Upgrading of the anaerobic digestion of waste activated sludge by ...

Q IWA Publishing 2009 Water Science & Technology—WST | 59.1 | 2009

185

Upgrading of the anaerobic digestion of waste activated sludge by combining temperature-phased anaerobic digestion and intermediate ozonation T. Kobayashi, Y. Y. Li, H. Harada, H. Yasui and T. Noike

ABSTRACT Upgrading of the anaerobic digestion of waste activated sludge (WAS) by the combination of temperature-phased two-stage digestion and intermediate ozonation was investigated by a continuous experiment with two processes, TM and TOM. The TM process is a temperaturephased two-stage system, which consists of a thermophilic digester and a mesophilic digester in series. The TOM process is a temperature-phased two-stage process with the intermediate ozonation. Two processes were operated at hydraulic retention times of 30 days for over 123 days. Waste activated sludge taken from wastewater treatment plant was fed as a substrate. Microbial community structure in each digester was analysed with molecular tools. Despite of less amount of ozone dose in TOM than ozone pre-treatment process, better effect of ozonation on performance improvement was obtained in TOM. TOM had the highest methane yield and CODCr reduction among comparative processes. Furthermore, flocculation efficiency of TOM

T. Kobayashi Y. Y. Li H. Harada Department of Civil and Environmental Engineering, Tohoku University, 6-6-06 Aza-Aoba, Aramaki, Aoba-ku, Sendai, Miyagi 980-8579, Japan E-mail: [email protected] Y. Y. Li Department of Urban and Environmental Engineering, Tianjin Institute of Urban Construction, Jinjinggonglu 26, Tianjin 300384, China

followed that of mesophilic digestion. Quality of dewatered supernatant is comparable to mesophilic digestion. Key words

| anaerobic digestion, intermediate ozonation, temperature phase, waste activated sludge

H. Yasui Faculty of Environmental Engineering, The University of Kitakyushu, 1-1 Hibikino, Wakamatsu, Kitakyushu-city, Fukuoka 808-0135, Japan T. Noike ARISH, Nihon University, 2-1 Kudan-kita 4-chome, Chiyoda-ku, Tokyo 102-0073, Japan

INTRODUCTION Huge amount of waste activated sludge (WAS) is produced

WAS, many efforts by mechanical, chemical and thermal

through the wastewater treatment system, which contains

pretreatment have been achieved (Kang et al. 2000; Kim

activated sludge process. Reduction of WAS with recover-

et al. 2003). Many researches focused on the optimum

ing of energy by anaerobic digestion has been of interest.

method of pretreatment and optimisation of pretreatment

However, it is well recognised that the degradation

condition. But it is also important to propose an optimum

efficiency of WAS is at lower level than those of other

process by combining known unit operations although

organic solids such as primary sewage sludge (Lafitte-Trou-

there is far less researches on the latter subject than that of

que & Forster 2002; Arnaiz et al. 2006). Anaerobic digestion

the former.

process consists of several stages: hydrolysis, acidogenesis,

Anaerobic digestion has well-known two optimum

methanogenesis. For WAS digestion, hydrolysis step is

temperature, one of which ranges around 358C (mesophilic)

rate-remitting (Li & Noike 1992). To promote hydrolysis of

and another around 558C (thermophilic). It is reported that

doi: 10.2166/wst.2009.510

186

T. Kobayashi et al. | Anaerobic digestion and intermediate ozonation of waste activated sludge

thermophilic digestion has higher hydrolysis rate of WAS than mesophilic digestion despite of worse dewaterability and the accumulation of soluble COD (Sangsan et al. 2007). However, in resent years, the development of temperaturephased anaerobic digestion (TPAD) system (Streeter et al. 1997) enabled to make an improvement of dewaterability with high hydrolysis rate of thermophilic digestion. So far several studies on TPAD process have been reported (Schmit & Ellis 2001; Vandenburgh & Ellis 2002; Sung & Santha 2003), but no study was reported for treating WAS. Ozonation is one of the pre-treatment techniques to improve hydrolysis of sewage sludge for anaerobic digestion (Weemaes et al. 2000). Generally, the amount of ozone dose for a treatment is determined on the basis of TS or SS concentration of sludge. In order to economize energy consumption, the amount of ozone dose should be as small as possible. The previous study revealed that the ozone post-treatment and recycling process, in which half of anaerobic digested sludge was ozonized and brought back to the digester, had higher effect on improvement of biologic degradability despite of less amount of ozone dose compared to ozone pretreatment process. More effective oxidation of difficult degradable substances was carried out on digested sludge than on WAS because the former consists of difficult degradable substances while the latter

Water Science & Technology—WST | 59.1 | 2009

METHODS Operation of two anaerobic digestion processes Continuous experiment with two processes (Figure 1), TM and TOM was carried out over 123 days. The TM process is a temperature-phased two-stage process, which consists of a thermophilic digester (558C) and a mesophilic digester (358C) in series. The TOM process is a temperature-phased two-stage process with the intermediate ozonation, which consists of a thermophilic digester (558C), an ozone treatment reactor and a mesophilic digester (358C) in series. The digested sludge from thermophilic digester was ozonized in batch in an ozone treatment reactor with 5 L volume. Ozone dose was set at 0.02 g/g-TS. Digesters used in this study were completely stirred tank reactors with a working volume of 5 L. Each digester were operated at a hydraulic retention time (HRT) 30 days respectively. A substrate tank kept at 48C fed WAS to thermophilic digesters of two processes by a time-controlled pump. Seed sludge for thermophilic digester was anaerobic sludge treating WAS at 558C, and that for mesophilic digester was sludge treating WAS at 358C. The WAS used in this study was taken from Sen-en WWTP every 3 –4 week period and then stored at 48C.

consists of both easy and difficult degradable substances. For further improvement in performance of TPAD process, we investigated the effect of the temperaturephased anaerobic digestion process with intermediate ozo-

Chemical analysis TS, VS, SS, VSS and PO32 4 ZP were measured in accordance

nation. In our study, the thermophilic digested sludge was

with the testing method for wastewater (JSWA 1994),

ozonized and fed to the mesophilic digester. Like post

and CODCr was measured with APHA Standard Methods

treatment and recycling, intermediate ozonation of digested

(APHA 1995). Carbohydrates were measured by Phenol

sludge would improve anaerobic digestablity more effi-

H2SO4 method, Proteins were measured by Lowry

ciently than pre-treatment with less amount of ozone dose.

method and Lipids were measured by Bligh-Dyer method.

But unlike that, intermediate ozonation treats all of digested

The biogas production was measured with a wet gas

sludge from thermophilic digester.

meter (SINAGAWA). Gas composition was detected

TM

Substrate tank (4°C)

Thermophilic digester (55°C)

Mesophilic digester (35°C)

TOM

Substrate tank (4°C)

Thermophilic digester (55°C)

Ozonation (0.02g/g-TS)

Figure 1

|

Schemes of two anaerobic digestion processes, TM and TOM.

Mesophilic digester (35°C)

187


by the gas chromatograph (Shimadzu, GC-8A). VFAs were measured by the gas chromatograph with FID (Agilent, 6890). Ammonium concentration was determined by the capillary electrophoresis (OHTSUKA, Photal CAPI-3200). Flocculation efficiency was measured by using

polyamidine-base

high

molecular

flocculants

for dewatering of sludge (Yonemoto & Sakano 1995). The value of flocculation efficiency was based on the addition of flocculants until the sludge appeared solidliquid separation.


RESULTS AND DISCUSSION Characteristics of operation of the two processes The time courses of biogas production rate, VFA and pH in the thermophilic digester and mesophilic digester of TM were shown in Figure 2, and those of TOM were shown in Figure 3. After day 90, the operating conditions of all digester appeared stable, so that each digester was considered to be in a steady state condition. Table 1 indicates the performance of each digester during the steady state condition. As shown in Figure 2(b) and Figure 3(b), the pH was in the suitable range 7– 8 for anaerobic digestion in

Clone analysis and real-time quantitative PCR

the all digesters. The pH in thermophilic digesters remained

Digested sludge samples for clone analysis were taken from

at higher level than that in mesophilic digesters. VFA had

each digester after 100 days of operation. Genomic DNA,

an average of 244 mg-HAc/L at the thermophilic digester of

extracted from samples with Ultra Clean Soil DNA

TM, 173 mg-HAc/L in the thermophilic digester of TOM

Isolation Kit (MO-BIO), followed amplification with the

while little VFA was detected in the mesophilic digesters

primers EUB338-mixF (Amann et al. 1990; Daims et al. 1999) and UNIV1500R (Weisburg et al. 1991) for bacteria. Thermal cycling of PCR consisted of 30 s denaturing

range 2,200 – 2,300 mg/L in each digester. Mesophilic digesters of two processes showed higher flocculation efficiency than thermophilic digesters. 90% of the total

2 min. PCR products were cloned with TOPO TA Cloning

(a)

kit (invitrogen). Cloned DNA fragments obtained from

Biogas production rate (L /d.L)

at 948C, 40 s annealing at 508C and extraction at 728C for

in both TM and TOM. NHþ 4 ZN concentration was in the

randomly selected recombinants served as templates for sequencing analysis with a CEQ8000 (BECKMAN COULTER). Operational Taxonomic Units (OTUs) was classified according to Hae III cleavage patterns. Representative

1.5 1.2 0.9 0.6

0.0

sequences were searched by using the BLAST program Genomic DNA for Real-time quantitative PCR was extracted from digested sludge taken from digesters at day 95, 100 and 120 with Ultra Clean Soil DNA Isolation

(b)

0

20

40

Thermophilic (TM) 6

0

20

40

for 30 s. The DNA fragments amplified by using the primer set EUB338F and UNIV1500R from affiliate clones with

100

120

1,500 Thermophilic (TM)

Mesophilic (TM)

1,000 500 0

each phylogenetic group respectively used as a template for the standard curve.

60 80 Time (days)

Mesophilic (TM)

(c) TVFA (mg-HAc/L)

ing at 948C, 10 s annealing at 608C and extraction at 728C

120

7

primer sets for Coprothermobacter spp. were COP1029f

The amplification followed 40 cycles PCR with 5 s denatur-

100

8

performed with LightCycler (Roche Diagnostics). The

the binding of the SybrGreen to double stranded DNA.

60 80 Time (days)

9

Kit (MO-BIO). Amplification of 16S rRNA gene was

and COP1179r (Kobayashi et al. 2008). The signal was

Mesophilic (TM)

pH

(Altschul et al. 1997) to classify OTUs based on phylogeny.

Thermophilic (TM)

0.3

Figure 2

|

0

20

40

60 80 Time (days)

Time course of operating condition.

100

120


188

(a)

methane production rate in TM process was attributed to

Biogas production rate (L/d.L)

1.5

Thermophilic (TOM)

Mesophilic (TOM)

the thermophilic digester, and 78% of that in TOM process

1.2

was produced by the thermophilic digester.

0.9

Table 2 shows the comparison of the system perform-

0.6

ance in TM and TOM process during the steady state

0.3 0.0

condition. Two processes were evaluated in terms of 4 view 0

20

40

60 80 Time (days)

100

points: (1) Methane production yields, (2) Organic matter

120

reduction, (3) Flocculation efficiency of mesophilic digested

9

(b)

sludge, (4) Water quality of dewatered supernatants.

pH

8

Methane production yield through the process in TOM had an average of 0.307 ^ 0.029 L/g-VS, which was 1.13

7 Thermophilic (TOM) 6

0

20

40

(c) TVFA (mg-HAc/L)


Mesophilic (TOM)

60 80 Time (days)

100

times higher than that in TM. Focused on the organic matter reduction all items in TOM were superior to those in TM

120

excepting the lipids reduction. The reduction efficiency of proteins, which is a major organic component of WAS, was

1,500 Thermophilic (TOM)

Mesophilic (TOM)

higher than any other items. The flocculation efficiency was

1,000

1.16 g-flocculants/L in TM and 0.97 g-flocculants/L respect500 0

ively. This indicates that the dewaterability of digested sludge from TOM was higher than that from TM. Dewa0

20

40

60

80

100

120

tered supernatant was obtained by 10min centrifugation (3,000 rpm) of dewatered sludge from the flocculation

Time (days) |

Figure 3

efficiency test. There is no significant difference between

Time course of operating condition of TM of TOM.

the TM and the TOM in terms of the water quality of

Table 1

|

Performance of each digesters during the steady state condition

TM

TS

g/L

TOM

Thermophilic

Mesophilic

Thermophilic

Mesophilic

30.6 ^ 2.2

26.9 ^ 2.2

30.5 ^ 0.5

22.3 ^ 0.8

VS

g/L

20.2 ^ 2.0

16.5 ^ 2.6

19.7 ^ 0.6

13.3 ^ 1.1

SS

g/L

25.1 ^ 1.2

19.9 ^ 1.6

25.0 ^ 0.4

16.9 ^ 1.0

VSS

g/L

14.9 ^ 1.0

13.6 ^ 1.4

15.2 ^ 1.4

12.1 ^ 1.1

T-CODCr

g/L

33.0 ^ 0.9

26.3 ^ 1.6

32.7 ^ 1.9

21.4 ^ 1.4

S-CODCr

g/L

13.7 ^ 2.0

9.0 ^ 0.8

13.6 ^ 0.4

7.5 ^ 0.5

NHþ 4 -N

mg/L

2410 ^ 64

2460 ^ 210

2320 ^ 51

2270 ^ 95

–

7.78 ^ 0.07

7.66 ^ 0.07

7.84 ^ 0.09

7.69 ^ 0.14

mg-HAc/L

244 ^ 115

27 ^ 54

173 ^ 51

N.D.

pH TVFA Flocculation efficiency

g/L

1.64 ^ 0.21

1.16 ^ 0.22

1.71 ^ 0.30

0.97 ^ 0.18

Gas production rate

L/L.d

0.93 ^ 0.10

0.09 ^ 0.02

0.91 ^ 0.09

0.20 ^ 0.05

N2

%

1.6 ^ 0.8

1.8 ^ 0.7

0.9 ^ 0.3

4.8 ^ 3.1

CH4

%

61.3 ^ 2.9

69.8 ^ 4.0

62.5 ^ 1.9

71.4 ^ 4.1

Gas content

CO2

%

36.9 ^ 2.4

30.5 ^ 3.1

36.7 ^ 1.2

27.7 ^ 3.1

H2S

ppm

740 ^ 141

140 ^ 57

600 ^ 30

250 ^ 71


189

Table 2

|


The comparison of the system performance

(1) Methane production yield

(2) Organic matter reduction

(3) Flocculation efficiency

TOM

CODCr

L/g-VS

0.272 ^ 0.026

0.307 ^ 0.029

VSS

%

56.5 ^ 2.2

63.6 ^ 1.7

%

58.2 ^ 5.3

62.5 ^ 3.3

Carbohydrates

%

51.8 ^ 5.4

58.7 ^ 4.4

Proteins

%

59.5 ^ 4.0

64.4 ^ 5.3

Lipids

%

53.3 ^ 6.6

52.9 ^ 3.6

CODCr

(4) Dewatered supernatant

TM

g/L

1.16 ^ 0.22

0.97 ^ 0.18

g/L

4.2 ^ 0.1

4.3 ^ 0.1

NH2 4N PO32 4 P

mg/L

2260 ^ 102

2120 ^ 87

mg/L

1150 ^ 42

1074 ^ 73

TVFA

mg-HAc/L

N.D.

N.D.

dewatered supernatant. From what has been discussed

of accumulated soluble COD in the thermophilic digester

above, we can conclude that the TOM showed higher

were removed in the mesophilic digester, but little particle

performance than TM from the three view points containing

COD (P-COD) removal was observed. On the other hand,

methane production yield, organic matter reduction and

in the TOM process (Figure 4(b)), half of accumulated

flocculation efficiency.

soluble COD and 28% of particle COD in the thermophilic digested sludge were removed in the mesophilic digester respectively. These results suggest that intermediate ozona-

CODCr mass balances among digesters in

tion improve degradability of both soluble and particle

TM and TOM process

COD of thermophilic digested sludge.

In order to understand the bioconversion process of organic matter in the digesters, COD mass balance in TM and TOM were illustrated in Figure 3(a) and (b) respectively.

Microbial community in digesters

Approximately 40% of influent COD was converted into

TOM process had higher P-COD and VSS reduction than

methane in the thermophilic digesters while in which the

TM. These results suggest that the intermediate ozonation

soluble COD accumulated to approx. 24% of influent total

promoted the hydrolysis of digested sludge from primary

COD. In the TM process shown in Figure 4(a), portion

digester, which consists of residue of thermophilic digestion

(b) 100 25.7

80

39.8 45.1

60 24.3

40

74.3

20

33.5

0

WAS P-COD

Figure 4

|

15.7 30.2

Thermophilic Mesophilic S-COD

CODCr mass balances.

Methane

Recovery from influent (%)

Recovery from influent (%)

(a)

100 80

25.7

40.8 53.8

60 40

23.7 74.3

20 0

13.1 33.5

WAS P-COD

24.3

Thermophilic Mesophilic S-COD

Methane

190



and thermophilic bacteria increased in primary digester. In this study, to understand the behaviour of thermophilic bacteria from primary digester in the mesophilic digester, the bacterial community structure was analysed with molecular approach including 16S rRNA gene (rDNA) clone analysis and real-time quantitative PCR. Results of clone analysis were shown in Figure 5. In the previous study, we reported the clone library constructed from the thermophilic anaerobic digested sludge treating WAS (Kobayashi et al. 2008), so that only approx. 20 clones from thermophilic digested sludge were analysed in this study. As shown in previous study, the predominance of

Figure 6

|

16S rRNA gene concentration of Coprothermobacter spp.

the clones affiliated with Coprothermobacter proteolyticus, which was thermophilic proteolytic bacteria, was observed in both thermophilic digesters of TM and TOM. In the mesophilic digester of TOM, no clone affiliated with Coprothermobacter was detected although in the mesophilic digester of TM, 15.3% of total clones affiliated with Coprothermobacter proteolyticus. Results of Real-time quantitative PCR analysis were shown in Figure 6. As well as results of clone analysis, TOM showed the less rDNA concentration of Coprothermobacter than TM. Judging from this, thermophilic bacteria increased in the primary digester might be killed by intermediate ozonation. For this reason, TOM would have higher P-COD and VSS reduction.

Performance comparison Tables 3 and 4 show comparison of system performance between this study and previous studies. First of all, we will focus our attention on the effect of temperature-phased digestion. TM and TOM have better flocculation efficiency and quality of dewatered supernatant than thermophilic digestion. The most likely cause for this is that the temperature phased two-stage digestion enabled the decrease of soluble COD by secondary mesophilic digester as shown in Figure 5. Furthermore, TM and TOM have the higher CODCr reduction than other processes. These results indicates that the temperature phased digestion, which was the combination of higher rate of hydrolysis by thermophilic digestion and the decrease of soluble COD by secondary mesophilic digester, resulted in the higher CODCr reduction with improvement of the flocculation efficiency and water quality of dewatered supernatant compared to the thermophilic digestion. We will now discuss the effect of intermediate ozonation. Amount of ozone dose for TOM, Ozone pretreatmentmesophilic and Ozone pretreatment-thermophilic shown in Table 4 reached 0.61, 1.16, 1.16 g/L respectively. However, as shown in Table 4, CODCr reduction in TOM was 1.13 times higher than that in TM while CODCr reduction in Ozone pretreatment-mesophilic and thermophilic digestion was 1.07 and 0.96 times higher than those in mesophilic and thermophilic digestion respectively. These results suggest that despite of less amount of ozone dose in TOM than two ozone pretreatment processes, better effect of ozonation on

Figure 5

|

Bacterial 16S rRNA gene clone libraries.

performance improvement was obtained. TOM had the

191

No.

|

Process flow diagrams

Processes

Flow diagrams of each processes

Reference

1 TM

WAS

Thermophilic digester

Mesophilic digester

This study

2 TOM

WAS


Mesophilic

WAS

Mesophilic digester

Sangsan et al. (2007)

Thermophilic

WAS


Sangsan et al. (2007)

Ozone pretreatment-Mesophilic

WAS

Ozonation

Mesophilic digester

Sangsan (2006)

Ozone pretreatment-Thermophilic

WAS

Ozonation


Sangsan (2006)

Ozonation

Mesophilic digester

This study

3

4


Table 3

5 Water Science & Technology—WST | 59.1 | 2009

6



–

processes. Furthermore, flocculation efficiency of TOM

1.16 ^ 0.01

–

1.78 ^ 0.31

48.2 ^ 3.1

30

0.256 ^ 0.037

6

highest methane yield and CODCr reduction among these Thermophilic†

Ozone pretreatment-

192

followed that of mesophilic digestion. The quality of dewatered supernatant is comparable to mesophilic digestion. These results indicate that TOM is the best process in

–

1.16 ^ 0.01

–

0.73 ^ 0.10

50.3 ^ 2.3

CONCLUSIONS The overall conclusions obtained from this study are

–

1. From the three terms comparison of methane production yield, organic matter reduction and flocculation effi-

N.D.

8.0 ^ 0.3

1.85 ^ 0.37

summarised as: 50.3 ^ 6.5

0.263 ^ 0.029

30 30

0.262 ^ 0.031

5 4

Mesophilic† Thermophilicp

Ozone pretreatment-

terms of total performance.

ciency, the system performance of TOM was higher than the TM process and the TOM process for the water –

quality of dewatered supernatant. 2. In the temperature phased two-stage digestion, the

N.D.

4.0 ^ 0.1

0.65 ^ 0.06

46.8 ^ 2.8

30

0.257 ^ 0.022

3

Mesophilicp

that of TM. There was no significant difference between

combination of higher rate of hydrolysis by thermophilic

N.D.

0.61 ^ 0.01

4.3 ^ 0.1

0.97 ^ 0.18

63.6 ^ 1.7

with improvement of the flocculation efficiency and water quality of dewatered sludge compared to the thermophilic digestion.

– N.D. mg-HAc/L

4. Results of clone analysis and real-time quantitative PCR

pre-treatment. suggest that the mesophilic digester of TOM showed less g/L

rRNA gene concentration of Coprothermobacter, which was predominant bacteria in the thermophilic digesters,

TVFA

than that of TM.

†

Sangsan et al. (2007). Sangsan (2006). ‡ At standard state. § 0.02 g per 1 g-TS of sludge.

REFERENCES

p

Flocculation efficiency

Dewatered supernatant

ozonation had better effect of ozonation on performance improvement in terms of COD reduction than ozone

Amount of ozone dose§

4.2 ^ 0.1 g/L

1.16 ^ 0.22

ozonation than ozone pre-treatment, the intermediate

CODCr

g/L

56.5 ^ 2.2 CODCr reduction

Methane production yield‡

HRT

No.

mesophilic digester resulted in higher COD reduction

3. Despite of less amount of ozone dose in the intermediate

%

30

0.307 ^ 0.029 0.273 ^ 0.026

30 Days

L/g-VS

2 1

Unit Item

Table 4

|

Comparison of the performance of six processes

TM

TOM

digestion and the decrease of soluble COD by secondary

Altschul, S. F., Madden, T. L., Schaffer, A. A., Zhang, J., Zhang, Z., Miller, W. & Lipman, D. J. 1997 Gapped BLAST and PSIBLAST: a new generation of protein database search programs. Nucleic. Acids Res. 25, 3389 –3402. Arnaiz, C., Gutierrez, J. C. & Lebrato, J. 2006 Biomass stabilization in the anaerobic digestion of wastewater sludges. Bioresour. Technol. 97, 1179 –1184.

193


Amann, R. I., Binder, B. J., Olson, R. J., Chisholm, S. W., Devereux, R. & Stahl, D. A. 1990 Combination of 16S rRNA-targeted oligonucleotide probes with flow cytometry for analyzing mixed microbial populations. Appl. Environ. Microbiol. 56, 1919 –1925. APHA 1995 Standard methods 19th Edition, American Public Health Association, Washington, DC. Daims, H., Bruhl, A., Amann, R., Schleifer, K. H. & Wagner, M. 1999 The domain-specific probe EUB338 is insufficient for the detection of all bacteria: development and evaluation of a more comprehensive probe set. Syst. Appl. Microbiol. 22, 434 –444. JSWA 1994 Examination Method for Wastewater and Sludge. Japan Sewage Works Association, Tokyo. Kim, J., Park, C., Kim, T. H., Lee, M., Kim, S., Kim, S. W. & Lee, J. 2003 Effects of various pretreatments for enhanced anaerobic digestion with waste activated sludge. J. Biosci. Bioeng. 95, 271 –275. Kobayashi, T., Li, Y. Y. & Harada, H. 2008 Analysis of microbial community structure and diversity in the thermophilic anaerobic digestion of waste activated sludge. Water Sci. Technol. 57(8), 1199 –1205. Lafitte-Trouque, S. & Forster, C. F. 2002 The use of ultrasound and gamma-irradiation as pre-treatments for the anaerobic digestion of waste activated sludge at mesophilic and thermophilic temperatures. Bioresour. Technol. 84, 113 –118. Li, Y. Y. & Noike, T. 1992 Upgrading of anaerobic digestion of waste activated sludge by thermal pretreatment. Water Sci. Technol. 26(3-4), 857 –866. Kang, N. Y., Hwang, K. & Song, W. 2000 Mechanical pretreatment of waste activated sludge for anaerobic digestion process. Water Res. 34, 2362 – 2368.


Sangan, T., Li, Y. Y., Noike, T. & Yasui, H. 2007 Comparison between mesophilic and thermophilic anaerobic digestion of waste activated sludge. J. Jpn Sewage Works Assoc. 44(12), 124– 134. Sangsan, T. 2006 Characterization of High Solids Anaerobic Digestion in treating the Waste Activated Sludge and Effect of Pre-treatment on Anaerobic Digestion. PhD Thesis, Department of Civil and Environmental Engineering, Tohoku University. Schmit, K. H. & Ellis, T. G. 2001 Comparison of temperature-phased and two-phase anaerobic co-digestion of primary sludge and municipal solid waste. Water Environ. Res. 73, 314 –321. Streeter, M. D., Dague, R. R. & Main, R. E. 1997 Evaluation of a field application, temperature-phased anaerobic digestion, residuals and solids management, Proceedings of the Water Environment Federation 71st Annual Conference and Exposition, Water Environ. Fed., Chicago, IL, 181– 190. Sung, S. & Santha, H. 2003 Performance of temperature-phased anaerobic digestion (TPAD) system treating dairy cattle wastes. Water Res. 37, 1628 – 1636. Vandenburgh, S. R. & Ellis, T. G. 2002 Effect of varying solids concentration and organic loading on the performance of temperature phased anaerobic digestion process. Water. Environ. Res. 74, 142– 148. Weemaes, M., Grootaerd, H., Simons, F. & Verstraete, W. 2000 Anaerobic digestion of ozonized biosolids. Water Res. 34, 2330 –2336. Weisburg, W. G., Barns, S. M., Pelletier, D. A. & Lane, D. J. 1991 16S ribosomal DNA amplification for phylogenetic study. J. Bacteriol. 173, 697– 703. Yonemoto, R. & Sakano, K. 1995 Application of polyamidine based flocculents on poorly dewaterable sludge. Annu. Tech. Conference Sewage Works 32, 736 –738.