This article was downloaded by: [University of California, San Francisco]On: 05 September 2014, At: 04:08Publisher: Taylor & FrancisInforma Ltd Registered in England and Wales Registered Number: 1072954 Registered office: MortimerHouse, 37-41 Mortimer Street, London W1T 3JH, UK
Journal of Environmental Science and Health, PartA: Toxic/Hazardous Substances and EnvironmentalEngineeringPublication details, including instructions for authors and subscription information:http://www.tandfonline.com/loi/lesa20
Effects of Mixture Ratio and Hydraulic RetentionTime on Single-Stage Anaerobic Co-digestion of FoodWaste and Waste Activated SludgeNam Hyo Heo a b , Soon Chul Park b & Professor Ho Kang aa Department of Environmental Engineering , Chungnam National University , Daejon,Koreab Biomass Research Center , Korea Institute of Energy Research , Daejon, KoreaPublished online: 16 Aug 2006.
To cite this article: Nam Hyo Heo , Soon Chul Park & Professor Ho Kang (2004) Effects of Mixture Ratio and HydraulicRetention Time on Single-Stage Anaerobic Co-digestion of Food Waste and Waste Activated Sludge, Journal ofEnvironmental Science and Health, Part A: Toxic/Hazardous Substances and Environmental Engineering, 39:7, 1739-1756,DOI: 10.1081/ESE-120037874
To link to this article: http://dx.doi.org/10.1081/ESE-120037874
PLEASE SCROLL DOWN FOR ARTICLE
Taylor & Francis makes every effort to ensure the accuracy of all the information (the “Content”) containedin the publications on our platform. However, Taylor & Francis, our agents, and our licensors make norepresentations or warranties whatsoever as to the accuracy, completeness, or suitability for any purpose ofthe Content. Any opinions and views expressed in this publication are the opinions and views of the authors,and are not the views of or endorsed by Taylor & Francis. The accuracy of the Content should not be reliedupon and should be independently verified with primary sources of information. Taylor and Francis shallnot be liable for any losses, actions, claims, proceedings, demands, costs, expenses, damages, and otherliabilities whatsoever or howsoever caused arising directly or indirectly in connection with, in relation to orarising out of the use of the Content.
This article may be used for research, teaching, and private study purposes. Any substantial or systematicreproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in anyform to anyone is expressly forbidden. Terms & Conditions of access and use can be found at http://www.tandfonline.com/page/terms-and-conditions
JOURNAL OF ENVIRONMENTAL SCIENCE AND HEALTH
Part A—Toxic/Hazardous Substances & Environmental Engineering
Vol. A39, No. 7, pp. 1739–1756, 2004
Effects of Mixture Ratio and Hydraulic Retention Time
on Single-Stage Anaerobic Co-digestion of
Food Waste and Waste Activated Sludge
Nam Hyo Heo,1,2 Soon Chul Park,2 and Ho Kang1,*
1Department of Environmental Engineering,
Chungnam National University, Daejon, Korea2Biomass Research Center, Korea Institute of Energy Research, Daejon, Korea
ABSTRACT
The biochemical methane potential (BMP) test was used to evaluate the anaerobic
biodegradabilities of food waste (FW), waste activated sludge (WAS), and the
mixtures having the ratios of 10:90, 30:70, 50:50, 70:30, and 90:10 (FW:WAS) on
a volatile solid (VS) basis. The carbon/nitrogen (C/N) ratio and the biodegrad-
ability of the mixtures improved from 6.16 to 14.14 and increased from 36.6 to
82.6% as the FW proportion of the mixture increased from 10 to 90%,
respectively. The stability and performance of the single-stage anaerobic digester
(SSAD) for the co-digestion of FW and WAS were investigated, operated at the
hydraulic retention times (HRTs) of 10, 13, 16, and 20 days with five mixtures at
35�C, respectively. During all the experiments, there were no indication of failure
such as low pH, insufficient alkalinity, ammonia inhibition, and the accumulation
of volatile fatty acids (VFAs) in any of the digesters, and the buffer capacity was
the highest in the digester fed with a feed mixture of 50:50. The optimum
operating conditions of the SSAD were found to be an HRT of 13 days and a
mixture of 50:50 in terms of the buffer capacity of the digester and the effluent VS
*Correspondence: Ho Kang, Professor, Department of Environmental Engineering,
Chungnam National University, 220 Gungdong, Yuseongku, Daejon 305-764, Korea;
Fax: (82) 42-822-5610; E-mail: [email protected].
1739
DOI: 10.1081/ESE-120037874 1093-4529 (Print); 1532-4117 (Online)
Copyright & 2004 by Marcel Dekker, Inc. www.dekker.com
Dow
nloa
ded
by [
Uni
vers
ity o
f C
alif
orni
a, S
an F
ranc
isco
] at
04:
08 0
5 Se
ptem
ber
2014
ORDER REPRINTS
concentration, the methane content of the biogas produced and the specific
methane production (SMP). The VS removal efficiency, biogas production rate
(GPR), and SMP in this condition achieved 56.8%, 1.24m3m�3d�1 and 0.321m3
CH4 kg�1VS�1
fed with an organic loading rate (OLR) of 2.43 kgVSm�3d�1.
Key Words: Anaerobic co-digestion; Biochemical methane potential (BMP)
test, Biogas production; Food waste; Mixture waste; Performance; Stability;
Waste activated sludge.
INTRODUCTION
In 2001, food waste (FW) generated in Korea was about 11,091 t d�1, of which56.7% is recycled as aerobic composting, feedstuff, and methanization and the rest isdisposed by landfill and incineration.[1] In 2005, landfilling of FW, which causesvarious problems such as a foul odor, emission of greenhouse gas (CH4, CO2) at thelandfill site and surface contamination by leachate, will be prohibited. Food waste isconsiderably difficult to treat or recycle by composting or feedstuff because itcontains high level of sodium salt and moisture as well as a sanitary problem asanimal feed.[2] Whereas, most of particulate materials in FW consist of easilybiodegradable organic components such as grains, vegetables, fruits, fish, and meat.[1]
Therefore, anaerobic digestion has been considered as a feasible alternative ofwaste volume reduction and a renewable energy recovery as the methane. PresortedFW with total solid (TS) of 15–30% has a high volatile solid (VS) content (88–92%of TS), and anaerobic biodegradability in batch BMP (Biochemical MethanePotential) test was estimated to be 86% with the methane potential of 0.472m3
CH4 kg�1 VS�1
fed.[3] A characteristic of FW that contains highly soluble organic
materials, the soluble organics would be rapidly converted to volatile fatty acids(VFAs) in the early stage of digestion. Therefore, the reactor configuration has beenconsidered a two-phase anaerobic digestion system, consisting of acid and methanephases, for the effective treatment of FW with a high organic loading rate (OLR)and methane production.[3–5]
One of the interesting alternatives for the treatment of FW is the anaerobicco-digestion together with sewage sludge in existing digester of municipal wastewatertreatment plant (MWTP).[6–8] A major concept of the co-digestion involves thetreatment of several wastes to get higher biogas production in a single treatmentfacility that makes the operation of biogas plants to be more economically feasible.The feasibility of anaerobic co-digestion of waste activated sludge and simulatedorganic fraction of municipal solid waste (OFMSW) was examined by Poggi-Varaldo and Oleszkiewicz.[9] The co-digestion has been successfully applied in full-scale for the treatment of OFMSW and sewage sludge.[10] In 2002, the number ofMWTP operated in Korea reach to 183 facilities. About 80% of the MWTP are notin full operation. Therefore, adding FW to existing anaerobic digester in the MWTPmay be a good alternative without significant investments.
The objective of this study was to evaluate the feasibility of the anaerobicco-digestion of FW and WAS. The specific objectives of this study were as follows:(a) to evaluate the anaerobic biodegradability based on cumulative methane
1740 Heo, Park, and Kang
Dow
nloa
ded
by [
Uni
vers
ity o
f C
alif
orni
a, S
an F
ranc
isco
] at
04:
08 0
5 Se
ptem
ber
2014
ORDER REPRINTS
productions of FW, WAS, and different mixtures using the batch BMP test; (b) toinvestigate the effects of hydraulic retention time (HRT) and mixture ratio of twobiowastes on the stability and performance of the single-stage anaerobic digestionprocess.
MATERIALS AND METHODS
Feedstocks and Mixtures
To simulate FW generated in Korea, a traditional Korean food called Bibimbab,which has a similar composition to FW, was used in this study. Food waste had anaverage 18% TS and consisted of boiled rice (10–15%), vegetables (65–70%), andmeat and eggs (15–20%). The FW was crushed down to 2–4mm using a cook mixerand the concentration of TS was controlled by adding tap water. WAS was obtainedfrom the secondary clarifier in MWTP located in Daejon, Korea, and itsconcentration was adjusted to 3% TS using a centrifuge. The mixture ratios ofFW and WAS (FW:WAS) were 10:90, 30:70, 50:50, 70:30, and 90:10 on a VS basis,respectively, and the mixtures were used in experiments.
Biochemical Methane Potential (BMP) Test
To evaluate the anaerobic biodegradability of FW, WAS and the mixtures, theBMP test was performed in 500mL Erlenmyer flask at 35�C using a modifiedmethod of Owen et al.[11] The inoculum used in the BMP test, in which themicroorganisms were well adapted to the mixture of FW and sewage sludge, wasobtained from a laboratory scale anaerobic co-digestion process operated at 35�C inthe Korea Institute of Energy Research. The characteristics of WAS, SFW, andinoculum used in the BMP test are shown in Table 1. The inoculum was filled to250mL in each flask, and the S/I ratio was brought to 0.2 with the addition of FW,
Table 1. Characteristics of the feedstocks and inoculum used in the BMP test.
Item
Feedstocks of the mixture
WAS FW Inoculum
TCOD (gL�1) 35.5 64.4 23.8
SCOD (gL�1) 0.26 11.5 0.31
TS (gL�1) 30.3 50.8 21.7
VS (g L�1) 22.2 46.7 14.2
pH 6.80 5.50 7.40
Alkalinity (gCaCO3 L�1) 0.70 0.25 3.65
NHþ4 -N (gL�1) 0.01 0.03 0.81
aVFAs (gL�1) 0 0.15 0
aVFAs: C2–C6.
Anaerobic Co-digestion of FW and WAS 1741
Dow
nloa
ded
by [
Uni
vers
ity o
f C
alif
orni
a, S
an F
ranc
isco
] at
04:
08 0
5 Se
ptem
ber
2014
ORDER REPRINTS
WAS and the mixtures, and the assays were purged with nitrogen gas to eliminateair. The S/I ratio is given by the ratio of the VS mass of feed substrate to the VS massof inoculum added. All assays were shaken by hand three times per day andincubated for 40 days in an incubator. Biogas produced from each assay wasmeasured with a glass syringe of 10mL volume. The biogas volume was thennormalized on standard temperature and pressure (0�C, 1 atm) by compensatingboth the thermal expansion according to Charles’s law and the volume occupied bywater vapor.
The anaerobic biodegradability was calculated by Eq. (1),[3,11,12] in which VC iscumulative methane production (CMP) obtained from the BMP test, VT istheoretical methane production (TMP). Generally, the organic wastes arerepresented with a generalized formula of the form CaHbOcNd. Assuming completeconversion of the biodegradable organic constituents to CO2 and CH4, the TMYcan be estimated using Eq. (2).[13,14]
Biodegradability ð%Þ ¼VC
VT� 100 ð1Þ
CaHbOcNd þ 1=4ð4a� b� 2cþ 3d ÞH2O ! 1=8ð4aþ b� 2c� 3d ÞCH4 þ
1=8ð4a� bþ 2cþ 3d ÞCO2 þ dNH3
ð2Þ
Single-Stage Anaerobic Co-digestion of the Mixtures
The characteristics of the feedstocks and five mixtures fed to the digester areshown in Table 2, and the mixtures were prepared every week and kept in a
Table 2. Physico–chemical characteristics of the feedstocks and five mixtures.
Items
Feed stocksc dMixture ratios (gVSFW:gVSWAS)
FW WAS 10:90 30:70 50:50 70:30 90:10
TCOD (gL�1) 110 36.8 43.3 48.7 60.0 77.8 97.4
SCOD (gL�1) 42.6 0.21 2.58 3.44 11.4 19.1 33.5
TS (gL�1) 85.0 30.0 3.16 3.34 3.81 4.80 6.28
VS (gL�1) 80.0 22.0 2.40 2.60 3.18 4.04 5.57aVFA (gL�1) 0.75 0 0.72 0.41 0.44 0.92 2.21
pH 4.50 6.80 6.10 5.40 4.10 4.07 4.03bAlkalinity (g L�1) 0.15 0.80 0.87 0.53 0 0 0
NHþ4 -N (g L�1) 0.29 0.03 0.07 0.14 0.16 0.15 0.14
C/N ratio 16.2 5.60 5.97 6.99 8.90 11.0 14.7
aVFA: C2–C6.bAlkalinity is expressed as CaCO3.cNumbers are the mean measured during all experiments.dNumbers are the mean measured during the start-up period of each feed mixture.
1742 Heo, Park, and Kang
Dow
nloa
ded
by [
Uni
vers
ity o
f C
alif
orni
a, S
an F
ranc
isco
] at
04:
08 0
5 Se
ptem
ber
2014
ORDER REPRINTS
refrigerator at 4�C. A single-stage anaerobic co-digester (SSAD) made ofPlexiglass—a semi-continuously fed and mixed reactor (SCFMR), was used in thisstudy. The volume of the digester was 4.0 L and its working volume was 3.5 L. FourSSADs were arranged in a water bath and inoculated with the same inoculum usedin the BMP test. The digester was operated with a fill and draw method andmechanically stirred at 80 rpm by an electric motor. The water bath was maintainedat 35�C by circulating water through a water jacket by a temperature controlledcirculator (Haake, Karlsruhe, Germany). The start-up period of each mixture was 50days to obtain reliable results and the HRT of four digesters were maintained from10 day to 13, 16 and 20 days with various ranging of the OLRs, respectively. Thebiogas produced from each digester was collected in Tedlar bags. The biogas volumewas measured by a wet gas meter (Sinagawa, Tokyo, Japan) and then normalized onstandard temperature and pressure (0�C, 1 atm). The influent, effluent, and biogasproductions of the co-digester were measured three times per week during thestart-up of each mixture.
Analytical Methods
The pH and temperature were monitored. Chemical oxygen demand (COD), TS,VS, alkalinity, NHþ
4 -N concentrations of the samples were determined accordingto Standard Methods.[15] The sample for the analysis of soluble COD (SCOD),NHþ
4 -N, VFA was prepared by filtration using a 0.45 mm cellulose acetate membraneafter centrifugation at 15,000 rpm for 15min. Elemental composition of the feed-stocks were analyzed by elemental analyzer (CHN-1000, LECO Co., USA) andsulfur analyzer (SC-432DR, LECO Co., USA). The percentages of methane andcarbon dioxide were analyzed using a gas chromatography (HP-5890A GC, USA)equipped with a thermal conductivity detector (TCD) and a 6 ft stainless columnpacked with Hayesep Q (80/100mesh). The injection and detector temperatures were120 and 150�C, respectively, and the column oven operated isothermally at 60�C.The concentration of VFA was determined using the same GC equipped with a flameionization detector (FID) and a capillary column (25m� 0.2mm� 0.33 mm; HewlettPackard-FFAP, USA). The injection and detector temperatures were 200 and 220�C,respectively. The initial temperature of the column oven was 80�C and increasedgradually by 13�Cmin�1, reaching a final temperature of 180�C. Helium was usedas the carrier gas.
RESULTS AND DISCUSSION
BMP Test Results of Feedstocks and Five Mixtures
C/N Ratio and Theoretical Methane Production (TMP)
The elemental compositions of WAS, FW, and five mixtures were analyzed toassess the theoretical methane production (TMP) and anaerobic biodegradability,and those C/N ratios and TMP were summarized in Table 3. The C/N ratios of WASand FW were 5.79 and 16.1, respectively, and that of FW was about three times
Anaerobic Co-digestion of FW and WAS 1743
Dow
nloa
ded
by [
Uni
vers
ity o
f C
alif
orni
a, S
an F
ranc
isco
] at
04:
08 0
5 Se
ptem
ber
2014
ORDER REPRINTS
higher than that of WAS. The TMP of WAS and FW, which were estimated byEq. (2) based on the elemental compositions of the sample, were 0.542 and 0.489m3
CH4 kg�1 VS�1
fed, respectively. The C/N ratio of the mixtures having differentproportion of FW improved from 6.16 to 14.14 as the mixture ratio of FW wasincreased from 10 to 90%, on the other hand, the TMP rather decreased from 0.522to 0.493m3 CH4 kg
�1 VS�1fed with increasing proportion of FW.
Evaluation of the Biodegradability by BMP Test
The BMP test, which is considered the most suitable method for a relatively easyevaluation of the anaerobic digestibility,[3,11,12,16] was used as a tool for evaluatingthe methane production and biodegradability of the feedstocks and five mixtures.Figure 1 shows the cumulative methane production obtained from the BMP test forthe feedstocks and five mixtures. The methane production obtained at the incubationof 40 days for the WAS and FW were 0.159 and 0.489m3 CH4 kg
�1 VS�1fed and those
biodegradabilities were estimated to be 29.3 and 86%, respectively. The biodegrad-ability of FW, of which was similar to that obtained by Cho et al.,[3] was about threetimes higher than that of WAS, it was evident that most of the particulate materialsconsisting of the FW were easily biodegradable components. Therefore anaerobicdigestion was found to be a good alternative for effective treatment of the Koreanfood waste, with a considerable reduction of the volatile solids and production of themethane.
As shown in Fig. 1, the methane production of the mixtures having differentratio of FW to WAS increased with increasing proportion of FW, and a significantdifference in methane production with each of the mixture was observed. At theincubation of 40 days, cumulative methane production increased from 0.191 to0.407m3 CH4 kg
�1 VS�1fed as the proportion of FW was increased from 10 to 90%,
and the biodegradability ranged from 36.6 to 82.6%. Considering for lower methaneproduction of the mixture having higher proportion of WAS, the result demonstratesthe fact that the hydrolysis of WAS is the rate-limiting step on the anaerobicbiodegradation.[17]
Table 3. Elemental compositions, C/N ratios, and theoretical methane productions (TMP) of
WAS, SFW, and five mixtures.
Wastes
Elemental compositions (wt% TS) and C/N ratiosTMP
(m3 CH4kg�1 VS�1
fed)C H O N C/N
FW 45.60 6.68 38.80 2.84 16.1 0.489
WAS 38.75 5.39 21.35 6.69 5.79 0.542
10:90 38.90 5.52 24.15 6.32 6.16 0.522
30:70 39.60 5.65 26.50 5.53 7.16 0.515
50:50 41.00 5.87 29.60 4.89 8.38 0.505
70:30 43.20 6.21 33.04 4.00 10.80 0.503
90:10 45.12 6.58 37.23 3.19 14.14 0.493
1744 Heo, Park, and Kang
Dow
nloa
ded
by [
Uni
vers
ity o
f C
alif
orni
a, S
an F
ranc
isco
] at
04:
08 0
5 Se
ptem
ber
2014
ORDER REPRINTS
A linear correlation (R2¼ 0.977) between anaerobic biodegradability and
mixture ratio of FW is given in Fig. 2. This result is consistent with the studies ofCecchi et al.[18] and Hamzawi et al.[6] on anaerobic co-digestion of OFMSW andsewage sludge. Cecchi et al.[18] found that an almost linear decrease of the organicmatter removal efficiency occurred with an increase in the percentage of sewagesludge. Hamzawi et al.[6] reported that the biogas production of the mixtureincreased with increasing proportion of OFMSW. Therefore, it becomes clear that
y = 10.946x + 24.073
R2
= 0.9732
0
20
40
60
80
100
10% 30% 50% 70% 90% 100%
Mixture ratio of food waste (% FW)
Ana
erob
ic b
iode
grad
abili
ty (
%)
Figure 2. Relationship between anaerobic biodegradability vs. mixture ratio of food waste
(FW).
0.0
0.1
0.2
0.3
0.4
0.5
0 5 10 15 20 25 30 35 40
Time (days)
Cum
ulat
ive
met
hane
pro
duct
ion
(m3
CH
4 kg
−1 V
S fed
−1)
WAS FW 10% FW 30%FW 50% FW 70% FW 90%FW 100%
Figure 1. Cumulative methane production of the feedstocks (WAS, FW 100%) and five
mixtures vs. reaction time in the BMP test.
Anaerobic Co-digestion of FW and WAS 1745
Dow
nloa
ded
by [
Uni
vers
ity o
f C
alif
orni
a, S
an F
ranc
isco
] at
04:
08 0
5 Se
ptem
ber
2014
ORDER REPRINTS
increased biodegradability could be attributed to the increased proportion of readilybiodegradable matter by the addition of FW. Accordingly, using a co-substrate bythe mixing of FW and WAS, the anaerobic co-digestion process may be more stableowing to the improved nutrient balance and C/N ratio, and also better methane yieldcould be achieved due to increased load of biodegradable organic matter.
Continuous Single-Stage Anaerobic Co-digestion of Different Mixture
In this work, the laboratory experiments using the SSAD maintained at theHRT ranging from 10 day to 13, 16, and 20 days were conducted to investigate theeffects of HRT and the mixture ratio of FW and WAS on the stability andperformance of the digester. During the start-up of the mixtures having differentproportion of FW at different HRTs, an operating period of about 15–20 days wasrequired to reach the first steady-state condition. Indicators such as pH, alkalinity,NHþ
4 -N, and VFAs concentrations were monitored to estimate the digester stability,and the COD and VS removal efficiencies, methane content of biogas, biogasproduction rate (GPR) and specific methane production (SMP) were investigatedto determine the operating performance of the digester.
C/N Ratios of Feed Mixtures
The C/N ratio of the substrate, of which is the prerequisite for a stableperformance and microbial growth and metabolism, is of particular importance inanaerobic digestion process. The mixing of OFMSW e.g., food waste, paper waste,food industry waste, etc. together with sewage sludge in the anaerobic co-digestionprocess, it is beneficial due to a different compositional characteristics of twobiowastes that are complementary. The high concentration of macro- andmicronutrients in the sewage sludge could compensate the deficiency of thosecompounds in OFMSW, and the low C/N ratio of sewage sludge could be increasedby the addition of OFMSW.[8,9,13,19–21] As shown in Table 2, the C/N ratios of FWand WAS were 16.2 and 5.60, respectively, and those of five mixtures were improvedfrom 5.97 to 14.07 as the FW proportion of the mixture increased from 10 to 90%.Sosnowski et al.[13] reported that the C/N ratio of the mixture increased from 9.3 to14.2 as 25% OFMSW was added to 75% sewage sludge and the biogas volumeproduced from the mixture was two times higher than that obtained from the sewagesludge only. In high solid anaerobic co-digestion of OFMSW and sewage sludge,the optimum C/N ratio for methane production with no adverse effect on theperformance was found to be in the range of 25–30.[20,21]
Evaluating Stability of SSAD at Steady-State Condition
The parameters of digester stability, such as pH, alkalinity, VFAs and ammoniaconcentration, are of particular importance in the process control and have to besimultaneously considered to understand the behavior of the anaerobic digestion
1746 Heo, Park, and Kang
Dow
nloa
ded
by [
Uni
vers
ity o
f C
alif
orni
a, S
an F
ranc
isco
] at
04:
08 0
5 Se
ptem
ber
2014
ORDER REPRINTS
process. Figure 3 shows the pH (A), alkalinity (B), and VFAs (C) concentration ofthe digesters operated at different HRTs with five mixtures, and these threeparameters are presented the average value measured at the steady-state condition ofthe digester. In anaerobic digestion, the methanogenic reactor is generally stable inthe pH range 6.5–7.5 and in the alkalinity range 2000–5000mgCaCO3L
�1.[22]
As shown in Fig. 3, the pH and alkalinity increased when the HRTs becomeslonger and were the highest at a mixture of 50:50 among five mixtures. During all theexperiments with five mixtures, the digesters operated at different HRTs maintainedwithin a stable pH range 7.2–7.5 and sufficient alkalinity ranging from 3.1 to
2.5
3.0
3.5
4.0
4.5
5.0
Alk
alin
ity (
g C
aCO
3 l−
1 ) (B)
7.1
7.2
7.3
7.4
7.5
7.6
pH
(A)
0.00
0.04
0.08
0.12
0.16
0.20
10:90 30:70 50:50 70:30 90:10
Mixture ratio (g VSFW:g VSWAS)
VFA
as
acet
ic a
cid
(g l−
1 )
10 day 13 day 16 day 20 day
(C)
Figure 3. pH (A), alkalinity (B), and VFAs (C) concentration of the SSAD operated at the
HRTs of 10, 13, 15, and 20 days with five mixtures.
Anaerobic Co-digestion of FW and WAS 1747
Dow
nloa
ded
by [
Uni
vers
ity o
f C
alif
orni
a, S
an F
ranc
isco
] at
04:
08 0
5 Se
ptem
ber
2014
ORDER REPRINTS
4.74 gCaCO3L�1. The VFA was not detected in the digester fed with the mixtures of
10:90, 30:70, and 50:50 as shown in Fig. 3, while its concentrations for treating themixtures of 70:30 and 90:10 ranged 0.026–0.036 gL�1 and 0.059–0.188 gL�1 as aceticacid, respectively. The increase in fatty acids in the anaerobic digester is generallydue to the high OLR, the alkalinity rapidly decrease when the VFAs accumulate inthe digester. The VFAs as acetic acid to alkalinity (V/A) ratio, which can be a goodtool for the observation of the stability of the anaerobic process, is good to be below0.3 for good stability of the digester.[23] The V/A ratio of the digesters operated atdifferent HRTs with five mixture was zero or much less than 0.3 during the entireoperation. Therefore all the digesters were also stable with respect to the V/A ratio.
Figure 4 shows the effluent NHþ4 -N concentrations of the digesters as a function
of HRT with five mixtures. The ammonia nitrogen�ammonium ion (NHþ4 ) and free
ammonia (FA, NH3), which depends on pH and temperature, exists in two formsin the liquid phase. The FA was reported to have more significant inhibition effectthan ammonium ion, and the FA of about 100mgL�1 and ammonium ion of3000mgL�1 caused inhibition in anaerobic system.[22,24] As shown in Fig. 4, theNHþ
4 -N concentration of the digester was a significant effected by the FWproportion of the mixture as well as pH and alkalinity and increased when the HRTsbecomes longer. Especially, the pH, alkalinity, and NHþ
4 -N concentration in all thedigesters simultaneously decreased as the FW proportion of the mixture increasedfrom 50 to 70 and 90%. It was likely due to a low organic nitrogen i.e., owing tonitrogen deficiency with a high C/N ratio by increasing FW proportion of themixture as shown in Tables 2 and 3.
The buffer capacity of the digester results from the ammonia formed from thebiodegradation of organic nitrogenous components and bicarbonate (HCO�
3 ) from
0.4
0.6
0.8
1.0
1.2
1.4
0 50 100 150 200 250
Start-up period (days)
NH
4+-N
(gl
−1)
20 day 16 day 13 day 10 day
10:90 30:70 50:50 70:30 90:10
Figure 4. NHþ4 -N concentration of the SSAD operated at the HRTs of 10, 13, 15, and 20
days with five mixtures.
1748 Heo, Park, and Kang
Dow
nloa
ded
by [
Uni
vers
ity o
f C
alif
orni
a, S
an F
ranc
isco
] at
04:
08 0
5 Se
ptem
ber
2014
ORDER REPRINTS
the carbon dioxide solubilization. The contemporary presence of ammonia andbicarbonate gives rise to the formation of a buffer system, the so-called carbonate-acetic buffer.[23] Therefore, a proper ammonia level is rather beneficial to provide thebuffer capacity to the digestion process by the formation of NH4HCO3. In thisstudy, the NHþ
4 -N concentration was the highest at 1.17 gL�1 when the digesteroperated at a HRT of 20 days with a feed mixture of 50:50, and then FAconcentration would be 37mgL�1, using the equation presented by Kayhanian[25] asa function of pH 7.5 at 35�C. Therefore, the FA and NHþ
4 -N concentrations wereestimated to be below the concentration that inhibits methanogenic activitymentioned in the literature.[22,24] As a result of this study, in terms of the stabilityparameters of SSAD, there was no inhibition of anaerobic process due to pH drop,insufficient alkalinity, accumulation of VFA, or methanogenic inhibition by theFA and NHþ
4 -N concentration during the whole experiments.
Evaluating Performance of SSAD at a Steady-State Condition
As shown in Fig. 5 (C), stepwise increase in the organic loading rates (OLRs) ofthe mixtures of 10:90, 30:70, 50:50, 70:30, and 90:10 were utilized in the SSAD,reducing the HRTs of the digester from 20 days to 16, 13, and 10 days, respectively.The OLR which was based on the VS concentration was gradually increased asthe HRT became shorter or the FW proportion of the mixture increased;the concomitant OLR ranges of 1.20–2.37, 1.34–2.61, 1.64–3.14, 2.02–4.04, and2.79–5.57 kgVSm�3d�1, respectively.
The effluent VS concentrations as a function of HRT are compared with theSSAD operated at different HRTs with five mixtures as shown in Fig. 5 (A). Inparallel the effluent VS concentration increased as the HRT became shorter with thesame mixture. The effluent VS concentration was the lowest in the digester fed with amixture of 50:50 among five mixtures, while its concentration was increased at all theHRTs when the digester was operated with the mixtures of 70:30 and 90:10, and itstrend was similar to that of effluent TCOD. Therefore, it was found that the effluentorganic concentration of the digester depended upon an operating HRT and the FWproportion of the mixture.
Figure 5 (B) shows the VS removal efficiencies of the SSAD operated withvarious OLRs of five mixtures as a function of HRT. The difference in the VSremoval efficiencies was not significant between the digesters operated at a HRT of10 days and at a HRT of 20 days. However, the magnitude in the VS removalefficiencies was remarkably different among five mixture at the same HRT.Therefore, it became clear that the VS removal efficiencies were more affected by theFW fraction of the mixture than the influence of the HRT; that is, the resultdemonstrates that the VS removal efficiency was strongly affected by thebiodegradability of the mixture as shown in the BMP test. Among five mixtures,the VS removal efficiencies were the highest at a mixture of 90:10, and the maximumVS removal of 71.0% was achieved when the digester was operated at the HRT of 20days with an OLR of 2.79 kgVSm�3 d�1. Even at a shorter HRT of 10 days with anOLR of 5.57 kgVSm�3 d�1, the VS removal of 67.6% was reached.
Anaerobic Co-digestion of FW and WAS 1749
Dow
nloa
ded
by [
Uni
vers
ity o
f C
alif
orni
a, S
an F
ranc
isco
] at
04:
08 0
5 Se
ptem
ber
2014
ORDER REPRINTS
Figure 6 shows the methane content of the biogas produced, biogas productionrate (GPR) and specific methane production (SMP) of the SSAS operated atdifferent HRTs with five mixtures. The composition of the biogas produced, in termsof methane and carbon dioxide, is of fundamental importance to evaluate thestability of the process. During the anaerobic digestion, the methanogenic reactor isgenerally stable in the methane content range of 60–70%, and a decrease in methanecontent of the biogas produced is generally due to the high OLR or a shorter HRT ofthe digester.[23] As shown in Fig. 6 (A), it became clear that the methane content ofthe biogas gradually decreased as the HRT became shorter or the FW proportion
13
14
15
16
17
18
19
20
Eff
luen
t VS
conc
. (gl
−1) 10:90 30:70 50:50 70:30 90:10
(A)
20
30
40
50
60
70
80
VS
rem
oval
(%
)
(B)
0
1
2
3
4
5
6
7
0 50 100 150 200 250
Start-up period (days)
OL
R (
kg V
S m
−3 d
ay−1
)
20 day 16 day 13 day 10 day
(C)
Figure 5. Effluent VS concentrations (A), VS removals (B), vs. organic loading rates (C),
of each mixture as a function of HRT.
1750 Heo, Park, and Kang
Dow
nloa
ded
by [
Uni
vers
ity o
f C
alif
orni
a, S
an F
ranc
isco
] at
04:
08 0
5 Se
ptem
ber
2014
ORDER REPRINTS
of the mixture increased. For the mixtures of 10:90, 30:70, 50:50, and 70:30, the
methane contents of the digester operated at different HRTs were in the ranges of
69–78, 66–74, 62–69, and 58–65%, respectively, while its content ranged from 55 to
59% for a mixture of 90:10. In view of the methane content of the biogas mentioned
in the literature for a stable process,[23] it is likely that the mixture of 50% FW and
50% WAS might be a good substrate for the stability of the anaerobic digestion
process. Especially, the methane content of the digesters fed with a mixture of 50:50
0.5
0.6
0.7
0.8
0.9
1.0
Met
hane
con
tent
(×
100,
%)
(A)
10:90 30:70 50:50 70:30 90:10
0.0
1.0
2.0
3.0
4.0
5.0
GPR
(m
3 m
−3 d
−1) (B)
0.0
0.1
0.2
0.3
0.4
0.5
0 50 100 150 200 250
Start-up period (days)
SMP
(m3
CH
4 kg
−1 V
S fed
−1)
10 day 13 day 16 day 20 day
(C)
Figure 6. Methane content of the biogas produced (A), biogas production rate (B), and
specific methane production (C) of the SSAD operated at the HRTs of 10, 13, 15, and 20 days
with five mixtures.
Anaerobic Co-digestion of FW and WAS 1751
Dow
nloa
ded
by [
Uni
vers
ity o
f C
alif
orni
a, S
an F
ranc
isco
] at
04:
08 0
5 Se
ptem
ber
2014
ORDER REPRINTS
was higher than those of 60% and 50% obtained by Mata-Alvarez et al.[26] andDel Borghi et al.[27]
The GPR, which is expressed as the volume of biogas produced per volume ofdigester per day, increased as the HRT became shorter or the FW proportion of themixture increased as shown in Fig. 6 (B). That is, the GPR, which is directly relatedto the OLR, increased in proportion to the OLR applied to the digester. Among fivemixtures, the biogas production rates were the highest at feed mixture of 90:10. Thesignificant difference in GPR was found between the digesters operated at a HRTof 10 days and a HRT of 20 days with the same mixture. The maximum GPR of3.33m3m�3d�1 was achieved when the digester was operated at the HRT of 10 dayswith an OLR of 5.57 kgVSm�3d�1.
The SMP, which is a good parameter to estimate the biodegradability of asubstrate fed to the digester and to define the operational mode of the digester, isexpressed as the methane volume produced per unit mass of VS fed to the digester.The specific methane productions (SMPs) as a function of HRT are compared withthe SSAD operated at different HRTs with five mixtures as shown in Fig. 6 (C). Thedifference in the SMPs was no significant between the digesters operated at a HRTof 10 days and at a HRT of 20 days with the same mixture, but the SMP increased asthe FW proportion of the mixture was higher. Therefore it became clear that theSMP was significantly affected by the FW proportion of the mixture than theinfluence of the digester HRT. Especially, the SMP of each mixture slightlydecreased when the HRT of the digester became shorter or longer than 13 days.
The maximum SMPs were 0.336 and 0.353m3 CH4 kg�1 VS�1
fed at the OLRs of3.10 and 4.27 kgVSm�3d�1 when the digester was operated at an HRT of 13 dayswith the mixtures of 70:30 and 90:10, respectively. Even at an HRT of 13 days withmixtures of 50:50, the maximum SMP of 0.321m3 CH4 kg
�1 VS�1fed was achieved at an
OLR of 2.43 kgVSm�3 d�1, despite much lower the OLR, VS removal efficiency andSGP than those of the mixtures of 70:30 and 90:10. The SMP obtained at an HRT of13 days with a mixture of 50:50 are similar to those obtained by Schmit and Eills,[28]
which reported that the maximum SMP of 0.332m3 CH4 kg�1 VS�1
fed was reachedwhen the digester was operated at an HRT of 15 days with a mixture of 40:60, usingtwo-phase anaerobic co-digestion process with different mixtures of 0:100, 20:80,40:60, 60:40, and 80:20 (OFMSW:sewage sludge). No significant difference in themaximum SMP was found between the digesters fed with a mixture of 50:50 and amixture of 90:10 at an HRT of 13 days. On the basis of the above results, theoptimum mixture ratio and HRT were estimated to be a ratio of 50% FW and50% WAS and 13 days, in terms of the methane content of the biogas producedand the SMP.
The operating conditions, reactor characteristics and performances of SSADoperated at an HRT of 13 days with five mixtures are summarized in Table 4.The data presented are the mean values at the steady-state condition of the digester.The results obtained in this work are comparable to previous research for theanaerobic co-digestion of OFMSW and sewage sludge (SS). Mata-Alvarez et al.[26]
investigated the mesophilic (35�C) anaerobic co-digestion of a mixture of 50%OFMSW and 50% sewage sludge. They reported that the VS of 57% and the SMPof 0.36m3 CH4 kg
�1 VS�1fed were achieved when the digester operated at the HRT of
14.5 days with an OLR of 2.80 kgVSm�3 d�1, respectively. The thermophilic (55�C)
1752 Heo, Park, and Kang
Dow
nloa
ded
by [
Uni
vers
ity o
f C
alif
orni
a, S
an F
ranc
isco
] at
04:
08 0
5 Se
ptem
ber
2014
ORDER REPRINTS
anaerobic co-digestion for a mixture of 50% OFMSW and 50% sewage sludge was
performed by Del Borghi et al.,[27] who reported that the VS removal and the specific
biogas production (SGP) were 64% and 0.36m3 kg�1 VS�1fed in the digester operated
at the HRT of 12 days with an OLR of 4.0 kgVSm�3 d�1, respectively. Sosnowski
et al.[13] also obtained, when the thermophilic (56�C) two-stage anaerobic
co-digestion process was operated at the HRT of 11.1 days with an OLR of
3.084 kgVSm�3 d�1, the SGP of 0.532m3 kg�1 VS�1fed and bioefficiency of 62.7% by
the addition of a mixture having the volume ratio of 25% OFMSW and 75% sewage
sludge. Also, the effect of the mixture ratio on anaerobic co-digestion of OFMSW
and sewage sludge was examined by several researchers, who observed the best
performance in terms of SGP and VS removal with feedstock having an
OFMSW:sewage sludge ratio in the range of 80:20 on a TS basis,[29] or a volume
ratio of 25% OFMSW and 75% sewage sludge.[6]
The anaerobic co-digestion of OFMSW with sewage sludge, using existing
anaerobic digester in MWTP, seems to be especially attractive and potentially could
reduce capital and operating costs.[6] The benefits of co-digestion include dilution
of potential toxic compounds coming from co-substrate, synergistic effects of
microorganisms, better methane yield per unit of digester volume by increased OLR
of biodegradable matter and improved balance of nutrient.[6–10,13,18–21,26–29]
Table 4. Results of SSAD operated at a HRT of 13 days with five mixtures.
Mixture ratios (gVSFW:gVSWAS)
10:90 30:70 50:50 70:30 90:10
Operating conditions
HRT (day) 13
OLR (kgTCODm�3d�1) 3.35 3.78 4.71 5.96 7.47
OLR (kgVSm�3d�1) 1.83 2.01 2.43 3.10 4.27
Reactor characteristics
TCOD (gL�1) 27.4 27.1 26.4 27.7 31.8
SCOD (gL�1) 0.85 0.67 0.79 0.86 0.61
TS (gL�1) 22.8 22.8 20.7 22.7 24.6
VS (gL�1) 15.1 15.5 14.1 15.3 17.2
pH 7.28 7.40 7.42 7.38 7.32
Alkalinity(g CaCO3 L�1) 3.30 3.83 4.42 3.87 3.54
NHþ4 -N (gL�l) 0.73 0.88 1.04 0.80 0.65
VFAs (gCODL�l) NDa ND ND 0.08 0.23
Digester performances
TCOD removal (%) 36.8 44.6 56.8 64.4 69.1
VS removal (%) 36.2 42.6 55.8 62.1 67.4
Methane content (%) 72.3 68.5 64.4 60.2 56.4
GPR (m3m�3d�1) 0.470 0.652 1.239 1.797 2.690
SGP (m3 kg�1 VS�1fed) 0.257 0.332 0.503 0.558 0.633
SMP (m3 CH4kg�1 VS�1
fed) 0.186 0.215 0.321 0.336 0.346
aNot detected.
Anaerobic Co-digestion of FW and WAS 1753
Dow
nloa
ded
by [
Uni
vers
ity o
f C
alif
orni
a, S
an F
ranc
isco
] at
04:
08 0
5 Se
ptem
ber
2014
ORDER REPRINTS
CONCLUSIONS
In this study, the anaerobic biodegradability of FW, WAS, and five mixtures
was estimated by the batch BMP test, and the effects of HRT and mixture ratio of
two biowastes on the stability and performance of the SSAD were investigated. In
the BMP test, it was found that the cumulative methane production of the mixtures
of FW and WAS increased with increasing proportion of the FW. The C/N ratio of
the mixture improved from 6.16 to 14.14 as the mixture ratio of FW was increased
from 10 to 90%, and the biodegradability also increased from 36.6 to 82.6%. A
linear correlation (R2¼ 0.977) was observed between anaerobic biodegradability
and the mixture ratio of FW.The SSAD which maintained at the HRT ranging from 10 to 13, 16, and 20 days
was quite efficient for treating five mixtures having SFW:WAS ratios of 10:90, 30:70,
50:50, 70:30, and 90:10. The pH, alkalinity, and NHþ4 -N concentration of the
digester were significantly affected by the FW fraction of the mixture and the HRT.
However, the digesters operated at different HRTs maintained within a stable pH
range 7.2–7.5 with sufficient alkalinity ranging from 3.1 to 4.74 g CaCO3L�1 without
the accumulation of VFAs and ammonia inhibition during all the experiments with
five mixtures. The buffer capacity was the highest in the digester fed with a feed
mixture of 50:50. The optimum operating conditions of the SSAD were found to be
an HRT of 13 days and a mixture of 50:50 in terms of the buffer capacity of the
digester and the effluent VS concentration, the methane content of the biogas, and
the SMP. The VS removal efficiency, GPR and SMP in this condition achieved
56.8%, 1.24m3m�3 day�1 and 0.321m3 CH4 kg�1 VS�1
fed with an OLR of 2.43 kg
VSm�3 day�1. Based on the results of this study, the application of the single-stage
anaerobic co-digestion process appears to be technically feasible, and the co-
digestion of FW together with sewage sludge seems to be an attractive method for
treating Korean food waste.
ACKNOWLEDGMENTS
This work was supported by a grant from the Ministry of Commerce, Industry
and Energy Republic of Korea and Halla Energy & Environment Ltd.
REFERENCES
1. Korea Ministry of Environment (KMOE, 2002). Homepage of KMOE (http://
www.me.go.kr).2. Yun, Y.S.; Park, J.I.; Suh, M.S.; Park, J.M. Treatment of food waste using
slurry-phase decomposition. Bioresour. Technol. 2000, 73, 21–27.3. Cho, J.K.; Park, S.C.; Chang, H.N. Biochemical methane potential and solid
state anaerobic digestion of Korean food waste. Bioresour. Technol. 1995, 52,
245–253.
1754 Heo, Park, and Kang
Dow
nloa
ded
by [
Uni
vers
ity o
f C
alif
orni
a, S
an F
ranc
isco
] at
04:
08 0
5 Se
ptem
ber
2014
ORDER REPRINTS
4. Lee, J.P.; Lee, J.S.; Park, S.C. Two-phase methanization of food waste in pilotscale. Appl. Biochem. Biotech. 1999, 77–79, 585–593.
5. Shin, H.S.; Han, S.K.; Song, Y.C.; Lee, C.Y. Performance of UASB reactortreating leachate from acidogenic fermenter in the two-phase anaerobicdigestion of food waste. Wat. Res. 2001, 35 (14), 3441–3447.
6. Hamzawi, N.; Kennedy, K.J.; McLean, D.D. Technical feasibility of anaerobicco-digestion of sewage sludge and municipal solid waste. Environ. Technol.1998, 19 (10), 993–1003.
7. Mata-Alvarez, J.; Mace, S.; Llabres, P. Anaerobic digestion of organic solidwaste. An overview of research achievement and perspectives. Bioresour.Technol. 2000, 74, 3–16.
8. Hartmann, H.; Angelidaki, I.; Ahring, B.K. Co-digestion of the organicfraction of municipal waste with other waste types. In Biomethanization of theOrganic Fraction of Municipal Solid Wastes; Mata-Alvarez, J., Ed.; IWAPublishing: London, 2003; 181–199.
9. Poggi-Varaldo, H.M.; Oleszkiewicz, J.A. Anaerobic co-composting ofmunicipal solid waste and waste sludge at high total solids levels. Environ.Technol. 1992, 13, 409–421.
10. Rintala, J.A.; Jarvinen, K.T. Full-scale mesophilic anaerobic co-digestion ofmunicipal solid waste and sewage sludge: methane production characteristics.Waste Management and Research 1996, 14 (2), 163–170.
11. Owen, W.P.; Stuckey, D.C.; Healy, J.B.; Young, L.Y.; McCarty, P.L. Bioassayfor monitoring biochemical methane potential and anaerobic toxicity. Wat.Res. 1979, 13, 485–492.
12. Penaud, V.; Delgenes, J.P.; Moletta, R. Thermo-chemical pretreatment of amicrobial biomass: influence of sodium hydroxide addition on solubilizationand anaerobic biodegradability. Enzyme and Microbial. Technol. 1999, 25,258–263.
13. Sosnowski, P.; Wieczorek, S.; Ledakowicz, S. Anaerobic co-digestion of sewagesludge and organic fraction of municipal solid waste. Advances in Environ.Res. 2003, 7, 609–616.
14. Tchobanoglous, G.; Theisen, H.; Vigil, S. Biological and chemical conversiontechnologies. Integrated Solid Waste Management: Engineering Principles andManagement Issues; McGraw-Hill: New York, 1993; 671–716.
15. APHA/AWWA/WEF. Standard Methods for the Examination of Water andWastewater, 20th Ed.; Washington DC, USA, 1998.
16. Lin, J.G.; Ma, Y.S.; Chao, A.C.; Huang, C.L. BMP test on chemicallypretreated sludge. Bioresour. Technol. 1999, 68, 187–192.
17. Eastman, J.A.; Ferguson, J.F. Solubilization of particulate organic carbonduring the acid phase of anaerobic digestion. J. Water Pollut. Control Fed.1981, 53, 352–366.
18. Cecchi, F.; Traverso, P.G.; Perin, G.; Vallini, G. Comparison of co-digestionperformance of two differently collected organic fractions of municipal solidwaste with sewage sludge. Environ. Technol. Lett. 1988, 9, 391–400.
19. Rivard, C.J.; Vinzant, T.B.; Adney, W.S.; Grohmann, K. Nutrient require-ments for anaerobic and aerobic digestion of processed municipal solid waste.J. Environ. Health. 1989, 52, 96–100.
Anaerobic Co-digestion of FW and WAS 1755
Dow
nloa
ded
by [
Uni
vers
ity o
f C
alif
orni
a, S
an F
ranc
isco
] at
04:
08 0
5 Se
ptem
ber
2014
ORDER REPRINTS
20. Kayhanian, M.; Tchobanoglous, G. Computation of C:N ratios for variousorganic fractions. Biocycle 1992, 33 (5), 58–60.
21. Kayhanian, M.; Rich, D. Sludge management using the biodegradable organicfraction of municipal solid waste as a primary substrate. Water Environ. Res.1996, 68 (2), 240–252.
22. Stronach, S.M.; Rudd, T.; Lester, J.N. Influence of environmental factors, toxicsubstances in anaerobic digestion. Anaerobic Digestion Processes in IndustrialWastewater Treatment; Springer-Verlag: Berlin, 1986; 59–92.
23. Cecchi, F.; Traverso, P.; Pavan, P.; Bolzonella, D.; Innocenti, L. Characteristicsof the OFMSW and behaviour of the anaerobic digestion process. InBiomethanization of the Organic Fraction of Municipal Solid Wastes; Mata-Alvarez, J., Ed.; IWA Publishing: London, 2003; 141–179.
24. McCarty, P.L.; Mckinney, R.E. Salt toxicity in anaerobic digestion. J. WaterPollut. Control Fed. 1961, 33, 399–414.
25. Kayhanian, M. Ammonia inhibition in high-solids biogasification: an overviewand practical solutions. Environ. Technol. 1999, 20, 355–365.
26. Mata-Alvarez, J.; Cecchi, F.; Pavan, P.; Liabres, P. The performances ofdigesters treating the organic fraction of municipal solid wastes differentlysorted. Biological Wastes 1991, 33, 181–199.
27. Del Borghi, A.; Converti, A.; Palazzi, E.; Del Borghi, M. Hydrolysis andthermophilic anaerobic digestion of sewage sludge and organic fraction ofmunicipal solid waste. Bioprocess Engineering 1999, 20, 553–560.
28. Schmit, K.H.; Ellis, T.G. Comparison of temperature-phased and two-phaseanaerobic co-digestion of primary sludge and municipal solid waste. WaterEnviron. Res. 2001, 73 (3), 314–321.
29. Demirekler, E.; Anderson, G.K. Effect of sewage sludge addition on the startupof the anaerobic digestion of OFMSW. Environ. Technol. 1998, 19 (8),837–843.
1756 Heo, Park, and Kang
Dow
nloa
ded
by [
Uni
vers
ity o
f C
alif
orni
a, S
an F
ranc
isco
] at
04:
08 0
5 Se
ptem
ber
2014
Request Permission/Order Reprints
Reprints of this article can also be ordered at
http://www.dekker.com/servlet/product/DOI/101081ESE120037874
Request Permission or Order Reprints Instantly!
Interested in copying and sharing this article? In most cases, U.S. Copyright Law requires that you get permission from the article’s rightsholder before using copyrighted content.
All information and materials found in this article, including but not limited to text, trademarks, patents, logos, graphics and images (the "Materials"), are the copyrighted works and other forms of intellectual property of Marcel Dekker, Inc., or its licensors. All rights not expressly granted are reserved.
Get permission to lawfully reproduce and distribute the Materials or order reprints quickly and painlessly. Simply click on the "Request Permission/ Order Reprints" link below and follow the instructions. Visit the U.S. Copyright Office for information on Fair Use limitations of U.S. copyright law. Please refer to The Association of American Publishers’ (AAP) website for guidelines on Fair Use in the Classroom.
The Materials are for your personal use only and cannot be reformatted, reposted, resold or distributed by electronic means or otherwise without permission from Marcel Dekker, Inc. Marcel Dekker, Inc. grants you the limited right to display the Materials only on your personal computer or personal wireless device, and to copy and download single copies of such Materials provided that any copyright, trademark or other notice appearing on such Materials is also retained by, displayed, copied or downloaded as part of the Materials and is not removed or obscured, and provided you do not edit, modify, alter or enhance the Materials. Please refer to our Website User Agreement for more details.
Dow
nloa
ded
by [
Uni
vers
ity o
f C
alif
orni
a, S
an F
ranc
isco
] at
04:
08 0
5 Se
ptem
ber
2014
