microwave, ultrasonic and chemo-mechanical pretreatments for enhancing methane potential of pulp...

Bioresource Technology 102 (2011) 7815–7826

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Bioresource Technology

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Microwave, ultrasonic and chemo-mechanical pretreatments for enhancingmethane potential of pulp mill wastewater treatment sludge

Mithun Saha a, Cigdem Eskicioglu a,⇑, Juan Marin b

a School of Engineering, University of British Columbia, Okanagan Campus, Kelowna, BC, Canada V1V 1V7b Department of Civil Engineering, University of Ottawa, Ottawa, ON, Canada K1N 6N5

a r t i c l e i n f o a b s t r a c t

Article history:Received 22 February 2011Received in revised form 10 June 2011Accepted 13 June 2011Available online 2 July 2011

Keywords:Sludge pretreatmentMicrowaveUltrasoundMicroSludge�

Anaerobic digestion

0960-8524/$ - see front matter � 2011 Elsevier Ltd. Adoi:10.1016/j.biortech.2011.06.053

⇑ Corresponding author. Tel.: +1 250 807 8544; faxE-mail addresses: [email protected] (M.

.ca (C. Eskicioglu), [email protected] (J. Marin).

Microwave (2450 MHz, 1250 W), ultrasonic (20 kHz, 400 W) and chemo-mechanical (MicroSludge� with900 mg/L NaOH followed by 83,000 kPa) pretreatments were applied to pulp mill waste sludge toenhance methane production and reduce digester sludge retention time. The effects of four variables(microwave temperature in a range of 50–175 �C) and sonication time (15–90 min), sludge type (primaryor secondary) and digester temperature (mesophilic and thermophilic) were investigated. Microwavepretreatment proved to be the most effective, increasing specific methane yields of WAS samples by90% compared to controls after 21 days of mesophilic digestion. Sonication solubilized the sludge sam-ples better, but resulted in soluble non-biodegradable compounds. Based on the laboratory scale data,MicroSludge� was found the least energy intensive pretreatment followed by sonication for 15 min alter-native with net energy profits of 1366 and 386 kWh/tonne of total solids (TS), respectively. Pretreatmentbenefits were smaller for thermophilic digesters.

� 2011 Elsevier Ltd. All rights reserved.

1. Introduction

Pulp and paper is the third largest environmental polluter inUSA and Canada by releasing well over a 100 million kg of toxicpollution each year. A typical Canadian pulp mill produces an aver-age of 40 oven-dry tonnes of sludge per day, which is dewateredand then either landfilled or incinerated (Statistics Canada,2002). However, the treatment and ultimate disposal of pulp millexcess sludge generated from aerobic wastewater treatment pro-cesses currently represent a rising challenge due to more stringentrequirements regarding landfilling, ocean disposal, agriculturaluse, and incineration. Therefore, there is a considerable recent ef-fort to explore and develop strategies and technologies for reduc-ing municipal and industrial sludge excess pulp mill sludgeproduction in the wastewater line (energy uncouplers, alternatingoxic and anoxic environments), sludge line (pretreatment methodsbefore anaerobic digestion) and final disposal line (incinerationand pyrolysis) at the wastewater treatment plants (WWTPs).Among these technologies, recently advanced anaerobic digestionwith emerging pretreatment technologies has created interest forenhanced biogas/methane production (Mahmood and Elliott,2006; Deublein and Steinhauser, 2008).

ll rights reserved.

: +1 250 807 8850.Saha), cigdem.eskicioglu@ubc

In the last decade, different pretreatment technologies havebeen proposed and tested for primary and secondary sludges gen-erated mostly by municipal treatment plants. Secondary or wasteactivated sludge (WAS) contains exocellular polymeric substances(EPS) and microbial cells that are resistant to anaerobic digestiondue to slow and incomplete hydrolysis. Therefore, large anaerobicdigesters needed to provide long sludge retention times (SRTs) forWAS treatment are capital intensive and not economic restraining.To overcome this limitation, studies have focused on the sludgeline processes to disrupt the WAS floc structure and microbial cellmembranes by external forces, such as mechanical (Baier andSchmidheiny, 1997), chemical (Chiu et al., 1997), ultrasonic (Hoganet al., 2004; Wood et al., 2009), enzymatic (Barjenbruch andKopplow, 2003), and thermal (Dereix et al., 2006) methods andcombinations (Valo et al., 2004; Dogan and Sanin, 2010) to increasethe soluble organic material available for anaerobic digestion. Pre-treatment of WAS can enhance the methane production for bothmesophilic (43–145%) and thermophilic (4–58%) anaerobic diges-tion conditions (Hamer et al., 2004; Eskicioglu et al., 2007; Doganand Sanin, 2010). Previous studies concluded that the major impactof pretreatment would be in a low-rate system such as a meso-philic digester. Thermophilic digestion is usually more efficient atvolatile solids (VS) reduction and methane production than meso-philic digestion. Therefore, a reduced benefit of pretreatment couldbe expected for thermophilic systems (Gavala et al., 2003). Theadvantage of sludge solubilization prior to anaerobic treatment is2-fold. First, increasing the amount of released soluble substrate

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7816 M. Saha et al. / Bioresource Technology 102 (2011) 7815–7826

significantly enhances volatile fatty acids (VFA) generation forimproved gas production during anaerobic digestion. Second, pre-treatment decreases the viscosity of the sludge, permitting greatersolids concentration to be fed to an anaerobic digester. Highersolids concentration in the feed either enables increased digestiontimes in an existing digester or allows for a smaller digestervolume (Elliot and Mahmood, 2007).

As one of the newest sludge pretreatment technologies, micro-wave (MW) pretreatment has been previously tested for municipalWAS. Recent studies indicate that irradiation at 2.45 GHz can effec-tively break down the EPS and divalent cation network in biosolids(Eskicioglu et al., 2007). In addition to thermal effect, MWs can alsocause an athermal effect by polarizing macromolecules, aligningwith the electromagnetic field poles that may cause the possiblebreakage of hydrogen bonds (Loupy, 2002; Eskicioglu et al.,2008). As a result of enhanced sludge disintegration and hydroly-sis, MW irradiation increases the rate/extent of anaerobic diges-tion, improves dewaterability (Park et al., 2004; Eskicioglu et al.,2007; Dogan and Sanin, 2010), and inactivates fecal coliformsand Salmonella spp. to produce Class A sludge (Pino-Jelcic et al.,2006).

Microwave irradiation has been studied at both below andabove boiling temperatures with kitchen type and industrial typeMW units with high temperature/pressure vessels, respectively.Under boiling temperatures with municipal sludges, both batchand continuous flow anaerobic digestion were studied, yieldingimproved digestion (20–30% increase in biogas production) anddewaterability (40% increase in rate of dewaterability quantifiedby capillary suction timers) with MW pretreatment at 96 �C com-pared to controls (Park et al., 2004; Eskicioglu et al., 2007). Morerecent studies with a bench-scale industrial MW unit equippedwith pressure-sealed vessels at 175 �C achieved 31% more biogasand dewaterability of pretreated municipal sludge after digestionwas enhanced by 75% (Eskicioglu et al., 2009).

Sonication has been used for many years in research laborato-ries to disrupt cellular matter. Sonication has been investigatedto solubilize municipal sludge to improve anaerobic digestion overthe last decades. So far, sonication of municipal biosolids have beenstudied at lab-scale (Bougrier et al., 2006), pilot-scale, and full-scale (Xie et al., 2007). It has been shown to be effective at solubi-lizing organic matter, as well as improving biogas production(Grönroos et al., 2005; Bougrier et al., 2007; Wood et al., 2009)To the best of our knowledge, ultrasound has been used only once,recently at the lab scale, to disintegrate two different (a sulfite anda kraft pulp mill) Canadian pulp mill secondary sludges (Woodet al., 2009). At 20 kHz and 1 W/mL sonication intensity, the solu-ble to total COD ratio increased from 11% (control, not pretreated)to 23% for sulfite mill and from 1.3% (control) to 5.0% for kraft millsecondary sludges. Among conventional heating, chemical andultrasonic pretreatment methods, sonication for 30 min and 1 W/mL achieved the lowest level of solubilization (Wood et al.,2009). In this study, the sonication time (30 min) and intensity(1 W/mL) was constant which did not allow researchers to reporton a large spectrum of pretreatment conditions.

MicroSludge� is a chemical and mechanical sludge pretreat-ment process which claims to result in more complete and rapidVS destruction. MicroSludge uses alkaline pretreatment (900 mg/L NaOH for 1 h) to weaken the cell walls, and a high pressurehomogenizer (pressure � 12,000 psi) for a sudden pressure changeto lyse the cells of WAS prior to anaerobic digestion. MicroSludgewas trialed at the Chilliwack WWTP (BC, Canada) and resulted inimproved VS reduction in mesophilic digesters by 18 percentagepoints from 60% to 78% (Rabinowitz and Stephenson, 2005).

The pretreatment technologies above have been applied for mu-nicipal sludges. A difficult to degrade industrial waste, pulp millwaste sludge contains residual cellulose, hemi-cellulose, lignin

and chemical components from the pulping process (Kyllönenet al., 1988). Since there are significant compositional differencesbetween municipal and pulp mill sludges, a systematic comparisonof pretreatment technologies using pulp mill primary sludge andsecondary sludge is required to assess the viability of using high-rate anaerobic digesters to reduce sludge volume while producingmore methane. Consequently, the objective of this study is to com-pare the impact of three pretreatment technologies (microwave,ultrasound and MicroSludge�) on the extent of anaerobic digestionof waste sludges from a bleached chemo–thermo mechanical pulpmill (BCTMP) in Quesnel, Canada.

2. Methods

2.1. Waste sludge samples

Pulp mill WAS and WAS + PS (40:60% v/v) mixed sludge sam-ples were collected from Quesnel River Pulp (QRP) Mill Company,BC, Canada. QRP is a BCTMP mill, producing pulp for printing andwriting papers, specialty papers, folding boxboard, higher qualitymechanical printing papers (MPP), and tissue and towel. QRP millWWTP has a primary treatment unit (a rotary screen and a primaryclarifier), and a pre-acidification tank, followed by two-stage mem-brane bioreactors (MBBRs). The effluent from MBBRs flows to anaerobic stabilization basin (HRT of 2 days and SRT of 6–9 days), fol-lowed by a secondary clarifier for further polishing. Primary sludgecollected in the primary clarifier and WAS from the secondary clar-ifier are mixed and stored in a sludge storage tank before dewater-ing. Sludge is dewatered by a belt press and is sent to a nearbylandfill. Upon sampling at QRP, WAS and mixed sludge sampleswere shipped within 24 h to the laboratory in coolers packed withdry ice. Table 1 presents characterization of two different WASsamples taken on 11/21/2009 and 05/17/2010, as well as mixedsludge sampled on 11/23/2009 from QRP.

2.2. Inoculum sample

Mesophilic inoculum was taken from the effluent line of theanaerobic sludge digesters (at SRT of 15–20 days), treating a mix-ture of PS and thickened WAS, at Robert O. Pickard EnvironmentalCenter (ROPEC) sewage treatment plant, located in Gloucester (ON,Canada). ROPEC has preliminary and primary treatment, followedby a conventional aerobic activated sludge unit operated at anaverage SRT of 5 days. Thickened WAS and PS are blended in a58:42 (v/v) ratio and undergo mesophilic (35 �C) anaerobic sludgedigestion to produce a stabilized biosolids product for disposal.

Thermophilic inoculum was taken from the effluent line of thethermophilic sludge digesters at Annacis Island WWTP in Vancou-ver (BC, Canada). The Annacis plant, the largest of the five MetroVancouver treatment plants, contains physical, biological, andchemical treatment units. Preliminary treatment (screening andgrit removal) and primary sedimentation is followed by tricklingfilters and secondary clarifiers. Thickened PS and WAS are mixedand undergo an extended (SRT > 20 days) thermophilic (55 �C)anaerobic process to ensure pathogen reduction. Sludge is dewa-tered by centrifuges to 30% TS before disposal (over 100 wet tonnesof biosolids per day).

Before setting up the methane potential tests with inocula, foursemi-continuous flow digesters (2, 2.5 L of mesophilic and 2, 3.5 Lof thermophilic) were run for more than 4 months to acclimatizemesophilic and thermophilic inocula to QRP pulp mill WAS andmixed sludge in order to minimize inhibition or lag-phase. Twoof the digesters (one mesophilic and one thermophilic) were fedwith QRP WAS, and the other two were fed with QRP mixed sludge.All of the semi-continuous digesters were run at an approximately

Table 1Characterization of Quesnel River Pulp (QRP) mill WAS and mixed sludge.a

Parameters QRP–WAS (11/21/2009) QRP–WAS (05/17/2010) QRP–mixed sludge(11/23/2009)

pH [–] 6.5 (0.2;2)* 6.9 (0.1;2) 6.2 (0.0)Total solids [%, w/w] 2.50 (0.01;2) 2.44 (0.01;2) 2.21 (0.01;2)Volatile solids [%, w/w] 1.99 (0.01;2) 1.80 (0.01;2) 1.84 (0.01;2)VS/TS⁄100 [%] 80 (0.0;2) 73.7 (0.3;2) 83 (0.0;2)Ammonia [mg/L] 37 (0.5;2) 3.5 (0.0;2) 11 (0.4;2)Alkalinity [mg CaCO3/L] 1520 (20;2) 1200 (0.0;2) 1010 (90;2)

Total fractionChemical oxygen demand [TCOD, mg/L] 39,579 (0.0;2) 34,229 (0.0;2) 34,229 (0.0;2)Proteins [mg/L] 164 (179;2) 176 (15;2) 104 (179;2)Humic acids [mg/L] 973 (16.1;2) 1564 (42;2) 837 (16.1;2)Sugars [mg/L] 202 (16;2) 354 (44;2) 1186 (38;2)Phosphorus [mg/L] 792 (47;2) – 419 (100;2)Dewaterability [Capillary Suction Time, sec] 18 (4;5) 16 (1.4;5) 13 (6;5)

Soluble fraction (filtered through 0.45 lm filter paper)Chemical oxygen demand [SCOD, mg/L] 1926 (71;2) 1235 (0;2) 3637 (71;2)Proteins [mg/L] 102 (5.2;2) 49 (7;2) 87.8 (6;2)Humic acids [mg/L] 37 (5;2) 153 (3;2) 192 (5;2)Sugars [mg/L] 15 (0;2) 20 (1;2) 29 (0;2)

Volatile fatty acids (VFA) in supernatant phaseAcetic acid [mg/L] 340 (1;2) 23 (1;2) 705 (1;2)Propionic acid [mg/L] 201 (1;2) 81 (12;2) 320 (1;2)Butyric acid [mg/L] 87 (1;2) 48 (11;2) 48 (1;2)

a QRP, Quesnel River Pulp; VS, volatile solids; TS, total solids; COD, chemical oxygen demand.* Data represent arithmetic mean of duplicate; (absolute difference between mean and duplicate).

M. Saha et al. / Bioresource Technology 102 (2011) 7815–7826 7817

20 d-SRT. Organic loading rates (OLR) of the acclimation digestersfed with WAS and mixed sludge were 1.98 ± 0.02 g TCOD/L.d and1.71 ± 0.02 g TCOD/L.d, respectively. The digester feed concentra-tions were 2.5% TS (w/w) and 2.2% TS (w/w) for WAS and mixedsludge, respectively.

2.3. Pretreatment of sludge sample

Since there are significant differences among MW, sonication andMicroSludge processes in terms of the nature of disintegration mech-anisms, more or less similar levels of maximum particulate COD dis-integration were targeted with all pretreatments. This allowed forbetter comparison of dewaterability as well as methane yields.

2.3.1. Microwave pretreatmentAn industrial MW unit [Microwave Accelerated Reaction

System (MARS-5), 0–1250 W, 2450 MHz, temperature range25–260 �C, CEM Corporation, Matthews, NC, USA], equipped withfiber optic temperature and pressure control within pressuresealed vessels, was used. MARS-5 operates with a focused MWirradiating beam and is capable of heating and holding desiredcooking (or temperature ramp) rates and holding times. Since thisis the first research on the effects of MW pretreatment on pulp millsludge, a wide temperature range (50–175 �C) was studied. Sludgesamples (400 g) were irradiated to 50, 75, 100, 125, 150 and 175 �Cat different temperature ramp rates in 10 Teflon vessels (40 g WASor mixed sludge per vessel) rotating on the carousel. Since the pulpmill biosolids were expected to be difficult to disintegrate, slowramping rates (1.35–4.47 �C/min) with long duration of heating(20–130 min) were used. The input energies for samples (�2.5%TS, w/w) irradiated to 50, 75, 100, 125, 150 and 175 �C were138,045, 287,521, 411,552, 515,040, and 717,680 kJ/kg TS, respec-tively. After target temperatures were reached, samples wereremoved from the heating source, and were cooled to room tem-perature in closed vessels to avoid evaporation of organics, andthen stored at 4 �C.

2.3.2. Ultrasound pretreatmentUltrasound (US) pretreatment was performed using a Digital

Sonifier� Unit Models S-450D (Branson Inc., Danbury, CT, USA) to

pretreat 400 mL of each sludge sample at 20 kHz and power den-sity of 1 W/mL. Ultrasonic vibration was delivered through a 0.5in titanium horn tip. To compare methane yields after differentpretreatments, a similar level of particulate COD solubilizationwas targeted for both MW and ultrasound treatments. This re-quired different sonication times for both WAS and mixed sludgesamples. Ultrasound pretreatment for 15, 30, 60 and 90 min re-sulted in specific energy delivered to samples (�2.5% TS, w/w) of17,234, 45,877, 83,758, and 117,719 kJ/kg TS, respectively. Thetemperature of the treated samples was kept below 55 �C usingice pellets around the beaker.

2.3.3. MicroSludge� pretreatmentFor MicroSludge� processing, a fresh WAS sample was obtained

from QRP on 05/17/2010. MicroSludge� pretreatment of the thick-ened QRP WAS was performed in a MicroSludge� pilot unit by Par-adigm Environmental Technologies Inc. in Vancouver. As a firststep, 900 mg/L NaOH was added to the thickened WAS and thesample was kept at room temperature for 1 h. As a second step,the sludge sample was processed at a pressure of 83,000 kPa(12,000 psi) using a high pressure homogenizer. The energy inputduring homogenization of WAS (�2.5% TS, w/w) was 4000 kJ/kgTS (Stephenson, 2011). A high pressure homogenizer is capableof producing very high shear force which breaks the cell mem-branes to release extracellular and intracellular substances. Upontreatment, both control (untreated WAS) and MicroSludge� treatedWAS samples were shipped to the University laboratory in coolerspacked with ice via overnight carrier for methane potential tests.MicroSludge� treatment was applied to the WAS sample only sincethere is no practical benefit of MicroSludge processing paper fibers(Stephenson, 2011).

2.4. Analytical methods

The sludge samples were analyzed for solids content (TS andVS), pH, alkalinity, ammonia, total and soluble chemical oxygendemand (TCOD and SCOD), total and soluble biopolymers (pro-teins, humic acids, sugars), and total volatile fatty acids (TVFAs).To separate soluble from total solid fractions, sludge samples werecentrifuged at 10,000 rpm for 45 min using a Dupont Instruments


Sorvall SS-3 automatic centrifuge, and filtered through amembrane with 0.45 lm pore size.

Standard methods (APHA, 2005) were used for TS and VSdetermination. The closed reflux colorimetric method (APHA,2005) was used for COD measurement. The phenol–sulfuric acidassay was performed according to the method developed by Du-bois et al. (1956) to measure soluble and total sugars. Absorbencieswere measured at 490 nm using Beckman DU� 50 series spectro-photometer (Beckman Instruments, Mississauga, ON, Canada).Total and soluble proteins along with humic acids were measuredusing modified Lowry protein assay (Frolund et al., 1995) at750 nm using a Beckman DU� 50 series spectrophotometer. Dis-solved ammonia concentrations were measured in the supernatantof centrifuged sludge samples. Ammonia measurements were car-ried out using an ORION Model 95–12 ammonia gas sensing elec-trode connected to a Fisher Accumet pH meter model 750 (FisherSci., Ottawa, ON) according to Standard Methods 4500D procedure(APHA, 2005) and reported as ammonia-N. Sample alkalinity wasdetermined according to Standard Method 2320B (APHA, 2005).Particle size analysis was carried out by using a dynamic particlesize analyzer, DPA 4100 (Brightwell Technologies Inc., ON, Canada).Dewaterability of control and pretreated samples were analyzed bya capillary suction timer (CST). This method provides a quantitativemeasure, reported in seconds, of how readily sludge releases itswater. The method needs at least 5 replicates injections from eachsample (APHA, 2005). All analyses were done at room temperature.

Anaerobic biodegradability of control, MW, US, and Micro-Sludge� pretreated sludge samples were determined by batch bio-chemical potential (BMP) test (Owen et al., 1979; Angelidaki et al.,2009) at mesophilic (33 ± 2 �C) and thermophilic (53 ± 2 �C) tem-peratures. In all cases, food to microorganism ratio (g VS sub-strate/g VS inoculum) of 0.5 was used. In each BMP bottle,sample VS and TCOD concentrations were 0.82 ± 0.2% (w/w) and15 ± 1 g TCOD/L, respectively, for the WAS (mesophilic and ther-mophilic) at the beginning of the assay. For mixed sludge, the feedconcentrations were 0.90 ± 0.2% (w/w) VS and 16 ± 2 g TCOD/L. Forthe BMP assays, nutrient media (25–30 mL) was based on Angeli-daki et al. (2009), and inocula (50–60 mL) were placed into serumbottles, and then sludge samples (25–30 mL) were added. Serumbottles (165 mL) were sealed after adding equal parts of NaHCO3

and KHCO3 to achieve an alkalinity of 4000 mg/L (as CaCO3). Mes-ophilic and thermophilic serum bottles were kept in two differentdarkened temperature controlled incubator shakers (PhycroTherm,New Brunswick Scientific Co. Inc., NB, Canada) at 90 rpm until theystopped producing biogas. Assays were performed in duplicate andproceeded for up to 43 days. Biogas production was measured dai-ly by inserting a needle into the serum bottles attached to amanometer. Biogas volume determinations were made by allowingthe manometer to equilibrate between the bottle and atmosphericpressure (�1 atm). The mesophilic and thermophilic biogas vol-umes were then corrected to standard temperature and pressure(STP, 0 �C at 1 atm). Biogas and methane production from blanks(inocula only) were subtracted from biogas and methane produc-tion of the treated samples and controls to eliminate backgroundmethane overestimation.

Digester pH, TVFAs (summation of acetic, propionic and butyricacids) and biogas composition (nitrogen, methane and carbondioxide percentage) were monitored weekly during anaerobicdigestion. TVFAs were determined by an internal standard method(Ackman, 1972), using a Hewlett–Packard 5840A GC (Agilent)equipped with a flame ionization detector, a 5840 model integra-tor, and a Chromosorb 101 packed column (304.8 cm � 2 mm ID,80/100 mesh size). The oven and injection temperatures were180 and 250 �C, respectively, and the detector temperature wasmaintained at 350 �C. The flow rate of the formic acid saturatedhelium carrier gas was 15 mL/min. Samples were first centrifuged

at 5000 rpm for 8 min in a micro-centrifuge, and the supernatantwas diluted with an equal volume of internal standard containing2000 mg/L isobutyric acid. Methane content in biogas samples wasdetermined according to van Huyssteen (1967), using a Hewlett–Packard 5710a gas chromatograph (GC) (Agilent, Santa Clara, CA,USA), equipped with a thermal conductivity detector, a 3380Amodel integrator, and a Porepak T column (6.35 mm � 304.3 cm)at 70 �C with a helium gas carrier flow rate of 40 mL/min. Once bio-gas production ceased, the serum bottles were removed from theshakers.

2.5. Statistical analysis

Standard statistical procedures were used, including standarddeviation (for more than duplicate data points), mean averaging,and absolute difference between main and duplicate data points.When significant difference analysis was required, a single factoranalysis of variance (ANOVA) or a one-sided t-test was used, withp 6 0.05 considered significantly different.

3. Results and discussion

3.1. Effect of pretreatment on solubilization of waste sludge

The effect of pretreatment intensity (MW temperature, US con-tact time, and MicroSludge�) on SCOD/TCOD ratios was investi-gated. As expected, solubilization of the WAS samples was muchhigher than the mixed sludge samples after both MW and US pre-treatments, since the primary sludge portion of the mixed sludgecontained high concentrations of lignin, cellulose, and hemi-cellu-losic components. Among the three pretreatments used, US wasfound to be most effective in converting particulate to solubleCOD. The SCOD/TCOD ratio in the WAS samples pretreated withUS for 60 and 90 min increased by a factor of 2.6 ± 0.1 and8.4 ± 0.5, respectively, compared to the controls (Fig. 1).

Pulp mill secondary sludge samples indicated a linear relation-ship between the intensity of pretreatment and sludge disintegra-tion. For example, SCOD/TCOD ratios of WAS samples increasedfrom 5.0 ± 0.3% (control) to 11 ± 0.6%, 16 ± 0.8%, 26 ± 1.3%,31 ± 1.6%, 35 ± 1.8% and 41 ± 2.1% for MW 50, MW 75, MW 100,MW 125, MW 150, and MW 175 samples, corresponding to energyinputs of 138,045, 287,521, 411,552, 515,040, and 717,680 kJ/kgTS, respectively. These results corresponded to 2.2 ± 0.1, 3.2 ± 0.3,5.2 ± 0.2, 6.2 ± 0.2, 7.0 ± 0.1 and, 8.2 ± 0.4 fold increases in theSCOD/TCOD ratios at MW temperatures of 50, 75, 96, 120, 150,175 �C compared to the control pulp mill WAS.

Previous MW studies indicate that in addition to the ultimateMW temperature reached, the heating profile (slow versus fastheating) plays an important role in enhancing hydrolysis and sub-sequent methane production (Eskicioglu et al., 2007, 2009). Thereis lack of literature regarding the effects of MW irradiation on pulpmill biosolids. Therefore, this study used similar MW heating pro-files (1.35–4.47 �C/min) as applied previously for municipal bioso-lids for comparison. One of the previous studies on municipal WASwith the identical heating profiles reported 1.3 ± 0.1, 2.3 ± 0.1,2.7 ± 0.2, 2.7 ± 0.1, 3.1 ± 0.3, and 3.9 ± 0.1 fold increase in theSCOD/TCOD ratio at MW temperatures of 50, 75, 100, 125, 150,175 �C compared to controls (Eskicioglu et al., 2009). The resultsindicate that MW irradiation with 1.35–4.47 �C/min heating ratesand in a temperature range of 50–175 �C can solubilize the BCTMPpulp mill WAS better compared to municipal WAS.

Ultrasound pretreated WAS samples achieved SCOD/TCOD ra-tios of 25 ± 1.3%, 29 ± 1.5%, 39 ± 1.2%, and 42 ± 2.1% at US 15, US30, US 60, and US 90 min contact times, corresponding to 17,234,45,877, 83,758, and 117,719 kJ/kg TS, respectively. In other words,

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Fig. 1. Solubilization of MW, US and MicroSludge� pre-treated WAS and mixed sludge samples (data represent the mean and error bars represent absolute differencebetween mean and duplicates).


these values correspond to increases of 5.0 ± 0.3, 5.8 ± 0.2, 7.8 ± 0.4,and 8.4 ± 0.4 times SCOD/TCOD at US 15, US 30, US 60, and US90 min contact times over the control WAS, respectively. Woodet al. (2009) recently reported a 4.5-fold increase in the solubiliza-tion ratio of a Canadian kraft mill WAS after 30 min of sonicationtime and a sonication density of 1 W/mL. Therefore, the amountof solubilization of WAS samples after sonication was in line orbetter than that from limited studies on pulp mill biosolids.

For MicroSludge� pretreated WAS processed with 900 mg NaOH/L and then processed at 83,000 kPa with a homogenizer, correspond-ing to an energy level of 4000 kJ/kg TS at 2.5% TS; the SCOD/TCOD in-crease was from 11 ± 0.6% (control) to 37 ± 1.9% (Fig. 1).

3.2. Effect of pretreatment on mineralization of waste sludge

Disintegration of pulp mill WAS and mixed sludge floc structureswas also evaluated by analyzing the solids in terms of TS, VS, andfixed solids (FS = TS–VS). High mineralization, or decrease in theVS concentration of sludge, which may occur after intense pretreat-ment is not desirable since the mineralized sludge will lose its meth-ane potential. Although VS/TS concentrations changed withincreased MW temperature or US contact time, the differences werenot statistically significant (p > 0.05). For WAS samples, the VS/TS ra-tios (w/w) were 80 ± 4.0%, 79 ± 3.4%, 78 ± 3.2%, 78 ± 3.0%, 76 ± 3.8%,75 ± 2.8%, and 73 ± 4.1%, in the control, MW 50, MW 75, MW 100,MW 125, MW 150, and MW 175 samples, respectively. Similarly,VS/TS ratio of the WAS sample was 80 ± 4.0%, 82 ± 4.1%, 83 ± 3.9%,84 ± 3.8%, and 83 ± 2.4% for the control, US 15, US 30, US 60, andUS 90 samples, respectively. Furthermore, for the mixed sludge sam-ple, US 30 and US 90 samples contained 8.0 ± 0.4% and 5.0 ± 0.3% lessFS compared to the control. The mineralization results obtained inthis study were in agreement with the results obtained by previousstudies which also used sonication for pulp mill sludge disintegra-tion (Wood et al., 2009).

The MicroSludge� processed sample, on the other hand,contained higher FS [0.87 ± 0.03% (w/w)] compared to the control[0.64 ± 0.03% (w/w)]. After MicroSludge� pretreatment and sampletransport to the University lab, the VS concentration in WASdecreased by 23 ± 1.2% compared to the control. This VS loss is notfactored into any other interpretation of results in this study.Sodium hydroxide addition as part of the MicroSludge� process con-tributed to the increase of FS.

It is also necessary to emphasize that the apparent loss of 23% ofVS in the MicroSludge� treated samples is exacerbated by shippingthe pretreated samples to the laboratory. This error is minimized

for the MW and US treated samples, since pretreatments were ap-plied at the lab scale and no shipping was necessary upon pretreat-ment. The loss of bioavailable VS in the MicroSludge� pretreatedsample may understate its methane potential to an unknownextent.

Apart from MW 175 and MicroSludge� processed samples, theFS concentration in other pretreated samples did not change(p > 0.05). The results indicate that for the majority of the pretreat-ments, organic matter was mainly solubilized rather than beingmineralized. Together, the COD solubilization and mineralizationresults indicate that the pretreatments tested were more efficientin converting particulate solids to soluble solids in WAS than inmixed sludges.

3.3. Effect of pretreatment on soluble biopolymer of waste sludge

After pretreatment, soluble organic fractions of biosolids con-tain intra-cellular (within the bacterial cell) and extra-cellular(within the polymeric network) biopolymers, such as proteins,sugars, lipids and nucleic acids. These biopolymers are releasedinto the supernatant to an extent that depends on the intensityof pretreatment. Soluble biopolymers have been found to be a goodmeasure of the solubilization of organic matter in biologicalsludges. Therefore, these have been extensively used by previoussludge pretreatment studies (Wood et al., 2009; Eskicioglu et al.,2009). In this study, solubilization of biopolymers was monitoredby protein, sugar and humic acid analyses. Relative soluble/totalfraction results are displayed in Fig. 2.

The largest increase in soluble protein concentrations werefound in samples pretreated with MW compared to US and Micro-Sludge� treated samples (for both WAS and mixed sludge). ForWAS, soluble/total protein (%) increased from 15 ± 0.3% (control)to 65 ± 4.3%, 78 ± 4.8%, 91 ± 4.9%, 95 ± 3%, 99 ± 5%, and 94 ± 5% forMW 50, MW 75, MW 100, MW 150, and MW 175 samples, respec-tively (Fig. 2a). US pretreated samples showed improved proteinsolubilization ratios with increased ultrasound intensity. For theWAS sample, protein solubilization improved from 15 ± 0.3% (con-trol) to 18 ± 4.3%, 44 ± 4.3%, 50 ± 4.8% and, 54 ± 4.9% for US 15, US30, US 60, and US 90, respectively. For MicroSludge� pretreatment,protein solubilization (from 20 ± 0.3% to 23 ± 4.3%) was not statis-tically significant at the 95% confidence limits (p > 0.05).

Soluble/total sugar (%) indicates the hydrolysis of large insolu-ble sugars (for example cellulose) to produce soluble monosaccha-ride components (for example glucose), and the destruction of EPSholding bacterial flocs together. The concentration of soluble

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fractions increased after pretreatment compared to control sam-ples (Fig. 2b). Again MW pretreatment increased the portion ofsoluble sugars more than US and MicroSludge� (except for WASpretreated at US 60). For WAS, the soluble to total sugar ratio (re-ported as a percentage) increased significantly above the boilingpoint temperature of sludge. The ratios significantly (p < 0.05)changed from 8 ± 0.4% (control) to 8 ± 0.3%, 22 ± 1.0%, 46 ± 2.3%,47 ± 2.4%, 44 ± 2.2%, and 45 ± 2.4% for MW 50, MW 75, MW 100,MW 150, and MW 175 samples, respectively. The mixed sludgesample contained notably less soluble sugar compared to WAS.For the US pretreated sample, sugar solubilization increasedslightly from 3.0 ± 0.3% (control) to 5.0 ± 0.2%, 7.0 ± 0.5%,7.0 ± 0.4%, and 6.0 ± 0.3% for US 15, US 30, US 60, and US 90, respec-tively. These smaller (compared to sugar solubilization in WAS) in-creases in the soluble sugar fractions were associated with largeamounts of complex and difficult to disintegrate substances inthe mixed sludge sample. However, changes in the soluble sugarconcentrations of mixed sludge after US pretreatments were stillstatistically significant (p < 0.05).

Humic acid substances are generally formed by microbial deg-radation of dead organic matter that is difficult to metabolize (Liet al., 2009). In this study, all of the pretreatment methodsincreased the concentration of humic acid like substances in thesoluble phase (results are not shown). Like protein and sugar, asa general trend, soluble/total humic acid (%) increased withpretreatment intensity. However, for MW pretreated WAS sample,the ratios first decreased from 22 ± 2.2% (control) to 7 ± 0.4%,21 ± 1.3% for MW 50 and MW 75. When the pretreatment temper-

ature went above the boiling point however, the ratios climbed to29 ± 1.5%, 41 ± 2.0%, 46 ± 3%, and 47 ± 3.5% for MW 100, MW 125,MW 150, and MW 175, respectively. A similar trend was obtainedfrom the mixed sludge samples, as well. US pretreatment solubi-lized humic acids to nearly the same extent for WAS and mixedsludge samples. For MicroSludge� pretreated WAS, humic acid sol-ubilization increased from 4.0 ± 0.5% (control) to 31 ± 2.0%, whichis lower than MW, but higher than US solubilization ratios in thesecondary sludge samples.

Overall, MW solubilized biopolymers the most. WAS sampleswere solubilized to a greater extent than mixed sludge in mostcases. This may be due to WAS being easier to disintegrate. Themixed sludge sample likely contained a higher degree of non-bac-terial matter such as lignin and cellulose which may be resistant tosolubilization.

3.4. Effect of pretreatment on particle size distribution of waste sludge

According to the waste sludge disintegration concept, the parti-cle size of solid particles should be decreased and become moreuniform after pretreatment. In this study, the effect of MW andUS pretreatment on the particle size of pulp mill waste sludgesamples was analyzed by the floc images captured from a particlesize analyzer that utilizes flow microscopy technology and shapeanalysis. Due to malfunctioning of the instrument when the Micro-Sludge� processed samples were analyzed, the images could not becaptured for the MicroSludge� pretreated samples. The selectedimages (among 12 pictures) for both MW and US treatments are


presented in Fig. 3 for WAS and mixed sludge samples,respectively. As expected, the image of the control WAS sampleindicated more microbial flocs wrapped with filamentous-like bac-teria compared to the control mixed sludge sample.

The effect of pretreatment on floc structure is apparent. Forboth WAS and mixed sludge sample, as pretreatment intensity in-creased, solid particles became smaller. This is in agreement withthe solubilization results presented previously. Maximum sludgedisintegration was obtained for the sludge samples with pretreat-ment conditions of MW 175 �C and US 90 min contact time. It isnecessary to emphasize that MW irradiation created a more grad-ual decrease in particle size with an increase in temperature, whileUS pretreatment disintegrated the microbial flocs very intensivelyeven after the shortest exposure time of 15 min (Fig. 3). Overall, USpretreatment broke down the floc structures into much smallerstructures than MW pretreatment for both WAS and mixed sludgewhich was also in agreement with the COD solubilization resultsdiscussed previously.

In addition to reduction in size, pretreatments also increasedthe circularity of the solid particles (results are not shown). Shapeanalysis was performed by the particle size analyzer in terms ofvariation of aspect ratio with different equivalent circular diameter(ECD) of the WAS and mixed sludge particles, respectively. The as-pect ratio quantifies the ‘‘squareness’’ or ‘‘roundness’’ of an object.For example, a circle or square has the aspect ratio of 1, while a linehas an aspect ratio close to 0. On the other hand, ECD of an object isexpressed in microns (lm) and represents the diameter of a spherethat occupies the same two dimensional surface areas as the parti-cle. In this study, ECD of the particles decreased as the intensity ofthe pretreatment increased. For example, the majority of WASparticles captured had an ECDC 6 30 lm in the US 90 min samples,

Fig. 3. Images of sludge flocs in WAS and mixed sludge samples (a) befor

while the majority of the particles of control samples had anEDC 6 92 lm. Compared to controls, aspect ratios were increasedat same ECDs for both MW 175 and US 90 pretreated samples. Thisindicates that sludge particles became smaller and more uniformafter pretreatment.

3.5. Effect of pretreatment on sludge dewaterability before anaerobicdigestion

It is postulated by previous studies that advanced hydrolysispretreatment of sludge samples releases some of the water thatis originally bound to the EPS and enhances dewaterability byaltering the hydration zone (Eskicioglu et al., 2007). In this study,the CST results showed that both WAS and mixed sludge dewater-ability decreased (results are not shown) after MW, US and Micro-Sludge� pretreatment (tests performed prior to anaerobicdigestion). Dewaterability was decreased by 0.39 ± 0.01, 3.2 ± 0.2,2.4 ± 0.4, 6.2 ± 0.3, 7.7 ± 0.5, and 9.3 ± 0.5 fold compared to theWAS control after MW 50, 75, 100, 125, 150, and 175 pretreat-ments, respectively. Furthermore, ultrasound significantly(p < 0.05) deteriorated dewaterability of the samples. For theWAS sample, US 15, 30, 60 and 90 min pretreatments reduceddewaterability (pre-digestion) by 65 ± 3.4, 73 ± 4.0, 83 ± 6, and93 ± 8 fold, respectively, compared to the control.

Rates of decrease in dewaterability (pre-digestion) were higherfor WAS compared to mixed sludge samples. This may be becausepretreated WAS containing more soluble polymeric substanceswhich clog the filter paper during the CST test. For MW pretreatedmixed sludge samples, dewaterability was decreased by0.20 ± 0.02, 0.83 ± 0.04, 1.1 ± 0.4, 1.8 ± 0.2, 2.1 ± 0.5, and 2.6 ± 0.8fold after MW 50, 75, 100, 125, 150, 175, respectively, compared

e, (b) after MW, and (c) after US pretreatment (magnification: 4.9�).

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Fig. 4. Specific methane yields at STP (1 atm, 0 �C) from MW treated WAS and mixed sludge at (a and c) mesophilic and (b and d) thermophilic temperatures (data representthe mean and error bars represent absolute difference between mean and duplicates).


to control. On the other hand, US pretreatment decreased dewater-ability by 7.0 ± 0.9, 11 ± 0.4, 12 ± 0.6 and, 20 ± 0.4 fold compared tomixed sludge control for US 15, 30, 60, and 90 min sonication time.

Similarly, the MicroSludge� pretreated WAS sample took longertime to filter during the CST test, therefore indicated a deteriorateddewaterability pre-digestion. It was interesting to notice thatpretreatments involving mechanical disruption (sonication or highpressure as in MicroSludge� process) deteriorated the dewaterabil-ity of samples more than the thermal pretreatment, such as MWheating. These results are in agreement with other pretreatmentstudies which report similar behavior for municipal biosolids(Eskicioglu et al., 2007; Feng et al., 2009). It is necessary to empha-size that in this study, dewaterability was measured only after pre-treatment, prior to anaerobic digestion. At full-scale, dewaterabilityof digested sludges (mostly after combined with the PS) is importantsince pretreatment is followed by mesophilic or thermophilic sludgedigestion which will change the dewaterability characteristics ofsludge.

3.6. Effect of pretreatment on anaerobic digestion of waste sludge

The effects of different pretreatment methods on anaerobicdigestion of WAS and mixed sludges were investigated by batchlab scale mesophilic and thermophilic anaerobic digesters. The re-sults are presented in terms of specific methane yield (SMY)/mgTCODadded (Figs. 4, 5 and 6). Compared to the control samples with-out pretreatment, batch digesters fed with pretreated waste sludgeshowed higher methane yields, and the methane yields increasedwith increased pretreatment intensity. Higher methane yieldscan be explained by the fact that pretreated sludge samples

contained higher levels of soluble organic matter which microor-ganisms were able to use immediately.

The first week of batch digestion is critical because maximumsubstrate utilization generally occurs at 5 to 7 days. Fig. 4(a–d)shows the effect of MW pretreatment on pulp mill WAS and mixedsludge for mesophilic and thermophilic conditions. Prior to settingup the methane potential tests, both mesophilic and thermophilicinocula, from municipal sludge digesters were acclimatized to raw(not pretreated) QRP waste sludge samples to minimize inhibitionto methanogens. As shown in Fig. 4, none of the digesters showed alag phase at the beginning of the batch tests. Until a digestion timeof 7 days, the control and pretreated bottles had very similar SMYs.Subsequently, the methane generation rates in the control bottlesslowed down compared to the pretreated samples. SMYs increasedwith an increase in MW temperature irrespective of waste sludgetype and anaerobic digestion temperature. Microwave at 175 �Cpretreatment proved to be the most effective in terms of accelerat-ing the biodegradation, increasing SMY of WAS samples by 90%compared to control after 21 days of digestion (Fig. 4a). In theMW pretreated WAS samples, although thermophilic SMY rateswere higher than mesophilic SMY rates, mesophilic bottles pro-duced biogas/methane for a longer digestion period, indicated byhigher ultimate SMYs.

After 43 days of digestion, SMYs in WAS samples increased by63 ± 3.2% and 13 ± 0.7% (Fig. 4a and b) compared to controls formesophilic and thermophilic MW 175 samples, respectively. LowerSMY improvement for thermophilic conditions over the controlsample was associated with higher methane production by thethermophilic control sample. Also, thermophilic ultimate methaneyields were lower than with mesophilic digestion. This may be dueto greater sensitivity of thermophilic microorganisms to denatured

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Fig. 5. Specific methane yields at STP (1 atm, 0 �C) from US treated WAS and mixed sludge at (a and c) mesophilic and (b and d) thermophilic temperatures (data representthe mean and error bars represent absolute difference between mean and duplicates).


compounds formed after the pretreatments. As expected, based onthe initial sludge characterization before and after solubilization,SMYs were lower for the mixed sludge samples compared toWAS (Fig. 4c and d). After 43 days of digestion, SMYs increasedby 46 ± 2.7% for MW 125 and 38 ± 2.0% for MW 175 compared tocontrol mixed sludge samples for mesophilic and thermophilicconditions, respectively. The peak value of ultimate methane yieldwas 0.13 ± 0.02 mL/g TCODadded (290 mL/g VSadded) at STP (0 �C,1 atm) for the MW 175 WAS sample under mesophilic tempera-ture. Similar methane yield results were obtained by previousstudies on mesophilic digestion of MW pretreated municipalWAS (Eskicioglu et al., 2009). The ultimate SMY from MW 125WAS sample was quite similar to that of MW 175 WAS (0.13 mL/g TCODadded), although SMY in terms of VS was little lower(262 mL/g VSadded).

Despite higher solubilization achieved by sonication (Fig. 1), forUS pretreated WAS, ultimate SMYs were lower than MW pre-treated WAS (Fig. 5a and b). After 43 days of digestion, SMYs in-creased by 51 ± 2.6% for mesophilic samples and 28 ± 1.4% forthermophilic samples compared to controls for US 90 pretreatedWAS, respectively. The results obtained were similar to the studydone by Wood et al. (2009) for mesophilic US 30 pretreated pulpmill WAS. US pretreatment improved ultimate methane potentialof mixed sludge more than WAS. This may be due to presence ofmore difficult to disintegrate components in mixed sludge com-pared to WAS. After 43 days of digestion, SMYs in the mixed sludgesamples increased by 43 ± 3.5% for mesophilic and 38 ± 2.2% forthermophilic conditions compared to controls for US 90 pretreatedsamples, respectively (Fig. 5c and d). The maximum ultimate SMYfrom the mixed sludge was 0.10 ± 0.04 mL/g TCODadded for US 60sample digested at mesophilic temperature.

MicroSludge� pretreatment produced the lowest SMY amongthe three pretreatment techniques used for both mesophilic and

thermophilic anaerobic digestion conditions (Fig. 6a and b) with-out taking into account the 23% loss in VS with the MicroSludgeprocessed sample. After 42 days of digestion, the MicroSludge�

pretreated WAS samples produced 34 ± 2.2% more methane formesophilic (Fig. 6a) and 16 ± 2.2% for thermophilic (Fig. 6b) condi-tions compared to the controls. It was observed that given suffi-cient digestion time, pretreated samples did not show substantialimprovement in methane production for thermophilic condition.

The BMP results suggested that, although US pretreatment sol-ubilized a higher fraction of COD and proteins, MW pretreatedsamples produced methane at a higher rate under mesophilicanaerobic digestion. This suggests that the US pretreatment pro-duced more digestible as well as recalcitrant soluble compoundscompared to MW. Results also suggest that within an extendeddigestion time of 15–43 days, the sludge pretreatment technolo-gies enhanced the ultimate degradability of the pulp mill sludge.

3.7. Organic removal efficiency

At the end of biochemical methane potential test; the residualVS, TS, and TCOD were measured to estimate organic removal effi-ciencies after anaerobic digestion (Table 2). After mesophilic BMPof WAS and mixed sludge, substantial increases in VS, TS, andTCOD removals in the pretreated bottles compared to the controlswere measured. Organic removal percentages were higher for WAScompared to mixed sludge, which is in agreement with the meth-ane yield results. For MW pretreated WAS samples, removal rateswere highest for MW 175. Compared to the control, MW 175 in-creased the amount of VS, TS and TCOD removal efficiency from23 ± 3.0% (control) to 29 ± 4.0%, 21 ± 4.0% (control) to 23 ± 5.0%,and, 23 ± 2.0% (control) to 33 ± 2.0%, respectively. Also, removalefficiency values for US 90 pretreated samples were increased from23 ± 3.0% (control) to 30 ± 4.0%, 21 ± 4.0% (control) to 25 ± 3.0%,

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Fig. 6. Specific methane yields at STP (1 atm, 0 �C) from MicroSludge� treated WASat (a) mesophilic, and (b) thermophilic temperatures (data represent the mean anderror bars represent absolute difference between mean and duplicates).

Table 2Percent of VS, TS and TCOD removed after anaerobic digestion of raw and pre-treatedwaste sludge samples.

Sample type % Removal

Sample WAS Mixed sludge

VS TS TCOD VS TS TCOD

MesophilicControl 23 (3)a 21 (4) 23 (2) 10 (2) 10 (2) 12 (4)MW 50 26 (5) 23 (6) 24 (5) 21 (2) 15 (4) 14 (4)MW 75 31 (4) 27 (5) 28 (4) 23 (1) 25 (2) 28 (2)MW 100 23 (3) 24 (4) 29 (3) 24 (2) 17 (6) 58 (6)MW 125 34 (5) 22 (5) 32 (5) 16 (3) 11 (2) 45 (2)MW 150 33 (3) 28 (4) 25 (1) 19 (3) 20 (4) 14 (2)MW 175 29 (4) 23 (5) 33 (2) 21 (6) 18 (4) 23 (3)US 15 30 (2) 32 (4) 25 (2) 15 (5) 6 (1) 18 (3)US 30 26 (3) 27 (3) 29 (4) 19 (5) 18 (3) 20 (2)US 60 30 (5) 24 (3) 29 (3) 17 (3) 18 (5) 21 (1)US 90 30 (4) 25 (3) 33 (5) 23 (1) 20 (3) 22 (4)MicroSludge� 26 (3) 23 (2) 30 (3)

ThermophilicControl 18 (4) 10 (1) 13 (3) 9 (2) 10 (4) 12 (3)MW 50 21 (5) 19 (2) 21 (3) 12 (4) 11 (1) 13 (3)MW 75 23 (3) 16 (4) 20 (4) 16 (5) 14 (4) 15 (4)MW 100 24 (2) 21 (1) 19 (4) 19 (3) 17 (5) 17 (1)MW 125 19 (4) 17 (5) 22 (2) 15 (2) 12 (2) 18 (3)MW 150 25 (3) 21 (4) 20 (1) 18 (2) 22 (4) 20 (2)MW 175 26 (3) 21 (2) 24 (4) 21 (1) 19 (3) 20 (2)US 15 24 (2) 17 (2) 16 (5) 10 (1) 10 (5) 12 (3)US 30 25 (1) 16 (4) 17 (3) 19 (4) 17 (5) 18 (1)US 60 24 (1) 20 (3) 19 (2) 16 (3) 22 (3) 17 (1)US 90 26 (4) 18 (2) 17 (4) 18 (2) 21 (3) 20 (4)MicroSludge� 21 (3) 15 (3) 23 (3)

a Data represent arithmetic mean of duplicate (absolute difference betweenmean and duplicate).


and 23 ± 2.0% (control) to 33 ± 5.0% for VS, TS, and TCOD parame-ters, respectively. MicroSludge� processed organic removalefficiencies were lowest among the three pretreatment techniques.VS, TS and TCOD removal efficiencies were increased from23 ± 3.0% (control) to 26 ± 3.0%, 21 ± 4.0% (control) to 23 ± 2.0%,and 23 ± 2.0% (control) to 30 ± 3.0%, respectively, during meso-philic digestion. Among TS, VS, and TCOD parameters, only TCODremovals indicated statistically significant improvements frompretreated sludge samples over controls under mesophilic temper-atures (Table 2).

Thermophilic removal efficiencies were lower than for meso-philic digestion, in agreement with the methane potential results.For thermophilic WAS, VS, TS, and TCOD removal rates were in-creased from 18 ± 4.0% (control) to 26 ± 3.0%, 10 ± 1.0% (control)to 21 ± 2.0%, and 13 ± 3.0% (control) to 24 ± 4.0%, respectively, forMW 175. For the US 90 sample, VS, TS, and TCOD removal efficien-cies were increased from 18 ± 4.0% (control) to 26 ± 4.0%, 10 ± 1.0%(control) to 18 ± 3.0% and 13 ± 3.0% (control) to 17 ± 5.0%, respec-tively. Similarly, for most of the scenarios, only improvements inTCOD removals were statistically significant (Table 2).

For mixed sludge samples (Table 2), similar trends were ob-tained. MW 175 and US 90 were the best pretreatment conditions.MW performed slightly better than the US in terms of TCODremoval. The results suggest that sludge pretreatment might par-tially hydrolyze the solids or disrupt their structure and madethe sludge samples more accessible to hydrolytic enzymes. Thefraction of TS left undigested in both mesophilic and thermophilicanaerobic digestions of WAS and mixed sludges represented the

fraction of solids that either required longer residence times fordigestion or were not biodegradable.

3.8. Energy analysis of pretreatment techniques

A simplified energy analysis was performed for the pretreat-ment techniques in terms of operating cost (electrical) versus po-tential savings associated with methane production. The resultsare presented in Table 3 for both mesophilic and thermophilicdigesters. All sludge samples had around 2.5% TS (w/w) duringthe pretreatments. Since pretreatment would be done on moreconcentrated sludge samples for more energy efficient sludge dis-integration at full-scale, energy input values were also presented inTable 3 for sludge samples pretreated at 10% TS (w/w). In that case,an additional �64 kWh/ dry tonne of TS as well as polymer addi-tion would be required for all of the pretreatment scenarios to con-centrate the waste sludge from 2.5% to around 10% TS before fullscale pretreatment. Since pretreatment increases the temperatureof sludge, the digester will not need to be additionally heated. Itwas also assumed that thermally pretreated samples near or abovethe boiling point (MW 75–175 �C) will not need an additional en-ergy to cool down the feed to the digester temperatures. At fullscale, this extra heat may be recovered to contribute positively tothe energy balance.

The analysis based on the lab scale data, presented in Table 3,suggested MicroSludge� as the most economical pretreatmentalternative with net (output–input) energy profits of 1366 kWh/tonne TS and 1211 kWh/tonne TS at mesophilic and thermophilictemperatures, respectively, for WAS samples pretreated at 10%TS. US 15 pretreatment was the second most economical pretreat-ment option for WAS (10% TS) at both mesophilic and thermophilicconditions with 386 kWh/tonne TS and 380 kWh/tonne TS net en-ergy profits, respectively. Although the overall methane production

Table 3Energy analysis of sludge pretreatment methods.a

Sample Input (kWh/tonne TS) Output (CH4) Output–Input @ 10% TS Output (CH4) Output–Input @ 10% TS

@ 2.5% TS @ 10% TS (kWh/tonne TS) (kWh/tonne TS)

Pretreatment Mesophilic WAS digestion Mesophilic mixed sludge digestionMW 50 38,346 9587 1279 �8308 1206 �8381MW 75 79,867 19,967 1648 �18,319 1354 �18,613MW 100 114,320 28,580 1847 �26,733 1353 �27,227MW 125 143,067 35,767 2052 �33,715 1502 �34,265MW 150 198,800 49,700 2090 �47,610 1509 �48,191MW 175 218,000 54,500 2206 �52,294 1530 �52,970US 15 4787 1197 1583 386 1373 176US 30 12,744 3186 1371 �1815 1223 �1963US 60 23,266 5817 1472 �4345 1432 �4385US 90 32,700 8175 1780 �6395 1362 �6813MicroSludge 1111 b 278 1644 1366 – –

Pretreatment Thermophilic WAS digestion Thermophilic mixed sludge digestionMW 50 38,346 9587 1467 �8120 995 �8592MW 75 79,867 19,967 1479 �18,488 1041 �18,926MW 100 114,320 28,580 1607 � 26,973 1128 �27,452MW 125 143,067 35,767 1634 �34,133 1170 �34,597MW 150 198,800 49,700 1687 �48,013 1285 �48,415MW 175 218,000 54,500 1905 �52,595 1357 �53,143US 15 4787 1197 1577 380 961 �236US 30 12,744 3186 1358 �1828 986 �2200US 60 23,266 5817 1513 �4304 1037 �4780US 90 32,700 8175 1890 �6285 1091 �7084MicroSludge 1111 b 278 1489 1211 – –

a Energy value of methane = 35.85 kJ/L; 1 kWh = 3600 kJ; absolute differences between the mean and duplicate readings were less than 10%, therefore removed from theTable for clarity.

b Stephenson (2011).


and organic removal efficiency were higher for the MW pretreatedWAS samples, high electrical energy input to operate the benchscale MW unit made this pretreatment the most energy intensiveoption. For mixed sludge at both mesophilic and thermophilic con-ditions, neither the MW nor the US pretreatment options were en-ergy economical due to much smaller methane yields compared tothe energy input.

It is necessary to emphasize that, MW and US lab-scale equip-ment were not designed to heat WAS or PS while MicroSludge pro-cess was applied at the pilot scale. Also, very slow MW heating(ramping) rates (1.35–4.47 �C/min, corresponding heating timesof 20–130 min) were used to maximize the pulp mill sludge solu-bilization. These may have caused overestimation of MW and USenergy inputs for the pulp mill samples. Therefore, a more compre-hensive cost-benefit analysis that takes into account the energy in-puts of full scale equipment plus other factors, such as capital costsavings for smaller digesters and operational cost savings for betterdewatering and lower sludge disposal costs are necessary to vali-date these results. Furthermore, larger scale MW studies with915 MHz generators (more efficient conversion from electricpower to electromagnetic energy) or better sonication systemswith more focused disintegration and to pretreat more concen-trated sludge may achieve improved energy balances since inputpower is the main determining factor for economical sludgepretreatment.

4. Conclusions

� Among ultrasound, microwave and MicroSludge�, within CODsolubilization range tested (5–43%); MW pretreatment wasthe most effective in terms of biodegradation rate/extentenhancement.� Methane yields and COD removals under mesophilic and ther-

mophilic conditions improved after pretreatment. However,pretreatment benefits were smaller for thermophilic digesters.

� Mechanical pretreatments deteriorated the dewaterabilitymore than microwave pretreatment before digestion.� MicroSludge� process was least energy intensive pretreatment

for mesophilic and thermophilic digestion of WAS.� For mixed sludge, neither microwave nor ultrasound were

energy economical due to much smaller methane yields com-pared to the high energy inputs tested.

Acknowledgements

The authors thank Dr. Kevin J. Kennedy at University of Ottawa(ON, Canada) for his contributions during data generation. Theauthors also thank Ms. Anna Rankin (Environmental Coordinator)from the Quesnel River Pulp for her help in providing sludge sam-ples and Dr. Rob Stephenson (Chief Scientific Officer/inventor ofMicroSludge�) from Paradigm Environmental Technologies Inc.for MicroSludge� processed pulp mill samples. This study wasfunded by the NSERC Discovery, NSERC ENGAGE, and BritishColumbia Ministry of Labour and Citizens, Student Led ResearchGrants.

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