International Journal of Environmental Science and Development, Vol. 4, No. 5, October 2013
Efficiencies of Mesophilic Two-Phase Anaerobic
Digestion of Pretreated Surplus Sludge
Guang Li, Jing Li, and Xiangkui Han

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 easymethanogenic 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
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
anaerobic digestion, ethanol-type fermentation.
two-phase
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; revised October 30, 2013. This
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:
[email protected];
[email protected]).
Jing Li is with Jilin Jian Yuan Design Group fifth Institute, Changchun,
China (e-mail: [email protected]).
DOI: 10.7763/IJESD.2013.V4.416
576
International Journal of Environmental Science and Development, Vol. 4, No. 5, October 2013
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.
10
10
5
5
2
II. EXPERIMENT MATERIALS AND METHODS
6
7
1
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.
3
2
6
1
8
8
3
4
water seal
mixer
9
inlet sludge
outlet sludge
water tank
temperature controller
acidogenic phase
circulating water pump
methanogenic phase
wet gas meter
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
Fig. 1. Schematic diagram of experimental apparatus.
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.
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.
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.
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International Journal of Environmental Science and Development, Vol. 4, No. 5, October 2013
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.
methanogenic phase
pH
7
6
5
4
3
0
10
20
30
40
50
running time(d)
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.
alkalinity (mgCaCO
3/L)
inlet sludge
3000
acidogenic phase
methanogenic phase
2500
2000
1500
500
0
10
20
30
running time(d)
40
acidogenic phase
methanogenic phase
removal rate of acidogenic phase
removal rate of methanogenic phase
18
16
14
12
10
8
6
4
2
0
70
60
50
40
30
20
10
0
0
10
20
30
running time(d)
40
50
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.
1000
0
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.
removal rate(%)
acidogenic phase
VS(mg/L)
inlet
8 sludge
50
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
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International Journal of Environmental Science and Development, Vol. 4, No. 5, October 2013
2
25
20
1.5
15
10
1
0.5
5
0
0
5
14
19
23 28 35 39
running time(d)
42
acidogenic phase
64
15
62
10
60
58
56
14
19
23 28 35
running time(d)
39
42
8
30
20
10
2
54
5
40
4
methane content of gas(%)
gas production(L/d)
66
0
10
6
methane content of gas
68
5
50
12
46
20
60
14
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%.
methanogenic phase
removal rate of methanogenic phase
70
16
Fig. 5. Acidogenic phase gas production content and gas components
changes.
25
methanogenic phase
removal rate of acidogenic phase
18
removal rate(%)
35
30
2.5
VS(mg/L)
gas production(L/d)
acidogenic phase
40
hydrogen content of gas(%)
hydrogen content of gas
3
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].
46
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,
579
0
0
50
60
70
80
90 100 110 120 130 140 150 160
running time(d)
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
inlet sludge
668
263
614
226
636
acidogenic phase
746
342
685
294
706
17.
47.
23.
55.
33.
8
4
6
7
6
methanogenic phase
Po
25
2
31
5
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
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.
inlet sludge
100
acidogenic phase
methanogenic phase
SV（%）
80
60
40
20
50
60
70
80
90 100 110 120
running time（d）
130
140
150
160
Fig. 8. The change of SV in two-phase system during the running period.
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.
50
inlet sludge
acidogenic phase
methanogenic phase
40
30
CST(s)
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.
20
10
0
50
60
70
80
90
100 110 120
running time(d)
130
140
150
160
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
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
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.