Bw

La

b

a

ARRA

KEBO

1

pfdmisnmtdc

1d

Chemical Engineering Journal 172 (2011) 321– 325

Contents lists available at ScienceDirect

Chemical Engineering Journal

j ourna l ho mepage: www.elsev ier .com/ locate /ce j

iological pretreatment applied to industrial organic fraction of municipal solidastes (OFMSW): Effect on anaerobic digestion

.A. Fdez.-Güelfoa,∗, C. Álvarez-Gallegoa, D. Sales Márquezb, L.I. Romero Garcíaa

Department of Chemical Engineering and Food Technology, Faculty of Science, University of Cádiz, 11510 Puerto Real, Cádiz, SpainDepartment of Environmental Technologies, Faculty of Sea and Environmental Sciences, University of Cádiz, 11510 Puerto Real, Cádiz, Spain

r t i c l e i n f o

rticle history:eceived 7 January 2011eceived in revised form 27 May 2011ccepted 6 June 2011

eywords:nhancement anaerobic digestioniological pretreatmentrganic fraction of municipal solid wastes

a b s t r a c t

The application of the anaerobic digestion for the treatment of the industrial organic fraction of municipalsolid waste (OFMSW) has currently been of special interest. The main barrier in the treatment of thistype or organic waste is its biotransformation, due to the complexity of organic material. Therefore, thefirst step must be its physical, chemical and biological pretreatment for breaking complex moleculesinto simple monomers, to increase solubilization of organic material and improve the efficiency of theanaerobic treatment in the second step. This article describes the application of biological pretreatmentbased on mature compost addition, in order to enhance organic matter solubilization and, hence, thebiogas and methane production and the organic matter removal during the anaerobic digestion of theindustrial OFMSW.

Laboratory-scale experiments were carried out in semicontinuous completely mixed reactor, 5-L capac-ity. Optimal conditions for organic matter solubilization in the first step of pretreatment were: maturecompost as biological agent, 2.5% (v/v) of inoculation percentage and 24 h as incubation period. Theanaerobic digestion efficiency of the industrial OFMSW, with and without pretreatment, was evaluatedfor a solid retention time of 15 days. Under those conditions the organic matter removal percentage, interms of eliminated dissolved organic carbon and volatile solids, was increased up to 61.2% and 35.3%

respectively over the control without pretreatment. As consequence, the biogas and methane productionare improved up to 60.0% and 73.3% respectively. The highest cumulative biogas and methane produc-tion under anaerobic digestion were obtained with pretreated waste: 178.6 L and 82.6 L against 61.6 Land 34.2 L of the control without pretreatment. The results have shown that biological pretreatmentwith mature compost, followed by dry-thermophilic anaerobic digestion, provides the best results forstabilizing the industrial OFMSW.

. Introduction

Anaerobic digestion can be an attractive option, both as a dis-osal route and as a source of alternative energy. In the lastew years, much effort has been made at introducing anaerobicigestion processes for treating the industrial organic fraction ofunicipal solid waste (OFMSW) [1]. However, the main obstacle

n spreading this technology is the lower biodegradation rate ofolid wastes (due to the chemical composition and structure of lig-ocellulosic materials) in comparison to liquid ones. Generally, theethanogenic stage is the rate-limiting step of the anaerobic diges-

ion processes of liquid wastes. However, studies on the anaerobicigestion of primary sludge and organic complex substrates haveoncluded that the hydrolysis of organic matter to soluble substrate

∗ Corresponding author. Tel.: +34 956016379.E-mail address: alberto.fdezguelfo@uca.es (L.A. Fdez.-Güelfo).

385-8947/$ – see front matter © 2011 Elsevier B.V. All rights reserved.oi:10.1016/j.cej.2011.06.010

© 2011 Elsevier B.V. All rights reserved.

is the rate-limiting step for solid waste degradation [2]. Therefore,physical, chemical or biological pretreatment methods (or theircombination) are required, in order to reduce the rate of such alimiting step.

Thus, in his work on cellulose fermentation, Pavlostathis [3]found a negligible accumulation of hydrolyzed products in the reac-tor and concluded that the conversion of cellulosic matter to solubleproducts was the rate-limiting step in the overall process. Lee andDonaldson [4] also observed a low concentration of soluble com-pounds in cellulose fermentation processes and thus, the hydrolyticstage was considered the limiting stage of the process as well. Bymeans of activity assays of soluble and insoluble fraction of slaugh-terhouse effluents (considered complex wastes for their ruminantremains and fat content), Galisteo et al. [5] determined that the

hydrolysis of organic-complex material was also a limiting stage ofthe degradation process.

Hence, several methods have been developed to improve thebiodegradability of complex wastes through the application of dif-

322 L.A. Fdez.-Güelfo et al. / Chemical Engineering Journal 172 (2011) 321– 325

Table 1Features of the industrial OFMSW and the inoculum.

Parameter Industrial OFMSW Inoculum

pH 7.98 7.85Density (kg/m3) 650 1.10 × 103

Alkalinity (g CaCO3/L) 18.1 22.6Ammonia (g NH3–N/L) 0.79 0.59Total nitrogen (g/kg) 29.0 16.5Total solids (g/g sample) 0.71 0.34Total volatile solids (g/g sample) 0.16 0.08

fccsmaw

tptTaatrappstc

2

2

t(treimrcC

2O

pmmrTw

crw

Dissolved organic carbon (mg/g sample) 11.9 3.64Total volatile fatty acids (mg AcH/L) 32.6 6.82

erent types of pretreatments: physical, chemical, biological andombinations of them [6–16]. For complex substrates such as ligno-ellulosic materials, pretreatment causes a deep modification in thetructure of complex material and a decrease in the degree of poly-erization, thus weakening the molecular bonds between lignin

nd carbohydrates and increasing the superficial area of particulateastes.

This article examines biological pretreatments applied to indus-rial OFMSW to enhance the hydrolysis process in a first stagerior to the thermophilic-dry anaerobic digestion process in ordero increase the gas production in this anaerobic second stage.hermophilic processes and particularly thermophilic hydrolyticctivity of bacterial populations have been investigated 80 yearsgo, mainly at a temperature of 55 ◦C [17]. Thermophilic condi-ions generally result in an increase of the organic solids destructionate, attributed to increased hydrolytic activity. Ge et al. [18] evalu-ted thermophilic against mesophilic pretreatment (HRT of 2 days)rior to mesophilic anaerobic digestion (HRT of 13–14 days) forrimary sludge. An increase of 25% on the methane production andolids destruction was observed. Model based analysis indicatedhat the improved performance was due to an increased hydrolysisoefficient rather than an increase in inherent biodegradability.

. Materials and methods

.1. Inoculum source

The inoculum that it was employed in this study was adaptedo the waste (industrial OFMSW) and the operational conditionsdry-thermophilic anaerobic digestion) previously for a long periodo this assay. It was effluent from a semicontinuous stirred tankeactor (SSTR) that it was fed with the same industrial OFMSWmployed in this work. The features of the inoculum are detailedn Table 1. The semicontinuous SSTR was previously used to deter-

ine the optimal conditions (solid retention time, organic loadingate, etc.) for thermophilic-dry anaerobic digestion of the OFMSWoming from an 880 t/d industrial MSW treatment plant called “Lasalandrias” and placed in Jerez de la Frontera (Cádiz, Spain).

.2. Semicontinuous study with non-pretreated industrialFMSW

Initially, a semicontinuous biodegradability assay with non-retreated OFMSW was developed to determine the biogas andethane production and the organic matter removal at ther-ophilic regime of temperature (55 ◦C). A 5-L completely mixed

eactor was loaded to 90% of its volume with industrial OFMSW.he total solids content was adjusted to 30% (dry fermentation)ith inoculum.

The reactor was jacketed and thermostatically controlled using airculating 7 L-bath. A ball valve was fitted to the lower part of theeactor in order to discharge the contents and in the cover thereere several ports: central hole for agitation system, a pH probe, a

Fig. 1. Anaerobic 5-L reactor.

biogas collector, an opening for the addition of feedstock and twofurther hoses for the pH-control (Fig. 1).

About the agitation, reduction of mixing levels may be used asan operational tool to stabilize unstable digesters. For this reason,the agitation was fixed in the minimal mixing level of the stirrer(13 rpm). The aim is to avoid the disintegration of the microbialconsortia established.

The pH was adjusted using an on/off controller with 5 N NaOHand 1 N H3PO4 solutions. The operating range of the pH controllerwas 6.5–8, which is suitable for the methanogenic microorganisms.

A solid retention time (SRT) of 15 days was established, suitableto the dry-thermophilic anaerobic digestion of industrial OFMSWwith this technology Fdez-Güelfo et al. [19,20]. These authorsreported maximum methane specific production and removal per-centage of organic matter at 15 days-SRT. The SRT was maintainedfor 3 times to stabilize the system.

The feedstock used in this study coming from an industrial facil-ity receiving MSW without separate collection or source selection.It consisted in industrial OFMSW of the MSW treatment plant calledLas Calandrias and the point in which the waste was taken waslocated behind “Selection and Reject” steps (i.e., manual sorting,trommel, magnetic splitter, etc.). The average particle size of theselected waste was 30 mm and its features are shown in Table 1. Theinorganic fraction has not been taken into account for the experi-mentation. It is very important to emphasize that the OFMSW usedin this research work has not been processed (milled or sieved) inthe laboratory previously to feed the digesters. From the method-ological point of view it is an important difference with respect toother works since the processing modifies the original characteris-tics of the waste.

According to Pavan et al. [21], this industrial OFMSW can be clas-sified as non-easily biodegradable, since the VS/TS ratio is lowerthan 0.7. In this study, the VS/TS ratio is 0.22, which, based on theassumptions of the author, implies the following: first, the systemcan work with a high organic loading rate without the occurrence

of acidification problems, and, second, the biogas and methaneproduction decreases considerably compared with the productionachieved from an easily biodegradable substrate. In this sense, the

L.A. Fdez.-Güelfo et al. / Chemical Engine

Table 2Features and composition of the compost pile in dry base.

Parameter Data

Industrial OFMSW (kg) 10.9 × 103

Sewage sludge (kg) 993pH 7.27Ammonia (mg/g sample) 0.08 × 10−1

Conductivity (mS/cm) 4.92Organic matter (%) 25.2Carbon (%) 14.6NO3

− (mg/g) 0.14Total Nitrogen (%) 0.74Dissolved organic carbon (mg/g sample) 21.4

ip

2

iaoFtp

cawtma

faabt

2

tS

sm(cuosftwg6

imaip

P2O5 (mg/g sample) 0.26C/N ratio 19.7

mplementation of biological pretreatment should increase the gasroductivity.

.3. Semicontinuous study with pretreated industrial OFMSW

Next, the reactor was daily fed with biologically pretreatedndustrial OFMSW in order to accelerate the hydrolytic ratend increase the anaerobic biodegradability of the waste. Theperational variables of the pretreatment were set according todez-Güelfo et al. [22]: mature compost as biological agent, roomemperature, inoculation percentage of 2.5% (v/v) and incubationeriod of 24 h.

The biological agent was mature compost (density = 0.58 g/mL)oming from a windrow composting system, which consisted in

pile of MSW and anaerobically digested sewage sludge fromastewater treatment plant (WWTP). The pile was constituted by

he following amounts on a wet basis: 10.9 × 103 kg of OFMSWixed with 993 kg of sewage sludge. The features of the compost

re detailed in Table 2.It is very important to emphasize that the only remarkable dif-

erence between sequential semicontinuous studies (Sections 2.2nd 2.3) is that feeding is pretreated in this last case. No furthercclimatation of the initial inoculum to the pretreated waste muste expected in these conditions. Indeed, any change in fed condi-ions could be negative for the process.

.4. Sampling and analysis

The analytical determinations to characterize the wastes ando check the 5-L reactor behavior were performed according totandard Methods [23].

The volume of gas produced in the reactor was directly mea-ured using a high precision flow gas meter (WET DRUM TG 0.1bar–Ritter) through the 40L-Tedlar bag. The gas composition

hydrogen, methane and carbon dioxide) was determined by gashromatography (SHIMADZU GC-14 B) with a stainless steel col-mn packed with Carbosive SII (diameter of 3.2 mm and lengthf 2 m) and a thermal conductivity detector (TCD). The injectedample volume was 1 mL and the operational conditions were asollows: 7 min at 55 ◦C; ramped at 27 ◦C min−1 until 150 ◦C; detec-or temperature: 255 ◦C; injector temperature: 100 ◦C. The carrieras helium and the flow rate used was 30 mL min−1. A standard

as (by Carburos Metálicos, S.A.; composition: 4.65% H2; 5.33% N2;9.92% CH4 and 20.10% CO2) was used for the system calibration.

Total and individual volatile fatty acids (VFA) (from C2 to C7,ncluding iC4, iC5 and iC6) levels were determined by gas chro-

atography (SHIMADZU GC-17 A) with a flame ionization detectornd a capillary column filled with Nukol (polyethylene glycol mod-fied by nitro-terephthalic acid). The temperatures of the injectionort and detector were 200 ◦C and 250 ◦C, respectively. Helium was

ering Journal 172 (2011) 321– 325 323

the carrier gas at 50 mL min−1. In addition, nitrogen gas was usedas make up at 30 mL min−1 flow rate.

3. Results and discussion

3.1. Organic matter removal and gas production

As seen in Table 3, when the OFMSW is pretreated the organicmatter removal percentage, expressed in terms of eliminated DOCand VS, is improved up to 61.2% and 35.3% respectively over thecontrol reactor without pretreatment. Therefore, the pretreatmentmust improve digestibility of complex wastes. Basically, the mech-anism of action of the biological agent may be due that the maturecompost has a consortium of microorganisms, aerobic and anaer-obic, with hydrolytic enzymes whose are capable to solubilizedifficulty-biodegradable organic matter. It should be kept in mindthat the compost used coming from a pile with a high proportionof industrial OFMSW and therefore, the microorganisms result-ing of the composting stage are able to hydrolyze and solubilizerapidly (24 h). Hence, the structural properties of the waste withoutpretreatment (which have not been partially hydrolyzed) difficultthe anaerobic degradation, since limiting surface area for microbialattack and anaerobic decomposition.

As consequence, the daily biogas and methane production areincreased up to 60.0% and 73.3% for the pretreated waste. It isimportant to note that if the pretreatment increases the solubleorganic matter in the liquid phase, it can promote the generationof new biomass in higher proportion and, as consequence, the gasproduction increase.

In Table 4, methane production are reported when differentbiological pretreatment methods are applied. Noted that there isnot literature where biologically pretreated industrial OFMSW isdigested in anaerobic reactor at semicontinuous regime of feeding.

This fact highlights the innovative aspect of this research. Mostof the research works are focused on pretreatment and anaer-obic biodegradability of sewage sludge. In Table 4 are reportedincrements of methane production between 11% and 100%. Theincrement of methane production achieved in this study is approx-imately in the bottom half of this range (73.3%). However, this is asuccessful data if it is taken into account that the 30%TS-OFMSWis a semi-solid waste (not liquid) and hence the organic matterbiomethanisation in semi-solid state can be affected by mass trans-fer phenomena. The waste composition and the unavoidable smallparticles sizes of non-biodegradable materials present in the waste(since it is not source-selected) contributed to the results presentedin this research work.

Similar results were summarized for gas yield. The specificmethane production (SMP) increases from 0.31 m3 CH4/kgVSr

to 0.42 m3 CH4/kgVSr for non-pretreated and pretreated wasterespectively, i.e., it is improved up to 35.5%. According with Pavanet al. [21], when the waste is not pretreated, it behaves as anon-easily biodegradable substrate. Therefore, the microorgan-isms cannot easily metabolize the organic matter to generatebiogas. Therefore, the pretreated digester reached higher methaneamounts than control, which suggests that pretreatment shouldimprove anaerobic digestion, not only increasing organic solubi-lization but also surface area available for enzymatic action as aresult of fiber swelling.

In this study, the SMP obtained for non-pretreated OFMSW(0.31 m3 CH4/kgVSr) is very similar to that reported by Hart-mann and Ahring [29] for semi-continuous dry co-digestion of

non-pretreated OFMSW and manure at thermophilic range of tem-perature and at 15-day SRT. These authors registered a SMP of0.34 m3 CH4/kgVSr. On the other hand, the SMP obtained whenthe OFMSW is pretreated (0.42 m3 CH4/kgVSr) is similar to that

324 L.A. Fdez.-Güelfo et al. / Chemical Engineering Journal 172 (2011) 321– 325

Table 3Main values of the anaerobic biodegradation parameters at 15 days-SRT.

Non-pretreated industrial OFMSW Pretreated industrial OFMSW

Dissolved organic carbon (DOC) (% removal) 44.8 72.1Total volatile solids (TVS) (% removal) 10.8 14.7Biogas production (L/Lreactor·day) 0.25 0.40Methane production (L CH4/Lreactor·day) 0.15 0.26Specific methane production (m3 CH4/kgTVSremoval) 0.31 0.42

Mean values in the last (of three) 15 days-SRT.

Table 4Results of anaerobic biodegradability with different biologically pretreated substrates.

Substrate Biological pretreatment: conditionsa Anaerobic digestion of pretreated substrate Reference

Conditions Results

Activated sludge 70 ◦C 7 days Batch 55 ◦C CH4 production of 10.9 mmol g−1

VSin (no influence)[24]

Primary sludge 70 ◦C 7 days Batch 55 ◦C Increase of CH4 production from13.7 mmol g−1b to 25.5 mmol g−1

VSin (+86%)Activated sludge 70 ◦C 2 days CSTR, HRT: 13 days (15 days

without pretreatment) 55 ◦CIncrease of CH4 production from40 mL L−1 d−1b to 55 mL L−1 d−1

(+28%)

[25]

Primary sludge 70 ◦C 2 days CSTR, HRT: 13 days (15dayswithout pretreatment) 55 ◦C

Increase of CH4 production from146 mL day−1b to 162 mL day−1

(+11%)Activated sludge 70 ◦C 9 h Batch 55 ◦C Increase of biogas production

(+58%)[26]

Mixed sludge 70 ◦C 9, 24, 48 h CSTR, HRT: 10 days 55 ◦C Increase of CH4 production from0.15 mL g−1b to 0.18 mL g−1 VSin

(+20%) Increase of energyproduction (+60 to 100%)

[27]

Primary sludge 70 ◦C 2 days CSTR, HRT: 13 days (15dayswithout pretreatment) 55 ◦C

Increase of CH4 production from13.6 mmol g−1b to 20.1 mmol g−1

VS (+48%)

[28]

rmaa

3

ltbmisttc

Fa

a Conditions are referred to temperature and incubation period.b Performance of anaerobic digestion without pretreatment.

eported by Del Borgui et al. [30] when a biologically pretreatedix of OFMSW and sewage sludge was digested in thermophilic

naerobic reactor at fed-batch regime. The value obtained by theseuthors was 0.40 m3 CH4/kgVSr.

.2. Cumulative gas production

Fig. 2 shows the comparison of the evolutions of the cumu-ated biogas and methane volumes as a function of time whenhe industrial OFMSW is pretreated and non-pretreated. As cane seen from the shape of the curves, no step-wise biogas andethane production were observed when the industrial OFMSW

n non-pretreated, as described by Aiba et al. [31]. On the contrary,

tep-wise methane production is clearly reported when the indus-rial OFMSW is pretreated [32]. It must be taken into account thathe methane production rate depends on the oxidation state of theompound that it is being degraded, i.e., carbohydrates are con-

ig. 2. Behavior of biogas and methane production when the OFMSW is pretreatednd non-pretreated at 15 days-SRT.

in

verted into equals amount of CH4 and CO2 and the lipids producehigher proportion of CH4. The result seems to show that the dif-ferent constituents of the waste have been partially hydrolyzedby means of the pretreatment. This fact suggests that these par-tially hydrolyzed fractions (carbohydrates, lipids, etc.) are moresusceptible to be degraded and converted to methane more easily.

When the OFMSW is pretreated, the system reaches a maxi-mum cumulative biogas and methane production of 178.6 L and82.6 L respectively. On the other hand, if the waste was not pre-treated, the obtained values were 61.6 L and 34.2 L respectively,i.e., the cumulative biogas and methane production are increasedup to 190.0% and 141.6% respectively when the biological pretreat-ment is applied. These results are in the same order of magnitude tothose obtained by López and Espinosa-Llorénsa [32]. These authorsreported an increment in the cumulative methane production of100% when OFMSW was pretreated and subsequently digested onfed-batch regime.

4. Conclusions

The biological pretreatment with mature compost improvesanaerobic digestion. The organic matter removal percentage, mea-sured in terms of eliminated DOC and VS, is improved up to 61.2%and 35.3% over the control without pretreatment. As consequence,the biogas and methane production, expressed as (L/Lreactor)d, areincreased up to 60.0% and 73.3% over the control. Also, the specificmethane production is improved up to 35.48% and the cumula-

tive biogas and methane production are enhancement 190.0% and141.6% respectively. From the above results, a procedure for thetreatment of the organic fraction of the municipal solid waste canbe designed. The two-stage design must involve biological pre-

ngine

tdeoatsdtf

A

a1AD

R

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

L.A. Fdez.-Güelfo et al. / Chemical E

reatment, followed by dry-thermophilic anaerobic digestion. Theigestate is a final product that may be employed as compost tonrich agricultural soils. It is a stabilized product from microbi-logical point of view since the anaerobic process was operatedt thermophilic regime of temperature (55 ◦C). On the other hand,he organic matter removal percentage, measured in terms of dis-olved organic matter, is higher than 60%. This is important sinceissolved organic matter is easily assimilated by plants or crops. Inhis sense, the obtained digestate is also a final product stabilizedrom the organic matter point of view.

cknowledgements

This work was supported by the Spanish Ministry of Sciencend Innovation (Projects CTM2007-62164/TECNO and CTM2010-7654), the Ministry of Innovation, Science and Enterprise ofndalusia (Project P07-TEP-02472) and the European Regionalevelopment Fund (ERDF).

eferences

[1] M. Wu, K. Sun, Y. Zhang, Influence of temperature fluctuation on thermophilicanaerobic digestion of municipal organic solid waste, J. Zhejiang Univ. Sci. B(2006), ISSN 1673-1581 (print), ISSN 1862-1783 (online).

[2] P. Chulhwan, L. Chunyeon, K. Sangyong, Ch. Yu, C.H. Howard, Upgrading ofanaerobic digestion by incorporating two different hydrolysis processes, J.Biosci. Bioeng. 100 (2) (2005) 164–167.

[3] S.G. Pavlostathis, Preliminary conversion mechanisms in anaerobic digestionof biological sludge, J. Environ. Eng. Div. Proc. Am Soc. Civ. Eng. 114 (1988)575–592.

[4] D. Lee, T.L. Donaldson, Anaerobic digestion of cellulosic wastes, Biotechnol.Bioeng. Symp. 14 (1984) 503–505.

[5] M. Galisteo, M. Mallo, J. Martínez, Degradabilidad anaerobia de efluentes com-plejos, in: Proceeding Fifth Latin-American Workshop-Seminar. WastewaterAnaerobic Treatment, Vinas del Mar, Chile, 1998.

[6] H. Carrère, C. Dumas, A. Battimelli, D.J. Batstone, J.P. Delgenès, J.P. Steyer, I.Ferrer, Pretreatment methods to improve sludge anaerobic degradability: areview, J. Hazard. Mater. 183 (2010) 1–15.

[7] L. Coulibaly, G. Gourene, N. Agathos, Utilization of fungi for biotreatment of rawwastewaters, Afr. J. Biotechnol. 2 (2003) 620–630.

[8] A.T.W.M. Hendriks, G. Zeeman, Pretreatments to enhance the digestibility oflignocellulosic biomass, Bioresour. Technol. 100 (2009) 10–18.

[9] M. López Torres, Ma. Espinosa Lloréns, C. del, Effect of alkaline pretreatment onanaerobic digestion of solid wastes, Waste Manage. 28 (11) (2008) 2229–2234.

10] K. Hwang, E. Shin, H. Choi, A mechanical pre-treatment of waste activatedsludge for improvement of anaerobic digestion system, Water Sci. Technol. 36(12) (1997) 111–116.

11] R. Rodríguez-Vázquez, G. Villanueva-Ventura, E. Rios-Leal, Sugarcane bagasse

pith dry pre-treatment for single cell protein production, Bioresour. Technol.39 (1992) 17–22.

12] M. Ropars, R. Marchal, J. Pourquié, J.P. Vandecasteele, Large-Scale enzymatichydrolysis of agricultural lignocellulosic biomass. Part 1: pre-treatment proce-dures, Bioresour. Technol. 42 (1992) 197–204.

[

[

ering Journal 172 (2011) 321– 325 325

13] H. Rughoonundun, C. Granda, R. Mohee, M.T. Holtzapple, Effect of thermo-chemical pretreatment on sewage sludge and its impact on carboxylic acidsproduction, Waste Manage. 30 (2010) 1614–1621.

14] H. Wang, Ho. Wang, W. Lu, Y. Zhao, Digestibility improvement of sorted wastewith alkaline hydrothermal pretreatment, Tsinghua Sci. Technol. 14 (3) (2009)378–382.

15] Ch. Ying-Chih, Ch. Cheng-Nam, L. Jih-Gaw, H. Shwu-Jiuan, Alkaline and ultra-sonic pre-treatment of sludge before anaerobic digestion, Water Sci. Technol.36 (11) (1997) 155–162.

16] J. Zhu, C. Wan, Y. Li, Enhanced solid-state anaerobic digestion of corn stover byalkaline pretreatment, Bioresour. Technol. 101 (2010) 7523–7528.

17] W. Rudolfs, H. Heukelelian, Thermophilic digestion of sewage sludge solids. I.-Preliminary paper, Ind. Eng. Chem. 22 (1930) 96–99.

18] H. Ge, P.D. Jensen, D.J. Batstone, Pre-treatment mechanisms duringthermophilic–mesophilic temperature phased anaerobic digestion of primarysludge, Water Res. 44 (1) (2010) 123–130.

19] L.A. Fdez-Güelfo, C. Álvarez-Gallego, D. Sales Márquez, L.I. Romero García,Determination of critical and optimum conditions for biomethanization ofOFMSW in a semi-continuous stirred tank reactor, Chem. Eng. J. (2011) 096,doi:10.1016/j.cej.2011.03.

20] L.A. Fdez-Güelfo, C. Álvarez-Gallego, D. Sales Márquez, L.I. Romero García, Dry-thermophilic anaerobic digestion of simulated organic fraction of municipalsolid waste: process modeling, Bioresour. Technol. 102 (2011) 606–611.

21] P. Pavan, P. Battistoni, J. Mata-Álvarez, F. Cecchi, Performance of thermophilicsemi-dry anaerobic digestion process changing the feed biodegradability,Water Sci. Technol. 41 (3) (2000) 75–82.

22] L.A. Fdez-Güelfo, C. Álvarez-Gallego, D. Sales Márquez, L.I. Romero García, Theuse of thermochemical and biological pretreatments to enhance organic matterhydrolysis and solubilization from organic fraction of municipal solid waste(OFMSW), Chem. Eng. J. 168 (2011) 249–254.

23] APHA, AWWA, WEF, Standard Methods for the Examination of Water andWastewater, ninteenth ed., Washington DC/New York, USA, 1995.

24] H.N. Gavala, U. Yenal, I.V. Skiadas, P. Westermann, B.K. Ahring, Mesophilicand thermophilic anaerobic digestion of primary and secondary sludge.Effect of pre-treatment at elevated temperature, Water Res. 37 (19) (2003)4561–4572.

25] I.V. Skiadas, H.N. Gavala, J. Lu, B.K. Ahring, Thermal pre-treatment of primaryand secondary sludge at 70 ◦C prior to anaerobic digestion, Water Sci. Technol.52 (1–2) (2005) 161–166.

26] M. Climent, I. Ferrer, M.D. Baeza, A. Artola, F. Vazquez, X. Font, Effects of thermaland mechanical pretreatments of secondary sludge on biogas production underthermophilic conditions, Chem. Eng. J. 133 (1–3) (2007) 335–342.

27] I. Ferrer, E. Serrano, S. Ponsa, F. Vazquez, X. Font, Enhancement of thermophilicanaerobic sludge digestion by 70 ◦C pre-treatment: energy considerations, J.Residuals Sci. Technol. 6 (1) (2009) 11–18.

28] J.Q. Lu, H.N. Gavala, I.V. Skiadas, Z. Mladenovska, B.K. Ahring, Improving anaer-obic sewage sludge digestion by implementation of a hyper-thermophilicprehydrolysis step, J. Environ. Manage. 88 (4) (2008) 881–889.

29] H. Hartmann, B.K. Ahring, Anaerobic digestion of the organic fraction of munic-ipal solid waste: Influence of co-digestion with manure, Water Res. 39 (2005)1543–1552.

30] A. Del Borghi, A. Converti, E. Palazzi, M. Del Borghi, Hydrolysis and thermophilicanaerobic digestion of sewage sludge and organic fraction of municipal solidwaste, Bioprocess Eng. 20 (1999) 553–560.

31] S. Aiba, A. Humphrey, N. Millis, Biochemical Engineering, Instituto del Libro,Ciencia y Tecnica (Book Institute, Science and Technical), La Habana, Cuba,1970.

32] M. López-Torres, M.C. Espinosa-Llorénsa, Effect of alkaline pretreatment onanaerobic digestion of solid wastes, Waste Manage. (2008) 2229–2234.