Chemical Engineering Journal 193–194 (2012) 10–15

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Chemical Engineering Journal

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Mesophilic anaerobic digestion of the organic fraction of municipal solidwaste: Optimisation of the semicontinuous process

J. Fernández Rodríguez a, M. Pérez b, L.I. Romero a,⇑a Department of Chemical Engineering and Food Technology, Faculty of Science, University of Cadiz, 11510 Puerto Real, Cadiz, Spainb Department of Environmental Technologies, Faculty of Marine and Environmental Sciences, University of Cadiz, 11510 Puerto Real, Cadiz, Spain

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

Article history:Received 1 December 2011Received in revised form 3 April 2012Accepted 7 April 2012Available online 16 April 2012

Keywords:Anaerobic digestionBiogasSRTOFMSWMesophilic

1385-8947/$ - see front matter � 2012 Elsevier B.V. Ahttp://dx.doi.org/10.1016/j.cej.2012.04.018

⇑ Corresponding author. Tel.: +34 956016379.E-mail address: luisisidoro.romero@uca.es (L.I. Rom

Dry mesophilic anaerobic digestion of the organic fraction of municipal solid waste (OFMSW) is a wide-spread technology. However, OFMSW is a very heterogeneous waste and therefore specific studies areneeded to optimise the AD process with each type of OFMSW that will be used. The main operationalvariables to optimise are the solids retention time (SRT) and the organic loading rate (OLR), which areclosely related to each other when the concentration of organic matter of OFMSW remains relatively con-stant. In this work three different experimental conditions (corresponding to the SRT of 30, 20 and15 days) were tested in a semi-continuous stirred tank reactor, operating at mesophilic range (35 �C)and high solids concentration (20% TS). The OLR corresponding to the above mentioned SRT were 22.8,27.3 and 35.9 mg DOC/Lreactor/d, respectively.

The results obtained in this study indicate that 20 days is the optimum SRT for the dry mesophilic anaer-obic digestion of the OFMSW used. Thus, in general, all the parameters analysed show better performancefor 20-day SRT with regard to 30-day and 15-day SRT. More specifically, it can be pointed out than at SRT of20 days (27.3 mg DOC/Lreactor/d), both the highest productivity of methane (0.11 L CH4/g waste-fed) andthe highest organic matter removal rate (66.3% DOC removal) were reached.

� 2012 Elsevier B.V. All rights reserved.

1. Introduction

In recent years, municipal waste generation in Spain has in-creased as a result of improved standards of living. Managementmeasures are required to minimise the impact of municipal wasteon the environment. Between 40% and 45% of the total municipalwaste produced in Spain is of organic nature, known as the organicfraction of municipal solid waste (OFMSW), and is suitable for usein producing electric power through anaerobic digestion.

Anaerobic digestion is a biological process suitable for organicwaste treatment that degrades the organic material without oxy-gen, generating methane and a digested waste similar to aerobi-cally produced compost [1]. In 2006, Spain treated approximately8.5 million tons of biological waste in treatment plants [2].

Anaerobic digestion is influenced by operating variables such asfeeding, the solids content of the waste and the temperature. Themesophilic temperature range for conducting the process (35 �C)is more stable [3], requires less energy and has less risk of inhibi-tion by ammonium [4,5] and long-chain fatty acids. Thus, on anindustrial scale, the most widely used temperature range for theanaerobic treatment of organic waste is the mesophilic range

ll rights reserved.

ero).

because it requires less energy and provides greater stability tothe process.

Anaerobic digestion can be classified with respect to the amountof solids as either wet (between 4% and 10% Total Solids – TS) or dry(between 20% and 30% TS). Dry anaerobic digestion is the most com-monly applied treatment for OFMSW.

The solids retention time (SRT) and organic loading rate (OLR)are the most important factors in the control of anaerobic digestionsystems [6], and they are strongly related. The SRT determines thefeed rate to the system and determines the chemical and biologicalbalances that occur during the anaerobic process. The OLR is corre-spond to the amount of organic matter supplied to the system perunit of reactor and unit time. High organic loads in the absence ofinhibitors result in the high-volume production of biogas. Resis-tance to certain inhibitors can increase the organic load [7]. How-ever, the increased load can cause instability, especially in the caseof an ‘‘overload’’ point, which involves the accumulation of volatilefatty acids [8].

Several authors have described the behaviour of mesophilicanaerobic systems by varying the SRT and the Organic LoadingRate (OLR) with different wastes [9–14]. Cuetos et al. [9] conductedexperiments at mesophilic range (34 �C) in a continuous stirredtank reactor CSTR, co-digesting slaughterhouse waste and OFMSWwith an SRT of 50 days and an organic loading rate of 1.85 kg VS/m3/d. After acclimatisation of the inoculum to the waste, it was

Glossary and notations

CSTRs continuously stirred tank reactorsDOC dissolved organic carbon, expressed as mg/LOFMSW organic fraction of municipal solid wasteOLR fed organic loading rate, expressed as mg DOC/Lreactor/day

or g VS/Lreactor/day15-day SRT 15 days of solid retention time in the reactor20-day SRT 20 days of solid retention time in the reactor

30-day SRT 30 days of solid retention time in the reactorSRT solid retention time, expressed as daysSSTR semi-continuously stirred tank reactorTS total solids, expressed as %VS volatile solids, expressed as %

J. Fernández Rodríguez et al. / Chemical Engineering Journal 193–194 (2012) 10–15 11

possible to decrease the hydraulic residence time (HRT) whilegradually increasing the organic load to values of 25 days and3.70 kg VS/m3/d, respectively. The plants that are currently operat-ing in Europe for the co-digestion of OFMSW and livestock wasteby mesophilic anaerobic digestion operate at SRT in the range be-tween 16 and 30 days [10]. The SRT values found in the literaturefor the mesophilic treatment of other wastes with a high solidscontent ranged between 12 and 25 days for animal manure and10–20 days for pig slurry [11]. For slaughterhouse waste frompoultry production, the SRT values found in the literature rangedbetween 50 and 100 days, and there is accumulation of long-chainVFAs when the SRT is decreased to between 13 and 25 days [12].Farmed salty sludge has also shown excellent results for an SRTof 30 days in a semi-continuous mesophilic reactor [13]. Fdez-Güelfo et al. [14] have developed a study about the treatment ofOFMSW operating at thermophilic range. In this paper a series ofSRT were tested and it was determined than 15 days is the opti-mum SRT for biomethanization of OFMSW in a semi-continuousstirred tank reactor at thermophilic (55 �C) temperature.

As can be deducted from the above mentioned papers, the re-search about the optimum SRT and OLR utilizable for AD of com-plex wastes is a very important topic and it is necessary tooptimisation of the anaerobic industrial process. Recently, Fdez-Güelfo et al. [15] have studied the efficiency of the anaerobic diges-tion of the OFMSW during a destabilization episode caused byoverloading and washing-out phenomena (high OLR and lowSRT). Also it was developed a study about the determination ofcritical and optimum conditions for biomethanization of OFMSWin a semi-continuous stirred tank reactor in thermophilic condi-tions [14].

The heterogeneity of the wastes, specially the OFMSW, makesnecessary a deep research about the processes. Researches can helpto resolve any future problems working in higher scale and to pre-dict the performance of the processes successfully.

The goal of this research is to determinate the optimum solidsretention time SRT (and the related organic loading rate OLR) forthe mesophilic anaerobic digestion of OFMSW in dry anaerobicconditions (20–30% TS) testing different solids retention times(SRTs) of 30, 20 or 15 days.

2. Materials and methods

2.1. Experimental equipment

In this study a semicontinuous lab-scale stirred tank reactor,operating at thermophilic range, was used. The equipment consistsof a reactor with a stainless steel vessel that is agitated and heatedand that has a total volume of 5 L and a working volume of 4.5 L(Fig. 1). The reactor features a lid that allows it to be sealed tomaintain anaerobic conditions within the reactor.

The stainless steel lid has several openings (for the output ofbiogas, insertion of a pH probe, insertion of a temperature probe,

two inputs to correct the pH balance, a power input and an agita-tion system). The bottom of the reactor has a release valve used forsampling the material inside the reactor, which is made possible bythe sealing system between the vessel and the cap. The assemblyincludes an agitator (operating at 15 revolutions per minute) thatachieves the homogenisation of waste using stainless steel bladescrapers. To maintain the operating temperature, the reactor isheated by recirculating water through a thermostatic jacket. Biogasis collected in 40-L Tedlar bags, and a special syringe is used forsampling gases.

2.2. Operational conditions

The test was developed in dry conditions, with a solids contentbetween 20% and 30% TS. The reactor was initially loaded with amixture of inoculum and OFMSW, resulting in a final concentrationof 20% total solids, which is considered optimum for biogas pro-duction, according to previous studies of dry anaerobic digestion[16,17]. The inoculum coming from a full scale mesophilic digesterfor the treatment of waste sludge of a WWTP. Once the inoculumwere mixed with the organic waste, OFMSW, the system remainedunfed for a period of 5 days in order to acclimatise the inoculum tothe waste at the selected temperature (35 �C).

The reactor was fed with OFMSW diluted with water until itcontained 20% TS. Based on information found in the literatureand the previous experience of the group, SRTs of 30, 20 and15 days were selected for study. Each condition was maintainedfor an operational period of 3 times the HRT (Hydraulic RetentionTime) to ensure that Steady-state conditions were reached.

2.3. Wastes characterisation

To characterise the waste and the inoculum, as well as to mon-itor the effluent of the process, the following parameters weredetermined: pH, volatile fatty acids (VFAs), dissolved organic car-bon (DOC), total solids (TS) and volatile solids (VS). These analyseswere conducted in accordance with standardised methods [18]adapted for waste with high solids content [19] and based on theprevious leaching of the waste in an aqueous medium. The biogasvolume and composition were determined using chromatographicmethods [16].

Table 1 shows the characteristics of the OFMSW that was usedin terms of the organic matter content. The solid percentage ofOFMSW coming from industrial treatment plant was 57.17% (highhumidity degree) and the maximum original size of particles ofOFMSW was 30 mm. For lab-scale studies, a previous stage of sizereduction was required to increase the specific surface and to ob-tain more homogeneous samples. Hence, pretreatment of OFMSW,consisting in drying, crushing and shredding until obtaining parti-cle sizes of 10 mm approx., was required to provide a suitable re-fined digested material, reaching 82–83% TS in the samples.Previous several studies showed the influence of the pretreatmentin the anaerobic digestion of OFMSW [20,21]. Later, mesophilic

Fig. 1. Photograph and scheme of the semi-continuously stirred tank reactor (SSTR).

Table 1Characteristics of the OFMSW and the Organic Loading Rate (OLR) in each condition.

SRT (d) TS (%) VS (%) DOC (ppm) OLR (mg DOC/Lreactor/d) OLR (g VS/Lreactor/d)

30-day SRT 30 82.98 36.29 684.12 22.80 2.4220-day SRT 20 83.19 29.49 546.43 27.32 2.9515-day SRT 15 82.19 30.69 538.84 35.92 4.09

Table 2Inoculum characteristics.

pH Density (kg/L) TS (%) VS (%) DOC (ppm) Total acidy (g AcH/L) Alkalinity (g Ca CO3/L)

7.46 0.959 4.12 1.63 623.98 0.280 1.21

7

830-day SRT

20-day SRT 15-day SRT

12 J. Fernández Rodríguez et al. / Chemical Engineering Journal 193–194 (2012) 10–15

inoculum with 3.64% of TS was added, until the selected condi-tions. Table 1 also shows the OLR of feeding into the digester ateach stage of the study, which is expressed in terms of DOC andVS. The characteristics of the inoculum used in the start-up processare shown in Table 2.

Time (days)0 50 100 150 200

unit

pH

5

6

Fig. 2. pH evolution in a semicontinuous mesophilic reactor.

3. Results and discussion

This section discusses the evolution of the main variables dur-ing the semicontinuous mesophilic anaerobic digestion process,such as pH, total acidity, dissolved organic carbon (DOC) and bio-gas production and composition.

Discussions are based on the comparison of the system perfor-mance for three different SRT conditions (and consequently differ-ent OLR) tested in the mesophilic dry anaerobic digestion ofOFMSW. These three conditions were: 30-day SRT, 20-day SRTand 15-day SRT, (corresponding to a range of OLR 22.8–35.95 mgDOC/Lreactor/d and 2.42–4.09 g VS/Lreactor/d, respectively).

In the different figures used in the following discussion, verticallines have been included to indicate the different SRT imposed tothe system: 30, 20 and 15 days, respectively.

3.1. Physiochemical parameters

3.1.1. pHpH is a fundamental parameter in the control of the anaerobic

degradation process [22]. Fig. 2 shows the evolution of pH during

the semicontinuous mesophilic experiments conducted in thisstudy.

Operating at 30-day SRT, the pH decreases during the first fewdays from 6.70 to 6.23 due to the hydrolysis of the waste. Afterthe pH was adjusted with 6 M NaOH, the pH began to increaseand approached to 8.02 on day 50. Subsequently, the system regu-lates itself at a pH of 7.22. With a 20-day SRT, the pH value stabi-lises at approximately 7.23, the optimum pH for the activity ofmethanogenic microorganisms. Operating at 15-day SRT an initialdecrease in pH was observed during the first 10 days, until 6.44, asa result of increasing the OLR. Subsequently, the pH stabilised at

J. Fernández Rodríguez et al. / Chemical Engineering Journal 193–194 (2012) 10–15 13

7.38, without the addition of an external agent. The sharp declinein pH values in the first days of each of the new SRT is related withdestabilization of the system as a consequence of the increasedOLR. Specifically, the initial decrease in pH may be due to the initialimbalance between the metabolic activities of microbial groups.When the added organic load is increased, the acidogenic microor-ganisms respond quickly, given their high specific growth rate, andgenerate more VFA. However, methanogenic archaea (with lowergrowth rate) require more time to reach the population size neces-sary to degrade the excess of VFA. During this process, the pH de-creases as a result of the accumulation of VFAs in the reactor.

The decrease of pH is more important when working with high-er OLR as the imbalance between the activities of the differentmicroorganisms groups is more pronounced. Therefore, the initialdecrease in pH in the transition from 20 to 15 day-SRT is especiallyhigh.

3.1.2. Total acidityThe total acidity that represents the total amount of VFAs, ex-

pressed as acetic acid, exhibits stable daily values in the effluentfrom the mesophilic reactor, i.e., 425 ppm for a 30-day SRT(Fig. 3a). When the SRT is changed to 20 days, a significant differ-ence in average total acidity is observed, with a value of 42 ppm.Finally, in 15-day SRT, the VFA concentration increases to values>850 ppm due to the increased organic load supplied to the sys-tem. This trend illustrates the initial destabilisation caused bythe reduction in the SRT, as discussed above. However, at the endof the 15-day SRT, the average acidity values are close to 60 ppmof acetic acid (Fig. 3b). The change to 15-day SRT supposed anaccumulation of the acids during the first days. At the final of thisstage the level of VFA decreased considerably, without reachingthe low level of 20-day SRT. The 15-day SRT seem to be a time

Time (days)0 50 100 150 200

Tot

al A

cidi

ty (

mg

AcH

/L)

0

200

400

600

800

1000

1200

30-day SRT

20-day SRT

15-day SRT

Time (days)

0 50 100 150 200

VFA

con

cent

ratio

n (m

g/L

)

0

200

400

600

800AceticPropionicButiric

30-day SRT

20-day SRT

15-day SRT

(a)

(b)

Fig. 3. Acidity evolution in a semicontinuous mesophilic reactor: (a) total acidityand (b) individual acids.

commitment to the stability of the system. The 20-day SRT is thecondition more stable and which shows less acidity concentrationin the effluent.

pH and VFA evolution (as seen in Figs. 2 and 3) are reversed. Alow pH stimulates the acidogenic activity (VFA-production) andmakes methanogenic activity difficult (VFA-consumption). Thiscould explain the observed VFA evolution in 30-day SRT and, also,it indicates the proper pH for the process. The trend of the ratioacidity/alkalinity (Fig. 4) shows it has remained below 0.25 formost tests in the stable period, and hence malfunction of the sys-tem, by severe decline in the alkalinity, was not detected [23].The industrial OFMSW has a ratio acidity/alkalinity adequate forthe process because it presents low organic content. This supposesgreater stability of the system due to low acid production duringthe hydrolysis-acidogenesis period.

3.1.3. Dissolved organic carbon (DOC)Fig. 5 shows the DOC evolution in the mesophilic reactor for dif-

ferent SRTs. The start up of the process (30-day SRT) shows a highfluctuation due to adaptation of the inoculum to the waste. Thiscan be reached in the first 30 days, and later the trend shows lowervariations. For the 30-day SRT, DOC reaches a value of 300 ppmaprox. in the stable phase of the operation. For the 20-day SRT,DOC reaches a value below 185 ppm. However, for the 15-daySRT, the DOC concentration reaches a peak during the first 15 daysat 568 ppm and later decreases to 230 ppm in the stable phase. Asmentioned previously, the DOC evolution in the 15-day SRT testssuggests that the reactor needs to adapt to the new conditions ofincreased organic loading.

The best results for DOC concentration, in terms of the quality ofthe effluent, were obtained at 20-day SRT. Thus, as can be seen inTable 3, the DOC removal percentage was the highest in the 20-daySRT. It can be observed the results are similar to those obtained forFdez-Rguez et al. [16,17]. These authors reached DOC remove of67.53% in a system operating with 20% TS and mesophilicconditions.

3.2. Biogas and methane

An increased production of biogas was detected in 20-day and15-day SRT, with an average of 1.03 L/Lreactor/d and 1.07 L/Lreactor/d,respectively. 30-day SRT resulted in a lower average, 0.54 L/Lreactor/dduring the stable phase. Fig. 6 shows the evolution of methaneproduction as L/Lreactor/d. The trend is similar to that for total biogasproduced, because methane represents approximately 60–70% ofthe biogas. It can be seen a sharp initial drop in methane productionat the transition from 20-day SRT to 15-day SRT. This evolution

Time (days)

0 50 100 150 200

Rat

io A

cidi

ty/A

lkal

inity

0.0

0.2

0.4

0.6

0.8

30-day SRT

20-day SRT 15-day SRT

Fig. 4. Acidity/alkalinity ratio in a semicontinuous mesophilic reactor.

Tiempo (días)0 50 100 150 200

Car

bono

Org

ánic

o D

isue

lto (

mg/

L)

0

200

400

600

800

1000

1200

30-day SRT

20-day SRT

15-day SRT

Fig. 5. DOC evolution in a semicontinuous mesophilic reactor.

Table 3Percentage of DOC removal under the conditions studied.

DOCfed (ppm) DOCexit (ppm) % Remove

30-day SRT 684.1 303.8 55.620-day SRT 546.4 184.2 66.315-day SRT 538.8 228.9 57.5

Time (days)0 50 100 150 200

Met

hane

pro

duct

ion

(L/L

reac

tor/d

)

0

1

2

3

4

5

6

30-day SRT20-day SRT

15-day SRT

-day SRT

Fig. 6. Evolution of daily methane production in a semicontinuous mesophilicreactor.

Time (days)

0 50 100 150Acc

umul

ated

met

hane

(L

CH

4/L

reac

t)

0

20

40

60

80

30-day SRT 20-day SRT 15-day

SRT

Fig. 7. Evolution of the accumulated methane per litre of reactor in a semicontin-uous mesophilic system.

Table 4Methane accumulation rate and estimated productivity of methane for mesophilicprocessing and different SRTs.

30-day SRT 20-day SRT 15-day SRT

Slope (L CH4/Lreactor/d) 0.4788 1.0611 0.9902R2 0.9840 0.9918 0.9743L CH4/d 2.1546 4.7750 4.4559L CH4/g DOCfed 21.00 38.84 27.56L CH4/gfed_waste 0.0718 0.1061 0.0743L CH4/g VSfed 0.1979 0.3598 0.2420

14 J. Fernández Rodríguez et al. / Chemical Engineering Journal 193–194 (2012) 10–15

coincides with the increased production of acids and it can be re-lated with the peak in organic matter solubilisation (expressed asDOC) and subsequent decrease in pH because to the increase of or-ganic load on the system.

The cumulative methane per litre for each SRT tested (Fig. 7)provides valuable information. As it was indicated, all the SRTshave been maintained for 3 operational periods of HRT, so the finalamounts of organic matter supplied to the system were similar forall cases. As Fig. 7 shows, the maximum cumulative methane pro-duction for the 20-day SRT is above 60 L CH4/Lreactor. For the 30-dayand 15-day SRTs, the cumulative methane productions are onlyapproximately 40 L CH4/Lreactor. After a short adaptation period,there is an almost linear increase in cumulative methane produc-tion with time. Thus, a comparison of the rates of cumulativemethane production for each SRT tested (represented by the slopesof the linear trends shown in Fig. 7 for the stable stage of each con-dition) can also provide relevant information about the process.

Table 4 shows the rate of methane accumulation for each of thetested conditions. The methane productivity per unit of soluble or-ganic matter that is fed into the system is calculated as a functionof this parameter and the organic load that is fed. The productivi-ties are calculated using the volume of methane and the amount oforganic matter initially fed (TS and VS).

For the 30-day SRT, there is an initial period (20 days) in which nomethane production occurs because of the hydrolysis of the waste andacclimatisation of the mesophilic inoculum. After 20 days, methaneproduction begins, with a rate of production of 0.48 L CH4/Lreactor/d.For the 20-day SRT, the rate is 1.06 L CH4/Lreactor/d. The subsequentchange to the 15-day SRT requires a new adjustment period of9 days, followed by a stable phase during which cumulativemethane production follows a linear trend with a slope of0.99 L CH4/Lreactor/d. The highest methane productivity,38.84 L CH4/gDOCfed, is achieved with the 20-day SRT, followed bythe 15-day SRT, with a methane productivity of 27.56 L CH4/g DOCfed.

The highest production of methane per gram of waste fed intothe system was obtained with the 20-day SRT (0.11 L/g), whilefor operation at 30 and 15 days, the values obtained were similarand 45% lower than the value obtained for 20 days. The highestmethane production per unit weight of fed VS occurred with the20-day SRT (0.36 L CH4/g VSfed). The methane production level for15 and 30 days reached 0.24 and 0.20 L CH4/g VS, respectively.These results are according with reports of methane yields foundin the literature. Capela et al. [24] reported that for a mixture of86% OFMSW and 14% industrial sludge, methane production pergram of VS reached 0.23 L CH4/g VSfed under mesophilic conditions.The production of methane varied from 0.18 to 0.73 L CH4/g VSfed forfruit and from 0.19 to 0.40 L CH4/g VSfed for vegetables. Other authorshave reported methane yields between 0.1 and 0.7 L CH4/g VSfed forvarious types of waste [25–28].

4. Conclusions

The following conclusions may be drawn from the abovediscussion:

J. Fernández Rodríguez et al. / Chemical Engineering Journal 193–194 (2012) 10–15 15

� In 20-day SRT operation, a higher level of methane productionwas reached with respect to the waste fed into the system,0.11 L CH4/gfed_waste against of 0.07 L CH4/gfed_waste in the otherconditions.� Also, 20-day SRT got higher removal of organic matter, 66.3%

DOC removal in contrast to 55.6% and 57.5% in 30-day and15-day SRT, respectively. This is 16.14% higher than for theother conditions.� According to these results, the best operating conditions for the

mesophilic anaerobic digestion of OFMSW in dry conditions areachieved with a 20-day SRT.

Acknowledgments

This study was conducted by the Environmental Technologiesresearch group at the University of Cadiz, a group of excellenceof the ‘‘Plan Andaluz de I + D + i’’ TEP-181. It was funded by theSpanish ‘‘Ministerio de Ciencia e Innovación’’ (Project CTM2010-17654), the ‘‘Consejería de Innovación, Ciencia y Empresa deAndalucía’’ (Project P07-TEP-02 472) and the European RegionalDevelopment Fund (ERDF).

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