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Dry thermophilic anaerobic digestion of the organic fraction of municipal solid wastes: Solid retention time optimization J. Fernández-Rodríguez a,, M. Pérez b , L.I. Romero a a Department of Chemical Engineering and Food Technology, Faculty of Science, University of Cádiz, Spain b Department of Environmental Technology, Faculty of Sea and Environment Sciences, University of Cádiz, Spain highlights Anaerobic digestion of organic fraction of municipal solid waste has been studied. The OFMSW comes from non-selective collection systems. Thermophilic temperature, dry condition and semicontinuous regime have been selected. The Solid Retention Time (SRT) was progressively decreased to optimize the process. The methane yield and the organic material removal in each SRT were analyzed. article info Article history: Received 4 November 2013 Received in revised form 31 March 2014 Accepted 18 April 2014 Available online 2 May 2014 Keywords: Dry anaerobic digestion Thermophilic Methane SRT OFMSW abstract The thermophilic temperature has showed operational advantages in the Anaerobic Digestion process (AD). The increasing of the rate of hydrolysis, which suppose an acceleration of the overall process, and a higher hygienization of the final digestate are one of them. Dealing with OFMSW (Organic Fraction of Municipal Solid Waste) from non-selective collection systems involves additional difficulties due to their heterogeneity. The AD process of OFMSW is conducted usually in the range of 20–35% TS (dry AD). The main objective of this study was to determine the optimum Solids Retention Time (SRT) for the dry thermophilic AD of OFMSW operating in semi-continuous regime. Thus, the following SRTs have been tested: 15, 10, 8, 6, 5, 4 and 3 days. The main results have showed that the process is feasible in SRTs lower that those found in literature. Probably the history of the reactor, joined to the lower organic content in the waste, can be a key factor and the progressive decreasing of SRT is a strategy that permits to maintain stable conditions for lower SRT. The best operating conditions for thermophilic semicontinuous systems ranged in 8–5 days, with a yield of 0.33–0.34 LCH 4 /gVS added and a concentration of Volatile Fatty Acids (VFA) around 100 mg HAc/L. The process becomes unstable for SRTs lower than 4 days, showing a clear decrease in methane produc- tivity (lower than 0.2 LCH 4 /gVS added ) and an accumulation of VFA (more than 500 mg HAc/L). In general, SRTs below 4 days were not suitable for single stage of dry AD of OFMSW. Ó 2014 Elsevier B.V. All rights reserved. 1. Background In the last years, waste production has increased at global scale [1]. The high amounts produced today require management mea- sures in order to minimize its impact on the environment. It is esti- mated that the 40–45% is the organic fraction, known as Organic Fraction of Municipal Solid Waste (OFMSW) and it can be valorized both energetically as agronomically by Anaerobic Digestion (AD). The AD or biomethanization is a biological treatment capable to remove the organic matter in absence of oxygen, generating meth- ane and a digestate with similar characteristics to the compost pro- duced aerobically [2]. The process is based on four steps: hydrolysis, acidogenesis, acetogenesis and methanogenesis and it is conditioned by operat- ing variables such as feed rate, the solids content and temperature. Based on the temperature range, different studies have been car- ried out in order to compare the performance of the mesophilic (35 °C) versus thermophilic (55 °C) range of AD of OFMSW [3]. Each one presents advantages and disadvantages. The operation can be carried out at thermophilic range, which is more effective due to http://dx.doi.org/10.1016/j.cej.2014.04.067 1385-8947/Ó 2014 Elsevier B.V. All rights reserved. Corresponding author. Current address: Department of Chemistry and Soil Science, Faculty of Science, University of Navarra, Spain. Tel.: +34 948 425 600x806271. E-mail addresses: [email protected], [email protected] (J. Fernández-Rodríguez). Chemical Engineering Journal 251 (2014) 435–440 Contents lists available at ScienceDirect Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej

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Page 1: Dry thermophilic anaerobic digestion of the organic fraction of municipal solid wastes: Solid retention time optimization

Chemical Engineering Journal 251 (2014) 435–440

Contents lists available at ScienceDirect

Chemical Engineering Journal

journal homepage: www.elsevier .com/locate /cej

Dry thermophilic anaerobic digestion of the organic fractionof municipal solid wastes: Solid retention time optimization

http://dx.doi.org/10.1016/j.cej.2014.04.0671385-8947/� 2014 Elsevier B.V. All rights reserved.

⇑ Corresponding author. Current address: Department of Chemistry and SoilScience, Faculty of Science, University of Navarra, Spain. Tel.: +34 948 425600x806271.

E-mail addresses: [email protected], [email protected](J. Fernández-Rodríguez).

J. Fernández-Rodríguez a,⇑, M. Pérez b, L.I. Romero a

a Department of Chemical Engineering and Food Technology, Faculty of Science, University of Cádiz, Spainb Department of Environmental Technology, Faculty of Sea and Environment Sciences, University of Cádiz, Spain

h i g h l i g h t s

� Anaerobic digestion of organic fraction of municipal solid waste has been studied.� The OFMSW comes from non-selective collection systems.� Thermophilic temperature, dry condition and semicontinuous regime have been selected.� The Solid Retention Time (SRT) was progressively decreased to optimize the process.� The methane yield and the organic material removal in each SRT were analyzed.

a r t i c l e i n f o

Article history:Received 4 November 2013Received in revised form 31 March 2014Accepted 18 April 2014Available online 2 May 2014

Keywords:Dry anaerobic digestionThermophilicMethaneSRTOFMSW

a b s t r a c t

The thermophilic temperature has showed operational advantages in the Anaerobic Digestion process(AD). The increasing of the rate of hydrolysis, which suppose an acceleration of the overall process,and a higher hygienization of the final digestate are one of them. Dealing with OFMSW (Organic Fractionof Municipal Solid Waste) from non-selective collection systems involves additional difficulties due totheir heterogeneity. The AD process of OFMSW is conducted usually in the range of 20–35% TS (dry AD).

The main objective of this study was to determine the optimum Solids Retention Time (SRT) for the drythermophilic AD of OFMSW operating in semi-continuous regime. Thus, the following SRTs have beentested: 15, 10, 8, 6, 5, 4 and 3 days.

The main results have showed that the process is feasible in SRTs lower that those found in literature.Probably the history of the reactor, joined to the lower organic content in the waste, can be a key factorand the progressive decreasing of SRT is a strategy that permits to maintain stable conditions for lowerSRT. The best operating conditions for thermophilic semicontinuous systems ranged in 8–5 days, with ayield of 0.33–0.34 LCH4/gVSadded and a concentration of Volatile Fatty Acids (VFA) around 100 mg HAc/L.The process becomes unstable for SRTs lower than 4 days, showing a clear decrease in methane produc-tivity (lower than 0.2 LCH4/gVSadded) and an accumulation of VFA (more than 500 mg HAc/L). In general,SRTs below 4 days were not suitable for single stage of dry AD of OFMSW.

� 2014 Elsevier B.V. All rights reserved.

1. Background both energetically as agronomically by Anaerobic Digestion (AD).

In the last years, waste production has increased at global scale[1]. The high amounts produced today require management mea-sures in order to minimize its impact on the environment. It is esti-mated that the 40–45% is the organic fraction, known as OrganicFraction of Municipal Solid Waste (OFMSW) and it can be valorized

The AD or biomethanization is a biological treatment capable toremove the organic matter in absence of oxygen, generating meth-ane and a digestate with similar characteristics to the compost pro-duced aerobically [2].

The process is based on four steps: hydrolysis, acidogenesis,acetogenesis and methanogenesis and it is conditioned by operat-ing variables such as feed rate, the solids content and temperature.Based on the temperature range, different studies have been car-ried out in order to compare the performance of the mesophilic(35 �C) versus thermophilic (55 �C) range of AD of OFMSW [3]. Eachone presents advantages and disadvantages. The operation can becarried out at thermophilic range, which is more effective due to

Page 2: Dry thermophilic anaerobic digestion of the organic fraction of municipal solid wastes: Solid retention time optimization

Nomenclature

AD Anaerobic DigestionCODs soluble chemical oxygen demand (mg/L)DOC Dissolved Organic Carbon (mg/L)HRT hydraulic retention time (days)MSW Municipal Solid WasteOFMSW Organic Fraction of Municipal Solid WasteOLRadded Organic Loading Rate added to reactors (gVS/Lreactor/

day)

SMP specific methane productivity (LCH4/gVSadded)SRT Solid Retention Time (days)SSTR Semicontinuous Stirred Tank ReactorsTCD thermal conductivity detectorTS Total Solids (%)VFA Volatile Fatty Acids (mg/L)VS volatile solids (%)

436 J. Fernández-Rodríguez et al. / Chemical Engineering Journal 251 (2014) 435–440

the higher metabolic rates of microorganisms [4] and besides, ahigher destruction of pathogens. However, thermophilic processmay be associated with higher concentrations of propionic acidin the effluent [5] that can inhibit the process. In the field of thebiomethanization of the Municipal Solid Waste (MSW), the dryAD (between 20% and 30% Total Solid-TS) is the most commonlyapplied treatment [6,7]. With respect to solids content of the sys-tem, previous studies have demonstrated than 20% TS offers a bet-ter performance than 30%TS [8,9].

Once the temperature of operation has been selected, the SolidsRetention Time (SRT) and the Organic Loading Rate (OLR) are themost important factors to control AD systems [10]. SRT and OLRare closely linked with each other. In fact, if characteristics of feedinghave been constant, the SRT and OLR are inversely proportional forCSTRs. The SRT determines the mean permanence time of solids intothe reactor whereas the OLR represents the input rate of organicmatter to the system. High organic loads, in the absence of inhibitors,provide high volumes of biogas and even resistance to certain inhib-itors may increase with the organic load [11]. However, the increaseof OLR can lead to instability and organic overloading of the system,which involves an accumulation of volatile fatty acids [12].

Moreover, the effect of SRT on process performance is closelyrelated to the hydraulic retention time (HRT), only for reactorswithout microorganisms retention. According to previous studiesconducted at thermophilic range for the anaerobic digestion ofliquid wastewaters, the minimal HRT for semicontinuous stirredtank reactors without microorganisms recycle is near to 3 daysfor wine-vinasses degradation [13]. However, the use of attachedgrowth reactors-anaerobic filters and fluidized bed-permit toreduce the HRT to the range 0.5–2 days [14,15].

Obviously, when the waste has a high content of suspended sol-ids, biofilm technologies are not usable. In these cases, as indicatedin the literature, the HRT should be higher. Thus, minimum HRTfor thermophilic anaerobic digestion of a mixture of primary andsecondary sludge are around 4 days [16]. Moreover, Cecchi et al.[17] working in 8 days HRT thermophilic reactors with municipalsolid waste (23% TS), have not reported any problems and they gotthe process stability under those conditions. Fernández-Güelfoet al. [7] have studied the dry thermophilic degradation of OFMSWcoming from a 30 mm trammel. These authors have worked in dryconditions (30% TS) and they have determined that 15 days is theoptimum SRT. At industrial scale, the SRT applied is usually, almostthe double of the laboratory scale. That is around 25–35 days. Thereason, perhaps, is to avoid the problems derived to the inhibitionof the system when it is working close to the optimum. Furthermore,the industrial plants operate in mesophilic regime and not in ther-mophilic, what supposes higher SRT [3]. The OLR investigated previ-ously is 4.4–22.2 g TVS/L/d operating with real OFMSW [7].

The aim of this study is to optimize the SRT of the thermophilicAD conducted in Semicontinuous Stirred Tank Reactors (SSTR) forthe treatment of OFMSW in dry conditions (20–30% TS). The tested

Solids Retention Times (SRT) have been: 15, 10, 8, 6, 5, 4 and3 days. The optimum operation time will be selected according tothe maximum biogas production and the minimum content oforganic matter in the digestate.

2. Materials and methods

2.1. Equipment used

In order to achieve the overall goal, a laboratory scale reactor(Fig. 1) operating in semicontinuous and thermophilic range wasused (as described previously by Fernández-Rodríguez et al. [6]),a Semi-continuously Stirred Tank Reactor (SSTR). The equipmentconsists of a 5 L stainless steel reactor with mixing and heatingdevices and a working volume of 4.5 L.

In addition, the system consists of a stainless steel cover with dif-ferent inlet/outlet ports (biogas output, pH probe, temperatureprobe, input of chemical agents for pH control, feed input and inputfor the shaft of the stirring system). Finally, the bottom of the reactorcontains a valve for sampling and discharging. The mechanical stir-ring allows homogenization inside the reactor. To maintain the tem-perature, the reactor is equipped with a heating system, byrecirculation through a thermostatic jacket. The biogas produced iscollected in Tedlar bags of 40 L and a sample from the Tedlar bagsis taken using a gas-syringe, to determine the biogas composition.

Fig. 1. Sketch of the Semi-continuously Stirred Tank Reactor (SSTR).

Page 3: Dry thermophilic anaerobic digestion of the organic fraction of municipal solid wastes: Solid retention time optimization

J. Fernández-Rodríguez et al. / Chemical Engineering Journal 251 (2014) 435–440 437

2.2. Wastes

The wastes used in this study were OFMSW, coming from thecomposting plant ‘‘Las Calandrias’’ located in Jerez de la Frontera(Cádiz, Spain), and inoculum from a thermophilic stable reactoradapted to degradation of OFMSW. Exactly, the inoculum was col-lected from a tank reactor operating at 15-days SRT. So, the goodacclimatization of the inoculum to the substrate has been a keyfactor to carry out the experiment.

Experimentation was conducted in the dry anaerobic digestionrange, which is characterized by total solids content between 20%and 30%. In this regard, the reactor initially was filled with a mix-ture of inoculum and OFMSW resulting in a final total solids con-centration around 20%, which has been considered the optimalfor the dry AD of this waste from previous biomethanization stud-ies [8,9]. Lately, the system was kept without feeding for a periodof a few days, around 3 days, to get the total acclimatizationbetween the inoculum and the new OFMSW. One time the produc-tion of biogas started, the system was ready to operate.

The daily feeding of the reactors has been OFMSW diluted withtap water to achieve the required conditions (approx. 20% TS) topreserve the same condition inside the reactor. The following ser-ies of retention times have been tested: 15, 10, 8, 6, 5, 4 and 3 days.Each retention time was maintained at least 3 times to ensure thatstable conditions were reached.

2.3. Characterization of the wastes

The characterization of wastes and the inoculum used in theprocess is shown in Table 1. Due to the long term of the study, itwas necessary to take samples of OFMSW in different moments.Since the waste was not the same during the overall experiment,the characterization of the OFMSW for each condition studiedcan be observed in the table. The pH, Volatile Fatty Acids (VFA),Dissolved Organic Carbon (DOC), soluble chemical oxygen demand(CODs), Total Solids (TS) and volatile solids (VS) were determinedaccording to Standard Methods [18]. However, the methods havebeen adapted to high solids samples by leaching in water accordingto the procedure proposed by Alvarez-Gallego [19]. Exactly, it con-sists in leaching 1 g of OFMSW per 10 mL distilled water, for 2 h.After that, the mixture is filtered to separate de liquid part whichcontains the soluble organic matter. According to the results ofthe characterization, the industrial OFMSW from non-selective col-lection systems used in this work has low organic matter content.

Furthermore, the production and the composition of the biogashave been determined by gas chromatography, as described Ferná-ndez-Rodríguez et al. [3]: using a gas chromatograph (ShimadzuModel 2014, Japan) equipped with a thermal conductivity detector(TCD) and a Carbosieve S-II packed column 3 m � 1/8 in. InternalDiameter (Supelco, USA).

The samples of the effluent were taken three times per week,and the biogas samples, daily.

Table 1Characterization of the thermophilic inoculum and the OFMSW used in the differentconditions studied.

SRT (d) TS (%) VS (%)* DOC (ppm) CODs (ppm)

Inoculum – 3.9 ± 0.5 10.5 ± 0.3 27.0 ± 0.5 93.0 ± 0.5

OFMSW 15 83.0 ± 0.5 36.0 ± 0.6 684.0 ± 0.5 2277.5 ± 0.510 82.0 ± 0.5 29.5 ± 0.6 545.0 ± 0.5 2226.5 ± 0.5

8 85.0 ± 0.5 34.5 ± 0.5 577.5 ± 0.5 2337.5 ± 0.56 85.0 ± 0.5 34.5 ± 0.5 577.5 ± 0.5 2337.5 ± 0.55 82.0 ± 0.5 32.0 ± 0.5 536.5 ± 0.5 1591.0 ± 0.54 82.0 ± 0.5 32.0 ± 0.5 536.5 ± 0.5 1591.0 ± 0.53 82.0 ± 0.5 32.0 ± 0.5 536.5 ± 0.5 1591.0 ± 0.5

* % VS of the dry waste.

3. Results and discussion

3.1. Characterization the feeding of the reactors

The main results about the optimization the SRT of thermo-philic AD are showed in this section. Table 2 shows the OrganicLoading Rate added (OLRadded) to the digester in each of the differ-ent experimental tests, expressed as gVS/Lreactor/day. It can beobserved that the decrease of the SRT supposes an increment inthe OLR. In the range of 15–3 days SRT, the OLR feeding is locatedin the range 4.84–20.46 gVS/L/d.

3.2. Effect of operating conditions

Initially, the process was conducted at 15 days SRT (the sameconditions of the inoculation reactor) to facilitate the adaptationof microorganisms. Later, the SRT was gradually decreased (andconsequently the OLR increased) in order to analyze the effectsof the organic load increase on the anaerobic thermophilic system.As mentioned previously, the following SRTs were tested: 15, 10, 8,6, 5, 4 and 3 days. Each of these SRTs were maintained at least forthree times to ensure the system stability.

3.2.1. pHThe pH of the system was maintained close to 7.5 for all SRTs

tested, except 4 days, which had an average value close to neutral-ity. This fact is probably due to the heterogeneity of the OFMSWsince at 3-days SRT the pH is slightly higher (Table 4). Thus,although changing SRT causes a transient state until the systemadapts to the new conditions, in no case the system reached valuesbelow 6.5, which has been indicated by Batstone et al. [20] asinhibitory for the anaerobic digestion. According with this authorsthe pH below 6 impedes the methanogenic activity and onlyhydrolytic and acidogenic activities can be found in this conditions.However, in this study, the methanogenic activity was very high inall the series of SRTs tested.

3.2.2. Total acidity and Volatile Fatty Acids (VFA)Fig. 2 shows the evolution of the total acidity expressed as

mgHAc/L. There is a slight increase in acidity between 15 and5-day SRT. However, the increase is noticeable in the transitionfrom 5 to 4-day SRT, reaching an increase of 217.8% over the aver-age acidity values. Operation at 3-day SRT also implies an increasein the average concentration of VFA (25.72%) with respect to theaverage values for 4-day SRT. These data are in according withliterature, since decreasing of SRT (and hence an increase in OLR)leads to the increase in system acidity. Fernández-Güelfo et al.[7] have reported the same behavior for semicontinuous anaerobicdigestion of both synthetic and industrial OFMSW at thermophilicrange. Moreover, Linke [21] study about the treatment of wasteswith high content in solid, exactly potato processing industrywastes, has found similar trends: increase in the total acidity, whenthe OLR is increasing. Table 3 shows the concentration of theindividual VFA at different SRT. Butyric and valeric acids are themore accumulation present when the OLR is increasing. Also, arelationship between the total VFA and COD has been calculated.Higher SRTs present a higher value for this relation, thus is, thetotal acidity is accumulate in the reactor.

3.2.3. Ratio acidity/alkalinityThe industrial OFMSW from non-selective collection systems, as

used in this work, has low organic matter content. This implies lowproductivity of biogas but also high stability associated with lowacid production during the hydrolysis and acidogenesis stepsand, hence, the process overloading by organic matter requires

Page 4: Dry thermophilic anaerobic digestion of the organic fraction of municipal solid wastes: Solid retention time optimization

Table 2Characterization of the Organic Loading Rate added(OLRadded) at different SRTs.

SRT (d) OLRadded (gVS/Lreactor/day)

15 4.8 ± 0.110 5.9 ± 0.1

8 8.8 ± 0.06 12.5 ± 0.35 13.0 ± 0.24 16.5 ± 0.13 20.0 ± 0.2

Tota

l Aci

dity

(mg

Ac/L

)

0

200

400

600

800

15-day SRT

10-day SRT

4-daySRT

5-day SRT

6-day SRT

8-day SRT

3-daySRT

Time (days)0 50 100 150 200

Fig. 2. Evolution of total acidity at different SRTs.

DO

C (m

g/L)

0

200

400

600

800

SRT 15d

SRT 10d

SRT 4d

SRT 5d

SRT 6d

SRT 3d

SRT 3d

Time (days)0 50 100 150 200

Fig. 3. Evolution of DOC at different SRT.

438 J. Fernández-Rodríguez et al. / Chemical Engineering Journal 251 (2014) 435–440

the operation at very low SRT. Thus, the ratio acidity/alkalinity wasmaintained between about 0 to 0.1, except for 4 and 3-day SRT,where acid production is very high because the organic overload.Under those conditions, the ratio increases to 0.4, indicating thedestabilization of the system.

3.2.4. Dissolved Organic Carbon (DOC)The evolution of the DOC for the different SRTs is shown in

Fig. 3. There was an increase of 33.27% in the DOC concentrationfrom 15 to 5-day SRT. For 4 and 3-day SRT, the concentrations ofDOC in the effluent were increased 188.96% and 209.88% respec-tively compared to the average data for 5-day SRT. Thus, in thesame way that for total acidity, the data from soluble COD indicatethan the destabilization of the system was reached from 4-day SRT.

3.2.5. Volatile solids (VS)The lowest percentage of VS in effluent of the reactor was

detected at 15-day SRT. The 86.22% removal is obtained under thiscondition. This result is comparable to that obtained by Davidssonet al. [22], who achieved VS removal rates around 80% workingwith OFMSW in thermophilic reactors operating at 15-day SRT.Moreover, Moen et al. [16], working with thermophilic reactorsfed with a mixture of primary and secondary sludge observed thatthe removal percentage of VS is increased from 53% to 66% byincreasing the 6-day SRT at 20-day SRT. The monitorization ofthe VS is difficult due to the OFMSW heterogeneity, which can

Table 3Production of volatile fatty acids at different SRT (ppm) and the relation between total VFA/COD.

SRT

15 10 8 6 5 4 3

Acetic 16.0 ± 0.4 11.4 ± 0.7 9.1 ± 0.4 7.8 ± 0.2 11.8 ± 0.5 17.3 ± 0.4 19.3 ± 0.7Propionic 7.6 ± 0.2 5.7 ± 0.8 11.2 ± 0.4 20.7 ± 0.7 7.5 ± 0.3 68.4 ± 0.1 86.5 ± 0.2I. Butyric 0.0 ± 0.0 3.9 ± 0.8 1.5 ± 0.6 6.3 ± 1.0 1.6 ± 0.3 15.8 ± 0.2 22.9 ± 0.1Butyric 0.0 ± 0.0 6.3 ± 1.0 3.7 ± 1.0 5.9 ± 1.0 10.6 ± 0.8 38.9 ± 0.3 32.0 ± 0.6I. Valerc 0.0 ± 0.0 4.7 ± 1.0 2.7 ± 0.6 8.1 ± 0.5 2.9 ± 0.3 39.1 ± 0.2 42.3 ± 0.1Valeric 0.0 ± 0.0 0.6 ± 0.4 0.6 ± 0.0 6.4 ± 0.7 8.8 ± 0.3 0.0 ± 0.0 0.0 ± 0.0I. Caproic 0.0 ± 0.0 1.3 ± 0.8 0.0 ± 0.0 0.0 ± 0.0 0.0 ± 0.0 0.0 ± 0.0 0.0 ± 0.0Caproic 0.0 ± 0.0 3.5 ± 1.0 0.9 ± 0.3 3.5 ± 0.9 2.3 ± 0.8 16.8 ± 0.6 24.3 ± 0.6Heptanoic 0.0 ± 0.0 12.5 ± 0.9 0.3 ± 0.2 0.0 ± 0.0 0.0 ± 0.0 0.0 ± 0.0 0.0 ± 0.0Total VFA/COD 0.1 0.1 0.1 0.1 0.1 0.5 0.4

Table 4Slope of the rate of methane accumulation for the different SRTs tested and specific methane productivity (SMP) with respect the organic load added and other importantparameters.

SRT (days) 15 10 8 6 5 4 3

SMP Slope (LCH4/Lreactor/day) 1.45 2.02 2.37 3.80 4.19 3.29 3.87R2 1.00 0.99 1.00 1.00 0.99 0.97 1.00LCH4/day 1.00 0.99 1.00 1.00 0.99 0.97 1.00LCH4/gVSadded – 9.10 8.12 9.76 12.69 8.28 7.30pH 7.7 7.4 7.4 7.4 7.3 7.0 7.1Ammonia 6.4 6.0 6.9 5.5 6.9 6.5 7.9

Page 5: Dry thermophilic anaerobic digestion of the organic fraction of municipal solid wastes: Solid retention time optimization

/Lre

acto

r)

60

80

SRT 4d

SRT 5dSRT 6d

SRT 8d

J. Fernández-Rodríguez et al. / Chemical Engineering Journal 251 (2014) 435–440 439

contain different fraction like crystals, metals, etc. Anyway, overthe average values in the present study a trend can be observed.The decrease from 15-day SRT and 3-day SRT results an increasein VS concentration from 5.31% to 8.65%.

Time (days)0 20 40 60 80 100 120 140 160

Accu

mul

ated

met

hane

(LC

H4

0

20

40

SRT 15d

SRT 10d

SRT 3d

Fig. 5. Evolution of accumulated methane per liter of reactor at different SRT.

3.2.6. AmmoniaNitrogen is an essential nutrient for microorganisms. The evolu-

tion of N-ammonia during the study shows high variability. Thelowest value was obtained for 6-day SRT (5.49 mg/g of waste or549 mg/L). This result is equivalent to that obtained by Davidssonand la Cour Jansen [23], who achieved stable ammonium concen-trations of 500 mgNH3-N/L, working at 15-day SRT with OFMSWin a thermophilic pilot plant. In the present study, at 15-day SRTthe N ammonia concentration in the effluent was higher(655 mg/L), decreasing after to reach the minimum value for the6-day SRT and increasing gradually for SRT lower than 6 days.

Values lower 6-day SRT show an increase of nitrogen due to thehigher amount of organic matter fed. In this way, the discharge ofnitrogen from the protein content of the organic wastes is nottotally used for microbial growth, and it is accumulated in the reac-tor. The average concentration of nitrogen for 3-day SRT was895 mg/L. Due to the pH is lower than in the rest of the SRT tested,the present ammonia does not appear in the most toxic form [24].

3.2.7. Biogas productionIn general, it was found that higher daily production of biogas

was related to the decrease in SRT, due to the increment of theOLR. This occurred until the system started to accumulate theVFA and the biogas production stabilized. Hansen et al. [25] indi-cated a decrease in methane production by increasing the totalVFA concentration in the effluent. Fernández-Güelfo et al. [26] alsoobserved the same effect in a study of thermophilic digestion ofOFMSW under different SRTs. However, in this study the 3-daySRT has generated a higher biogas production (7.98 L/Lreactor/day)even though the VFA content was the highest.

In fact, the increase of acidity from 4 to 3-day SRT has notinvolved a reduction in methane production (Fig. 4). Even, it canbe observed a higher methane production than at 4-day SRT. How-ever, the highest production of methane (measured as stable aver-age of the SRT) for all conditions tested has been 5-day SRT.

Fig. 5 shows the evolution of accumulated methane per liter ofreactor for different SRTs. It can be observed that the greatestcumulative production has occurred in the 6-day SRT (withapproximately 67 L) followed closely by the conditions 10, 8 and5-day SRT (approximately 60 L in each case). For assays at 4 and

Time (days)0 50 100 150 200

Dai

ly m

etha

ne p

rodu

ctio

n (L

/Lre

act/d

ay)

0

2

4

6

8

SRT 15d

SRT 10d

SRT 4dSRT 5d

SRT 6d

SRT 8d

SRT 3d

Fig. 4. Evolution of the daily production of methane at different SRT.

3-day SRT, a decrease of the accumulation of methane happened,reaching only 40 L approx.

Analysis of the slope of the accumulated methane curves canpermit to extract relevant information of the AD process (Table 4).The slopes, which have been calculated for the last period of eachSRT tested, are different depending on the corresponding condi-tions. The highest value has been obtained for 5-day SRT, with4.19 LCH4/day, indicating that these are the most favorable condi-tions for the generation of methane.

3.2.8. Methane yieldSpecific methane productivities (SMP) have been calculated

according to the values of the slopes and considering the organicload added to the system.

According to the total organic content in the reactor, the bestconditions to carry out the AD of the waste are 6-day SRT and5-day SRT. When the system was operating with these conditionsthe yield obtained was around 0.34 LCH4/gVSadded. A decrease inthe SRT supposed a decrease in the methane yield. In this sense4-day SRT and 3-day SRT, achieved 0.21 and 0.19 LCH4/gVSadded

respectively.

4. Conclusions

The data obtained in this study about semicontinuous thermo-philic dry AD process of industrial OFMSW, indicate that OFMSWfrom non-selective collection systems can be treated over a widerange of SRTs without causing an irreversible acidification byorganic overloading. That can be due its low organic content andadequate alkalinity. The best operating conditions for thermophilicrange are at the interval from 8 to 5 days SRT as it relates toremoval percentage of organic matter or generated methane pro-ductivity with respect to the organic matter (both added orconsumed).

The process becomes unstable for SRT lower than 4 days. Itshows a marked decrease of methane productivity with a substan-tial accumulation of VFA and dissolved organic matter inside thereactor (200% higher than the previous conditions). SRT lower than4 days has shown not to be suitable for operation of single-stageAD.

Acknowledgements

This study was conducted by the Environmental Technologiesresearch group at the University of Cadiz, a group of excellence of

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440 J. Fernández-Rodríguez et al. / Chemical Engineering Journal 251 (2014) 435–440

the ‘‘Plan Andaluz de I+D+i’’ TEP-181. It was funded by the Spanish‘‘Ministerio de Ciencia e Innovación’’ (Project CTM2010-17654)and the European Regional Development Fund (ERDF).

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