Abstract—With surplus sludge after being treated for 30

minutes at 5MPa, 150℃ as matrix, the operating efficiency of

mesophilic two-phase anaerobic digestion system was examined.

The result showed that the removal rate of sludge VS decreased

as organic load increased and the removal rate of the system VS

was 59.4%, 54.7%, 50.21%, and 42.2% respectively when the

organic load was 0.98, 1.57, 2.35 and 3.13kgVS/m3•d. If

acidogenic phase controlled in ethanol-type fermentation period,

the specific hydrogen production of acidogenic phase in this

condition achieved 0.018 L•H2/g•VS. When the two phase

anaerobic digestion system organic load at 3.13 kgVS/(m3•d), the

removal rate of acidification phase and the two phase system VS

removal is 15.7% and 42.2% respectively. In anaerobic

digestion acidification stage portion EPS from activated sludge

floc in vivo release, such as the release of methane-producing

bacteria Extracelluler Polymer Substances （EPS） could not be

timely use will result in the increase of sludge EPS impact sludge

settling and dewatering performance.

Index Terms—Surplus sludge, mesophilic two-phase

anaerobic digestion, ethanol-type fermentation.

I. INTRODUCTION

Anaerobic digestion is the most widely used stabilizing

sludge treatment process, in the absence of anaerobic

digestion principle the presence of oxygen, anaerobic bacteria

through degradation of the organic carbon in the sludge into

CH4 and CO2, the sludge is stabilized. Anaerobic digestion is

a complex biochemical process, the current is more

recognized in 1979 by Bryant et al. proposed three-stage

anaerobic digestion theoretical model will be divided into

anaerobic digestion hydrolysis fermentation stage, the stage

of hydrogen acetogenic and methanogenic phase.

Conventional anaerobic digestion process will be three stages

of reaction in a reactor for, but the three stages of strain

generation cycle, the speed of degradation of organic matter

and its environmental requirements vary, so that bacteria can

not simultaneously each stage in the best physical condition,

affecting the operational efficiency of the anaerobic digestion

process. Poland, who in 1971 proposed a two -phase

anaerobic digestion process, the anaerobic digestion process

Manuscript received October 12, 2013

work was supported in part by the financial support of the Jilin Architectural

and Civil Engineering Institute. The doctoral scientific research fund and

Youth Science and Technology Development Fund, Jilin province college

students’ innovation and entrepreneurship training program..

Guang Li and Xiangkui Han are with the Key Laboratory of Songliao

Aquatic Environment, Ministry of Education, Ji Lin Jian Zhu University,

Changchun, China (e-mail: liguang1908@163.com;

han.xiangkui@163.com).

Jing Li is with Jilin Jian Yuan Design Group fifth Institute, Changchun,

China (e-mail: lijing1916@163.com).

of the first and second phase and the third phase were

controlled within the two reactors, each form acid-producing

bacteria, methanogens optimum environmental conditions

acidogenic phase and methanogenic phase separation, can

significantly improve system capacity and operational

stability [1].

For two-phase anaerobic digestion process, the operational

performance of the acidogenic phase methanogenic phase

directly affects the subsequent operation stability. Ethanol

type fermentation acid production phase is considered the

best type of fermentation [2], [3]. It not only provides an easy-

methanogenic phase microbial transformation of organic

substrates (ethanol and acetic acid -based short-chain fatty

acids), while its fermentation process can also be a byproduct

of hydrogen as a clean energy for recycling [4]. In this study,

the temperature two -phase anaerobic digestion process, the

acidogenic phase control in ethanol type fermentation stage,

the high pressure combined with thermal hydrolysis

pretreatment of sludge remaining after the stabilization

process.

Organic load can directly reflect the amount of organic

pollutants and microbial relationship between the biological

treatment systems is designed to run in one of the important

parameters. Anaerobic biological treatment sludge dosing

rate is usually represented by the system of organic load.

However, due to the moisture content of sludge and organic

matter content increased with treatment process and

geographical changes are not consistent, the dosing rate can

not reflect the actual organic load, so the use of organic load

unit reactor volume per day to accept the weight of organic

matter more accurate. This research into the mud by adjusting

the amount of daily organic load control system was

investigated in the temperature two -phase anaerobic

digestion process sludge performance.

In recent years, there is much research for surplus sludge

two-phase anaerobic digestion technology, but less on

acidogenic phase fermentation. Operation efficiency in acid

directly influences the operation stability of subsequent

methanogenic-phase. Ethanol-type fermentation is believed

to be the optimal fermentation type in acidogenic phase [5],

[6]. It not only supply organic substrate which could easy to

transformation in methanogenic phase improve [7], the

by-product hydrogen during the fermentation process could

also act as a clean energy for recycling.

This paper adopts mesophilic two-phase anaerobic

digestion technology, controlling the acidogenic phase in

ethanol-type fermentation; this study discussed the operating

characteristic of mesophilic two-phase anaerobic digestion,

researched the change of sludge VS, settling property and

dewatering performance as well as species distribution of the

Efficiencies of Mesophilic Two-Phase Anaerobic

Digestion of Pretreated Surplus Sludge

Guang Li, Jing Li, and Xiangkui Han

International Journal of Environmental Science and Development, Vol. 4, No. 5, October 2013

576DOI: 10.7763/IJESD.2013.V4.416

; revised October 30, 2013. This

heavy metals in sludge after anaerobic digestion, built a

dynamic model of surplus sludge anaerobic digestion and

studied the property of surplus sludge after ethanol-type

fermentation with supernatant as denitrifying carbon source.

II. EXPERIMENT MATERIALS AND METHODS

A. Surplus Sludge Properties

Residual sludge used in this experiment from Beijing a

sewage treatment plant, sludge in the investment and added to

the original two-phase anaerobic digestion were carried out

prior to pretreatment, pretreatment conditions 5MPa, 150 ℃

treatment 30min. Before and after pretreatment sludge

properties are shown in Table I.

TABLE I: SLUDGE PROPERTIES

Parameters Original Sludge Pretreatment Sludge

TS(g/L) 20.2～23.7 18.4～21.2

VS(g/L) 14.5～16.6 12.1～14.9

SS(g/L) 20.3～22.5 11.7～13.6

VSS(g/L) 10.4～14.5 5.5～6.9

TCOD(g/L) 21.8～23.9 20.6～22.4

SCOD(mg/L) 998～1145 6104～6539

NH3-N(mg/L) 98.4～118.4 326.4～367.7

TKN(g/L) 970.3～1233.6 876.2～1133.8

pH 6.65～7.32 6.54～6.93

Alkalinity (mg/L) 903.2～997.3 1547.6～1788.4

B. Seed Sludge Properties

Test two-phase anaerobic digestion process as seed sludge

acidogenic phase test their own rich culture domesticated type

of ethanol fermentation bacteria sludge, MLSS was 24.87g / L,

MLVSS was 18.69g / L.

Methanogenic sludge as inoculum Beijing Municipal

Education Commission pilot school-enterprise cooperation

projects hydrolysis acidification tank sludge and Gaobeidian

sewage treatment plant sludge anaerobic digestion a mixed

digester sludge. After mixing the sludge MLSS is 22.34g / L,

MLVSS was 17.58g / L.

C. Experimental Device and Operation Parameters

Acidogenic phase and methanogenic phase reactor are

made of a continuous stirred tank reactor (CSTR), CSTR

reactor consists of glass processing, acidogenic phase reactor

effective volume 9.0L, methanogenic phase reactor effective

volume of 31.5L. Outside of the reactor with water bath sets,

and with a water tank, submersible pump control tank and the

water bath comprising water cycle, controlled by the

temperature inside the reactor temperature in the range of 35

± 1 ℃ , the test apparatus shown in Fig. 1. During the

two-phase anaerobic digestion system startup HRT remained

at 16.20d, the system of organic load of 0.98 kgVS/m3 • d.

water seal

2

3

2

3

1

4 9

5 5

10 10

6 6

7

8 8

1

inlet sludge outlet sludge

mixer

temperature controller wet gas meter

water tank

acidogenic phase

circulating water pump

methanogenic phase

Fig. 1. Schematic diagram of experimental apparatus.

D. Analysis Method

Sludge TS and VS adopts gravimetric determination

method; TCOD and SCOD adopts phosphoric acid

distillation titration determination method; pH adopts pH

meter; Alkalinity adopts using pH potentiometric titration

method. Organic acid using phosphoric acid distillation

titration determination; Volatile fatty acids (VFAs) the gas

chromatography (GC) determination with FFAP capillary

column (30m×0.32mm×0.5mm) Daojin - 2010, and detector

is FID sludge samples operational controlling parameters are:

5,000 turn/min, 15min centrifugal, filtrate supernatant fluid

by 0.45μm filter membrane, and then filtrate by 0.20μm filter

membrane; using 6mol/L formic acid to control pH value

below 2. During the determination, using 2μl nitrogen as the

incoming sample carrier and control the injection temperature

for 220 ℃,using program warming for detector temperature:

initial temperature 120 ℃, maintain 5min, heating rate 10 /

min, intermediate temperature 160 ℃ , maintain 1min.

Component of anaerobic digestion gas production: gas

chromatography (GC) - 2010 determination, TDX - 01

supporter, 60 - 80 order, the carrier for nitrogen, 0.1 MPa

former pressure column, TCD detector, column temperature

70 ℃, inlet temperature 200 ℃, incoming sample content 80

~ 100μl.

III. RESULTS AND DISCUSSION

A. The Change of pH Value and Alkalinity in Starting

Process

pH value in environment has a great influence on microbial

activity, each microbial has its appropriate pH range. Under

the optimal pH range and other appropriate conditions, the

activity of microbial enzymes will reach the highest point and

the growth rate also the fastest. Generally believed that the

most suitable ethanol-type fermentation bacteria pH range

between 4.0 and 4.5, and the most suitable methanogenic

phase bacteria pH range between 6.8 and 7.2. After

monitoring the changeable of pH value during the two phase

starting periods, the results were shown in Fig. 2.

International Journal of Environmental Science and Development, Vol. 4, No. 5, October 2013

577

3

4

5

6

7

8

0 10 20 30 40 50

running time(d)

pH

inlet sludge acidogenic phase methanogenic phase

Fig. 2. The change of pH Value in two-phase system.

Based on ethanol-type fermentation process of

domestication, the author found that when water alkalinity

reached more than 1500mg/L, because of stronger ability into

the water cushion, ethanol fermentation bacteria can't

maintain acidogenic phase in its appropriate scope of the pH

value by itself. And after the pretreatment sludge alkalinity

stable at 1500mg/L or so. Before put the sludge in the

acidogenic phase reactor, the pH value should be adjust 5.2 ~

5.8 in order to ensure acidogenic phase pH value in 4.0 ~ 4.5.

At the first 7 days of the starting period, without any treatment,

the acidogenic phase sludge enter the methanogenic phase,

which make methanogenic phase pH value stable at less than

6.0. In order to guarantee the normal metabolism of methane

bacteria, from the first 8d, we began to adjust acidogenic

phase sludge pH value at 6.0 or so, and then put it into the

methanogenic phase. From Fig. 2 we can see that from the

first 12 days of methanogenic phase starting period, the pH

value is stable at 6.5 ~ 7.3.

Alkalinity refers to the neutralization reaction that can

accept H+ ions and can carry on neutralizations reaction. For a

buffer system, alkalinity can reflect the system's buffering

capacity to acid, and it also can effectively relieve the pH

value of rapid swings, which belongs to an important control

parameters. Fig. 3 shows the change of alkalinity in two-phase

system during the starting period.

0

500

1000

1500

2000

2500

3000

0 10 20 30 40 50running time(d)

alk

ali

nit

y (

mg

CaC

O 3/L

)

inlet sludge acidogenic phase methanogenic phase

Fig. 3. The change of alkalinity in two-phase system.

It can be seen from the Fig. 3, the alkalinity of inlet sludge

is stable at 1400～1650mg/L, during the first 15 days of

starting period, the alkalinity of acidogenic phase gradually

raised, with the extension of time, the alkalinity gradually

stabilized in 2000 ~ 2200mg/L. Although acidogenic phase

produce numerous volatile acid, but the alkalinity not reduced,

it is mainly because sludge produced a great deal of volatile

acid, at the same time protein hydrolysis into amino acids,

which make alkalinity rise. The alkalinity of methanogenic

phase is increasing gradually at the starting period, after

running 27 days, and it stable at 2057～2660mg/L, the reason

of alkalinity increases is that in methanogenic phase reactor

methanogens bacteria turn liquid VFA into CH4, which make

VFA restoration.

B. The Change of VS in Two-Phase System during the

Starting Period

This study adopts two-phase anaerobic digestion system to

stabilize the surplus sludge. At the starting period, the system

organic loading has been controlled at 0.98kgVS/m3•d, and

the starting period is not finished until the removal rate of VS

stops rising. Fig. 4 reflects the change of VS in two-phase

system during the starting period.

0

2

4

6

8

10

12

14

16

18

0 10 20 30 40 50

running time(d)

VS

(mg

/L)

0

10

20

30

40

50

60

70

rem

ov

al

rate

(%)

acidogenic phase methanogenic phase

removal rate of acidogenic phase removal rate of methanogenic phase

Fig. 4. The change of VS in two-phase system during the starting period.

It can be seen from Fig. 4, at the first 18 days in starting

period, the concentration of acidogenic phase VS gradually

slow down, and it is stable at 12.73g/L after the system

running 22d, the removal rate of acidogenic phase VS rang

from 16.5% to 22.4% before it finished the starting period. At

the first 18 days in the starting period, the concentration of

methanogenic phase VS gradually slow down and it is sharply

reduced after 9d. It is stable at 12.73g/L after the system

running 32d, the removal rate of acidogenic phase VS rang

from 59.4% to 61.6% before it finished the starting period.

C. Gas Production Content and Gas Components

Changes

Fig. 5 and Fig. 6 reflect gas production content and gas

components changes in starting period of two-phase

anaerobic digestion system. It can be seen from Fig. 5, with

time increased acidogenic phase gas production content and

hydrogen content gradually increasing. After operation 23d,

the content of gas production and hydrogen respectively

stable at 2.3L/d and 36.6% or so, and the specific hydrogen

production of acidogenic phase in this condition achieved

0.018 L•H2/g•VS.

International Journal of Environmental Science and Development, Vol. 4, No. 5, October 2013

578

0

0.5

1

1.5

2

2.5

3

5 14 19 23 28 35 39 42 46running time(d)

gas

pro

du

cti

on

(L/d

)

0

5

10

15

20

25

30

35

40

hy

dro

gen

co

nte

nt

of

gas(

%)

hydrogen content of gas acidogenic phase

Fig. 5. Acidogenic phase gas production content and gas components

changes.

It can be seen from Fig. 5, with time increased

methanogenic phase gas production content gradually

increasing. After operation 35d, the content of gas production

is stable at 15L/d or so. The methanogenic phase gas

production is stable at more than 58%, and after operation 35d,

the content of methane production range from 63% to 68%.

0

5

10

15

20

25

5 14 19 23 28 35 39 42 46running time(d)

gas

pro

du

cti

on

(L/d

)

54

56

58

60

62

64

66

68

meth

an

e c

on

ten

t o

f g

as(

%)

methanogenic phase methane content of gas

Fig. 6. Methanogenic phase gas production content and gas components

changes.

D. The Change of VS in Two-Phase System during the

Running Period

Anaerobic digestion is mainly through the degradation of

organic matter in the sludge, the sludge stabilization. Sludge

volatile solids content of organic matter available (VS)

content expressed. Thus, during the trial investigated the

anaerobic digestion of sludge VS changes, the results shown

in Fig. 7.

It can be seen from the Fig. 7, when the two phase

anaerobic digestion system organic load of 0.98 kgVS/(m3•d),

the acid production phase and two phase system VS average

removal rate of 20.1% and 59.4% respectively, with the

increase of organic load, after acidification phase and the two

phase system VS removal rate are declining, organic load

when the system reaches 3.13 kgVS/(m3•d), the removal rate

of acidification phase and the two phase system VS 15.7%

and 42.2% respectively. Wang Zhijun single-phase CSTR

reactor by thermal hydrolysis and the joint treatment of

residual sludge and organic load in the system of similar cases,

the study of sludge VS removal rate lower than the results in

this study, based on two-phase anaerobic digestion can

obviously improve the removal efficiency of sludge VS [8].

0

2

4

6

8

10

12

14

16

18

50 60 70 80 90 100 110 120 130 140 150 160running time(d)

VS

(mg

/L)

0

10

20

30

40

50

60

70

rem

ov

al

rate

(%)

acidogenic phase methanogenic phase

removal rate of acidogenic phase removal rate of methanogenic phase

Fig. 7. The change of VS in two-phase system during the runnting period.

E. The Change of SV in Two-Phase System during the

Running Period

Anaerobic digestion of sludge settling and dewatering

sludge stabilization is an important assessment indicator.

Traditional theory holds that anaerobic digestion, anaerobic

digestion of sludge in the sludge produced during the gap gas

becomes large, thereby increasing the specific surface area of

the sludge, changing the degree of integration of sludge and

water, the sludge settling and dehydration is improved.

However, studies have shown that treatment sludge settling

and dewatering performance and anaerobic digestion for

sludge retention time, within a certain range, sludge settling

and dewatering performance as sludge retention time

increased with the increase, when the sludge retention time

reaches a certain value, which by the sludge retention time has

little effect. Conversely, when the sludge retention time is

insufficient, anaerobic digestion and dewatering sludge

settling performance even worse than the original sludge [9].

Mainly because of the sludge settling and dewatering sludge

addition to performance gaps related, but also with activated

sludge extracellular polymeric substances (EPS) for [10],

[11].

TABLE II: THE CHANGE OF EPS DURING THE RUNNING PERIOD

OLR(kgVS/m3d) 1.57 2.35 3.13

EPS(mg/L) Pr Po Pr Po Pr Po

inlet sludge 668 263 614 226 636 25

2

acidogenic phase 746 342 685 294 706 31

5

methanogenic phase 17.

8

47.

4

23.

6

55.

7

33.

6 78

EPS and activated sludge with a combination of a cation as

a substrate, the microorganism containing, organic and

inorganic substances, including the composition [12].

International Journal of Environmental Science and Development, Vol. 4, No. 5, October 2013

579

Although most of the extracellular polymeric substances

stored in the sludge floc body, but in the acidification stage

anaerobic digestion part from the activated sludge floc EPS

released from the body, such as the release of EPS can not be

timely use of methane-producing bacteria, can cause EPS

rises affecting sludge sedimentation and sludge dewatering

performance. During the test sludge EPS of different organic

load changes as shown in Table II.

F. The Change of SV in Two-Phase System during the

Running Period

In this study, settlement ratio (SV, also known 30min

sedimentation rate) indicates the sludge settling properties,

Fig. 8 reflects the Central Plains during the test sludge,

pretreated sludge acidogenic phase and methanogenic sludge

SV, changes. Fig. 8 shows the original residual sludge SV

average of 91%, by high pressure combined with thermal

hydrolysis pretreatment significantly improved sludge settling

properties, SV average of 53%, and after acidogenic phase

fermented sewage mud settling performance degradation, and

finally by the methanogenic phase digested sludge settling

properties and a slight improvement, and the whole process of

anaerobic digestion sludge settling properties are better than

the original sludge. Can also be seen from the figure, with the

increase of organic loading methanogenic sludge settling

performance is getting worse. This is mainly because the heat

treatment can be released from the sludge colloid bound water,

while improving sludge settling [13], after fermentation by

acidogenic phase release large amounts of dissolved organic

matter in the sludge, resulting in deterioration of the sludge

settling properties, and finally by the methanogenic phase

digestion, sludge EPS dropped significantly (see Table II),

methane phase sludge settling properties in acidogenic phase.

20

40

60

80

100

50 60 70 80 90 100 110 120 130 140 150 160running time（d）

SV（

%）

inlet sludge acidogenic phase methanogenic phase

Fig. 8. The change of SV in two-phase system during the running period.

G. The Change of CST in Two-Phase System during the

Running Period

This study adopts the capillary suction time (CST) sludge

dehydration nature [14], a test during the sludge CST change

as shown in Fig. 9.

As can be seen from the figure, the sludge during the

two-phase anaerobic digestion process dewatering

performance and settling performance has a similar trend, i.e.,

after the high pressure combined with thermal hydrolysis

pretreatment significantly improved sludge dewatering, the

two-phase anaerobic digestion of sludge dewatering

performance deterioration, but still better than the original

sludge. By observing the testing process sludge settling and

dewatering of changes can be found in high pressure

combined with thermal hydrolysis pretreatment can

significantly improve the performance of residual sludge

settling and dewatering performance, after the temperature

two-phase anaerobic digestion of sludge settling properties

and Although the dewatering performance deterioration, but

are better than the original sludge.

0

10

20

30

40

50

50 60 70 80 90 100 110 120 130 140 150 160

running time(d)

CS

T(s

)

inlet sludge acidogenic phase methanogenic phase

Fig. 9. The change of CST in two-phase system during the running period.

IV. CONCLUSIONS

Using mesophilic two-phase anaerobic digestion system to

process the excess sludge of high pressure combined with

thermal hydrolysis. Controlling parameters are as follows:

HRT 16.20d, system organic loading 0.98kgVS/m3•d, the VS

removal rate can stable at 59.4 ～ 61.6%, which can be

achieved the sludge stabilization.

If acidogenic phase controlled in ethanol-type fermentation

period, the specific hydrogen production of acidogenic phase

in this condition achieved 0.018 L•H2/g•VS.

When the two phase anaerobic digestion system organic

load at 3.13 kgVS/(m3•d), the removal rate of acidification

phase and the two phase system VS removal is 15.7% and

42.2% respectively.

In anaerobic digestion acidification stage portion EPS from

activated sludge floc in vivo release, such as the release of

methane-producing bacteria EPS could not be timely use will

result in the increase of sludge EPS impact sludge settling and

dewatering performance.

ACKNOWLEDGMENT

The authors gratefully acknowledge the financial support

of the Jilin Architectural and Civil Engineering Institute The

doctoral scientific research fund and Youth Science and

Technology Development Fund (J201 11026). Ji Lin

province college students' innovation and entrepreneurship

training program.

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Guang Li was born in December 12, 1982, comes from

Jilin, China, have full-time postgraduate education

background, and have got Ph.D in China University of

Geosciences in 2011. His major interest is

Environmental Science and Engineering. His study

orientation is technology of water treatment.

Jin Li graduated from Ji Lin Jian Zhu University in 2005. She is an engineer

of Jilin Jian Yuan Design Group fifth Institute, She's major interests are

sustainable development and systems-level solutions in engineering and the

environment, particularly in water resources, asset management and water

distribution systems.

Xiangkui Han is a dean of the school of municipal and environmental

engineering, Jilin Jianzhu University, environmental science in Jilin

province committee and deputy director of the secretary-general. He is the

three northeast provinces council vice director of water supply and drainage

technology and the international water association member and small

wastewater treatment plant design and operation panel experts.

International Journal of Environmental Science and Development, Vol. 4, No. 5, October 2013

581