influence of mixing proportion on the solid-state anaerobic co-digestion of distiller's grains...

ww.sciencedirect.com

b i o s y s t em s e n g i n e e r i n g 1 1 2 ( 2 0 1 2 ) 1 3 0e1 3 7

Available online at w

journal homepage: www.elsevier .com/locate/ issn/15375110

Research Paper

Influence of mixing proportion on the solid-state anaerobicco-digestion of distiller’s grains and food waste

Li-Hong Wang a,b, Qunhui Wang a,*, Weiwei Cai a, Xiaohong Sun c

aDepartment of Environmental Engineering, University of Science and Technology, Beijing 100083, People’s Republic of ChinabDepartment of Architectural Engineering, Handan Polytechnic College, Handan, Herbei 056001, People’s Republic of ChinacBeijing Agro-Biotechnology Research Center, Beijing 100081, People’s Republic of China

a r t i c l e i n f o

Article history:

Received 3 February 2012

Received in revised form

13 March 2012

Accepted 15 March 2012

Published online 9 April 2012

* Corresponding author. Tel./fax: þ86 010 623E-mail address: [email protected] (Q.

1537-5110/$ e see front matter ª 2012 IAgrEdoi:10.1016/j.biosystemseng.2012.03.006

The solid-state anaerobic co-digestion (AC) of distiller’s grains (DG) and food waste (FW) for

biogas production was investigated. The effects of different DG/FW (TS) ratios (10/1, 8/1, 6/

1, 4/1, 1/0, and 0/1) were examined. Co-digestion with the above DG/FW ratios was found to

be superior to that of mono-digestion. The AC of DG and FW had a good synergistic effect of

the lower range of propionate/acetate ratio and VFA/alkalinity ratio. No methane was

produced when the propionate/acetate ratio was >0.1 � 0.01, but this inhibition was

reversible. When the propionate/acetate ratio was <0.08 � 0.01, methane production began

to recover. When the volatile fatty acids (VFA)/alkalinity ratios were <0.9 � 0.05, the AC of

the influents successfully proceeded in a stable manner. However, when the VFA/alkalinity

and propionate/acetate ratios were >1.25 � 0.21 and 0.1 � 0.01, respectively, the AC system

reached an acidification crisis and failed. Therefore, the propionate/acetate and VFA/

alkalinity ratios may be used as important indices for controlling anaerobic digestion (AD).

ª 2012 IAgrE. Published by Elsevier Ltd. All rights reserved.

1. Introduction volume, lower energy requirement for heating, minimal

Biogas technologies are attractive and well established alter-

natives that allow the production of energy while processing

different organic wastes or biomass and obtaining a solid

product that can be used as an organic fertiliser or conditioner

(Mata-Alvarez, Mac, & Llabres, 2000). Therefore much

research work has been carried out on biogas technologies

(Kotsopoulos, Karamanlis, Dotas, & Martzopoulos, 2008; Liew,

Shi, & Li, 2011; McGrath & Mason, 2004). Solid-state anaerobic

digestion or dry anaerobic digestion is characterised by a high

solid content of feedstock (typically greater than 15%) to be

digested (Li, Park, & Zhu, 2011; Zhu, Wan, & Li, 2010). This

process has been claimed to be better than liquid anaerobic

digestion for a number of reasons, including smaller reactor

32778.Wang).. Published by Elsevier Lt

material handling, and lower total parasitic energy loss

(Montero, Garcia-Morales, Sales, & Solera, 2008). The anaer-

obic co-digestion (AC) of organic wastes also offers the

advantages of increased process stability and biogas yield,

better handling of mixed waste streams, balanced nutrient

supply, stable pH, optimised C/N ratio, and improved buffer

capacity (Mata-Alvarez et al., 2000; Ponsa, Gea, & Sanchez,

2011).

Food waste (FW) and lignocellulosic waste are two basic

components of municipal solid waste. During the early stage

of anaerobic digestion, FW is prone to hydrolysis acidifica-

tion, which is likely to produce volatile fatty acids (VFA).

Consequently, the anaerobic digestion of FW is inhibited

(Guendouz, Buffiere, Cacho, Carrere, & Delgenes, 2008).

d. All rights reserved.

mailto:[email protected]

www.elsevier.com/locate/issn/15375110

www.elsevier.com/locate/issn/15375110

http://dx.doi.org/10.1016/j.biosystemseng.2012.03.006



Nomenclature

AC Anaerobic co-digestion

AD Anaerobic digestion

DG Distiller’s grains

FW Food waste

TC Total carbon

TN Total nitrogen

TS Total solids

VFA Volatile fatty acids

VS Volatile solids

b i o s y s t em s e ng i n e e r i n g 1 1 2 ( 2 0 1 2 ) 1 3 0e1 3 7 131

However, the anaerobic digestion of lignocellulosic waste to

produce methane is controlled by a slow hydrolysis process

(Vavilin, Fernandez, Palatsi, & Flotats, 2008). The hydrolysis

rate of mixed lignocellulosic waste and FW is higher than that

of pure lignocellulosic waste or pure FW (Qu, He, Shao, &

Bouchez, 2008). However, studies on AC of these two wastes

are limited.

Our research group, in a study of sludge AD, found that the

propionate/acetate ratio is a more sensitive index than VFA

(Wang, Kuninobu, Ogawa, & Kato, 1999;Wang et al., 1997). The

sudden increase in this ratio indicates the need to prepare for

AD failure. Some experiments have also shown that the VFA/

alkalinity ratio can be used to measure the process stability

(Appels, Baeyens, Degreve, & Dewil, 2008; Callaghan, Wase,

Thayanithy, & Forster, 2002). A ratio of 0.3e0.4 is generally

regarded as optimal for liquid AD, and a ratio exceeding 0.8 is

regarded as indicative of overfeeding (Sanchez, Borja,

Travieso, Martın, & Colmenarejo, 2005). However, there are

few studies on the most favourable ratios of propionate/

acetate and VFA/alkalinity in solid-state AD.

The aim of this work was to study the effect of the mixing

proportion of FW and distiller’s grains (DG) on their solid-state

AD under batch conditions. The methane yield as well as the

variations in VFA, alkalinity, and ammonia-N concentration

was determined. Themost favourable propionate/acetate and

VFA/alkalinity ratios were also identified. These results may

aid future studies on or applications of solid-state AD.

Table 1 e Composition of the raw materials.

Parameter Foodwaste (TS)

Distiller’sgrains (TS)

Inoculum(TS)

Protein (%) 14.82 9.83 14.50

Carbohydrate (%) 45.01 75.02 68.15

C (%) 43.21 42.94 27.53

N (%) 2.37 1.57 2.40

C/N 18.2 28.3 11.4

Fat (%) 32.1 6.5 e

Hemicellulose (%) 12.32 22.86 47.17

Cellulose (%) 4.43 19.92 18.47

Lignin (%) 2.83 13.82 14.50

Ash (%) 8.07 8.65 17.35

Note: Units were based on dry base. Data are presented as themean

value of three replicates.

2. Methods & materials

2.1. Raw materials

FWwas obtained from a school canteen in Beijing, China, and

DG was from a distillery also in Beijing, China. Before the

experiments, FW and DG were stored in a refrigerator below

4 �C. The anaerobic sludge used as inoculumwas from Beijing

Agro-Biotechnology Research Center. The sludge was allowed

to acclimatise for 2 months to the degrading ability of FW and

DG before the experiments. The chemical characteristics of

the wastes and sludge are shown in Table 1.

2.2. Setup

The DG/FW mixture was anaerobically digested in the labo-

ratory through batch tests. The experimental apparatus

comprised a jar, a 0.5 l graduated container, and an electric

constant temperature air bath. Six influents with DG/FW (TS)

ratios of 1/0, 10/1, 8/1, 6/1, 4/1, and 0/1 were tested. The

inoculum contained 7.82% total solids (TS), 5.91% volatile

solids (VS). The ratio of inoculums to substrate was 0.45:1 on

a VS basis. The total solid (TS) concentration of fermentation

liquid in the six mixtures was 20% (Li et al., 2011; Zhu et al.,

2010). Reactors were sealed after the addition of a mixture

containing equal parts of dry sodium bicarbonate (NaHCO3)

and dry potassium bicarbonate (KHCO3) to achieve an alka-

linity of about 10 g l�1 (as CaCO3). Batch bottles were kept in

mesophilic temperature programmable air bath shakers

(60 rpm) to allow sufficient bacteriaesubstance mixing and

contact. The bottles needed to be manually shaken two times

for at least 5 min per day. Biogas production was measured

daily, and the VFA components were measured every other

day. Similarly, alkalinity, ammonia, and biogas composition

(N2, CH4, and CO2) were measured every 5 days.

2.3. Analytical techniques

The wastes were characterised by analysing TS, VS, total

nitrogen (TN), total carbon (TC), protein, fat, carbohydrate,

cellulose, hemicellulose, lignin, and stability parameters

(alkalinity, ammonia, and VFA content) in accordance with

standard methods. About 5 ml of fermented liquid was

centrifuged (4000 rpm, 30 min), and 0.1 ml of the filtered

supernatant was analysed for VFA by high-performance liquid

chromatography (Shimadzu LC-20A e Shimadzu Interna-

tional Trading Co., Ltd., Shanghai, China) using 20 ml injection

volumes. Biogas composition was analysed using a gas chro-

matograph (Varian CP 3800 GC e Shimadzu International

Trading Co., Ltd., Shanghai, China) with a thermal conduc-

tivity detector as well as CP-Molvsieve5A (CP-Molvsieve5A e

Shimadzu International Trading Co., Ltd., Shanghai, China)

(15m� 0.53mm� 15ml) and CP-Porabond Q (CP-Molvsieve5A

e Shimadzu International Trading Co., Ltd., Shanghai, China)

(25 m � 0.53 mm � 10 ml) columns.




3. Results and discussion

3.1. Analysis of VFA variations and methane yield

The daily methane yields and VFA variations for each co-

digestion test are shown in Fig. 1. Six influents showed

various degrees of methane production in the initial phase of

AC. As fermentation proceeded, methane production was

inhibited by the constantly increasing VFA. The pure FW

fermentation (0/1 DG/FW) had a long inhibitory period

because FW has more easily degradable organics that yield

more VFA. On the 30th day, it began to producemethane again

by relying on its own adaptability. Its cumulative methane

production potential was 41.44 ml g�1 TS for 48 days of

fermentation time, similar to previous results (Wang &Wang,

1996). The 1/0 DG/FW influent had the shortest repressed

time. It recovered the capability of methane production only

on the 11th day. After 48 days of fermentation, its cumulative

methane production potential reached 95.98 ml g�1 TS. The

methane production recovery times of the other four influ-

ents, 10/1, 8/1, 6/1, and 4/1 DG/FW, after inhibitionwere on the

14th, 13th, 16th, and 20th days of digestion, respectively. Their

accumulated methane productions within 48 days were

152.23, 159.74, 141.70, and 108.25 ml g�1 TS, respectively. The

influent, 8/1 DG/FW had not only the highest cumulative

methane production but also the lowest VFA concentration.

One of the important benefits of co-digestion is a syner-

gistic effect. Themethane productions for the ADwith DG/FW

ratios of 10/1, 8/1, 6/1, 4/1, 1/0, and 0/1 were obtained directly

from experiments. The methane productions for DG fraction

and FW fraction in co-digestions were calculated in terms of

the methane productions of mono-digestions with DG alone

(1/0) and FW alone (0/1), respectively. The experimental and

calculating results are shown in Table 2. It was found that

compared to the mono-digestions at four DG/FW ratios, the

co-digestions achieved 25e75% more methane production.

The differences were significant. This means that based on

the same amount of DG and FW, more bioenergy can be

generated when the co-digestion process is applied. The

02468101214161820

0 6 12 18 24 30 36 42 48

Meth

an

e p

ro

d. p

oten

tial

(m

l g

-1T

S d

-1)

Digestion time (days)

a

Fig. 1 e Daily methane production and total VFA of

increase in biogas production is considered to be from the

synergetic effect in the co-digestion process. The co-digestion

of 8/1 DG/FW had an obvious comparative advantage on the

cumulative methane production, rate of methane production,

and recovery of acidification (Fig. 1 and Table 2).

3.2. Analysis of the relationship between the propionate/acetate ratio and methane production

The evolution of VFA plays an important role in maintaining

efficient anaerobic digestion because it strongly affects the pH

value, alkalinity, and activity of methanogens (Buyukkamaci

& Filibeli, 2004). The irreversible acidification of the digestion

resulting from the rapid hydrolysis and acidogenesis is the

major challenge in anaerobic digestion considering that it can

inhibit methanogenesis or dictate the failure of the digestion

(Wang, Zhang, Wang, & Meng, 2009). VFA with concentrations

above 2 g l�1 have reportedly led to the inhibition of cellulose

degradation, whereas VFA concentrations >4 g l�1 have only

slightly inhibited glucose degradation (Siegert & Banks, 2005).

Some experiments have shown that total VFA concentrations

>25 g l�1 did not result in an inhibition of anaerobic digestion

(Zhu et al., 2010). Hence, relying solely on VFA for judging the

effect AD inhibition is not prudent. A number of studies have

indicated that the propionate/acetate ratio is an index of

predicting inhibition, and is even more sensitive than VFA

(Li, Chen, & Gu, 2008; Wang et al., 1997). A drastic increase in

the propionate/acetate ratio could be used as reliable warning

for impending failure in anaerobic digestion (Wang et al.,

1997). The concentrations of formic, acetic, propionic,

butyric, and lactic acids (Fig. 2), as well as the propionate/

acetate ratio (Fig. 3) during the fermentation were analysed.

Figure 2 shows that acetic acid is the dominant VFA. There

is no significant accumulation of propionic and butyric acids

probably because of sufficient propionate- and butyric-

degrading syntrophs in the inoculum, which can rapidly

convert both into acetic acid. Acetic acid rapidly increased

after the test started and eventually reached a maximum.

During this period, the acetic acid production rate was

05101520253035404550

0 6 12 18 24 30 36 42 48

TV

FA

(g

l-1)


b

the six influents during anaerobic co-digestion.



Table 2 e Synergistic effect of the co-digestion of DG with FW at different DG/FW ratios.

Total drymatter (g)

DG/FW Dry matterof DG (g)

Dry matter ofFW (g)

Methane production (ml g�1) Increase (%)

Co-digestion Single DG Single FW

58.30 1/0 58.30 0 97.08 0

58.30 10/1 53.00 5.30 152.23 88.26 3.77 65.42

58.30 8/1 51.82 6.48 159.74 86.30 4.60 75.73

58.30 6/1 49.98 8.32 141.70 83.21 5.92 58.97

58.30 4/1 46.64 11.66 108.25 77.67 8.29 25.94

58.30 0/1 0 58.30 0 41.44

Note: Data are presented as the mean value of three replicates.


apparently higher than the acetic acid consumption rate.

Afterwards, the acetic acid concentration rapidly dropped.

Each group began to produce methane after acetic acid drop-

ped to w7 g l�1. The digestion reached the stabilisation stage

(methanogenesis) with a balance between the production and

consumption of acetic acid. A constant methane content of

05101520253035

In

div

id

ua

l V

FA

(g

l-1)

1/0

05101520253035

In

divid

ual V

FA

(g

l-1) 8/1

05101520253035

In

divid

ual V

FA

(g

l-1)


4/1

Fig. 2 e Changes in the individual VFA conc

50%e60% was obtained, for example, in the AC of 10/1, 8/1,

and 6/1 DG/FW.

Figure 3 shows the changes in the propionate/acetate ratio

and the cumulative methane production potential during AC.

The propionate/acetate ratio initially increased and then

decreased in the process of fermentation. During digestion,

10/1

6/1


0/1

entrations during anaerobic digestion.



Cu

mu

lati

ve M

eth

an

e p

ro

d.

po

ten

tial (m

l g

-1T

S)

10/1

pro

pio

na

te

/ac

eta

te

ra

tio

1/0

0

0.05

0.1

0.15

0.2

pro

pio

nate/a

cetate r

atio

8/1

0

50

100

150

200

Cu

mu

lativ

e M

eth

an

e p

ro

d.

po

ten

tia

l (m

l g

-1T

S)

6/1

0

0.05

0.1

0.15

0.2

pro

pio

na

te

/a

ce

ta

te

ra

tio


4/1

Cu

mu

la

tiv

e M

eth

an

e p

ro

d.

po

te

ntia

l (m

l g

-1T

S)


0/1

Fig. 3 e Changes in the ratio of propionate/acetate and the cumulative methane production potential during anaerobic co-

digestion.


the propionate/acetate ratio of all influents was >0.1 � 0.01

when they did not produce methane, and was <0.08 � 0.01

when they produced methane. Therefore, the propionate/

acetate ratio of 0.1 � 0.01 may bring about a period where the

influents do not producemethane during their AC. In these six

influents, the propionate/acetate ratio of 8/1 DG/FW had the

slowest change in the early stages of fermentation and the

fastest change in the subsequent fermentation time, which

perhaps was one of the reasons for highest rate of methane

yield.

3.3. Analysis of the alkalinity, VFA/alkalinity ratio, andammonia-N of the influents

VFAproduced during AD tend to reduce the pH. This reduction

is normally countered by the activity of methanogenic

bacteria, which also produce alkalinity in the form of carbon

dioxide, ammonia, and bicarbonate. Previous laboratory

studies on mesophilic and thermophilic anaerobic organic

waste digestion have reported a range of 2 g l�1 to 4 g l�1 partial

alkalinity as being typical for properly operating digesters

(Chen, Cheng, & Creamer, 2008; Sharma, Testa, Lastella,

Cornacchia, & Comparato, 2000). In this experiment the

initial partial alkalinity ranged between 10 and 11 g l�1,

whereas the final rangewas between 9.8 and 11.4 g l�1 (Fig. 4a).

The alkalinity initially decreased, increased, and then stabi-

lised. The increase could be attributed to the added NaHCO3

and dry KHCO3 at the beginning of the experiment and the

generation of NHþ4 during the digestion of protein in FW. This

addition resulted in an increased digester buffering capacity,

and consequently, the stability of the digesters. This result is

similar to that of Liew (Liew et al., 2011), who have found that

a partial alkalinity ranging from 2 g l�1 to 4 g l�1 does not occur

at all when the AD process is going well. A study on the solid-

state AD of corn stover and fallen leaves by Liew et al., (2011)

revealed that a high alkalinity reaching 21 g l�1 significantly

enabled the smooth progress of the AD. This finding is an

interesting cost-effective approach. In the current study, the

relatively high alkalinity concentration assured the successful

progress of the solid-state AC of FW with DG. The alkalinity

(Fig. 4a) and methane production (Table 2) of the 8/1 and 10/1

DG/FW were also higher.



7

8

9

10

11

12

0 6 12 18 24 30 36 42 48

Alk

alin

ity (g

l-1)


0

1

2

3

4

5

6

0 6 12 18 24 30 36 42 48

VF

A / A

lka

lin

ity r

atio


a b

Fig. 4 e Alkalinity and VFA/alkalinity ratio variations during anaerobic co-digestion.


The stability criterion for anaerobic digestion is often

expressed by the ratio of the total VFA to the buffering

capacity measured as alkalinity, i.e., the total VFA/alkalinity

ratio (Koch, Lubken, Gehring, Wichern, & Horn, 2010). A ratio

of 0.3e0.4 is generally regarded as optimal for liquid AD, and

a ratio exceeding 0.8 is regarded as indicative of overfeeding

(Callaghan et al., 2002). However, there are few studies on the

ratios of VFA/alkalinity in solid-state AD. In this solid-state AD

experiment, as shown in Fig. 4b, the initial total VFA/alkalinity

ratios of all influents during start-up in all reactors was

approximately 0.5. Subsequently, the ratios initially increased

and then decreased. This trend could be due to the VFA and

alkalinity changes as the organic matter degraded. In the

current experiment, the influents could reproduce methane

within 30 days of inhibition if the VFA/alkalinity ratio of the

influents was <5.4. As the AC proceeded, all VFA/alkalinity

ratios gradually decreased. The AC system became endan-

gered by acidification when the VFA/alkalinity ratios were

>1.25� 0.21. However, the ACwas successful and stable when

0.3

0.6

0.9

1.2

1.5

1.8

2.1

0 6 12 18 24 30 36 42 48

am

mo

nia n

itro

gen

co

nc

en

tratio

n (g

l-1)


a

Fig. 5 e The variations of ammonia-N concentratio

the VFA/alkalinity ratios were <0.9 � 0.05. It can also be seen

that the VFA/alkalinity ratio of 8/1 DG/FW had the minimum

range. This was perhaps one of the reasons it obtained the

highest rate of methane production.

Ammonia is essential for bacterial growth but also inhibits

the anaerobic digestion process if present in high concentra-

tions (Sung & Liu, 2003). Inhibition has been reported to start

at a total ammonia-N level of between 1.5 and 2.0 g l�1

(Hashimoto, 1986). An ammonia-N tolerance of between 3 and

4 g l�1 for an adapted process has also been reported

(Angenent, Sung, & Raskin, 2002). Among all the influents, the

ammonia concentration increased with increased FW added,

which could help enhance the pH of the fermentation liquid to

promote the stability of gas production. The ammonia

concentration in each treatment was about 0.4 g l�1 to 1.8 g l�1

(Fig. 5a). After about 24 days of fermentation, the ammonia

concentrations of 8/1, 6/1, 4/1, and 0/1 were >1.5 g l�1, but did

not lead to inhibition. This phenomenon may be related to

sludge acclimatisation before the experiment, and was

4.5

5

5.5

6

6.5

7

7.5

8

0 6 12 18 24 30 36 42 48

pH


b

n and pH value during anaerobic co-digestion.



Fig. 6 e Variations in VFA/alkalinity, methane yield,

propionate/acetate ratio, pH value and ammonia-N

concentration during the anaerobic co-digestion of 8/1

DG/FW.


perhaps the major driving force that ensured AC was ach-

ieved. Appropriate concentration of ammonia is essential for

8/1 DG/FW with the highest rate of methane yield.

Figure 5b shows pH values as a function of time for

various experiments. Generally, the pH dropped rapidly at the

beginning of each experiment as the easily digestible fraction

of organic matter was hydrolysed and converted to fatty

acids. After the initial drop, the pH began to rise gradually as

the fatty acids were transferred to the methane phase reac-

tors, consumed by methanogens, and evaporated into the gas

phase. The fluctuation of pH during the experiments was due

to the periodic accumulation of fatty acids in the reactor and

the subsequent transfer and consumption of VFA by meth-

anogenesis. In addition, the high alkalinity played a buffer

role to pH changes. The pH values for the co-digested wastes

of DG/FW gradually increased and stayed high, resulting in

high alkalinity of the substrate, rapidly hydrolysis acidifica-

tion of FW, slow hydrolysis process of DG and gas production.

On the other hand, the pH of the single wastes of FW

increased slowly, indicating high VFA from the rapidly

hydrolysis of FW. The pH of the single wastes of DG increased

also slowly, due to the slow hydrolysis of DG and slow gas

production.

As mentioned earlier, the evolution of VFA plays an

important role in maintaining efficient anaerobic digestion,

given that it strongly affects alkalinity, pH andmethane yield.

The ammonia-N concentration also affects alkalinity, VFA

degradation, pH and methane yield; hence, these three are

closely related to one another. Figure 6 shows the mutual

relations of these parameters during the AC of 8/1. At the

beginning of the experiment, VFA concentration and VFA/

alkalinity were increasedwith organic substance degradation.

This also produced the rapid decline of pH value. As the fatty

acids were transferred to the methane phase reactors,

consumed by methanogens, and evaporated into the gas

phase, alkalinity and pH value were increased gradually. After

the protein degraded, the ammonia-N concentration

increased. Consequently, the alkalinity increased, the VFA/

alkalinity decreased, pH value increased, and the AD system

became stable. In the present AC system, methane is stably

produced when the VFA/alkalinity and propionate/acetate

ratios were <0.9 � 0.05 and 0.08 � 0.01, respectively. The AC

system could have failed due to acidification when the VFA/

alkalinity and propionate/acetate ratios were>1.25� 0.21 and

0.1 � 0.01, respectively.

4. Conclusions

The AC of 8/1 DG/FW (C/N 23.81) had the highest methane

production at 38 � 1 �C and pH 7.5 after 48 digestion days.

Compared to mono-digestion, 25e75% more biogas produc-

tions were obtained at AC with four DG/FW ratios due to the

synergistic effect. The AC of DG and FWhad a good synergistic

effect with the appropriate range of propionate/acetate ratio

and VFA/alkalinity ratio.

The propionate/acetate ratio was an important indicator of

the degree of inhibition of methane production, and was even

better than VFA. In the current experiment, when the propi-

onate/acetate ratio was >0.1 � 0.01, the process of methane

production stopped, but when the value was <0.08 � 0.01, the

process was restored.

The VFA/alkalinity ratio was also an important indicator of

the stability of the fermentation process. Methane production

could still be restored within 30 days as long as the VFA/

alkalinity ratio was <5.4. When methane production was

restored and reached a steady state, the VFA/alkalinity ratio of

each treatment was <0.9 � 0.05. However, when the VFA/

alkalinity was >1.25 � 0.21, the AC systemwas exposed to the

danger of acidification and failure.

Acknowledgement

This study was supported by the National High-Tech R&D

Program (863) of China (2008AA06Z34) and the National

Natural Science Foundation (No. 50978028).

r e f e r e n c e s

Angenent, L. T., Sung, S., & Raskin, L. (2002). Methanogenicpopulation dynamics during startup of a full-scale anaerobicsequencing batch reactor treating swine waste. WaterResearch, 18(36), 4648e4654.

Appels, L., Baeyens, J., Degreve, J., & Dewil, R. (2008). Principlesand potential of the anaerobic digestion of waste-activatedsludge. Progress in Energy and Combustion Science, 6(34),755e781.

Buyukkamaci, N., & Filibeli, A. (2004). Volatile fatty acid formationin an anaerobic hybrid reactor. Process Biochemistry, 11(39),1491e1494.

Callaghan, F. J., Wase, D. A. J., Thayanithy, K., & Forster, C. F.(2002). Continuous co-digestion of cattle slurry with fruit andvegetable wastes and chicken manure. Biomass and Bioenergy,1(22), 71e77.

Chen, Y., Cheng, J. J., & Creamer, K. S. (2008). Inhibition ofanaerobic digestion process: a review. Bioresource Technology,10(99), 4044e4064.




Guendouz, J., Buffiere, P., Cacho, J., Carrere, M., & Delgenes, J. P.(2008). High-solids anaerobic digestion: comparison of threepilot scales. Water Science and Technology, 9(58), 1757e1763.

Hashimoto, A. G. (1986). Ammonia inhibition of methanogenesisfrom cattle wastes. Agricultural Wastes, 4(17), 241e261.

Koch, K., Lubken, M., Gehring, T., Wichern, M., & Horn, H. (2010).Biogas from grass silage e measurements and modeling withADM1. Bioresource Technology, 21(101), 8158e8165.

Kotsopoulos, T. A., Karamanlis, X., Dotas, D., &Martzopoulos, G. G. (2008). The impact of different naturalzeolite concentrations on the methane production inthermophilic anaerobic digestion of pig waste. BiosystemsEngineering, 1(99), 105e111.

Li, H., Chen, Y., & Gu, G. (2008). The effect of propionic to aceticacid ratio on anaerobiceaerobic (low dissolved oxygen)biological phosphorus and nitrogen removal. BioresourceTechnology, 10(99), 4400e4407.

Li, Y., Park, S. Y., & Zhu, J. (2011). Solid-state anaerobic digestionfor methane production from organic waste. Renewable andSustainable Energy Reviews, 1(15), 821e826.

Liew, L. N., Shi, J., & Li, Y. (2011). Enhancing the solid-stateanaerobic digestion of fallen leaves through simultaneousalkaline treatment. Bioresource Technology, 19(102), 8828e8834.

Mata-Alvarez, J., Mac, S., & Llabres, P. (2000). Anaerobic digestionof organic solid wastes. An overview of researchachievements and perspectives. Bioresource Technology, 1(74),3e16.

McGrath, R. J., & Mason, I. G. (2004). An observational method forthe assessment of biogas production from an anaerobic wastestabilisation pond treating farm dairy wastewater. BiosystemsEngineering, 87(4), 471e478.

Montero, B., Garcia-Morales, J. L., Sales, D., & Solera, R. (2008).Evolution of microorganisms in thermophilic-dry anaerobicdigestion. Bioresource Technology, 8(99), 3233e3243.

Ponsa, S., Gea, T., & Sanchez, A. (2011). Anaerobic co-digestionof the organic fraction of municipal solid waste with severalpure organic co-substrates. Biosystems Engineering, 4(108),352e360.

Qu, X., He, P., Shao, L., & Bouchez, T. (2008). Effect of wastecomposition on anaerobic methanization of municipal solidwaste. Zhongguo Huanjing Kexue/China Environmental Science,8(28), 730e735, In Chinese.

Sanchez, E., Borja, R., Travieso, L., Martın, A., & Colmenarejo, M. F.(2005). Effect of organic loading rate on the stability,operational parameters and performance of a secondaryupflow anaerobic sludge bed reactor treating piggery waste.Bioresource Technology, 3(96), 335e344.

Sharma, V. K., Testa, C., Lastella, G., Cornacchia, G., &Comparato, M. P. (2000). Inclined-plug-flow type reactor foranaerobic digestion of semi-solid waste. Applied Energy,1e4(65), 173e185.

Siegert, I., & Banks, C. (2005). The effect of volatile fatty acidadditions on the anaerobic digestion of cellulose and glucosein batch reactors. Process Biochemistry, 11(40), 3412e3418.

Sung, S., & Liu, T. (2003). Ammonia inhibition on thermophilicanaerobic digestion. Chemosphere, 1(53), 43e52.

Vavilin, V. A., Fernandez, B., Palatsi, J., & Flotats, X. (2008).Hydrolysis kinetics in anaerobic degradation of particulateorganic material: an overview. Waste Management, 6(28),941e953.

Wang, Q., Kuninobu, M., Ogawa, H. I., & Kato, Y. (1999).Degradation of volatile fatty acids in highly efficient anaerobicdigestion. Biomass and Bioenergy, 6(16), 407e416.

Wang, Q., Noguchi, C. K., Kuninobu, M., Hara, Y., Kakimoto, K.,Ogawa, H. I., et al. (1997). Influence of hydraulic retention timeon anaerobic digestion of pretreated sludge. BiotechnologyTechniques, 2(11), 105e108.

Wang, G., & Wang, W. (1996). Study on the acidification period inthe anaerobic digestion of municipal solid waste. China Biogas,2(14), 11e14, In Chinese.

Wang, Y., Zhang, Y., Wang, J., & Meng, L. (2009). Effects of volatilefatty acid concentrations on methane yield and methanogenicbacteria. Biomass and Bioenergy, 5(33), 848e853.

Zhu, J., Wan, C., & Li, Y. (2010). Enhanced solid-state anaerobicdigestion of corn stover by alkaline pretreatment. BioresourceTechnology, 19(101), 7523e7528.



influence of mixing proportion on the solid-state anaerobic co-digestion of distiller's grains...

Documents