Journal of Environmental Chemical Engineering 2 (2014) 773–779

Contents lists available at ScienceDirect

Journal of Environmental Chemical Engineering

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / j e c e

Acidic thermal post-treatment for enhancing anaerobic digestion of sewage sludge

M. Takashima

a , * , Y. Tanaka

b

a Department of Architecture and Environmental Engineering, Fukui University of Technology, 3-6-1 Gakuen, Fukui 910-8505, Japan b Technology Development Division, Hokukon Ltd., 66-20-2 Imaichi, Fukui 910-8152, Japan

a r t i c l e i n f o

Article history:

Received 22 August 2013

Accepted 27 February 2014

Keywords:

Acidic thermal treatment

Anaerobic digestion

Phosphate release

Post-treatment

Sewage sludge

a b s t r a c t

Acidic thermal post-treatment (ATPT) was examined for treatment conditions in a batch study, and was

demonstrated to enhance anaerobic digestion of sewage sludge in a continuous study. In the batch study

where anaerobically digested sludge was the substrate in view of a post-treatment mode, higher ATPT

temperatures between 25 and 180 ◦C improved volatile suspended solids (VSS) destruction and methane

production, but generated color significantly at 180 ◦C. Lower ATPT pH between 2 and 6 enhanced sludge

dewaterability (as capillary suction time), and slightly suppressed color generation. In the continuous study,

two single-stage anaerobic digestion processes were operated at 35 ◦C and 20 days hydraulic retention

time. For one of the processes, ATPT at 170 ◦C and pH 5–6 for 1 h was incorporated in the recycle line.

Approximately 75% of VSS destruction was achieved in the ATPT process, which was 2–2.5 times more than

that in the control process, 30–37%. The ATPT process also showed 14–21% more methane production and

22–23% better dewaterability, but formed around three times more color, compared to the control process.

Sulfuric acid as the acidifying agent caused more release of phosphate from the digested sludge, which

enables efficient phosphorus recovery. c © 2014 Elsevier Ltd. All rights reserved.

Introduction

Anaerobic digestion is suited for treating organic waste with high

water contents, such as industrial wastewater, sewage sludge, and

municipal and agricultural solid waste. It enables the decomposition

of organic matter as well as the production of fuel gas, methane. Fur-

thermore, many other valuable materials can be produced or recov-

ered through anaerobic digestion, including hydrogen, volatile acids

and phosphorus. In light of these facts, anaerobic digestion has been

recognized as one of the core technologies for recycling energy and

materials from waste, and contributing to sustainable developments.

In the case of sewage sludge, however, the organic fraction con-

verted into biogas by the current anaerobic digestion technology is

not high enough, while the production of sewage sludge is explo-

sively increasing worldwide. Waste activated sludge has been par-

ticularly paid attention to, because of its more recalcitrant nature

to anaerobic biodegradation than primary sludge. A classic method

to overcome the difficulty is the application of mechanical, thermal,

chemical, biological or other pre-treatment methods to anaerobic di-

gestion. Several literature reviews on this topic have been published

in recent years, indicating the world ’ s technological, environmental

Abbreviations: ATPT, acidic thermal post-treatment; COD, chemical oxygen de-

mand; CST, capillary suction time; HRT, hydraulic retention time; PO 4 -P, phosphate

phosphorus; SS, suspended solids; T-P, total phosphorus; TS, total solids; VS, volatile

solids; VSS, volatile suspended solids.

* Corresponding author.

E-mail address: takasima@fukui-ut.ac.jp (M. Takashima).

2213-3437/ $ - see front matter c © 2014 Elsevier Ltd. All rights reserved.

http://dx.doi.org/10.1016/j.jece.2014.02.018

and economical requirements for sludge minimization and biogas

maximization [ 1 –5 ].

So far, the major pre-treatment methods reported to be employed

in full-scale installations have been thermal hydrolysis, enzyme hy-

drolysis and ultrasound, and in particular, the Cambi and BioTHELYS

processes that combine thermal hydrolysis with mechanical dehydra-

tion of influent sludge, have been most widely applied in the world [ 6 ].

Thermal pre-treatment of sewage sludge has been demonstrated to

enhance not only sludge dewaterability but also anaerobic digestibil-

ity [ 1 –5 ]. Most of the previous studies report optimal temperatures of

160–180 ◦C and treatment durations of 30–60 min, although the latter

appears to have a secondary effect [ 4 ]. Drawbacks are associated with

the generation of hardly degradable chemical oxygen demand (COD),

including colored compounds, and possibly increased fine suspended

solids (SS) [ 2 , 4 ].

A recent study conducted by the authors implicated that the ther-

mal treatment at acidic pH is preferable, particularly from the point

of view of the enhanced dewaterability of digested sludge and the

mitigation of color generation [ 7 ]. Alkaline conditions are generally

more compatible with anaerobic digestion due to the increase of al-

kalinity than acidic conditions [ 3 ], and therefore have been favored

as pre-treatment in the previous investigations. However, Devlin et

al. [ 8 ] recently reported that acid pre-treatment can speed up the hy-

drolysis of waste activated sludge, increase methane yield and reduce

cationic polymer addition for dewatering.

On the other hand, there is another aspect to be taken into con-

sideration, when applying any treatment method, that is, process

774 M. Takashima, Y. Tanaka / Journal of Environmental Chemical Engineering 2 (2014) 773–779

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onfiguration. One of the authors has examined some process config-

rations for mesophilic anaerobic digestion of sewage sludge, when

ncorporated with thermal treatment of 120 ◦C and for 1 h [ 9 ]. The

ost- and interstage-treatment configurations were superior in terms

f organic matter destruction and methane production (here, the

ost-treatment configuration refers to a single-stage process with

he thermal treatment of recycled digested sludge, and the interstage-

reatment configuration a two-stage process with the thermal treat-

ent placed between the first and second stage). The author con-

luded that the thermal treatment as pre-treatment tends to act on

egradable parts of sewage sludge, and that it is more effective on

naerobic digestibility to apply the thermal treatment after sewage

ludge is digested once. This is further supported by the recent two

apers on the interstage-treatment, one with mixed sludge [ 10 ] and

nother with waste activated sludge [ 11 ].

Considering the energy-conservative nature of heat, another ad-

antage of the thermal post-treatment configuration is that the heat

pent for it can be reused for the digester heating. In addition, the

igester operation will be more economical when thickening the di-

ested sludge recycled, because the volume reduction can save heat-

ng energy and thus costs for the thermal post-treatment.

This research focused on acidic thermal treatment and its appli-

ation to anaerobic digestion of sewage sludge as the post-treatment

onfiguration. Firstly, treatment conditions of acidic thermal post-

reatment (ATPT) were examined in a batch anaerobic study, as in-

estigation of the thermal treatment at acidic pH has not been done

n detail. Secondly, the effectiveness of ATPT was demonstrated in a

ontinuous experiment. Centrifugation was applied for solids separa-

ion to add acidic thermal treatment to digested sludge in the recycle

ine.

ethods

atch study

The temperature and pH conditions of ATPT were examined by a

atch test employing 120 mL serum bottles. Anaerobically digested

ludge was used as the substrate, in particular, to observe the effects

n the anaerobically refractory part of sewage sludge. The digested

ludge was taken from a municipal anaerobic digester in Fukui City,

apan, and also served as the seed sludge.

The acidic thermal treatment was performed with a stainless steel

utoclave, which had a total volume of 2.5 L and a maximum pressure

f 1.5 MPa (TZA100-15K-LG, Unicontrols, Tokyo, Japan). The temper-

ture and initial pH of acidic thermal treatment were varied at three

evels each; 25, 100 and 180 ◦C for temperature, and 2, 4 and 6 for

H with hydrochloric acid. The treatment duration of 1 h and mixing

peed of 300 rpm were fixed. The headspace was air, meaning there

as no substitution with other gases.

25 mL each of the substrate and seed sludge was adjusted to the

H of 7.0–7.3, and placed in the serum bottles. Then, the bottles were

lled with a mixture of 80% N 2 and 20% CO 2 , sealed with rubber caps

nd aluminum stoppers, and incubated in a water bath (MMS-1, Eyela,

okyo, Japan) at 35 ◦C and 40 strokes / min. Duplicated bottles were

sed for each condition. For 20 days, the volume of methane gas pro-

uced was measured in appropriate time intervals by means of gas

nalysis with a gas chromatograph and subsequent water replace-

ent in a 100 mL graduated cylinder [ 12 ]. The cylinder contained

n acidic salt solution to minimize the absorption of carbon dioxide.

lso, the solids concentrations, capillary suction time (CST) and color

ere analyzed before and after the batch test.

ontinuous study

As drawn in Fig. 1 , two single-stage anaerobic digestion processes

ere prepared: the control process, and the process combined with

ATPT in the recycling line, called ATPT process hereafter. The hydraulic

retention time (HRT) was equally set at 20 days based on the flow

rate of influent sewage sludge. In the ATPT process, the solids in the

digested sludge were separated by centrifugation (2000 rpm for 10

min; LCO6-SP, Tomy Seiko, Tokyo, Japan), stored at 4 ◦C, and received

ATPT once a week. Then, the thermally treated sludge was recycled

to the digester at the recycle ratio of 30%. This reduced the actual

HRT of the digester to 15.4 days, because the biomass in the recycled

digested sludge is killed and converted into substrate by the ATPT.

The employed flow rates and sampling volumes are also summarized

in Fig. 1 .

Erlenmeyer flasks with the effective volume of 2.0 L 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. These

digesters were placed in a constant temperature room maintained at

about 35 ◦C, and were operated in a daily draw and fill mode at the

rotating speed of 100 rpm. The biogas produced was collected in an

aluminum-coated gas bag (CCK, GL Science, Tokyo, Japan), and then

quantified with a wet gas meter (WS-1A, Sinagawa, Tokyo, Japan).

The autoclave mentioned above was also used for the ATPT in this

continuous study. Based on the results of the batch study, the ATPT

conditions of 170 ◦C and pH 5–6 were employed, as will be described

later. The duration and mixing speed were identical to those for the

batch study. Hydrochloric acid was used to decrease the pH in Run 1,

and sulfuric acid in Run 2. During the thermal treatment, charcoal-like

materials were often produced and adhered to the inner surface of the

autoclave. Larger particles of them were removed from the recycled

sludge to minimize adverse effects on the digester operation.

The substrate was gravitationally thickened sewage sludge taken

from a municipal combined wastewater treatment plant located in

Fukui City, Japan. It had TS of about 2.5%, and stored in a refrigerator

at 4 ◦C. For feeding, the substrate was warmed up to > 30 ◦C to prevent

temperature shocks to the digesters. The seed sludge was obtained

from lab-scale digesters, which were operated in a similar fashion.

The characteristics of the seed sludge were TS of 35.0 g / L, VS of 17.9

g / L and the VS / TS ratio of 0.51. TS, VS, suspended solids (SS), volatile

suspended solids (VSS), total and soluble COD, total phosphorus (T-

P), phosphate phosphorus (PO 4 -P), CST, color, pH and the volume and

constituents of biogas were analyzed once a week.

Analytical procedures

Most of the analyses were performed in accordance with Stan-

dard Methods [ 13 ]. The soluble fraction of sludge samples was pre-

pared through centrifugation (15,000 rpm and 10 min; CF15R, Hi-

tachi, Tokyo, Japan) and membrane filtration (0.45 μm cellulose-

ester; A045A047A, Advantec, Tokyo, Japan). The closed reflux col-

orimetric method (Standard Methods 5220D) was employed for COD

using a spectrophotometer DR / 4000U (Hach, Colorado, USA). The de-

waterability of sludge was investigated by the CST (Standard Methods

2710G) using a CST meter (304B, Triton Electronics, Essex, England).

PO 4 -P and color in the sludge 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 spec-

trophotometer DR / 4000U with appropriate dilution. For the analysis

of T-P, samples were digested by the alkaline persulfate digestion

method (Standard Methods 4500E-N C). Biogas produced was ana-

lyzed by a gas chromatograph with a flame ionization detector (col-

umn: Parapak Q of 80–100 mesh and 2 m length, carrier gas: Ar at 40

mL / min, oven temp.: 40 ◦C, injection temp.: 120 ◦C, detector temp.:

120 ◦C; GC-9A, Shimadzu, Kyoto, Japan).

M. Takashima, Y. Tanaka / Journal of Environmental Chemical Engineering 2 (2014) 773–779 775

Fig. 1. A schematic of the experimental processes employed.

Table 1

Characteristics of the digested sludge used in the batch study.

TS (g / L) 26.1

VS (g / L) 15.4

SS (g / L) 25.2

VSS (g / L) 15.0

COD (g / L) 26.6

Sol. COD (g / L) 0.27

Color (ADMI) 300

CST (s) 203

pH 7.5

Fig. 2. Major results of the batch study.

Results and discussion

Batch study

The characteristics of the digested sludge used in this batch study

is summarized in Table 1 . Fig. 2 shows solids destruction as VSS%, cu-

mulative methane production, CST and color at the end of the batch

experiment, using the average values of two serum bottles. The con-

trol serum bottles fed the substrate without any treatment were used

for comparison. The cumulative methane production was increased

from 0.11 gCOD-CH 4 / gCOD-substrate for the control to 0.12–0.18,

0.18–0.25 and 0.30–0.32 gCOD-CH 4 / gCOD-substrate for the acidic

thermal treatment temperature of 25, 100 and 180 ◦C, respectively.

The VSS destruction was also increased to 3.3–6.7, 8.0–11.3 and 24.7–

26.0% for the ascending acidic thermal treatment temperature, re-

spectively, against 3.3% for the control. Thus, methane production

and particulate organic destruction of anaerobically digested sludge

were improved, as the treatment temperature was raised. Lowering

treatment pH seems to have an influence on those parameters at

lower treatment temperatures.

Fig. 2 also shows that the dewaterability of sludge as CST was

improved, as the initial pH of the acidic thermal treatment was low-

ered at any temperature. Therefore, lowering treatment pH has a

significant influence on dewaterability. In addition, the CST value

was decreased at the thermal treatment temperature above 100 ◦C.

The reduction in particulate solids concentration by the acidic ther-

mal treatment is partly attributable to this improvement. Based on

Neyens and Baeyens [ 14 ] and Bougrier et al. [ 15 ], at least 150 ◦C ap-

pears to be required for an apparent improvement of dewaterability.

Although the pH influence of thermal treatment on dewaterability

has been rarely found in the literature, Chen et al. [ 16 ] and Neyens et

al. [ 17 ] confirmed that decreasing pH of thermal hydrolysis improves

the dewaterability of activated sludge and thickened sludge, respec-

tively. Strong acidic treatment removes extracellular polymers from

activated sludge, which results in the improvement of dewaterability

[ 16 ].

The color of the sludge filtrates stayed constant at the treatment

temperature up to 100 ◦C, but was increased sharply at 180 ◦C. It

was also observed that lower pH tended to suppress color slightly,

as reported before [ 7 ]. The color generation by the thermal treat-

ment above 100 ◦C has been known to be explained by the so-called

Maillard reaction, in which reduced sugars and amino acids form the

colored compounds, melanoidins [ 18 ]. In the food system, alkaline

pH is reported to accelerate the Maillard reaction, as the open chain

form of the sugar and the unprotonated form of the amino group, the

primal reactive forms, are predominated [ 19 ]. Therefore, the results

of this batch study are consistent with the literature.

776 M. Takashima, Y. Tanaka / Journal of Environmental Chemical Engineering 2 (2014) 773–779

Fig. 3. Time course of major parameters in the continuous study.

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As reviewed above, the reported temperature optima are in the

ange of 160–180 ◦C, and the durations of 30–60 min seem to be ad-

quate. On the basis of this batch test and those previous studies, the

reatment temperature of 170 ◦C for 1 h was employed in the follow-

ng continuous experiment. For the treatment pH, the weak acidic

ange of 5–6 was selected to expect minor improvement in dewat-

rability and color generation as well as to avoid severe corrosion to

eating devices.

ontinuous study

The continuous experiment was run for 63 days for each run. The

ime course of major parameters is shown in Fig. 3 . Also, the experi-

ental results and performance for the whole period of each run are

ummarized in Tables 2 and 3 , respectively. The solids concentration of digested sludge, expressed as VSS, is

hown in Fig. 3 . After the start-up period of about 20 days in Run 1, the igester VSS became quite stable in both processes, and so was the gas

roduction; 8.4–10.2 g / L and 10.6–13.1 g / L of digester VSS, and 0.56–.78 L / day and 0.64–0.91 L / day of gas production for the control and

TPT process, respectively. Since the ATPT process was operated as

quasi-closed one regarding particulate solids, the performance like SS destruction and COD recovery is calculated as follows by applying

the material balance shown in Table 3 (unit: g / day):

Destruction ( % ) =

Influent − ( Accumulation + Supernatant + Sample + Charcoal )

Influent × 100

(1)

Recovery ( % ) =

Accumulation + Supernatant + Sample + Charcoa l

Influent × 100

(2)

Particulate charcoal-like materials, expressed as charcoal in Table 3

and the above equations, were found to be produced during the ther-

mal treatment. Larger particulate materials were removed from the

recycled ATPT sludge due to their possible affects on the operation and

analysis, although they were included in the calculation as charcoal.

Using the average values for each run, the control process showed

30–42%, 30–37% and 31–46% of VS, VSS and COD destruction respec-

tively, while the destruction (%) in the ATPT process was almost dou-

bled, that is, 66–67%, 75% and 67–71%, respectively. The low solids

destruction % obtained in the control process can be accounted for

by an inherent nature of the influent sludge originating from com-

bined sewer systems. Previously, one of the authors verified exper-

imentally that post-treatment is advantageous over pre-treatment,

when thermal treatment is combined with anaerobic digestion [ 9 ].

As pointed out therein, the post-treatment configuration is capable

of transforming part of the non-digestible sewage sludge and syn-

thesized anaerobic microorganisms into the substrate. Enhancement

by post-treatment is also reported for ozone treatment [ 20 , 21 ] and

thermo-oxidative treatment using hydrogen peroxide [ 22 ].

Methane production averaged for each run was in the range of

0.92–1.15 gCOD / day and 1.11–1.31 gCOD / day for the control and

ATPT process, respectively, indicating the improvement by 14–21%

with the ATPT. Therefore, there appears to be no inhibition at all to

the anaerobic microorganisms by the ATPT under the conditions of

20 days HRT and 30% recycle ratio for the digester and of 170 ◦C, pH

5–6 and 1 h for the ATPT. But the methane recovered was much lower

than expected from the solids destruction obtained. This is because,

during the thermal treatment, part of organic matters were converted

to non-biodegradable compounds, such as soluble colored ones, and

were lost by self-burning at the temperatures higher than 150 ◦C [ 23 ],

as shown by the lower COD recovery in Table 3 . The averaged COD

loss during the ATPT is calculated to be 22.4% and 13.9% of the influent

COD in Run 1 and Run 2, respectively. It is reasonable to think that

biodegradable organics were contained in this COD fraction lost.

Additionally, in the batch study, the thermal post-treatment at 180 ◦C showed the cumulative methane production of 0.30–0.32 gCOD-

CH 4 / gCOD-substrate and VSS destruction of 24.7–26.0%. In the con-

tinuous study, compared to the control process, the ATPT process was

limited to the increased methane production of 0.060–0.064 gCOD-

CH 4 / gCOD-substrate (as digested sludge), whereas the VSS destruc-

tion was improved by 37–45%. In the ATPT process, as the digested

sludge receives the thermal post-treatment more than once with the

progress of operation, non-biodegradable solids are gradually accu-

mulated in the digester. Accordingly, it is presumed that methane

production will be decreased and solids destruction enhanced with

time. Off course, the difference in the post-treatment temperature

and in the characteristics of the digested sludge post-treated can ex-

plain the different results between the batch and continuous study to

some extent.

The elevated temperature imposed on the ATPT could bring about

high energy consumption and thus high costs. However, ATPT can

serve as heat supply for an anaerobic digester to maintain the

mesophilic temperature regime. As a simple example calculation, if

we assume the influent temperature of 20 ◦C, the ATPT temperature

of 170 ◦C, the recycle ratio of 30% and heat efficiency of 70% around

the digester from a literature [ 24 ], the digester temperature will be

(1 × 20 ◦C + 0.3 × 170 ◦C) / 1.3 × 0.7 = 38 ◦C. So, the heat require-

ment for the digester can be satisfied with the heat recycled from the

M. Takashima, Y. Tanaka / Journal of Environmental Chemical Engineering 2 (2014) 773–779 777

Table 2

Summary of the continuous study. The values are expressed as the average and standard deviation for each run ( = 9 analyses).

Influent

Control

digested sludge

ATPT

digested sludge Supernatant ATPT sludge

Run 1

TS (g / L) 24.5 ± 0.6 25.0 ± 3.4 32.8 ± 3.6 3.1 ± 0.2 70.0 ± 8.4

VS (g / L) 16.7 ± 0.4 11.7 ± 1.0 14.8 ± 0.7 1.8 ± 0.2 34.5 ± 3.7

SS (g / L) 21.8 ± 1.4 23.3 ± 3.7 28.7 ± 2.8 1.4 ± 0.3 51.6 ± 5.7

VSS (g / L) 14.6 ± 0.8 10.3 ± 1.1 12.5 ± 0.5 0.9 ± 0.2 21.2 ± 1.8

COD (g / L) 27.1 ± 1.7 18.8 ± 2.7 24.9 ± 1.6 2.2 ± 0.4 57.6 ± 4.3

Sol. COD (g / L) 2.9 ± 0.9 0.3 ± 0.1 0.7 ± 0.2 − −T-P or PO 4 -P (mg / L)* 321 ± 3 6 ± 2 9 ± 4 − −CST / SS (s / g / L) 6.1 ± 4.3 7.7 ± 1.8 6.0 ± 1.4 − 0.5 ± 0.1

Color (ADMI) 880 ± 250 420 ± 50 1090 ± 320 − 9950 ± 1580

pH 6.02 ± 0.05 7.08 ± 0.08 7.06 ± 0.13 − 5.81 ± 0.25

Gas production (L / day) − 0.587 ± 0.083 0.684 ± 0.116 − −CH 4 content (%) − 58.7 ± 5.5 60.8 ± 4.0 − −CO 2 content (%) − 30.1 ± 1.6 32.7 ± 1.5 − −

Run 2

TS (g / L) 26.0 ± 3.0 19.9 ± 1.1 32.4 ± 2.0 3.0 ± 0.3 83.1 ± 11.7

VS (g / L) 17.7 ± 2.0 10.3 ± 0.6 15.0 ± 0.7 1.9 ± 0.2 38.1 ± 4.5

SS (g / L) 21.7 ± 2.6 17.2 ± 1.3 27.3 ± 2.6 1.1 ± 0.2 60.3 ± 10.2

VSS (g / L) 14.5 ± 1.2 9.1 ± 1.0 12.0 ± 0.8 0.8 ± 0.3 23.2 ± 3.7

COD (g / L) 28.3 ± 3.3 15.2 ± 3.6 22.4 ± 2.1 2.2 ± 0.4 56.3 ± 5.2

Sol. COD (g / L) 3.8 ± 0.3 0.3 ± 0.03 0.9 ± 0.1 − −T-P or PO 4 –P (mg / L) * 351 ± 42 6 ± 1 58 ± 20 − −CST / SS (sec / g / L) 3.4 ± 2.1 10.6 ± 1.6 8.2 ± 1.0 − 0.5 ± 0.2

Color (ADMI) 1190 ± 250 380 ± 30 1390 ± 40 − 7920 ± 1040

pH 6.14 ± 0.14 7.14 ± 0.09 7.13 ± 0.07 − 5.53 ± 0.17

Gas production (L / d) − 0.688 ± 0.062 0.824 ± 0.064 − −CH 4 content (%) − 62.8 ± 0.7 59.7 ± 1.4 − −CO 2 content (%) − 30.9 ± 0.6 34.4 ± 0.6 − −

∗T-P for influent, and PO 4 -P for others.

Table 3

Summary of performance in the continuous study. The values are those calculated for the whole period of each run.

Influent Control ATPT

Accumulation Effluent Methane Destruction Recovery Accumulation Supernatant

Samples

and

charcoal * Methane Destruction Recovery

(g / day) (g / day) (g / day) (g / day) (%) (%) (g / day) (g / day) (g / day) (g / day) (%) (%)

Run 1

VS 1.67 −0.23 1.17 − 30.3 − −0.10 0.15 0.43 − 65.8 −VSS 1.46 −0.14 1.03 − 29.7 − −0.06 0.07 0.30 − 74.6 −COD 2.71 −0.26 1.88 0.92 30.5 93.7 −0.07 0.19 0.71 1.11 66.8 71.5

Run 2

VS 1.77 −0.01 1.03 − 42.0 − −0.03 0.16 0.44 − 66.5 −VSS 1.45 −0.03 0.91 − 37.3 − −0.01 0.07 0.30 − 74.6 −COD 2.83 −0.02 1.52 1.15 46.4 93.8 0.03 0.18 0.68 1.31 70.7 76.6

∗Particulate charcoal-like materials removed from the recycled ATPT sludge.

ATPT, and there is little need for surplus heating. This is an advantage

of thermal treatment from an energetic point of view, as mentioned

before. Additionally, the improved solids destruction and methane

production can add economic benefits to the ATPT process.

The amount of strong acid added for the ATPT resulted in 3.3

mmol / day for hydrochloric acid in Run 1 and 1.8 mmol / day for sul-

furic acid in Run 2. Although methane precursors can be consumed by

microbial sulfate reduction in Run 2, the effect of the type of strong

acid added on the ATPT performance is unclear. This is probably

because temperature has the largest influence. An additional factor

could be the varied characteristics of the sewage sludges used, i.e., the

sewage sludges used in Run 2 appear to be more degradable and thus

produce more methane. As observed in the control process, methane

conversion was 34% in Run 1 against 41% in Run 2 based on COD.

Dewatering is an important unit operation in the whole sludge

handling process. Here, the CST divided by SS, namely CST / SS, is used

for comparison to correct the difference in solids concentration. De-

waterability of digested sludge was better by 22–23% on average in

the ATPT process than the control. As seen in Table 2 , the recycled

ATPT sludge had superior dewaterability, 12–21 times the digested

sludges. In the ATPT process, therefore, the dewaterability of digested

sludge was improved as a consequence of mixing the ATPT sludge.

This observation indicates the possible use of gravity sedimentation

for solids separation instead of centrifugation.

778 M. Takashima, Y. Tanaka / Journal of Environmental Chemical Engineering 2 (2014) 773–779

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According to Table 2 , the color intensity in the filtered solution of

igested sludge was around three times higher in the ATPT process

ompared to the control. This is attributable to brown-colored, non-

egradable organic compounds that are generated by the Maillard re-

ction, as written before. The formation of these soluble compounds

as been recognized as a disadvantage of thermal treatment. Lower-

ng the thermal treatment temperature, and lowering the treatment

H as well, is an alternative to reduce the color generation. For in-

tance, Dwyer et al. [ 25 ] reported that decreasing thermal hydrolysis

emperature of activated sludge from 165 ◦C to 140 ◦C was able to

educe effluent color without decreasing anaerobic degradability.

In Run 2, an interesting phenomenon was noted with respect to

hosphorus. Phosphorus is a material predicted to be exhausted glob-

lly within several decades at the soonest [ 26 ]. As shown in Fig. 3 and

able 2 , the phosphate concentration in the filtrate of digested sludge

n Run 1 was quite low, < 10 mg / L on average, and was comparable

etween the control and ATPT process. In Run 2 where sulfuric acid

as used for the ATPT, the phosphate concentration was gradually

ncreased, and finally reached 88 mg / L in the ATPT process, while it

tayed low in the control process. Considering that sulfate added is

onverted to sulfide by microbial sulfate reduction, Takashima and

anaka [ 10 ] postulated the replacement of phosphate in iron phos-

hate with sulfide and subsequent release of phosphate into the so-

ution, as described by the following equation.

FePO 4 + 3 S 2 − → 2 FeS + S + 2 P O 4 3 − or 2 FeS 2 + 2 P O 4

3 − (3)

From this equation, 1 mol of phosphate requires 1.5 mol of sul-

de to be released. In Run 2, since sulfuric acid of 1.8 mmol / day or

3.8 mmol / L was added to the digester, phosphate phosphorus of

.2 mmol / L or 285 mg / L can be released in theory for the ATPT pro-

ess. Takashima and Tanaka [ 10 ] further indicated that the phosphate

ncrease was partly caused by the enhanced destruction of particu-

ate organic matter. This phosphate-releasing phenomenon can offer

n opportunity for recovering phosphorus efficiently from the super-

atant of the ATPT process.

onclusions

ATPT was focused on as measures to enhance anaerobic digestion

f sewage sludge. In the batch study, higher ATPT temperatures be-

ween 25 and 180 ◦C were more effective for solids destruction and

ethane formation, while lower ATPT pH between 2 and 6 improved

ewaterability. The color in the sludge solution was significantly gen-

rated at 180 ◦C, but was slightly suppressed at lower pH. In the

ontinuous study, the mesophilic anaerobic process combined with

he ATPT at 170 ◦C and pH 5–6 for 1 h showed superior VSS de-

truction, methane production and dewaterability, compared to the

ontrol process. It is noteworthy that the VSS destruction of the ATPT

rocess reached approximately 75%. A drawback was seen in color

eneration. Sulfuric acid used for the ATPT caused more release of

hosphate from the digested sludge. As a future study, closer exam-

nation on the optimum ATPT conditions may be suggested to max-

mize methane production and to be economically more attractive.

lso, continuous efforts are required to overcome the deteriorated

haracteristics of digested sludge filtrates.

cknowledgement

A part of this study was supported by a grant-in-aid to support

rivate universities building up their foundations of strategic research

rom Ministry of Education, Culture, Sport, Science and Technology

MEXT), Japan.

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