Desalination 246 (2009) 275–284

0011-9164/09/$– See front matter © 2008 Elsevier B.V. All rights reserved

*Corresponding author.

Disintegration of excess activated sludge by hydrogen peroxideoxidation

Tak-Hyun Kima*, Sang-Ryul Leea, Youn-Ku Nama, Jeongmok Yangb,Chulhwan Parkc, Myunjoo Leea

aRadiation Research Center for Industry and Environment, Korea Atomic Energy Research Institute, Jeongeup, 580-185, Korea

Tel. +82 (63) 570-3343; Fax+82 (63) 570-3348; email:tkhk@kaeri.re.krbGreen Engineering Team, Korea Institute of Industrial Technology, Chonan, 330-825, Korea

cDepartment of Chemical Engineering, Kwangwoon University, Seoul, 139-701, Korea

Received 7 December 2007; accepted revised 12 June 2008

Abstract

As the result of a conventional wastewater treatment such as activated sludge process, an excess sludge isproduced and must be disposed of safely. Recently, increased attention has been given to a minimization of wastesludge in a wastewater treatment process. In this study, hydrogen peroxide oxidation was applied for an excessactivated sludge reduction and an alkaline pretreatment method was used to enhance the efficiency of hydrogenperoxide oxidation of sludge. Sludge particle disintegration by hydrogen peroxide oxidation was confirmed throughthe evaluations of total solid concentration and particle size distribution. The solubility (SCOD/TCOD) of sludgewas increased, while the viscosity was decreased with hydrogen peroxide oxidation and alkaline hydrolysis. Thesettleability of sludge was improved. When alkaline hydrolysis was applied as a pretreatment for hydrogen peroxideoxidation, the sludge particle disintegration, viscosity decrease, and settleability improvement were accelerated.From the results of this experiment, it can be concluded that hydrogen peroxide oxidation combined with alkalinepretreatment was useful for an excess sludge reduction.

Keywords: Excess activated sludge; Particle disintegration; Alkaline hydrolysis; Hydrogen peroxide oxidation

1. Introduction

A large quantity of sludge is produced from awastewater treatment plant. An activated sludgetreatment of sewage wastewater also results in the

generation of a considerable amount of excesssludge. This sludge must be disposed of safely.Commonly used disposal practices comprise ofincineration, landfilling and land application. Thecost for sludge treatment is highly dependent onthe volume and water content of the produced

doi:10.1016/j.desal.200 .0 08 6. 23.

T.-H. Kim et al. / Desalination 246 (2009) 275–284276

sludge. Treatment and disposal of an excess sludgein a biological wastewater treatment system re-quires an enormously high cost which accountsfor approximately 35–60% of the whole opera-tion cost of a wastewater treatment [1]. Land ap-plication or agricultural use of sewage sludge ishighly debated, and the landfilling of sewagesludge is now restricted in Korea. And an incin-eration is quite expensive. Moreover the conven-tional disposal methods of landfilling or incin-eration cause secondary pollution problems.Therefore, interest in methods to reduce the vol-ume and mass of excess sludge has been increased.

Various studies have reported the use of dif-ferent physical, chemical and biological processesto reduce a sewage sludge production. Severaldisintegration methods have been investigatedsuch as thermal treatment [2], mechanical treat-ment using ultrasounds [3,4] and mills [5], chemi-cal treatment using ozone [6,7], acid or alkali [8],biological hydrolysis with or without enzyme ad-dition [9]. An alkaline hydrolysis was tradition-ally studied for the sludge reduction and the en-hancement of sludge biodegradability. Ozoneoxidation is one of powerful sludge destructionprocesses. While the cost to disintegrate the sludgethrough a chemical treatment is typically moreexpensive than through physical or biologicaltreatment processes.

Recently, some researchers reported a processcombined with biological process to minimize asludge production. A combined system of an ac-tivated sludge process and an ozone oxidation forexcess sludge reduction has been successfullydeveloped [10,11]. Sludge reduction by alkaline-thermal hydrolysis has also been investigated[1,12,13]. Tokumura et al. [14] and Neyens et al.[15] examined the photo-Fenton process and theperoxidation for reducing an excess sludge, re-spectively. Yoon et al. [3] incorporated an ultra-sonic treatment with a membrane bioreactor, andSong et al. [10] combined ozonation with a mem-brane bioreactor.

Hydrogen peroxide (H2O

2) oxidation, which

is one of the advanced oxidation processes(AOPs), may offer a promising technology for theminimization of excess sludge. Unlike conven-tional inorganic oxidizing agents such as chlorineand hypochlorite, hydrogen peroxide yields nonoxious or polluting byproducts. Because its onlybyproducts are water and oxygen, it provides asecondary benefit-dissolved oxygen, which stimu-lates the activity of aerobic organisms [16]. Hy-drogen peroxide can oxidize a wide variety ofaqueous pollutants such as cyanide, nitrite andhypochlorite, phenol aromatics, formaldehyde,sulfite, thiosulfate and sulfide compounds [17].Recent research has focused on the use of hydro-gen peroxide to reduce odors and destroy patho-genic organisms [18]. Hydrogen peroxide can beused alone or with a UV light, ozone and a cata-lyst such as Fe(II) [19]. Due to the effectivenessof hydrogen peroxide as a strong oxidative chemi-cal in a wastewater treatment, the suitability ofthis AOP technique was tested in this work for avolume and mass reduction of sludge.

In this study, hydrogen peroxide oxidation andalkaline hydrolysis were considered as post treat-ment processes for sludge disintegration. Char-acteristics of the sludge such as total solid con-tent, solubility, particle size distribution, viscos-ity and settleability were examined after apply-ing hydrogen peroxide oxidation and alkalinehydrolysis to sludge in laboratory experiments.

2. Materials and methods

2.1. Sludge sampling

The waste sludge was collected from a mu-nicipal wastewater treatment plant located inJeongeup, Korea. Sludge sample was collectedfrom the secondary settling tank after activatedsludge treatment process. Table 1 shows the char-acteristics of the original waste activated sludge.The waste activated sludge contained approxi-mately 16200 mg/l of total solid (TS), 12340 mg/lof total chemical oxygen demand (TCOD), and

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370 mg/l of soluble chemical oxygen demand(SCOD). Total-nitrogen (T-N) and total phospho-rus (T-P) concentration in the raw sludge were 65and 38 mg/l, respectively.

2.2. Hydrogen peroxide oxidation and alkaline hy-drolysis

Hydrogen peroxide oxidation and alkalinehydrolysis of sludge were carried out in a 1 l bea-ker. A mechanical stirrer was used to ensure a suf-ficient mixing of sludge. The mixing speed wasmaintained at 200 rpm, which dispersed the re-agents and prevented the sludge from settling.Hydrogen peroxide oxidation was tested withoutand with alkaline hydrolysis pretreatment. Hydro-gen peroxide, 35%, was used and four differentconcentrations of hydrogen peroxide (0.4, 0.8, 1.2and 1.6 M) were added to the sludge. For the ex-periments of pH effect on a sludge reduction, ini-tial pH of the sludge was set up to 3.0, 5.0, 7.0,9.0 and 11.0, respectively, by adding 10 N so-dium hydroxide (NaOH) and 10 N sulfuric acid(H

2SO

4). Samples were collected after 2 h reac-

tion. Experiments were carried out at an ambienttemperature (25°C).

2.3. Sludge characterization

The physicochemical characteristics of wasteactivated sludge including pH, TS, SCOD, TCOD,

Table 1Chemical characteristics of the waste activated sludge

Item Range Average

pH TS (mg/l) TSS (mg/l) VSS (mg/l) TCOD (mg/l) SCOD (mg/l) T-N (mg/l) T-P (mg/l)

6.3–6.8 12300–23700 11000–22000 7490–10000 9940–17360 40–700 34–88 24–42

6.6 16200 13500 8745 12340 370 65 38

T-N and T-P were measured by Standard Meth-ods [20]. The pH value was measured by using apH meter (Orion Ross ultra pH, Thermo ElectronCorporation). The term of solubilization meansthe transfer of COD or solids from the particulatefraction of sludge to the soluble fraction of sludge.In order to determine a sludge solubilization, sev-eral measurements were carried out on thesamples. The ratio of SCOD/TCOD was appliedas an indicator to evaluate a change of sludge bio-degradability. COD was measured for the totalsludge (TCOD) and for the filterate (SCOD). Sol-ids contents were also measured by heating asludge at 105°C during 24 h for total solids.

Particle size of sludge was analyzed by usinga LS Particle Size Analyzer (Beckman Coulter LS13 320, USA). Results were expressed as a me-dian diameter (d), and the particle size distribu-tion was determined by using a sphere of samevolume. Viscosity measurements were carried outby using a rotational viscometer Brookfield DV-2+ PRO (Brookfield Engineering Laboratories,Inc.). Settleability of sludge was evaluated bycarrying out a sludge volume index (SVI) testaccording to Standard Method [20]. Zeta poten-tial measurements were also performed in orderto interpret the obtained results. The determina-tion of zeta potential was carried out by using azeta potential meter (Zeta-meter 3+, AST Co.,USA).

3. Results and discussion

3.1. Reduction of total solids

Hydrogen peroxide oxidation and alkalinehydrolysis not only can reduce the sludge vol-ume but can also solubilize the particulate com-ponent of sludge into the soluble form. In gen-eral, organic matter is released and solubilizedduring the disruption of the sludge floc and cellby hydrogen peroxide oxidation and alkaline hy-drolysis. It can be quantified using some param-eters such as TS content and SCOD/TCOD ratioof the sludge.

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TS contents of sludge were measured to con-firm the effect of hydrogen peroxide oxidation onsludge reduction. As shown in Fig. 1a, TS con-tent of sludge was decreased from 14850 mg/l to9960 mg/l (33% removal of TS) with an increaseof hydrogen peroxide dose from 0 M to 1.6 M.Furthermore, when the pretreatment of alkalinehydrolysis was applied (pH 11) prior to hydrogenperoxide oxidation, TS removal efficiency was en-hanced. It decreased from 14670 mg/l to 7410 mg/l

0.0

0.2

0.4

0.6

0.8

1.0

1.2

0.0 0.4 0.8 1.2 1.6 2.0

H2O2 Conc. (M)

TS

/TS

0

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1 3 5 7 9 11 13

pH

TS

/TS

0

Fig. 1. Effects of hydrogen peroxide oxidation (a) and alkaline hydrolysis (b) on total solids reduction, hydrogen perox-ide oxidation only (), hydrogen peroxide oxidation at pH 11 (), alkaline hydrolysis only (), and alkaline hydrolysisat 1.6 M H

2O

2 addition ().

(a)

(b)

(49% removal of TS) with an increase of hydro-gen peroxide dose.

From Fig. 1b, the alkaline hydrolysis alone didnot change TS content. Hydrogen peroxide solu-bilized the sludge and thus decreased TS content.At the same dose of hydrogen peroxide, the ef-fect of initial pH on sludge reduction was insig-nificant (Fig. 1b). TS content of sludge decreaseda little more at acidic and alkaline pH than at neu-tral pH. At extremely high pH values of medium,

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the cell loses its viability, it cannot maintain anappropriate turgor pressure and disrupts. Alkaliadded to the cell suspension reacts with the cellwalls in several ways, including a saponificationof the lipids in the cell walls, which leads to asolubilization of the cell membrane. Disruptionof the sludge cells leads to a leakage of intracel-lular material out of the cell [1]. Katsiris et al.stated that the bacterial surfaces were negativelycharged as the pH of sludge samples increased.This creates high electrostatic repulsion whichcauses desorption of some part of extracellularpolymers [21].

3.2. Solubilization of sludge

In order to investigate the effect of hydrogenperoxide oxidation on a solubilization (SCOD/TCOD ratio) of sludge, a series of experimentswas carried out at different hydrogen peroxideinitial concentrations. In these experiments, con-centration of SCOD and TCOD were measuredafter 2 h reaction. Fig. 2 shows the effects of hy-drogen peroxide oxidation and acid/alkaline hy-drolysis on a sludge solubilization. From Fig. 2a,the SCOD/TCOD of raw sludge was 1.6%, it in-creased to 54.7% by hydrogen peroxide oxida-tion (1.6 M). However, when alkaline hydrolysisof pH 11 was employed before the hydrogen per-oxide, the SCOD/TCOD of sludge increased from28.3% to 60.8% as the dose of hydrogen perox-ide was increased from 0 to 1.6 M. Furthermore,the increasing rates of solubilization were foundto decrease at a higher hydrogen peroxide dose.This indicates that an easy-to-be-oxidized frac-tion inside or outside sludge cell was solubilizedfirst and then other fractions were gradually solu-bilized. Lipid was known as the hard-to-be-de-graded fraction whereas protein and carbohydratewere known as the easy-to-be-oxidized fractionsin a sludge [7]. Hydrogen peroxide can effectivelyimprove the biodegradability. Ksibi reported thatthe intermediates such as a short-chain carboxy-lic acid produced from ramified aliphatic chain

by hydrogen peroxide oxidation are easily de-graded by microorganisms [19].

The effects of initial pH of sludge on the sludgesolubilization were also investigated. As shownin Fig. 2b, the solubilization efficiency (SCOD/TCOD) was 6.3% at pH 3 and 0.4% at pH 7, andit increased to 27.6% as the pH was increased topH 11. When hydrogen peroxide was applied tooxidize the sludge after the acid or alkaline treat-ment, the solubilization efficiencies were en-hanced. The solubilization efficiencies showed52.6–57.4% regardless of the sludge pretreatmentpH conditions. Therefore, the effects of acid oralkaline hydrolysis pretreatment on the hydrogenperoxide oxidation were insignificant with respectto a sludge solubilization. Production of solubleCOD (SCOD) during alkaline hydrolysis and hy-drogen peroxide oxidation was confirmed, andthere was a reasonable correlation between thedestruction of TS and the increase in SCOD (Figs. 1and 2). Ksibi et al. [19] reported that hydrogenperoxide can effectively improve the biodegrad-ability of sludge because the intermediates, suchas short-chain carbon carboxylic acids producedfrom a ramified aliphatic chain are easily de-graded.

Comparison of SCOD/TCOD ratios reveals ageneral indication of the extent of hydrolysis. Inthis study, the SCOD/TCOD ratio increased from0.02 (raw sludge) to 0.55 and 0.61 by hydrogenperoxide oxidation (1.6 M H

2O

2) and by hydro-

gen peroxide oxidation pretreated with alkalinehydrolysis (pH 11 NaOH + 1.6 M H

2O

2), respec-

tively. From other studies of an activated sludgehydrolysis, SCOD/TCOD ratios increased from0.06 (raw sludge) to 0.15 using a microwave treat-ment to 96°C, and to 0.27 using a conventionalthermal treatment to 96°C [2]. Chu et al. [22] re-ported that an ultrasound treatment achievedSCOD/TCOD ratio of 0.2 after 120 min of a soni-cation (20 kHz, 110 W) for waste activated sludge.Otherwise, a combination of chemical treatmentwith thermal pretreatment method could providea better disintegration of sludge. SCOD/TCOD

T.-H. Kim et al. / Desalination 246 (2009) 275–284280

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.0 0.4 0.8 1.2 1.6 2.0

H2O2 Conc. (M)

SC

OD

/TC

OD

(a)

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

1 3 5 7 9 11 13

pH

SC

OD

/TC

OD

(b)

Fig. 2. Effects of hydrogen peroxide oxidation (a) and alkaline hydrolysis (b) on the sludge solubilization (SCOD/TCOD),hydrogen peroxide oxidation only (), hydrogen peroxide oxidation at pH 11 (), alkaline hydrolysis only (), andalkaline hydrolysis at 1.6M H

2O

2 addition ().

ratio of 0.8 was obtained at 170°C (autoclave) atpH 12 with potassium hydroxide (KOH) [23].Bougrier et al. [24] compared the COD solubili-zation of sludge by sonication, ozone oxidation,and thermal treatment, they achieved SCOD/TCOD ratio of 0.15 for sonication (6250–9350 kJ/kg TS), 0.20–0.25 for ozone oxidation(0.10–0.16 g O

3/kg TS), and 0.40–0.45 for ther-

mal treatment (170–190°C).

The production of soluble COD from a partialoxidation of an excess activated sludge has the po-tential to provide a good carbon source for a deni-trification, and leads to substantial cost savings inthe reactions that are carbon-limited. Many research-ers have studied on a solubilization and a reuse ofsewage sludge for a biological denitrification. Ozoneoxidation, microwave treatment, alkaline hydroly-sis and thermal hydrolysis were employed [25–28].

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3.3. Particle size reduction

In order to investigate the effect of hydrogenperoxide and alkaline treatment on a sludge par-ticle size reduction, particle size distribution wasanalyzed and is presented in Fig. 3. In this studythe median size of the raw sludge was around34.5 μm. It was reduced to 13.5 μm with hydro-gen peroxide oxidation (decrease of 61%), andreduced to 10.8 μm with alkaline hydrolysis andhydrogen peroxide oxidation (decrease of 68%).On the other hand, the alkaline hydrolysis alonedid not affect the sludge particle size reduction.Hydrogen peroxide treated sludge samplesshowed a significant increase in the number ofsmaller particles. The results from the particle sizedistribution showed that hydrogen peroxide oxi-dation was effective for a sludge particle reduc-tion. The active forms of oxidant are generallythe hydroxyl radicals (OH•) and hydroperoxylradicals (HO

2•) generated from the decomposi-

tion of hydrogen peroxide. These radicals attackon sludge particles leads to the destruction of cellwall of microorganism, resulting in not only SSreduction and generation of soluble COD but alsothe sludge particle reduction.

0

1

2

3

4

5

6

0.01 0.1 1 10 100 1000

Particle diameter (μm)

Vo

lum

e (%

)

Fig. 3. Effects of alkaline hydrolysis and hydrogen peroxide oxidation of sludge on the distribution of particle diameter,raw sludge (); alkaline hydrolysis at pH 11 (); hydrogen peroxide oxidation at 1.6M H

2O

2 (); alkaline hydrolysis

and hydrogen peroxide oxidation at pH11 and 1.6M H2O

2 ( ).

The formation model of sludge floc consistsof primary particles (~2.5 μm), microflocs (~13 μm)and porous flocs (~100 μm) [29]. The particles ina raw domestic sewage wastewater range in sizefrom less than 1 nm to over 100 μm, however, thesize of settled sewage sludge ranges in usuallyless than 50 μm [30]. Particle size distribution ofthis study shows that most of the porous flocs aredestroyed by hydrogen peroxide oxidation.

3.4. Viscosity and zeta potential

Sludge treatments had effects on the rheologyof sludge such as viscosity and surface charge ofparticle. The viscosities and zeta potential ofsludge particle according to different reactionconditions are depicted in Figs. 4a and 4b, respec-tively. The viscosity of raw sludge was 4.2×10–2 g/cm·s, it decreased to 3.9×10–2 g/cm·s by al-kaline hydrolysis (pH 11), to 1.5×10–2 g/cm·s byhydrogen peroxide oxidation (1.6 M H

2O

2), and

finally to 1.3×10–2 g/cm·s by the combination ofalkaline hydrolysis and hydrogen peroxide oxi-dation (pH 11 + 1.6 M H

2O

2). Hydrogen peroxide

oxidation of sludge decreases its viscosity andconsequently decreases the hydraulic shearing

T.-H. Kim et al. / Desalination 246 (2009) 275–284282

force on the particles. At a higher hydrogen per-oxide concentration, the viscosity of sludge waslower, which implies that less energy is requiredfor a mixing operation [30].

The sludge surface charge affects the physi-cal-chemical characteristics of sludge such assludge settleability, viscosity and dewaterability.Distributions of zeta potential according to dif-ferent reaction conditions are presented in Fig. 4b.The raw sludge retained a negative charge of –8.1 mV. The zeta potential of the sludge increasedto 6.5 mV for alkaline hydrolysis, to 36.0 mV forhydrogen peroxide oxidation, and finally to66.5 mV for the combined treatment of alkalinehydrolysis and hydrogen peroxide oxidation. Thesludge surface charge is most likely a result ofthe ionic groups of an extracellular polymericsubstance (EPS), such as amino groups (–NH

2),

hydroxyl groups (–OH), carboxyl groups (–COOH)and the adsorption of ions from the water phase[30]. The charge of these groups depends on thenature of the groups and the pH. At neutral pH,functional groups such as carboxylic groups havea negative charge, while amino groups have apositive charge. Kwon et al. [31] reported that the

0

1

2

3

4

5

Vis

co

sit

y (

10-2

g/c

m s

)

raw pH 11 H2O2 1.6M pH11 + H2O2 1.6M

-10

0

10

20

30

40

50

60

70

Ze

ta p

ote

nti

al

(mV

)

raw pH 11 H2O2 1.6M pH11 + H2O2 1.6M

(a) (b)

Fig. 4. Effects of alkaline hydrolysis and hydrogen peroxide oxidation on the viscosity (a) and the zeta potential (b) ofsludge.

surface charge of an ozonated sludge was con-verted from negative to positive.

3.5. Sludge settleability

Hydrogen peroxide oxidation can also enhancethe sludge settleability [18]. The SVI was inves-tigated for this purpose after hydrogen peroxideoxidation and after hydrogen peroxide oxidationwith alkaline hydrolysis pretreatment (Fig. 5). TheSVI is typically used to monitor the settling char-acteristics of sludge and other biological suspen-sions [20]. The SVI is the volume in millilitersoccupied by 1 g of suspension after 30 min set-tling. When 1.6 M H

2O

2 was applied to oxidize

sludge, SVI decreased from 67.6 ml/g to 62.9 ml/g.About 7% settleability enhancement could beobtained. In addition, the settleability was enhancedmore when alkaline hydrolysis was employed asa pretreatment of hydrogen peroxide oxidation.As hydrogen peroxide dose was increased from 0to 1.6 M after alkaline hydrolysis (pH 11), SVIdecreased to 23.4 ml/g. Settleability enhancementof 66.7% was achieved. From these data, it canbe inferred that the sludge settleability enhance-

283T.-H. Kim et al. / Desalination 246 (2009) 275–284

0

20

40

60

80

100

0.0 0.4 0.8 1.2 1.6

H2O2 dose (M)

SV

I (m

l/g)

Fig. 5. Effects of alkaline hydrolysis and hydrogen peroxide oxidation on the sludge volume index (SVI), hydrogenperoxide oxidation only (1.6 M H

2O

2) (); alkaline hydrolysis and hydrogen peroxide oxidation (pH 11 + 1.6 M H

2O

2) ().

ment is attributable to the particle size reductionand the viscosity reduction by hydrogen perox-ide oxidation, which are described in Figs. 3 and4, respectively.

4. Conclusion

This paper investigated the effects of alkalinehydrolysis and hydrogen peroxide oxidation onan excess activated sludge reduction, and com-pared the efficiencies of each process and a com-bination of two treatment processes for more ef-fective sludge reduction.

The combined process of alkaline hydrolysisand hydrogen peroxide oxidation leads to the de-struction of sludge particles to be solubilized intosoluble organic products, resulting in TS andTCOD reduction and generation of SCOD. Un-der the conditions of pH 11 and 1.6M H

2O

2 dose,

approximately 49% of the TS content and 69.1%of the viscosity of initial untreated sludge weredecreased, and SCOD/TCOD ratio improved to57.4%. Also 66.7% of the settleability (SVI) isenhanced. The median diameter of sludge particlealso decreased from 34.5 μm to 10.8 μm. As a

result of the experimental investigations, it canbe concluded that the combined process of alka-line hydrolysis and hydrogen peroxide oxidationis efficient for reducing the amount of residualsludge from the biological wastewater treatmentfacilities.

Acknowledgements

This research was supported by Ministry ofScience and Technology of Korea. The authorsdeeply appreciate their financial support.

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