Journal of Industrial and Engineering Chemistry 19 (2013) 197–206

Synergistic effects of sono-alkaline pretreatment on anaerobic biodegradabilityof waste activated sludge

Serkan Sahinkaya a,*, Mehmet Faik Sevimli b

a Department of Environmental Engineering, Nevs ehir University, Nevs ehir, Turkeyb Department of Environmental Engineering, Selcuk University, Konya, Turkey

A R T I C L E I N F O

Article history:

Received 8 May 2012

Accepted 4 August 2012

Available online 10 August 2012

Keywords:

Anaerobic digestion

Pretreatment

Sonication

Sono-alkaline

Ultrasonic

Waste activated sludge

A B S T R A C T

The individual and combined effects of alkaline and ultrasonic pretreatment on both physical and

chemical properties and anaerobic biodegradability of waste activated sludge (WAS) were investigated

comprehensively in this study. The experimental results showed that both disintegration and anaerobic

biodegradability of WAS were significantly improved by the combination of alkaline and ultrasonic

(sono-alkaline) pretreatment. Besides, it was determined that the hydraulic retention time in anaerobic

digester can be shortened by half using this combined pretreatment. However, it was also determined

that sono-alkaline pretreatment was not feasible economically due to its high energy requirement.

Crown Copyright � 2012 Published by Elsevier B.V. on behalf of The Korean Society of Industrial and

Engineering Chemistry. All rights reserved.

Contents lists available at SciVerse ScienceDirect

Journal of Industrial and Engineering Chemistry

jou r n al h o mep ag e: w ww .e lsev ier . co m / loc ate / j iec

1. Introduction

Activated sludge process is commonly used to treat industrialand municipal wastewaters due to their easy operation, hightreatment efficiency and low operating cost. However, productionof excess sludge, which is also called as ‘‘waste activated sludge(WAS)’’, is an inevitable drawback inherent for this process. TheWAS is one of the most serious challenges encountered in thewastewater treatment plants. It should be treated owing to itspathogen content and highly putrescible nature, prior to its finaldisposal. However, its treatment is the major economic problem inthe treatment plants. The treatment and disposal of the WAS costabout half, and even up to 60%, of the total expense of wastewatertreatment [1]. Therefore, the sludge minimization and treatmentare crucial in terms of both environmental and economical aspects.

Anaerobic digestion is a widely utilized technique to treat andminimize the WAS especially in big treatment plants, because ithas some great advantages such as reducing the sludge volume,generating energy-rich gas in the form of methane (CH4) andyielding a nutrient-containing final product. However, theextremely slow hydrolysis of the microorganism cells present inthe WAS is limiting the efficiency of sludge digestion processes.Thus, typical digestion times in the high rate digesters take 20 and

* Corresponding author. Tel.: +90 384 228 10 69; fax: +90 384 228 10 37.

E-mail addresses: serkansahinkaya@gmail.com, sahinkaya@selcuk.edu.tr

(S. Sahinkaya).

1226-086X/$ – see front matter . Crown Copyright � 2012 Published by Elsevier B.V. on be

http://dx.doi.org/10.1016/j.jiec.2012.08.002

more days [2]. To accelerate the rate limiting hydrolysis and toimprove the sludge digestion, various sludge pretreatmentmethods have been widely studied in the previous studies [3].Among these methods, alkaline and ultrasonic (US) pretreatmentsare the most effective sludge disintegration methods. For thisreason, alkaline pretreatment (alkalization), US pretreatment(sonication) and the combination of these methods werecomprehensively studied in the present study.

Alkalization is a commonly examined method owing to its easyoperation, simplicity and high efficiency. Its disintegrationmechanism is based on the disruption of sludge flocs, thedestruction of cell walls and membranes by several ways includingthe saponification of lipids, losing natural shapes of proteins,hydrolysis of RNA and the transfer of extracellular and intracellularpolymeric substances into the aqueous phase by hydroxyl anions(OH�) [4]. Recently, alkalization has been combined with otherpretreatment methods such as microwave [5], thermal [4] andultrasonic [6,7] pretreatment methods to enhance the solubiliza-tion of sludge, before the sludge digestion.

Sonication is a well known method which has been extensivelyinvestigated due to its effectiveness and simple operation. It isbased on the application of US irradiation into the WAS. When thesludge is exposed to US irradiation; hydro-mechanical shear forces,which is the predominant mechanism in sludge disintegration [8],are generated by acoustic cavitation into the WAS. The hydro-mechanical shear forces disintegrate the sludge flocs, break out thecell walls and membranes, and thus, extract the extracellular andintracellular organic substances into the aqueous phase of sludge

half of The Korean Society of Industrial and Engineering Chemistry. All rights reserved.

S. Sahinkaya, M.F. Sevimli / Journal of Industrial and Engineering Chemistry 19 (2013) 197–206198

[9]. The rise in US energy improves the sludge disintegration [7],which is evidenced by a reduction in particle size and an increase insoluble organic matter fractions. As a result of the increment in thesolubility of sludge, the hydrolysis of sludge is accelerated [2].Therefore, both the degradation of organic matters and methaneproduction can be improved in the anaerobic sludge digesters [8].However, since sonication is an energy-intensive process [10], itsmajor disadvantage is its high energy consumption. To lessen itsenergy consumption, sonication can be applied together withalkaline agents [6,7]. The sono-alkaline pretreatment (sono-alkalization), which is the simultaneous combination of US andalkaline pretreatment, aims to enhance the sludge disintegrationby using different disintegration mechanisms of two separatemethods. Thereby, the main objective of the present work is toprovide more insights into the disintegration of sludge by thecombined method, to reduce US energy consumption and toexamine the influences of sono-alkaline pretreatment on subse-quent anaerobic digestion. For these aims, comprehensive experi-ments were conducted in three parts in this study. The individualand combined effects of alkaline and US pretreatment on thedisintegration of sludge were first investigated separately. In thesecond part of experiments, the effects of sonication and sono-alkaline pretreatment on methane production and anaerobicbiodegradability of the sludge were investigated by biochemicalmethane production (BMP) assay in batch mesophilic anaerobicreactors. In the final part, the influences of the combinedpretreatment were experienced with semi continuous anaerobicreactors working at different operational conditions and comparedwith control reactors fed with the raw (untreated) sludge.

2. Materials and methods

2.1. Waste activated sludge (WAS) and anaerobic inoculum

The fresh WAS samples used in this study were obtained fromBasarakavak domestic wastewater treatment plant in Konya,Turkey. The waste sludge samples were taken from the returnactivated sludge line connecting the clarifier to the aeration tank.The raw WAS had a water content of 99.6%, pH of 7.5, totalchemical oxygen demand (tCOD) of 3735 mg/L, soluble chemicaloxygen demand (sCOD) of 55 mg/L, total solid (TS) of 3935 mg/L,volatile solid (VS) of 2450 mg/L, suspended solid (SS) of 3350 mg/Land volatile suspended solid (VSS) of 2230 mg/L. Before theexperiments, the samples were concentrated by gravity settling,the concentrations of which were 1.0% TS content, since it wasfound to be the best solid matter content for both alkalization andsonication in our previous study [9].

For each anaerobic experimental set, the fresh mesophilicanaerobic seed sludge samples were supplied from an anaerobicsludge digester of an industrial wastewater treatment plant(Konya Sugar Factory, Konya, Turkey) using activated sludgeprocess. The anaerobic inoculum used in the BMP assay had a pH of7.5, tCOD of 12,475 mg/L, TS of 23,700 mg/L, VS of 3230 mg/L, SS of20,410 mg/L and VSS of 2805 mg/L; while the anaerobic seedsludge utilized in the semi-continuous digesters had a pH of 7.6,tCOD of 48,050 mg/L, TS of 76,590 mg/L, VS of 12,210 mg/L, SS of65,185 mg/L and VSS of 10,675 mg/L. The WAS and anaerobic seedsludge samples were kept in dark at +4 8C, before their usage.During the maximum 7-day storage time, significant dissolution inthe sludge samples was not observed.

2.2. Preliminary sludge pretreatment studies

US sludge disintegration experiments were performed using anultrasonic homogenizer (Bandelin, Germany), which was equippedwith a TT 13 titanium tip probe (Bandelin, Germany). The

experiments were conducted at 20 kHz fixed frequency, whichis the most effective US frequency for sludge disintegration [8].Ultrasonic power density applied into the sludge was adjusted byinput power setting of the ultrasonic homogenizer. The actualpower delivered to the sludge was not measured. For each USexperiment, 100 mL sludge sample at room temperature was filledin a custom-made pyrex-glass beaker and the US probe wasimmersed at 2 cm in the middle of the WAS [11]. The sludgesamples were sonicated at 0.5, 1.0 and 1.5 W/mL US densities fordifferent periods up to 10 min. During ultrasonic pretreatmentexperiments, since rise in temperature may contribute to thedisintegration of the WAS [12], the sludge temperature was notcontrolled. Temperature and pH of the sludge were measuredbefore and after the experiments. It was determined that thetemperature increased significantly from room temperature up toabout 70 8C depending on sonication conditions; while the sludgepH was changed negligibly. Specific ultrasonic energy (Es) appliedinto the sludge was described as product of the ultrasonic power(P) and the ultrasonic time (t) divided by the sample volume (V)and the initial total solids concentration (TS) and it ranged from 0to 22,500 kJ/kg dried sludge, as given in Eq. (1):

Es ¼P � t

V � TS(1)

Alkalization experiments were performed by using sodiumhydroxide (NaOH) as an alkaline source, because NaOH is one ofthe most effective alkaline agents for the sludge disintegration[12]. These experiments were conducted at 0.01, 0.03, 0.05, 0.08and 0.10 N NaOH dosages for the pretreatment period of 30 min atroom temperature. 250 mL of sludge was mixed at 90 rpm in a jartest apparatus (Velp, Italy). The desired NaOH concentrations inthe sludge were adjusted by using 5 N NaOH solution. At the end ofalkalization period, pH of the alkalized sludge was adjusted toabout 7.5 by using 5 N H2SO4 solution.

According to Jin et al. [6], the simultaneous combination ofalkalization and sonication is the most effective combinationalternative. Thus, sono-alkalization was experienced as a simulta-neous combination of these methods in this work. In thiscombination, the effect of US irradiation period up to 10 min atdifferent US densities (0.5, 1.0 and 1.5 W/mL) and at a constantNaOH dosage (0.05 N) on sludge disintegration was first investi-gated. Then, the influences of varying base dosages in the range of0.01–0.10 N on physical and chemical properties and anaerobicbiodegradability of sludge were examined at US density of 0.5, 1.0and 1.5 W/mL for shorter US irradiation periods (0.5, 1 and2.5 min). In sono-alkalization experiments, the sludge was mixedand immediately sonicated after the addition of the desired dosageof NaOH into 100 mL sludge. Then, the sonicated sludge was keptat room temperature in the remaining part of the pretreatmentperiod. At the end of the total pretreatment period of 30 min, thepretreated sludge was neutralized by using 0.1 and 5 N solutions ofH3PO4.

2.3. Biochemical methane production (BMP) assay

BMP assay was conducted in the batch mesophilic reactors toobserve biodegradability differences resulted from the pretreat-ment conditions. Therefore, following sludge disintegration bysonication and sono-alkalization, the relative increases in biogasand methane productions and the anaerobic biodegradability ofuntreated (raw), sonicated and sono-alkalized sludge sampleswere investigated by the BMP assay in 125 mL screw cap bottlessealed with butyl rubber stoppers and lid.

For the batch anaerobic biodegradability experiments, the BMPassay was conducted according to the procedure defined byEskicioglu et al. [13,14]. Thus, 20 mL of anaerobic seed sludge and

S. Sahinkaya, M.F. Sevimli / Journal of Industrial and Engineering Chemistry 19 (2013) 197–206 199

80 mL of the pre-treated sludge samples were placed into thebottles. A mixture containing equal parts of NaHCO3 and KHCO3

was then added into the reactors to obtain an alkalinity of4000 mg/L (as CaCO3). After the batch reactors were immediatelysealed with stoppers and screw lids, the reactors were purged withnitrogen (N2) gas to remove oxygen (O2) from the system. The BMPassay was carried out in triplicate reactors in an incubator in darkat 37 � 1 8C. Control reactors digesting the raw WAS were set up withthe same methodology; but the sludge was not gone through anypretreatment.

At the beginning of the batch anaerobic tests, gas compositionof the headspace was assumed to be 100% N2 on the zeroth day,since the reactors were closed immediately after purging thebottles. Feeding or wasting from the batch reactors was not done.Total gas production and methane content were measured duringthe operation of reactors once in two days. For proper contact ofseed and substrate, manual shaking was done twice a day. Theexperiments were continued until the gas production becamenegligible. Then, the operations of batch reactors were ceased andthe reactors were opened, since the analyses of tCOD, TS and VS inthe digested sludge were performed.

2.4. Semi-continuous anaerobic sludge digestion

Five 1.2 L laboratory-scale single-stage high-rate glass reactorswith 1 L effective volume were employed to examine the effects ofoptimized sono-alkalization pretreatment on the anaerobic sludgedigestion. The anaerobic reactors were continuously mixed byusing magnetic stirrers. Temperature of the reactors was keptconstant at 37 � 1 8C which is appropriate for mesophilic anaerobicdigestion. Two reactors were operated as control reactors and fedwith the raw WAS, whereas the other three reactors were fed with thesono-alkalized sludge. Control and pretreatment reactors werenamed as Control 1 (C1), Control 2 (C2) and Pretreatment 1 (P1),Pretreatment 2 (P2), Pretreatment 3 (P3). C1 and C2 were thereplicates of the P1 and P2, respectively.

At the beginning of the start-up phase, the reactors were filledwith the anaerobic inoculum and sewage sludge in the ratio ofinoculum (w):sludge (w) was 1:2. Then, they were sealed withrubber stopper and were immediately purged with N2 to eliminateO2 from the system, by using ports on the top of the reactors.Initially, anaerobic seed and activated sludge were mixed andacclimated for 7 days without any feeding/wasting. After theacclimation period, daily wasting and feeding were started. Theoperational parameters in the anaerobic digesters were hydraulicretention time (HRT) (which was equivalent to solids retentiontime (SRT)) and organic loading rate (OLR). The P1 and C1 wereoperated at a HRT of 14 days and at an OLR of 1.0 kg VS/m3, whilethe HRT and OLR used in the C2 and P2 were 7 days and 1.0 kg VS/m3, respectively. The HRT and OLR of C3 were set at 7 days and0.5 kg VS/m3, respectively.

During the operation of the reactors, the reactor pH, temperature,alkalinity and biogas production were daily analyzed. Methane gasvolume, mixed liquor volatile suspended solids (MLVSS), TS, VS,tCOD and total volatile fatty acids (tVFA) were measured in every 2days. VS and tCOD concentrations of the reactors were monitored inorder to determine the assessment of steady state. When thevariation in the effluent VS and tCOD data was smaller than 10%, itwas assumed that the reactors were reached the steady stateconditions. After reaching steady state and collecting enough steadystate data, operating of reactors was ceased.

2.5. Kinetic study

With the same methodology of the BMP assay, anaerobickinetic studies were performed to determine the influences of

sono-alkalization on the rate and efficiency of anaerobicbiodegradability (based on the tCOD and VS removal). For thisaim, kinetic studies were carried out in eight batch reactors withand without pretreatment. When the methane production wasnegligible, the kinetic study was ceased.

2.6. Analyses

Sludge samples were utilized directly for the measurements ofsCOD (after filtration through 0.45 mm), tCOD, SS and TS byfollowing the Standard Methods [15]. The VS and VSS wereanalyzed according to the method of Tiehm et al. [16]. The pH andtemperature of sludge were determined by a multi-parameterinstrument (WTW, Germany).

To evaluate the sludge disintegration efficiencies, the disinte-gration degree (DDCOD) is calculated as the ratio of sCOD increaseby pretreatment to the maximum possible sCOD increase, as givenin the following Eq. (2) [17]:

DDCOD ð%Þ ¼ sCODPr � sCOD0

tCOD � sCOD0

� �� 100 (2)

where sCODPr is the sCOD of the pretreated sludge (mg/L), sCOD0 isthe sCOD of the raw sludge (mg/L) and tCOD is the tCOD of the rawsludge (mg/L).

Right before and after the sludge pretreatment experiments,microscopic observations were carried out by using a lightmicroscope (Olympus, Germany) to observe the impacts ofpretreatment used in this study on microorganism flocs andfilamentous bacteria which have a significant negative effect onthe physical properties of the WAS.

In the batch and semi-continuous anaerobic reactors, gasproductions were measured with liquid displacement method. Themeasurement of total gas volume was performed by passing itthrough a liquid containing 2% (v/v) H2SO4 and 10% (w/v) NaCl[18]. Methane gas volume was determined by using a liquidcontaining 3% NaOH to scrub out the carbon dioxide (CO2) from thebiogas [19]. Total alkalinity was analyzed by titrating a samplewith 0.1 N of standard H2SO4 solution to a pH of 4.3 [15]. tVFAconcentration was measured simultaneously by titrimetric method[20]. During the anaerobic sludge digestion in the semi-continuousreactors, sulfate (SO4

2�) was analyzed by gravimetric methodaccording to the Standard Methods [15], while ammonia (NH3) andortho-phosphate were measured using Hach Lange test kits(Germany).

3. Results and discussion

3.1. Preliminary sludge pretreatment studies

3.1.1. The effect of alkalization on sludge disintegration

Since alkaline dosage is the most important operatingparameter which influences both the disintegration efficiencyand operating cost of alkalization, the effect of NaOH dosage wasexamined separately to determine its possible contribution to thecombined pretreatment. With increase in the base dosage from0.01 N to 0.10 N, pH of the alkalized sludge samples were raisedfrom 7.5 to 8.7, 9.7, 12.1, 12.4 and 12.6, respectively. With raisingthe sludge pH, the disintegration degree of sludge (DDCOD) was alsoincreased to 3.9, 7.5, 14.4, 15.3 and 16.1% in the dosage range from0.01 to 0.10 N, respectively. Thus, it was observed that the sludgedisintegration was enhanced significantly until reaching pH of 12.1(at 0.05 N NaOH). However, the higher dosages have a limitedinfluence on the sludge disintegration. Therefore, the optimalNaOH dosage was found to be 0.05 N NaOH. This result wascompatible with the results of the study of Li et al. [12] who found

Fig. 1. Effect of sonication period on the disintegration of sludge in sonication and sono-alkalization.

S. Sahinkaya, M.F. Sevimli / Journal of Industrial and Engineering Chemistry 19 (2013) 197–206200

that the optimum base dosage was 0.05 N for 0.5-h alkalinepretreatment. That is why the first part of sono-alkalizationexperiments was performed at a fixed NaOH concentration of0.05 N.

3.1.2. The effect of sonication on sludge disintegration

Individual sonication experiments were carried out to investi-gate the effect of sonication period on the sludge disintegrationand to compare the disintegration efficiencies of sonication andsono-alkalization. During US pretreatment of WAS, the disintegra-tion of sludge was improved steadily, as shown in Fig. 1. USpretreatment at the densities of 0.5, 1.0 and 1.5 W/mL resulted in aconsiderable increment in the sCOD concentration from 65 to 500,825, 850 mg/L in the first 2.5 min and 1330, 1695, 1890 mg/L in thetotal sonication period of 10 min, respectively. The DDCOD valueswere calculated as 5.9, 10.3 and 10.6% for 2.5 min sonication and17.1, 22 and 22.7% for 10 min sonication at 0.50, 1.00 and 1.50 W/mL densities. Thus, the sludge was rapidly disintegrated in the first2.5 min of sonication, whereas the disintegration rate was sloweddown in the next 7.5 min. This rapid disintegration achieved in thefirst 2.5 min was likely resulted from the rapid cavitational effectarising from the collapse of powerful transient bubbles generatedin fractions of microseconds close to microbial cells [21]. Inaddition, sonication at higher densities in shorter periods wasdetermined as a more effective alternative for US sludgepretreatment [22,23]. Therefore, these outcomes suggested thatUS pretreatment could be performed in periods shorter than2.5 min to reduce both capital and operating costs of thepretreatment process.

3.2. The synergistic effects of sono-alkalization

3.2.1. The effect of sono-alkalization on sludge disintegration

In this study, the influence of ultrasonic irradiation period in thecombined pretreatment was first investigated to develop a processconsuming less energy. For this reason, the first part of sono-alkalization experiments was conducted at the constant 0.05 NNaOH dosage.

As seen from Fig. 2, the degrees of sludge disintegration wereapparently higher in the combined method, compared to sonica-tion alone. With alkalization alone at 0.05 N NaOH for 0.5 h, theDDCOD value was only 14.4%. After sonication of sludge at 0.5, 1.0and 1.5 W/mL, the DDCOD values were reached to 5.9, 10.3 and10.6% in 2.5 min and 17.1, 22 and 22.7% in 10 min, respectively.However, by sono-alkalization at 0.5, 1.0 and 1.5 W/mL densitieswith 0.05 N NaOH dosage, the DDCOD values were increasedsignificantly to 22.8, 26.7 and 31.2% with US irradiation of 2.5 min,while 32.7, 38.8 and 41.2% with US irradiation of 10 min. Thus, theefficiency of sono-alkalization was higher than the total of thesetwo different methods. With prolonging US irradiation period, thesludge disintegration was enhanced by synergistic effects. The

mechanism of the synergistic effects may be explained as that thesludge flocs were first disintegrated by hydro-mechanical shearforces and thus, microbial cells released from the disrupted sludgeflocs were more effectively exposed to OH� ions. Besides, sono-alkalization with US irradiation of 2.5 min resulted in a moresludge disintegration, compared to 10 min sonication alone.Therefore, it can be said that it is possible to provide a significantreduction in the US energy consumption by combining sonicationwith alkalization. On the other hand, the sludge disintegration wasrapidly achieved with US irradiation of 2.5 min which correspondsto rapid disintegration stage, and then, the sludge solubilizationrate was decelerated. Therefore, US irradiation of 2.5 min in sono-alkalization was found enough to disintegrate the sludge.Consequently, a comprehensive optimization was carried out atvarying US irradiation periods up to 2.5 min in the second part ofsono-alkalization experiments.

Under different alkaline and ultrasonic conditions, the syner-gistic effects were experienced in order to optimize all processparameters. As shown in Fig. 2, sono-alkaline pretreatmentresulted in a sharp increase in sCOD concentration even in muchless US energy consumption, compared to US pretreatment alone.The increase in US density from 0.5 to 1.0 W/mL enhanced the USsludge disintegration, however further rise in US density changednegligibly the disintegration of WAS. On the other hand, while theincrease in the NaOH concentration up to 0.05 N improved thedisintegration efficiency; further dosages caused an insignificantenhancement in the sludge solubilization (Fig. 2). However, it isneeded to optimize the process parameters by taking theincrement in methane gas production and the US energyconsumption into account. Therefore, BMP assay was performedto determine the most effective operating parameters for thesecond part of sono-alkalization experiments.

3.2.2. The effect of sono-alkalization conditions on biodegradability of

WAS

The energy consumption in US pretreatment is the mainparameter that determines both the process performance and thetotal cost of sludge treatment. Hence, the potential methaneproduction and biodegradation of pre-treated WAS relative tountreated sludge were investigated in triplicate batch mesophilic(at 37 � 1 8C) reactors with BMP assay. The biodegradation of sludgewas evaluated with the reductions of VS and tCOD. The aim of BMPassay was to choose the most effective operating conditions based onthe anaerobic biodegradability of WAS and the energy consumption.

During 80 days of digestion period in the batch reactors, thecomprehensive results of cumulative total biogas and methaneproductions are summarized in Table 1 together with US specificenergy (Es) consumptions. As a result of the increase in the sludgesolubilization, the methane production and sludge biodegradabili-ty were promoted in the batch reactors with pretreatment.However, the relative increments in methane production and

Fig. 2. Effect of NaOH dosages on sCOD and DDCOD in sono-alkalization at 0.5 (a), 1.0 (b) and 1.5 W/mL (c) US densities.

S. Sahinkaya, M.F. Sevimli / Journal of Industrial and Engineering Chemistry 19 (2013) 197–206 201

sludge biodegradability were negligible with higher US densitythan 1.0 W/mL and/or higher base dosages than 0.05 N, whencompared with Es applied to the sludge (Table 1). Therefore, theoptimal conditions for sono-alkaline pretreatment were deter-mined as the combination of ultrasonic irradiation at 1.0 W/mL for1 min and alkalization at 0.05 N NaOH for 30 min. Under thesedetermined optimum conditions, the highest relative increase inmethane production (15.5%) at the lowest energy consumptionwas achieved at the end of 80-day operational period, compared toraw sludge. The relative VS and tCOD reductions were 37.1% and47.7% as well, respectively. Consequently, the WAS samples to befed into the semi-continuous anaerobic digesters were decided tobe pre-treated by sono-alkalization under the optimum conditionsdetermined by the results of BMP assay.

3.3. Anaerobic kinetic assays

As seen from Fig. 3, sono-alkaline pretreatment clearlyaccelerated and enhanced the sludge digestion process. This wasan expected result due to the remarkable increase in the sludgesolubilization with the pretreatment, prior to the anaerobicdigestion. The pre-treated sludge was therefore digested morerapidly in the first 10 days of digestion and the maximum substrateutilization occurred in this initial period of digestion (Fig. 3). After10th day of incubation period, the degradation rate of pre-treatedsludge was slowed down. On the other hand, as expected, thedigestion rate of raw sludge was slower in the first 10-day period,

because the hydrolysis of raw sludge was the rate limiting step inthe anaerobic digestion. However, the digestion rate of raw sludgewas improved as sludge was hydrolyzed with prolonging digestiontime. In addition, by improving the hydrolysis phase of anaerobicdigestion, sono-alkalization increased the organic matter degra-dation in anaerobic sludge digestion, as shown in Fig. 3.

Due to the increase in tCOD and VS reduction, cumulativebiogas and methane productions in the batch reactors alsoincreased (Fig. 4). The methane production in the pre-treatedreactor was about 24% more as a result of a higher degree of sludgedisintegration.

For the determination of the kinetics of tCOD and VS removals,zero, first and second order kinetics and Monod model wereapplied to the experimental data which are shown in Fig. 3. Thecoefficients of determination (R2) are summarized in Table 2. Thus,it was found that Monod model was appropriate for tCOD and VSremovals in the pre-treated reactors and tCOD removal in thecontrol reactor, while tCOD removal in the control reactor obeyedthe zero order kinetics.

3.4. The effects of sono-alkalization on semi-continuous anaerobic

digesters

It was assumed that steady state was achieved in the reactors atthe time when the variations in tCOD and VS were less than 10%.P1, P2 and P3 digesters were considered as they were at steadystate after 21st, 35th and 21st days, respectively; hence the data

Table 1The effect of pretreatment conditions on biodegradability of WAS in the batch reactors.

US density (W/mL) Sonication

time (min)

Es (kJ/kg DS) NaOH dosage (N) Relative increases in VS reduction (%) tCOD

reduction (%)

Biogas (%) Methane (%)

0.50 0.5 1500 0 1.90 1.91 33.6 40.8

0.01 5.1 5.6 34.5 41.4

0.03 5.6 5.0 34.9 42.6

0.05 6.6 5.0 35.4 44.4

0.08 8.4 8.0 35.6 45.0

0.10 8.5 7.9 35.7 46.2

1.0 3000 0 3.4 3.4 34.3 41.4

0.01 4.2 4.3 35.3 42.3

0.03 5.1 5.3 35.8 44.0

0.05 7.6 7.4 35.7 45.1

0.08 9.6 9.3 35.9 46.2

0.10 10.3 9.7 36.5 46.4

2.5 7500 0 5.1 4.0 35.3 43.0

0.01 6.1 5.5 36.5 44.4

0.03 6.1 5.7 36.6 45.0

0.05 9.6 9.7 36.4 46.2

0.08 9.9 9.5 36.6 47.1

0.10 13.2 12.4 37.4 47.6

1.00 0.5 3000 0 6.0 5.3 34.2 41.7

0.01 5.1 4.9 29.0 42.5

0.03 7.8 7.2 35.5 42.8

0.05 10.2 9.8 36.5 45.8

0.08 12.3 11.2 38.6 47.6

0.1 13.2 12.3 37.8 49.0

1.0 6000 0 6.6 6.0 34.4 42.6

0.01 6.7 7.6 34.7 43.4

0.03 11.1 10.9 36.2 43.8

0.05 15.3 15.5 37.1 47.7

0.08 16.2 16.0 37.3 49.0

0.10 17.6 17.4 38.0 50.2

2.5 15,000 0 7.6 7.8 35.7 43.8

0.01 7.8 8.2 35.5 45.1

0.03 12.3 11.8 37.4 45.5

0.05 16.3 15.9 37.8 49.2

0.08 16.7 16.3 37.7 50.0

0.10 18.2 18.5 38.3 50.6

1.50 0.5 4500 0 6.6 6.9 34.7 42.3

0.01 8.4 9.4 35.2 43.0

0.03 10.7 9.4 35.9 43.9

0.05 12.3 11.8 36.5 46.7

0.08 14.9 12.7 37.2 48.0

0.10 15.7 14.5 38.0 49.8

1.0 9000 0 7.3 6.8 35.3 42.9

0.01 10.4 10.5 36.0 43.8

0.03 12.6 12.5 36.6 44.9

0.05 18.1 17.1 37.8 48.8

0.08 17.5 17.2 37.5 50.5

0.10 18.2 17.2 38.4 51.0

2.5 22,500 0 9.2 9.3 35.9 43.8

0.01 11.2 10.4 36.8 45.7

0.03 13.7 13.2 37.7 46.4

0.05 19.3 18.8 38.1 49.7

0.08 18.6 17.5 37.9 50.8

0.10 19.0 18.2 38.7 51.3

Fig. 3. The removal of tCOD and VS in the batch reactors with and without pretreatment.

S. Sahinkaya, M.F. Sevimli / Journal of Industrial and Engineering Chemistry 19 (2013) 197–206202

Fig. 4. The cumulative methane productions in the batch reactors with and without

pretreatment.

S. Sahinkaya, M.F. Sevimli / Journal of Industrial and Engineering Chemistry 19 (2013) 197–206 203

collected between days 21st and 52nd, 35th and 63rd, and 21st and38th for Set 1, Set 2 and P3 were evaluated to compare theperformances of reactors with and without sono-alkaline pre-treatment.

The effluent tCOD and VS concentrations of each reactor areshown in Figs. 5 and 6, respectively. Since the pre-treated andcontrol reactors were fed with the same concentration of VS andtCOD during the operations of reactors, the differences observed inthe reactors were only due to sono-alkaline pretreatment. Thepercentages of tCOD and VS reductions and methane productionyields were calculated based on the steady state data of influent

Fig. 5. tCOD and VS reductions in the

and effluent concentrations. P1 and C1, operated with a HRT of 14days and an OLR of 1 kg/m3, were reached their steady state within21st and 35th days. These reactors were run for 35 days (except foracclimation period of 7 days). As shown in Table 3, the tCOD and VSreductions were 43.9 and 35.2% in P1 and 50.6 and 43% in C1,respectively. P2 and C2 were run with a HRT of 7 days and an OLR of1 kg/m3. P2 and C2 were assumed to attain their steady statewithin 35th and 42nd days, respectively and operated for 63 days.In P2 and C2, tCOD reductions were 43.9 and 35.2%, while 43.7 and33.5% were observed in VS reductions, respectively. P3, operatedwith a HRT of 7 days and an OLR of 0.5 kg/m3, were reached asteady state within 21 days and operated for 38 days. The tCOD andVS reductions in P3 were 47.8% and 46.7%. Therefore, it wasdetermined that the pretreatment improved obviously theperformance of anaerobic digester, as a result of the increase inthe sludge solubilization. Besides, HRT can be shortened from 14days to 7 days without reducing the treatment efficiency with theapplication of sono-alkaline pretreatment. On the other hand, thedaily gas productions from the beginning of steady state till the endof the reactor operation were much more in the pre-treatedreactors than those of control reactors. Methane production yieldswere also increased and presented in Table 3, based on the tCODand VS reductions.

The concentrations of carbohydrate, protein, COD, NH3-N andPO4-P in soluble phase were examined at steady state, before thereactors were opened. The experimental results are given in Table 3.The hydrolysis products such as carbohydrate and protein, whichare the main components of COD, might be much more in the

pre-treated and control reactors.

Table 2The determination coefficients of the kinetic models.

tCOD VS

Pretreated reactor Non-treated reactor Pretreated reactor Non-treated reactor

Monod model 0.9741 0.9812 0.9934 0.9689

Zero order kinetic 0.8868 0.9465 0.9210 0.9286

First order kinetic 0.9443 0.9624 0.9506 0.9469

Second order kinetic 0.9805 0.9732 0.9718 0.9626

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pre-treated digesters owing to two reasons: (1) the physico-chemical hydrolysis of sludge with pretreatment and (2)extremely slow hydrolysis of sludge in the control reactors,contrast to pre-treated reactors. However, sCOD concentrations inthe effluent were less because of the increase in the bioactivity ofthe pre-treated reactors, compared to reactors without pretreat-ment. NH3-N concentrations were increased due to the hydrolysisof more nitrogenous organic matters in the pre-treated reactors[24]. Similarly, PO4-P concentrations were higher due to bothhydrolysis of more organic matters and usage of H3PO4 toneutralize the sludge in sono-alkaline pretreatment. Besides,

Fig. 6. Daily and cumulative methane productio

SO42� concentrations in all reactors were less than 150 mg/L and

could not affect toxically in the reactors.

3.5. Economical analysis

As it can be concluded from the results presented above, thecombined pretreatment has many advantages for sludge biodeg-radation process. However, it is necessary to prepare benefit costanalysis for environment friendly and economical pretreatment tobe applied in a wastewater treatment plant since US pretreatmentis a high energy expending process.

ns in the pre-treated and control reactors.

Table 3Steady state data for pretreated and non-treated semi-continuous reactors.

Parameters P1 C1 Relative improvement (%) P2 C2 Relative improvement (%) P3

Daily methane production (mL) 146.2 109.7 33.3 111.2 89.5 24.2 111.7

Daily biogas production (mL) 209.2 192.2 8.8 169.2 152.1 11.2 162.2

Methane content (%) 69.9 57.2 22.2 65.8 58.9 11.7 68.7

VS reduction (%) 50.6 43 17.6 43.7 33.5 30.4 46.7

Methane yield (mL CH4/g VSeff) 761.0 583.8 30.4 435.7 318.4 36.8 349.8

tCOD reduction (%) 50.4 44.4 13.5 43.9 35.2 24.7 47.8

Methane yield (mL CH4/g tCODeff) 409.3 337.4 21.3 276.1 214.0 29.0 182.2

Carbohydrateeff (mg/L) 40 20 – 55 25 – 55

Proteineff (mg/L) 75 55 – 90 70 – 95

sCODeff (mg/L) 225 325 – 260 380 – 270

NH3-N (mg/L) 470 310 – 435 260 – 260

Ortho-phosphate (mg/L) 85 55 – 100 65 – 90

Table 4Summary of cost analysis for baseline and upgraded situation.

Baseline situation Upgraded situation

PS WAS Total PS WAS Total

Capital cost (s) 0 0 0 0 320.000 320.000

Operating costs (s/year)

US irradiation 0 0 0 0 17.515 17.515

Alkaline application 0 0 0 0 8.181 8.181

Transportation cost 16.412 13.150 29.562 16.412 12.139 28.551

Disposal cost 16.412 13.150 29.562 16.412 12.139 28.551

The sum of operating costs (s/year) 32.824 26.300 59.124 32.824 49.974 82.798

Biogas revenue (s/year) 14.202 13.665 27.867 14.202 18.919 33.121

Sludge treatment cost (s/year) (=biogas revenue � the sum of operating cost) S31.257 S49.677

S. Sahinkaya, M.F. Sevimli / Journal of Industrial and Engineering Chemistry 19 (2013) 197–206 205

In this section of the study, economical applicability ofcombined pretreatment was investigated by using experimentaldata presented in Table 3 for control reactor without pretreatment(C1) and reactor with pretreatment (P1). Cost analysis wasperformed by predicating an upgraded sono-alkaline pretreatmentfor a wastewater treatment plant which was assumed to serve to apopulation equivalent to 50,000 people. According to thisassumption, WAS which is more difficult to biodegrade will beapplied pretreatment, while primary sludge (PS) will not be gonethrough any pretreatment.

In order to perform benefit cost analysis, first of all, it is necessaryto classify expenses and inputs. Since same anaerobic sludge digesterswill be processed under same conditions in both situations by thisassumption, first investment and operating costs of sludge digestersare not included in accounts. According to this, main expenses in theplant without pretreatment are transportation and disposal of sludge.When setting up pretreatment unit, operating costs including energyand chemicals (used for pretreatment) together with the cost fordisposal and transportation of sludge are taken into account. In both ofthe situations, main input in benefit cost analysis is CH4 gas producedas a result of anaerobic biodegradation. When pretreatment is applied,decreasing the mass of digested sludge is another important incomefor the reactor with pretreatment. In these calculations, Euro (s) wasused as the unit of currency.

For the combined pretreatment of sludge, it was accepted to usea US reactor with 25 L capacity and 30 kW power at 20 kHzconstant frequency. First investment cost of this reactor havingdecadal life is 300,000 s [25]. Moreover, since alkaline dosing willbe applied to the sludge at the entrance of US reactor, if it isconsidered that the sludge coming out of US reactor will be keptwaiting in a tank and will be neutralized in another tank at the endof 30-min pretreatment process, one each dosage pumps arerequired for acids and bases and one sludge pump is required totransfer entire mixed two tanks and sludge to anaerobic digesters.Total set up cost of these support units was expected to be

20,000 s. Thus, first set up cost of pretreatment was calculated as320,000 s.

It was assumed that digested sludge will be transferred to aplace that is 10 km away from the plant and it will be spread on thearea. The cost for transfer of sludge and spreading it on the areawas accepted as 50 s/tons of sludge [26]. The energy potential ofmethane gas produced in anaerobic digesters was taken as6.5 kWh/m3 [27]. In order to calculate energy cost, unit price ofelectricity was assumed as 0.07 s/kWh. Moreover, the amount ofmethane gas produced during digesting of PS was supposed to be20% more when compared to WAS [26].

The results of benefit cost analysis for both baseline (withoutpretreatment) and upgraded (with pretreatment) situations aregiven in Table 4. It was determined with this benefit cost analysisthat total operating cost of setting up a pretreatment for sono-alkaline process will increase from 31,257 s to 49,677 s and thiscannot be compensated by the increase in methane gas aftersetting up a pretreatment. Therefore, application of pretreatmentwas found feasible in terms of economy. This unexpected resultmay result from carrying out experimental studies in bench-scalereactors having small volumes.

4. Conclusion

In this comparative study, alkalization, sonication and sono-alkalization were comprehensively investigated to develop a moreeffective sludge pretreatment. Based on the experimental results,the following conclusions are obtained:

(I) Although alkalization and sonication are powerful sludgepretreatment methods, the combination of these two methodssignificantly improves the sludge disintegration due to thesynergistic effects of these different disintegration mecha-nisms. The efficiency of sono-alkalization was therefore morethan the total efficiencies of alkalization and sonication

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individually. The experimental results of sono-alkalinepretreatment showed that it is also possible to improve thesludge disintegration at even lower US energy consumption,compared to sonication alone.

(II) The anaerobic biodegradation in the batch reactors wasimproved with pretreatment. However, the increase in USdensity and/or NaOH concentration did not cause a significantincrease in the methane production and organic matterdegradation. Hence, sono-alkalization was optimized by takinginto account the most increment in methane production atlowest US energy consumption. Thus, the optimum conditionsof sono-alkalization were determined as the US irradiation at1.0 W/mL density for 1 min and alkalization at 0.05 N NaOHdosage in the total pretreatment period of 30 min. Under theseconditions, the degree of sludge disintegration was 24.4%.

(III) It was determined with the kinetic assays that the sono-alkalization obviously accelerated the hydrolysis of sludge andso improved the anaerobic biodegradability.

(IV) Since the significant improvement in sludge solubilizationwas obtained by sono-alkaline pretreatment, the dailymethane productions were increased by 33.3 and 24.2% inthe pre-treated semi-continuous reactors with hydraulicretention times (HRTs) of 14 and 7 days, while the VS andtCOD reductions were enhanced by 17.6 and 30.4%, and 13.5and 24.7%, respectively. Thus, sono-alkalization significantlyimproved the anaerobic digestion process. In addition, the HRTcan be also shortened by half and so, the capital cost ofanaerobic digesters can be reduced considerably.

(V) Based on the economical analysis, sono-alkalization was foundto be unfeasible owing to their high energy requirements,compared to the conventional (control) reactor.

Acknowledgments

This work was supported by Scientific Research Fund (BAP) ofSelcuk University (grant number: 08101005). This study wasderived from Ph.D. thesis of Serkan Sahinkaya.

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