〈一般研究課題〉 水熱処理による生活廃水（汚泥）の減容化に関する研究
助 成 研 究 者

名古屋大学

小林 信介

水熱処理による生活廃水（汚泥）の減容化に関する研究小林 信介（名古屋大学）

Effect of hydrothermal Reaction conditions on volume
reduction and filtration performance of sewage sludge
Nobusuke Kobayashi(Nagoya University)

Abstract：
Hydrothermal treatment was carried out with sewage sludge, and the effect of hydrothermal
conditions, such as treatment temperature and treatment period, on dewatering performance byfiltration was evaluated. The properties of hydrothermally treated slurry was also analyzed bymeasuring particle sized distribution, pH and ζ potential to understand physical filtration
mechanism. With increasing of treatment temperature, filtration capability of the treated slurrysignificantly increased. Treatment period also affected on filtration performance, and the filtration
performance increased with longer treatment period. Even though particle size decreased withhigher treatment temperature by a hydrolysis reaction, the filtration capability increased withincreasing hydrothermal treatment temperature or period. The agglomeration effect on thefiltration performance might be small, but the characteristic change between particles and filtrateby the decomposition of organic material in sewage sludge, such as polysaccharide and polymeric
protein, might influence the filtration efficiency significantly.
INTRODUCTION
Sludge treatment is very important for the comfort and sanitary city life, however, energy
consumption in a conventional sludge treatment system is huge, and lack of disposal site of sludge－ 153 －

is also one of the biggest problems in sewage treatment process in Japan. In response to thesesituations, lots of alternative sludge treatment system has been developed of recent years to reduceenergy consumption and sludge volume (Ishikawa 1999, Imai et al., 2008, Nakazawa et al., 2009,
Yoshida et al., 2001, Hishio and Nakashimada, 2007)
Hydrothermal treatment is applied for one of volume reduction techniques of high moisture
content sludge, because organic materials, which are mostly metabolite and dead shape of
microscopic organism (Voet and Voet, 1995), are decomposed by a hydrolysis reaction. Therefore,
many research works regarding hydrothermal treatment of sewage sludge has been conducted andclarified the relationship between hydrothermal conditions and hydrolysis behavior of various kindof sludge in many countries (Murakami et al., 1999, Okuda et al., 2001, Ohmura et al., 2004, catallo
and Comeaux, 2008). However, significant increase of water treatment load, if the hydrothermallytreated liquid returns to aerating tank, was pointed out as the disadvantage of high temperaturehydrothermal treatment of sludge. This problem can be solved by decreasing treatment temperature,
but volume reduction ratio decreases with decreasing operation temperature. Further research iscontinuing to find out optimum hydrothermal treatment condition to increase volume reduction andreduce water treatment load.
Another advantage of hydrothermal treatment is improvement of dewatering performance oftreated slurry. Easy and efficient dewatering technique in sludge treatment system is important not
only for the reduction of energy consumption, but also fuel saving in sludge combustion process.
Many filtration experiments have been conducted with no-treated sludge and clarified thatmetabolite of microscopic organism, which was polysaccharide and high molecular protein, wasmajor cause of deterioration of the filtration performance (Judd, 2004 ). One the other hand,
applying hydrothermal treatment on sludge increasing filtration performance was known empirically(Miyakawa et all, 2003, Kobayashi et al., 2009a). Improvement of filtration performance by applyinghydrothermal treatment would estimate
However, detail analysis on the relationship between hydrothermal treatment conditions andfiltration performance has not been reported so far. Therefore, in this study, hydrothermal
treatment was conducted on sewage sludge and effect of hydrothermal treatment conditions on
dewatering performance by a filtration was clarified. In this experiment, filtration experiment was
carried out with hydrothermally treated sludge and effect of treatment temperature and treatment
period on filtration performance was evaluated. To understand filtration mechanism, treated slurry
characteristics, such as particle size distribution, pH and ζ potential of the treated slurry, wasalso analyzed.
EXPERIMENTAL
Hydrothermal experiment of sewage sludge was conducted in microwave irradiation type batch
reactor (Milestone General, Ethos One). Schematic diagram of the experiment unit is shown in Fig.
1 . Sludge samples in a reactor are heated to arbitrary temperature by microwave irradiation.

－ 154 －

Microwave frequency is 2.455 GHz, and maximum power iscoating thermocouple. Heating rate of the sample is also adjusted
1000 W. The reactor is made by Teflon, which volume isby controlling microwave output power. To create homogeneous
100 ml and maximum pressure capacity of the reactor is 10
reaction condition, a stirring bar was put in the reactor with theMPa. Temperature of the sample in the reactor is able tosample. The stirring bar usually starts moving around 373 K withmeasure with a Teflon coating thermocouple. Heating rate
100 rpm, when the sludge sample turns soft.
of the sample is also adjusted by controlling microwave
Dewatering digestive sludge was used as theoutput power. To create homogeneous reaction condition, aexperimental sample, which moisture content was around 82 %.
stirring bar was put in the reactor with the sample. The
40 g-wet of sludge, which filled about half of the reactor, wasstirring bar usually starts moving around 373 K with 100
Fig. 1 Schematic
diagram ofhydrothermal
loaded in the Teflon reactor, and temperature of the sample wasFig. 1 Schematic
diagramofrpm, when the sludge sample turns soft.
treatment equipment
raised by irradiating microwave. Treatment temperature was sethydrothermal treatment equipment
Dewatering digestive sludge was used as the453, 473, 493 and 513 K, and heating rate was controlled 100
experimental sample, which moisture content was around
K/min in all experiments. The pressure in the reactor would be summation of water vapor pressure at the
82 %. 40 g-wet of sludge, which filled about half of the reactor, was loaded in the Teflon reactor,
treatment temperature and generated decomposed gas, but in this experiment, pressure of decomposed gas
and temperature of the sample was raised by irradiating microwave. Treatment temperature was
expected vanishingly small from the result of pre-experiment. After the temperature of the sludge sample
set 453, 473, 493 and 513 K, and heating rate was controlled 100 K/min in all experiments. The
reached the target temperature, the sample kept arbitrary treatment period. Treatment period set 5, 10, 20 and 30
pressure in the reactor would be summation of water vapor pressure at the treatment temperaturemin. After the hydrothermal treatment, the reactor was immersed into an ice water immediately to prevent theand generated decomposed gas, but in this experiment, pressure of decomposed gas expected
secondary reaction. Then the reaction slurry was taken out from the reactor, when the treated slurry became
vanishingly small from the result of pre-experiment. After the temperature of the sludge sample
normal temperature.
reached the target temperature, the sample kept arbitrary treatment period. Treatment period set
5 , 10 ,Obtained
20 and 30
min.separated
After thetreatment,
thea reactor
was immersed
intoAnyan iceslurrythe hydrothermal
solid residue andliquid byconstant-pressure
filtration.
othertreatment
was not conducted
beforethethesecondary
filtration, butnote thatThen
the slurry
concentrationdifferent
depends
water immediately
to prevent
reaction.
the reaction
slurrywas
wastaken out
fromonthethehydrothermal
condition.
Schematic
diagramofnormalfiltration
system is shown in Fig. 2. Cylinder type strainer isreactor, when
the treatedslurrybecametemperature.
madeObtainedby stainlesssteel,separated
which diameteris 2.56
cm andand
the liquid
length by
is 20
Filtration area isfiltration.
around 5.14Any
cm3.
slurrythe solid
residuea cm.
constant-pressureThreedifferent
size wasof glass
filters paperbefore
were usedin the filtration,and thatthe opening
size concentrationof the filter waswas
1, 7
other
treatment
not conducted
the filtration,
but notethe slurryand
20 ȝm. Wellagitatingtreatedslurry was filled
into thestrainer andthe strainer
was sealed.
Afterissealingdifferent
depends
on thehydrothermal
condition.
Schematic
diagramof filtration
systemshownthestrainer,
filtration
conducted
withbynitrogengas steel,at normalof sealedin Fig.dead-end
2 . Cylinder
typewasstraineris madestainlesswhichtemperature.
diameter isThe
2 . pressure
56 cm andthe3strainerwas
keptis 0.1
MPa.5.14
The cmweightof filtrate
againstwaspapermeasured
lengthduring
is 20 thecm. filtration
Filtrationarea
around. Threedifferent
sizefiltrationof glasstime
filters
werebymicrobalance.
volumewasopening
calculated
withthe filtratedensityfiltration
Cake andused in the Filtrate
filtration,and thesizeof thefilter was
1, 7after
and the
20 μm.
Wellexperiment.
agitating treatedfiltrate
was
dried
after
thethefiltration
experimentcalculatesolubilization
ratio.
Dryingwas 353 K
slurrywasfilledinto
strainerand the tostrainerwassealed. Aftersealingthetemperaturestrainer, dead-end
anddrying period
was 48 h. Experimental
conditions
were summarized
in Fig. 3.
ratio
filtration
was conductedwith nitrogenflow
gas andat normal
temperature.
The pressureof Solubilization
sealed strainer
wascalculated
as following
duringthe filtration
wasequation.
kept 0.1 MPa. The weight of filtrate against filtration time was measured
by microbalance. Filtrate volume was calculated with the filtrate density after the filtration
experiment. Cake and filtrate was dried after the filtration experiment to calculate solubilization
ratio. Drying temperature was 353 K and drying period was 48 h. Experimental flow and conditions
To evaluate the characteristics of hydrothermally treated slurry at different treatment condition, particle size
were summarized in Fig. 3. Solubilization ratio was calculated as following equation.
distribution, pH and ȗ potential of the slurry was also analyzed. Analysis was applied the treated slurry, which（）

was not separated solid and liquid. The particle size distribution
Deposit [g] was measured by a laser-diffraction particle（

Residue [g]－ CE [g]×）[g]diameteranalyzer (LA-920ˈHORIBA).
For the measurement of
pH,×100
pH meter (HM-20P, Toa DKK) was used.
Solubilization
ratio[%]= 1－
Filtrate
Dried sludge [g]－ 155 －

To evaluate the characteristics of hydrothermally treated slurry at different treatment condition,
particle size distribution, pH and ζ potential of the slurry was also analyzed. Analysis was applied
the treated slurry, which was not separated solid and liquid. The particle size distribution wasmeasured by a laser-diffraction particle diameter analyzer (LA-920，HORIBA). For theȝ potential was measured with § potential analyzer (Model-502, Nihon Rufuto Co. Ltd.). Because obtained
measurement of pH, pH meter (HM-20P, Toa DKK) was used. ζ potential was measured with ζslurry concentration was too high to measure with this ȗ potential analyzer, treated slurry was diluted with 3
potential analyzer (Model-502, Nihon Rufuto Co. Ltd.). Because obtained slurry concentration wasȝ potential
measuredwithpotential
analyzer (Model-502,
Nihon
RufutoBecausetimes
distilledwas
waterof slurry
total§weight.
Biochemical
oxygen demand(BOD)
andCo.
totalLtd.).
organic
carbonobtained(TOC)
too high to measure with this ζ potential analyzer, treated slurry was diluted with 3 times
slurrywas too highto measurewaterwith treatment
this ȗ potential
treatedtreatment.
slurry wasBOD
diluted3ofthe concentration
filtrate also measured
to understandload inanalyzer,hydrothermal
andwithTOC
distilled water of slurry total weight. Biochemical oxygen demand (BOD) and total organic carbon
times distilledwater of slurrytotal
weight.
Biochemical
oxygen demand
and totalorganic carbon (TOC)
analysis
was conductedwith JIS
method,which
is JIS K 0120-21
and JIS (BOD)
K 0102-22,
respectively.
(TOC) of the filtrate also measured to understand water treatment load in hydrothermal treatment.
of the filtrate also measured to understand water treatment load in hydrothermal treatment. BOD and TOC
BOD and TOC analysis was conducted with JIS method, which is JIS K 0120 - 21 and JIS K
analysis was conducted with JIS method, which is JIS K 0120-21 and JIS K 0102-22, respectively.
0102-22, respectively.
61pressurize
1 filter
6 2 sludge slurry

2
1

3

3 filter paper

pressurize

4 beaker
1 filter
5 balance
2 sludge slurry
6 regulator
3 filter paper

234

5 4 beaker

5 balance
6 regulator

4
5

Fig.2 Schematic diagram of
constant-pressure filtration
Fig.2Schematic
Schematic diagram
Fig.2
diagramofofconstant-Fig.3 Experimental flow and experimental conditions

pressure filtration

constant-pressure filtration

Fig.3
flowflow
and experimental
conditions
Fig.3Experimental
Experimental
and experimental
conditions

Filtrate volume
Filtrate[cm3volume][cm3]

RESULTS AND DISCUSSIONRESULTS AND DISCUSSION
25
Fig.4 shows the effect of hydrothermal treatment
Fig. AND
4 shows
the effect of hydrothermal treatment
RESULTSDISCUSSION
temperature on filtration performance. Treatment period was 520
temperature on filtration performance. Treatment period
Treatment Temp. [K]25shows
the waseffectof Dependhydrothermal
minutes Fig.4
and filter
opening
1 Ǎm.
on the treatment453
473
was 5 minutes and filter opening was 1 μm. Depend on the
15
493
513
temperature
filtration
Treatment
periodwas4735
temperature, on
filtration
speedperformance.
was significantly
changed.
Under
20
Treatment Temp. [K]treatment temperature, filtration speed was significantly
10
minutesand filter
opening
Ǎm. Depend
the treatment
K treatment,
filtration
was wasnot 1progressat all onin thisfiltration
453
473
changed. Under 473 K treatment, filtration was not
15
493
513
temperature,
speedperformance
was significantly
changed.
Under with473
condition,
butfiltration
the filtration
drastically
improved
5
progress at all in this filtration condition, but the filtration
10
K
filtration
wasslurry.
not progressat allin (Kobayashi
this filtration
hightreatment,
temperature
treatment
In previous
paper
etperformance drastically improved with high temperature
0
0
200
400
600
800
condition,
but authors
the filtration
performance
drastically
with
al.,
2011b),
conducted
a filtration
with improvedhydrothermal
5treatment slurry. In previous paper (Kobayashi et al.,Filitration time [s]high temperature
treatmentslurry.
paper (Kobayashi
ettreatmentslurry andclarifiedthat Intheprevious
slurry treatedover 473 K,
02011 b), authors conducted a filtration with hydrothermal
Fig.4
Effect of hydrothermal treatment
0
200
400
600
800
al.,
2011b),
authors
conducted
a filtration
with hydrothermal
Fig.4
Effecthydrothermal
which
is around
decomposition
temperatureof organic
material, wastemperature
onoffiltration
speed,
treatment slurry and clarified that the slurry treated over
Filitrationtime[s]hydrothermal treatment periods: 5 mintreatmentslurry filtrationand clarified
that the However,slurry treated473473K,
treatment temperature
on filtrationable
to improve
performance.
slurryover
treatedconstant.
473 K, which is around decomposition temperature of
Fig.4 Effectof hydrothermal
which
is around
decomposition
temperatureof organic
material,
was
speed,hydrothermal
treatment
K in thisexperiment
was not filtrated
easily.
Since thedifference
organic material, was able to improve filtration performance. However, slurry treated 473 K in thistreatment
temperature
on filtration
able tosludge
improvefiltration
slurry treated473
periods:
5 min constant.
thosesamplesareperformance.
initial slurryHowever,concentrationand slurryexperiment was not filtrated easily. Since the difference those sludge samples are initial slurry
speed, hydrothermal treatment
K in this experiment
was hydrolysis
not filtratedreaction
easily. ofSince
the difference
concentrationinfluences
organic
material,
concentration and slurry concentration influences hydrolysis reaction of organic material, filtration
periods:reaction.
5 min constant.
thosesludgesamples might
are initialslurry concentration
slurry of hydrolysis
filtration
performance
be effectedby the degreeandof progress
With increasing
concentrationinfluencesof organic
treatment
temperature
fromhydrolysis
493 to 513reaction
K, filtration
speed ofmaterial,
513 K increased 6 times more than that of the speed－ 156 －

filtration
might bethat
effectedby resistancethe degreeofoftreatedprogressof athydrolysis
reaction.
With
increasingof 493 K.performanceThis result indicatedthe cakeslurry513 K wasdrastically
improvedbytreatment temperature from 493 to 513 K, filtration speed of 513 K increased 6 times more than that of the speedperformance might be effected by the degree of progress of hydrolysis reaction. With increasing
treatment temperature from 493 to 513 K, filtration speed of 513 K increased 6 times more thanthat of the speed of 493 K. This result indicated that the cake resistance of treated slurry at 513
K was drastically improved by hydrothermal treatment. But, even though the slurry of 513 K,
filtration speed was getting decrease with growing the cake resistance.
Fig. 5 shows filtration volume against filtration time with hydrothermally treated slurry at
different treatment periods. Treatment temperature of the slurries was 473 K, and filter size was
1 μm. Depend on the treatment period, filtration performance significantly changed. With 5
minutes treatment, filtration was not progressed, but with increasing treatment periods, filtration
performance was improved drastically. Filtration volume at the same filtration time increased
linearly against treatment period, which presumes that cake resistance of the slurries decreased
linearly with increasing treatment period at temperature of 473 K. Because 473 K is around thedecomposition temperature of saccharine and microscopic organism tissue (Voet and Voet, 1995),
reaction period influences on decomposition ratio of organic materials, and the decomposition ratio
may effect on the filtration performance. In consequence, filtration performance of 30 minutes
treatment closed to the filtration performance of 493 K treatment of 5 minutes. Without increasing
treatment temperature, filtration performance was able to improve by prolonging treatment time,
but further experiment is necessary to clarify the optimum treatment period at different treatment
temperature.
Fig. 6 also shows the filtration volume against filtration time, but different size of filters wasused in this case. Treatment period of this slurry was 5 minutes, and filter size was 1, 7 and 20
μm. Depend on the filter size, difference was not observed with the treated slurries of 513 and453 K. Since filter resistance is deferent between the filters, cake resistance dominates thefiltration performance with these slurries. However, the filtration mechanism expected to bedifferent between the slurries of 513 and 453 K, because filtration performance was totallydifferent. It is assumed that 513 K is much higher temperature than decomposition temperature oforganic substance, resistance of the cake itself influences the filtration performance. On thecontrast, because 453 K is much lower temperature than decomposition temperature, organic
substance, such as polysaccharide and high molecular protein, is still remains in filtrate and thosematerials hinders filtration performance. On the other hand, the slurry treated at 473 and 493 K
showed the difference in filtration performance depends on filter opening size. Difference in 473 K
was negligibly small, but the slurry treated at 493 K indicated significant difference between thefilter sizes. Increasing the opening size of filter, filtration speed increased in case of 493 K,
especially filter of 1 and 7μm. However, the filtration speed was not increased linearly with filter
opening size or area, and filtration performance gradually deteriorated with bigger filter opening.
This filtration performance may be also explained by the decomposition of organic material.
Decomposition material dissolves in the liquid at 493 K, but the organic material in liquid is notdecomposed as small as in the case of 513 K condition and some organic substance is not－ 157 －

decomposed at 493 K in 5 minutes treatment. Since filtration resistance is also determined bydecomposition ratio and size of decomposed organic materials, the difference is showed up in thefiltration performance, because the slurry at 493 K may be having broad molecular size distributionof organic substance. Further research is necessary to understand the relationship between
molecular size and filtration performance.

25

25
Treatment perods [min]
5

10

20

20

30

Filtrate volume [cm3]Filtrate volume [cm3]

20
15
10
5
0

0

200

400

600

800

15
10
5
0

0

200

Filitration time [s]Fig.5 Effect of hydrothermal treatment periods on filtration
Fig.5
Effect of hydrothermal treatment periods on
speed, hydrothermal treatment temperature: 473 K

filtration speed, hydrothermal treatment
temperature: 473 K

400

600

800

Filitration time [s]Filter size [ȝm]
1

7

20

453
473
493
513

453
473
493
513

453
473
493
513

Fig.6 Effect of filter size on filtration speed,

Fig.6
Effect of filter
size periods:
on filtrationhydrothermal
treatment
5 min speed,hydrothermal treatment periods: 5 min
One of the biggest factors, which influence the filtration performance and cake resistance, is
One of the biggest factors, which influence the filtration performance and cake resistance, is particle
particle size in treated slurry. To understand the hydrolysis effect on the particle size andsize in treated slurry. To understand the hydrolysis effect on the particle size and filtration performance, particle
filtration performance, particle size distribution of the slurry was measured. Fig. 7 shows thesize distribution of the slurry was measured. Fig. 7 shows the particle size distribution of treated slurry. Thisparticle size distribution of treated slurry. This figure shows the particle size distribution
figure shows the particle size distribution measured with both ultrasonic wave irradiation and non-ultrasonicmeasured with both ultrasonic wave irradiation and non-ultrasonic wave irradiation. 3 minutes
wave irradiation. 3 minutes ultrasonic wave was applied the sample. Agglomeration of the particles in initial
ultrasonic wave was applied the sample. Agglomeration of the particles in initial sludge sample wassludge sample was observed as expected, because of the addition of agglomerating agent in dewatering process
observed as expected, because of the addition of agglomerating agent in dewatering process ofof digestive sludge. Agglomerated particle easily break into a small particle by the ultrasonic wave irradiation,
digestive sludge. Agglomerated particle easily break into a small particle by the ultrasonic wave
and a number of lager particle decreased. By applying hydrothermal treatment, particle size decreased with
irradiation, and a number of lager particle decreased. By applying hydrothermal treatment, particle
increasing reaction temperature, especially, a number of fine particles were increased. But significant difference
size decreased with increasing reaction temperature, especially, a number of fine particles were
was not observed between the slurries, even though the treatment temperature was different. The particle size
increased. But significant difference was not observed between the slurries, even though the
distribution was not also changed by even the ultrasonic wave irradiation, except 513 K treatment. This means
treatment temperature was different. The particle size distribution was not also changed by eventhat the agglomeration effect of the particle in the slurries is very small, or coagulation of the particle is very
the ultrasonic wave irradiation, except 513 K treatment. This means that the agglomeration effectstrong not enough to break by ultrasonic wave irradiation. But, considering the 50 % particle size, the particleof the particle in the slurries is very small, or coagulation of the particle is very strong not enoughsize is around 120 Ǎm, it is very difficult to consider that the particle is not coagulated at all. Less difference into break by ultrasonic wave irradiation. But, considering the 50 % particle size, the particle size isparticle size distribution was observed between the slurries, however, the filtration performance showedaround 120 μm, it is very difficult to consider that the particle is not coagulated at all. Less
significant difference between the slurries. Usually, filtration performance deteriorated with smaller particle, butdifference in particle size distribution was observed between the slurries, however, the filtration
this result indicated that the particle size of the slurries was less influence on the filtration performance.

Therefore, improvement of the filtration performance was not effect of particle agglomeration, but
158 －be more importance to the filtration performance of thedecomposition ratio and size of organic material－would

treated slurry.

performance showed significant difference between the slurries. Usually, filtration performance
deteriorated with smaller particle, but this result indicated that the particle size of the slurries
was less influence on the filtration performance. Therefore, improvement of the filtration
performance was not effect of particle agglomeration, but decomposition ratio and size of organic
material would be more importance to the filtration performance of the treated slurry.
Particle size distribution of the slurries treated in different time period was shown in Fig. 8.
Treatment temperature was 473 K. Increasing the treatment periods, particle sized decreased
gradually. Especially, a number of small particles increased. Since particle size decreased withtreatment period, hydrolysis reaction would progress with the periods. But, significant difference

progress
But,depend
significant
differencein particle
sizeEffectwas notofobserved
dependinwould
particle
size withwas thenot periods.
observedon thetreatment
periods.
ultrasonicwaveon thetreatment periods.
Effectof ultrasonicirradiation
was also
particle size
irradiation
was also
negligible.
Even wave
thoughthe particle
size negligible.
decreased Even
with though
longer thetreatment
decreased
longerperformancetreatment period,
the filtration
performance
drastically,
seen asalso
in fig.5.
period,the withfiltration
increased
drastically,
as seen increased
as in fig.5.
Reactionas period
Reaction periodalso influences
decompositionof andorganic
materials, anddecomposition
ratio maysignificantly
influences
decompositionof organic
materials,
decomposition
ratio
may significantly
infectoninfecton the filtration
performance.
thefiltration
performance.

Comulative distribution [%]100
Ultrasonic wave
irradiation
㽢䖣

75

Treatment Temp.
Initial
453
473
493
513

50

25

0

Initial
453
473
493
513

1

10

100

1000

Particle size [ȝm]Fig.7 Effect of hydrothermal treatment temperature on particle size distribution, hydrothermal treatment periods: 5 min,
Filter size:
1μm
Fig.7 Effect of hydrothermal treatment temperature
on particle
size distribution, hydrothermal treatment periods:

5 min, Filter size: 1Ǎm

Comulative distribution [%]100
Ultrasonic wave
irradiation

75

㽢
50

25

0

䕿

Period [min]

1

5

5

10

10

20

20

30

30

10

100

1000

Particle size [ȝm]Fig.8
Effectof hydrothermal
periodsperiods
on particle
distribution,hydrothermal
treatment temperature:
K, Filter size:
Fig.8
Effectof hydrothermal
on size
particle
size distribution,hydrothermal
treatment 473
temperature:
473 K,
1μm.

Filter size: 1Ǎm.

To evaluate treated slurry characteristics, pH and ȝ potential of the slurry was measured. Tables 1 and2 show the pH value and absolute ȝ potential－of159
slurries.
Table 1 shows those values in different treatment－
temperature. The pH value decreased with increasing the treatment temperature, but over 493 K, the slurry pH
value increased on the other hand. Decrease of the pH value at lower treatment temperature would be by theTo evaluate treated slurry characteristics, pH and ζ potential of the slurry was measured.
Tables 1 and 2 show the pH value and absolute ζ potential of slurries. Table 1 shows those values
in different treatment temperature. The pH value decreased with increasing the treatment
temperature, but over 493 K, the slurry pH value increased on the other hand. Decrease of the pH
value at lower treatment temperature would be by the hydrolysis reaction of organic material.
Since hydrolysis reaction creates an acid substance, such as formic acid and acetic acid, this kindof acid production decreases the pH value. On the other hand, increase of the pH value may be theproduction of ammonia. Because, nitrogen content in sludge is high, ammonia production influences
the pH value during the hydrothermal treatment. Since the decomposition of high molecular weightorganic
progressedat thetemperature
treatment of
temperatureof 493K, filtration
was bymaterialmaterial
progressedat the treatment
493 K, filtration
performance
was performance
drastically improved
drastically
improved
by theresistance.
inhabitance of filtration resistance.
the inhabitanceof filtration
ζpotential of all slurries had negative electrification. The value of absolute ζ potential alsoȝpotential of all slurries had negative electrification. The value of absolute ȝ potential also changed
changed with the treatment temperature, and it decreased with increasing treatment temperature,with the treatment temperature, and it decreased with increasing treatment temperature, but ȝ potential alsobut ζ potential also turned increased over 473 K. Usually pH value and ζ potential has a strong
turned increased over 473 K. Usually pH value and ȝ potential has a strong relationship, but the pH value
relationship, but the pH value decreased from 473 to 493 K against ζ potential slightly increased
decreased from 473 to 493 K against ȝ potential slightly increased from 473 to 493 K. This result indicated thatfrom 473 to 493 K. This result indicated that the surface characteristics of the sludge changed by
the surface characteristics of the sludge changed by the hydrolysis reaction. Looking into the value of absolutethe hydrolysis reaction. Looking into the value of absolute ζ potential, the value obtained fromȝ potential, the value obtained from non-treated slurry was also very low. One of the reasons of the low ȝ
non-treated slurry was also very low. One of the reasons of the low ζ potential value ispotential value is agglomeration agent. Since the treated slurries are diluted before ȝ potential analysis, theagglomeration agent. Since the treated slurries are diluted before ζ potential analysis, the dilution
dilution also influenced the value of ȝ potential. However, in general, ȝ potential value of this range
also influenced the value of ζ potential. However, in general, ζ potential value of this range
encourages the particle agglomeration. But, significant agglomeration was not observed by the particle size
encourages the particle agglomeration. But, significant agglomeration was not observed by the
analysis. This result suggested that the treated particles were coagulated strongly and the particles couldn’t break
particle size analysis. This result suggested that the treated particles were coagulated strongly andinto small particles by ultrasonic wave irradiation.
the particles couldn’t break into small particles by ultrasonic wave irradiation.

Table 11 pHζ potentialof hydrothermallytreated slurry
Table
pHvaluevalueandandabsoluteabsolute
ȝ potentialof hydrothermallytreated slurry

Treatment
temperature

K

293

453

473

493

513

pH

-

-

7.14

6.94

6.25

6.49

Absolute ȗ potential

mV

1.94

1.56

1.18

1.23

1.63

* Treatment period: 5 min.
Table 2 shows the pH value and absolute ȝ potential of the slurry treated in different treatment period.
Table
shows
value andabsolute
potentialdecreasedof the slurry
treated intreatment
differentperiod.
treatment
The
pH 2value
wasthenotpHchangeddrastically,butζgraduallywith increasing
Minimum
period.
pH notvalue
was notchangeddrastically,
but decrease
gradually
decreasedwith increasing
value ofThe
pH wasobservedin thistreatment
period. Gradualof thepH value would
be the hydrolysis
treatment
period.
Minimumvalueof pH would
was not
observedin thisperformance.
treatment ȝpotential,
period. Gradualreaction, andthe gradualprogressof reaction
improvethe filtration
keeping low
decrease
pH value
would
thechangedhydrolysis
reaction,
gradualreaction wasvalue in ofthistheoperation
range,
wasbenotdependon theandpH thevalue.
Since progress
filtration of
performance
would
improvethe filtration
ζpotential,
valuegraduallyin this operation
range,
changedwith treatment
period,performance.
the particle-liquidinteractionkeeping
might below
changedwith reaction
period bywas
not changed
dependon thematerial,
pH value.
filtration performance
was notchangedwith treatment
decomposing
solubleorganic
butSince
particle-particle
interaction might
be changed
drastically bychanging treatment periods.
－ 160 －

Table 2 pH value and absolute ȝ potential of hydrothermally treated slurry

The pH value was not changed drastically, but gradually decreased with increasing treatment period. Minimum
value of pH was not observed in this treatment period. Gradual decrease of the pH value would be the hydrolysis
reaction, and the gradual progress of reaction would improve the filtration performance. ȝpotential, keeping low
value in this operation range, was not changed depend on the pH value. Since filtration performance waschangedthe
withparticle-liquid
treatment period,interaction
the particle-liquidinteraction
mightgradually
be changedwith
gradually
with reaction
period,might
be changed
reaction
periodperiod
by bydecomposing soluble
soluble organic
butbutparticle-particle
interaction
might notbe changed
drastically bydecomposing
organicmaterial,material,
particle-particle
interaction
might
not be changed
changing treatment
periods.
drasticallyby changing
treatment periods.

Table 22 pHζ potentialof hydrothermallytreated slurry
Table
pHvaluevalueandandabsoluteabsolute
ȝ potentialof hydrothermallytreated slurry

Treatment
periods

min

5

10

20

30

pH

-

6.94

7.04

6.54

6.52

Absolute  mVȗ potential

1.44

1.35

1.34

1.38

* Treatment temperature: 473 K.
The solubilizaiont ratio, deposit ratio, BOD and TOC of slurries or filtrates was summarized inTable 3 . Treatment period of this experiment was 5 minutes and 1 μm filter was used.
Solubilization ratio increased with increasing treatment temperature. Only 7 % of the initial solid
substance decomposed to liquid soluble substance at the treatment temperature of 453 K, butaround 15 % at 513 K. Comparing previous results (Kobayashi et al, 2011b), which conducted low
slurry concentration, around 35 % of solubilization ratio was achieved. In this process, since slurry
concentration was higher than previous one, proton concentration during hydrolysis reaction was
less, and limitation of solubility of decomposition material would effect on the solubilization ratio.
Since the solubiliztion ratio increased with the reaction temperature, deposit ratio in the filtratealso increased with increasing treatment temperature. Only 0 .02% of initial substance passed
though the 1 μm filter at the temperature of 453 K, but the around 0.22 % of initial substancepassed though at 473 K, because decomposition of organic material in sludge starts around 473 K.
Significant difference was also observed between 473 and 493 K with the progress of
decomposition. At 513 K treatment, deposit ratio increased more. Since decomposition starts
around 473 K, BOD and TOC value at 473 K increased significantly. However BOD and TOC value
was not changed between 473 and 513 K, even though the deposit ratio increase. It indicates that
not only organic material, but also the other inorganic material was passing through the filter as
liquid soluble substance, and the soluble inorganic material also increased with increasing treatment
temperature. Regarding the carbon ratio in filtrate, it was not changed the total amount withincreasing treatment temperature, but the chemical form of the carbon would be different dependon the treatment temperature. At high treatment temperature, decomposition of organic material
progress and molecular weight and size would become small. Therefore, filtration performance
improved with increasing reaction temperature, even though the particle size decreased.
Consequently, chemical form of carbon, molecular weight and size would influence the filtration
performance significantly.

－ 161 －

organic material progress and molecular weight and size would become small. Therefore, filtration performance
improved with increasing reaction temperature, even though the particle size decreased. Consequently, chemical
form of carbon, molecular weight and size would influence the filtration performance significantly.

Table
3 Deposit,
BODBODand TOCfiltrate
Table
3 Deposit,
and inTOC
in filtrate

Treatment
Temperature

K

453

473

493

513

Solubilization
ratio

%

7.42

8.73

11.75

15.39

Deposit ratio

%

0.02

0.22

2.73

3.07

BOD

g/L

5.8

25

26

27

TOC

g/L

13

29

28

30

* Treatment period: 5 min, filter size: 1Ǎm.

CONCLUSIONSCONCLUSIONS
Filtration experiment with hydrothermally treated slurry was carried out, and filtration
Filtration experiment with hydrothermally treated slurry was carried out, and filtration performance
performance significantly improved by increasing treatment temperature and treatment period.
significantly improved by increasing treatment temperature and treatment period. Over 493 K in 5 minutes
Over 493 K in 5 minutes hydrothermal treatment, filtration speed was drastically improved. Withhydrothermal treatment, filtration speed was drastically improved. With increasing treatment period, filtration
increasing treatment period, filtration speed of the slurry also improved linearly against treatment
speed of the slurry also improved linearly against treatment period at 473 K.
period at 473 K.
Particle diameter was decrease with increasing treatment temperature and period, and theparticle in treated slurry might be coagulated strongly. The effect of particle size would not besignificantly influence on the filtration performance.
Decomposition of organic materials, evaluated from pH, BOD and TOC, started around 473 K,
and the decomposition ratio, weight and size of organic materials might strongly influence thefiltration performance of the treated slurry.
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