526c
516c
282
313
308
58.2
59.8
59.9
MSW and
biosolids
1
2
3
60.0
54.5
61.9
64.2
57.0
69.3
557d
534d
492d
320
319
296
57.3
59.7
60.2
Paper and
biosolids
1
2
3
55.3
50.2
52.2
57.5
51.8
58.8
457e
507e
504e
261
300
304
57.2
59.4
60.3
Food waste
–
86.0
88.1
609
350
57.5
*
Values within the same type of waste followed by different letters are significantly different at P < 0.05.
1128
B. Zhu et al. / Bioresource Technology 100 (2009) 1122–1129
Methane Content of Biogas (%)
100
90
MSW
MSW and Paper
Paper and Biosolids
Food waste
MSW and Biosolids
80
70
60
50
40
30
20
10
0
1
2
3
4
5
6
7
8
9 10 11 12 13 14 15 16 17 18 19 20
Digestion Time (d)
Fig. 7. Methane concentrations of the biogas produced from RDR treated solid
wastes and from food waste.
recommendation for the retention time in RDR for a specific type of
waste should be based on combined considerations of both the organic recovery through RDR process and biogas yield of the organic
material recovered from wastes.
3.3. Methane content of biogas and methane yield
For the RDR treated wastes, biogas methane concentrations
showed similar trends for different retention times in the drum
(Fig. 7). On the first day of anaerobic digestion, the methane contents were around 52%, with trace amounts of hydrogen. After
the second day, the methane contents stabilized at about 61%,
while headspace hydrogen concentration was negligible. It must
be noted that the experimental conditions for this study were designed for complete biomass degradation. Bio-hydrogen production could be distinct if higher initial loading rate had been
selected. The methane content of biogas produced by food waste
continued increasing from 37% to 68% until the 11th day of
digestion.
The methane yields of RDR treated wastes and food waste were
calculated (Table 6) from daily biogas yield and methane content of
biogas. The methane yields of RDR treated wastes were in the
range of 261–320 mL g VS1, while food waste presented higher
methane yield of 350 mL g VS1. Among the four types of solid
wastes evaluated in this study, the organic material recovered from
RDR treated mixtures of MSW and biosolids showed the highest
methane production potential of 320 mL g VS1.
3.4. Solids reduction
As shown in Table 6, the organic materials separated from MSW
presented the highest TS and VS reduction which were in the range
of 52.3–65.0% and 59.0–66.8%, respectively. The mixture of paper
and biosolids showed the lowest TS and VS reductions of 50.2–
55.3% and 51.8–58.8% between the RDR treated wastes and food
wastes. The wastes containing biosolids showed comparatively
higher TS and VS reduction than the wastes containing paper.
The above results suggest that MSW consists of more easily biodegradable materials than biosolids and paper. Food waste presented
higher TS and VS reduction (86.0% and 88.1%, respectively) than all
the RDR treated wastes.
4. Conclusions
The results of this study showed that the RDR process could be
used as an effective technology for separation and pre-treatment of
the organic materials in MSW prior to anaerobic digestion. For the
four types of waste materials studied, the organic fractions recovered from the RDR process showed similar characteristics with regard to the anaerobic digestion, in terms of biogas and methane
yields which ranged 457–557 mL g VS1 and 261–320 mL g VS1,
respectively, after 20 d digestion at 50 ± 1 °C. Methane content of
biogas ranged from 57.3% to 60.6%. The organic material recovered
from the mixture of MSW and biosolids with 1-d retention time in
the RDR showed the highest biogas yield (557 mL g VS1), while
the organic material recovered from paper and biosolids mixture
with 1-d retention time in the RDR showed the lowest biogas yield
(457 mL g VS1). About 90% of the total biogas yield was achieved
in the first 10 d of digestion for most of the samples. The TS and VS
reductions in the various RDR treated wastes after 20 d anaerobic
digestion were 48.4–65.0% and 51.8–69.3%, respectively. The biogas yield potential estimated by the nonlinear Gompertz equation
showed significant decrease (a = 0.05) on the wastes that contained papers when the retention time in RDR varied from 1 to
3 d. One-day retention time is preferable among the three retention times tested based on the test results of organic fractions as
shorter retention time translates to either smaller drums or fewer
drums and better economics. However, further studies are recommended to determine the minimum retention time requirements
in the drum for different types of wastes, and to assess the separation efficiencies of the organic fraction under different retention
times.
Acknowledgements
This research was conducted at University of California, Davis
with funding support from the California Integrated Waste Management Board. The authors wish to thank the Pinetop-Lakeside
Sanitary District, Lakeside, Arizona, and particularly the operations
manager Mr. Phil Hayes for providing technical and management
assistance for this project. The authors also wish to thank Chris
Choate of Norcal Waste Systems, Inc., California, for his guidance
and support of this study.
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