Bioresource Technology 101 (2010) 545–550

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Bioresource Technology

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Case Study

Thermophilic anaerobic co-digestion of cattle manure with agro-wastesand energy crops: Comparison of pilot and full scale experiences

C. Cavinato a,*, F. Fatone b, D. Bolzonella c, P. Pavan a

a Department of Environmental Sciences, University Ca’ Foscari of Venice, Dorsoduro 2137, I-30123 Venice, Italyb Department of Biotechnology, University of Verona, Strada Le Grazie 15, I-37134 Verona, Italyc Department of Science, Technology and Market of Wine, University of Verona, via della Pieve 70, 37020 San Floriano, Verona, Italy

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

Article history:Received 27 May 2009Received in revised form 24 July 2009Accepted 7 August 2009Available online 10 September 2009

Keywords:Anaerobic co-digestionThermophilicBiogasCattle manureEconomics

0960-8524/$ - see front matter � 2009 Elsevier Ltd. Adoi:10.1016/j.biortech.2009.08.043

* Corresponding author. Tel.: +39 0422 321037; faxE-mail address: cavinato@unive.it (C. Cavinato).

The paper deals with the benefits coming from the application of a proper process temperature (55 �C)instead of a ‘reduced’ thermophilic range (47 �C), that is often applied in European anaerobic co-digestionplants. The experimental work has pointed out that biogas production improve from 0.45 to 0.62m3/kg VS operating at proper thermophilic conditions. Moreover, also methane content was higher: from52% to 61%. A general improvement in digester behaviour was clear also considering the stability param-eters comparison (pH, ammonia, VFA content). The second part of the study takes into account theeconomic aspects related to the capital cost of anaerobic digestion treatment with a 1 MW co-generationunit fro heat and power production (CHP). Moreover, the economic balance was also carried out consid-ering the anaerobic supernatants treatment for nitrogen removal. The simulation showed how a pay-back-time of 2.5 yr and between 3 and 5 yr respectively could be determined when the two options ofanaerobic digestion only and together with the application of a nitrogen removal process wereconsidered.

� 2009 Elsevier Ltd. All rights reserved.

1. Introduction

The need to reduce the emissions of green house gases, espe-cially carbon dioxide, and to develop a reliable alternative to thefossil fuel economy, is conveying the interest of policy makers to-ward the renewable energy sources. In fact, together with the in-creased efficiency in the energy sector (that is a reduction ofconsumption), renewable energy sources can contribute to thereduction in fossil fuel using and carbon dioxide emissions. Besidesolar, hydro, wind or geothermal-energy, the biogas productionfrom organic waste is knowing a renaissance after the interest ofthe past (Ghosh and Pohland, 1974; Cecchi and Traverso, 1988;Chynoweth et al., 1990; Kayhanian and Tchobanoglous, 1993; Cec-chi et al., 1994). Anaerobic co-digestion of agricultural wastes andenergy crops, in particular, is supposed to be one of the main alter-native in this sector, as stated also by the United Nations Develop-ment Programme (UNPD) that consider this technology as one ofthe most useful decentralised sources of energy supply, especiallyif used with energy crops and all substrates easily available inmany farms. Moreover, considering the complete waste-to-energytransformation, anaerobic processes can be considered a way to re-duce the organic content of biowaste giving low-CO2 emission.

ll rights reserved.

: +39 0422 326498.

Agricultural wastes can be considered as a primary substrate forthese aims, in particular considering co-digestion of manure andother specific biomass coming from cultivations. Manure, in partic-ular, is a resource easily available in many farms all over the world.However, the reduced biogas yield of this material, sometimesdoes not justify the capital costs for farm-scale plants. However,the biogas productivity can be dramatically increased by introduc-ing energy rich co-substrates to the anaerobic digester (maize,grass, bread, fruit, etc.).

Several experiences have shown how the thermophilic range oftemperature should be preferred for the co-digestion process be-cause of its superior performances compared to a mesophilic pro-cess (Mladenovska and Ahring, 2000; Ahring et al., 2001; Van Lieret al., 2001; Angelidaki et al., 2006) as well as its sanitisationcapability.

In central and north Europe, in particular, anaerobic digestion iswidely applied in the agricultural sector. In Denmark, for example,there are some 20 centralised biogas plants and more than 60farm-scale plants treating livestock manure. Most of the centra-lised biogas plants treat manure together with other organicwastes; the preferred temperature range is thermophilic and theHRT is 11–22 d (Nielsen and Angelidaki, 2008). The annual amountof manure and other biomass treated is about 1.5 million ton/yr,producing a biogas equivalent of about 39,000,000 m3 CH4/yr(Angelidaki and Ellegaard, 2003).

546 C. Cavinato et al. / Bioresource Technology 101 (2010) 545–550

Also in Sweden the choice of co-digestion was implemented inmore than 200 sites: ten of them are centralised plants wheremanure is co-digested together with various kind of waste, usuallyoriginated from the food-processing industries or the source sepa-rated collection of restaurant wastes (Lantz et al., 2007), all theothers are farm-scale plants treating manure and crop residues.This situation can be ascribed to the fact that Sweden is so spreadout that full utilisation of this energy potential by centralised slur-ry based technology is difficult (Svensson et al., 2005), therefore,decentralised plants are preferred.

Usually, on farms, bioreactors may be subjected to temperaturefluctuations due to large variations in outdoor temperature andreactor feeding, especially in highland and northern climates, ornon optimal set-up ranges. All these problems can cause instabilityand disturbances in all the main parameters of the process (Alvarezand Lidé, 2008; Lettinga, 2004; Massé et al., 2003), leading to nota-ble yields reduction.

As a general remark we can say that often a not optimal temper-ature range is applied in these plants.

Ahring (1994) showed that the optimal temperature for ther-mophilic digestion was found to be 60 �C. However, for practicaloperation of full scale plants, a temperature between 52 and56 �C will be preferable, allowing a slight variation in the temper-ature without fatal consequences for some of the active microbes.

Following this evidences, this paper consider the anaerobic co-digestion of manure with other agro-waste in mesophilic and ther-mophilic conditions at both pilot and full scale: in particular the re-sults coming from a 0.38 m3 stirred reactor operating at 55 �C arecompared to those coming from a 1400 m3, two-stage process,operating at 47 �C.

2. Methods

The experimental plan was divided in two phases. During thefirst part, the operational conditions of a full scale plant werereproduced at pilot scale in order to study the stability of the pro-cess under the same temperature conditions (47 �C). On the otherhand, during the second part of the experimental work, the tem-perature of the pilot-scale process was increased at 55 �C, withthe aim of improving performances and process yields.

Fig. 1. Simplified flow diagra

2.1. Full scale plant configuration

The full scale plant based in Marcon-Venice and shown in Fig. 1,was composed by a mechanical feed system for manure, maize andother food waste residues (bread, fruit, grass, etc.), a storage tank(900 m3 volume) for the liquid manure feeding the reactor and alsofor the recycle operations, an anaerobic digester with volume of1400 m3 operating at an average temperature of 47 �C, a storagetank for treated manure (4300 m3 volume), a dewatering systemand an unit for the co-generation of heat and power (340 kW hCHP unit).

The plant treated 140,000 kg/d of cattle manure mixed withagro-industrial wastes and 25 m3/d of liquid manure (inclusive ofeffluent recycling). The organic loading rate (OLR) applied wassome 5 kg TVS/m3 d with a HRT of some 34 d while the storagetank allowed for other 3 months of retention time for furtherstabilisation.

2.2. The pilot-scale plant

The experiment was carried out in a CSTR reactor of stainlesssteel, with a working volume of 380 l. The reactor was heated byhot water circulation. The reactor was inoculated with the anaero-bic sludge coming from the full scale plant of Marcon (122 g/kg TSand 92 g/kg TVS) in order to reproduce the same conditions of thefull scale reactor.

2.3. Analytical schedule and methods

The effluent of the full scale plant was monitored 2 times/weekwhile the pilot reactor was monitored 5 times/week.

The effluent of both reactors was monitored in terms of total(TS) and volatile solids (VS) content, chemical oxygen demand(COD), total Kjiendhal nitrogen (TKN), total phosphorus (TP), andstability parameters (pH, alkalinity, ammonia and volatile fattyacids (VFA) content), all in accordance with the Standard Methods.

The VFA content was monitored using a gas chromatograph(Carlo Erba instruments) with hydrogen as gas carrier, equippedwith a Fused Silica Capillary Column (Supelco NUKOLTM,15 m � 0.53 mm � 0.5 lm film thickness) and with a flame ioniza-

m of the full scale plant.

Table 1Characterisation of influent substrates.

Substrate TS, g TS/kg w.w. TVS, g TVS/kg w.w. VS, % COD, g COD/kg TS TKN, g N/kg TS Ptot, g P–PO4/kg TS VFA, mg COD/l N–NH3, mg N/l

Solid manure 361 304 84 904 30.3 4.33 25.9 –Liquid manure 97 74 76 877 34.2 12.00 756.6 4482Maize 351 327 93 1061 16.4 2.35 40.5 –Fruit 219 211 96 1091 16.6 1.88 910.6 –Bread 828 748 90 1143 24.8 1.06 n.d. –

0

1

2

3

4

5

6

7

0 20 40 60 80 100 120 140

dayskg

TV

S/m

3d

4647484950515253545556

°C

OLR temperature

Fig. 2. OLR behaviour during the experimental work, with a focus on the changetemperature period.

Table 2Characteristics of digested sludge, operational conditions and process yield.

Full scale Pilot plant

47 �C Run 1 (47 �C) Run 2 (55 �C)

Characterisation of digested sludgepH – 7.94 8.06 8.17N–NH3 mg N/l 4762 3647 2429VFA mg COD/l 592 1777 313TS g TS/kg w.w. 122.0 82.5 83.4TVS g TVS/kg w.w. 91.9 62.8 64.1

Operational conditionsOLR kg VS/m3 d 5.39 5.67 4.66HRT d 33.0 34.5 34.2

C. Cavinato et al. / Bioresource Technology 101 (2010) 545–550 547

tion detector (200 �C). The temperature during the analysis startedfrom 80 �C and reach 200 �C trough two other steps at 140 and160 �C, with a rate of 10 �C/min. The analyzed samples were centri-fuged and filtrated with a 0.45 lm membrane. Gas production inthe pilot plant was monitored continuously and on line by a gasflow meter (Ritter Company, drum-type wet-test volumetric gasmeters), while the biogas composition (CO2–CH4–H2S) was definedby a portable infrared gas analyser (geotechnical instrument,model GA2000).

2.4. Substrates characterisation

The reactors were fed with a mixture of cattle manure (solidand liquid), maize, fruit-processing waste (marc) and bread of27%, 18%, 37% and 18%, respectively, on wet weight basis, to obtaina suitable solid content in the digester feeding (some 10–12% totalsolids).

Table 1 shows the average values found for the characterisationof each substrate.

With reference to data shown in Table 1, the solid fraction ofmanure and maize showed similar characteristics, with a TS con-tent of some 35–36% and a volatile fraction of 84% and 93%, respec-tively, while fruit showed a lower content in terms of total solids(22%) but a higher VS concentration (96%). As for bread, thisshowed a solid content of some 83%, 90% volatile while the liquidfraction of manure showed a TS content <10%. All the substrateswere characterized by a COD/TS ratio in the range 0.9–1.0. As forthe nutrients content it turned out clearly that manure wasresponsible for high nitrogen and phosphorus contents. Liquidmanure, in particular, showed high presence of ammonia–nitro-gen: some 4–5 g/l.

Process yieldGPR m3/m3 d 2.4 2.9 2.7SGP m3/kg VS 0.45 0.54 0.62CH4 % 52.3 58.8 61.6H2S ppm 884 483 549

3. Results and discussion

3.1. Anaerobic co-digestion at pilot and full scale

As mentioned above, the pilot scale test was divided in twoperiods, the first carried out at 47 �C and the second at 55 �C. Forboth those temperatures, were reached stable steady state condi-tions, however the high variability of the substrates characteristicsinfluenced the stability of the process (Fig. 2).

The organic loading rate applied to the pilot scale reactor wasmaintained at about 5 kg TVS/m3 d (as in the full scale) duringthe steady state periods.

As for temperature changes, the most common strategies ofadapting mesophilic reactors to thermophilic temperatures aretwo: a one-step or a step-wise temperature increase. These wereboth widely described in literature and were often applied in realcases (Bouskova et al., 2005). Nevertheless, there is a lack of studieswhere strategies for temperature change are compared (from mes-ophilic to thermophilic range), especially treating maize, cattlemanure and other similar organic substrates.

In this experimental work, according to Cecchi et al. (1993), aquick change of thermal conditions was performed and the diges-ter feeding interrupted in order to not upset the bacterial food-

chain (Bolzonella et al., 2003a,b). Fig. 2 shows this particularsituation.

Table 2 reports the main operational conditions and the processyield as well as a detailed characterisation of the digested sludge.

Considering the period at 47 �C and comparing the results at fulland pilot scale, it turns out clear that the two reactors operated atsimilar OLR and HRT and the stability parameters were similar: inparticular, the pH showed an average value of eight and the samewas for ammonia concentration (some 4 g/l).

On the other hand, the TS and TVS content at pilot scale waslower than the full scale while the VFA concentration was higher:this let suppose a better degradation of the substrates, confirmedalso by the results in terms of biogas production.

In fact, the specific gas production was a slightly higher in thepilot scale reactor than at full scale, 0.54 m3/kg TVSfed instead of0.45 m3/kg TVSfed and with a methane content of 58.8% insteadof 52.3%. These better results can be probably ascribed to the goodmixing achieved in the pilot scale reactor: in fact, as already shown

0

1000

2000

3000

4000

5000

6000

0 20 40 60 80 100 120 140days

mgN

/l

N-NH4 digestor N-NH4 liquid manure

6

6.5

7

7.5

8

8.5

0 20 40 60 80 100 120 140

days

pH

pH digestor pH liquid manure

a

b

Fig. 3. Ammonia concentration (a) and pH (b) of digested sludge and influent liquidmanure in pilot plant.

0.20.30.30.40.40.50.50.60.60.70.7

0 20 40 60 80 100 120 140 160days

m3/

kgV

S

SGP pilot plant SGP full scale

35

40

45

50

55

60

65

70

75

0 20 40 60 80 100 120 140days

%

% CH4 pilot plant % CH4 full scale plant

a

b

Fig. 4. Comparison between biogas specific production (a) and methane content (b)of pilot plant and full scale plant.

548 C. Cavinato et al. / Bioresource Technology 101 (2010) 545–550

by Stroot et al. (2000), this allowed for an improvement in thesubstrates degradation and release of the biogas from the bulk,resulting in an overall increased biogas production.

Ammonia and pH values of the pilot scale reactor were stronglydependent on the substrates characteristics, in particular liquidmanure (Fig. 3a and b).

As for nitrogen concentration, which reached levels of some4–5 g/l, it should be pointed out that at these pH and temperaturelevels, the values for free ammonia (as NH3) were calculated in0.84, 0.80 and 0.90 g N/l, respectively, for the full and pilot-scaleruns. These values were lower than the value found to cause inhi-bition by Hansen et al. (1998) and equal to 1.1 g N/l. On the otherhand, the authors themselves, experienced the operation of fullscale reactor operating at nitrogen concentrations of some 8 g N/lin the bulk and producing some 180 m3 biogas per ton of raw bio-waste treated, therefore showing that no inhibition was occurring(Bolzonella et al., 2006).

From day 70 to day 110 (Fig. 4) the pilot plant specific gas pro-duction was variable. This behaviour was probably due both to thestrategy adopted to achieve the real thermophilic condition, and tothe great heterogeneity of influent wastes combined to the feedingstrategy (i.e., whey with low TVS content, melon and water-melonduring summer period).

In the second pilot-scale run, carried out at 55 �C, pH was con-stant (8.2) while ammonia concentration firstly decreased, mainlybecause of the decrease of the inlet ammonia content, achieving anaverage value of 2429 mg N/l. During the steady state conditions,the specific gas production reached a SGP value of 0.62 m3/kg TVSwith a methane content of 61.6% despite a decrease in the organicloading rate, which passed from some 5.5 to 4.7 kg TVS/m3 d. Thetrends of the stability parameters and the yields obtained at55 �C, confirmed a better digester behaviour operating in theseconditions compared to the results obtained working at 47 �C:VFA decreased to some 300 mg/l compared to the mesophilic con-dition while TS and TVS concentrations in the reactor were thesame observed during the first run.

According to the results of this study it was shown that theanaerobic co-digestion process carried out at 55 �C showed anincrease of 15% in biogas production (SGP from 0.54 to 0.62m3/kg TVS) while all the stability parameters (pH, ammonia andVFA concentrations) showed the reliability of the process. More-over, also the importance of a proper mixing (at pilot scale) wasemphasized.

3.2. Economics of the anaerobic co-digestion strategy

After the experimental evaluation of the optimum workingtemperature, the economic aspects of the anaerobic co-digestionstrategy were considered both in terms of capital costs and ofeffluent treatment costs and the net present value (NPV) wasdetermined.

To calculate the net present value was applied the followingequation:

NPVn ¼ �C0 þ ðbn � cnÞð1:035Þn � 1ð1:035Þn � 0:035

ð1Þ

where n is referred to the year considered, bn is the total amount ofannual benefit, cn the total annual costs and C0 the capital cost.

In this paper the bn was determined as the benefit coming fromthe electric energy selling, also considering the credits for renew-able energy (green certificates).

Taking into consideration the design parameters of the full scaleplant of Marcon and literature data (Bonazzi, 2001), the net presentvalue for implementation of the anaerobic digestion process in amedium size farm was evaluated.

The average inlet flow treated was of 38 m3/d of liquid manurefrom pigs/milk cows farming, and about 70 ton/d of maize. Withthis loading conditions, and considering the optimal specific gasproduction (at 55 �C, 0.62 m3/kg TVS) the anaerobic process ledto a daily biogas production of 10,200 m3/d. After the evaluationof the benefit of the electric energy selling and the cost of the

Table 3Evaluation of capital cost and profit of a medium size biogas plant.

Capital cost € 3,000,000m3 biogas produced m3/d 10,200Cost EE + GC €/kW h 0.22Electric energy produced MW h/yr 8789

BenefitElectric energy selling + benefit from GC €/yr 1,933,473

Total €/yr 1,933,473

CostService CHP unit €/kW h 0.01

€/yr 87,885Staff (2 qualified labor) €/yr 70,000Maize cultivation €/yr 420,000EE used €/yr 52,000General €/yr 90,000

Total €/yr 719,885

0 1 2 3 4 5 6 7 8 9 10 11 12 13

years

Fig. 5. Net present value of the investment for the biogas plant.

0 1 2 3 4 5 6 7 8 9 10 11 12 13

years

30%

60%

90%

Fig. 6. Net present value of the investment for the biogas plant considering theeffluent treatment cost for nitrogen removal.

C. Cavinato et al. / Bioresource Technology 101 (2010) 545–550 549

Green Certificates (0.22€/kW h, Italian financial law 2008), in Table3 are reported the economics input and output for the estimationof the net present value (Fig. 5).

The capital cost estimated was 3 million €, inclusive of all theequipments (among the others, 1 MW CHP unit and two digesterswith a total volume of about 5000 m3), but, considering the benefitcoming from the electric energy selling, the net present value was2.5 yr.

Following the Nitrates Directive (Commission of EC, 1991)where vulnerability of the water bodies was taken into account,the effluent treatment for nitrogen removal become the mainproblem for farmers. It is well known that the anaerobic digestionprocess convert the protein’s organic nitrogen into high amount ofammonia, and for cattle manure and similar waste, the averageammonia content is about 4–4.5 g N/l (Hansen et al., 1998; Ahringet al., 2001).

There are several papers in scientific literature where differentbiological and chemical alternatives for nitrogen removal werestudied and compared, for example some authors (van Loosdrechtand Salem, 2006; STOWA, 1996) have analysed the biological treat-ments of sludge digester liquids, comparing processes like theSHARON process, ANNAMOX process, N-removal over nitrite, alsoin economics terms. Bortone (2008) studied the N-removal effi-ciency from piggery wastewater, through the integrated anaero-bic/aerobic treatment showing the economics advantage ofcoupling anaerobic digestion and SBRs.

These studies presented an average cost for nitrogen removal of4€/kg N removed: this value was used in this work to estimate theannual cost for the digester effluent treatment. Considering thesame biogas plant data, the annual nitrogen production was158 ton/yr.

In Fig. 6 are shown the net present values of the investment inthree different situations: removal of 30%, 60% or 90% of nitrogenload in the anaerobic digestion effluent.

The same calculation of the net present value was made takinginto account the nitrogen removal cost. The pay-back time in thefirst condition (30% nitrogen removal) was similar to the initialcondition (3 yr); when the percentage of removed nitrogen riseup to 90% the pay-back time became about twice the amount.

4. Conclusions

The results of the present study showed that biogas productionfrom the co-digestion of cattle manure and other organic wasteswas increased when operating at proper thermophilic conditions(55 �C) and also a general improvement in digester behaviour isclear considering the stability parameters. Further, the economicaspects of the co-digestion strategy were considered: the resultsshowed that the net present value of the investment, consideringonly the anaerobic digestion, was 2.5 yr. If the effluent treatmentfor nitrogen removal was also considered in the calculation for a30%, 60% and 90% efficiency, the net present value of the invest-ment observed was 3–5 yr, respectively.

Acknowledgements

This research was realized thanks to the funding of the ItalianMinistry of University and Research under the Project PRIN2007.Andretta’s farm, Marcon-Venice, where the full scale plant is lo-cated, is also acknowledged for the kind hospitality during theexperimental work.

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