Evaluation of the Anaerobic Co-Digestion of SewageSludge and Tomato Waste at Mesophilic Temperature

Siham Belhadj & Yassine Joute & Hassan El Bari &Antonio Serrano & Aida Gil & José Á. Siles &Arturo F. Chica & M. Ángeles Martín

Received: 24 September 2013 /Accepted: 10 February 2014# Springer Science+Business Media New York 2014

Abstract Sewage sludge is a hazardous waste, which must be managed adequately.Mesophilic anaerobic digestion is a widely employed treatment for sewage sludge involvingseveral disadvantages such as low methane yield, poor biodegradability, and nutrient imbal-ance. Tomato waste was proposed as an easily biodegradable co-substrate to increase theviability of the process in a centralized system. The mixture proportion of sewage sludge andtomato waste evaluated was 95:5 (wet weight), respectively. The stability was maintainedwithin correct parameters in an organic loading rate from 0.4 to 2.2 kg total volatile solids(VS)/m3 day. Moreover, the methane yield coefficient was 159 l/kg VS (0 °C, 1 atm), and thestudied mixture showed a high anaerobic biodegradability of 95 % (in VS). Although theammonia concentration increased until 1,864±23 mg/l, no inhibition phenomenon was deter-mined in the stability variables, methane yield, or kinetics parameters studied.

Keywords Sewage sludge . Tomatowaste .Mesophilic anaerobic co-digestion .Methaneyield .

Organic loading rate

Introduction

Water treatment systems started to become popular in the 1990s as a consequence of a higherand higher influence of environmental consciousness in society reaching an 80 % in Europeregarding wastewater [1]. Wastewater plants generate a high level of potentially dangeroussludge. At the end of the 1980s, sludge started to become a problem as it increased considerably,so water treatment was a matter to bear in mind. In particular, sludge production has increased a28 % in the last 25 years in Europe [2]. Biological sludge is a waste that ought to be managed

Appl Biochem BiotechnolDOI 10.1007/s12010-014-0790-9

S. Belhadj : Y. Joute : H. El BariLaboratory of Environmental Biotechnology and Quality, Faculty of Sciences, University Ibn Tofail(Morocco), BP 133, Kenitra, Morocco

A. Serrano : A. Gil : J. Á. Siles : A. F. Chica :M. Á. Martín (*)Department of Chemical Engineering, Campus Universitario de Rabanales, University of Cordoba (Spain),Edificio Marie Curie (C-3), Ctra. N IV, Km 396, 14071 Cordoba, Spaine-mail: iq2masam@uco.es

properly due to the high volume generated (8.3 million tons of dry solid matter in 2006), as wellas the elevated concentration in heavy metals, organic polluting agents, and pathogens; thereby, itconstitutes a problem for the environment and a risk for the human health [3].

The traditional technique used for the sludge treatment has been to deposit the waste inlandfill sites [4]. Nowadays, the directive tends to propose new treatment technologies such asincineration, autothermal thermophilic aerobic digestion (ATAD), composting or anaerobicdigestion [5]. The techniques used in Europe vary depending on the legislation and themanagement strategies of each country. More than two thirds of the total volume of sludgeis used in the agriculture in Spain and Ireland, whereas in Holland, Germany, and Belgium,incineration is used as the main technique. Other countries, such as Greece and Malta, stilldeposit the waste in landfills, as well as other developing countries [6].

This technique is one of the most extended options, although it involves wasting organicmatter and nutrients that can be found in residues [7]. Besides, green house gasses aregenerated in the landfills, such as CO2 and CH4, as well as unpleasant odor compounds witha consequent impact on the environment and the human health. For that reason, the Directive1999/31/EC [4] on landfills settles an objective on reducing the municipal biodegradable wastedeposited by 35 % for 2016.

Incineration is a technique that permits generating energy and reducing the volume ofwaste. However, the sewage has a high content in water, so it makes it a barely viabletechnique. Likewise, the gas produced during the incineration could contain environmentallyhazardous compounds such as dioxins, SO2, NOx, which could cause acid rain and environ-mental pollution [8]. On soils that are poor in organic matter, sewage treatment is a frequentorganic solution, as well as the composting treatment and its use in agriculture, although itinvolves drawbacks such as the presence of heavy metals, specially cadmium, and othercompounds that could have an impact on human health [9]. Regarding ATAD, this emergingtechnology presents drawbacks such as an elevated consumption of oxygen and energy [10].

Anaerobic digestion is a broadly used technology, which is viable to stabilize and treat thesewage sludge and further organic waste [11]. This technique is characterized by low levels ofbiological sludge generation, low nutrient requirements, high efficiency, the production ofmethane, which can be used as an energy source, and stabilized fertilizers, recovering N and Pby the soil [7, 12]. However, the single anaerobic digestion of sewage sludge implies a lowbiodegradability and methane production rate [13].

On the other hand, co-digestion implies digesting several substrates to improve the viabilityof the anaerobic digestion. It is a long-established process in Europe, as Germany andScandinavia were pioneers and have 20 years experience in the process now [14, 15]. Thistechnique allows improving the methane production and diluting some inhibitory compoundsthat are present in digested organic residues [16, 17]. Jansen, Gruvberger, Hanner, Aspegren,and Svärd [18] described an increase in the methane production of a 21 % in the mesophilicanaerobic digestion of sewage sludge by adding a 20 % of food waste in total volatile solids(VS). Another co-substrate that is extensively employed at full scale is the organic fraction ofmunicipal solid waste (OFMSW), which allows increasing the methane yield since OFMSWprovides carbon and compensates the excess of nitrogen from sewage sludge [18].

Amongst the possible co-substrates for the sewage sludge digestion, agricultural residuesare an interesting alternative compared to other possibilities, such as purines, due to theircomposition and widespread location [19]. An example would be fruit and vegetable waste inMediterranean areas. The production of tomatoes is worldwide (reaching an internationalproduction of 159 million tons per year). China is the leading producer with a 30 % (48 milliontons per year). The next in the top list of producers is India, with a 10 % of production. Othercountries, such as Morocco, have a 0.7 % (1 million tons) [20]. However, tomato

Appl Biochem Biotechnol

has a high level of humidity and a high content in nitrogen and cellulose (12.5 %),hemicellulose (7.9 %), lignin (1.4 %) and a low content in heavy metals [21]. Tomatowaste would be interesting to combine with sludge, as it would dilute the content inheavy metals it has, and it would allow improving a balance in the main nutrients such ascarbon, nitrogen, and phosphorus.

In this research study, anaerobic co-digestion with residual tomato in mesophilic conditionshas been proposed to improve sewage sludge treatment. The addition of residual tomatogenerated in farms or green houses allows treating and revaluing this residue, avoiding theenvironmental impact that storing in landfills provokes. This research may be considered ofspecial interest for rural areas and developing countries where both residues are generated.

Materials and Methods

Experimental Setup

The experimental setup used for the anaerobic co-digestion consisted of two 3.5-l Pyrexcomplete mixing reactors working in parallel under mesophilic temperature (35 °C). It workedin semicontinuous mode and with recirculation of the solid fraction of the digestate (whichincluded microorganisms and nonbiodegraded substrate). The reactors were equipped withfour connections in order to load feedstock, ventilate the biogas, inject inert gas (nitrogen) tomaintain the anaerobic conditions and remove effluent. The content of the reactors wasmechanically stirred and temperature was maintained by means of a thermostatic jacketcontaining water at 37 °C under mesophilic conditions. The volume of methane producedduring the process was measured using 2-l Boyle–Mariotte reservoirs connected to eachreactor. To remove the CO2 produced during the process, tightly closed bubblers containinga NaOH solution (6 N) were connected between the two elements. The volume of methanedisplaced an equal measurable volume of water from the reservoirs. This volume was correctedin order to remove the effect of the water steam pressure, and the measured methane was thenexpressed at standard temperature and pressure conditions (STP: 0 °C and 1 atm).

The reactors were inoculated with methanogenically active granular biomass obtained froma full-scale anaerobic reactor used to treat brewery wastewater from the Heineken S.A. Factory(Jaen, Spain; total mineral solids (MS) 14,945 mg/kg, VS 53,680 mg/kg). The inoculum wasselected on the basis of its high methanogenic activity [22]. The methane production rateobserved in the employed inoculum reached a value of 58 mlSTP CH4/g added chemicaloxygen demand (CODadded) h.

Substrate

The raw materials used as substrate were sewage sludge and tomato waste. The sewage sludgewas collected from an aerobic reactor from a wastewater treatment plant of the city of PuenteGenil, Spain. The sewage sludge was dehydrated in the plant by centrifugation after itscollection. A previous coagulation and flocculation was carried out to facilitate this stage.The tomato waste was obtained from a local marketplace, which dismisses the tomatoes thatare considered unfit for human consumption. Tomato waste presenting a COD:N:P ratio of119:1:1 was previously blended, homogenized, and conserved under freezing conditions. Themain analytical characteristics of the substrates are shown in Table 1.

The substrate mixture studied consisted of sewage sludge and tomato waste mixture at aratio of 95:5, respectively, in wet basis. The mixture was blended to facilitate handling and the

Appl Biochem Biotechnol

feeding process of the digesters, thus improving the homogenization of the mixtures andavoiding organic overload as previously described by other authors [23]. The studied propor-tion corresponds with the generation ratio of both substrates in the studied area. The mainanalytical characteristics of the mixture are also shown in Table 1.

Anaerobic Digesters: Experimental Procedure

The reactors were initially loaded with 7 g VS/l of anaerobic granular sludge as inoculum.Likewise, nutrients (mainly nitrogen and phosphorus) and the trace element solutions de-scribed by Fannin [24] and Field, Sierra-Alvarez, and Lettinga [22] were added when thesludge was loaded in order to reach a nutrient balance close to 300:5:1 for the correct start-upof the process as described by Aiyuk, Forrez, Lieven, van Haandel, and Verstraete [25]. Bothsolutions are very important for activating bacterial growth and metabolism at the beginning ofthe process. Additionally, 1 g KHCO3/l was added to increase the buffer capacity in thereactors during the initial phases of the process.

In order to bio-stimulate the biomass prior to the experiments, the reactors were first fed with asynthetic solution composed of glucose, sodium acetate, and lactic acid at concentrations of 50 g/l,25 g/l, and 21ml/l, respectively. During this initial period, the organic load added to the reactors wasgradually increased from0.50 to 1.00 gCOD/l, with intervals of 0.25 gCOD/l, over a 15-day period.

Subsequently, biomass acclimatization was carried out. The reactors were then fed with loadsof 1.00 g COD/l, in which the percentage of the waste mixture in the feeding was increased from25 to 100 % after several loads. During this acclimatization period, the volume of methane wasmeasured as a function of time. The maximum duration of each assay in this stage was 26 h andcorresponds to the time interval required for the maximum gas production and substrate removal.

Subsequently, during each set of experiments with the waste mixture, the organic loadadded to the reactors was gradually increased from 0.5 to 3.0 g VS/l, which corresponds withan organic loading rate (OLR) interval from 0.4 to 2.2 kg VS/m3 day; each load was carriedout at least in duplicate. In all cases, the volume of methane was measured as a function oftime, and samples were taken and analyzed before and after feeding. Furthermore, the resultshave been referred to the allowed OLR due to its wide use in this research field. To determinethe allowed OLR, we considered the time required to reach 95 % of the total methaneproduction for each load added to the reactors.

Chemical Analyses

The following parameters were determined in the effluents of each load: pH, COD (milligramO2 per kilogram), total solids (TS, milligram per kilogram), MS (milligram per kilogram), VS

Table 1 Analytical characterization of tomato waste, sewage sludge and tomato-sewage sludge mixture (wetbasis)

Tomato waste Sewage sludge Tomato waste-sewage sludge mixture

Moisture (%) 94.4±0.9 88.4±2.3 89.0±0.1

CODt (grams O2 per kilogram) 87±2 210±6 204±1

N-NH4+ (grams per kilogram) 1.11±0.10 10.51±1.69 8.65±0.15

PT (grams per kilogram) 0.73±0.09 6.87±0.32 6.56±0.18

COD/N-NH4+ 78 23 24

COD:N-NH4+:PT 119:1:1 31:2:1 31:1:1

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(milligram per kilogram), volatile acidity (VA, milligram acetic acid per liter), alkalinity (Alk,milligram CaCO3 per liter), ammoniacal nitrogen (N-NH4

+, milligram per liter), and solublephosphorus (Psoluble, milligram per liter). All analyses were carried out in accordance with theStandard Methods of the APHA [26]. On the other hand, the same parameters and the moistureand total phosphorus (PT, in gram per kilogram, were analyzed to characterize the sewagesludge, the tomato waste, and their mixture, following the test methods for the examination ofcomposting and compost developed by the US Department of Agriculture and the USComposting Council [27].

Software

Sigma-Plot software (version 11.0) was used to create graphs to perform the statistical analysisand to fit the experimental data presented in this work.

Results and Discussion

Stability, methane yield coefficient, biodegradability, process kinetics, and inhibitory com-pounds were studied to evaluate biomethanization of sewage sludge and tomato mixture.

Monitoring Parameters and Stability

The stability of the process has been monitored through the pH, volatile fatty acids (VFA)concentration, and alkalinity (Alk) in the effluents of the reactors at the end of each load. TheVFA and Alk has been evaluated according to the VFA/Alk ratio described by the WaterPollution Control Federation [28], which established a value of 0.30 as a limit for stable workin anaerobic reactors. Figure 1 shows the evolution of the pH and the VFA/Alk ratio againstthe OLR in the digesters. The pH was almost constant along the process, with a mean value of7.5±0.1. The pH values were always in the limits usually established for the methanogenicbacteria in literature, from 7.3 to 7.8 [12, 24]. The VFA/Alk ratio is a widely used variable formonitoring the stability in the anaerobic digestion processes. According to literature, this valuemight not be higher than 0.30–0.40 in order to avoid an excessive concentration of VFAwhichcould entail an acidification process [29]. As it can be seen in Fig. 1, the VFA/Alk ratio valueswere very lower than the inhibition thresholds, in a range from 0.11 to 0.08 for the differentOLR studied. These values of VFA/Alk are more stable than the described for the singlebiomethanization of tomato, which reach a mean value of 0.25 under mesophilic conditions[30]. The enhancement of the stability was derived from the high buffer capacity provided bythe sewage sludge [31].

Methane Yield and Biodegradability

Figure 2 shows the total methane volume generated against the OLR in the digesters. As it maybe appreciated, the methane volumes raised after increasing the OLR. Besides, the methaneyield coefficient was calculated regarding the load added (expressed as VS) to the digestersreaching a value of 159 lSTP/kg VS added. No inhibition phenomenon was determined throughthe evolution of the methane production in the studied range. The obtained methane yieldcoefficient was higher than those described by Lee and Han [32], who obtained a methaneyield coefficient of 67 lSTP CH4/kg VS for the individual anaerobic digestion of sewage sludgeat mesophilic conditions at lab scale. On the other hand, the methane yield coefficient

Appl Biochem Biotechnol

described in literature for the anaerobic digestion of tomato waste was remarkably higher in arange from 211 to 420 lSTP CH4/kg VS under mesophilic conditions [33, 34]. These values areconsiderably higher than those described for the sewage sludge biomethanization, providedthat most of the vegetable wastes are more readily digestible co-substrates [35]. For futureresearch, it might be interesting to increase the tomato proportion in the studied mixture inorder to enhance the methane yield without any stability decay. Nevertheless, the studiedproportion might allow absorbing the generation of tomato waste of a small population by thewastewater treatment plant there or even a punctual production excess of this vegetablegenerated in a bigger population.

Other fundamental operational variable, which has been determined, was the biodegrad-ability for the proposed mixture in the studied conditions. The biodegradability was defined asthe relationship between the VS removed and the VS added to the digesters. The sewagesludge and tomato mixture showed a biodegradability value around 95 % (in VS) along theprocess. This value is markedly higher than the one described in the single biomethanization ofsewage sludge under mesophilic conditions. For example, Mottet, François, Latrille,Steyer, Déléris, and Vedrenne [36] determined higher biodegradability values forsewage sludge anaerobic digestion, about 54–66 % under mesophilic conditions. Onthe other hand, the biodegradability of tomato waste under mesophilic conditionsvaried from 60 to 70 % in VS [34, 37]. As it can be seen, the proposed co-digestion process enhances considerably the biodegradability, so it can be a viabletreatment method for these hazardous wastes.

Kinetics

A first order kinetic model was used to characterize each set of experiments kinetically as wasdescribed by Borja, Martín, Banks, Alonso, and Chica [38]. This kinetic model fit theexperimental methane production volumes against the time for low substrate concentrations.The kinetic characterization allows comparing the proposed anaerobic co-digestion with other

OLR (kg VS/m3·d)

0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4

VF

A/A

lk (

eq A

cetic

Aci

d/eq

CaC

O3)

0.00

0.05

0.10

0.15

0.20

0.25

0.30

pH

0

2

4

6

8

10

VFA/AlkpH

Fig. 1 Variation in pH and volatile acidity/alkalinity ratio (eq acetic acid/eq CaCO3) in the effluents of thereactors as a function of the organic loading rate (OLR)

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ones. According to Winkler [39], the variation in biodegradable substrate with time can berepresented by the following first-order differential equation:

−dSb�dt ¼K�Sb�X ð1Þ

where Sb is the biodegradable substrate (gram VS per liter), K is the specific kinetic constant(liter per gram VS·per hour), X is the concentration of sludge in the reactors (gram VS perliter), and t is the time (hour). Separating variables and integrating them with the hypothesisthat X remained constant across the experiments due to the low biomass yield coefficient inanaerobic processes [12] and considering that the yield for the conversion of biodegradablesubstrate into methane (Y Sb=CH4

, milligram per milliliter CH4·per liter) is defined as:

YSb=CH4¼ −dSb

�dG

� � ð2Þ

The following expression may be obtained [40]:

G ¼ Gm� 1−e−K�X�t� � ð3Þ

Equation 3 allows relating the accumulated volume of methane (G, ml) with time (t) oncethe concentration of sludge (X) and the kinetic constant (K) are known. Moreover, the previousequation can be ordained in the form shown in Eq. 4 as the microorganism concentration isconsidered to be constant [K×X=K′, where K′ (per hour) is an apparent kinetic constant]:

G ¼ Gm� 1−e−K0�t

� �ð4Þ

The K′ andGm values for each load were calculated numerically from the experimental dataobtained by nonlinear regression using Sigma plot (version 11.0). To evaluate the variations inexperimental data, the theoretical values of maximum methane production (Gm) were

OLR (kg VS/m3·d)

0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4

Met

hane

(L

STP/m

3 )

0

100

200

300

400

500

600

Fig. 2 Variation of the experimental maximum methane volume produced with the organic loading rate (OLR)

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calculated using Eq. 4 and plotted against their corresponding experimental values (Fig. 3).These calculations were performed so as to give an error band of 1 %. The deviations obtainedwere less than 1 % in all the cases, suggesting that the proposed model can be used to predictthe behavior of the co-digestion process accurately.

On the other hand, Fig. 4 represents the values of the apparent kinetic constant (K′) againstthe load added to the reactors. The K′ values remained almost constant, as the waste

Gexp (mLSTP CH4)

0 500 1000 1500 2000 2500

Gm

(m

LST

P C

H4)

0

500

1000

1500

2000

2500

m = 1.117 r² = 0.9898Confidence Interval 99%

Fig. 3 Comparison between the experimental maximum methane production values (Gexp) and the theoreticalvalues (Gm) predicted by Eq. (3)

OLR (kg VS/m3·d)

0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4

K' (

h-1 )

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

Fig. 4 Variation of the apparent kinetic constant (K′) against the organic loading rate (OLR)

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mixture added to the digesters was increased, with a mean value of 0.06±0.01 h-1.Therefore, no inhibition phenomenon was determined through the kinetic parametersin line with the stability variables and the methane production. Martín, Fernández,Serrano, and Siles [41] reported that K′ values decreased from 0.39 to 0.16 h-1 withincreasing loads in the anaerobic co-digestion of orange peel waste and crude glycerolat a proportion 1:1 in COD. These values are clearly higher due to the fact thatglycerol is an easily biodegradable substrate instead of the sewage sludge employed inthe present research study.

Moreover, the following expression for the methane production rate (rG, milliliter per hour)may be obtained from Eq. 4:

rG ¼ Gm � K 0½ � � e−K0�t ð5Þ

This expression allows determining the methane production rate for the differentorganic loads added to the digesters. It was expressed as a function of the reactorsvolume in order to become independent of the experimental setup (liter of STP CH4

per cubic meter per day). Figure 5 shows the values of this variable against the OLRfor each load added to the reactors. As it may be observed the methane rate increasedin line with the OLR added to the reactors, reaching a value close to 400 lSTP CH4/m3 day for a OLR of 2.2 kg VS/m3 day. The difference could be explained by thefact that tomato contributes providing easily biodegradable matter into the digesters.Furthermore, the methane yield coefficient was also calculated by fitting the valuepairs of methane rate and OLR, which showed a correct linear relationship betweenboth variables with a value of 156 lSTP/kg VS. This value is in accordance with theone described in “Monitoring Parameters and Stability”. In particular, around 85 % ofthe value pairs fitted correctly within a confidence interval of 95 %.

OLR (kg VS/m3·d)

0.0 0.5 1.0 1.5 2.0 2.5 3.0

r G (

LST

P C

H4/

m3 ·

d)

0

100

200

300

400

500

600

m = 156 LSTP CH4/kg VS

r² = 0.8400Int Conf 95%

Fig. 5 Variation of the methane production rate (rG) as a function of the organic loading rate (OLR)

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Nutrients and Inhibitors

Inhibiting compounds are obtained from the treated waste. The phosphorus and ammoniacalnitrogen were monitored along the process in order to ensure the correct working of theprocess and to avoid possible inhibitory effects.

Figure 6 shows the variation of Psoluble concentration in the reactors throughout the process.The phosphorus concentration values were relatively constant with a mean value of 666±67 mg/l. Thus, this compound was not accumulated in the digester at increasing the OLR.Moreover, the concentration of soluble phosphorus was remarkably higher than the limitsdescribed in literature for the correct working of the anaerobic process, which has beenestablished within a range from 5 to 8 mg P-PO4/l [42, 43]. Additionally, the high phosphorusconcentration present in the digestate can be recycled as different compounds, such as calciumor magnesium salts, which are valuable fertilizers for agriculture and hence provide probableeconomic benefits [44, 45].

Nevertheless, the application of digestate from mesophilic anaerobic digestion as agricul-tural amendment could be problematic due to the accumulation of heavy metals, organicmicropollutants, or pathogens in this fraction. In that sense, it might be advisable to stabilizethe digestate through a posttreatment, such as composting, although its application in soils ishighly dependent on the local legislation.

The ammoniacal nitrogen contained in the sewage sludge is a well-known inhibitory agentin the biomethanization of this hazardous waste. Ammonia is produced during the degradationof nitrogenous matter, mainly proteins and urea [12]. Ammonium (NH4

+) and free ammonia(NH3) are the two most predominant forms of soluble inorganic nitrogen present in thedigesters [14]. The hydrophobic ammonia molecule may diffuse passively into the microbialcells used as innoculum, causing proton imbalance and/or potassium deficiency [46, 47] withthe consequent inhibitory effect. Figure 6 shows the evolution of the ammoniacal nitrogen andfree ammonia in the digesters at the end of each load (once biogas production and VS removalare finished).

Free ammonia was calculated as a function of the ammoniacal nitrogen concentration, thetemperature, and the pH following the procedure described by Emerson, Russo, Lund, and

OLR (kg VS/m3·d)0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0

Nitr

ogen

(m

g/L

)P

solu

ble (

mg/

L)

0

500

1000

1500

2000

N-NH4+

Free ammonia

Psoluble

Fig. 6 Variation in the ammoniacal nitrogen, free ammonia, and soluble phosphorus concentration against theorganic loading rate (OLR)

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Thurston [48]. As it can be seen, the concentration of both compounds followed a similartrend. The concentration raised in line with the OLR added to the digesters.

Particularly, the ammoniacal nitrogen concentration increased from 937±11 to 1,864±23 mg/l. Thus, the highest concentrations were higher than the inhibitory thresholds describedin the literature. For example, Sung and Liu [49] reported that a free ammonia concentrationaround 565 mg/l may cause a 50 % inhibition of methanogenic bacteria at pH 7.6 underthermophilic conditions. Nevertheless, other authors have reported higher inhibitory values forammoniacal nitrogen in a wide range from 1.7 to 14 g/l [12]. This difference depends onseveral factors such as the pH, the presence of other anions, or specially a correct acclimationprocess [50]. Although the ammonia concentration was in the limit described previously, thestability and the methane yield coefficient were maintained throughout the process.

Conclusions

The results obtained through this research study reveal that sewage sludge and tomato wasteanaerobic co-digestion was stable in a OLR range from 0.4 to 2.2 kg VS/m3 day, reaching amethane production yield of 159 mlSTP/g VS and a biodegradability of 95 % VS. Although theammonia concentration increased until 1,864±23 mg/l, no inhibition phenomenon was deter-mined in the stability variables, methane yield, or kinetic parameters, which remain almostconstant at different OLRs (K′ mean value of 0.06±0.01 h-1). Moreover, the methaneproduction rate increased in line with the OLR added to the reactors, reaching a value closeto 400 lSTP CH4/m

3 day at the highest OLRs. Consequently, this valorization process could bea viable option for the centralized management of the studied wastes.

Acknowledgments The authors are very grateful to the Spanish Ministry of Science and Innovation for co-funding this research through Project CTM2011-26350 and to the AECID for the economic support through theProjects D/024687/09, D/030888/10, and A1/039699/11.

References

1. Vall, M.P. (2001).Waste water in European countries. Statistics in Focus: Environment and Energy, 14, 8–14.2. Wieland, U. (2003). Water use and waste water treatment in the EU and in Candidate Countries. Statistics in

Focus: Environment and Energy, 13, 8–13.3. Hendrickx, T. L. G., Elissen, H. J. H., & Buisman, C. J. N. (2009). Bioresource Technology, 100, 4642–4648.4. Council Directive of 26 April 1999 on the landfill use of waste. (Directive 1999/31/EC). Council of the

European Communities.5. Council Directive of 4 December 2000 on the incineration and the co-incineration of industrial and municipal

solid waste (Directive 2000/76/EEC). Council of the European Communities.6. EUROSTAT (2013) http://epp.eurostat.ec.europa.eu/tgm/table.do?tab=table&init=1&plugin=0&language=

en&pcode=ten00034. Accesed 05/17/2013.7. Koroneos, C. J., & Nanaki, E. A. (2012). Integrated solid waste management and energy production – a life

cycle assessment approach: The case study of the city of Thessaloniki. Journal of Cleaner Production, 27,141–150.

8. Deng, W., Yan, J., Li, X., Wang, F., Chi, Y., & Lu, S. (2009). Emission characteristics of dioxins,furans and polycyclic aromatic hydrocarbons during fluidized-bed combustion of sewage sludge.Journal of Environmental Sciences, 21, 1747–1752.

9. Dean, R. B., & Suess, M. J. (1985). The risk to health of chemicals in sewage sludge applied to land. WasteManagement and Research, 3(25), 1–278.

10. Staton, K.L., Alleman, J.E., Pressley, R.L., & Eloff, J. (2001). 2nd Generation Autothermal ThermophilicAerobic Digestion: Conceptual Issues and Process Advancements. WEF/AWWA/CWEA joint residuals andbiosolids management conference biosolids 2001: Building public support.

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11. Iacovidou, E., Ohandja, D. G., &Voulvoulis, N. (2012). Foodwaste codigestion with sewage sludge-realising itspotential in the UK. Journal of Environmental Management, 112, 267–274.

12. Wheatley, A. (1990). Anaerobic digestion: A waste treatment technology. London: Elsevier.13. Buendía, I. M., Fernández, F. J., Villaseñor, J., & Rodríguez, L. (2009). Feasibility of anaerobic co-digestion as a

treatment option of meat industry wastes. Bioresource Technology, 100, 1903–1909.14. Appels, L., Baeyens, J., Degrève, J., & Dewil, R. (2008). Principles and potential of the anaerobic digestion

of waste-activated sludge. Progress in Energy and Combustion, 34, 755–781.15. Environment Agency (2010) Renewable energy potential for the water industry. https://connect.innovateuk.

org/c/document_library/get_file?folderId=2023104&name=DLFE-20141.pdf16. Chen, Y., Chen, J. J., & Creamer, K. S. (2008). Inhibition of anaerobic digestion process: A review.

Bioresource Technology, 99, 4044–4064.17. Jansen, J., Gruvberger, C., Hanner, N., Aspegren, H., & Svärd, A. (2004). Digestion of sludge and organic

waste in the sustainability concept for Malmö, Sweden. Water Science and Technology, 49, 163–169.18. Sosnowski, P., Wieczorek, A., & Ledakowicz, S. (2003). Anaerobic co-digestion of sewage sludge and

organic fraction of municipal solid wastes. Advances in Environmental Research, 7, 609–616.19. Marañón, E., Fernández, Y., & Castrillón, L. (2009). Manual de Estado del Arte de la Co-digestión

Anaerobia de Residuos Ganaderos y Agroindustriales (2nd ed.). Oviedo: Universidad de Oviedo.20. FAOSTAT. http://faostat.fao.org/site/567/DesktopDefault.aspx?PageID=567#ancor.21. Rossini, G., Toscano,G., Duca,D., Corinaldesi, F., Pedretti, E. F., &Riva, G. (2013). Analysis of the characteristics

of the tomato anufacturing residues finalized to the energy recovery. Biomass and Bioenergy, 51, 177–182.22. Field, J., Sierra-Alvarez, R., & Lettinga, G. (1988). 4° Seminario de Depuración Anaerobia de Aguas

Residuales. Valladolid: University of Valladolid.23. Cheng, F., Boe, K., & Angelidaki, I. (2011). Anaerobic co-digestion of by-products from sugar productions

with cow manure. Water Research, 45, 3473–3480.24. Fannin, K. F. (1987). In D. P. Chynoweth & R. Isaacson (Eds.), Anaerobic digestion of biomass: Vol. 1. Start-

up, operation, stability and control (pp. 171–196). London: Elsevier.25. Aiyuk, S., Forrez, I., Lieven, D. K., van Haandel, A., & Verstraete, W. (2006). Anaerobic and complemen-

tary treatment of domestic sewage in regions with hot climates – a review. Bioresource Technology, 97,2225–2241.

26. APHA. (1989). Standard methods for examination of water and wastewater (17th 6 ed.). Washington, DC:American Public Health Association.

27. Thompson, W. H., Leege, P. B., Millner, P. D., & Watson, M. E. (2001). Test methods for the examination ofcomposting and compost. Bethesda: US Composting Council’.

28. Water Pollution Control Federation (WPCF) (1967). Anaerobic sludge digestion. Manual of practice No. 16.Alexandria, VA: Water Environment Federation.

29. Balaguer, M. D., Vicent, M. T., & Paris, J. M. (1992). Anaerobic fluidized bed reactor with sepiolite assupport for anaerobic treatment of vinasses. Biotechnology Letters, 14, 433–438.

30. Gonzalez-Gonzalez, A., Cuadros, F., Ruiz-Celma, A., & López-Rodríguez, F. (2013). Energy-environmentalbenefits and economic feasibility of anaerobic codigestion of Iberian pig slaughterhouse and tomato industrywastes in Extremadura (Spain). Bioresource Technology, 136, 109–116.

31. Bouallagui, H., Lahdhed, H., Romdan, E. B., Rachdi, B., & Hamdi, M. (2009). Improvement of fruit andvegetable waste anaerobic digestion performance and stability with co-substrates addition. Journal ofEnvironmental Management, 90, 1844–1849.

32. Lee, I., & Han, J. I. (2013). The effects of waste-activated sludge pretreatment using hydrodynamiccavitation for methane production. Ultrasonics Sonochemistry, 20, 1450–1455.

33. Nallathambi, V. (2004). Biochemical methane potential of fruits and vegetable solid waste feedstocks.Biomass and Bioenergy, 26, 389–399.

34. Sarada, R., & Joseph, R. (1996). A comparative study of single and two stage processes for methaneproduction from tomato processing waste. Process Biochemistry, 31, 337–340.

35. Van Assche, P., Poels, J., & Verstraete, W. (1983). Anaerobic digestion of pig manure with cellulose as co-substrate. Biotechnology Letters, 5, 749–754.

36. Mottet, A., François, E., Latrille, E., Steyer, J. P., Déléris, S., & Vedrenne, F. (2010). Estimating anaerobicbiodegradability indicators for waste activated sludge. Chemical Engineering Journal, 160, 488–496.

37. Hills, D., & Nakano, K. (1984). Effects of particle size on anaerobic digestion of tomato solid wastes.Agricultural Wastes, 10, 285–295.

38. Borja, R., Martín, A., Banks, C. J., Alonso, V., & Chica, A. (1995). A kinetic study of anaerobic digestion ofolive mill wastewater at mesophilic and thermophilic temperatures. Environmental Pollution, 88, 13–18.

39. Winkler, H. (1983). Biological treatment of wastewater. Chichester: Elis Horwood.40. Gujer, W., & Zehnder, A. J. (1983). Conversion processes in anaerobic digestion. Water Science and

Technology, 15, 123–167.

Appl Biochem Biotechnol

41. Martín, M. A., Fernández, R., Serrano, A., & Siles, J. A. (2013). Semi-continuous anaerobic co-digestion oforange peel waste and residual glycerol derived from biodiesel manufacturing. Waste Management, 33,1633–1639.

42. Alphenaar, P. A., Sleyster, R., Reuver, P., Ligthart, G. J., & Lettinga, G. (1993). Phosphorusrequirement in high-rate anaerobic wastewater treatment. Water Research, 27, 749–756.

43. Britz, T. J., Noeth, C., & Lategan, P. M. (1988). Nitrogen and phosphate requirements for the anaerobicdigestion of a petrochemical effluent. Water Research, 22, 163–169.

44. Wild, D., Kisliakova, A., & Siegrist, H. (1997). Prediction of recycle phosphorus loads from anaerobicdigestion. Water Research, 31, 2300–2308.

45. Marti, N., Ferrer, J., Seco, A., & Bouzas, A. (2008). Optimisation of sludge line management to enhancephosphorus recovery in WWTP. Water Research, 42, 4609–4618.

46. Sprott, G. D., & Patel, G. B. (1986). Ammonia toxicity in pure cultures of methanogenic bacteria system.Applied Applied Microbiology, 7, 358–363.

47. Gallert, C., Bauer, S., & Winter, J. (1998). Effect of ammonia on the anaerobic degradation of protein by amesophilic and thermophilic biowaste population. Applied Microbiology and Biotechnology, 50, 495–501.

48. Emerson, K., Russo, R. C., Lund, R. E., & Thurston, R. V. (1975). Aqueous ammonia equilibriumcalculation: Effect of pH and temperature. Journal of the Fisheries Research Board of Canada,32, 2379–2383.

49. Sung, S., & Liu, T. (2003). Ammonia inhibition on thermophilic anaerobic digestion.Chemosphere, 53, 43–52.50. Bujoczek, G., Oleszkiewicz, J., Sparling, R., & Cenkiwski, S. (2000). High solid anaerobic digestion of

chicken manure. Journal of Agricultural Engineering Research, 76, 51–60.

Appl Biochem Biotechnol