War. Sci. Tech. Vol. 35, No.8. pp. 209-215, 1997.C 1997 IAWQ. Published by Elsevier Science LId

Prinled in Greal 8nlalO.0273-1223J97 S17'00 +0-00

PD: S0273-1223(97)OO169-8

EFFECTS OF THERMOCHEMICALPRETREATMENT ON THE ANAEROBICDIGESTION OF WASTE ACTIVATEDSLUDGE

~ Pergamon

Shuzo Tanaka*, Toshio Kobayashi*, Ken-ichi Kamiyama*and Ma. Lolita N, Signey Bildan**

• Departmento/Civil Engineering, Meisei University, 2·/·/, Hodokubo, Hino-shi,Tokyo /9/ Japan•• Environmental Engineering Division, Asian Institute o/Technology,G.P.D. Box 2754, Bangkok 1050/, Thailand

ABSTRACT

Effects of pretreatment on the anaerobic digestion of waste activated sludge (WAS) were investigated in terms of VSSsolubilization and methane production by batch experiments. The methods of pretreatment studied are NaOH addition(chemical), heating (thermal) and heating with NaOH addition (thermochemical) to the domestic WAS and to thecombined WAS from domestic, conunercial and industrial wastewaters. The thermochenucal pretreatment gave the bestresult among three methods in the combined WAS, i.e., the VSS was solubilized by 40-50% and the methane productionincresed by more than 200% over the control when the WAS was heated at 130°C for S minutes with the dose 0.3 gNaOHlg VSS. In the domestic WAS, the VSS solubilization rate was 70-80% but the increase of the methaneproduction was about 30% after thermochemically pretreated. The domestic WAS consists of 41 % protein, 25% lipidand 14% carbohydrate on COD basis, and the solubilization rate of protein, which is the largest constituent of theWAS, was 63% in the thermochemical pretreatment. Although the effect of the thermochemical pretreatment on themethane production was higher to the combined WAS than to the domestic WAS, the methane production rate was 21.9ml CHJg VSSw,u·day in the domestic WAS and 12.8 ml CHJg VSSWAS·day in the combined WAS.<C> 1997 IAWQ. Published by Elsevier Science Ud

KEYWORDS

Alkali addition; anaerobic digestion; heating; thermochemical pretreatment; waste activated sludge

INTRODUCTION

Most of the substrate in waste activated sludge (WAS) is enclosed in cell membranes, which cause its resistance tobiodegradation. Because of this, the conventional anaerobic treatment requires such a long hydraulic retention time as20-30 days. Eastman and Ferguson (1981) reported that a rate luniting step during the acid phase ofanaerobic digestionwas the hydrolysis (solubilization) of the WAS. If the solubilization of the WAS is accelerated. then the subsequentacidogenesis and methanogenesis can be greatly enhanced.

Several methods to treat the WAS prior to biological process have been studied to accelerate the solubilization ofsubstrate in anaerobic digestion. Haug et a1. (1977, 1978, 1983) thermally pretreated the WAS at 100-250°C for 30mimutes. Digestion of the thermally pretreated sludge resulted to an increase in methane production over unpretreatedsludge of 60-70% at 175°C, but higher temperatures resulted in decreased gas production. Pinnekamp (1989) alsoconfirmed that at temperatures below 170°C the gas yields were lower than at 170-180°C, but the gas yields sharplydecreased above 18OOC. Hiraoka et a1. (1985) investigated the thermal pretreatment at temperatures below 100°C andrevealed an increase of more than 30% in gas production at lower temperatures such as 60 and 80°C, but the lowtemperature pretreatment necessitated a longer contact time than the high temperature treatment. Li and Noike (1989)

209

210 S. TANAKA et al.

reported that the optunum temperature and contact time for the WAS were 170°C and 60 min., respectively.

Chemical pretreatment at ambient temperatures using low levels of aIka1i was evaluated by Rajan et aI. (1989) and Rayet aI. (1990) on its effects on solubilization and gas production. Solubilization rates of more than 45% of particulateCOD were achieved at 30 meq!l NaOH and the gas production increased by 112% over control.

The effect of combined thermal and chemical (thermochemical) pretreatment on anaerobic biodegradability of the WASwas studied by Stuckey and McCarty (1984). It was found that the effect of thermochenucal pretreatment onbloconvertlbility to methane was marginal around 175°C, but prevented the sharp decline in blocovertibility noted inthermal pretreatment alone at high temperatures.

These studies have shown that the solubihzation increases with increasing the pretreatment temperature as does themethane production. However, Pinnekamp (1989) noted that differences in gas YJeld mcrcase were not very great attemperatures between 120°C and 180°C. Optimum conditions ofalkali dose and contact time as well as temperature areyet necessary to be established pnor to applicatIon of these pretreatments to the anaerobic digestion of the WAS.Moreover, characteristics of the WAS may need to be considered, because they vary depending on wastewaters to treatand whether or not the primary sludge nuxed.

The objectives ofthis study are to optinuze operating conditIons of chemical, thermal and thermochenucal pretreatmentsto enhance solubilization and methane production, and evaluate these three pretreatment methods in the anaerobicdigestion of two different types of WAS The WAS studied are the domestic WAS from the plant treatmg domesticwastewater and the combined WAS from the plant which treats wastewaters from residentIal, commercial and industrialarea without primary settling tank. The study also includes to investigate the individual solubilIzation of organicconstituents 10 the WAS.

MATERIALS AND METHODS

Two different types of WAS from return sludge lines were used in this study, i.e, the domestic WAS collected from theKitano wastewater treatment plant 10 Tokyo (Japan), which mainly treats wastewater from households, and thecombined WAS collected from the Navanakom wastewater treatment plant near Bangkok (Thailand), which treats all ofthe wastewaters from reSIdential, commercial and IOdustnal area Influent to the Navankom plant IS expected not tocontam any toxic materials as these are required to be removed from the point source of the wastewater. Characteristicsof both WAS are summarized 10 Table I Smce the Navanakom plant has no pnmary settling tank, the morgaruc 5Sconcentration of the combmed WAS is high as indicated by a low VSS/SS ratio of 0.53

Table I Characteristics ofwaste activated sludge (WAS)Type ofWAS T-COD D-COD SS VSS VSS/SS

(mgil) (mgil) (mg~) (mgil)

Domestic WAS 9,200 500 7,000 6,100 0.89Combined WAS· 6,700 50 8,400 4,500 053• Combm.:d WAS from domeshc, COnunc:rclal and mdustnal wastewaters

Pretreatment conditions investigated had the followmg ranges of alkali dose, temperature and contact time: (I) alkalidose 0.05-1.5 g NaOHlg VSS, (2) temperature 1I5-180°C and (3) contact time 5-200 minutes. In the chemicalpretreatment, well-nuxed WAS was transferred to flasks, added with the required NaOH dose, and mixed for therequired contact tune by magnetic stirrers. In the thermal pretreatment, well-nuxed WAS was Plpetted into teflon bottles,purged with Nz gas to prevent oxidation of readily oxidizable organics, and autoclaved at the required temperature andcontact time. Heating to the set temperature took about 30 minutes for 1I5-180°C and the set contact time wasactivated only when the set temperature was reached. In the thermochemical pretreatment, well-mixed WAS waspipctted into teflon bottles, added with the required alkali dose, and followed the procedure of the thermal pretreatment.

Batch experiments were conducted to anaerobically digest the WAS after pretreatments Pretreated WAS and seedsludge (obtained from a cultivation tank) were fed to serum bottles at a food to microorganism ratio,F{VSSWAS)IM{VSSSEED), of 0.45-0.50. A control digester was fed with raw WAS together with the seed sludge at thesame FIM ratio. By adding HC) before feeding, pH of all feed sludge were adjusted to pH 7±O I. The dIgesters wereincubated at 37°C in a water bath.

Anaerobic digestion of waste activated sludge 211

Parameters assayed were pH, solids (SS and VSS), COD, volatile fatty acids (VFA), gas composition, organicconstituents (protein, lipid, carbohydrate). VFA and gas composition were analyzed by gas chromatography with FlOand TCD detectors, respecnvely. In organic constituents of the WAS, analyzed were protein by the modified Lowry'smethod, lipid by the dichromate method after ether extraction, and carbohydrate by the phenol-sulfuric acid method.Other parameters followed Sewage Test Methods of Japan and/or Standard Methods of U.S.A.

RESULTS AND DISCUSSION

Comparison of various pretreatments. Three different methods of pretreatment were compared in tenns of VSSsolubilization and methane production of the combmed WAS. Results are sho\\1l m Fig.l- Flg.4, in which the methaneproducion in each pretreatment is evaluated by the relative value to that of the control in 8 day digestion.

In the chemical pretreatment(Fig.l), the VSS solubilization rate increased as the alkali dose increased upto 0.5-0.6 gNaOHlg VSS, but it became constant around 15% above the dose 0.6. The methane production was gradually improvedover the range of the dose studied, and about 50% increase was achieved over the control at the dose 1.0. In the thermalpretreatment (Fig.2), the VSS solubilization rate was around 15% in between 1l5-150°C and then increased furtherabove 160°C, reaching 30% at 180°C. The methane production showed the same behavior as the solubilization, i.e., itachieved about 40% increase in between 115-150°C and then increased further above 160°C, resulting in 90% increaseat 180°C.

20

--.-Solubil.-+-Methanc

3

o 0.2 0.4 0.6 0.8A1kab Dose (8 NaOH/g VSS)

Fig.1 Effects ofalkali dose on solubilization and methane productionin chemical pretreatment ofcombined WAS (I h contact time)

40

--.- Solubil.

-+-Methane

3

o I

cont. 120 130 140 150 160 170 180

Ternperature ( "C)

Fig.2 Effects of temperature on solubilization and methane productionin thermal pretreatment of combined WAS (I h heating)

212 S. TANAKAetal.

In the thermochemical pretreatment, the VSS solubilization rate increased as the alkali was added until the dose 0.25 gNaOHIg VSS, but was no longer improved above It when the domestic WAS was heated at 130°C, as shown in Fig.3.The value of pH was also constant around 12 above the dose 0.25. Therefore, the effect of temperature on solubilizationand methane production was studied at the constant dose of 0 3, which was equivalent to 34 roM NaOH (Fig.4). Bothsolubihzation and methane production were more improved than those ofthe chemical and the thermal methods. About45% of the VSS were solubilized and the methane production showed 220% increase over the control at 130°C. Theenhancement of solubilization and methane production, however, was leveled off above 130°C

100 14~

'if. 12fJl 80fJl

G 10

~ 60 8c ::r:0 6 c.

:~40 -e-Solubll.

~ -+-pH 4200 2fJl

0 00 0.1 0.2 03 0.4 0.5

Alkah Dose (g NaOHlg VSS)

Fig.3 NaOH dose in thermochemical pretreatment of domestIc WAS(5 min. at 130°C)

60 ,..------------, 3

~sofJlell

G40B

~ 306.~ 20

~IOell

.......-Solubil.-+-Methane

o I

conI. 120 130 140 ISO 160 170 ISOTemperature ('C)

Fig.4 Effects of temperature on solubilizatio and methane production in thermo•chemical pretreatment of combined WAS (5 min. heatmg at dose 0 3)

The effects of three different pretreatments on the anaerobic digestIon of the combmed WAS were evaluated under eachoptimum condJtion obtained in tlus study, i.e., the chemical at the dose 06, thc thermal at ISO°C and thethermochelDlcal at JJO°C with the dose 0.3. As shown in Flg.5, the thermochemical pretreatment gave the best result In

the solubilization and the methane production, though the cost of pretreatments had not been considered. Afterthermochemically pretreated, the methane was produced at a 2.2 times higher ratc than the control Without anypretreatment.

Evaluation of thermochemical pretreatment. Two dIfferent types of WAS, domestic WAS and combined WAS, werethermochemically pretreated under the optimum condition determined prevJously and then anaerobically digested for 20days. Evaluated were the VSS solubilization in terms of pretreatment time and organic components, and the methaneproduction in terms ofdJgestion time.

Anaerobic digestion of waste activated sludge 213

50 TC

#. 40 c

'" .~ TC'"~ ~ " T-gg£ 30 6: 1l;! u .~ Cc ;0

.S!20

oS ~.

] Vo::;:", I:E .~ .:" 10~ U

0::

0 0Solubilization MetIw1< Production

C Chemical (dose 0.6)T Thermal ( 180·C)Te Thennochemical (l30·C, dose 0.3)

Fig.5 Solubilization and methane production of combined WAS underoptimum conditions of various pretreatments

The relationship between solubilization and pretreatment time is depicted in Fig.6. The VSS solubilization was notgreatly influenced by the pretreatment time in the range of 5 to 60 minutes, particularly in case of the domestic WAS.The rate of solubilization was 70-80% in the domestic WAS, while 40-50% in the combined WAS. The low rate ofsolubilization in the combined WAS is probably due to complexity of organics including refractory components,because the WAS is from the plant which treats all of the wastewaters from residential, commercial and mdustrial area,and has no primary settling tank.

6050403020

100

~ KO'"'"~... 60

'":>::cc 40~]

__ Domestic WAS

" 20 -6-Combined WAS"0r/l

Pretreatment TIme (min.)

Fig.6 Solubilization of WAS by thermochemical pretreatments(130°C, dose 0.3)

The domestic WAS consists of 41% protein, 25% lipid, 14% carbohydrate and 20% unknown components on the basisof COD as shown in Fig.7. The solubilization rate of carbohydrate was the highest among three organic components,and the dissolved component occupied 90% in carbohydrate, 64% in protein and 61% in lipid after thermochemicallypretreated. Biodegradability of the three main organic components of the WAS was given by Pinnekamp (1989) as 65%,52% and 39% for lipid, carbohydrate and protein, respectively. Since protein, which is the largest constituent of theWAS, is the least biodegradable, the 63% solubilization of particulate protein by the pretreatment will greatly contributeto the subsequent digestion of the WAS.

The methane production increased with digestion time and the pretreated WAS gave higher production than the controlover 20 day digestion periods as shown in Fig.8. In 20 day digestion, the final rate of methane conversion increasedfrom 35% to 50% in the domestic WAS, and from 20% to 35% in the combined WAS. The behavior of methaneproduction, however, was different in between two types ofWAS. The methane production in the domestic WAS greatly

21-1 5, TANAKA et al

Raw Raw Prc Raw Prc Raw Prc100

~

80 ]~ "::>00 60~

~ -10'"0-E'"u 20

0TOlal Protem LIPid Carbohydralc

Fig 7 Solublhzatlon of domestic WAS components b\ thermochemIcalpretreatment (5 mIn, at 130°C. dose 03)

6 10

--~()

c

III'J

:;:

__ Pre (domestIC )

--.- Cont.(domcstic)

-.- Pre.{comblned)

......-Cont.{comblned)

10 12 I.) IR ~(I

I)lgc..,:lJon TIllh: \ da\ I

Pre Pretreated, ('ont('ontro1

FlgX Mcthane production from WAS In thermochemical pretreatment

increased upto day ]0 In dIgestion but leveled off thereafter. \\hlle It graduall\ Increased until day lo-IX III thecombIned WAS

Table 2 sunmlanzes the dall\ rate of methane productIon per 'iSS of WAS Introduced ('iSS, ) and per YSS of seedsludge (VSS",p) III !O day digestIon, \\Inch showed a hnear merease of methane production agaInst digestIon tnne Inthe control. the digestion of the domestic WAS produced the methane gas at a ralc of 17 2 ml CH ug 'iSS,\,da~ andthe combIned WAS dId at 02 ml CHJg VSS\\.wda\, \\hlch corresponded to about .to% of the domestIc In thepretreated WAS. the domestic produced the methane gas at 21 'i ml CHJg VSS,\,'da~ and the combmed dId at 12 X mlCHJg VSS\\,\s,day Thus. the thermochemical pretreatment Increased the methane production b\ I 27 limes m thedomestic WAS and by 2,00 tnnes In the combIned WAS oyer the control.

Table 2 Effects of Themlochemical Pretreatment on Methane Production In 10 Da\ DigestIonT\ pe of WAS Methane Productlon Rate COlli erslon Rate

Imll'll~ g \'SS'... ~, d) (m] ell I b \'SS !-b' d" (e( )1)""1

Domes!lc

CombIned

ControlPretreatedPre.:Cont

ControlPretreatedPre fCont

17221 'i

1270.2 (OJo)'

12 X((UX)'206

77'J X

1273 I (040)'o.t (005)'

206

II25

·Thc ratio of the (omhilleL! to the dOIllcstlC In the rndhJne proJUl.:tHm

CONCLUSIONS

Anaerobic digestion of waste activated sludge 215

The effects of three different pretreatments on the anaerobic digestion ofthe combmed WAS were evaluated under eachoptimum condition obtained in this study, i.e., the chemical at the dose 0.6 g NaOHIg VSS, the thermal at 180°C andthe thermochemical at 130°C for 5 minutes with the dose 0.3 g NaOHIg VSS. The thermochemical pretreatment gavethe best result among three, i.e., the VSS was solubilized by 40-50% and the methane production increased by morethan 200% over the control. In the domestic WAS, the VSS solubilization rate was 70-80% but the increase of themethane production was about 30% after thermo-chemically pretreated The domestic WAS consists of 41% protein,25% lipid and 14% carbohydrate on COD basis, and the solubilization rate of protein, which IS the largest constituentof the WAS, was 63% of initial particulate protein in the thermochemical pretreatment. Although the effect of thethermochemical pretreatment on the methane production was higher to the combined WAS than to the domestic WAS,the methane production rate was 21.9 ml ClWg VSSWAS·day in the domestic WAS, while 12.8 ml ClWg VSSWAS·day inthe combined WAS.

ACKNOWLEDGEMENT

This study was supported by the Special Research Grant of Meisei University and the Japan International CooperationAgency (JICA) through Asian Institute ofTechnology. The authors wish to express their gratitude to them.

REFERENCES

Eastman, J.A. and Ferguson, J.F. (1981), Solubilization of Particulate Organic Carbon during the Acid Phase ofAnaerobic Digestion, J. WPCF, 53, 352-366.

Haug, R.T. (1977), Sludge Processing to Optimize Digestibility and Energy production, J. WPCF, 49, 1713-1721.Haug, R.T., Stuckey, D. C., Gossett, J.M. and McCarty, P.L. (1978), Effects ofThermal Pretreatment on Digestibility

and Dewaterability ofOrganic Sludges, J. WPCF, SO, 73-85.Haug, R.T., leBrun, T. J. and Tortorici, L. D. (1983), Thermal Pretreatment of Sludges - a Field Demonstration,

J. WPCF, 55, 23-34.Hiraoka, M., Takeda, N., Sakai, S. and Yasuda, A. (1985), Highly Efficient Anaerobic Digestion with Thermal

Pretreatment, Waf. Sct. Tech., 17, 529-539.Li, Y. and Noike, T. (1989), The Effect of Thermal Pretreatment and Retention Time on the Degradation of Waste

Activated Sludge in Anaerobic Digestion, J. Waf. PollUf. Res., 11 (2), 112-121, (in Japanese).Pinnekamp, 1. (1989), Effects ofThermal Pretreatment of Sewage Sludge on Anaerobic Digestion, Waf. ScI. Tech., 21,

97-108.Rajan, R.Y., Lin, J. and Ray, B. T. (1989), Low-Level Chemical Pretreatment for Enhanced Sludge Solubilization,

J. WPCF,61, 1678-1683.Ray, B.T., Lin, J. and Rajan, R.Y. (1990), Low-Level Alkaline Solubilization for Enhanced Anaerobic Digestion,

J. WPCF, 62, 81-87.Stuckey, D.C. and McCarty, P.L. (1984), The Effect ofThermal Pretreatment on the Anaerobic Biodegradability and

Toxicity ofWaste Activated Sludge, Waf. Res., 18, 1343-1353.