Bioresource Technology 101 (2010) 3466–3473

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Bioresource Technology

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Freezing/thawing effects on anaerobic digestion of mixed sewage sludge

A. Montusiewicz *, M. Lebiocka, A. Ro _zej, E. Zacharska, L. PawłowskiInstitute of Environmental Protection Engineering, Faculty of Environmental Engineering, Lublin University of Technology, ul. Nadbystrzycka 40B, 20-618 Lublin, Poland

a r t i c l e i n f o

Article history:Received 30 September 2009Received in revised form 11 December 2009Accepted 19 December 2009Available online 27 January 2010

Keywords:Freezing/thawing pre-treatmentSludge disintegrationSludge solubilizationMixed sewage sludgeEnhanced biogas yield

0960-8524/$ - see front matter � 2010 Elsevier Ltd. Adoi:10.1016/j.biortech.2009.12.125

* Corresponding author. Tel.: +48 81 538 4325; faxE-mail address: A.Montusiewicz@wis.pol.lublin.pl

a b s t r a c t

This study examined the effects of freezing/thawing disintegration on the characteristics of mixed (pri-mary and waste) sewage sludge from municipal wastewater treatment plants. It also assessed the effectsof freezing/thawing on anaerobic digestion, and its consequences for biogas production and digestersupernatant quality. Freezing/thawing caused a decrease of more than 10% in the total chemical oxygendemand (COD), total nitrogen (TN), volatile solids (VS) and total solids (TS). A simultaneous doubling ofthe soluble COD and volatile fatty acids (VFA) occurred. Release of nitrogen and phosphorus compoundswas also found.

The biogas yield obtained from frozen/thawed sludge was 1.31 m3 kg�1 of removed VS; this exceededapproximately 1.5 times the value for the raw sludge.

On the basis of the global mass balance it was indicated that freezing/thawing of the mixed sewagesludge followed by anaerobic digestion could be considered as a ‘‘double-phase digestion” rather thana pre-treatment method.

� 2010 Elsevier Ltd. All rights reserved.

1. Introduction

Current thinking is that anaerobic digestion of sewage sludgeshould focus both on highly efficient sludge stabilization and onenhanced biogas production. Operational data relating to munici-pal wastewater treatment plants (WWTPs) have shown that anaer-obic digesters work with insufficient loads and possible capacityreserves reach as much as 30% of the digesters’ volume (Braun,2002). This has particularly been shown at high flow rates. In suchcases, co-fermentation of sewage sludge and other organic compo-nents could be introduced to improve biogas generation (Mont-usiewicz, 2008). Another fairly recent technological advancementthat enhances anaerobic digestion efficiency and increases biogasproductivity has been the development of pre-treatment tech-niques that accelerate the hydrolysis of sludge (Elliott and Mah-mood, 2007). Sludge pre-treatment incorporates differentdisintegration methods, such as mechanical (e.g., sonification,hydrodynamic cavitation), chemical (e.g., alkali treatment, ozona-tion), thermal (e.g., heat treatment, freezing/thawing) and biologi-cal (e.g., treatment by enzymes) (Erdinclerm and Vesilind, 2000;Saktaywin et al., 2005; Xue and Huang, 2007; Jan et al., 2008).Some of these methods are thought to improve the dewateringcapability. However, most pre-treatments for municipal activatedsludge lead to cell disruption (Bien et al., 2004) that promotes sol-ubilization of organic matter and releases intracellular and cellwall polymers (including polysaccharides, proteins, lipids and

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: +48 81 538 1997.(A. Montusiewicz).

other macromolecules) into the liquid phase. Interestingly, differ-ent mechanisms are responsible for membrane damage. Thehydrolysis of sludge can be accomplished by its exposure to wetmilling with small beads (mechanical disintegration), or to high-temperature and high-pressure gradients resulting from the rapidcollapse and expansion of microbubbles (sonification, hydrody-namic cavitation), or to highly oxidative conditions provided byozone, which reacts with components of cell membranes and con-verts them into smaller molecular-weight compounds (Elliott andMahmood, 2007). A high-temperature treatment sterilizes thesludge, which splits the cells due to pressure differences duringthe hydrolysis process (Keep et al., 2000). Treatment by enzymesincorporates primary and secondary cell lysate (formed in lysate-thickening centrifuges), which contains some enzymes, parts of en-zymes and cofactors that accelerate cellular degradation and sup-port bacterial growth (Dohányos et al., 1997). Freezing causes theformation of intracellular crystals, which mechanically damage cellmembranes (Thomashow, 1998).

Freezing/thawing processes frequently occur in nature, leadingto changes in soil characteristics (Eigenbrod, 1996). The composi-tion of sewage sludge that is treated and dumped outside in coldregions could also vary as a result of freezing (Rush and Stickney,1979; Reed et al., 1986). In municipal wastewater treatment plantsfreezing/thawing could be used for highly efficient dewatering(Martel, 1989). Sewage sludge dewatering by freezing is accom-plished through the separation of solid and liquid fractions duringthe formation of ice crystals. It is stated that freezing promotestransformation of the flocs into a more compact form (Chu et al.,1999; Jean et al., 2001). However, to obtain the best results, the

A. Montusiewicz et al. / Bioresource Technology 101 (2010) 3466–3473 3467

sludge must be completely frozen at a relatively slow rate sincebeneficial effects of colder temperatures, longer periods of freezingand slower freezing rates were noticed (Vesilind and Martel, 1990;Örmeci and Vesilind, 2001; Wang et al., 2001). According to Reedet al. (1986), sludge freezing may convert a non-drainable jelly-likeconsistency into a granular material that drains immediately whenit thaws. This is particularly important for sludge that containsalum due to its extremely low dewatering capability by gravitydrainage (Martel (2000), the aggregated particles found in frozen/thawed alum sludge are described as ‘‘coffee ground”). Precondi-tioning allows the achievement of more than 20% of dry mass insludge as soon as thawing is completed. Moreover, some furtherdrying provides concentration of solids of up to 50%, which is notpossible using mechanical devices.

Most studies concerning the mechanisms of freezing/thawingprocesses and their influence on the qualitative characteristics ofthe product relate to waste-activated sludge and alum sludge (Ves-ilind et al., 1991; Örmeci and Vesilind, 2001). When sludge is fro-zen, both the suspended and dissolved solids are rejected by agrowing ice front (Vesilind et al., 1991; Martel, 2000). This dis-placement gathers the solids into larger particles. As a result offreezing, the sludge is being converted to a matrix of ice crystalsand aggregated solid particles. Martel (2000) found differences inmorphology of the ice crystals. He reported that the alum sludgeice crystals grow in columns. Whereas growth of the activatedsludge ice crystals becomes dendritic (dendrites denote branchingtree-like structures), which was attributed to the presence of dis-solved solids in activated sludge. Water surrounding the particlestrapped in the ice creates a very thin layer, commonly called a‘‘transition layer”, that does not freeze at normal temperatures.Vesilind et al. (1991) stated that the transition layer is more easilyreplenished when the particles are small.

Chu et al. (1997), similar to Kawasaki and Matsuda (1995),investigated the effect of electrolyte (sodium chloride) additionon the subsequent freezing/thawing of the excess activated sludge,and thus on its dewater ability, settleability and residual moisture.The authors concluded that addition of sodium chloride retards thesludge particles migration and remarkably improves the sludgefilterability.

Örmeci and Vesilind’s studies (2001) regarding activated sludgeand alum sludge indicated that freeze–thaw conditioning causescell disruption and release of intracellular material to the sludgesupernatant. In activated sludge the process mentioned stronglyincreases the concentration of proteins, carbohydrates and cationsin the supernatant, however in alum sludge a noticeable release isnot observed. The authors concluded that to improve activatedsludge dewater ability the concentration of proteins, carbohy-drates and cations should previously be decreased.

The literature review by Pérez-Elvira et al. (2006) shows thatthe influence of sludge pre-treatment technology preceding itsanaerobic digestion on the enhanced biogas production, sludge re-moval efficiency and pathogen reduction was so far investigatedregarding different methods, although the freezing/thawing tech-nique was examined only with reference to the dewatering ability(Reed et al., 1986; Martel, 1989; Chu et al., 1997; Örmeci and Ves-ilind, 2001; Wang et al., 2001). Its involvement in anaerobic diges-tion seems to be counter-productive due to high energyconsumption. This does not apply to some countries that have spe-cific climate conditions, or that have access to natural or artificialice – thereby allowing freezing without additional operationalcosts. The practical aspects of this process and its effect on theanaerobic stabilization of sewage sludge should be examined. Ifthe production of biogas could be increased, the whole processwould become more cost-effective despite environmentalrestrictions.

Only a few studies regarding sludge pre-treatment via freezing/thawing in the enhancement of its anaerobic digesting efficiencywere found; furthermore, they concern merely waste-activatedsludge (Wang et al., 1995). Reports relating to the evaluation ofthe effects of such pre condition technique on the chemical compo-sition of mixed sewage sludge (which consists of primary sludgeand waste-activated sludge) were not found. Similarly, its influ-ence on anaerobic digestion process and biogas production wasnot investigated. Considering a specific character of the primaryand waste-activated sludge and taking into account that mixedsludge is a typical digester feed in the full-scale systems, these is-sues are worth researching.

In the present study, the influence of freezing/thawing disinte-gration on the change in characteristics of mixed sewage sludgewas evaluated. Moreover, its effect on anaerobic digestion wasanalyzed based on changes in bioreactor operational conditions,enhanced biogas production and the quality of the digester super-natant. The freezing/thawing technique followed by the mixedsludge anaerobic digestion was evaluated on the basis of the globalmass balance calculations and the question of whether freezing/thawing disintegration could be considered as a pre-treatmentmethod or as the first stage of the ‘‘double-phase digestion”.

2. Methods

2.1. Material characteristics

Sewage sludge that included two-source residues was obtainedfrom the Puławy municipal WWTP, Poland, using primary and sec-ondary treatments. Sludge originating from a gravity thickener (i.e.primary thickened sludge) and from a mechanical belt thickener(i.e. waste thickened sludge) was used as material for our study.The characteristics of thickened sludge from Puławy WWTP areshown in Table 1. It should be noted that wastewater from themeat industry, which supplied the municipal sewer system, influ-enced the sludge characteristics, especially with regard to the totalCOD value (high levels were observed).

2.2. Sample preparation procedure

Sludge was sampled daily in the Puławy municipal WWTP andthen provided immediately to the laboratory of the Lublin Univer-sity of Technology (Poland). Sludge from a gravity thickener andfrom a mechanical belt thickener was transported in separate con-tainers. Under laboratory conditions, sludge was mixed at a vol-ume ratio of 60:40 (primary:waste sludge), then homogenizedand partitioned. Such samples were considered to be raw sludge(RS). Some of the samples were disintegrated using a freezing/thawing technique. The sludge was frozen at �25 �C for 24 h in alaboratory freezer and then thawed for another 12 h at 20 �C inthe indoor air. Freezing/thawing conditions were selected similarlyto Jan et al.’s studies (2008); however, some modifications wereapplied. The preliminary experiments showed that the mixedsludge froze completely at �25 �C and at least 12 h were requiredto completely thaw the samples at room temperature. The as-sumed operational conditions complied with these results. Thesamples prepared using the freezing/thawing procedure were con-sidered to be frozen/thawed sludge (FTS).

2.3. Laboratory installation of sludge anaerobic digestion

The laboratory installation consisted of two anaerobic digestionsystems operating in parallel. Each of them included a completelymixed, hermetic reactor with a working volume of 40 dm3 insertedinto the heating jacket at a stable temperature and equipped with

Table 1Characteristics of thickened raw sludge from the Puławy (Poland) municipal wastewater treatment plant during 2008.

Value Primary thickened sludge Waste thickened sludge

TSg kg�1

VS% (of TS)

TSg kg�1

VS% (of TS)

Average 30.35 75.31 60.85 76.47Upper/Lower 95% mean 32.45/28.24 77.11/73.51 66.81/54.88 78.03/74.92

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the required instrumentation. Mixing was carried out using amechanical stirrer that has a rotational speed of 50 min–1. Bioreac-tor equipment included an influent peristaltic pump feeding the di-gester, as well as storage vessels both for influent and effluent(digested medium). At the top of the reactor, an attachment forbiogas installation was fitted. The gas system consisted of pipelineslinked with the pressure equalization unit and the drum gas meter.This was equipped with gas valves, a dewatering valve and a gassampler with a rubber septum, which enabled insertion of a syr-inge with a pressure lock. The laboratory installation is shown inPhoto 1.

2.4. Operational set-up

An inoculum for the laboratory reactors was taken from thePuławy WWTP as a collected digest from a mesophilic anaerobicdigester operating at 35–37 �C with a volume of 2500 m3 and ahydraulic retention time of about 18 days. The adaptation of the di-gester biomass in the systems under investigation was achievedafter 30 days.

The experiment was carried out simultaneously in two parallelsystems operating at a controlled mesophilic temperature of 35 �C.The semi-flow reactors were supplied regularly once a day with 2 Lof feed sludge. The reactor RS (control) was fed using raw sludge,whereas the reactor FTS (experimental) was supplied using fro-zen/thawed sludge. The hydraulic retention time reached 20 daysand the hydraulic loading rate was 0.05 d–1 for both reactors. How-ever, the organic loading rate (OLR) differed for the RS and FTSreactors and was 1.52–1.62 kg VS m–3 d–1 for the RS system and1.27–1.35 kg VS m–3 d–1 for the FTS system, respectively. The studywas run over 60 days.

Photo 1. Laboratory installation of sludge anaerobic digestion.

2.5. Analytical methods

For raw and frozen/thawed sludge, the following parameterswere analyzed each day: total chemical oxygen demand (COD),volatile fatty acids (VFA), total solids (TS), volatile solids (VS), totalnitrogen (TN), alkalinity, sludge density and pH level. The sameschedule was used for determining the values of parameters thatcharacterized supernatant (sludge liquid phase) before digestion:soluble COD (SCOD), total nitrogen in supernatant (TNs), ammonianitrogen (N–NHþ4 ), nitrite and nitrate nitrogen (N–NO�x ) and ortho-phosphates (P–PO3�

4 ). The supernatant samples were obtained bycentrifuging the sludge at 4000 r min–1 for 30 min.

In digested sludge, specified parameters were determined threetimes a week, in accordance with the assumed timetable. Similarly,the supernatant of digested sludge was examined using the sameschedule.

Most analyses were carried out in accordance with the proce-dures in the Polish Standard Methods. Some analyses were per-formed with FIASTAR 5000 using FOSS analytical methods:ammonium was determined according to ISO 11732, nitrite and ni-trate – according to ISO 13395 and ortho-phosphate in accordancewith ISO/FDIS 15681–1.

The anaerobic digestion efficiency was controlled by the dailyevaluation of biogas yield and its composition (CH4, CO2 and othergases). Biogas production was determined using Drum-type Gas-meter TG-Series (Ritter, Germany). The composition of the biogaswas measured using a gas chromatograph Shimadzu GC 14B cou-pled with a thermal conductivity detector (TCD) fitted withglass-packed columns. The Porapak Q column was involved todetermine CH4 and CO2 concentrations. The parameters used forthe analysis were as follows: injector 40 �C, column oven 40 �C,detector 60 �C and current bridge 150 mA. The carrier gas was he-lium with a flux rate of 40 cm3 min–1. Peak areas were determinedusing the computer integration program (CHROMA X).

3. Results and discussion

3.1. Effect of freezing/thawing on sludge characteristics

The characteristics of raw and frozen/thawed sludge that sup-plied, respectively, the RS (control) and FTS (experimental) systemare listed in Table 2 (average data are reported).

In comparison with the raw sludge, a 12% decrease in the aver-age total COD concentration was observed for the frozen/thawedmedium. Simultaneously, a decline in TS, VS (Table 2) and TN (Ta-ble 3) values of 16.1%, 16.9% and 15.1%, respectively, were shown.There was a more than twofold increase in SCOD compared withthe average concentration as a result of freezing/thawing disinte-gration. Concomitantly, a rapid rise in VFA concentration (morethan double), as well as a simultaneous drop in pH value to 5.82,was observed. An increase in alkalinity (as CaCO3) from 1100 to1400 mg L–1 and a decrease in sludge density from 1008 kg m–3

to 1006 kg m–3 were also noticed. The above results indicated arelationship between the increase in soluble contaminants and,at the same time, the decrease in total organic matter in frozen/thawed sludge. A high concentration of both SCOD and VFA and

Table 2Characteristics of raw (RS) and frozen/thawed sewage sludge (FTS) supplied to RS and FTS systems.

Parameter Unit Value Raw sludge (influent to RS reactor) Frozen/thawed sludge (influent to FTS reactor)

pH – Average 6.39 5.82Upper/Lower 95% mean 6.46/6.32 5.87/5.77

Alkalinity mg L–1 Average 1100 1400Upper/Lower 95% mean 1157/1043 1470/1330

COD mg L–1 Average 43,844 38,566Upper/Lower 95% mean 45,531/42157 42,586/34,546

SCOD mg L–1 Average 3190 6756Upper/Lower 95% mean 3411/2969 7638/5874

VFA mg L–1 Average 1757 3755Upper/Lower 95% mean 1804/1710 3845/3665

TS g kg–1 Average 41.0 34.4Upper/Lower 95% mean 42.3/39.7 35.5/33.3

VS g kg–1 Average 31.4 26.1Upper/Lower 95% mean 32.3/30.5 26.9/25.3

Table 3Concentration of nutrients in raw and frozen/thawed sewage sludge.

Parameter Unit Value Raw sludge (RS reactor) Frozen/thawed sludge (FTS reactor)

TN mg L–1 Average 950 806.7Upper/Lower 95% mean 1119.7/780.3 1031.4/582.0

TNs (supernatant) mg L–1 Average 190 340Upper/Lower 95% mean 288.7/91.3 419.2/260.8

N–NHþ4 (supernatant) mg L–1 Average 94.0 130.9Upper/Lower 95% mean 99.7/88.3 154.9/106.9

P–PO3�4 (supernatant) mg L–1 Average 86.4 185.2

Upper/Lower 95% mean 100.4/72.4 199.6/170.8

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a simultaneous decrease in pH level seems to confirm the solubili-zation of organic matter and the release of intracellular material.This observation is consistent with the research by Stabnikovaet al. (2008) regarding food waste and Örmeci and Vesilind(2001) regarding activated sludge. Moreover, another study simi-larly reported that freezing/thawing disintegration caused an in-crease in the soluble fraction of carbohydrates in increased SCODduring the treatment of waste sludge from food-processing waste-water (Jan et al. 2008). The same authors indicated that a criticalvalue of pH (5.5) was required for initiation of the methanogenicstage, however according to Vavilin et al. (2001) only hydrogeno-trophic methanogenesis is suspected to be possible at such acidicpH. In the presented research, the average value of pH was 5.82,thus the boundary requirements for the initiation of the methano-genic stage were fulfilled.

In the present study the complex mechanisms of freezing/thaw-ing disintegration could lead to the formation of the specific buffer-ing conditions. However, the increase of the VFA concentration washigher than alkalinity changes which, as a consequence, caused thepH decrease.

It is known that the biological effects of biomass freezing aredominated by destruction of living cells (resulting from directmechanical action of ice crystals) or by damaging them via changesin the composition of the liquid phase (e.g. concentration of thesolutes) (Pegg, 2007). On the other hand, it is commonly acceptedthat freezing causes lethal effects for some microorganisms. How-ever, recent studies relating to the freezing of food have indicatedthat some microorganisms (even those that are considered to besensitive for inactivation) are only inhibited at low temperatures(Kennedy, 2000; Miladi et al., 2008). It is worth noticing that somefood components, such as proteins or fats, tend to protect themicroorganisms and act as cryoprotectants (e.g. glycerol). More-over, it is stated that a combination of slow freezing and rapid

thawing could favour the survival of the microorganisms (Ken-nedy, 2000).

The research by Örmeci and Vesilind (2001) regarding activatedsludge indicated that freeze–thaw conditioning caused cell disrup-tion and release of intracellular material to the sludge supernatant.Additionally, they reported the start of anaerobic reactions in theactivated sludge for 12 h of thawing. Following this, the authorspropose a similar explanation for the mixed sludge and suggestthat the COD, TS, VS and TN losses, and the observed solubilization,could result from two possible mechanisms that might take placesimultaneously, even though one of them could prevail. The firstmechanism involves the reaction of both the immobilizing exoen-zymes present in the system and the endoenzymes released fromthe cells disrupted as an effect of extracellular and intracellularice formation. The second approach suggests that biomass activitycould not be excluded and degradation could be carried out by themicroorganisms that survived in frozen sludge. The presence of thecryoprotectant components (e.g. proteins and fats) in the raw sew-age sludge could have an influence on the microorganisms, pro-tecting them from freezing, thus biodegradation lasting for 12 hduring the thawing stage could occur.

The analysis of the effects of freezing on nutrient concentrationseems to confirm sludge solubilization. The concentration of nutri-ents in raw and frozen/thawed sludge that supplied, respectively,the RS and FTS systems is listed in Table 3. An average reductionin TN affected by sludge freezing was found at approximately143 mg L�1. However, an average concentration of TNs in thesludge supernatant showed almost a twofold increase for the fro-zen/thawed sludge, thus indicating the nitrogen release from thebiomass. Moreover, higher concentrations of ammonia nitrogenand ortho-phosphates were also observed. Their values increasedby approximately 37 mg L�1 for ammonia nitrogen and by 99 mgL�1 for phosphorus as ortho-phosphates. Such observations are

3470 A. Montusiewicz et al. / Bioresource Technology 101 (2010) 3466–3473

consistent with results obtained by Xue and Huang (2007) duringthermal treatment of excess sludge. The authors found that heatingthe sludge at a temperature of no less than 50 �C led to the efficientrelease of not only phosphorus and nitrogen compounds, but alsoorganics and some metal cations.

Considering the similarity of the freezing and thermal treat-ment effects (which were, in part, confirmed in our investigation),it could be expected that some metal cations would be releasedand would increase the alkalinity of the sample. Such an increasewas observed during our experiment in frozen/thawed sludge thatbuffered acidification resulting from an increase in VFA concentra-tion and a decrease in pH level. It should be noted that the concen-trations of nitrite and nitrate nitrogen were negligible during allexperiments that were undertaken.

3.2. Effect of freezing/thawing on operational conditions and biogasyields

Table 4 includes essential values of loading rates in the systemsunder investigation. It should be noted that the hydraulic loadingrate and hydraulic retention time were the same in both reactorsand were 0.05 d–1 and 20 d, respectively.

Based on the differences in the characteristics of raw and disin-tegrated sludge, presented and discussed before, the higher loadingrates relating to VS and COD were estimated in the RS reactor (con-trol) and compared to the FTS reactor. However, loading rates eval-uated for the FTS reactor with regard to SCOD and VFA exceededmore than twice the values for the RS digester, which was in accor-dance with a higher concentration of solutes via freezing. The VFA/VS influent ratios were 0.056 and 0.144, respectively, in the sys-tems supplied with raw (RS) and frozen/thawed (FTS) sludge.

It is commonly accepted that a higher organic loading rate(according to Tchobanoglous et al. (2003), a value of 4.8 kg m–3

d–1 should not be exceeded) and a higher concentration of VS fa-vours more efficient anaerobic digestion, whereas a decrease ofsuch parameters diminishes biodegradation effects. This was con-firmed during the present studies on the basis of removal of VSat the level of 51.9%, with the average OLR being 1.57 kg VS m–3

Table 4Operating conditions for anaerobic digestion.

Reactor Value Loading rate(kg

VS

RS Average 1.57Upper/Lower 95% mean 1.62/1.52

FTS Average 1.31Upper/Lower 95% mean 1.35/1.27

Table 5Effluent quality of the anaerobic digestion process.

Parameter Unit Value

pH – AverageUpper/Lower 95% m

Alkalinity mg L–1 AverageUpper/Lower 95% m

COD mg L–1 AverageUpper/Lower 95% m

SCOD mg L–1 AverageUpper/Lower 95% m

VFA mg L–1 AverageUpper/Lower 95% m

TS g kg–1 AverageUpper/Lower 95% m

VS g kg–1 AverageUpper/Lower 95% m

d–1, and of 44.5% with an OLR value of 1.31 kg VS m–3 d–1, respec-tively, in the RS and FTS reactors.

In the investigated systems, however, various loading rates didnot influence the parameters of digested sludge. Results showedthat effluent parameters, such as TS and VS (Table 5), and TN (Ta-ble 7, presented and discussed later in section 3.3), were similar inboth treatments and the value differences amounted only to 6.8%for TS, 4.0% for VS and 4.6% for TN. A greater difference of 45.8%was found for SCOD as a twofold lower concentration was ob-served in the FTS reactor – probably due to microorganism adapta-tion for solute uptake. Moreover, high alkalinity (above 4000 mg L–

1) and low VFA concentration (not exceeding 300 mg L–1) and afavourable pH value of 7.3–7.4 indicated that a methanogenicphase was stable. Comparable parameters were achieved despitethe differences in feed sludge that supplied reactors and the vari-ous loading rates (see Table 4).

The average biogas yields attained during investigations areshown in Fig. 1. The biogas production per kg of VS removed, whichwas calculated in the RS system and amounted to 0.86 m3 kg–1 VS,was in compliance with the range of 0.75–1.12 m3 kg–1 VS that wassuggested by Tchobanoglous et al. (2003) as an adequate yieldfrom sewage sludge anaerobic digestion. A comparison of such val-ues in both systems showed that the biogas yield obtained fromthe frozen/thawed sludge exceeded approximately 1.5 times thevalue achieved from the raw sludge and reached 1.31 m3 kg–1 ofvolatile solids removed. A similar tendency was observed with re-gard to the biogas yields based on the removed TS and COD. Theincreased values were achieved in the FTS system despite a lowersolids loading rate and a minor concentration of VS. The possibleexplanation is that the solubilization of organic matter resultedfrom sludge disintegration and this was confirmed by both thehigh concentrations of SCOD and VFA. An increased content ofthe solutes regarded as the substrate, being more available andeasy to convert for microorganisms, resulted in an improved biogasyield.

During experiments, a comparable biogas composition in bothreactors was observed (Fig. 2). The average methane content was61.5% and 63.0%, respectively, in the system fed with raw and fro-

m–3 d–1) refers to:

COD SCOD VFA

2.19 0.16 0.0882.28/2.10 0.17/0.15 0.09/0.0861.93 0.34 0.192.13/1.73 0.38/0.30 0.195/0.185

Reactor RS Reactor FTS

7.73 7.84ean 7.87/7.59 8.06/7.62

4275 4350ean 4304/4246 4411/4289

23,734 19,130ean 25,256/22,212 20,574/17,686

2342 1270ean 2407/2277 1337/1203

263 217ean 379/147 288/146

24.9 23.2ean 25.7/24.1 23.8/22.6

15.1 14.5ean 15.3/14.9 15.2/13.8

Fig. 1. The biogas yields obtained from RS (raw sludge) and FTS (frozen/thawed)systems.

Fig. 2. The biogas composition observed in RS and FTS systems.

Fig. 3. The methane yield achieved from RS and FTS systems.

A. Montusiewicz et al. / Bioresource Technology 101 (2010) 3466–3473 3471

zen/thawed sludge; an average difference of only 1.5% was ob-tained. These results indicated that disintegration of mixed (pri-mary and waste) sewage sludge via freezing/thawing influencedthe share of biogas components to a minor extent.

The average methane yields achieved during the studies arepresented in Fig. 3. It should be noticed that higher methane pro-duction was found from frozen/thawed sludge in comparison toraw sludge. The observed increases were 36.1%, 14% and 37.6%,respectively, with regard to the removed VS, COD and TS. Wanget al. (1995) found, similarly, that methane generation was im-proved from waste-activated sludge that was pre-treated by freez-ing; however, a lower increase of about 27% was achieved withregard to VS removed. Providing feedstock for anaerobic digestionconsisting of mixed sludge instead of waste-activated sludge couldexplain a higher increase in methane yield in the present study.Methane is a source of energy for producing electricity and heat,and it is especially profitable in terms of enhancing its generationfrom sewage sludge.

To verify results achieved during experiments, the calculationsof mass balance in the form of measurable parameters were under-taken for the organic material fluxes (that entered and left thereactors). According to Grady et al. (1999), COD should be usedas a parameter for organic material measurement in an anaerobicprocess related to methane generation. Araújo et al. (1998) pro-posed an extended approach and calculated the COD/VS ratio asa constant that reflected the conversion of VS into COD mass.Moreover, to convert methane into COD mass, it was consideredthat there is a stoichiometric relationship of 4 mg COD mg–1 CH4

(Grady et al., 1999) and that 1 L of CH4 at 35 �C represents a CODmass of 64/25.3 = 2.53 g.

The mass balance in the digester, based on a COD mass in suchfluxes as an influent (MSinf), an effluent (MSeff) and methane(MSCH4 ), was expressed according to Araújo et al. (1998) as follows:

- COD in digester influent

MSinf ¼ Q � ðCODinf þ fcv � VSinfÞ ð1Þ

- COD in digester effluent

MSeff ¼ Q � ðCODeff þ fcv � VSeffÞ ð2Þ

- COD in methane

MSCH4 ¼ 2:53 � Q CH4ð3Þ

It should be noted that CODinf and CODeff values refer to SCOD con-centration and the fcv value expressed the COD/VS ratio for bothexamined treatments.

Araújo et al. (1998) proposed the fcv constant value of 1.5 mgCOD mg–1 VS for waste-activated sludge. However, in our study,based on a mixed sludge (primary sludge was also included), thevalues of COD/VS ratio were determined using the experimentaldata both for supplied and digested medium. The data requiredand results achieved from mass balance are shown in Table 6.

The percentage fraction of recovered organic material in the di-gester (included in Table 6) was calculated according to theequation:

B ¼ ½ðMSeff þMSCH4 Þ=MSinf � � 100 ð4Þ

The calculations of mass balance were based on experimentaldata and were therefore subject to errors. Unimportant differenceswere found between the fluxes of organic material entering thereactors and leaving them. These differences amounted to 2.3%and 3.2% for the RS (control) and FTS digester, respectively. Thisshows them to reflect normal measurement uncertainty and leavesthe 14% increase of methane yield per COD removed in FTS aremarkable value for all the mass balance errors.

The results of mass balance calculations exclude the effects ofsludge transformations during freezing/thawing pre-treatment.However, the sludge solubilization and acidification, as well asthe concurrent losses of 12% for COD and 17% for VS noticed afterfreezing/thawing process, indicate conversion of the organic com-pounds into soluble forms or their degradation. Therefore, thefreezing/thawing process followed by anaerobic digestion couldbe considered as a ‘‘double-phase digestion” rather than a pre-treatment method, thus the whole process should be involved inthe global digestion calculations. The results obtained using suchan approach showed similar values for the biogas yields per kg ofVS removed, respectively of 0.9 m3 kg�1 and 0.86 m3 kg�1 for thefrozen/thawed sludge and the raw one. The first value was calcu-lated concerning the ‘‘double-phase digestion” and quantifyingthe volatile solids that were previously converted to SCOD andVFA in frozen/thawed sludge; the second one was obtained forconventional system (sludge anaerobic digestion without pre-treatment RS). The biogas yields calculated per VS fed were also

Table 7Concentration of nutrients in digested sludge.

Parameter Unit Value RS effluent FTS effluent

TN mg L–1 Average 756.7 793.3Upper/Lower 95% mean 821.0/692.4 912.2/674.4

TNs (supernatant) mg L–1 Average 426.7 466.7Upper/Lower 95% mean 491.0/362.4 540.3/393.1

N–NHþ4 (supernatant) mg L–1 Average 750.1 775.6Upper/Lower 95% mean 786.6/713.6 840.4/710.8

P–PO3�4 (supernatant) mg L–1 Average 137.2 133.6

Upper/Lower 95% mean 148.8/125.6 144.4/122.8

Table 6COD mass balance for anaerobic digesters – data and results.

Parameter Unit Reactor

RS FTS

Influent Effluent Influent Effluent

Mass balance dataQ L d–1 2 2 2 2SCOD g L–1 3.19 2.34 6.76 1.27VS g kg–1 31.4 15.1 26.1 14.5Sludge density kg m–3 1008 1002 1006 1002COD/VS – 1.4 1.6 1.5 1.3Qbiogas L d–1 28.35 30.75CH4 in biogas % 61.5 63.0QCH4 L d–1 17.44 19.37Mass balance resultsMSinf (Eq. (1)) g COD d–1 95.00 92.29MSeff (Eq. (2)) g COD d–1 53.10 40.32MSCH4 (Eq. (3)) g COD d–1 44.11 49.01COD mass balance (Eq. (4)) % 102.3 96.8

3472 A. Montusiewicz et al. / Bioresource Technology 101 (2010) 3466–3473

comparable in both systems; the achieved values reached 0.49 m3

kg�1 in the ‘‘double-phase” process and 0.45 m3 kg�1 in the con-ventional digestion. It is worth noticing that the removal of VSreached 53.8% for the combined process and was slightly higherthan the level of 51.9% observed in the conventional one. Globalmass balance carried out for the removed COD gave the biogasyield of 0.62 m3 kg�1 for the ‘‘double-phase digestion” and 0.7for the conventional system. To sum up, from a global point of viewthe effects of the combined process were similar to the conven-tional one.

3.3. Supernatant quality

The digester supernatant characteristics are presented in Ta-ble 7. Comparing data from Table 3 and Table 7, a higher releaseefficiency was achieved in the RS reactor due to the nutrients re-leased directly from raw sludge (without disintegration). The ob-served concentrations of released TNs, N–NHþ4 and P–PO3�

4 in thesupernatant increased approximately 2.3 times for TNs, almost 8times for N–NHþ4 and 1.6 times for P–PO3�

4 in comparison to thesupernatant composition of the raw sludge. Lower efficiency wasattained in the FTS reactor, as some nutrients were released previ-ously during the freezing/thawing pre-condition. In this case, theconcentration of released TNs and N–NHþ4 in the digester superna-tant increased almost 1.4 times and 6 times, respectively, in com-parison to the supernatant of the FTS reactor. However, a smalldecrease in P–PO3�

4 concentration (to about 52 mg L–1) was ob-served in the reactor that was fed with frozen/thawed sludge.

It was found that, despite the differences in nitrogen and phos-phorus released from freezing/thawing of sewage sludge (shown inTable 3), the characteristics of the digester supernatant were com-parable in both systems. Moreover, the observed N–NHþ4 valueswere similar to those seen by Song et al. (2004) during mesophilicanaerobic digestion of sewage sludge. The authors found that theconcentration was 630 mg L–1, whereas during our investigations

observed levels in supernatants were, respectively, 750.1 mg L–1

from the RS reactor and 775.6 mg L–1 from the FTS reactor. ForP–PO3�

4 concentrations, much lower values were achieved by Songet al. (42.4 mg L–1) compared to our observations at the levels of137.2 mg L–1 and 133.6 mg L–1, respectively, in the RS and FTSsupernatant.

The comparable N–NHþ4 concentrations obtained during our re-search could result from either of the following: total nitrogen lossduring freezing (Table 3) and a higher struvite (MgNH4PO4) precip-itation in the FTS reactor. The latter explanation is in accordancewith a lower P–PO3�

4 concentration (Table 7) and a higher pH value(Table 5) in the FTS supernatant.

It can be concluded that freezing/thawing of the mixed sewagesludge did not influence the digester supernatant quality. Suchobservations are essential as the supernatant quality is an impor-tant factor that could influence the nutrient mass balance duringadvanced wastewater treatment (Battistoni et al., 1998). The highloads of ammonia nitrogen and ortho-phosphates that suppliedthe wastewater treatment system in the supernatant stream couldaffect its overloading and a subsequent decrease of nutrient re-moval efficiency and an increase of total treatment cost.

4. Conclusions

The freezing/thawing disintegration altered the mixed sewagesludge characteristics. Decreases in the COD, TN, TS and VS valuesand simultaneously more than twofold increases in both the SCODand VFA concentrations were observed. A release of nitrogen andphosphorus compounds also occurred.

The average biogas yield from frozen/thawed sludge reached1.31 m3 kg�1 of removed VS and exceeded approximately 1.5 timesthe value from the raw sludge.

On the basis of the global mass balance it was indicated that thefreezing/thawing of the mixed sewage sludge followed by anaero-

A. Montusiewicz et al. / Bioresource Technology 101 (2010) 3466–3473 3473

bic digestion could be considered as a ‘‘double-phase digestion”rather than a pre-treatment method.

Acknowledgements

Authors thank the financial support from the Ministry of Sci-ence and Higher Education of Poland, PBZ-MEiN-3/2/2006.

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