mesophilic and thermophilic temperature co-phase anaerobic digestion compared with single-stage...

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Water Research 38 (2004) 1653–1662 Mesophilic and thermophilic temperature co-phase anaerobic digestion compared with single-stage mesophilic- and thermophilic digestion of sewage sludge Young-Chae Song*, Sang-Jo Kwon, Jung-Hui Woo Division of Civil and Environmental System Engineering, Korea Maritime University, 1, Dongsam-Dong, Yeongdo-Gu, Busan 606-791, South Korea Abstract The performance of thermophilic and mesophilic temperature co-phase anaerobic digestions for sewage sludge, using the exchange process of the digesting sludge between spatially separated mesophilic and thermophilic digesters, was examined, and compared to single-stage mesophilic and thermophilic anaerobic digestions. The reduction of volatile solids from the temperature co-phase anaerobic digestion system was dependent on the sludge exchange rate, but was 50.7–58.8%, which was much higher than 46.8% of single-stage thermophilic digestion, as well as 43.5% of the mesophilic digestion. The specific methane yield was 424–468 mL CH 4 per gram volatile solids removed, which was as good as that of single-stage mesophilic anaerobic digestion. The process stability and the effluent quality in terms of volatile fatty acids and soluble chemical oxygen demand of the temperature co-phase anaerobic digestion system were considerably better than those of the single-stage mesophilic anaerobic processes. The destruction of total coliform in the temperature co-phase system was 98.5–99.6%, which was similar to the single-stage thermophilic digestion. The higher performances on the volatile solid and pathogen reduction, and stable operation of the temperature co-phase anaerobic system might be attributable to the well-functioned thermophilic digester, sharing nutrients and intermediates for anaerobic microorganisms, and selection of higher substrate affinity anaerobic microorganisms in the co-phase system, as a result of the sludge exchange between the mesophilic and thermophilic digesters. r 2004 Elsevier Ltd. All rights reserved. Keywords: Anaerobic digestion; Thermophilic; Mesophilic; Co-phase; Sewage sludge 1. Introduction Single-stage mesophilic completely mixed anaerobic digestion has been widely used for the reduction in volume of organic sludge from wastewater treatment processes, and for obtaining energy in the form of methane gas. Here, the mesophilic digestion usually requires over a 20-day retention time, but is not so efficient in the reduction of volatile solids and the deactivation of pathogenic organisms. To overcome these limitations, interest in thermophilic digestion, using the higher metabolic rate of thermophilic micro- organisms, has increased [1–4]. Although better perfor- mance in the reduction of volatile solids and deactivation of pathogenic organisms can be obtained from thermophilic digestion, the effluent quality and ability to dewater the residual sludge are poor, and require additional energy to heat the digester [2,3,5]. Especially, thermophilic digestion is a little more sensitive to operational conditions, such as temperature, and the organic loading rate, as well as to the characteristics of the influent sludge [5,6]. Generally, anaerobic processes can be characterized from the digestion environments, microorganisms and process configuration, and each process has its unique advan- tages. According to previous studies [7–10], two-phase or two-stage anaerobic processes showed good perfor- mance in the effluent quality, methane yield, volatile ARTICLE IN PRESS *Corresponding author. Tel.: +82-51-410-4417; fax: +82- 51-410-4415. E-mail address: [email protected] (Y.-C. Song). 0043-1354/$ - see front matter r 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.watres.2003.12.019

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Page 1: Mesophilic and thermophilic temperature co-phase anaerobic digestion compared with single-stage mesophilic- and thermophilic digestion of sewage sludge

Water Research 38 (2004) 1653–1662

ARTICLE IN PRESS

*Correspond

51-410-4415.

E-mail addr

0043-1354/$ - se

doi:10.1016/j.w

Mesophilic and thermophilic temperature co-phase anaerobicdigestion compared with single-stage mesophilic- and

thermophilic digestion of sewage sludge

Young-Chae Song*, Sang-Jo Kwon, Jung-Hui Woo

Division of Civil and Environmental System Engineering, Korea Maritime University, 1, Dongsam-Dong, Yeongdo-Gu,

Busan 606-791, South Korea

Abstract

The performance of thermophilic and mesophilic temperature co-phase anaerobic digestions for sewage sludge, using

the exchange process of the digesting sludge between spatially separated mesophilic and thermophilic digesters, was

examined, and compared to single-stage mesophilic and thermophilic anaerobic digestions. The reduction of volatile

solids from the temperature co-phase anaerobic digestion system was dependent on the sludge exchange rate, but was

50.7–58.8%, which was much higher than 46.8% of single-stage thermophilic digestion, as well as 43.5% of the

mesophilic digestion. The specific methane yield was 424–468mL CH4 per gram volatile solids removed, which was as

good as that of single-stage mesophilic anaerobic digestion. The process stability and the effluent quality in terms of

volatile fatty acids and soluble chemical oxygen demand of the temperature co-phase anaerobic digestion system were

considerably better than those of the single-stage mesophilic anaerobic processes. The destruction of total coliform in

the temperature co-phase system was 98.5–99.6%, which was similar to the single-stage thermophilic digestion. The

higher performances on the volatile solid and pathogen reduction, and stable operation of the temperature co-phase

anaerobic system might be attributable to the well-functioned thermophilic digester, sharing nutrients and intermediates

for anaerobic microorganisms, and selection of higher substrate affinity anaerobic microorganisms in the co-phase

system, as a result of the sludge exchange between the mesophilic and thermophilic digesters.

r 2004 Elsevier Ltd. All rights reserved.

Keywords: Anaerobic digestion; Thermophilic; Mesophilic; Co-phase; Sewage sludge

1. Introduction

Single-stage mesophilic completely mixed anaerobic

digestion has been widely used for the reduction in

volume of organic sludge from wastewater treatment

processes, and for obtaining energy in the form of

methane gas. Here, the mesophilic digestion usually

requires over a 20-day retention time, but is not so

efficient in the reduction of volatile solids and the

deactivation of pathogenic organisms. To overcome

these limitations, interest in thermophilic digestion,

using the higher metabolic rate of thermophilic micro-

ing author. Tel.: +82-51-410-4417; fax: +82-

ess: [email protected] (Y.-C. Song).

e front matter r 2004 Elsevier Ltd. All rights reserve

atres.2003.12.019

organisms, has increased [1–4]. Although better perfor-

mance in the reduction of volatile solids and

deactivation of pathogenic organisms can be obtained

from thermophilic digestion, the effluent quality and

ability to dewater the residual sludge are poor, and

require additional energy to heat the digester [2,3,5].

Especially, thermophilic digestion is a little more

sensitive to operational conditions, such as temperature,

and the organic loading rate, as well as to the

characteristics of the influent sludge [5,6]. Generally,

anaerobic processes can be characterized from the

digestion environments, microorganisms and process

configuration, and each process has its unique advan-

tages. According to previous studies [7–10], two-phase

or two-stage anaerobic processes showed good perfor-

mance in the effluent quality, methane yield, volatile

d.

Page 2: Mesophilic and thermophilic temperature co-phase anaerobic digestion compared with single-stage mesophilic- and thermophilic digestion of sewage sludge

ARTICLE IN PRESS

C-Meso

Motor

Co-phase

C-Thermo

Feed sludge

Biogas

Biogas

35oC

55oC

Motor

S-Thermo

Biogas

Feed sludge55oC

S-Meso

Motor Biogas

Feed sludge

35oC

Motor

(a)(c)

(b)

Fig. 1. Schematic diagrams of the temperature co-phase

anaerobic digestion system (a), the single-stage mesophilic-

(b), and the thermophilic anaerobic digestion processes (c).

Y.-C. Song et al. / Water Research 38 (2004) 1653–16621654

solid reduction and process stability. This implies that

the performance of an anaerobic process could be

improved with the proper combination of the anaerobic

process characteristics. Recently, the temperature-

phased anaerobic digestion (TPAD) process, which

consists of thermophilic- and mesophilic digesters in

series, has been studied in order to incorporate the

advantages of both mesophilic- and thermophilic diges-

tion [11–14]. The TPAD process could be operated at

higher loading rates compared to single-stage processes

[12,13,15] and was better in the deactivation of

pathogenic organisms [13,15] and in its capability for

absorbing shock loadings like other two-stage or two-

phase anaerobic processes [8]. However, the first-stage

thermophilic digester of the TPAD process is still

sensitive to environmental conditions, and has possibly

an influence on the overall performance of the process as

well as the second-stage mesophilic digester. In addition,

the degree of maximum volatile solids’ reduction and

specific methane yield obtainable from the TPAD

process were not much different from that of single-

stage anaerobic processes with enough solid retention

time [12,15].

The purpose of this research is to test a new

configuration of anaerobic digestion, which consists of

a combination of mesophilic and thermophilic diges-

tions, for the more efficient sewage sludge stabilization.

For this, the performances of the single-stage completely

mixed mesophilic and the thermophilic digestions were

examined to clarify their unique characteristics, and

those of a newly conceived temperature co-phase

anaerobic digestion, which simultaneously uses two

temperature biochemical phases via the sludge exchange

between a spatially separated mesophilic and a thermo-

philic digester, were studied and compared with single-

stage anaerobic processes for sewage sludge.

2. Materials and methods

2.1. Experimental apparatus

The schematic diagrams of the anaerobic digestion

systems used for the experiments are shown in Fig. 1.

The temperature co-phase anaerobic digestion system

(a) consisted of a flow through mesophilic digester

(13.6 L) and a retention thermophilic digester (5 L). The

feeding of the influent sludge and the discharging of

the stabilized sludge from the co-phase system were

carried out at the mesophilic digester. An inter-circula-

tion line was installed to exchange the digesting sludge in

the spatially separated two temperature digesters. For

the single-stage anaerobic digestion systems, a comple-

tely mixed type mesophilic digester (b) and the same

type of thermophilic digester (c) were used, with effective

volumes of 12.2 and 5L, respectively. All the digesters

used in the experiments were made of transparent acrylic

tubes with conical style bottoms. A thermostat was

connected to a temperature sensor, which was inserted

into the digester. The outer wall of the digester was

coiled with an electrical heating material and covered

with an insulating material. The temperature of the

sludge during the digestion was maintained by the

thermostat at 3572�C for the mesophilic- and 5572�C

for the thermophilic conditions. The content of the

digester was completely mixed by a blade connected to a

vertical motor. There was a gas sampling port, with an

n-butyl rubber stopper, on the digester cover for the

analysis of the biogas composition. The biogas produc-

tion was monitored with a water displacement gas

collector, which was connected to the digester head

space. The water in the gas collector was acidified

with sulfuric acid and saturated with salt to prevent

the resolution of the biogas. During the experiments, the

wastewater sludge was uniformly supplied to the

digesters 4–8 times a day, using a peristaltic pump

equipped with a timer.

2.2. Experimental methods

The sewage sludge was obtained from a municipal

wastewater treatment plant in B metro city, and was

stored at 4�C in a refrigerator for less than 2 weeks prior

to its use as feed. The sludge was screened with a 1mm

sieve to prevent clogging problems during the transfer

from the feed tank to the digesters. The anaerobic

digestion sludge was taken from a single-stage meso-

philic anaerobic digester for sewage sludge at B metro

city. This sludge was also screened with a 1mm sieve to

remove impurities, and after analysis of the initial

characteristics was used as the inocula for the start-up

of both the anaerobic digestion systems. Table 1 shows

Page 3: Mesophilic and thermophilic temperature co-phase anaerobic digestion compared with single-stage mesophilic- and thermophilic digestion of sewage sludge

ARTICLE IN PRESS

Table 1

Characteristics of feed sludge and inocula and the experimental conditions

Characteristics pH VS (g/L) SCOD (g/L) Alkalinity

(mg/L as CaCO3)

NH4+-N (mg/L) Total coliform

(colonies/100mL)

Feed sludge 6.9370.3 25.0877.78 6.4571.5 24267521 406750 1� 106–4� 105

Seed sludge 7.5 9.9 1.8 — — —

Process Run HRT (days) SRT (days) Sludge exchange rate (L/d) OLR (gVS/L/d)

Meso- Thermo-

Co-phase I 21 2.38 1.0 7.35Q 0.8670.29

II 21 7.60 5.0 1.67Q 0.8670.10

III 21 4.68 2.5 3.33Q 1.0870.18

S-Meso — 20 20 — 1.4370.18

S-Thermo — 10 10 — 2.9070.37

Q: Flow rate of feeding sludge (L/d); S-Meso and S-Thermo: single-stage mesophilic and thermophilic anaerobic processes.

Y.-C. Song et al. / Water Research 38 (2004) 1653–1662 1655

the average characteristics of the feed sludge and

inocula, and the experimental conditions for the single-

stage and the co-phase anaerobic digestion systems. The

characteristics of the feed sludge, such as volatile solids,

chemical oxygen demand (COD) and alkalinity, varied

widely during the experiments.

During the experiments, the hydraulic retention times

(HRTs) and average organic loading rates for the single-

stage processes were 20 days and 1.43 gVS/L/d for

mesophilic digester, and 10 days and 2.90 gVS/L/d for

thermophilic digester, respectively. For the temperature

co-phase anaerobic digestion, the system HRTs were 21

days, and the various sludge exchange rates between the

mesophilic and the thermophilic digesters were 7.35,

3.33 and 1.67 times that of the inflow rate of the feed

sludge. Corresponding to each sludge exchange rate, the

sludge retention times (SRTs) for the thermophilic

digesters were the same as those of the HRTs, but were

reduced to 2.38, 4.68 and 7.60 days, respectively, for the

mesophilic digesters.

2.3. Analysis and calculations

To monitor the performance of the anaerobic

processes, digesting sludge samples were taken from

the single-stage and co-phase anaerobic digestion

systems daily. According to the Standard Methods

[16], the pH and alkalinity were measured daily and the

total solids (TSs), volatile solids (VSs), total COD

(TCODs) and soluble COD (SCODs) were analyzed

twice weekly. The volatile fatty acid (VFA) was also

measured daily by the titration method proposed by

Anderson and Yang [17], and the composition of the

VFA was analyzed twice weekly by a high-performance

liquid chromatography (DX-500), equipped with an

Aminex HPX-87H column (300� 7.8mm) employing

ultraviolet detection. The biogas production was mon-

itored daily, and its composition was analyzed by a gas

chromatography (Gow-Mac Series 580), equipped with

a stainless-steel column (Porapak Q, mesh size 807100,

6 ft� 1/800), employing thermal conductivity detection.

After the stabilization of the anaerobic processes, the

NH4+-N was measured according to the Standard

Methods [16]. The concentrations of sulfate and ortho-

phosphate were analyzed with ion chromatography

(DX-500) with an Ion-Pac column (AS 144� 250mm)

and a conductivity detector. The total coliform content

of the digested sludge was also measured using the

membrane filtration method, by counting the colonies

after 36 h of incubation at 35�C [16]. During the

experiments, the steady state indicating stable anaerobic

digestion was determined from the stability of certain

parameters, including pH, VS, the VFA-to-alkalinity

ratio and methane content of the biogas. The specific

hydrolysis rate, which was based on the unit particulate

COD (PCOD) loading rate, was estimated by the PCOD

removal rate per unit volume of the digester. The mean

values and standard deviations of the experimental

results during the steady-state operations of the anae-

robic processes were determined to compare the diges-

tion performances by the compensation of the variations

in the characteristics of the feed and digesting sludge.

3. Results and discussion

3.1. Single-stage mesophilic- and thermophilic digestion

During the 70 days of operation of the single-stage

anaerobic processes, the alkalinity level of the thermo-

philic digestion process was higher than that of the

mesophilic process, as shown in Fig. 2(a). It is well

known that the alkalinity in an anaerobic digestion can

be generated from the degradation of nitrogenous

Page 4: Mesophilic and thermophilic temperature co-phase anaerobic digestion compared with single-stage mesophilic- and thermophilic digestion of sewage sludge

ARTICLE IN PRESSY.-C. Song et al. / Water Research 38 (2004) 1653–16621656

organic compounds, sulfate reduction, release of ortho-

phosphate and an increase of VFAs [18–20]. In this

study, the ammonia nitrogen from the thermophilic

digestion process was 860mg/L, which was higher than

the 630mg/L of the mesophilic process (Table 2).

However, significant differences in the sulfate and

ortho-phosphate were not observed in the single-stage

mesophilic or thermophilic anaerobic digestions. This

indicates that the activity for the degradation of

nitrogenous organic compounds under the thermophilic

Time (day)0 15 30 45 60 75

pH

5

6

7

8

S-MesoS-ThermoFeed sludge

Alk

alin

ity(m

g/L)

as

CaC

O3

0

2000

4000

6000

8000

10000(a)

(b)

Fig. 2. Alkalinity (a) and pH (b) in the single-stage anaerobic

digestion systems.

Table 2

Effluent quality and performance of the single-stage mesophilic and t

Process pH Alkalinity

(mg/L as CaCO3)

NH4+-N

(mg/L)

SO

(m

S-Meso 7.6770.1 64127545 630 36

S-Thermo 8.0870.1 68757546 860 30

Process VFA VSs

Total

(mg HAc/L)

C2:C3:C4

(%)

Concentration

(g/L)

R

(%

S-Meso 579797 97.6:2.4:0 16.1872.7 43

S-Thermo 15877302 26.4:73.6:0 15.3471.18 46

conditions was higher than that under the mesophilic

conditions [19–21]. The pH value of the influent sludge

gradually decreased from 7.2 to 6.8 during the operation

period, as shown in Fig. 2(b). However, the pH value of

the mesophilic process increased from 7.2 to around

7.67, and was stable at this value. The pH of the

thermophilic process was generally higher, at 8.08, than

that of the mesophilic process. This was as a result of the

higher alkalinity of the thermophilic anaerobic digestion

process. The increased alkalinity, and thus pH, from the

degradation of nitrogenous compounds in our experi-

ments is in agreement with previous studies [22,23].

The SCOD level of the thermophilic process was a

little more dependent on the change in the influent

characteristics, and was generally higher than that in the

mesophilic process, as shown in Fig. 3(a). At a steady

state, the mean values of SCODs were 2555 and

5240mg/L, for the mesophilic and thermophilic pro-

cesses, respectively (Table 2). The VFA level in the

thermophilic process was generally higher than that

in the mesophilic process, which was consistent with

the SCOD data (Fig. 3(a)). This clearly shows that the

mesophilic digestion was superior to that of the

thermophilic digestion in terms of the effluent quality,

which can be explained by the low substrate affinity of

some thermophilic organisms [2,3,5,6]. The main com-

ponent of the VFA in the mesophilic process was

acetate, but in the thermophilic process it was propio-

nate (Table 2). From the literatures [2, 5, 6], the higher

level of propionate in the thermophilic digester occurred

under higher hydrogen partial pressures, and the acetate

from higher organic loading rate conditions. In this

study, the accumulation of propionate in the thermo-

philic digester was probably due to the wide variation in

the influent characteristics. This indicates that acetogens

and hydrogenotrophs under the thermophilic condition

are more sensitive to changes in the environments. The

VFA contents of the SCOD were around 22.7 and

30.3%, for the mesophilic and thermophilic digestion

processes, respectively. These were mainly as a result of

he thermophilic digestion processes

42�

g/L)

PO43�

(mg/L)

SCOD

(mg/L)

Total coliform

reduction (%)

.2 42.4 25557493 66.7

.4 39.5 524071404 99.7

Methane

eduction

)

Specific hydrolysis

rate (g PCOD/L/d)

CH4 (%) Specific yield

(mL/g VSrem)

.578.4 0.451 64.772.6 451.1745.0

.875.4 0.520 63.671.8 416.0767.1

Page 5: Mesophilic and thermophilic temperature co-phase anaerobic digestion compared with single-stage mesophilic- and thermophilic digestion of sewage sludge

ARTICLE IN PRESS

Time(day)

0 15 30 45 60 75

VF

A/A

lkal

inity

0.0

0.2

0.4

0.6

VF

A(m

g/L

as H

Ac)

0

1500

3000

4500

6000

SC

OD

(mg/

L)

0

2000

4000

6000

8000

10000

S-MesoS-ThermoFeed sludge

(a)

(b)

(c)

Fig. 3. SCOD (a), VFA (b) and the VFA-to-alkalinity ratio (c)

in the single-stage anaerobic digestion processes.

VS

(g/L

)

0

10

20

30

40

50

60

70

80

VS

red

uctio

n(%

)

10

20

30

40

50

60

70

80

Feed sludgeS-MesoS-ThermoS-Meso(%)S-Thermo(%)

(a)

Time (day)

0 10 20 30 40 50 60 70

Spe

cific

met

hane

yie

ld(m

lCH

4/g

VS

rem

oved

)0

200

400

600

800

1000

Met

hane

con

tent

(%)

0

10

20

30

40

50

60

70

80

90S-Meso(mL)S-Thermo(mL)S-Meso(%)S-Thermo(%)

(b)100

Fig. 4. VSs (a) and biogas (b) in single-stage anaerobic

processes.

Y.-C. Song et al. / Water Research 38 (2004) 1653–1662 1657

the higher level of propionate under thermophilic

condition.

The VFA to alkalinity ratio for the two single-stage

anaerobic systems were monitored to compare the

buffering capacities for the rapid change of pH

(Fig. 3(C)). It has been reported that the buffering

capacity was sufficient when the VFA-to-alkalinity ratio

was maintained below 0.4 [11]. In this study, the ratio of

the mesophilic process was almost constant at around

0.1, with the exception of the start-up period. For the

thermophilic anaerobic digestion process, this ratio was

a little unstable during the early stage of the process

operation. However, as the steady-state conditions were

approached, this value decreased to around 0.2 with

some fluctuations. The slightly higher VFA-to-alkalinity

ratio of the thermophilic process was primarily as a

result of the higher VFA concentration. This indicates

that the single-stage mesophilic anaerobic digestion

could have better buffering capabilities than the

thermophilic digestion.

The reduction in VSs during anaerobic digestion is

generally equal to the total amount of VFA converted

from the volatile dissolved solids. The dissolved VSs are

produced from the hydrolysis of suspended VSs, and the

VFA is finally converted to methane gas. The reduction

in the VSs can be expressed as the sum of the residual

VFA and the methane gas produced from the anaerobic

digester. Therefore, the hydrolysis of particulate organ-

ics has a significant influence on the reduction of VSs. In

this study, the levels of VSs were constantly maintained

throughout the operations at 16.18 g/L for the meso-

philic and 15.34 g/L for the thermophilic digester,

despite the wide variation in the influent characteristics

of the feed sludge, as shown in Fig. 4(a). Therefore, the

VS reduction of the thermophilic digester was consider-

ably dependent on the feed sludge characteristics. This

implies that the single-stage anaerobic digestion pro-

cesses have large potentials for the stable reduction of

VSs, which are not influenced by the temperature

conditions. However, the specific hydrolysis rate in the

thermophilic process during the steady state was

0.520 gPCOD/L/d, which was a little higher than the

0.451 gPCOD/L/d for the mesophilic process (Table 2).

This led to the higher reduction of volatile solids of

Page 6: Mesophilic and thermophilic temperature co-phase anaerobic digestion compared with single-stage mesophilic- and thermophilic digestion of sewage sludge

ARTICLE IN PRESS

Time (days)

Alk

alin

ity (

mg/

L as

CaC

O3)

0

1000

2000

3000

4000

5000

6000

0 20 40 60 80 100 120

pH

6

7

8

C-MesoC-ThermoFeed sludge

(a)

(b)

Run I Run II Run III

Fig. 5. Alkalinity (a) and pH (b) in the mesophilic/thermophilic

co-phase anaerobic digestion system.

Y.-C. Song et al. / Water Research 38 (2004) 1653–16621658

46.8% in the thermophilic digester compared to the

43.5% of the mesophilic digester. Maibaum et al. [3]

reported that the difference in the degradation rates of

solid substrates under the thermophilic and mesophilic

conditions become significant in relation to the decrease

in the retention time [3].

As shown in Fig. 4(b), the average methane content

of the biogas from the mesophilic process was just a

little higher, at around 64%, than that of the thermo-

philic process. This was probably a result of the reduced

solubility of carbon dioxide under the thermophilic

condition [22]. In previous studies, the methane content

of the biogas was mainly affected by the types of

substrate, rather than the temperature conditions,

for the anaerobic digestion [4,22,24]. However, the

specific methane yield of the mesophilic process, based

on the removed VS, was a little more sensitive to the

influent characteristics of the feed sludge, indicating the

higher capacity of mesophilic methanogens for coping

with the variation of the influent characteristics com-

pared to the thermophilic methanogens. The average

specific methane yield of the thermophilic process was

lower, at 416mLCH4/gVSremoved, than the

451mLCH4/gVSremoved of the mesophilic digester

(Table 2). This was presumably due to the higher

maintenance energy of the anaerobic thermophilic

microorganisms [5,6,25], as well as the higher hydrogen

content of the biogas [22].

Interest in the deactivation of pathogenic organisms,

for the production of Class A biosolids from the

digested residual sludge, from wastewater sludge during

anaerobic digestion has significantly increased in recent

years. In this study, the percentage of deactivation of

total coliform in the thermophilic digester was 99.7%,

which was much higher than the 66.7% in the

mesophilic digester (Table 2). The higher deactivation

of pathogenic organisms in the thermophilic process was

probably a result of the combined effects of the long

retention time of 10 days, under higher levels of free

ammonia and VFA, as well as the thermophilic

conditions. [14].

3.2. The mesophilic and thermophilic co-phase anaerobic

digestion

The alkalinity levels of the temperature co-phase

mesophilic and thermophilic digesters were influenced

by the alkalinity variation of influent sewage sludge, as

shown in Fig. 5(a). The average level of alkalinity in

the co-phase thermophilic digester was around 3500–

4700mg/L as CaCO3, which was a little higher than

3300–4400mg/L as CaCO3 in the co-phase mesophilic

digester. The higher alkalinity under the thermophilic

condition was similar to that of the single-stage

anaerobic processes, as shown in Fig. 2(a), and reflects

the higher degradation activity of nitrogenous organic

compounds, such as proteins, under the thermophilic

condition [22,23]. The pH levels of the co-phase

mesophilic and thermophilic digesters in the early stage

of the operation were a little unstable due to the wide

variation in the feed characteristics, as shown in

Fig. 5(b). On the 35th day, the pH level in the mesophilic

digester was stable at 7.5–7.6, which was similar to that

in the single-stage mesophilic anaerobic process. The pH

in the co-phase thermophilic digester during the initial

stage of the operation increased to over 8.0, which was

similar to the single-stage thermophilic digester. How-

ever, at the stable stage, the pH in the thermophilic

digester was slightly decreased to 7.7–7.8, which was a

more favorable pH condition for the thermophilic

anaerobic bacteria [8,26]. This was a result of the

exchange of the thermophilic sludge with the lower

alkalinity sludge from the mesophilic digester [27]. The

influence of the sludge exchange rate on the pH of the

co-phase system was not observed apparently. However,

the alkalinity levels of the co-phase mesophilic and

thermophilic digesters were slightly increased in relation

to the increase of the sludge exchange rate from 1.67Q to

over 3.33Q. This was a result of the enhanced degrada-

tion of nitrogenous compounds due to the extended

portion of the sludge passing through the thermophilic

digester at higher sludge exchange rate [22,23,26].

Page 7: Mesophilic and thermophilic temperature co-phase anaerobic digestion compared with single-stage mesophilic- and thermophilic digestion of sewage sludge

ARTICLE IN PRESS

Time (day)0 20 40 60 80 100 120

SC

OD

(m

g/L)

0

2000

4000

6000

8000

10000

VF

A(m

g/L

as H

Ac)

1000

2000

3000

4000

C-MesoC-Thermofeed

(a)

(b)

Run(I) Run(II) Run(III)

Fig. 6. VFA (a) and SCOD (b) in the temperature co-phase

anaerobic digestion system.

Y.-C. Song et al. / Water Research 38 (2004) 1653–1662 1659

Fig. 6(a) shows the VFA trends of the mesophilic and

thermophilic co-phase anaerobic digestion system. After

35 days of the operation, the levels of VFA in the co-

phase thermophilic digester became stable, as well as

that of the mesophilic digester, and were not influenced

by the wide change in the influent characteristics. At

stable state, the VFA levels in the co-phase thermophilic

digester were 339–679mgHAc/L, which were lower than

436–795mgHAc/L in the mesophilic digester for all

rates of the sludge exchange (Table 3). This indicates

that the thermophilic digester of the co-phase system

was stable and well functioned and the affinity of the

thermophilic sludge on VFA was quite higher than that

of the sludge from the single-stage thermophilic digester

(Fig. 3(b)). This seems to suggest that the higher

substrate affinity methanogenic bacteria were selected

and dominated in the co-phase thermophilic digester by

the sludge exchange between the mesophilic and

thermophilic digesters. In the case of the co-phase

mesophilic digester, the VFA level was also less than

that of the single-stage mesophilic digester, at 1.67Q and

3.22Q of the sludge exchange rates. However, at 7.36Q

of the sludge exchange rate, the VFA was increased to

around 795mgHAc/L, indicating that the methanogen-

esis was the rate limiting step in the overall anaerobic

reactions due to the reduced solid retention time of 2.38

days in the mesophilic digester. The main VFA

component of the co-phase mesophilic digester was

acetate, as the single-stage mesophilic process (Tables 2

and 3). However, in the co-phase thermophilic digester,

the propionate content was considerable, at 7.35Q of the

sludge exchange rate, as the single-stage thermophilic

process. This higher propionate content at the higher

sludge exchange rate in the co-phase thermophilic

digester was probably related to the higher hydrogen

partial pressure [5,22].

Fig. 6(b) shows the SCOD levels of the mesophilic and

thermophilic temperature co-phase digestion systems.

At steady state, the levels of SCOD in the co-phase

mesophilic and thermophilic digesters were 2100–2200

and 1700–2900mg/L, which were less than those of

single-stage mesophilic and thermophilic processes,

respectively. The good effluent quality in the SCOD

was mainly attributable to the low VFA in the co-phase

mesophilic and thermophilic digesters, probably as a

result of the higher methanogenic activity and higher

affinity of the anaerobic sludge on VFA in the co-phase

system.

Fig. 7(a) shows the VFA-to-alkalinity ratio required

to evaluate the buffering capacity of the temperature co-

phase anaerobic digestion system. At 7.35Q of the

sludge exchange rate, the VFA-to-alkalinity ratios were

stable, at 0.19 for the mesophilic- and 0.16 for the

thermophilic digester, except for the early stage of the

operation. As the sludge exchange rates were reduced to

3.33Q and 1.67Q, the VFA-to-alkalinity ratios were a

little more reduced and stabilized. These indicate that

the buffering capacity in the temperature co-phase

anaerobic system was sufficient for sewage sludge

digestion, as with the single-stage mesophilic anaerobic

processes [11]. The higher buffering capacity in the co-

phase thermophilic digester was attributable to both

higher alkalinity level from the enhanced degradation of

nitrogenous compounds and lower VFA level by the

higher substrate affinity of methanogens. The higher

buffering capacity in the co-phase thermophilic digester

also contributed to good buffering capacity in the

mesophilic digester through the sludge exchange be-

tween the mesophilic and thermophilic digesters.

Generally, the mesophilic anaerobic digestion requires

over a 20-day retention time to stabilize the wastewater

sludge, which was due to the slow growth rate of the

mesophilic anaerobic bacteria [1]. However, as presented

in Table 1, the SRTs of the mesophilic digester of the

temperature co-phase anaerobic digestion system were

varied between 2.38 and 7.60 days according to the

sludge exchange rate between the mesophilic and

thermophilic digesters. The overall specific methane

yields were as good as the single-stage mesophilic

anaerobic process, although some portion of the overall

yield was from the thermophilic digester of the co-phase

digestion system. Especially, at 7.36Q of the sludge

exchange rate, the SRT of the mesophilic digester was

Page 8: Mesophilic and thermophilic temperature co-phase anaerobic digestion compared with single-stage mesophilic- and thermophilic digestion of sewage sludge

ARTICLE IN PRESS

Table 3

Performance of the temperature co-phase anaerobic digestion system

Content Run I Run II Run III

Meso- Thermo- Meso- Thermo- Meso- Thermo-

Exchange rate (L/d) 7.35Q 1.67Q 3.33Q

pH 7.5370.07 7.7370.09 7.5270.06 7.7470.07 7.5570.06 7.7570.13

Alkalinity

(mg/L as CaCO3)

43087271 45207239 33557320 35137373 43577118 46217215

SCOD (mg/L) 21337148 22557193 21787168 29757103 2182788 1745775

VFA mg/L as Hac 795766 6977128 4467150 3397179 436768 364742

C2: C3: C4 (%) 86.1:10.2:3.7 77.4:20.5:2.1 67.7:32.3:0 93.8:6.2:0 83.8:6.6:9.6 96.7:3.3:0

Specific hydrolysis

rate (g PCOD/L/d)

Each digester 0.098 0.035 0.286 0.019 0.225 0.014

Overall 0.537 0.524 0.566

VSs Concentration (g/L) 10.2070.27 9.9770.32 8.8170.28 8.2070.18 10.1070.44 9.9370.12

Reduction (%) 58.472.7 50.773.2 58.872.0

Specific methane

yield (mLCH4/

gVSremoved)

459.7747.4 468.2734.7 424.2743.1

Destruction of

total coliform (%)

99.6 98.5 99.1

Y.-C. Song et al. / Water Research 38 (2004) 1653–16621660

only 2.38 days, but the specific methane yield was higher

at 460mLCH4/g VSremoved. This shows the possibility

that there are some temperature facultative anaerobic

microorganisms that are highly active under mesophilic,

as well as thermophilic conditions in the temperature co-

phase anaerobic digestion system with sludge exchange.

This is different from the widely held view that two

distinct groups of anaerobic bacteria exist at the

mesophilic and thermophilic temperature regimes. In a

previous study [13], it has been reported that they are the

temperature-tolerant anaerobic bacteria from the anae-

robic biomass activity tests under mesophilic and

thermophilic conditions.

The hydrolysis rate in the anaerobic digestion of

sewage sludge is an important parameter to evaluate the

process performance due to the higher content of the

particulate organics in the sewage sludge. The overall

specific hydrolysis rate of the co-phase anaerobic

digestion system, estimated as the PCOD removal rate

per unit volume based on the PCOD loading rate in the

digester, was 0.537 g PCOD/L/d at 7.35Q of the sludge

exchange rate, which was higher than 0.520 gPCOD/L/d

of single-stage thermophilic anaerobic digestion process

as well as 0.451 g PCOD/L/d of single-stage mesophilic

anaerobic digestion process (Table 3). However, the

specific hydrolysis rate for the thermophilic digester in

the co-phase digestion system was 0.035 gPCOD/L/d,

which was less than 0.098 gPCOD/L/d of the mesophilic

digester, indicating that most of the particulate organics

were hydrolyzed in the mesophilic digester. This is quite

different from that of the single-stage anaerobic

processes, where the hydrolytic activity of the thermo-

philic digester was higher than that in the mesophilic

digester. These interesting results might arise because the

intermediates including hydrolytic enzyme, alkalinity

and other nutrients were easily produced from the stable

and well-functioned thermophilic digester, and were

then transferred into the mesophilic digester through the

sludge exchange process to make the favorable meso-

philic condition for the hydrolytic enzyme, or anaerobic

microorganisms. When the sludge exchange rate was

decreased to 3.33Q and 1.67Q, the overall specific

hydrolysis rates were varied to 0.566 and

0.524 gPCOD/L/d. Independent of the sludge exchange

rate in the co-phase digestion system, however, the

specific hydrolysis rate of the mesophilic digester was

higher than that of the thermophilic digester.

The VSs in the co-phase mesophilic and thermophilic

digesters were stable, which were not influenced by the

VSs variation of the influent sludge, as shown in Fig.

7(c). The reduction of volatile solids was around 51% at

1.67Q of the sludge exchange rate, but increased to

around 59% when the sludge exchange rate was

increased to over 3.33Q as presented in Table 3. In the

literature [15], the reduction of volatile solids obtained

from TPAD process for waste-activated sludge was

about 50% at 28 days of SRT, which was around 10%

higher than that of the single-stage mesophilic digester.

In this study, the reduction of volatile solids that could

be obtained in the co-phase digestion system was over

15% higher than that of the single-stage mesophilic

digester and around 12% higher than the single-stage

thermophilic digester. The enhanced performance on the

VS reduction obtained from the temperature co-phase

Page 9: Mesophilic and thermophilic temperature co-phase anaerobic digestion compared with single-stage mesophilic- and thermophilic digestion of sewage sludge

ARTICLE IN PRESSS

peci

fic m

etha

ne y

ield

(m

L C

H4/

g V

S re

mov

ed)

0

200

400

600

800

1000

1200

Time (days)0 20 40 60 80 100 120

VS

(g/

L)

0

20

40

60

VS

red

uctio

n (%

)

0

20

40

60

80

100

C-MesoC-ThermoFeed sludgeVS reduction(%)

VF

A/A

lkal

inity

0.0

0.5

1.0

1.5

2.0

2.5

Run I Run II Run III(a)

(b)

(c)

Fig. 7. VFA-to-alkalinity ratio (a), specific methane yield (b)

and VSs (c) in the temperature co-phase anaerobic digestion

system.

Y.-C. Song et al. / Water Research 38 (2004) 1653–1662 1661

anaerobic digestion system was mainly attributable to

the higher hydrolytic activity of the mesophilic digester,

which was a result of the sludge exchange between the

mesophilic and thermophilic digesters. On the other

hand, the additional energy for the sludge exchange and

for heating the thermophilic digester in the co-phase

digestion system is possibly required. However, the

additional energy requirements could be compensated

by the advantages of the co-phase digestion system

including higher reduction of volatile solids, better

effluent quality and process stability and increased

methane production, compared to the single-stage

mesophilic- or the thermophilic processes.

The destruction of total coliform in the co-phase

anaerobic digestion system was increased from 98.5% to

99.6% in relation to the increase in the sludge exchange

rate, as presented in Table 3. This shows that the ratio of

sludge passing through the thermophilic digester in the

temperature co-phase system plays an important role in

the destruction of total coliform, rather than the

retention time, under the thermophilic condition.

4. Conclusions

From the studies on the characteristics of the single-

stage mesophilic- and thermophilic anaerobic processes,

the treating of sewage sludge, and the mesophilic and

thermophilic temperature co-phase anaerobic digestion

systems, the following conclusions were made. The

single-stage mesophilic anaerobic digestion in terms of

the specific methane yield, effluent quality and process

stability was superior to the thermophilic digestion, but

both VS reduction and total coliform destruction from

the single-stage thermophilic digestion were higher than

those of the mesophilic digestion. The performance of

the mesophilic and thermophilic co-phase anaerobic

digestions was dependent on the sludge exchange rate

between the mesophilic and thermophilic digesters, but

the advantages of single-stage mesophilic and the

thermophilic anaerobic digestions could be obtained

from the temperature co-phase anaerobic digestion

system. The effluent quality in terms of SCOD and

VFA, specific methane yield and process stability, which

could be obtained from the temperature co-phase

anaerobic digestion, were better than those of the

single-stage mesophilic anaerobic digestion. The patho-

gen destruction was similar to those of the single-stage

thermophilic digestion, but the reduction of volatile

solids was much higher than that of the single-stage

thermophilic digestion. These higher performances of

the temperature co-phase anaerobic digestion might be

mainly attributable to stable and well-functioned

anaerobic thermophilic digester, selection of the active

and higher substrate affinity of anaerobic microorgan-

isms and sharing the nutrients and intermediates for

anaerobic microorganisms, as a result of the sludge

exchange between the mesophilic and thermophilic

digesters.

Acknowledgements

This work was supported by the academic research

program (Grant No. 2001-N-BI03-P-04) of the Korea

Energy Management Corporation.

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