Application of acidic thermal treatment for one- and

two-stage anaerobic digestion of sewage sludge

M. Takashima and Y. Tanaka

ABSTRACT

M. Takashima (corresponding author)

Department of Civil and Environmental

Engineering,

Fukui University of Technology,

3-6-1 Gakuen,

Fukui 910-8505,

Japan

E-mail: takasima@fukui-ut.ac.jp

Y. Tanaka

Maintenance E&D Co, Ltd,

3-15-27 Tarumi,

Suita, Osaka 546-0062,

Japan

E-mail: y_tanaka@maintyo.co.jp

The effectiveness of acidic thermal treatment (ATT) was examined in a 106-day continuous

experiment, when applied to one- or two-stage anaerobic digestion of sewage sludge (4.3% TS).

The ATT was performed at 1708C and pH 5 for 1 hour (sulfuric acid for lowering pH). The one-

stage process was mesophilic at 20 days hydraulic retention time (HRT), and incorporated the

ATT as pre-treatment. The two-stage process consisted of a thermophilic digester at 5 days HRT

and a mesophilic digester at 15 days HRT, and incorporated the ATT as interstage-treatment. On

average, VSS reduction was 48.7% for the one-stage control, 65.8% for the one-stage ATT, 52.7%

for the two-stage control and 67.6% for the two-stage ATT. Therefore, VSS reduction was

increased by 15–17%, when the ATT was combined in both one- and two-stage processes. In

addition, the dewaterability of digested sludge was remarkably improved, and phosphate release

was enhanced. On the other hand, total methane production did not differ significantly, and color

generation was noted in the digested sludge solutions with the ATT. In conclusion, the anaerobic

digestion with ATT can be an attractive alternative for sludge reduction, handling, and

phosphorus recovery.

Key words | acidic thermal treatment, anaerobic digestion, dewaterability, one-stage process,

sewage sludge, two-stage process

INTRODUCTION

Anaerobic digestion achieves stabilization and reduction

of sewage sludge as well as the production of methane. It

has taken up a position as a core sludge treatment

technology and an alternative energy source. Furthermore,

the growing amount of sewage sludge and scarcity of

resources encourage the reuse of sewage sludge as

resources. Many treatment plants are therefore looking

for improved sludge management systems. Phosphorus,

an essential nutrient for all forms of life, is estimated to

be exhausted within several decades. A sustainable

way to reuse phosphorus is its recovery from sewage

treatment plants.

Traditional mesophilic anaerobic digestion has

problems such as low volatile solids (VS) destruction,

foaming, low pathogen reduction and poor dewatering

characteristics. Reducing the sludge volume and quantity

can reduce costs of disposal. In order to improve

performance of the anaerobic digestion of sewage sludge,

the application of pre-treatment, like mechanical, thermal,

chemical or biological treatment methods have been

studied for the last few decades. Since the particulate

nature of sewage sludge means that hydrolysis is the

rate limiting step, the pre-treatments are intended to

reduce the size of particles and promote hydrolysis of

organic matter (Weemaes & Verstraete 1998; Mullar 2001;

Delgenes et al. 2003).

For example, successful full-scale applications have

been reported with the Cambi thermal hydrolysis process,

which combines pre-centrifugation and thermal pre-

treatment with anaerobic digestion of sewage sludge

doi: 10.2166/wst.2010.490

2647 Q IWA Publishing 2010 Water Science & Technology—WST | 62.11 | 2010

(Kepp et al. 2000). Although the capital cost as well as

the operating and maintenance costs could be high

for the thermal pre-treatment method (Mullar 2001),

the enhancement of sludge stabilization and biogas

production makes the net energy production positive,

and also stabilized sludge with less pathogens and better

dewaterability is generated (Kepp et al. 2000).

On the other hand, process configurations have been

a matter of concern to effectively incorporate an addi-

tional treatment into anaerobic digestion. In a continuous

study conducted by the author (Takashima 2008), where

anaerobic digestion was combined with moderate thermal

treatment at 1208C, some process configurations other

than pre-treatment were examined including post-treatment

(the digested sludge is thermally treated in the recycle

line and returned to the digester) of one-stage anaerobic

digestion, and interstage-treatment of two-stage anaerobic

digestion. The VS destruction efficiency of these configura-

tions resulted in the following order: interstage-treatment

. post-treatment , pre-treatment . control. Similar pro-

cess configurations have been studied by applying alkaline

thermal treatment (Gossett et al. 1982), ozonation (Goel

et al. 2003) and thermo-oxidative treatment (Cacho Rivero

et al. 2006). The results from these studies may imply that

the effectiveness of an additional treatment is enhanced,

after sewage sludge is digested once.

So far, few studies have been reported on acidic

thermal treatment (ATT) in a continuous mode. Thermal

treatment under acidic conditions appears to have

advantages over that under alkaline conditions, including

improved dewaterability and less color generation in

batch vial tests (Takashima & Tanaka 2008). In addition,

there was an indication that sulfuric acid used for ATT

enhances phosphate release from digested sludge, thereby

enabling the recovery of phosphate contained in sewage

sludge (Takashima & Tanaka 2007). In this study, the

effectiveness of ATT at 1708C was examined in a continu-

ous experiment fed with sewage sludge, when incorpor-

ated into one-stage anaerobic digestion as pre-treatment

and two-stage anaerobic digestion as interstage-treatment.

Applying sulfuric acid for the ATT, this bench-scale

study also aimed to evaluate release of phosphate from

digested sludge.

METHODS

Sewage sludge

The sewage sludge used for this study was a mixed primary

and waste activated sludge, which was taken from a

municipal combined wastewater treatment plant located

in Fukui City, Japan. This local treatment plant employs the

conventional activated sludge process. The sewage sludge

taken was further thickened gravitationally to about 4–5%

TS in the laboratory, and was frozen to maintain its

chemical nature during storage. Prior to use, the frozen

sludge was thawed, smashed with a mixer, and stored at

48C. All the sludges used were warmed up to .308C to

prevent temperature shocks to the digesters.

Experimental apparatus and operation

The process configurations examined in this study are

summarized in Table 1. The one-stage process was

mesophilic, and the ATT was incorporated as pre-treatment.

The two-stage process was thermophilic-mesophilic

(i.e. temperature-phased anaerobic digestion; TPAD).

In this two-stage process, a unique feature of the ATT

application was that it was placed between the first and

Table 1 | Process configurations examined

# of stage Name Flowp

One Control Sewage sludge ! Digester (meso, 20 d)

ATT (pre-treatment) Sewage sludge ! Pre-treatment ! Digester (meso, 20 d)

Two Control Sewage sludge ! First digester (thermo, 5 d) ! Second digester (meso, 15 d)

ATT (interstage-treatment) Sewage sludge ! First digester (thermo, 5 d) ! Interstage-treatment ! Seconddigester (meso, 15 d)

pValues in the parenthesis indicate the operating temperature regime and HRT.

2648 M. Takashima and Y. Tanaka | Acidic thermal treatment for anaerobic digestion of sewage sludge Water Science & Technology—WST | 62.11 | 2010

second stage in what we call interstage ATT. A control that

received no ATT was prepared for comparison in both one-

and two-stage processes. All the configurations were

operated in a daily draw and fill mode at the total hydraulic

retention time (HRT) of 20 days.

Erlenmeyer flasks were used as the anaerobic digesters.

The flasks had a rubber stopper with two glass ports, each

for the inlet/outlet of sludge or for the outlet of gas. The

mesophilic digesters had the effective volume of 1.6 L for

the one-stage process and 1.2 L for the second stage of the

two-stage process. They were placed in a temperature

controlled room at 34–368C, and were rotated at 100 rpm.

The thermophilic reactor for the first stage of the two-stage

process had the effective volume of 1.0 L. It was maintained

at 54–568C using a ribbon-heater and thermostat, and was

rotated at 140 rpm. The biogas produced was collected in an

aluminium-coated gas bag (CCK, GL Science, Tokyo), and

then its volume was measured with a wet gas meter

(WS-1A, Sinagawa, Tokyo, Japan).

The ATT was performed at the temperature of 1708C and

pH of about 5 for 1 hour, using conc. sulfuric acid for lowering

pH. It was conducted once a week in an autoclave (TZA100-

15K-LG, Unicontrols, Tokyo, Japan). The seed sludges were

obtained from lab-scale digesters. The mesophilic one

(8.5 g/L VSS) was from a mesophilic anaerobic digester

treating sewage sludge, and the thermopilic one

(14.8 g/L VSS) was from a thermophilic anaerobic digester

treating the organic fraction of municipal solid wastes.

Phosphorus fractionation

Phosphorus fractionation was performed according to

Mederios et al. (2005). A first aliquot of sludge sample

(about 0.2 g DS after centrifugation) was mixed with 20 mL

of 1 M HCl, and shaken for 16 hours at room temperature.

The residue was treated as organic phosphorus. A second

aliquot of sludge sample was mixed with 20 mL of 1 M

NaOH, and shaken for 16 hours at room temperature. 10 mL

of the extract was mixed with 4 mL of 3.5 M HCl, and left

for 16 hours at room temperature. The extract corresponded

to non-Ca inorganic phosphorus. The residue is further

extracted with 20 mL of 1 M HCl, and shaken for 16 hours

at room temperature. The supernatant corresponded to

inorganic Ca phosphorus. Extractions were carried out

in 40 mL polypropylene tubes, which were also used for

centrifugation. After each extraction step, the supernatant

liquid was separated from the solid phase by centrifugation

at 15,000 rpm for 10 min. Phosphorus in those extracts was

measured as phosphate by the ascorbic acid method.

Analytical procedures

Most of the analyses were performed in accordance

with Standard Methods (APHA/AWWA/WEF 1998). The

soluble solutions of sludge samples were prepared through

centrifugation (15,000 rpm and 10 min) and membrane

filtration (0.45mm). The closed reflux colorimetric method

(Standard Methods 5220D) was employed for chemical

oxygen demand (COD) using a spectrophotometer

DR/4000U (Hach, Loveland, Colorado, USA). For the

analysis of total phosphorus (T-P), samples were digested

by an alkaline persulfate digestion method (Standard

Methods 4500E-N C). Phosphate and color in the filtrate

were measured by the ascorbic acid method (Standard

Methods 4500-P E) and the ADMI tristimulus filter method

(Standard Methods 2120E) respectively, using the spectro-

photometer DR/4000U with appropriate dilution. Biogas

produced was analyzed by a gas chromatograph with a

thermal conductivity detector (GC-9A, Shimadzu, Kyoto,

Japan). The dewaterability of sludges was investigated by

the capillary suction time (CST; Standard Methods 2710G)

using a CST meter (304B, Triton Electronics Ltd., Dunmow,

Essex, England). It was ultimately expressed as sec-CST per

g/L-SS of the sludge.

RESULTS AND DISCUSSION

Performance comparison

The experiment was run for 106 days. The sewage sludge

fed had 4.3% TS and 2.9% VS on average. The amount

of sulfuric acid added to adjust the pH of ATT sludge

to about 5 was 0.24 ^ 0.04 mL/week for the one-stage

ATT and 0.34 ^ 0.05 mL/week for the two-stage ATT.

Figure 1 shows the time-course of VSS concentration,

VSS/SS ratio, dewaterability, PO4-P concentration and

gas production. When the ATT was combined, the VSS

2649 M. Takashima and Y. Tanaka | Acidic thermal treatment for anaerobic digestion of sewage sludge Water Science & Technology—WST | 62.11 | 2010

concentration reached below 10 g/L, and the VSS/SS

ratio decreased to about 0.4 in both one- and two-stage

configurations. In addition, the dewaterability of digested

sludge, measured as sec-CST per g/L-SS, was superior,

and the phosphate released from digested sludge was

increased significantly. On the other hand, there were

smaller differences in gas production among the four

configurations.

After about the 70th day, stable methane production

was observed from the first stage of the two-stage process.

The data during the 71st–106th day are thus treated as

steady-state, and are summarized in Table 2 for perform-

ance comparison. The steady-state data shows that VSS

reduction was 48.7% for the one-stage control, 65.8% for

the one-stage ATT, 52.7% for the two-stage control and

67.6% for the two-stage ATT. Therefore, VSS reduction was

increased by 15–17%, when the ATT was combined in both

one- and two-stage processes. Comparing between the

controls, the two-stage process showed a slightly higher

VSS reduction of about 4%.

Table 3 summarizes the results of statistical analysis of

VSS reduction, using the t-statistic at a significance level of

0.05. The difference between the controls and between the

configurations with ATT was found to be not statistically

significant. Thus, VSS reduction of the four configurations

can be placed in the following order: two-stage ATT , one-

stage ATT . two-stage control , one-stage control. For

this sewage sludge, it is concluded that the ATT had greater

impact on particulate matter destruction than the staging.

The improvement by TPAD in this study seems to be

smaller, since Han et al. (1997) reported that VS removal

efficiency is increased by more than 10% under similar

operating conditions.

The dewaterability of digested sludge was remarkably

improved with ATT; 17.0, 6.3, 17.0 and 6.8 sec-CST

per g/L-SS for the one-stage control, one-stage ATT,

two-stage control and two-stage ATT respectively.

This seems to be totally attributed to the effectiveness of

ATT, as the ATT sludges showed improved dewaterability

of 0.6–0.7 sec-CST per g/L-SS. Dewaterability has been

reported to improve at the pre-treatment temperature of

1608C or higher (Elbing & Dunnebeil 1999). Lower pH

of ATT has been found to be beneficial to dewaterability

(Takashima & Tanaka 2008). Improved dewaterability

can make subsequent sludge handling easier and more

economical.

On the contrary, the total methane production lay in a

narrow range of 0.609–0.642 L/d, as shown in Figure 2.

Thus, methane production did not differ significantly among

the four configurations examined, despite the significant

enhancement of particulate destruction with ATT. This is

probably because methane precursors, such as volatile fatty

acids and hydrogen, were consumed due to microbial sulfate

reduction with the added sulfate for the configurations with

05

1015202530

VSS

(g/

L)

One-stage ATTOne-stage controlTwo-stage 1st digester Two-stage 2nd digester

controlTwo-stage 2nd digester ATT

0.30.40.50.60.70.80.9

VSS

/SS

0

10

20

30

40

Dew

ater

abili

ty(s

ec/g

/L)

0

100

200

300

400

0 20 40 60 80 100

Day

PO4-

P (m

g/L

)

0.0

0.3

0.6

0.9

1.2

1.5

Gas

pro

duct

ion

(L/d

)

Figure 1 | Time course of operating results.

2650 M. Takashima and Y. Tanaka | Acidic thermal treatment for anaerobic digestion of sewage sludge Water Science & Technology—WST | 62.11 | 2010

Table 2 | Summary of performance (average ^ standard deviation for the last 6 analyses)

Two stage

One stage 1st stage 2nd stage

Control ATT Control ATT

Influent sludge Digested sludge ATT sludge Digested sludge Digested sludge Digested sludge ATT sludge Digested sludge

TS (g/L) 43.0 ^ 3.8 29.6 ^ 2.4 40.5 ^ 3.1 24.6 ^ 2.2 40.2 ^ 2.6 28.2 ^ 2.6 39.1 ^ 2.0 23.8 ^ 2.0

SS (g/L) 39.8 ^ 2.0 28.1 ^ 1.2 30.6 ^ 0.6 21.5 ^ 1.4 38.1 ^ 1.2 26.0 ^ 2.6 29.8 ^ 1.9 20.2 ^ 1.5

VS (g/L) 28.5 ^ 2.9 14.4 ^ 0.8 26.0 ^ 2.7 12.1 ^ 0.9 26.4 ^ 1.9 13.4 ^ 1.4 23.6 ^ 1.3 11.2 ^ 0.9

VSS (g/L) 26.1 ^ 1.5 13.4 ^ 0.8 17.1 ^ 0.5 8.9 ^ 0.7 21.6 ^ 0.6 12.3 ^ 0.9 15.8 ^ 1.4 8.5 ^ 0.9

COD (g/L) 46.4 ^ 1.7 22.2 ^ 0.9 44.6 ^ 3.1 19.0 ^ 1.5 44.4 ^ 2.1 21.7 ^ 2.4 42.6 ^ 2.1 18.4 ^ 1.1

S-COD (g/L) 4.4 ^ 1.3 0.6 ^ 0.4 14.5 ^ 0.2 2.8 ^ 0.4 9.6 ^ 0.8 0.9 ^ 0.4 16.2 ^ 0.5 3.8 ^ 0.4

T-P (mg/L) 606 ^ 42 – – – – – – –

PO4-P (mg/L) 18 ^ 6 18 ^ 6 75 ^ 13 163 ^ 24 48 ^ 11 12 ^ 4 124 ^ 24 207 ^ 50

Color (ADMI) 2,060 ^ 300 2,430 ^ 470 11,300 ^ 1,800 6,220 ^ 230 5,000 ^ 1,640 1,850 ^ 500 15,500 ^ 3,100 6,370 ^ 280

Dewaterability (sec/g/L) 8.0 ^ 2.5 17.0 ^ 0.9 0.6 ^ 0.1 6.3 ^ 2.4 18.6 ^ 3.9 17.0 ^ 1.2 0.7 ^ 0.4 6.8 ^ 1.5

pH 5.8 ^ 0.1 7.2 ^ 0.1 4.9 ^ 0.1 7.3 ^ 0.1 5.5 ^ 0.3 7.3 ^ 0.1 4.9 ^ 1.0 7.3 ^ 0.1

Gas production (L/d) – 0.99 ^ 0.03 – 1.08 ^ 0.08 0.08 ^ 0.03 0.95 ^ 0.09 – 1.02 ^ 0.11

Gas CH4 (%) – 63.0 ^ 0.8 – 60.4 ^ 1.0 35.4 ^ 4.6 64.9 ^ 1.4 – 62.0 ^ 1.3

Gas CO2 (%) – 35.3 ^ 0.8 – 37.6 ^ 1.0 57.7 ^ 1.8 32.9 ^ 1.5 – 35.4 ^ 1.1

Gas H2S (ppm) – 13 ^ 2 – 155 ^ 45 489 ^ 296 10 ^ 3 – 295 ^ 123

VS reduction (%) – 49.5 ^ 2.9 – 57.5 ^ 3.1 – 53.2 ^ 4.8 – 59.6 ^ 3.7

VSS reduction (%) – 48.7 ^ 3.1 – 65.8 ^ 2.8 – 52.7 ^ 3.4 – 67.6 ^ 3.3

COD reduction (%) – 52.1 ^ 2.0 – 59.0 ^ 3.2 – 53.3 ^ 5.2 – 60.3 ^ 2.4

COD recovery (%) – 94.7 ^ 1.4 – 89.8 ^ 5.1 – 94.2 ^ 4.6 – 89.1 ^ 5.4

CH4 conversion (%) – 46.8 ^ 1.1 – 48.9 ^ 4.3 – 48.5 ^ 3.3 – 49.4 ^ 4.1

PO4-P release (%) – 3.0 ^ 0.9 – 26.9 ^ 4.0 – 2.0 ^ 0.6 – 34.2 ^ 8.2

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ATT. Also, color generation of approximately 3 times more

was observed in the digested sludge solutions with ATT, as

shown in Table 2. This phenomenon is caused by the

Maillard reaction, and has been pointed out as a disadvan-

tage of thermal treatment (Mullar 2001; Delgenes et al.

2003). The products of the Maillard reaction are known to

be inhibitory in methane fermentation as well. Lowering the

pH during thermal treatment was beneficial to the mitiga-

tion of color generation (Takashima & Tanaka 2008).

Recently, Dwyer et al. (2008) reported that decreasing the

thermal pre-treatment temperature from 165 to 1408C can

reduce the color intensity to one third without significant

impact on the anaerobic degradability of sewage sludge.

The effect of thermal treatment is dependent on the type

of sludge being processed (Weemaes & Verstraete 1998).

Thermal pre-treatment has more positive effect on waste

activated sludge rather than primary sludge, because

thermal treatment destroys the cell walls and makes the

soluble organics accessible for anaerobic degradation

(Haug et al. 1978; Mullar 2001). In the continuously operated

anaerobic digestion of waste activated sludge, solids

reduction was almost doubled with thermal pre-treatment

(Pinnekamp 1989; Li & Noike 1992; Bougrier et al. 2006).

In contrast, there was a case in which thermal pre-treatment

does not improve degradability of primary sludge during

anaerobic digestion (Haug et al. 1978). According to Kepp

et al. (2000), the typical degree of hydrolysis is approxi-

mately 20–25% for primary sludge, 35–60% for waste

activated sludge, and 20–45% for mixed primary and waste

activated sludge after thermal pre-treatment (135–1808C

for about 30 min), and their first full-scale installation

showed increased solids reduction of mixed sludge by

about 20% after anaerobic digestion. In our previous

studies, the mixed sludge collected from this local treatment

plant has shown relatively low anaerobic degradability with

a typical VSS reduction of about 40% (Takashima &

Tanaka 2007; Takashima 2008). The particulate destruction

achieved with ATT in this study appears to be limited by the

characteristics of the sewage sludge used.

Phosphate release

As shown in Figure 1 and Table 2, the PO4-P concentration

in digested sludge was 10–20 mg/L for the controls,

whereas it reached to 163 mg/L for the one-stage ATT and

207 mg/L for the two-stage ATT. The ratio of PO4-P

released against influent T-P is calculated to be 26.9% for

the one-stage ATT and 34.2% for the two-stage ATT. This

phosphate-releasing phenomenon can be explained by the

replacement of phosphate in iron phosphate with sulfide,

and subsequent release of phosphate into the solution

(Oshita et al. 2005; Oshita 2009), as described by the

following equation.

2FePO4 þ 3S22 ! 2FeS þ S þ 2PO324 or

2FeS2 þ 2PO324

ð1Þ

In this experiment, most of the sulfide came from the

sulfate-reducing reaction with the added sulfate. From the

above equation, 1 mol of phosphate requires 1.5 mol of sulfide

Table 3 | Statistical comparison of VSS reduction

One-stage control One-stage ATT Two-stage control Two-stage ATT

t-statistic compared to one stage-ATT – 10.04 2.12 10.25

t-stastics compared to two stage-control – – 7.29 0.98

t-statistic compared to two stage-ATT – – – 7.71

Note: The underlined values indicate that there is a statistically significant difference at the significance level of 0.05, when the t-statistic is larger than 2.228.

0.609 0.635 0.630 0.642

0.0

0.2

0.4

0.6

0.8

1.0

Tot

al C

H4

prod

uctio

n (L

/d)

Two-stageATT

Two-stagecontrol

One-stageATT

One-stagecontrol

Figure 2 | Total methane production (average ^ standard deviation for the last

6 analyses).

2652 M. Takashima and Y. Tanaka | Acidic thermal treatment for anaerobic digestion of sewage sludge Water Science & Technology—WST | 62.11 | 2010

to be released. On a basis of the amount of sulfate added,

170 mg/L and 240 mg/L are calculated to be released by this

method for the one-stage ATT and two-stage ATT respectively.

Therefore, phosphate was efficiently released by the sulfuric

acid addition in this study. When hydrochloric acid was used

for ATT, significant phosphate release was not observed

(Takashima & Tanaka 2007). As demonstrated here, the use

of sulfuric acid for ATT can enhance phosphate release from

digested sludge, while sacrificing methane production because

of the concomitant sulfate reduction.

Figure 3 shows the results of phosphorus fractionation

for digested sludge. The particulate inorganic non-Ca

fraction is associated with aluminium and iron phosphorus

mostly, and so the particulate inorganic Ca fraction with

calcium phosphorus (Mederios et al. 2005). There is a clear

tendency that the fractions of both particulate inorganic

non-Ca and particulate organic phosphorus were

decreased, when the ATT was combined. It is postulated

that the PO4 increase was due not only to the PO4 releasing

reaction by sulfide, but also to enhanced destruction of

particulate organic matter. Figure 3 also indicates that the

particulate inorganic Ca fraction was increased with ATT.

The final PO4 concentration is postulated to be controlled

by the precipitation with calcium, as reported by Wild et al.

(1997) and Komatsu et al. (2008).

CONCLUSIONS

The effectiveness of ATT (1708C, pH 5, 1 hour and

sulfuric acid for lowering pH) was examined in a 106-day

continuous experiment, when applied to one- or two-stage

anaerobic digestion of sewage sludge (4.3% TS).

(1) VSS reduction obtained at steady-state was 48.7% for

the one-stage control, 65.8% for the one-stage ATT,

52.7% for the two-stage control and 67.6% for the two-

stage ATT. Statistical analysis of VSS reduction

showed that the configurations with ATT are superior

to the controls without ATT. Dewaterability was also

improved with ATT, less than half of the controls level,

measured as sec-CST per g/L-SS.

(2) Methane production was similar among the four

configurations, primarily because of the COD loss by

microbial sulfate reduction in the configurations with

ATT. Also, color was produced with ATT by approxi-

mately three times the controls.

(3) PO4-P release was enhanced with ATT, from

12–18 mg/L of the controls to 163–207 mg/L. The

results of phosphorus fractionation indicated that

the ATT decreased the fractions of particulate inor-

ganic non-Ca and particulate organic phosphorus.

In conclusion, the anaerobic digestion with ATT can be

an attractive alternative for sludge reduction and handling.

Furthermore, phosphate release is enhanced with ATT,

which enables greater phosphorus recovery. Further studies

are necessary to confirm the effectiveness of ATT at lower

treatment temperatures.

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2654 M. Takashima and Y. Tanaka | Acidic thermal treatment for anaerobic digestion of sewage sludge Water Science & Technology—WST | 62.11 | 2010