effect of substrate concentration and temperature on the anaerobic digestion of piggery waste in a...

Process Biochemistry 37 (2001) 483–489

Effect of substrate concentration and temperature on theanaerobic digestion of piggery waste in a tropical climate

E. Sanchez a, R. Borja b,*, P. Weiland c, L. Travieso a, A. Martın d

a Di�ision de Consultores Ambientales (CONAM), A�e 23, No. 802, Esq. B, Vedado, La Habana, Cubab Instituto de la Grasa (CSIC), A�da. Padre Garcıa Tejero, 4, E-41012 Se�illa, Spain

c Institut fur Technologie und Biosystemtechnik (FAL), Bundesallee 50, D-38116 Braunschweig, Germanyd Departamento de Ingenierıa Quımica, Facultad de Ciencias, Edificio C-3, Campus Uni�ersitario de Rabanales, Ctra. Madrid-Cadiz, km. 396,

E-14071 Cordoba, Spain

Received 13 March 2001; received in revised form 31 May 2001; accepted 9 June 2001

Abstract

A study of the effect of substrate concentration and temperature variation on batch anaerobic digestion of piggery waste wascarried out in laboratory-scale completely mixed reactors. The variation of chemical oxygen demand (COD), total volatile fattyacids (TVFA), alkalinity, pH and methane production with digestion time followed the same pattern at mesophilic (35 °C) andambient temperatures (16.8–29.5 °C). The process was more stable at mesophilic than at ambient temperatures. The rate of CODremoval correlated with the digestion time through an equation of the Grau kinetic model type. An increase in the initialconcentration of organic matter caused a reduction of COD removal rate. In addition, COD removal rates at ambienttemperatures were significantly lower than those obtained at mesophilic temperature. © 2001 Elsevier Science Ltd. All rightsreserved.

Keywords: Anaerobic digestion; Piggery waste; Substrate concentration; Mesophilic and tropical climate temperatures

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1. Introduction

Anaerobic digestion of piggery waste is commonlyapplied in some tropical climate countries for energypurposes on the farm. During digestion the variation oftemperature is not measured or controlled. The re-sponse of the system at a variation of the ambienttemperature and of the influent substrate concentra-tions has not been studied under tropical climateconditions.

The anaerobic digestion process can be developedover different temperature ranges including: mesophilictemperatures of around 35 °C and thermophilic tem-peratures in the range 55–60 °C. Methanogenic activ-ity has been reported by psychrophilic anaerobicbacteria at temperatures below 15 °C [1–3] and byextremophiles at temperatures above 65 °C. The con-

ventional anaerobic digesters operate either atmesophilic or thermophilic temperatures and in the caseof tropical countries at ambient temperatures. In tropi-cal countries the control of temperature is not a com-mon practice and the digestion process will depend onthe changes of temperatures between the day and thenight and the weather conditions. The changes of tem-perature in these countries can be in the range of5–10 °C between the day and the night. In the dryseason (November–April) the maximum average tem-perature reaches around 28 °C during the day and theminimum average achieved during the night is around18 °C while during the rain season (May–October), themaximum and minimum average temperatures arearound 32 and 24 °C, respectively.

The temperature at which digestion occurs can sig-nificantly affect the conversion, kinetics, stability,effluent quality and consequently the methane yield. Ithas been demonstrated that the anaerobic degradationrate of organic matter increases with temperature whenpsychrophilic, mesophilic and thermophilic processes

* Corresponding author. Tel.: +34-95-469-2516; fax: 34-95-469-1262.

E-mail address: [email protected] (R. Borja).

0032-9592/01/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved.

PII: S 0 0 3 2 -9592 (01 )00240 -0

E. Sanchez et al. / Process Biochemistry 37 (2001) 483–489484

are compared. Gallert et al. [4,5] reported that inhibitiondue to ammonia accumulation affects the mesophilicprocess more than the thermophilic one. Mackie andBryant [6] studied the anaerobic digestion of cattle wastesat mesophilic and thermophilic temperatures and foundthat methane yield was more affected in mesophilicdigestion when the organic load was increased andretention time reduced in a completely mixed digester ona laboratory scale. Sanchez et al. [7] demonstrated anincrease of the methane yield when mesophilic andthermophilic reactors treating cattle manure on a labora-tory scale were compared. The rate of organic nitrogendegradation and phosphorus assimilation also increasedwith the temperature. Some authors claim that anaerobicdigestion at 25 °C is 10% less efficient than at 35 °C [8].Bodik et al. [9] compared various types of anaerobicdigesters [upflow anaerobic sludge blanket (UASB),upflow anaerobic filter and anaerobic suspended reac-tors] treating municipal wastewater under psychrophilicconditions (9–23 °C). The upflow anaerobic filter andanaerobic suspended reactor were more effective than theUASB reactor. The comparison of anaerobic digestionof sewage sludge at different temperatures (20, 25 and35 °C) and retention times demonstrated that when theretention time decreased, the organic matter removal rateincreased when temperature was augmented.

There are other factors to be considered related to theefficiency of the process: the effects of the changes oftemperature and of the increase of the organic matterconcentration. In the present work, results were obtainedon the effect of changes of temperature and substrateconcentration on the anaerobic digestion of piggerywaste under tropical climate conditions.

2. Materials and methods

2.1. Wastewater

A mixture of swill and molasses was used for feedingthe animals. The characteristics of the animal food isgiven in Table 1 [10]. Due to the utilization of molassesthe pH of the food is acid. The wastewater containingthe urine, feces and food residues was collected in theeffluent channel before its disposal. The characteristics ofthe piggery wastewater used in the experiments are shownin Table 2. The pig farm was located close to thelaboratory and the samples of wastewater were stored at4 °C until utilized.

2.2. Experimental design

The batch anaerobic digestion process was carried outin 10-l glass vessels closed by rubber caps provided withtwo connections, one to take the samples of waste of thedigestion and the other for biogas outlet to a collection

Table 1Chemical composition of the typical feed used in Cuba for pigs (swilland molasses) based on % of dry matter

Number of samplesAverage�S.D.Composition (%)

16.0�0.3 353Protein2.5�0.2 89Raw fibres

Fixed solids 35310.2�0.2213Ca 2.1�0.1

0.5�0.3 213PMg 2130.6�0.1

2131.0�0.1Na2130.7�0.1K244pH 4.5�0.135325.1�1.1Dry matter

flask. Biogas produced during the digestion was bubbledthrough a solution of 10% (v:v) NaOH to absorb H2Sand CO2 and allow the measurement of methane gasproduction. The volume of methane was determinedindirectly from the amount of water displaced by the gasin Boyle-Mariotte flasks. The digesters were mechani-cally mixed by magnetic stirrers.

2.3. Inoculum

Digesters were inoculated with well digested sludgeobtained from a full scale anaerobic digester in operationfor more than 1 year. The total and volatile solidsconcentrations of the inoculum were 3.7 and 63% (drybasis) respectively. The sludge used as inoculum wasdivided into two portions of 10 l, one was maintained at35 °C and the other at ambient conditions until methanegas production ceased. A total of 2-l of sludge acclimatedat 35 °C and ambient temperature were used as inocu-lum in mesophilic and ambient conditions digesters,respectively.

Table 2Characteristics of the waste used in the experiment

Average value Number ofParameter Standarddeviation samples

2458COD (mg/l)a 8019 990BOD (mg/l)b 10 100 1297 48

1421TS (mg/l)c 6112 100VS (mg/l)d 548029500

584 535500TSS (mg/l)e

750VSS (mg/l)f 515200740Total N (mg/l) 56 23

27380 27P (mg/l)1000.7pH 6.0

a COD, chemical oxygen demand.b BOD, biochemical oxygen demand.c TS, total solids.d VS, volatile solids.e TSS, total suspended solids.f VSS, volatile suspended solids.

E. Sanchez et al. / Process Biochemistry 37 (2001) 483–489 485

Table 3Influent substrate concentration, S0 (g COD/l), and operating temper-ature for the different reactors used

Temperature (°C)Run S0 (g COD/l)

A1 353.3Ambient*B1 3.3

A2 357.0Ambient*B2 7.0

A3 3512.0Ambient*B3 12.0

A4 3519.3Ambient*B4 19.3

A5 3526.3Ambient*B5 26.3

* Ambient temperature ranged between 16.8 and 29.5 °C (reactorsB1, B2, B3, B4 and B5).

Fig. 1. Variation of the operational parameters (COD, TVFA, alka-linity, pH and methane production) with the digestion time (days) forthe experimental run A1 (influent substrate concentration, S0=3.3 gCOD/l and mesophilic temperature).

alkalinity, pH and methane production for mesophilicand ambient temperature anaerobic digestions, respec-tively in the first experimental run (S0=3.3 g COD/l).COD and TVFA values decreased as a function of thedigestion time. In contrast, alkalinity, pH and methaneproduction increased with digestion time. COD andTVFA removals and methane production were higherin digestion A than in B. Specifically, the COD removalvalues were around 85 and 75% in digestions A and B,respectively, and methane gas production was 12%higher in the former experiment than in the second one.These results were coincident with those obtained byFannin [8]. Figs. 3 and 4 show the variation of the sameparameters but at higher initial concentration of COD(S0=7.0 g COD/l, run 2). The increase of initial CODconcentration did not determine a significant change inthe removal efficiency compared with run 1 and com-paring digestions A and B. A similar variation of COD,TVFA, alkalinity, pH and methane gas production with

2.4. Experimental procedure

Five experimental runs were carried out at eachtemperature. The influent substrate concentration (S0)was changed by dilution of the waste with distilledwater. Table 3 shows a summary of the operatingconditions during the experiments carried out. Eachdigester was loaded with 2 l of inoculum (Table 3) and8 l of the waste (Table 2). All experiments were carriedout in triplicate and the results expressed as meanvalues. The experiments were carried out during thesame period of time, during the months of January–February, a period of low temperatures in Cuba.

2.5. Frequency, procedure of sampling and analysis

Samples of the digester liquors were taken twice perweek during the duration of the experiments (33 days).The experiments were concluded when no significantvariation of chemical oxygen demand (COD) and accu-mulative methane production was observed. Analysesof COD, alkalinity, total volatile fatty Acids (TVFA)and pH were carried out according to standard meth-ods of APHA [11]. Accumulative methane gas produc-tion was measured using graduated cylinders.

The ambient temperature changed from 16.8 to29.5 °C during the experiment. Gradients of tempera-ture between the interior and the exterior of the di-gesters were neglected.

3. Results and discussion

3.1. Variation of the operational parameters (COD,TVFA, alkalinity, pH and methane production) duringthe experiments

Figs. 1 and 2 show the variation of COD, TVFA,

Fig. 2. Variation of the operational parameters (COD, TVFA, alka-linity, pH and methane production) with the digestion time (days) forthe experimental run B1 (influent substrate concentration, S0=3.3 gCOD/l and ambient temperature).




digestion time was observed for this initial substrateconcentration. Methane production in run 2 wasaround two times higher than that obtained in run 1and around 12% higher in digestion A than in B. Figs.5 and 6 show the behaviour obtained for the thirdexperimental run carried out (initial substrate concen-tration, S0=12.0 g COD/l, run 3). The increase of theinitial concentration of organic matter determined adecrease of the COD removal efficiency of around 20%for digestions A and B. The same pattern in variationof all parameters evaluated was observed. The CODremoval efficiency was 3% higher in digestion A whencompared to digestion B. Methane production was alsohigher when compared with that obtained in run 2. Forthis initial substrate concentration, methane productionwas 7% higher in digestion A than in the second one.Figs. 7 and 8 show the results obtained for the fourthexperimental run carried out (initial substrate concen-tration, S0=19.3 g COD/l). The increase of the initialCOD concentration to 19.3 g/l caused a significant

reduction in the COD removal efficiency observed inthe digester B. The increase in the concentration didnot affect the removal efficiency of the anaerobic diges-tions at mesophilic and ambient temperatures. TheCOD removal efficiency at the end of the operatingtime was 63% in digestion A but only 47% in digestionB. The volume of methane accumulated reached valuesof around 25 and 20 l in digestions A and B, respec-tively, which means a gas production 25% higher in theformer process than in the second one. TVFA valuedecreased in the course of both digestions but the finalvalues were significantly higher than those observed inprevious runs. In consequence, the pH values wereconsiderably lower than those observed for the runs 1,2 and 3, throughout the digestion time.

Figs. 9 and 10 show the results obtained for run 5,which was carried out with the most concentratedinfluent (S0=26.3 g COD/l). An increase of 6 g/l in theinitial COD concentration caused a reduction of theorganic matter removal efficiency of a 10% for both






digestions. The difference in the COD removal effi-ciency between digestion A and B was 17%. This differ-ence determined a methane gas production 45% higherin digestion A than in B. The pattern of the variation ofall control parameters with digestion time was similarto that observed in previous runs. However, the valuesof TVFA and alkalinity were higher compared to thoseobserved in the previous experiments. The pH and thealkalinity values were also higher in digestion A com-pared to the values of digestion B, while the TVFAvalues were lower in digestion B than those observed inA.

Comparing all the runs carried out, it was observedthat an increase in the initial substrate concentrationcaused a reduction of the COD removal efficiency andan increase of the methane production. The increase ofthe initial concentration of organic matter determinedan increase of the differences in the substrate removalefficiency between the anaerobic digestion at mesophilic

and ambient temperatures. In addition, the increase inthe initial substrate concentration caused an increase inthe TVFA and alkalinity concentration and theVFA:Alkalinity ratio with a consequent reduction ofthe pH.

3.2. Kinetic e�aluation

In order to characterize each experiment kineticallyand to study the influence of substrate concentrationand indirectly of the temperature variation on the ki-netics of the process the following first-order modeldeveloped by Grau et al. [12] was used:

S/S0=exp− (k1X0t/S0) (1)

where: S is the concentration of organic matter at anydigestion time (g COD/l); S0 is the initial substrateconcentration (g COD/l); k1 is a first-order kineticconstant (g COD/l day); X0 is the initial concentrationof microorganisms expressed as g of volatile suspended


Fig. 10. Variation of the operational parameters (COD, TVFA,alkalinity, pH and methane production) with the digestion time(days) for the experimental run B5 (influent substrate concentration,S0=26.3 g COD/l and ambient temperature).


Table 4Values of the slopes (p) calculated by using Eq. (3)

Run Slope values (p, in days−1)S0 (g COD/l)

A (35 °C) B (ambient temperature)

Average VC (%)* Average VC (%)*

0.064 7.31 0.0503.3 5.62 7.0 0.071 6.6 0.053 8.93 12.0 0.030 4.9 0.025 6.0

0.029 8.119.3 0.0184 9.20.025 9.3 0.0115 9.826.3

* VC, variance coefficients.

Table 5Values of the kinetic constant (k �1) of the Grau equation

S0 (g COD/l) Values of k �1 (g COD/l day)Run

A (35 °C) B (ambient temperature)

VC (%)*Average AverageVC (%)*

5.61 0.163.3 0.21 7.38.92 7.0 0.370.49 6.6

0.30 6.04.90.3612.038.10.5519.3 9.24 0.35

0.64 10.39.35 0.2826.3

* VC, variance coefficients.

solids (VSS)/l. Taking into account that the value of X0

was constant during the experiment and, hence, k1X0 isa constant (k �1) the Eq. (1) is simplified to:

S/S0=exp− (k �1t/S0) (2)

In order to calculate the value of the kinetic constantof the first-order model, the Eq. (1) is transformed asfollows:

−Ln(S/S0)=pt (3)

where p is equal to k �1/S0.As p and S0 are constants for each experimental

condition studied, plotting −Ln(S/S0) against t, willyield a straight line. By fitting the data to a linearfunction, using the least-squares method, the slopes (p)of the lines can be calculated. Finally, k �1 can be deter-mined by the equation:

k �1=pS0 (4)

Table 4 summarizes the values of the slopes (p) andits variance coefficients obtained through the plot of theexperimental data according to Eq. (3). The values of pare proportional to the values of k �1, in such a manner,that multiplying the values of p and S0, the values of thekinetic constant, k �1, are obtained. The calculated valuesof k �1 and its corresponding variance coefficients aregiven in Table 5. The k �1 values were significantly higher

for the mesophilic digestion than for the digestion atambient temperature. The values of k �1 ranged from 0.21to 0.64 g COD/l day at mesophilic temperature whilethey varied between 0.16 and 0.28 g COD/l day atambient temperature. The differences between the k �1values observed at mesophilic and ambient temperatureswere increased when the initial substrate concentrationincreased.

The values of k �1 increased when the initial concentra-tion of organic matter increased from 3.3 to 7.0 g COD/lfor both temperature ranges studied, and lower differ-ences were observed for higher initial substrate concen-trations. The rate of reaction given by the expression[12]: −dS/dt=k �1S/S0, was strongly influenced by theinitial concentration of organic matter. Therefore, anincrease of the initial substrate concentration (S0)caused a reduction of the substrate removal rate.

4. Conclusions

The results of this study demonstrate that the varia-tion of COD, TVFA, alkalinity, pH and methane pro-duction with time during the batch anaerobic digestionof piggery wastes followed the same pattern both atmesophilic and ambient temperatures.

The COD removal efficiency was a function of theoperating temperature and initial substrate concentra-


tion. The concentration of TVFA and alkalinity in-creased when the influent concentration increased from3.3 to 26.3 g COD/l. The concentration of TVFA wasalways higher in the digesters operating at ambienttemperature while the pH values were lower than thoseobserved at mesophilic temperature.

A first-order kinetic model was adequate to fit theexperimental data obtained. By using this model, it canbe demonstrated that the increase of the initial sub-strate concentration affects anaerobic digestion more atambient temperature than that at the mesophilic tem-perature. An increase of the initial concentration oforganic matter caused a reduction of the COD removalrate. In addition, the COD removal rates at ambienttemperatures were significantly lower than those ob-tained at mesophilic temperature.

Acknowledgements

The authors want to acknowledge the support of theAlexander Von Humboldt Foundation, Germany, andthe ‘‘Consejeria de Educacion y Ciencia’’ of ‘‘Junta deAndalucia’’, Spain, to develop the present work.

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