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Evaluation of pre-treatment processes for increasingbiodegradability of agro-food wastesD. Hidalgo a , E. Sastre a , M. Gmez a & P. Nieto aa Centro Tecnolgico CARTIF, Valladolid, SpainAccepted author version posted online: 13 Feb 2012.Published online: 16 Mar 2012.

To cite this article: D. Hidalgo , E. Sastre , M. Gmez & P. Nieto (2012): Evaluation of pre-treatment processes for increasingbiodegradability of agro-food wastes, Environmental Technology, 33:13, 1497-1503

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Environmental TechnologyVol. 33, No. 13, July 2012, 14971503

8TH IWA SYMPOSIUM ON WASTE MANAGEMENT PROBLEMS INAGRO-INDUSTRIES-AGRO2011

Evaluation of pre-treatment processes for increasing biodegradability of agro-food wastes

D. Hidalgo, E. Sastre, M. Gmez and P. Nieto

Centro Tecnolgico CARTIF, Valladolid, Spain

(Received 11 October 2011; nal version received 30 January 2012 )

Anaerobic digestion (AD) technology can be employed for treating sewage sludge, livestock waste or food waste. Generally,the hydrolysis stage is the rate-limiting step of the AD processes for solid waste degradation. Therefore, physical, chemicaland biological pre-treatment methods or their combination are required, in order to reduce the rate of such a limiting step. Inthis study, four methods (mechanical shredding, acid hydrolysis, alkaline hydrolysis and sonication) were tested to improvemethane production and anaerobic biodegradability of dierent agro-foodwastes and their mixtures. The kinetics of anaerobicdegradation and methane production of pre-treated individual wastes and selected mixtures were investigated with batch tests.Sonication at lower frequencies (37 kHz) proved to give the best results with methane productivity enhancements of over100% in the case of pig manure and in the range of 1047% for the other wastes assayed. Furthermore, the ultimate methaneproduction was proportional, in all the cases, to the specic energy input applied (Es). Sonication can, thus, enhance wastedigestion and the rate and quantity of biogas generated. The behaviour of the other pre-treatments under the conditionsassayed is not signicant. Only a slight enhancement of biogas production (around 10%) was detected for whey and wasteactivated sludge (WAS) after mechanical shredding. The lack of eectiveness of chemical pre-treatments (acid and alkalinehydrolysis) can be justied by the inhibition of the methanogenic process due to the presence of high concentrations of sodium(up to 8 g l1 in some tests). Only in the case of WAS did the acid hydrolysis considerably increase the biodegradabilityof the sample (79%), because in this case no inhibition by sodium took place. Some hints of a synergistic eect have beenobserved when co-digestion of the mixtures was performed.

Keywords: anaerobic digestion; batch assays; co-digestion; organic waste; pre-treatments

IntroductionThe production of organic waste can be considered anintegral part of developed society. These wastes (solid orsemi-solid) are generated from agriculture, food process-ing, drinks manufacture or even in the form of domesticwastes, and their quantities are appreciable. Over the years,an array of ideas for the utilization of these wastes has beenput forward. However, anaerobic digestion (AD) of organicwastes to produce energy in the form of biogas is the mostlikely option to be of commercial interest, provided thatthe economics are favourable [1,2]. The degradation rate ofcomplex particulate organic matter in anaerobic digesters isgenerally controlled by the rate of the rst, and usually lim-iting, step: the hydrolysis, where the cell wall is broken, thusallowing the organic matter inside the cell to be availablefor biological degradation. In order to increase the wastedegradation rate and methane production, the pre-treatmentoptions, such as physical, chemical or even thermal ones,have yielded promising results [3,4].

Pre-treatment breaks down the complex organic struc-ture into simpler molecules that are more susceptible tomicrobial degradation. Alkaline pre-treatment destroys cellwalls by hydroxyl anions and causes natural shape loss

Corresponding author. Email: dolhid@cartif.es

of proteins, saponication of lipids and hydrolysis ofRNA (ribonucleic acid). Many alkalis have shown theireectiveness in waste solubilization, with an eciency of(NaOH > KOH > Mg(OH)2 andCa(OH)2). However, toohigh concentrations of Na+ or K+ may cause subsequentinhibition of AD [5]. It has been used to solubilize varioussubstrates, such as lignocellulosic materials, waste acti-vated sludge (WAS) or food waste [69]. The addition ofalkali increases the mineral content of digested waste [10].Acidication to solubilize wastes has been suggested andshown in the laboratory to be a possibility [1114]. Theconditions for achieving this scenario are very intensive interms of extreme pH values: 0.51.0 gH2SO4/gTSS [15].Devlin et al. [16] showed that chemical oxygen demand(COD) and volatile solids (VS) reduction and gas produc-tionwere enhancedwhenwastes were pre-treatedwithHCl.However, one concern with acidication is the need to addalkalinity.Ultrasonic disintegration is awell-knownmethodfor breaking-upmicrobial cells to extract intracellularmate-rial [17,18]. There are two mechanisms associated withultrasonic treatment: cavitation, which is favoured at lowfrequencies, and chemical reactions due to the formation ofradicals at high frequencies [19]. The hydrolysis of cellular

ISSN 0959-3330 print/ISSN 1479-487X online 2012 Taylor & Francishttp://dx.doi.org/10.1080/09593330.2012.665488http://www.tandfonline.com

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membranes can also be achieved by mechanical rupturingtechniques (i.e. shredding, grinding, etc.). Mechanical pre-treatment is generally based on the disruption ofmechanicalcell walls by shear stress. This method has also been shownto improve the anaerobic digestibility of wastes [20,21].

As previously mentioned, many studies have investi-gated pre-treatment of waste, mainly activated sludge, forAD and also dealt with a single pre-treatment method incomparison with non-pre-treatment. However, few reportshave been published on various pre-treatment methods forAD [22], and even fewer have been applied to other typolo-gies of waste apart from activated sludge. For this reason,an in-depth study of the anaerobic biodegradability of sixdierent wastes from four agro-food industries and theirmixtures was undertaken, with special emphasis placed onthe hydrolysis step. Dierent approaches were attempted:mechanical (shredding and sonication) and chemical (acidand base addition) pre-treatment to improve methane pro-duction. To show the comprehensive eciency of thepre-treatment methods, the kinetics of anaerobic degra-dation and methane production was examined with batchtests.

Materials and methodsWaste selectionSix dierent types of wastes from four agro-food indus-tries dairy (cheese production), jam and sauce production,candy manufacturing and swine farming were chosenfor this project, namely: whey (WH), fresh cheese (FC)waste, physical-chemical sludge (PCS) from dairy wastew-ater treatment, sauce and jam (SJ) waste, WAS from candywastewater treatment and pig manure (PM). The selectioncriteria relied upon aspects such as the relative geographicaldistance between each producer and theoretical informa-tion related to biomethane generation/kg of organic waste.Four mixtures of four selected individual wastes amongthe above-mentioned oneswere prepared, considering thosewith more diculties to be used directly as sub-products inother industrial processes. Table 1 shows the content of eachwaste in the selected mixtures. Results are presented on aVS percentage basis.

Chemical analysisSolid concentrations (total and volatile), pH and totalphosphorus were determined following standard methods

Table 1. Composition of the waste mixtures.

Waste M1 (%) M2 (%) M3 (%) M4 (%)

WAS 25 15 15 35PCS 25 15 55 15SJ 25 55 15 15PM 25 15 15 35

(APHA, 1998) recommendations. C, N, H and S contentswere determined byUNE-CEN/TS15104EXwith a LECOTruspec CHN(S) elemental determinator. Oxygen contentwas not measured directly, but was estimated assuming thatno other elements (apart from the measured C, H, N, S andP) were present in the wastes.

Biochemical methane potential testsBatch experiments were run in glass serum bottles witha liquid volume of 300 ml (1000 ml of total volume).Anaerobic sludge from a municipal wastewater treatmentplant (WWTP) with a concentration of 12 gVS l1 wasused as inoculum for the anaerobic test. The concen-tration of the inoculum in all the assays was 5 g l1.Substrate/inoculum (S/X) ratios in the range of 0.30 to0.50 gVSwaste gVS1inoculum were performed in triplicatefor all the wastes. A set of triplicate blank assays with-out any waste (only inoculum) was also performed forendogenous methane production determination. To avoidacidication of the assay, NaHCO3 was added as a buer(5 g NaHCO3 l1). The pH was adjusted to 7.5 in allcases by the addition of HCl or NaOH. After the set-upof each reactor, the headspace was ushed with nitrogenfor 3 minutes in order to remove the oxygen. All theexperiments were carried out at 35 1C in a thermo-static room and constant agitation was provided by a stirrertable.

Biogas production was measured manually by a pres-sure transducer (Druck, PTX 1400, range 1 bar) in thehead space of each reactor. To avoid reaching overpressure,biogas in the head space was periodically released. Pres-sure dierences were converted to biogas volume, usingthe ideal gas law and standard conditions (P = 1 bar andT = 0C). Biogas composition was measured before eachrelease with a Varian CP-4900 Micro-GC with a ThermalConductivity Detector. Methane production was calculatedby subtracting the amount of the methane produced bythe blank assay from the methane production of eachassay.

Theoretical methane potentialAnalysis of the elemental composition of thewaste providesinformation about its potential anaerobic degradation andcomposition of the biogas [23], by a stoichiometric con-version to methane and carbon dioxide, using Buswellsequation (1). This equation assumes methane productionfrom the complete degradation of a certain waste witha given elemental composition, where CnHaObNc repre-sents the chemical formula of the biodegradable organiccompound subjected to the anaerobic degradation process,and the production of methane considered herein is themaximum stoichiometrically possible. For a CnHaObNccompound, the theoretical methane potential (at standard

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Table 2. Pre-treatment conditions.

Experiment 1 Operating conditions

Pre-treatment Power (kW) Frequency (kHz) Bath volume (l) Time (min) Sample

Shredding 850 - 0.5 1.5 Raw wasteSonication 200 37 0.5 60 Raw waste

pH Solution Time (min) Sample

Alkaline hydrolysis 12 5 g l1 NaOH 60 Raw wasteAcid hydrolysis 2 30ml l1 H2SO4 60 Raw waste

*Raw waste: WH, FC, WAS, PCS, SJ and PM

Experiment 2 Operating conditions

Sonication Power(kW) Frequency (kHz) Bath volume (l) Time (min) Sample

Assay 2A 200 37 0.5 60 Waste mixtureAssay 2B 200 37 0.5 30 Waste mixtureAssay 2C 1500 40 35 60 Waste mixtureAssay 2D 2500 40 35 60 Waste mixture

* Waste mixture: WAS, PCS, SJ and PM in %: M1 (25/25/25/25); M2 (15/15/55/15); M3 (15/55/15/15);M4 (35/15/15/35)

conditions for temperature and pressure) is:

B0,Th =

(n2

+ a8

b4

3c8

) 22.4

12n + a + 16b + 14c |=|L CH4g VS

(1)where the parameters n, a, b, c refer to the stoichiometryindex of C, H, O and N, respectively.

Biodegradability factorThe eciency of the pre-treatments can be calculated usingthe biodegradability factor (BF) as methane potential ofthe pre-treated sample (PreT ) to methane potential of thesample not pre-treated ratio (Non-PreT ) (Equation (2)), theBF being a measure of ultimate biodegradability extent:

BF = (ml CH4/g VS)Pr eT(ml CH4/g VS)nonPr eT

(2)

Pre-treatment conditionsTwo experiments were carried out. The rst was designedto determine the pre-treatment method giving the high-est biodegradability enhancement when the raw wasteswere individually analysed. Four methods (mechanical,acid hydrolysis, alkaline hydrolysis and sonication) weretested. For mechanical treatment a domestic shredder wasused. The ultrasonic irradiation of wastes was performed intwo systems (Elmasonic S 40H and Tierra Tech SET 1500)that emit 37 and 40 kHz, respectively. For each sonicationtest, a 100ml sample was lled in a beaker and then placedin the equipment water bath. The specic input energy (Es)(Equation (3)) was dened as a function of ultrasonic power

(P), ultrasonic time (t), total volume (V ) and total solidconcentration (TS):

Es = P tV TS | = |

kJg TS

(3)

Since sonication was the method showing the best resultsduring the rst experiment, the second approach was tostudy the pre-treatment of selected waste mixtures underdierent ultrasonic irradiation conditions in order to ndout the preferential ones. Pre-treatment conditions for bothexperiments are summarized in Table 2.

Before entering the biochemical methane potential(BMP) test, samples proceeding from acid and alkali pre-treatmentswere neutralizedwithNaOHandH2SO4, respec-tively, in order to avoid extreme pH values inside the batchtests.

Results and discussionWaste compositionLumped parameters as TS, VS, COD, total organic car-bon (TOC) and total Kjeldahl nitrogen (TKN) are the mostfrequently analysed, since they are the key when determin-ing treatment optimization. Furthermore, analysis of theelemental composition provides information about the the-oretical methane potential of a given waste. Table 3 gathersthe results obtained from the characterization of the selectedraw wastes.

The B0,Th values for the degradation of the six wastestreams under study were calculated on the basis ofBuswells equation (1), where the specic content in C,H, O and N of each waste is the origin of the dierences inthe theoretical methane potentials calculated.

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Table 3. Wastes characterization.

Physico-chemical parameters Elemental compositiona Theoretical methane potentialc

Waste pH- TS g l1 VS g l1 VS/TS- %C %H %N %P %S %Ash %Ob B0,Th mLCH4 gVS1

WH 5.0 49.1 43.6 0.9 46.5 5.0 3.3 1.1 0.4 11.2 32.5 504FC 5.2 364.5 256.4 0.7 47.0 7.1 2.0 0.8 0.3 29.7 13.1 835PCS 5.1 445.2 427.8 1.0 60.4 8.9 6.9 4.0 0.4 3.9 15.5 782SJ 3.9 194.4 179.3 0.9 44.9 5.9 1.6 0.7 0.2 7.8 38.9 480WAS 6.5 25.6 23.1 0.9 46.4 6.4 6.9 0.5 0.4 9.8 29.6 523PM 7.7 44.9 30.7 0.7 34.5 4.7 2.8 0.4 0.7 31.6 25.3 516

a% dry-matter weight basis.b%O estimated.cCalculated assuming a CaHnObNc composition (S and P neglected)

From the theoretical point of view, in accordance withBuswells equation, FC and PCS give wastes with the high-est methane potential, as shown in Table 3. These two typesof waste are also expected to have the highest lipid content.Lipids are one of the most energetic organic compounds,with a high specic biogas potential. Consequently, addi-tion of wastes with a high lipid content into a biogas reactorcan result in signicant increase of the biogas production.

Eect of pre-treatmentsPre-treatment of raw wasteThe eect of sonication, shredding, acid hydrolysis andalkaline hydrolysis pre-treatmentwas evaluated by compar-ing the experimental results obtained, in terms of the extentof biogas production, when BMP assays for pre-treatedand non-pre-treated samples were run. Table 4 shows thebiodegradability factors for the six wastes selected and thefour pre-treatmentmethods assayed. These data refer to ulti-mate biodegradability and the behaviour of the tests duringthe incubation time is not reected here.

Comparison of the biogas production after 1.5 minutesshredding thewastes shows that the inuence ofmechanicalpre-treatment on anaerobic biodegradability under the con-ditions assayed is not signicant. Only a slight enhancementof biogas production was detected for WH and WAS. Themaximum values of the biodegradability factor for thesewastes were, respectively, 1.11 and 1.10, while for the othersamples this factor remained close to the unit.

The acid and alkaline pre-treatments were carried outat pH 2 and 12, respectively, using NaOH and H2SO4 aspH modiers. In these cases, a lack of eectiveness was

Table 4. Biodegradability factor for raw wastes.

Sample

Pre-treatment WH FC PCS SJ WAS PM

Shredding 1.11 1.05 1.00 1.02 1.10 1.00Sonication 1.31 1.10 1.27 1.24 1.47 2.54Alkaline hydrolysis 0.67 0.59 0.37 1.01 0.94 0.05Acid hydrolysis 0.60 0.50 0.38 0.40 1.79 0.00

observed with a decrease in methane productivity in mostof the assays, clearly due to inhibition processes related tothe nature of the chemicals used during pre-treatment. Onlyin the case of WAS did the acid hydrolysis considerablyincrease the biodegradability of the sample.

Sonication pre-treatment was performed at the fre-quency of 37 kHz and 60 minutes of sonication time. Thebest results were obtained with this method, improving theAD by a factor of 1.102.54. Better results were obtainedfor waste samples with less TS content (PM and WAS).This is an indication that the degree of waste disintegrationis directly related to the specic energy input expressed asthe amount of energy consumed in relation to the total solidscontent.

In conclusion, from the rst experiment it can beassessed that methane productivity was enhanced by soni-cation pre-treatment in all the cases. Meanwhile, methaneproductivity remained constant regardless of the shreddingpre-treatment (only a slight enhancement was detected).It would seem that shredding favours particle deagglom-eration without achieving the destruction of bacteria cells.On the other hand, the lack of eectiveness of alkali andacid pre-treatments, with decrease of methane productiv-ity in some cases, can be justied by the inhibition of themethanogenic process due to the presence of high concen-trations of sodium of up to 8 g l1 in some cases. Accordingto Chen et al. [24], sodium concentrations ranging from 3to 16 g l1 cause 50% inhibition in AD processes. Althoughsulfuric acid was also added during both pre-treatmentprocesses, no competition between sulfate-reducing andmethanogenic bacteria was observed, since the presence ofH2S in the biogas was not detected. In the case of WAS, thepositive eect of the acid hydrolysis might be due to the factthat the acidication of WAS required a smaller amount ofacid and, consequently, a smaller amount of alkali for laterneutralization. As a consequence, non-inhibition by sodiumtook place.

Eect of ultrasonic treatment on co-digestionFour mixtures of four selected individual wastes were pre-pared as indicated in Table 1. Figure 1 shows the average

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Figure 1. Cumulative specic methane production curves of the waste mixtures (M1, M2, M3 and M4) at dierent conditions (2A, 2B,2C, 2D and non-pre-treatment). All data are seed-blank corrected.

cumulative specic methane production of the wastes at thedierent conditions assayed (2A, 2B, 2C, 2D), where AN-Prefers to non-pre-treatment conditions. All the biodegrad-ability assays were conducted in triplicate and the maxi-mum deviation reported was 10mlCH4 gSV1 for everyassay set.

It can be observed that the results obtained withpre-treated samples, no matter the conditions applied,always show higher methane generation potential thannon-pre-treated samples. Focusing on the tests carried outwith non-pre-treated samples, it can be observed that exper-imental values obtained from the biodegradability tests after600 hours operation (Figure 1) are always lower than thetheoretical ones obtained by applying Buswells equation(in mlCH4 gVS1: B0,Th(M1) = 575, B0,Th(M2) = 537,B0,Th(M3) = 658, B0,Th(M4) = 553), which could meanthat the maximum theoretical conversion has not beenreached during the experimentation, but also (more prob-able) that with Buswells equation neither the use ofsubstrate nor other routes of conversion of organic mat-ter are taken into consideration for the production of

bacterial biomass. Furthermore, it is known that in thepresence of specic inorganic donors (such as nitrate, sul-fate or sulte) the production of methane can decrease.During the reported period, as expected, the mixture withthe highest percentage of high theoretical methane yieldwaste (M3 with 55% PCS content) also shows, in general,higher specic methane potential than the other mixtures(M1, M2 and M4), with a lower percentage of high the-oretical methane yield wastes (25%, 15% and 15% ofPCS, respectively). This behaviour is common to pre-treated and non-pre-treated samples. For the other mix-tures, where the order of theoretical methane potential,B0,Th is (B0,Th(M1) > B0,Th(M4) > B0,Th(M2)), the situa-tion changes in the absence of pre-treatment, the exper-imental results obtained being B0,Th(M2) > B0,Th(M1) >B0,Th(M4). Here a clear synergistic eect is observed whenco-digesting mixture M2, in accordance with Nieto et al.[25], although this eect is not so evident when samples arepre-treated.

Mechanisms of the ultrasonic process are clearlyinuenced by three factors: supplied energy, ultrasonic

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frequency and nature of the treated material, as the experi-mental results show.

The optimal scenario assayed corresponds to 2A, closelyfollowed by scenarios 2B and 2D and, nally, 2C. Theassays without pre-treatment gave the worst results. Theultimate methane production was proportional to the spe-cic energy input applied, Es, in all the mixtures, but higherspecic methane potentials were found at lower frequency(37 kHz, assays 2A and 2B) in accordance with Bourgrieret al. [26]. The disintegration of samples was also mosteective at the lower frequency, as can be deduced fromthe largest increase in turbidity, derived from the paral-lel increase in COD solubilization. The soluble COD ratio(dened as soluble COD divided by total COD) increasedin all the cases from 15% to 38% for 37 kHz frequency andonly from 9% to 25% for 40 kHz. This can be explainedbecause low-frequency ultrasound creates large cavitationbubbles that, upon collapse, initiate powerful jet streamsexerting strong shear forces in the liquid. The decreasingdisintegration eciency observed at higher frequencies isattributed to smaller cavitation bubbles, which do not allowthe initiation of such strong shear forces. On the other hand,methane generation increased with increasing sonicationtime in M2 and M4. Here, short sonication times result inparticle deagglomeration without the destruction of bacte-ria cells, while longer sonication brought about the break-upof cell walls. The waste particles are disintegrated and dis-solved compounds release. However, in the case of M1 andM3 (the mixtures with the highest composition of PCS),the eect of the sonication times assayed on the cumulativespecic methane production is not appreciable, since thegures for assays 2A and 2B are similar. It is possible thata higher sonication time would have given better resultsfor this specic waste, which seems more resistant to dis-integration. When maintaining the parameters frequency,total volume (V ) and sonication time (t) constant during thesonication pre-treatment, an increment in the same test onthe power input, P, positively aects the nal AD processbehaviour, resulting in an increment in the total methaneproduction, as shown when comparing assays 2C and 2D.

In absolute terms, M1 is the mixture most aected byultrasound pre-treatments, since it shows that the highercumulativemethane production increaseswhen thismethodis applied in comparison with the non-pre-treated assays(104%, 103%, 75% and 102% of increment for assays 2A,2B, 2C and 2D, respectively).

Biogas compositionDuring the early stages of the assays, methane content in thebiogas was low, but it increased until reaching the amountsshown in Table 5. Higher methane content was observedin the biogas of the pre-treated samples as compared tothe control, due to the better biodegradation of disinte-grated samples. Again the optimal scenario corresponds toassay 2A.

Table 5. Biogas composition.

Methane percentage

M1 M2 M3 M4

Non-pre-treatment

62.2 1.5 60.0 1.8 62.0 1.2 63.8 1.0Assay 2A 70.4 2.0 67.5 1.5 70.6 1.5 67.8 1.3Assay 2B 70.2 1.9 66.6 2.1 70.4 0.9 66.4 1.6Assay 2C 68.7 1.6 68.0 1.5 69.7 2.4 66.4 1.9Assay 2D 68.0 1.6 66.8 1.7 70.4 1.0 66.4 1.9

Increase in the methane percentage of pre-treatedsamples over untreated ones may suggest that wastecomponents with higher methane production potential(polymeric material, such as lipids, proteins and carbohy-drates) were the most aected in terms of disintegration(conversion into smallermolecules) bypre-treatment activi-ties. These substrateswere solubilized in themedia thanks topre-treatment actions and the bacteria present in the systemwere able to access and transform them into methane, whilein un-treated samples, some of these substrates were ableto remain insolubilized and, thus, not contribute to biogasgeneration. No inhibition of methane generation by ammo-nia metabolization proceeding from soluble proteins wasobserved.

ConclusionIn this study, several pre-treatment methods (shredding,sonication, acid hydrolysis and alkaline hydrolysis) appliedto dierent wastes and their mixtures were performed toimprove the AD eciency.

The results indicate that the digestion eciencies ofthe wastes were consequently improved by ultrasonic pre-treatment. Ultrasonic pre-treatment enhances the subse-quent AD, resulting in increased production of biogas. Ithas been shown that degradation of samples ismore ecientwhen using low frequencies and a correlation was found toexist between gas production and energy applied. Resultsalso showed that the co-digestion of the mixtures does nothave a clear eect on AD eciency, although suggestionsof a synergistic eect have been observed, depending on thewaste mixture formulation.

The development and establishment of the pre-treatmentof wastes prior to AD in order to accelerate the hydrol-ysis step can make this technology potentially more fea-sible for all kinds of wastes. Pre-treatment can, thus,enhance waste digestion and the rate and quantity of biogasgenerated.

AcknowledgementsThe authors gratefully acknowledge support of this work by theSpanish Ministry of Science and Innovation (project CTM2009-14330).

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