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Page 1: Screening pretreatment methods to enhance thermophilic anaerobic digestion of pulp and paper mill wastewater treatment secondary sludge

Chemical Engineering Journal 223 (2013) 479–486

Contents lists available at SciVerse ScienceDirect

Chemical Engineering Journal

journal homepage: www.elsevier .com/locate /cej

Screening pretreatment methods to enhance thermophilic anaerobicdigestion of pulp and paper mill wastewater treatment secondary sludge

1385-8947/$ - see front matter � 2013 Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.cej.2013.02.119

Abbreviations: ASP, activated sludge plant; BCTMP, bleached chemithermome-chanical pulp; COD, chemical oxygen demand; CTMP, chemithermomechanicalpulp; CSTR, continuously stirred tank reactor; FID, flame ionization detector; GC,gas chromatograph; HRT, hydraulic retention time; 5HMF, 5-hydroxymethylfurf-ural; WWTP, wastewater treatment plant; MWWTP, municipal wastewater treat-ment plant; OLR, organic loading rate; SCOD, soluble chemical oxygen demand;TCOD, total chemical oxygen demand; TMP, thermomechanical pulp; TS, totalsolids; TVFA, total volatile fatty acids; VFA, volatile fatty acids; VS, volatile solids.⇑ Corresponding author. Tel.: +358 40 805 3904; fax: +358 14 617 239.

E-mail address: [email protected] (S. Bayr).1 Present address: Tampere University of Technology, Department of Chemistry

and Bioengineering, P.O. Box 541, FI-33101 Tampere, Finland.

Suvi Bayr ⇑, Prasad Kaparaju, Jukka Rintala 1

University of Jyväskylä, Department of Biological and Environmental Science, P.O. Box 35, FI-40014 University of Jyväskylä, Finland

h i g h l i g h t s

� Anaerobic digestion of pulp and paper mill secondary sludge was studied.� Effect of 12 pretreatments (single or combinations) on methane yield was studied.� Pretreatment methods used were hydrothermal, enzymatic, ultrasound and chemical.� Hydrothermal pretreatment alone and in combinations increased methane yields.� Hydrothermal pretreatment alone and in combinations fastened methane production.

a r t i c l e i n f o

Article history:Received 17 July 2012Received in revised form 23 February 2013Accepted 27 February 2013Available online 8 March 2013

Keywords:Anaerobic digestionMethane yieldPretreatmentPulp and paper millSecondary sludge

a b s t r a c t

The effect of hydrothermal (150 �C for 10 min and 70 �C for 40 min), enzymatic (Accelerase 1500, 0.07 g/gvolatile solids (VS)), ultrasound (45 kHz for 30 min) and chemical pretreatments (HNO3 at pH 3 andNaOH at pH 12) alone or in combination on the chemical composition and methane yield of the pulpand paper mill secondary sludge was studied in batch assays at 55 �C. In total, 12 different pretreatmentcombinations were compared. Chemical analyses showed that all pretreatments except for HNO3 andultrasound pretreatments improved the organic matter solubilization. Among the studied pretreatments,hydrothermal (150 �C, 10 min) pretreatment alone or in combination with enzymatic and/or ultrasoundpretreatment had the highest impact on sludge solubilization and methane yield. The increase in meth-ane yield was 31% (from 108 ml/g VSoriginal to 141 ml/g VSoriginal). In addition, enzymatic pretreatmentalso improved the methane yields but only when combined with hydrothermal pretreatment at 150 �Cor ultrasound + hydrothermal pretreatment at 150 �C. On the other hand, ultrasound pretreatment didnot improve the methane yields while acid and alkaline pretreatments resulted in lower methane yieldsthan control. Improved hydrolysis and higher methane production rates noticed in assays subjected tohydrothermal pretreatment alone or in combination with enzymes and/or ultrasound could make thesetreatments more attractive in reducing the retention times required during full-scale anaerobic digestionof pulp and paper mill wastewater sludges.

� 2013 Elsevier B.V. All rights reserved.

1. Introduction ASPs, the amount and characteristics of the produced secondary

Aerobic activated sludge plants (ASPs) are commonly used totreat wastewaters in pulp and paper mills e.g. in Finland. In the

sludge vary according to the raw materials and processes used inthe pulp and paper manufacturing process, and also according tothe subsequent wastewater treatment process. In pulp and papermills, secondary sludge is typically mixed with primary sludge,thickened and dewatered and then disposed by landfilling or incin-eration. If landfilled, the value of these raw materials is lost andmay result in gaseous emissions and water pollution. On the otherhand, incineration might be energetically unfavorable due to thehigh water content of the secondary sludge. In response to the cur-rent general interest to promote sustainable renewable energyproduction and biorefinery concepts (e.g. [1]), anaerobic digestionis increasingly considered to convert the pulp and paper millsludge into renewable energy in the form of biogas and at the sametime to produce a stabilized product for further use (e.g. [2]).

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480 S. Bayr et al. / Chemical Engineering Journal 223 (2013) 479–486

Anaerobic digestion has been employed for decades to stabilizeprimary, secondary and mixed sludges from municipal wastewatertreatment plants (MWWTPs). However, the adoption of this tech-nology to treat sludges arising from the pulp and paper industrieshave been limited [e.g. 3]. The limited studies reported on anaero-bic digestion of various pulp and paper mill secondary sludgesindicate that these materials are typically low in organic matterand thus have low methane potentials. The low methane yieldsof pulp and paper mill secondary sludge can be attributed to thefact that the sludge is already subjected to partial degradation dur-ing the ASP process, as the ASPs in pulp and paper industry are typ-ically operated with low sludge loads [4] and primarily consists ofmicrobial biomass, residual cellulose, lignin and chemical compo-nents from the pulping process [5].

Depending upon the pulp and paper mill processes and ASP atthe mill, methane yields ranging from a very low 50 m3/t VSadded

for bleached kraft pulp and paper mill secondary sludge [6], 89–197 m3/t VSadded for thermomechanical (TMP) pulp mill secondarysludge, 159 m3/t VSadded for sulphite pulp mill secondary sludgeand, 145 m3/t VSadded for kraft pulp mill secondary sludge and97–199 m3/t VSadded for chemithermomechanical (CTMP) and kraftpulp mill secondary sludge [3] have been reported in batchexperiments under mesophilic conditions (35 �C). Temperaturehas also been shown to influence the methane yields from thesecondary sludge. Higher methane yields in batch assays have beenreported for bleached kraft pulp and paper mill secondary sludgeunder thermophilic (100 m3/t VSadded) than under mesophilic(50 m3/t VSadded) conditions [6].

Pretreatment technologies viz., physical, mechanical, thermal,chemical, biological and physico-chemical have been studied andapplied to increase anaerobic biodegradability and methane yieldsof various biomasses including wastewater treatment plant sludgesas recently reviewed e.g. by Carlsson et al. [7]. The aim of these pre-treatments was to improve the accessibility and/or solubilize theorganic fraction of sludges (see e.g. reviews [7,8]). In addition, pre-treatments may also disrupt cells by destroying the floc structureand by fragmenting the cell walls into low size particles, whichare more readily available for further hydrolysis [9]. However, somepretreatments, especially under acidic conditions, may also pro-duce compounds like furfural, 5-hydroxymethylfurfural (5-HMF)or phenolic compounds, which are the degradation products of pen-toses, hexoses and lignin, or acetic acid derived from the cleavage ofacetyl groups of the hemicelluloses [10]. These compounds canhave inhibition and/or toxic effect on anaerobic microorganism[10]. Therefore, the choice of the pretreatment and pretreatmentconditions should be designed with respect to the physical struc-ture and chemical composition of the substrate used.

Table 1Operational parameters in the experiments studying the effect ofsecondary sludge in batch assays at 55 �C.

Abbreviation Pretreatment

C1 Control: originalC2 Control: enzyme + bufferUS UltrasoundAL AlkaliAC AcidE EnzymeUS + AL Ultrasound + alkaliHT70 Hydrothermal 70 �CHT150 Hydrothermal 150 �CUS + E Ultrasound + enzymeHT150 + E Hydrothermal 150 �C + enzymeHT70 + E Hydrothermal 70 �C + enzymeUS + HT150 Ultrasound + hydrothermal 150 �CUS + HT150 + E Ultrasound + hydrothermal 150 �C + enz

Most of the studies reported on the evaluation of various pre-treatments on organic matter solubilization and methane yieldsof secondary sludges have been dealt with MWWTP sludges, andrelatively a few studies have actually compared the effect of differ-ent pretreatments on pulp and paper mill secondary sludge [11].With pulp and paper mill secondary sludge, increased methaneyields have been reported at least with microwave, hydrothermal,ultrasound and alkali pretreatments under mesophilic conditions[12,13] and microwave and ultrasound pretreatments under ther-mophilic conditions [13]. For example, hydrothermal pretreatmenthas shown to increase the biogas yield (methane not reported) ofthe kraft mill secondary sludge in batch assays by 280% and sulfitemill secondary sludge by 50% under mesophilic conditions [12].Furthermore, ultrasound pretreatment increased the methaneyield of the bleached chemithermomechanical pulp mill (BCTMP)secondary sludge by 51% and 28% in batch assays under mesophilicand thermophilic conditions, respectively [13]. However, someresearchers have reported lower or no improvement in methaneyields from pulp and paper mill sludges subjected to enzymatic,ultrasound and microwave pretreatments at 35 �C [3,13] andmicrowave pretreatment at 55 �C [13]. The lower impacts in ther-mophilic than in mesophilic assays was attributed to the highersensitivity of thermophilic micro-organisms to denatured com-pounds formed in pre-treatments [13]. Therefore, systematic stud-ies related to thermophilic anaerobic digestion of pulp and papermill secondary sludges subjected to various pretreatments arelimited.

Due to the compositional difference between the municipalsludges and pulp and paper mill secondary sludge, a systematiccomparison of different pretreatments on sludge solubilizationand methane yields is essential in order to assess the options intreating the pulp and paper mill secondary sludge. The objectiveof this study was to investigate the effect of hydrothermal, enzy-matic, ultrasound and chemical pretreatments alone or in combi-nation on organic matter solubilization and methane yields ofpulp and paper mill secondary sludge in batch assays at 55 �C.The pretreatment conditions studied were selected based on theliterature, previous experience e.g. on bioethanol process and dis-cussions with researchers at the pulp and paper industry, andkeeping in mind that the composition of the pulp and paper millsecondary sludge is different from municipal sludges [5]. Thermo-philic conditions were used as a previous study with the secondarysludge showed higher methane yields at 55 �C than at 35 �C insemi-continuously fed reactors [6]. Moreover, the surplus heatthat is commonly available at the pulp and paper mills can providethe energy required for the pretreatment as well as for biogasprocess.

pretreatments on methane yield of the pulp and paper mill

Operational parameters

–72 h, 50 �C30 min, 45 kHz24 h, 22 �C, pH 12 with 5 M NaOH24 h, 22 �C, pH 3 with 5 M HNO3

72 h, 50 �C30 min, 45 kHz + 24 h, 22 �C, 5 M NaOH40 min, 70 �C10 min, 150 �C30 min, 45 kHz + 72 h, 50 �C10 min, 150 �C + 72 h, 50 �C40 min, 70 �C + 72 h, 50 �C30 min, 45 kHz + 10 min, 150 �C

yme 30 min, 45 kHz + 10 min, 150 �C + 72 h, 50 �C

Page 3: Screening pretreatment methods to enhance thermophilic anaerobic digestion of pulp and paper mill wastewater treatment secondary sludge

Table 2Characteristics of the original and pretreated sludges in the experiments studying the effect of pretreatments on methane yield ofthe pulp and paper mill secondary sludge in batch assays at 55 �C.

Pretreatment SCOD (g/l) TS (%) VS (%) VS/TS (%) TVFA (g SCOD/l)

C1 1 4.7 3.9 83 0.7US 1 4.7 3.9 83 0.4AL 5 5.1 3.7 73 0.7AC 1 5.1 4.0 78 0.5US + AL 5 3.9 3.3 85 0.7HT70 4 4.3 3.6 84 0.6HT150 9 4.5 3.7 82 0.6US + HT150 9 4.6 3.8 83 0.5Enzymatic treatmentsC1 + E + buffera 1 4.0 3.3 83 0.6Ea 4 3.9 3.2 82 2.4US + Ea 4 4.0 3.3 83 N.d.HT150 + Ea 9 4.0 3.2 80 0.9HT70 + Ea 5 4.0 3.2 80 1.7US + HT150 + Ea 10 3.7 3.0 81 0.8

N.d. = not determined.a Estimated values based on determination of the values of enzyme and buffer solution, assumed that the values were

unchanged during the pre-treatments.

S. Bayr et al. / Chemical Engineering Journal 223 (2013) 479–486 481

2. Materials and methods

2.1. Substrate and inoculum

Secondary sludge from activated sludge plant treating waste-waters from an integrated bleached (chlorine dioxide, oxygen)kraft pulp (softwood and birch) and paper mill (producing coatedmagazine paper), Kaukas, Finland was used as substrate. The char-acteristics of the substrate are presented in Table 2. Digested mate-rial from a thermophilic biogas plant treating primary andsecondary sludges from a MWWTP (Stormossen, Finland) was usedas inoculum. The chemical composition of inoculum is as follows:2.9% total solids (TS), 1.7% VS, 3.4 g/l soluble chemical oxygen de-mand (SCOD) and 1.2 g/l NH4–N with a pH value of 8.1. The mate-rials were stored at 4 �C until further use.

2.2. Pretreatment methods

Five different pretreatment methods were used either alone orin different combinations. Pretreatments and the operationalparameters used in the study are summarized in Table 1. In total,12 different pretreatment combinations were evaluated. For chem-ical pretreatment, nitric acid (HNO3) and sodium hydroxide(NaOH) were used. HNO3 was chosen over H2SO4 or HCl althoughH2SO4 is more acidic than HCl and HNO3 on equal molar basisand as it was considered more useful also in other applications atthe studied pulp and paper mill. It must be noted that pulp and pa-per mill wastewaters has very low ammonium content and in facturea is commonly added to the wastewaters to facilitate theirtreatment in ASP. Dosage used in the NaOH chemical pretreat-ments were based on previous study on caustic treatment of pulpmill sludges [12].

Hydrothermal pretreatment was performed at two differentpretreatment conditions viz., 150 �C for 10 min (HT150) and70 �C for 40 min (HT70). The optimal pretreatment conditions forhydrothermal pretreatment for sludges from MWWTPs are at high-er temperatures of 160–180 �C and treatment times from 30 to60 min (e.g. reviewed by [8]). These conditions were generally ap-plied to improve the dewaterability and partial solubilization of or-ganic matter of the above sludges. In the present study, HT150 i.e.hydrothermal pretreatment at 150 �C for 10 min was chosen in or-der to avoid the production of inhibitory compounds due to carbo-hydrates degradation at high temperatures and long residencetimes. Previously, Wood et al. [12] have successfully demonstrated

hydrothermal pretreatment of sulfite mill and kraft mill secondarysludges at 170 �C for 1 h without production of inhibitory com-pounds. On the other hand, hydrothermal pretreatment at 70 �Cfor 40 min (HT70) was selected to evaluate if low temperature(<100 �C) thermal pretreatment prior thermophilic digestion haveany impact on COD solubilization and methane yields. Moreover,pretreatments at temperatures lower than 100 �C, albeit withslightly longer residence time, could also save energy costs.

For ultrasonic pretreatment (US), an operating frequency of45 kHz for 30 min was used as the solids loading in the presentstudy was high (4.7% TS). Traditionally, low frequencies of 20–40 kHz are commonly used in treatment of municipal sludges [8]with an optimal TS of 2.3–3.2% [14].

For enzymatic pretreatment (E), commercial enzyme Acceler-ase� 1500, an enzyme complex consisting of exoglucanase, endo-glucanase, hemi-cellulase and b-glucosidase, was used (GenencorInternational, The Netherlands). Enzyme loading rate of 0.07 g/g VS was as per the recommendations by the enzyme supplier forenzymatic hydrolysis of lignocelluloses [15]. Standard enzymatichydrolysis protocol i.e. incubation at 50 �C on a shaker (100 rpm)for 72 h was followed [16].

2.3. Experimental set-up

All the pretreatments were performed in closed systems to pre-vent the material losses during pretreatments. All the pretreat-ments, except HT pretreatments, were carried out in ten parallel118 ml serum bottles. After the pre-treatments six of the ten par-allel bottles were used for chemical analyses while the remainingfour bottles were used for biochemical methane potential (BMP)assays. For each pretreatment, 260 g of secondary sludge was used,which meant 26 g/bottle. For hydrothermal pretreatment, 260 g ofthe sludge was transferred to the hydrothermal reactor. After theHT pretreatment, treated sludge was mixed and distributed amongthe four BMP assays (26 g/bottle) and the remaining amount wasused for chemical analyses.

For ultrasonic pretreatment, ultrasonic apparatus Branson 5210was used with an operating frequency of 45 kHz for 30 min with-out any heating. During the pretreatment the open end of the bot-tles were closed with paraffin to prevent any losses of volatilecompounds. The temperature rise during the pretreatment wasless than 2 �C.

For alkali pretreatment, 5 M NaOH was added to the samples toreach pH 12 and incubated at room temperature for 24 h. Theamount of NaOH used was 0.38 g/g VS. After the pretreatment,

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482 S. Bayr et al. / Chemical Engineering Journal 223 (2013) 479–486

pH was adjusted to 7 with 5 M HNO3. For acid pretreatment,5 M HNO3 was used (0.48 g/g VS) to reach pH 3 and incubated atroom temperature for 24 h. After the treatment, pH was adjustedto 7 with 5 M NaOH.

Hydrothermal pretreatment was carried out using a high pres-sure (250 bars) and temperature (250 �C) reactor. After sample(260 g) loading, the headspace of the reactor was flushed withnitrogen gas to exclude oxygen and prevent any oxidation of theorganic compounds. The reactor contents were continuouslymixed by an in-built mixer. It took 35 min to reach 70 �C and55 min to reach 150 �C. During this period, the reactor pressureprogressively increased to reach a final pressure of 2 bars at70 �C and 5–6 bars at 150 �C. Upon completion, the samples wereallowed to cool to room temperature and the reactor was openedjust the following morning. In order to prevent the loss of organicvolatiles and also to avoid accidents and spillage of materials dueto high pressure and temperature, the reactor was not openedimmediately. Therefore, hydrolysis/solubilisation would have con-tinued during this cooling period.

In enzymatic pretreatment (E), commercial enzyme Accelerase�

1500 with an endoglucanase activity of 2200–2800 CMC U/g andthe b-glucosidase activity of 525–775 pNPG U/g was used. Enzy-matic hydrolysis was performed in ten 118 ml serum bottles. Toeach bottle, 26 g of sludge, 0.07 g/g VS of Accelerase� 1500 enzymecomplex and 5 ml of sodium citrate buffer (50 mM, pH 4.8) wasadded. The SCOD (0.8 g SCOD/l), volatile fatty acids (VFA), VS(0.22%), and TS (0.16%) of the added enzyme and buffer solutionwas analysed, and considered in assessing the impacts of enzy-matic pretreatments. The prepared assays were incubated at50 �C on a shaker (100 rpm) for 72 h in order to achieve 80–90%cellulose to glucose conversion efficiency.

After the pre-treatments, BMP assays were started by adding tothe four replicate 118 ml bottles 30 ml of inoculum to attain VSsub-

strate to VSinoculum ratio of 2. Water was added to get the desiredworking volume of 60 ml (65 ml in case of enzymatic pretreat-ments – due to 5 ml citrate buffer addition). NaHCO3 (0.2 g/bottle)was added as a buffer. The bottles were closed with butyl rubberstoppers and aluminum crimps. Prepared assays were flushed withnitrogen gas for 3 min. Assays with inoculum alone were used asblanks for all the treatments, except for assays with enzymatic pre-treatment. For enzymatic pretreatments, blank assays were pre-pared by inactivating the enzymes and citrate buffer mixer(boiled for 10 min) before the inoculation. Methane produced fromthe blank assays was subtracted from the respective sample assays.In addition, assays with untreated sludge and inoculum were usedas control assays. The prepared BMP assays were incubated stati-cally at 55 �C.

2.4. Analyses and calculations

TS and VS were analysed according to Standard Methods [17].pH was measured with a Metrohm 774 pH-meter (Metrohm, Swit-zerland). COD was analyzed according to SFS 5504 [18]. SCOD wasanalysed after filtration through a glass fiber filter paper (GF50,Schleicher and Schuell, Dassel, Germany, as against 0.45 lm mem-brane filter in the standard. Total VFA (TVFA) content was analyzedwith a gas chromatograph (GC) equipped with a flame ionizationdetector (FID; Perkin Elmer Autosystem XL GC, PE FFAP column30 m�0.32 mm�25 lm, carrier gas helium, oven 100–160 �C(20 �C/min), detector and injector 225 �C). VFA concentrationswere converted to SCOD with following coefficients: acetic acid1.066, propionic acid 1.512, iso-butyric and butyric acid 1.816,iso-pentanoic (iso-valeric) and pentanoic (valeric) acid 2.036 andhexanoic (caproic) acid 2.204 [19]. Cellulose, hemicellulose and lig-nin contents were analyzed as described elsewhere [6].

Gas samples were taken through stoppers from the gas phasewith a pressure-locked glass syringe (Supelco, Pressure-Lok� SeriesA-2 Syringe, Bellefonte, USA). Methane content in the gas sampleswas analysed with a GC equipped with a FID (Perkin Elmer ArnelClarus 500 GC, Perkin Elmer Alumina column 30 m�0.53 mm, car-rier gas argon, oven 100 �C, detector 225 �C and injector 250 �C).

Methane results were converted to standard conditions as de-scribed elsewhere [6]. Average methane yields of four replicateswere used to calculate methane yields as per VStreated and as perVSoriginal. Methane yield per VStreated is the amount of methane ob-tained from the added VS of the sludge after pretreatment. Meth-ane yield per VSoriginal is the amount of methane produced per VSof the untreated sludge. Hydrolysis was measured as solubilizationof organic matter and determined by the increase in SCODconcentration.

For methane yield comparisons, statistical analyses were donewith R. Shapiro–Wilk-test was used for testing if the results werenormally distributed. Since they were, ANOVA was used for study-ing if there were differences between treatments. Then Anova posthoc test Tukey was used to test which treatments differed from ori-ginal sludge in methane yields. P 6 0.05 was considered as the le-vel of significancy.

3. Results

The chemical composition of secondary sludge before and afterpretreatments is presented in Table 2. TS and VS values of un-treated sludge were 4.7% and 3.9% (in all enzyme pretreatmentsTS and VS values were 4.0% and 3.3% due to dilution with buffer),respectively, while some pretreatments were found to affect theTS and VS values. The major impact was noticed with combinedUS + AL treatment which resulted in decrease of TS to 3.9% andVS to 3.3%. However, US and AL treatments alone or in other com-binations did not result in such high changes. Enzyme pretreat-ments did not affect the TS and VS values. The increase in TSvalues in the chemically (AC, AL) pretreated sludge was obviouslydue to the addition of HNO3 and NaOH during chemical pretreat-ment and also to adjust the pH to 7 prior to incubation as also sug-gested by the decreased VS/TS ratio.

The SCOD concentration in the untreated sludge was 1 g/l(Table 2). All pretreatments, except for ultrasound and acid(HNO3) pretreatments, improved COD solubilization. SCODconcentration in pretreated sludge samples was increased up to4–10 g/l. Pretreatment combination of US + HT150 + E had thegreatest impact on COD solubilization. Effects of all pretreatmentson VFA concentrations were low, only enzymatic pretreatmentalone resulted in major increase in VFA concentration (Table 2).Thus the solubilized COD was other organic material than VFA.

Cellulose, hemicellulose and lignin contents in the untreatedand pretreated sludge are shown in Table 4. Pretreatments affectedthe concentrations of these biomass constituents. All pretreat-ments, except for HNO3 and NaOH pretreatments, resulted in a de-crease of cellulose content. Hydrothermal pretreatments (HT70and HT150) were the most effective pretreatments. Cellulose con-tent increased by 1.8 times in HT150 and US + HT150 pretreatedsludges. Correspondingly, a decrease in hemicellulose contentwas noticed in all pretreatments especially in hydrothermally pre-treated sludges (HT150 + E and US + HT150 + E). For lignin, pre-treatments resulted in a high variation: from a very low 110 g/kg VStreated noticed for US + AL pretreatment to a high 350 g/kg VStreated of lignin (HT150 + E).

Cumulative methane production rates and methane yields fromuntreated and pretreated sludge are presented in Fig. 1 and Table 3,respectively. Results show that methane production started imme-diately in all assays except for chemically pretreated assays. A lag

Page 5: Screening pretreatment methods to enhance thermophilic anaerobic digestion of pulp and paper mill wastewater treatment secondary sludge

Table 4Cellulose, hemicellulose and lignin contents of the original and treated sludges in theexperiments studying the effect of pretreatments on methane yield of the pulp andpaper mill secondary sludge in batch assays at 55 �C.

Treatment Cellulose (g/kgVS)

Hemicellulose (g/kgVS)

Lignin (g/kg VS)

C1 130 250 190US 160 220 160AL 110 250 190AC 110 240 190E 130 200 150US + AL 180 190 100HT70 150 250 210HT150 230 210 110US + E 160 200 130HT150 + E 110 130 300HT70 + E 170 230 160US + HT150 230 170 260US + HT150 + E 180 130 230

Table 3Methane yields (on d 11–12, on d 20–23 and final) and changes of the methane yields compared to original sludge in the experiments studying the effect of pretreatments onmethane yield of the pulp and paper mill secondary sludge in batch assays at 55 �C.

Treatment Methane yield (ml/g VSoriginal)

Change inmethane yield (%)

Methane yield (ml/g VSoriginal)

Change inmethane yield (%)

Methane yield (ml/g VSoriginal)

Change inmethane yield (%)

Methane yield (ml/g VStreated)

d 11–12 d 20–23 Final

C1 56 ± 1 – 67 ± 2 – 108 ± 5 – –US 58 ± 3 4 68 ± 4 1 114 ± 6 6 115 ± 6AL �5 ± 1 �109 11 ± 15 �84 86 ± 11a �20 91 ± 11AC �5 ± 0 �109 �3 ± 5 �99 61 ± 3a �44 59 ± 3E 44 ± 4 �21 66 ± 5 �1 114 ± 5 6 116 ± 5US + AL 23 ± 11 �59 31 ± 14 �54 67 ± 17a �38 77 ± 20HT70 59 ± 9 5 72 ± 9 7 112 ± 10 4 121 ± 10HT150 86 ± 1 54 97 ± 1 45 134 ± 2a 24 138 ± 2US + E 48 ± 1 �14 61 ± 1 �9 109 ± 1 1 108 ± 1HT150 + E 79 ± 3 41 101 ± 5 51 128 ± 6a 19 129 ± 6HT70 + E 59 ± 4 5 80 ± 3 19 124 ± 6 15 125 ± 6US + HT150 85 ± 5 52 105 ± 6 57 141 ± 6a 31 145 ± 6US + HT150 + E 88 ± 5 57 111 ± 6 66 131 ± 5a 21 140 ± 6

a Statistically significant (p 6 0.05).

S. Bayr et al. / Chemical Engineering Journal 223 (2013) 479–486 483

phase of 20–25 days was noticed in these assays indicating processinhibition. Methane production rates from assays subjected toHT150 treatment alone or in combination with enzymatic and/orultrasound pretreatments were faster than that of the controland other pretreatments (Fig. 1). For instance, HT150 treatmentalone and its combinations with enzymes and/or ultrasound pro-duced 52–57% more methane than control (56 ml/g VSoriginal) dur-ing the initial 11–12 days of incubation.

Untreated sludge had a methane potential of 108 ± 5 ml/g VSadded (Table 3). All pretreatments, except for chemical pretreat-ments (AL, AC and US + AL), resulted in higher methane yields thancontrol. The increase in methane yields was 4–31%. On methaneyields per original VS added (ml/g VSoriginal) basis, ultrasound fol-lowed by hydrothermal pretreatment at 150 �C for 10 min(US + HT150) was the best pretreatment. A statistically significantincrease in methane yields (p < 0.05) was noticed in all pretreat-ment combinations where hydrothermal pretreatment HT150was involved. The increase in methane yields was 19–31% (Table 3).In addition, a statistically significant increase in methane yieldswere also noticed when enzymatic pretreatment was used in com-bination with HT150 (HT150 + E) and with ultrasound and HT150pretreatment (US + HT150 + E). The methane yields obtained inthe above combinations were 128–131 ml/g VSoriginal (129–140 ml/g VStreated). However, no statistically significant increasein methane yields were noticed when enzymes were used alone(E) or in combination with US or HT70. Similarly, ultrasound pre-

treatment had no statistically significant impact on methane pro-duction as the methane yield of 114 ± 6 ml/g VSoriginal

(115 ± 6 ml/g VStreated) was obtained from the pretreated sludge.On the contrary, both chemical pretreatments (acid and alkaline)resulted in a lower methane yields (20–44%) than untreatedsludge.

4. Discussion

The impact of five different pretreatments and their combina-tions on the organic matter solubilization and methane yields ofpulp and paper mill secondary sludge under thermophilic condi-tions was evaluated. Among the studied pretreatments, hydrother-mal pretreatment (HT150) alone or in combination withultrasound and/or enzyme pretreatments increased the methaneyields of the pretreated sludge by 19–31% (Table 3). This increasedmethane yields in all assays subjected to HT150 pretreatment wasattributed to the improved COD solubilization. The fraction ofSCOD concentration in all HT150 pretreated sludge was increasedby more than 9 times after the pretreatment (Table 2). These re-sults are in accordance to those reported for hydrothermal pre-treatment of pulp and paper mill secondary sludge by Woodet al. [12]. In the above study, hydrothermal pretreatment at170 �C for 1 h increased the SCOD concentration by 6 times (from1.4 to 8.5 g/l) and biogas yield by 50% from sulfite mill secondarysludge and similar increase in SCOD value (from 0.3 to 6.5 g/l)and biogas yield by 280% from kraft mill secondary sludge. Thelower increase in biogas/methane yields in the present study com-pared to Wood et al. [12] study, albeit performed under mesophilicconditions, could be due to the difference in the pretreatment con-ditions and/or characteristics of sludges used in these studies. Inthe present study, hydrothermal pretreatment was performed at150 �C for 10 min while Wood et al. [12] applied thermal condi-tions of 170 �C for 1 h. Furthermore, the results of these two stud-ies are not strictly comparative as Wood et al. [12] measuredbiogas yields per added COD while in the present study methaneyields per VS were measured. Thermal pretreatment has beenstudied to improve sludge dewaterability and solubilization usinga wide range of temperatures ranging up to 200 �C with optimumtemperature in range of 160–180 �C and treatment times from 30to 60 min (reviewed by [8]). However, treatment time is oftenshown to have little effect at this temperature range [20], and evena one min thermal pretreatment at 170 �C has been proposed [21].Nevertheless, the increase in biodegradability and methane yieldsduring thermal pretreatment has been attributed to the fact that

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Control US AL AC E

US+AL HT70 HT150 US+E HT150+E

HT70+E US+HT150 US+HT150+E

(A)

(B)

Fig. 1. Methane yield (ml/g VSoriginal) of the original and treated secondary sludge samples during the whole experimental period (A) and within first 30 days and (B) in theexperiments studying the effect of pretreatments on methane yield of the pulp and paper mill secondary sludge in batch assays at 55 �C. Methane production of blanks(inoculum + inactivated enzymes and buffer in E treatment assays, and inoculum alone in other assays) was reduced from methane productions of the substrate assays.

484 S. Bayr et al. / Chemical Engineering Journal 223 (2013) 479–486

thermal treatment cause disrupting of chemical bonds in cell wallsand membranes and thus releasing intracellular organic materialfor further biological degradation [22]. In the present study, HTat 150 �C for 10 min seems reasonable as treatments at tempera-tures higher than 170–190 �C have shown to decrease sludge bio-degradability in spite of achieving high COD solubilization asreviewed by Carrere et al. [8]. The decreased biodegradability isdue to formation of toxic, refractory compounds at these high tem-peratures, which are hardly degradable by anaerobic microorgan-isms (e.g. [23]).

The relatively small and statistically insignificant increase inmethane yields (4%) noticed when hydrothermal pretreatmentwas employed at 70 �C for 40 min (HT70) indicates that the shortresidence times used in the present study compared to previousstudies [24,25] had low impact on COD solubilization. Previousstudies with municipal sludges have shown that thermal pretreat-ment at 70 �C may require long treatment times lasting several

hours to days to achieve significant increase in methane yield orCOD solubilization [24,25]. Moreover, studies on combination oflow temperature (<100 �C) thermal pretreatment prior thermo-philic digestion have been also reported to have notable impacton COD solubilization and methane yields. The mechanism of in-crease in COD solubilization at <100 �C is assumed to be enzymatichydrolysis [8].

In addition to hydrothermal pretreatment, enzymatic pretreat-ment also improved methane yields but only when combined withhydrothermal (HT150 + E) or ultrasound + hydrothermal(US + HT150 + E) pretreatments. The increased methane yield uponenzymatic pretreatment of the HT150 pretreated sludges (15–21%), was most probably due to the enhanced solubilization of or-ganic matter by the hydrothermal pretreatment and subsequenthydrolysis of the released cell contents by enzymes. The enzymeAccelerase 1500� used in the present study is an enzyme complexconsisting of exoglucanase, endoglucanase, hemi-cellulase and

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b-glucosidase. These enzymes hydrolyze the cellulose and hemi-celluloses to monomer sugars. This increased solubilization andhydrolysis was evident by 4–10 times increase in SCOD concentra-tion (Table 2). However, the lower or insignificant increase inmethane yields noticed after enzymatic pretreatment alone or incombination with ultrasound pretreatment, despite an increasein SCOD and VFA concentration, is unclear. The highest increasein VFA concentration (3.4 times) was noticed in enzymaticallytreated sludge (Table 2). It should be also noted that the pH inthe enzymatically pretreated sludge was around 6.8 and no pHadjustment was made prior to anaerobic incubation. In a similarstudy, Karlsson et al. [3] also did not observe any increase in meth-ane yields from enzymatically (mixture of cellulases, proteases andlipases) pretreated pulp and paper mill (kraft/CTMP) secondarysludge when the methane yield of enzymes and chelating mixturewas taken into account. The above authors suggested that the highviscosity of the reaction mixture in the enzymatically treatedassays could have limited the availability of binding sites for theenzymes. These results suggest standard enzymatic hydrolysisprotocol and dosage recommended for the enzymatic hydrolysisof lignocellulose for bioethanol production cannot be used for pre-treatment of secondary sludge for biogas production.

The possible reason for a slight increase (6%), although statisti-cally insignificant, in methane yields by the ultrasound pretreat-ment could be that the pretreatment conditions used in thepresent study were less effective to cause cavitation, which is fa-voured at low frequencies. According to Carrère et al. [8], low fre-quencies of 20–40 kHz have been shown to be the most efficient inmunicipal sludge treatment. The frequency used in the presentstudy was around 45 kHz. Moreover, the solids loading used inthe present study was slightly higher (4.7% TS) compared to theoptimal loading of 2.3–3.2% TS required for effective sonication[14]. Ultrasound pretreatment at high solids concentration hasshown to increase viscosity and thereby hindering cavitation bub-ble formation [14]. This was also evident in the chemical analyseswhere there was no increase in SCOD concentration upon ultra-sound pretreatment (Table 2). A similar observation where sonica-tion was least effective in improving COD solubilization ofsecondary sludge from pulp mill [12] and municipal sludge [26]were reported. The low impact of sonication on the solubilizationof sludge in the above study was attributed to the high levels ofsuspended solids in the sludge [12]. Unfortunately, suspended sol-ids were not analyzed in the present study. Nevertheless, mixed re-sults were reported in the literature on the impact of ultrasoundtreatment on COD solubilization and methane yields (see e.g.[8]). Wood et al. [12] reported no significant effect of ultrasoundpretreatment on COD solubilization and biogas yield of sulfite millsecondary sludge and kraft mill secondary sludge and also Karlssonet al. [2] reported low (0–10%) increase in methane yields of kraftand kraft and CTMP mill secondary sludge. On the contrary, Sahaet al. [13] reported 51% and 28% increase in methane yields fromultrasound pretreated BCTMP secondary sludge at both 35 and55 �C. The increased COD solubilization and methane yields duringultrasound pretreatment in the above studies was attributed to thefact that ultrasound causes thinning of the cell wall which contrib-utes the cell membrane breakage and release of cell contents forfurther hydrolysis [27].

The low impact of chemical pretreatments (HNO3 and NaOHpretreatment) on the COD solubilization and/or methane produc-tion could be due to ammonia inhibition as HNO3 acid was usedfor acid pretreatment. Addition of HNO3 may have resulted inammonia production (not analyzed) and thus inhibited the hydro-lysis and methane production. This was evident from the extre-mely long lag phase (25 d) and low rates of methane productionin the assays subjected to HNO3 pretreatment. Moreover, pH dur-ing the acid pretreatment has profound effect on the COD solubili-

zation [28]. In the present study, the pH in the HNO3 treated assayswas 3.2–3.6. Previous studies have shown that HCl acid pretreat-ment to pH 3 or 4 resulted only in a little increase in methane yieldwhile pH 2 was the most effective for improving the COD solubili-zation and increased the methane yield by 10.8% from thickenedMWWTP secondary sludge [28].

Despite an increase in SCOD concentration by 5 times, no signif-icant increase in methane yields from NaOH pretreated sludge wasnoticed (Table 3). This was apparently due to the ammonia inhibi-tion (not analysed) and was evident from the lag phase and lowmethane production rates from the NaOH pretreated sludge. UponNaOH pretreatment, HNO3 was used for adjusting the pH of pre-treated sludge from 12 to 7 prior to incubation. Nevertheless, sev-eral studies have shown to improve methane yields fromsecondary sludge subjected to alkaline pretreatments (see e.g.[8]). For instance, NaOH pretreatment was reported to increasebiogas yield of kraft secondary sludge by 280% and sulfite second-ary sludge by 18% [12]. Similarly, Navia et al. [29] also observedthat the NaOH/KOH pretreatment of kraft mill sludge increasedSCOD/total chemical oxygen demand (TCOD) ratio which probablywould increase methane yield of the sludge. The increase in meth-ane yields from the alkali pretreated biomass is mainly attributedto enhanced hydrolysis caused by the disruption of the ester bondscross-linking of lignin and xylan [30].

The possible sources for organic matter losses, noticed in somepretreatments in the present study, despite taking all precaution-ary measures, are due to evaporation of volatile compounds likeVFAs during thermal pretreatments and/or microbial degradationof the organic matter. Therefore, methane yields expressed as perVStreated were somewhat higher than those expressed as perVSoriginal. The obtained methane yields in the present study (67(22 days) and 108 ml/g VS original (150 days)) were not exception-ally low as referred to yields presented for pulp and paper second-ary sludges (see introduction), but anyway in the lower range.Besides the specific characteristics of the studied substrate, alsothe inoculum may effect the methane yield. The inoculum usedin the present study was digestate from a thermophilic biogasplant treating primary and secondary sludges from a municipalwastewater treatment plant, and thus not adapted for pulp and pa-per mill materials. The long duration of the assays should have alsofacilitated conversion of slowly degradable material, but anywayrecalcitrant materials cannot be degraded.

The increase in cellulose with corresponding decrease in hemi-cellulose content as seen in most pretreatments, except for chem-ical pretreatments, indicates that the pretreatments were effectivein solubilizing/removing hemicelluloses and thereby improvingthe accessibility of cellulose to further hydrolysis [31,32]. On thecontrary, the decrease in carbohydrates content, in case of chemi-cal pretreatments, indicate conversion of these organic compoundsto potential inhibitor compounds. Samuel et al. [32] studying theeffect of steam, lime and acid pretreatments on switch grass ob-served a decrease in hemicellulose concentration while celluloseand lignin concentrations either decreased or increased dependingon the treatment.

Finally, the faster methane production rates with higher meth-ane yields noticed in assays subjected to HT150, HT150 + E,US + HT150 and US + HT150 + E indicate the possibility to operatebiogas plants at a shorter HRTs than 20–30 days found in labora-tory studies for 55 �C CSTR with untreated pulp and paper millsludges as reviewed by [11]. According to the present study, HRTduring the thermophilic digestion of the studied substrate couldbe reduced by 50% (10–15 d) especially with hydrothermally pre-treated sludge (HT150). The possibility to shorten the HRT duringanaerobic digestion of pulp and paper mill secondary sludge mayprovide several techno-economical advantages and thus needs fur-ther studies on pretreatment and energy balance. It should be

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noted that in the present study, the energy balance for the pre-treatment has not been considered and thus could be a subject offurther study.

5. Conclusions

The effect of 12 different pretreatments (consisting of single orcombined treatment process) on methane yield of the pulp andpaper mill secondary sludge was studied. The studied pretreat-ments included hydrothermal, enzymatic, ultrasound and chemicalmethods. The highest methane yields were obtained with hydro-thermally (150 �C, 10 min) pretreated sludge, both alone and incombination with other pretreatments such as enzyme and/orultrasound. The increase in methane yields was by 31% (from108 ml/g VSoriginal to 141 ml/g VSoriginal). In addition, enzymaticpretreatment also improved the methane yields in combinationwith hydrothermal 150 �C and ultrasound + hydrothermal 150 �Cbut not alone. Ultrasound treatment had no effect while acid andalkaline pretreatments decreased the methane yields. Higher meth-ane production rate noticed with hydrothermally pretreated mate-rial alone or in combination with enzymes and ultrasound couldallow full-scale plants to be operated at a shorter retention times.

Acknowledgements

This project was part of the project BIOVIRTA (processing biogasplant digestates into value-added products, 40281/08) co-financedby The Finnish Funding Agency for Technology and Innovation(Tekes) and several companies. The authors greatly acknowledgeMervi Koistinen for her excellent laboratory work.

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