This article was downloaded by: [Aston University]On: 30 January 2014, At: 19:57Publisher: Taylor & FrancisInforma Ltd Registered in England and Wales Registered Number: 1072954 Registered office: Mortimer House,37-41 Mortimer Street, London W1T 3JH, UK

Environmental TechnologyPublication details, including instructions for authors and subscription information:http://www.tandfonline.com/loi/tent20

Mesophilic anaerobic co-digestion of sewage sludge andorange peel wasteAntonio Serranoa, José Ángel Siles Lópeza, Arturo Francisco Chicaa, M. Martina, FadouaKarouachb, Abdelaziz Mesfiouib & Hassan El Bariba Department of Inorganic Chemistry and Chemical Engineering, University of Cordoba,Cordoba, Spainb Waste Recovery Laboratory, University Ibn Tofail, Kenitra, MoroccoPublished online: 22 Nov 2013.

To cite this article: Antonio Serrano, José Ángel Siles López, Arturo Francisco Chica, M. Martin, Fadoua Karouach, AbdelazizMesfioui & Hassan El Bari (2014) Mesophilic anaerobic co-digestion of sewage sludge and orange peel waste, EnvironmentalTechnology, 35:7, 898-906, DOI: 10.1080/09593330.2013.855822

To link to this article: http://dx.doi.org/10.1080/09593330.2013.855822

PLEASE SCROLL DOWN FOR ARTICLE

Taylor & Francis makes every effort to ensure the accuracy of all the information (the “Content”) containedin the publications on our platform. However, Taylor & Francis, our agents, and our licensors make norepresentations or warranties whatsoever as to the accuracy, completeness, or suitability for any purpose of theContent. Any opinions and views expressed in this publication are the opinions and views of the authors, andare not the views of or endorsed by Taylor & Francis. The accuracy of the Content should not be relied upon andshould be independently verified with primary sources of information. Taylor and Francis shall not be liable forany losses, actions, claims, proceedings, demands, costs, expenses, damages, and other liabilities whatsoeveror howsoever caused arising directly or indirectly in connection with, in relation to or arising out of the use ofthe Content.

This article may be used for research, teaching, and private study purposes. Any substantial or systematicreproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in anyform to anyone is expressly forbidden. Terms & Conditions of access and use can be found at http://www.tandfonline.com/page/terms-and-conditions

Environmental Technology, 2014Vol. 35, No. 7, 898–906, http://dx.doi.org/10.1080/09593330.2013.855822

Mesophilic anaerobic co-digestion of sewage sludge and orange peel waste

Antonio Serranoa, José Ángel Siles Lópeza, Arturo Francisco Chicaa, M. Ángeles Martína∗, Fadoua Karouachb,Abdelaziz Mesfiouib and Hassan El Barib

aDepartment of Inorganic Chemistry and Chemical Engineering, University of Cordoba, Cordoba, Spain; bWaste Recovery Laboratory,University Ibn Tofail, Kenitra, Morocco

(Received 10 September 2013; final version received 9 October 2013 )

Mesophilic anaerobic digestion is a treatment that is widely applied for sewage sludge management but has several disadvan-tages such as low methane yield, poor biodegradability and nutrient imbalance. In this paper, we propose orange peel wasteas an easily biodegradable co-substrate to improve the viability of the process. Sewage sludge and orange peel waste weremixed at a proportion of 70:30 (wet weight), respectively. The stability was maintained within correct parameters throughoutthe process, while the methane yield coefficient and biodegradability were 165 L/kg volatile solids (VS) (0◦C, 1 atm) and76% (VS), respectively. The organic loading rate (OLR) increased from 0.4 to 1.6 kg VS/m3 d. Nevertheless, the OLR andmethane production rate decreased at the highest loads, suggesting the occurrence of an inhibition phenomenon.

Keywords: sewage sludge; orange peel waste; mesophilic anaerobic co-digestion; methane yield; organic loading rate

NomenclatureAlk alkalinity (mg CaCO3/L)COD chemical oxygen demand (g; g O2/kg)G cumulative methane volume (mL)Gm cumulative methane volume at infinite

time (mL)GT experimental maximum methane

volume (mLSTP)K specific kinetic constant (L/(g VS h))K ′ apparent kinetic constant (1/h)MS total mineral solids (mg/kg)N-NH+

4 ammoniacal nitrogen (mg/L; mg/kg)OLR organic loading rate (kg VS/m3 d)Pt total phosphorus (mg/kg)rG methane production rate (mL/h)Sb biodegradable substrate (g VS/L)STP standard temperature and pressure conditions

(0◦C, 1 atm)t time (h)TS total solids (mg/kg)VS total volatile solids (g VS/L; g; mg/kg)X concentration of sludge in the reactors (g VS/L)YCH4/S methane yield coefficient (mL CH4/g VS;

mL CH4/g COD)YSb/CH4 yield for the conversion of biodegradable

substrate into methane (g VS/(mL CH4 L))

IntroductionThe activated sludge process is one of the most commonlyused treatment technologies in municipal and industrial

∗Corresponding author. Email: iq2masam@uco.es

wastewater treatment plants (WWTPs). Although this bio-logical method is highly efficient for the removal of organicmatter and nutrients, it produces large amounts of wastesludge. In Europe, the estimated average of dry weight percapita production of sewage sludge resulting from primary,secondary and even tertiary treatment is 90 g per person aday.[1] Because sewage sludge from WWTPs often con-tains heavy metals, organic micropollutants and pathogens,strict legislation has been implemented concerning the useand recovery of sewage sludge.[2] The efficient treatmentof sewage sludge is therefore required to prevent environ-mental pollution and human health risks. Waste disposalin landfill sites is not a viable management option dueto its environmental impact. In fact, the European Unionhas set the target to reduce final waste disposal by 20%in 2010 and by 50% in 2050 (compared with 2000).[3] Itis necessary to develop an economically viable manage-ment process as sewage sludge treatment could accountfor 50–60% of the operational costs in small and mediumWWTPs.[4]

Anaerobic digestion is a highly efficient process whichproduces methane as a final product that can be used asan energy source for electricity and on-site heating dueto its high heating value (35,793 kJ/m3

STP).[5] Moreover,anaerobic digestion provides some additional advantagesover other treatment technologies, such as the possibilityof working at different temperature ranges, high organicload rates and the hygienization of the final effluent.[6]Nevertheless, the anaerobic treatment of sewage sludgeas a single substrate involves problems such as poor

© 2013 Taylor & Francis

Dow

nloa

ded

by [

Ast

on U

nive

rsity

] at

19:

57 3

0 Ja

nuar

y 20

14

Environmental Technology 899

degradability, which results in higher retention times andmixing costs, as well as lower biogas production.[7] How-ever, sewage sludge is poor in organic carbon comparedwith nitrogen and phosphorus content. Sewage sludgereaches a nutrient balance, which is not the recommendedrange for the anaerobic digestion process to work prop-erly, from 50:4:1 to 350:5:1 for COD:N:P (chemical oxygendemand [COD]), respectively.[8,9]

Given these limitations, different pretreatments or co-digestion processes have been proposed to improve theefficiency and viability of the anaerobic digestion of sewagesludge as a single substrate. The pretreatments enhancethe hydrolysis of the sewage sludge before its diges-tion, thus improving the methane production rate andbiodegradability. Different pretreatments such as hydrother-mal, enzymatic, sonication or microwave processes havebeen evaluated.[10,11] However, the energy requirementsmight not be compensated by the increase in methaneproduction, which could hinder the application of these pro-cesses in developing countries, particularly in rural areas.In this sense, co-digestion with agricultural waste couldimprove the biomethanization of sewage sludge in theseareas. The co-digestion of different organic substrates mightimprove the stability of the process due to an enhancementof biodiversity in the reactor and reduce the concentrationof inhibitors in the sewage sludge, thus producing synergis-tic effects such as an increase in methane production or theorganic loading rate (OLR).[12]

Due to its availability and composition, orange peelwaste might be an interesting substrate to be co-digestedwith sewage sludge, particularly as this combined treat-ment has not been reported previously in the literature.Previous research has shown the suitability of orange peelwaste when combined with crude glycerol [13] or fruitand vegetable waste.[14,15] Oranges are widely used forboth direct consumption and secondary processing (juices,marmalades, etc.) in many Mediterranean areas and devel-oping countries. In these processes, high volumes of orangepeel waste are generated (around 50–60% of the inputfruit is converted into waste).[16,17] The main purposeof this research study is to evaluate the viability of theanaerobic digestion of sewage sludge by adding orangepeel waste as a co-substrate at a proportion of 70:30, inwet weight, respectively. The study can be considered ofgreat interest for evaluating the viability of the combinedtreatment at pilot or full scale in rural areas or develop-ing countries where both forms of waste are generatedsimultaneously.

Materials and methodsExperimental set-upThe experimental set-up used for the anaerobic co-digestionconsisted of two 3.5-L Pyrex completely mixed reactorsworking parallel under mesophilic temperature (35◦C) and

in semi-continuous mode. The reactors were equipped withfour connections to load feedstock, ventilate biogas andinject inert gas (nitrogen) in order to maintain the anaerobicconditions and remove effluent. The content of the reactorswas mechanically stirred and temperature was maintainedby a thermostatic jacket containing water at 37◦C undermesophilic conditions. The volume of methane producedduring the process was measured using 2-L Boyle–Mariottereservoirs connected to each reactor. To remove the CO2produced during the process, tightly closed bubblers con-taining a NaOH solution (6 N) were connected betweenthe two elements. The volume of methane displaced anequal measurable volume of water from the reservoirs.This volume was corrected in order to remove the effect ofwater steam pressure and the measured methane was thenexpressed at STP conditions (STP: 0◦C and 1 atm).

The reactors were inoculated with methanogenicallyactive granular biomass obtained from a full-scale anaer-obic reactor used to treat sewage sludge from the urbanWWTP of Jerez de la Frontera (Cadiz, Spain) (totalvolatile solids (VS): 53,680 mg/kg and total mineral solids(MS): 14,945 mg/kg). The inoculum was selected on thebasis of its high methanogenic activity [18]: methaneproduction rate of 58 mLSTPCH4/gCODadded h (CODadded:added COD).

SubstrateThe raw materials used as substrate were sewage sludge andorange peel waste. The sewage sludge was collected fromthe urban WWTP of Puente Genil (Cordoba, Spain). TheWWTP generates a sewage sludge flow rate of 68.44 ton peryear, on dry basis. The sewage sludge was composed of pri-mary and secondary sludge. It was dehydrated in the plantby centrifugation after the addition of coagulant and floccu-lant. This waste presents a nutrient balance with a notabledeficit in carbon compared with nitrogen and phosphorus(31:2:1). The orange peel waste was supplied by the Cítricosdel Andévalo Company (Huelva, Spain), which generatesthis waste during orange juice manufacturing processes.The orange peel waste had a COD:N:P ratio of 249:4:1. Thewaste was previously blended, homogenized and subjectedto steam distillation to remove the d-limonene present inthe orange peel and ensure the stability of the anaerobicprocess. The d-limonene removal yield achieved with thispretreatment for 1 h was found to be 70% (out of 12.73–3.82 mL d-limonene/kg orange peel (wet basis)).[14] Themain analytical characteristics of both substrates are shownin Table 1.

The substrate mixture studied consisted of sewagesludge and orange peel waste at a ratio of 70:30 (wetweight), respectively. This proportion corresponds approxi-mately to the quantities in which both wastes are simultane-ously produced in the industrial area and allows enhancingthe organic matter proportion in the mixture (Table 1). Themixture was blended and distilled water was added at a

Dow

nloa

ded

by [

Ast

on U

nive

rsity

] at

19:

57 3

0 Ja

nuar

y 20

14

900 A. Serrano et al.

Table 1. Analytical characterization of orange peel waste, sewage sludge and orangepeel waste–sewage sludge mixture (wet basis).

Orange peel Sewage Orange peel waste–waste sludge sewage sludge mixture

Moisture (%) 91.3 ± 0.4 89.1 ± 0.3 93.9 ± 0.1COD (mg O2/kg) 109,145 ± 5510 209,315 ± 6125 89,455 ± 2560TS (mg/kg) 87, 430 ± 4685 109,285 ± 2960 60,660 ± 215MS (mg/kg) 3585 ± 122 34,355 ± 1610 12,480 ± 245VS (mg/kg) 83,845 ± 4564 74,930 ± 1765 48,180 ± 320N-NH+

4 (mg/kg) 1694 ± 157 10,505 ± 1690 3687 ± 252PT (mg/kg) 438 ± 28 6870 ± 350 1870 ± 20COD/N-NH+

4 64 20 25COD:N-NH+

4 :P 249:4:1 31:2:1 48:2:1

Table 2. Stability variables and methane production yield values during theacclimatization period.

GAL, % in COD 75 50 25

Waste mixture, % in COD 25 50 75

pH 7.55 ± 0.04 7.53 ± 0.05 7.35 ± 0.04VA (mg acetic acid/L) 681 ± 30 666 ± 35 595 ± 35Alkalinity (mg CaCO3/L) 5020 ± 56 4664 ± 66 4722 ± 215YCH4/S (mLSTP CH4/g COD) 310 ± 4 236 ± 24 216 ± 6

proportion of 1:1 in wet weight to facilitate handling andthe feeding process of the digesters, at least at lab scale, thusimproving the homogenization of the mixtures and avoidingorganic overload. The mixture was conserved under freez-ing conditions. The main analytical characteristics of themixture are also shown in Table 1. The addition of orangepeel waste to the sewage sludge improved the COD:N:Pratio, with a nutrient balance of 48:2:1 observed for themixture. The mixture entails a slight improvement in thebalance ratio compared with the sewage sludge as a singlesubstrate. Moreover, the proposed mixture allows the jointtreatment of both hazardous and polluting wastes in a cen-tralized digester in areas where these wastes are generatedsimultaneously.

Anaerobic digesters: experimental procedureThe reactors were initially loaded with 7 g VS/L of anaer-obic sludge as inoculum. In order to bio-stimulate thebiomass prior to the experiments, the reactors were firstfed with a synthetic solution composed of glucose, sodiumacetate and lactic acid at concentrations of 50, 25 and25.2 g/L, respectively. During this initial period, the organicload added to the reactors was gradually increased from 0.5to 1.0 g COD/L over a 15-day period.

Biomass acclimatization was then carried out. The reac-tors were fed with loads of 1.00 g COD/L in which thepercentage of mixture in the COD was increased from25% to 100% after three loads. During this acclimatizationperiod, the volume of methane was measured as a functionof time. The maximum duration of each assay in this stage

was 30 h and corresponds to the time interval required formaximum gas production and substrate removal. Table 2shows the performance values for the stability and themethane production during the acclimatization period.

During each set of subsequent experiments with thepure mixture, the organic load added to the reactors wasgradually increased from 0.5 to 4.5 g VS/L at intervals of0.5 g VS/L. Each load was carried out in duplicate. In allcases, the volume of methane was measured as a func-tion of time and samples were taken and analysed beforeand after feeding. The solid fraction of digestate (whichincluded microorganisms and non-biodegraded substrate)was recovered from the samples and recirculated into thedigesters after centrifugation at 2000 rpm. The maximumduration of each assay in this stage was 48 h, which cor-responds to the highest substrate concentration added tothe digesters. All the experiments, including the start-up,biomass acclimatization and waste treatment, were carriedout over an 80-day period.

Chemical analysesThe parameters determined in the effluents of each loadwere pH, COD (mg/L), total solids (TS, mg/L), MS(mg/L), VS (mg/L), volatile acidity (VA, mg aceticacid/L), Alk (alkalinity, mg CaCO3/L) and N-NH+

4(ammoniacal nitrogen, mg/L). All analyses were carried outin accordance with the Standard Methods of the AmericanPublic Health Association (APHA).[19] The same param-eters and the total phosphorus (PT , g/kg) were analysed tocharacterize the orange peel waste, the sewage sludge and

Dow

nloa

ded

by [

Ast

on U

nive

rsity

] at

19:

57 3

0 Ja

nuar

y 20

14

Environmental Technology 901

the orange peel–sewage sludge mixture following the testmethods for the examination of composting and compostdeveloped by the US Department of Agriculture and theUS Composting Council.[20]

Separate volatile fatty acids (acetic, propionic, butyric,isobutyric, valeric, isovaleric and caproic acids) were deter-mined using a Hewlett-Packard HP-5890 gas chromato-graph equipped with a 15 m × 0.53 mm (i.d.) Nukol-silicasemicapillary column and a flame ionization detector.

Calculation sectionStabilityThe following empirical expression was used to evaluatestability in the reactors:

Alk(mg CaCO3/L) − 0.7 · VA(mg acetic acid/L),

which should not be lower than 1500 for balanced digestionto take place.[14]

Kinetics of methane productionAccording to Winkler,[21] the variation in biodegradablesubstrate with time can be represented by the followingfirst-order differential equation:

dSb

dt= K × Sb × X , (1)

where Sb is the biodegradable substrate (g VS/L), K is thespecific kinetic constant (L/(g VS h)), X is the concentra-tion of sludge in the reactors (g VS/L) and t is the time(h). Separating variables and integrating with the hypoth-esis that X remained constant across the experiments dueto the low biomass yield coefficient in anaerobic processes[5,22] and considering that the yield for the conversion ofbiodegradable substrate into methane is defined as

YSb/CH4 =(−dSb

dG

), (2)

the following expression may be obtained:

G = Gm × (1 − e−K×X × t). (3)

Equation (3) allows relating the accumulated volume ofmethane (G, mL) with time (t) once the concentration ofsludge (X ) and the kinetic constant (K) are known. More-over, the previous equation can be reordered in the formshown in Equation (4), as microorganism concentration isconsidered to be constant K × X = K ′; where K ′ (1/ h) isan apparent kinetic constant

G = Gm × (1 − e−K ′× t). (4)

The K ′ and Gm values for each load were calculatednumerically from the experimental data obtained by non-linear regression using Sigma-Plot (version 11.0).

Moreover, the following expression for the methane pro-duction rate (rG , mL/h) may be obtained from Equation (4):

rG = [Gm × K ′] × e−K ′×t . (5)

This expression allows determining the methane pro-duction rate for different organic concentration loads addedto the digesters.

Free ammonia concentrationTotal ammonia in aqueous solution consists of two principalforms: the ammonium ion (NH+

4 ) and un-ionized ammonia(NH3). The relative concentrations of each are pH depen-dent as described by the following equilibrium equation:

Ka = [NH3][H+]NH+

4. (6)

The relative concentrations of the two forms are alsotemperature dependent [23]

pKa = 0.09108 + 2729.92273.2 + T

. (7)

Based on Equations (6) and (7), and using the pH andtemperature of the solution, free ammonia concentrationwas calculated from the following formula [24]:

[NH3][TAN]

=(

1 + 10−pH

10−(0.09018+(2729.92/T (K)))

)−1

, (8)

where [NH3] is the concentration of free ammonia (mg/L),[TAN] is the total ammonia concentration (mg/L) and T(K) is the temperature (Kelvin).

SoftwareSigma-Plot software (version 11.0) was used to creategraphs, perform the statistical analysis and fit the experi-mental data presented in this work.

Results and discussionStability of the anaerobic co-digestion processThe stability of the proposed co-digestion process was eval-uated based on the variation in pH, Alk, VA, the ratiobetween Alk and VA levels [14] and the concentrationof short-chain fatty acids (between two and six carbonatoms). Figure 1(a) shows the variation in VA at increas-ing loads added to the digesters. The VA concentrationwas quite constant until it reached a load of 3.0 g VS/L(620 ± 79 mg acetic acid/L). At higher loads, the VA con-centration increased up to a value of 1565 ± 128 mg aceticacid/L. Figure 1(b) shows the profile of short-chain fattyacids, which has between two and six carbon atoms and is

Dow

nloa

ded

by [

Ast

on U

nive

rsity

] at

19:

57 3

0 Ja

nuar

y 20

14

902 A. Serrano et al.

Load (g VS/L)

0 1 2 3 4 5 6

Aci

d co

ncen

trat

ion

(mg

acet

ic a

cid/

L)

0

200

400

600

800

1000

1200

1400

1600

Total AcidsC2C3C4C5C6

(a)

(b)Load (g VS/L)

0 1 2 3 4 5 6Alk

(m

g C

aCO

3/L

) -

0.7

x V

A (

mg

acet

ic a

cid/

L)

0

1000

2000

3000

4000

5000

6000

VA

(m

g ac

etic

aci

d/L)

0

500

1000

1500

2000

2500

Alk (mg CaCO3 /L) - 0.7 x VA (mg acetic acid/L)

VA (mg acetic acid/L)

Figure 1. (a) Variation in VA and ratio Alk (mg CaCO3/L) – 0.7VA (mg acetic acid/L) in effluents of the reactors as a functionof the load added and (b) variation in the concentration of C2–C6acids against the load added to the digesters.

expressed as mg C2/L. The total concentration of short-chain fatty acids also enhanced with the load added tothe reactors. The final concentration of total acids reacheda similar value to the VA concentration determined bytitration after subjecting the samples to steam distillation.Specifically, a marked enhancement of C2 concentrationwas observed at a load of 5.5 g VS/L. The inhibition ofthe methanogenic activity might explain the enhancementof the C2 concentration in the digesters.[12] However,the stability in the reactors was maintained even at thehighest loads in accordance with the empirical expres-sion reported by Lane.[14] As shown in Figure 1(a), thevalues determined throughout the experimental procedurefor this relationship were remarkably higher than the pro-posed limit, with values ranging from 4000 to 5500. Thesehigh values are in line with the high alkalinity concen-tration in the digesters, which were mainly provided bysewage sludge, and reached a mean value of 5345 ± 835 mgCaCO3/L. Moreover, the pH value remained fairly con-stant throughout the process with a mean value of 7.45 ±0.20. This value is considered to be in the optimal rangefor methanogenic archaea (7.3–7.8) in line with severalauthors.[5,25]

Load (g VS/L)

0 1 2 3 4 5 6

CH

4 (m

LST

P/L

)

0

200

400

600

800

1000

1200

1400

Y CH4/S = 165 mLSTP CH4/g VS

r² = 0.9092

Figure 2. Variation of the experimental maximum methane vol-ume produced with the VS added to obtain the methane yieldcoefficient of the process.

Methane yield and biodegradabilityAs shown in Figure 2, by fitting pairs of values of themaximum experimental volume of methane produced ineach load (mLSTP/L)-VS to a straight line, the methaneyield coefficient coincides with the slope of the regressionline, which was found to be 165 mLSTP CH4/g VS. Thisvalue is higher than those described by Lee and Han,[26]who obtained a methane yield coefficient of 67 mLSTPCH4/g VS for the individual anaerobic digestion of sewagesludge under mesophilic conditions at lab scale. How-ever, the methane yield is slightly lower than the valuereported for the mesophilic biomethanization of orange peelwaste, which reached a methane yield of 230 ± 16 mLSTPCH4/g VS.[17] As can be observed, the obtained value cor-responds to an intermediate value when both types of wasteare treated separately. This has also been reported by Daiet al.,[27] who described a linear enhancement in biogasproduction and a reduction in VS in the co-digestion ofdewatered sludge and food waste with higher ratios of foodwaste in the mixture.

COD and total VS were determined in the effluents ofthe digesters to evaluate the variation in organic matter con-centration. Figure 3 shows a linear relationship betweenboth variables in the samples taken from the effluents ofthe digesters. Specifically, around 90% of the value pairsshowed a good fit within confidence intervals of 99%.This relationship allows expressing the methane yield coef-ficient as a function of the added COD to the reactors,reaching a value of 101 mLSTP CH4/g COD. This value isequivalent to 30% of the theoretical maximum (350 mLSTPCH4/g COD).[28] This percentage might be explained bythe partial degradation of the added substrate. Addition-ally, the inoculum does not only degrade the added CODinto methane, but is also used for cell growth, as well asits metabolism and maintenance. Moreover, depending onthe operational conditions, carbon (not included in the CODbalance) is lost as CO2 in variable proportions in the biogas.

Dow

nloa

ded

by [

Ast

on U

nive

rsity

] at

19:

57 3

0 Ja

nuar

y 20

14

Environmental Technology 903

VS (mg/kg)

0 5000 10000 15000 20000 25000

CO

D (

mg

O2/

kg)

0

10000

20000

30000

40000

50000

m = 1.6358Confidence intervals of 99%

Figure 3. Comparison between COD (mg O2/kg) and VS(mg/kg) in effluents of the reactors.

The biodegradability of the sewage sludge and orangepeel waste mixture was defined as the relationship betweenthe VS removed and the VS added to the digesters, whichreached a value close to 76% in VS. Given that thereactors were adapted to the single treatment of sewagesludge with a biodegradability yield of 53%, the addi-tion of orange peel improved the treatment efficacy. Inaddition, the biodegradability of the mixture was consid-erably higher than the values reported by other authorsfor the anaerobic digestion of sewage sludge. Bolzonellaet al.,[7] for example, reported a 13–27% reduction in VSconcentration for the anaerobic digestion of waste acti-vated sludge at mesophilic temperatures. Other authors,however, have reported biodegradability values close to55% (VS). The wide variation in the previous values couldbe explained by the presence of toxic substances in thesewage sludge, such as metals, biocides and detergents,or even different operational conditions.[29] Specifically,the higher biodegradability obtained in the present researchstudy could be explained by the addition of an organicco-substrate that is easily digestible (orange peel waste).Other authors have described similar synergism by the addi-tion of vegetable or agro-industrial wastes to the sewagesludge anaerobic treatment. Bouallagui et al. [15] describeda biodegradability of 73–86% (VS) for the anaerobic diges-tion of different mixtures of fish waste, abattoir wastewater,waste-activated sludge and fruit and vegetable waste at labscale under mesophilic conditions. Habiba et al. [29] studiedthe anaerobic co-digestion of fruit and vegetable waste andactivated sludge at different mix proportions and determinedbiodegradability in a range of 65–88%, in VS.

Organic loading rateThe treatment capacity of the process was evaluated throughthe rate of substrate addition or allowed OLR. This oper-ational variable is especially important for the full-scaleapplication of anaerobic digestion processes as the OLR

Figure 4. Plot of the OLR (kg VS/m3 d) against the substrateconcentration (kg VS/m3).

determines the digester volume, as well as operational andinvestment costs. The allowed OLR is a variable that relatesthe quantity of substrate that can be added with the reactorvolume and time (kg VS/m3 d or kg waste mixture/m3 d).Figure 4 shows the variation in the OLR with the loadadded to the reactors. The allowed OLR was calculatedconsidering the minimum time required to reach 95% oftotal methane production for each experiment. This vari-able increased from 0.4 to 1.6 kg VS/m3 d when the addedload was increased from 0.5 to 3.5 kg VS/m3. Subsequently,the OLR values decreased to 1.0 kg VS/m3 d at the highestloads due to the slowdown of methane production at thehighest loads. Even under the worst conditions, the resultsare in line with those observed for the single biometha-nization of sewage sludge (1.1 kg VS/m3 d). These OLRresults were also in accordance with the OLR described bySaev et al. [30] for the anaerobic co-digestion of vegetablewaste (cucumbers, tomatoes and potatoes) and sewagesludge under mesophilic conditions. These authors deter-mined an OLR of 1.84 kg VS/m3 d for the highest methaneproduction, while Gómez et al.,[31] who studied the co-digestion process of fruits and vegetables wastes (banana,apple, orange, cabbage, potatoes, etc.) with primary sludgeunder mesophilic conditions, reported OLRs between 0.82at 1.10 kg VS/m3 d. Likewise, the addition of orange peelwaste enhanced the OLR compared with the value describedby Bolzonella et al. [7] for the single biomethanizationof sewage sludge. These authors described an OLR of1.0 kg VS/m3 d under mesophilic conditions and at fullscale.

Kinetics of methane productionIn order to characterize each set of experiments kinetically,and thus facilitate comparisons, the first-order kinetic modeldescribed by Borja et al. [32] was used to fit the experimen-tal methane production for low substrate concentrations.

Dow

nloa

ded

by [

Ast

on U

nive

rsity

] at

19:

57 3

0 Ja

nuar

y 20

14

904 A. Serrano et al.

Figure 5. (a) Comparison between the experimental maximummethane production values (GT) and the theoretical values (Gm)predicted by Equation (3) and (b) variation of the apparent kineticconstant (K ′) against the substrate concentration.

Specifically, to evaluate the variations in the experimentaldata, theoretical values of maximum methane production(Gm) were calculated using Equation (4) and adjusted totheir corresponding experimental values (GT) (Figure 5(a)).These calculations were performed so as to give an errorband of 1%. The deviations obtained were less than 1% inthe majority of cases (90%), suggesting that the proposedmodel can be used to accurately predict the behaviour ofco-digestion processes. Figure 5(b) shows the values of theapparent kinetic constant (K ′) against the load added to thereactors. The K ′ values decreased when the waste mixtureadded to the digesters was increased from 0.13 ± 0.02 1/hto 0.02 ± 0.001 1/h, in line with the variation observed inthe allowed OLR. Martín et al. [13] reported that K ′ valuesdecreased from 0.39 to 0.16 1/h with increasing loads inthe anaerobic co-digestion of orange peel waste and glyc-erol at a proportion of 1:1, in COD. These values are clearlyhigher due to the fact that glycerol is an easily biodegrad-able substrate unlike the sewage sludge employed in thepresent research study.

OLR (kg VS/m3 ·d)

0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8

r G (

LST

P C

H4/

kg V

Sin

ocul

um·d

)

50

100

150

200

250

300

350

400

450

Figure 6. Plot of the methane production rate (rG) against theOLR.

On the other hand, Figure 6 shows the values of themethane production rate determined from Equation (5)against the OLR for each load added to the reactors. As canbe seen, the methane production rate increased at increas-ing the OLR. The values obtained for the optimal OLRdetermined were close to 400 LSTP CH4/kg VS inoculum·d.

Influence of nitrogenAmmoniacal nitrogen is a well-known inhibitory agent inthe anaerobic digestion process. The hydrophobic ammo-nia molecule may diffuse passively into the cell, causingproton imbalance, and/or potassium deficiency [33] withthe consequent inhibitory effect. The ammoniacal nitro-gen concentration determined for the sewage sludge andorange peel waste employed in this research was very high,with a COD/N ratio of 64 and 20, respectively (Table 1).Therefore, the variation in the ammoniacal nitrogen and thefree ammonia was monitored in the digesters during theprocess in order to prevent a possible inhibitory process.Figure 7 shows the variation in ammoniacal nitrogen andfree ammonia in the digesters at the end of each load (oncebiogas production was exhausted). As can be seen, the con-centration of both compounds showed a similar trend, andincreased with increasing loads added to the reactors until aload of 3.0 g VS/L; around 1300 mg N/L for both nitrogenforms. At higher loads, the concentrations of ammoniacalnitrogen and free ammonia were virtually constant and bothvaried in a range from 1200 to 1400 mg N/L. These val-ues are slightly lower than the wide inhibitory thresholdsdescribed in the literature, which range from 1.7 to 14 gammoniacal nitrogen/L.[12] This difference depends onseveral factors such as pH, the presence of other anionsor a correct acclimatization process.[34]

Thus, K ′ and OLR decreased at increasing the loadadded to the reactors, while the stability and methaneyield coefficient were maintained throughout the process.

Dow

nloa

ded

by [

Ast

on U

nive

rsity

] at

19:

57 3

0 Ja

nuar

y 20

14

Environmental Technology 905

Load (g VS/L)0 1 2 3 4 5 6

Nit

roge

n (m

g/L

)

0

200

400

600

800

1000

1200

1400

1600

Free ammoniaAmmoniacal nitrogen

Figure 7. Variation in the ammoniacal nitrogen and free ammo-nia against the load added to the reactors.

This might be due to the increase in ammoniacal nitrogenconcentration in the digesters at the highest loads or evendue to the presence of d-limonene in the orange peel waste,as this compound was not completely removed to maintainthe economic viability of the process.

Conclusion• The anaerobic co-digestion of sewage sludge and

orange peel waste was stable, reaching a methaneproduction yield of 165 mLSTP/g VS and a biodegrad-ability of 76% (VS).

• The addition of orange waste improved the permit-ted OLR compared with the single biomethaniza-tion of sewage sludge, until it reached a value of1.6 kg VS/m3 d.

• A decrease in the OLR and methane production ratewas observed at high loads.

• The proposed management procedures could be aviable option for the valorization of these hazardouswastes in rural areas or developing countries.

FundingThe authors are very grateful to the Spanish Ministry of Sci-ence and Innovation for co-funding this research through Project[CTM2011-26350] and to the AECID for the economic sup-port through the Projects [D/024687/09, D/030888/10 andA1/039699/11].

References[1] Fytili D, Zabaniotou A. Utilization of sewage sludge in EU

application of old and new methods – a review. Renew SustEnergy Rev. 2008;12:116–140.

[2] Hendrickx TLG. Aquatic worm reactor for improved sludgeprocessing and resource recovery [Ph.D. dissertation].Wageningen: Wageningen University; 2009.

[3] Lundin M, Olofsson M, Pettersson G, Zetterlund H. Environ-mental and economic assessment of sewage sludge handlingoptions. Resour Conserv Recycl. 2004;41:255–278.

[4] Wei SY, Fan BY, Wang JM. A cost analysis of sewage sludgecomposting for small and mid-scale municipal wastewatertreatment plants. Resour Conserv Recycl. 2001;33:203–216.

[5] Wheatley A. Anaerobic digestion: a waste treatment tech-nology. London: Elsevier; 1990.

[6] Astals S, Venegas C, Peces M, Jofre J, Lucena F, Mata-Alvarez J. Balancing hygienization and anaerobic digestionof raw sewage sludge. Water Res. 2012;46:6218–6227.

[7] Bolzonella D, Pavan P, Battistoni P, Cecchi F. Mesophilicanaerobic digestion of waste activated sludge: influence ofthe solid retention time in the wastewater treatment process.Process Biochem. 2005;40:1453–1460.

[8] Thaveesri J. Granulation and stability in upflow anaerobicsludge bed reactors in relation to substrates and liquid surfacetension [Ph.D. dissertation]. Belgium: Ghent University;1995.

[9] Brunetti A, Boari G, Passino R, Rozzi A. Physico-chemicalfactors affecting start-up in UASB digestors. Proceedings ofEuropean Symposium on anaerobic wastewater treatment;Noordwijkerhout, The Netherlands; 1983. p. 317.

[10] Seng B, Khanal KS, Visvanathan C. Anaerobic digestion ofwaste activated sludge pretreated by a combined ultrasoundand chemical process. Environ Technol. 2010;31:257–265.

[11] Park JW, Ahn HJ. Optimization of microwave pretreatmentconditions to maximize methane production and methaneyield in mesophilic anaerobic sludge digestion. EnvironTechnol. 2011;32:1533–1540.

[12] Chen Y, Chen JJ, Creamer SK. Inhibition of anaerobicdigestion process: a review. Bioresour Technol. 2008;99:4044–4064.

[13] Martín MA, Fernández R, Serrano A, Siles AJ. Semi-continuous anaerobic co-digestion of orange peel wasteand residual glycerol derived from biodiesel manufacturing.Waste Manage. 2013;33:1633–1639.

[14] Lane GA. Laboratory scale anaerobic digestion of fruit andvegetable solid waste. Biomass Bioenergy. 1984;5:245–259.

[15] Bouallagui H, Lahdhed H, Romdan BE, Rachdi B, Hamdi M.Improvement of fruit and vegetable waste anaerobic diges-tion performance and stability with co-substrates addition. JEnviron Manage. 2009;90:1844–1849.

[16] Wilkins RM, Suryawati L, Maness ON, Chrz D. Ethanol pro-duction by Saccharomyces cerevisiae and Kluyveromycesmarxianus in the presence of orange-peel oil. World JMicrobiol Biotechnol. 2007;23:1161–1168.

[17] Martín MA, Siles AJ, Chica FA, Martín A. Biomethanizationof orange peel waste. Bioresour Technol. 2010;101:8993–8999.

[18] Field J, Sierra-Alvarez R, Lettinga G. Anaerobic assays(Ensayos anaerobios), in 4◦ Seminario de Depuración Anaer-obia de Aguas Residuales. Spain: Valladolid; 1988.

[19] American Public Health Association. Standard methods forexamination of water and wastewater, 17th ed. Washington(DC): American Public Health Association; 1989.

[20] US Composting Council. Test methods for the examinationof composting and compost, Bethesda and EEUU, 2001.

[21] Winkler H. Biological treatment of wastewater. Chichester:Elis Horwood Ltd; 1983.

[22] Siles AJ, Martín MA, Chica FA, Borja R. Kinetic mod-elling of the anaerobic digestion of wastewater derivedfrom the pressing of orange rind produced in orange juicemanufacturing. Chem Eng J. 2008;140:145–156.

[23] Emerson K, Russo CR, Lund ER, Thurston VR. Aque-ous ammonia equilibrium calculation: effect of pH andtemperature. J Fish Res Board Can. 1975;32:2379–2383.

[24] Østergaard N. Biogasproduktion i det thermofile temperatur-interval. STUB rapport nr. 21. Taastrup: Kemiteknik DanskTeknologisk Institut; 1985.

Dow

nloa

ded

by [

Ast

on U

nive

rsity

] at

19:

57 3

0 Ja

nuar

y 20

14

906 A. Serrano et al.

[25] Fannin FK. Start-up, operation, stability and control. In:Chynoweth PD, Isaacson R, editors. Anaerobic digestion ofbiomass. London: Elsevier; 1987. p. 171–196.

[26] Lee I, Han IJ. The effects of waste-activated sludge pretreat-ment using hydrodynamic cavitation for methane produc-tion. Ultrason Sonochem. 2013;20:1450–1455.

[27] Dai X, Duan N, Dong B, Dai L. High-solids anaerobicco-digestion of sewage sludge and food waste in compar-ison with mono digestions: stability and performance. WasteManage. 2013;33:308–316.

[28] Ramalho SR. Tratamiento de aguas residuales. London: EdReverte; 1996.

[29] Habiba L, Hassib B, Moktar H. Improvement of acti-vated sludge stabilisation and filterability during anaerobicdigestion by fruit and vegetable waste addition. BioresourTechnol. 2009;100:1555–1560.

[30] Saev M, Simeonov I, Koumanova B. Effect of organicloading rate on the anaerobic co-digestion of vegetable

wastes with activated sludge. Special Abstracts/J Biotech-nol. 2010;150S:S1–S576.

[31] Gómez X, Cuetos JM, Cara J, Morán A, García AI. Anaero-bic co-digestion of primary sludge and the fruit and vegetablefraction of the municipal solid wastes: conditions for mixingand evaluation of the organic loading rate. Renew Energy.2006;31:2017–2024.

[32] Borja R, Martín A, Alonso V, García I, Banks JC. Influ-ence of different pretreatments on the kinetics of anaerobicdigestion of olive mill wastewater. Water Res. 1995;29:489–495.

[33] Gallert C, Bauer S, Winter J. Effect of ammonia on theanaerobic degradation of protein by a mesophilic and ther-mophilic biowaste population. Appl Microbiol Biotechnol.1998;50:495–501.

[34] Bujoczek G, Oleszkiewicz J, Sparling R, Cenkiwski S. Highsolid anaerobic digestion of chicken manure. J Agric EngRes. 2000;76:51–60.

Dow

nloa

ded

by [

Ast

on U

nive

rsity

] at

19:

57 3

0 Ja

nuar

y 20

14