Bioresource Technology 102 (2011) 4131–4136

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Bioresource Technology

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Potential for methane production from anaerobic co-digestionof swine manure with winery wastewater

B. Riaño, B. Molinuevo, M.C. García-González ⇑Agricultural Technological Institute of Castilla y Léon, Ctra. Burgos, km 119, 47071 Valladolid, Spain

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

Article history:Received 19 October 2010Received in revised form 15 December 2010Accepted 16 December 2010Available online 24 December 2010

Keywords:Swine manureWinery wastewaterAnaerobic co-digestionMethane

0960-8524/$ - see front matter � 2011 Elsevier Ltd. Adoi:10.1016/j.biortech.2010.12.077

⇑ Corresponding author.E-mail addresses: bertariano@yahoo.es (B. Riaño

García-González).

This work examines the methane production potential for the anaerobic co-digestion of swine manure(SM) with winery wastewater (WW). Batch and semi-continuous experiments were carried out undermesophilic conditions. Batch experiments revealed that the highest specific methane yield was 348 mLCH4 g�1 COD added, obtained at 85.4% of WW and 0.7 g COD g�1 VS. Specific methane yield from SMalone was 27 mL CH4 g�1 COD added d�1. Furthermore, specific methane yields were 49, 87 and107 mL CH4 g�1 COD added d�1 for the reactors co-digesting mixtures with 10% WW, 25% WW and40% WW, respectively. Co-digestion with 40% WW improved the removal efficiencies up to 52% (TCOD),132% (SCOD) and 61% (VSS) compared to SM alone. These results suggest that methane can be producedvery efficiently by the co-digestion of swine manure with winery wastewater.

� 2011 Elsevier Ltd. All rights reserved.

1. Introduction

The wine industry is of major importance in Castilla y León(Spain). This industry generates large volumes of wastewater,which are mainly originated from various washing operations dur-ing the crushing and pressing of the grapes, as well as rinsing thefermentation tanks, barrels and other equipment or surfaces(Petruccioli et al., 2002). Winery wastewater presents an acidicpH, a high organic load and notable polyphenol, macronutrient,micronutrient and heavy metal content (Bustamante et al., 2005).Another important agro-industrial sector in Castilla y León is inten-sive pig production. In 2008, the estimation of pig production inthis region was 3.7 million heads (animals) (MARM, 2010). Thedevelopment of this sector has led to an increase of livestockwastes in small and located areas. The uncontrolled discharge ofproduced wastes may cause serious social, environmental andhealth problems. For this reason, it is necessary to minimize therisks.

Co-digestion has been defined as the anaerobic treatment of amixture of at least two different substrates with the aim of improv-ing the efficiency of the anaerobic digestion process (Álvarez et al.,2010). Thus, co-digestion process lies in balancing the car-bon:nitrogen (C:N) ratio in the co-substrate mixture, as well asmacro and micronutrients, pH, inhibitors/toxic compounds anddry matter (Hartmann et al., 2003). In general, the anaerobic diges-tion of agro-residues is associated with a low pH of the substrate

ll rights reserved.

), gargonmi@itacyl.es (M.C.

itself, poor buffering capacity and the possibility of high volatilefatty acid (VFA) accumulation during co-digestion (Banks andHumphreys, 1998; Campos et al., 1999). Co-digestion of manuresand other substrates overcomes those problems by maintaining astable pH within the methanogenesis range due to their inherenthigh buffering capacity. Additionally, manure presents high ammo-nia content and a wide variety of nutrients needed by the metha-nogens during the anaerobic process (Angelidaki and Ahring,1997; Molnar and Bartha, 1998). On the other hand, the co-diges-tion of manures with agro-residues will also aid in overcomingammonia inhibition related to pure manure digestion.

Nowadays, there is an increasing number of full-scale co-digestion plants treating manure and industrial organic wastes(Angelidaki and Ellegard, 2003), and there is an increasing interest,especially in Europe, in using this technology for biogas produc-tion. The manure specific methane potential has been improvedby co-digestion with other agro-residues (Ferreira et al., 2007;Fountoulakis et al., 2008; Panichnumsin et al., 2010). Some of thepossible technological, economic and ecological advantages of theanaerobic co-digestion are the better handling of mixed wastes,the use of common access facilities and the effect of economy scale(Martínez-García et al., 2007). However, it is not clear whethersome by-products might have adverse effects when added to astable digester or used in conjunction with other types of residues(Fountoulakis et al., 2008).

The objective of the present work was to develop the use ofwinery wastewater for co-digestion with swine manure. Batchexperiments were carried out based on a central composite design.The influence of the percentage of the winery wastewater in thesubstrate and the substrate/inoculum ratio were evaluated in

4132 B. Riaño et al. / Bioresource Technology 102 (2011) 4131–4136

terms of methane yield. Finally, the effect of the feed componentratio of winery wastewater to swine manure on process perfor-mance was investigated in a semi-continuously fed stirred tankreactor (CSTR) at 35 �C.

2. Methods

2.1. Origin of manure, winery wastewater and inoculum

Swine manure (SM) was obtained from a pig farm located inSegovia (Spain). Winery wastewater (WW) was collected in a cellarlocated in Valladolid (Spain). The anaerobic sludge (AS) used asinoculum was collected from an anaerobic digester in the munici-pal wastewater treatment plant of Valladolid (Spain). Thesubstrates and inoculum were individually homogenized andsubsequently stored at 4 �C for further use. The chemical character-isation of each waste and the sludge employed is shown in Table 1.

2.2. Batch experiments

Batch experiments were carried out at 35 ± 2 �C for 55 daysbased on a central composite design, which is a second order fac-torial design used when the number of runs for a full factorial de-sign is too large to be practical (Box and Wilson, 1951). This type offactorial design usually consists of a 2k factorial nucleus, six repli-cations of the central point and 2�k axial points, where k is thenumber of factors evaluated, two factors in this case. The factorsselected for the study were the solid concentration (SC) measuredin terms of substrate (g COD)/inoculum (g VS) ratio and the per-centage of winery wastewater in the substrate (% WW), measuredin terms of COD of WW in relation to the COD of the feed. The se-lected range for factor one (SC) was 0.3–3 g COD g�1 VS. The se-lected range for factor two (% WW) was 0–100%. Factorial designlevels were codified from +1 to �1. The central point was repli-cated six times in order to estimate experimental error. Axialpoints ensure design rotatability and their distance to the centralpoint (a) was calculated according to Eq. (1).

a ¼ 2k=4 ð1Þ

The selected response for analysis was the methane yield, mea-sured as volume of methane produced per unit of COD added. Thevariables, Xi, were coded as xi according to Eq. (2), such that X0 cor-responded to the central value:

xi ¼ Xi � X�i� �

=DXi; where i ¼ 1;2;3; . . . ; k; ð2Þ

where xi is the dimensionless coded value of an independent vari-able, Xi is the actual value of an independent variable for the ith test,X�i is the actual value of an independent variable at the centre pointand DXi is the step change (Chong et al., 2009). All the evaluatedlevels were combined in 9 different treatments (T1–T9). Codifiedand real values for both factors are presented in Table 2A.

Table 1Composition of the substrates in batch experiments and semi-continuous digestion: wine

Parameters Batch experiments

WW SM AS

pH 3.8 (0.01) n.d. n.d.TS (g L�1) n.d. 26.9 (0.2) 18.3 (VS (g L�1) n.d. 18.1 (0.2) 10.8 (TSS (g L�1) 0.1 (0.04) n.d. n.d.VSS (g L�1) 0.1 (0.03) n.d. n.d.TCOD (g L�1) 20.0 (0.1) 25.3 (0.1) 14.5 (SCOD (g L�1) 18.1 (0.1) 11.4 (0.2) 11.0 (NHþ4 -N (g L�1) 0.0 (0.01) 1.3 (0.1) 0.6 (0

Data are means of two replicates. Standard deviation is shown in brackets.n.d.: not determined.

For predicting the optimal point, a second order polynomialfunction was performed Eq. (3):

Y ¼ b0 þ b1X1 þ b2X2 þ b11X21 þ b22X2

2 þ b12X1X2; ð3Þ

where Y represents the predicted response, b0, b1, b2, b11, b22 and b12

are the regression coefficients. X1 and X2 are the evaluated factors(SC and % WW). The coefficient of determination (R2) was calculatedto achieve the proportion of data variability that is explained by themodel, thus the quality of fit to the model. The p-values of theparameter estimation were used to validate the model. p-values lessthan 0.05 indicate the significant model terms. Multiple regressionanalysis for the data sets collected was performed using Excel soft-ware (Excel 2003).

The anaerobic assays were conducted in 500 mL bottles with atotal liquid volume of 300 mL and an inoculum volume of111 mL. Three blanks containing 111 mL of inoculum and 189 mLof distilled water were also run to determine the endogenousmethane production of the anaerobic sludge. The bottles wereclosed with a septum and the headspace flushed with N2 to removethe oxygen. The biogas production was measured by the overpres-sure in the headspace with time frequency (Colleran et al., 1992).

2.3. Semi-continuous digestion of different mixtures

Semi-continuous co-digestion of SM with WW was carried outin two identical continuously stirred tank reactors (CSTRs) with atotal volume of 7 L and a working volume of 5 L, namely R1 andR2. A water bath was used to maintain the temperature of thedigesters at 35 ± 1 �C. Digesters were mounted separately on amechanical stirrer, stirring continuously at 55 rpm. The outletsprovided on the top of each digester were used for feeding influent,withdrawing effluent and for collecting biogas. Biogas was dailymeasured by displacement of water.

Feed TCOD was maintained constant during the whole experi-ment resulting in an organic loading rate (OLR) of 0.85 g COD L�1

d�1 (hydraulic retention time (HRT) of 12 days). Different feedTCOD ratios of SM and WW were evaluated. After inoculating thedigester with 5 L of digested anaerobic sludge (AS), R1 was usedto co-digest swine manure with winery wastewater in a feed TCODratio of 75% SM and 25% WW, whereas R2 was performed with SMalone. After 34 days, R1 was fed with swine manure and winerywastewater, in a feed TCOD ratio of 60% SM and 40% WW, whereasthe feed in R2 was made up 90% SM and 10% WW. The digesterswere fed once a day every weekday. Prior to each feeding, a volumeequal to the feeding volume was removed to maintain a constantdigester volume. The characteristics of substrates are shown inTable 3.

The composition of influent and effluent were determined twicea week except the pH which was monitored daily. The resultsfrom the analysis of each mixture at steady state were used for

ry wastewater (WW), swine manure (SM) and anaerobic sludge (AS).

Semi-continuous digestion

WW SM AS

3.1 (0.01) 7.7 (0.02) n.d.2.5) n.d n.d. 5.4 (0.01)1.5) n.d. n.d. 3.6 (0.02)

0.5 (0.04) 33.4 (0.6) n.d.0.5 (0.04) 20.9 (0.7) n.d.

0.04) 10.7 (2.3) 38.0 (0.5) n.d.0.1) 9.2 (0.1) 5.4 (0.7) n.d..1) 0.01 1.5 (0.1) n.d.

Table 2(A) Codified, real values and response for swine manure co-digestion in batch experiments. (B) Regression results for swine manure co-digestion in batch experiments.

Treatments Codified values Real values Real response Predicted response

SC (g COD g�1 VS) % WW SC (g COD g�1 VS) % WW Y (mL CH4 g�1 COD added) Y (mL CH4 g�1 COD added)

(A)T1 �1 1 0.70 85.36 348 (18) 334T2 �1 �1 0.70 14.64 233 (8) 222T3 1 1 2.60 85.36 190 (39) 216T4 1 �1 2.60 14.64 223 (11) 253T5 0 1.4142 1.65 100.00 257 (2) 252T6 0 �1.4142 1.65 0.00 210 (0) 199T7 1.4142 0 0.30 50.00 298 (24) 318T8 �1.4142 0 3.00 50.00 293 (9) 256T9 0 0 1.65 50.00 301 (7) 301

Y (mL CH4 g�1 COD added)

Coefficient Prob

(B)b0 301.3 <0.001b1 �21.9 0.003b2 18.6 0.010b11 �7.1 0.365b22 �37.9 <0.001b12 �37.2 <0.001

R2 = 0.7960, Adj. R2 = 0.7322, r = 0.8922F value = 12.48, Prob > F = 0.000047

Data are means of two replicates, except T9, which data are means of six replicates. Standard deviation is shown in brackets.R2, correlation coefficient; Adj. R2, adjusted correlation coefficient; r, regression coefficient; F value, value resulted from the F-test.

Table 3Characteristics of feedstocks at different mixture ratio used in CSTR experiment.

WW (%)

0 10 25 40

TCOD (g L�1) 11.7 (1.1) 10.7 (2.9) 10.6 (0.1) 9.8 (1.8)SCOD (g L�1) 0.8 (0.2) 1.2 (0.4) 2.9 (0.3) 3.5 (0.6)NHþ4 -N (g L�1) 0.4 (0.03) 0.4 (0.03) 0.3 (0.02) 0.3 (0.04)TKN (g L�1) 0.9 (0.3) 0.6 (0.3) 0.4 (0.2) 0.5 (0.3)TSS (g L�1) 11.8 (2.8) 10.6 (3.8) 6.6 (1.6) 6.5 (2.7)VSS (g L�1) 7.9 (3.7) 6.8 (1.7) 3.5 (2.4) 4.5 (1.4)

Standard deviation is shown in brackets.

B. Riaño et al. / Bioresource Technology 102 (2011) 4131–4136 4133

evaluating the effect of co-digestion on biodegradability, biogasproduction efficiency and process stability.

2.4. Analyses

Total solids (TS), volatile solids (VS), total suspended solids(TSS), volatile suspended solids (VSS), total and soluble chemicaloxygen demand (TCOD and SCOD), total Kjeldahl nitrogen (TKN),ammonium nitrogen (NHþ4 -N) and alkalinity were performed inaccordance with APHA Standard Methods (2005).

Biogas composition was analyzed using a gas chromatograph(Varian CP 3800 GC) with a thermal conductivity detector, pro-vided by a CP-Molvsieve5A column (15 m � 0.53 mm � 15 lm)followed by a CP-Porabond Q column (25 m � 0.53 mm � 10 lm).Hydrogen (13.6 mL min�1) was used as the carrier gas. The injec-tion port temperature was set at 150 �C and the detector tempera-ture was 175 �C. pH and temperature in the reactors weredetermined using a Multiline P4 Oxical-SL Universal Meter(WTW, Germany).

Total VFA were analyzed using a gas chromatograph (Varian CP3900) equipped with a CP-Was 58 Varian capillary column(25 m � 0.53 mm � 1 lm) and a flame ionization detector. The car-rier gases were nitrogen, hydrogen and air and the temperature ofthe injector was 250 �C. The temperature of the oven was set at40 �C for 2 min and thereafter increased to 180 �C.

3. Results and discussion

3.1. Chemical characteristics of winery wastewater and swine manure

There were significant differences in the composition of waste-waters (Table 1). Winery wastewater had high SCOD/TCOD ratios(0.86–0.91) compared to swine manure, which presented SCOD/TCOD ratios in the range of 0.14–0.45. WW, as shown for pH val-ues, was significantly acidic, whereas SM had a pH of 7.7. TheTCOD/NHþ4 -N ratio was very high for WW compared to SM. Thecharacterisation of the wastes indicated that co-digestion of WWwith SM could be a good solution to improve C:N ratio, overcomingthe problems of digesting both substrates separately.

3.2. Batch experiments

The experimental design data, real response and predicted re-sponse are presented in Table 2A. Regression analyses are shownin Table 2B and resulted in the following second order polynomialEq. (4):

YCH4 ¼ 301:3� 21:9ðSCÞ þ 18:6ð%WWÞ � 7:1ðSCÞ2

� 37:9ð%WWÞ2 � 37:2ðSCÞð%WWÞ ð4Þ

The regression showed that the model was significant becausethe value of F statistics of 12.48 was greater than the calculatedone (0.000047). However, the determined R2 coefficient obtainedwas 0.7960, meaning that the model explained 80% of the variabil-ity data. Moreover, the 0.8922 correlation coefficient indicated thatthe combination of both factors (SC and % WW) had a high impor-tance in the methane yield. p-values for the entire model termswere lower than 0.05, except for the quadratic term associatedwith SC (Table 2B). As an overall, the second order polynomialmodel fitted the experimental results corresponding to methaneyield quite well.

The averaged methane yield was 269 mL CH4 g�1 COD added.Methane content was above 68% (data not shown) for all treat-ments. The highest specific methane yield was 348 mL CH4 g�1

4134 B. Riaño et al. / Bioresource Technology 102 (2011) 4131–4136

COD added, obtained at values of 0.7 g COD g�1 VS and 85% forfactor SC and % WW, respectively (T1). To our knowledge, noprevious works have determined methane production of this mix-ture. Fountoulakis et al. (2008) evaluated the potential methaneproduction from the anaerobic co-digestion of a mixture of 50%wine-grape residues with slaughterhouse wastewater, reporting amaximum methane yield of 188 mL CH4 g�1 COD added. Thesedata suggested that swine manure was a better co-substrate thanslaughterhouse wastewater for the anaerobic co-digestion of win-ery by-products.

Fig. 1 illustrates the accumulated methane production through-out the co-digestion time for T1–T9. Regarding two of the treat-ments with a high percentage of winery wastewater as co-substrate (T3 and T5), the delay in methane production could havebeen the result of VFA accumulation due to the high biodegradabil-ity of winery wastewater, which resulted in partial inhibition ofthe system. This partial inhibition was overcome when the VFAwere consumed and then the methane production increased (Cam-pos, 2001). As can be seen in Fig. 1, at the end of the assays, the sta-bilization of the methane production was not achieved, so methaneproduction could have been underestimated. In spite of VFA accu-mulation, the pH value was maintained in the range of 7, due to theswine buffer capacity, as previously reported González-Fernándezet al. (2008).

With regard to treatments with a constant value for SC of1.65 g COD g�1 VS (T5, T6 and T9); T9 (with 50% WW) presentedthe maximum methane yield. Thus, co-digestion improved meth-ane production in comparison with anaerobic digestion of winerywastewater and swine manure alone. As described above, the delayin methane production in T5 could have been the result of VFAaccumulation, due to the high biodegradability of winery wastewa-ter, leading to a partial inhibition of the system and underestimat-ing methane yield. In the case of treatment T6, swine manurepresented lower biodegradability than winery wastewater. On

0

200

400

600

800

1000

1200

0 10 20 30 40 50 60Time (d)

Met

hane

(m

L)

T1 T2 T3 T4 T5 T6 T7 T8 T9

Fig. 1. Accumulated methane production for SM and WW co-digestion in batchexperiments.

Table 4Performance data of CSTR treating SM and three mixtures.

WW (%)

0

Biogas production rate (mL d�1) 256 (68)CH4 content (%) 49.4 (6.5Specific methane yield (mL g�1TCOD added d�1) 27 (7)TCOD reduction (%) 29.0 (5.8SCOD reduction (%) 36.8 (17.VSS reduction (%) 32.6 (8.0

Standard deviation is shown in brackets.

the other hand, treatments with a constant value of % WW (T7,T8 and T9) presented a similar methane yield, in the range of293–301 mL CH4 g�1 COD added. In this case, the evaluated rangefor factor SC seemed not to influence methane production.

3.3. Semi-continuous single-stage digestion of different mixtures

The summarized values of the monitored parameters are givenin Table 4. The results indicated that the highest biogas productionwas achieved when 25% WW was added to the feedstock. As can beseen from Table 4 and Fig. 2, the biogas production rate and thespecific methane yield of SM alone were 256 mL CH4 d�1 and27 mL CH4 g�1 TCOD added d�1, respectively, and these values in-creased up to 653 mL CH4 d�1 and 87 mL CH4 g�1 TCOD added d�1

with 25% WW addition. The methane content increased from49.4% with SM alone to 64.4% with a 40% WW addition. At thispoint, it is worth mentioning that the reactor fed with 40% WWshowed lower biogas production rate than the reactor fed with25% WW, although the composition in methane and the specificmethane yield were higher. The high variability in biogas produc-tion in the reactor fed with 40% WW (Table 4) could be due to thelower stability of the process. In spite of the fact that the pH valuesin the reactors were constant during the whole experiment,approximately in the range of 6.9–7.9, the reactor fed with 40%WW presented an average alkalinity ratio of 0.42, whereas thereactor fed with 25% WW showed an average ratio of 0.34. Ripleyet al. (1986) reported that successful digestion of poultry manureoccurred with an alkalinity ratio below 0.3.

Specific methane yields in this work were higher than those re-ported by Álvarez et al. (2010) who obtained a specific methaneyield of 16.4 mL CH4 g�1 COD added d�1 by digesting a mixturecontaining 88% pig manure, 4% fish waste and 8% biodiesel waste.In any case, the low biogas production rate could be attributed tothe characteristics of the swine manure itself, with a low SCOD/TCOD ratio (0.14), which indicated that the swine manure was par-tially degraded apparently due to long term storage (Table 1). Thespecific methane yield increased 45%, 69% and 75% compared tothat obtained from the digestion of swine manure alone when10%, 25% and 40% of WW was added, respectively. These resultswere in accordance with those found in literature that indicatedthat anaerobic co-digestion could increase CH4 production of man-ure digesters by 50–200%, depending on the operating conditionsand the co-substrates used (Callaghan et al., 1999; Murto et al.,2004; Amon et al., 2006; Ferreira et al., 2007; Soldano et al., 2007).

The higher methane yields achieved in co-digestion of WW withSM, as compared with those achieved with SM alone at the sameloading rate in the present study, were apparently due to the highermethane potential of WW, as demonstrated by batch experiments.This high methane potential achieved by the co-digestion of WWwas probably due to the high anaerobic biodegradability of theethanol and sugars, the main components of winery wastewater(Colin et al., 2005). On the other hand, the main components in typ-ical pig manures are carbohydrates (53% of TS), hemi-cellulose and

10 25 40

327 (48) 653 (58) 635 (135)) 58.8 (8.9) 58.6 (2.9) 64.4 (8.3)

49 (7) 87 (8) 107 (23)) 23.0 (12.1) 44.7 (5.5) 44.0 (9.9)0) 46.6 (17.6) 76.5 (2.6) 86.4 (1.8)) 39.6 (18.4) 26.2 (8.0) 52.5 (10.1)

0

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0 10 20 30 40 50 60 70

0 10 20 30 40 50 60 70

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-1 d

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Feed TCOD ratio: 75% SM+25% WW

Feed TCOD ratio: 60% SM+ 40% WW

R1

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R2

Fig. 2. Biogas production during anaerobic digestion of agro-industrial wastewatersduring operation of reactors.

B. Riaño et al. / Bioresource Technology 102 (2011) 4131–4136 4135

cellulose followed by proteins, fats and lipids and a small amount oflignin (4.4%) and starch (1.6%) (Iannotti et al., 1979). The biodegrad-ability of pig manure has been reported to be dependent upon thelignin content, which is not only considered as refractory to anaer-obic degradation, but also reduces the availability of other compo-nents, especially cellulose (Kaparaju and Rintala, 2005).

Likewise, the removal efficiencies of TCOD, SCOD and VSS grad-ually increased (Table 4). The TCOD, SCOD and VSS reduction of thereactor with SM alone were 29%, 37% and 33%, respectively. Themaximum reduction of TCOD, SCOD and VSS obtained at a WW ra-tio of 40% were 44%, 86% and 53%, respectively. The results showedthat co-digestion improved the efficiencies up to 52% (TCOD), 132%(SCOD) and 61% (VSS) as compared to SM alone. This increase wasapparently due to the increased amount of easily degradable com-pound in the feed, as it was reported by Panichnumsin et al. (2010),who co-digested cassava pulp with various concentrations of pigmanure.

Finally, NHþ4 -N concentrations in the present study were far toreach reported toxics levels of >4 g L�1 which would cause ammo-nia inhibition (De Baere et al., 1984). The high content of ammoniain swine manure makes it possible to degrade winery wastewaterbiologically without the addition of external alkalinity and withoutaddition of external nitrogen source (Table 1). On the other hand,as reported by previous study, significant increases in volumetricbiogas production can be achieved by adding carbon rich agricul-tural residues to the co-digestion process with swine manure(Wu et al., 2010). These authors found that the C:N ratio of 20:1was the best in terms of biogas productivity in the anaerobic co-digestion of swine manure with three crop residues as an externalcarbon source. Winery wastewater addition to swine manure wid-ened COD:TKN ratio from 13.0 to 26.5 in 0% WW and 25% WW,

respectively (Table 3). Resch et al. (2010) widened the COD:TKN ra-tio to 17:1 by adding COD in terms of glycerine to nitrogen animalby-products and concluded that no stable process could beachieved due to the fast hydrolysis of glycerine that led to theaccumulation of acetic and propionic acid. As mentioned, in thiswork, the swine buffer capacity contributed to the stability of theprocess.

Nevertheless, co-digestion in the present context should be con-sidered as a process for the simultaneous treatment of two differ-ent waste streams and as a solution for the problems of ammoniainhibition generally encountered during anaerobic digestion of pigmanure and ‘‘souring’’ of readily acidifying winery wastewaterwhich is basically low in pH (Riaño, 2008).

4. Conclusions

Co-digestion of SM with WW is very promising for the produc-tion of renewable energy in the form of methane. The specificmethane yield increased 45%, 69% and 75% compared to that ob-tained from the digestion of swine manure alone when 10%, 25%and 40% WW was added, respectively. Moreover, the addition ofwinery wastewater to the anaerobic digestion of swine manure in-creased organic matter removal.

The results of the present laboratory study revealed that the useof WW as co-substrate in the anaerobic digestion of swine manurehas other advantages: the improvement of the balance of theCOD:TKN ratio and efficient process stability.

Acknowledgements

The authors would like to thank to Javier Tascón and JairoMartín for their technical and analytical support, respectively.

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