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Renewable Energy 71 (2014) 263e270

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Renewable Energy

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Energy production from piggery waste using anaerobic digestion:Current status and potential in Cyprus

Elisavet Theofanous a, Nicoletta Kythreotou b, Gregoris Panayiotou a, Georgios Florides a,Ioannis Vyrides a,*

aCyprus University of Technology, Department of Environmental Science and Technology, 95 Eirinis Str., P.O. Box 50329, 3603 Lemesos, CyprusbDepartment of Environment of the Ministry of Agriculture, Natural Resources and Environment, Cyprus

a r t i c l e i n f o

Article history:Received 4 November 2013Accepted 6 May 2014Available online 10 June 2014

Keywords:Anaerobic digestionCyprusPiggery wasteEnergy generationBiogas production

* Corresponding author. Tel.: þ357 25 002218.E-mail address: ioannis.vyrides@cut.ac.cy (I. Vyrid

http://dx.doi.org/10.1016/j.renene.2014.05.0030960-1481/� 2014 Elsevier Ltd. All rights reserved.

a b s t r a c t

In this work the current status and the potential of biogas production and energy generation through theanaerobic digestion of piggery waste in Cyprus are presented. The onsite use of anaerobic digestion fortreating piggery waste not only generates renewable energy, but it is also a sustainable waste man-agement solution. The actual values of the biogas production (20,475 m3/day) and the energy generationare compared with the theoretical values, which are in line with several units. The value 20 m3/tonne ofpig waste was found to predict more accurately the biogas, heat and electricity production compared tothe value of 36 m3/tonne of pig waste. Moreover, an empirical equation (R2¼ 0.9939) is proposed forcalculating the biogas production per day, according to the volume of pig waste treated per dayBGP¼ 14.64 (PWT)þ 535. The potential biogas production from the total pig population of Cyprus equalsto 29,734,356 m3/yr and the potential thermal and electrical energy are calculated to be 90.85 GWhth/yrand 63.59 GWhel/yr, respectively. Finally future alternatives on anaerobic digestion in Cyprus are pre-sented such as co-digestion, centralized anaerobic digestion, hydrothermal pre-treatment, possible useof fuel cells and efficient utilization of pig slurry.

� 2014 Elsevier Ltd. All rights reserved.

1. Introduction

Cyprus has no indigenous energy sources and thus its powersystem is totally isolated and depends completely on imported oilwhich contributes to 96.4% of total primary energy supply and 100%of electricity production. Given the increasing prices of fossil fuelson a worldwide level, the need to include Renewable EnergySources (RES) into the current energy system is becoming imper-ative. During the past years, due to the efforts of both governmentalagencies and private investors, many RES production units havebeen installed in Cyprus. These units comprise of photovoltaic (PV)systems, wind parks and biomass-biogas production units. Ac-cording to data from the Cyprus Energy Regulatory Agency (CERA)[1], the production of electricity from RES for 2013 was as follows:230.61 GWh by wind parks, 35.83 GWh, by biomass-biogas unitsand 44.99 GWh by PV systems. The target for Cyprus is that by 2020renewable energy should account for 13% of the total energyconsumed compared to the 3% in 2005.

es).

In 2013, 11.5% of the electricity produced from RES was frombiomass-biogas units. However, the potential of biomass-biogasunits is considerably great and this is the main focus of this work.The first anaerobic digester in Cyprus was installed in 2007 for thetreatment of pig waste. According to the Department of Environ-ment of the Ministry of Agriculture, Natural Resources and Envi-ronment, today, there are 13 biomass-biogas units in Cyprus, ofwhich 12 units use anaerobic digestion (AD) (Fig.1) and 10 units useanimal waste as their main substrate. These 10 units are connectedwith the power distribution grid of the Electricity Authority ofCyprus (EAC) supplying their output. Noteworthy is that, 8 of theAD units, use mainly piggery waste. In Cyprus there are 78 pigfarms (2011) and thus the potential to use AD for biogas productionfrom piggery waste is highly attractive.

During 2003, biodegradable waste was estimated to be 3203tons and their separation by origin is depicted in Fig. 2 [2]. Animalwaste consisted of 59.7% of biodegradable waste of which 82% waspiggery waste. Indeed, Kythreotou et al. [3,4] assessed the totalbiomass potential in Cyprus available for energy production,through AD. They estimated that the total amount of solid andliquid biomass of the specified waste streams was 9.2 million tonswhile the potential biogas production was estimated in two ways,

0

2

4

6

8

10

12

14

2007 2008 2009 2010 2011 2012

Num

ber o

f AD

units

Year

Fig. 1. Number of AD units in Cyprus.

E. Theofanous et al. / Renewable Energy 71 (2014) 263e270264

the Chemical Oxygen Demand (COD) consumption method and theMass of Waste Digested method, was found to be 114 and 697million m3, respectively.

Anaerobic digestion is a natural process in which a variety ofspecies from two biological kingdoms, the Bacteria and theArchaea, work together to convert organic wastes, through variousintermediates into methane gas [5]. During the past 30 years, thetreatment of piggery waste through AD has increased considerablyand the amount of anaerobically digested substrate fromwaste hasincreased at an annual growth rate of 25% [6]. The main advantagesof AD are low biomass generation, low nutrient requirements andenergy production in the form of methane gas. Biogas production,of which about 60% is methane, has considerable potential becauseit may replace fossil fuels and vehicle fuels. Moreover, biogas hasadvantages, compared to other renewable energy alternatives: itcan be produced on demand, is easily stored and it is already amature technology [7]. Besides, the digestate (anaerobic sludge)from AD is a valuable fertilizer because of its high nitrogen content.AD considerably reduces the survival of pathogens, which isimportant when the digested residue is used as fertilizer [8].Inappropriate handling, storage and application of digestate asfertilizer can cause ammonia emissions, nitrate leaching

6.73% 0.18%

29.88%

3.42%0.12%

59.67%

Biodegradable municipal wastesWastewater sludgeFood and drink industriesAgriculture wastesUsed cooking oils

Fig. 2. Biodegradable wastes in Cyprus by source during 2009 [2].

phosphorus overload [7]. The nitrogen load on farmland is regu-lated inside the EU by the EU nitrate-directive (91/676/EEC nitrate).This piece of legislation aims to protect the ground and surfacewater environment from nitrate pollution. A more novel applica-tion is to transform the digestate into biochar, which can be furtherused as soil enhancer or as an adsorbent to purify wastewater orflue gas [6].

Some laboratory studies exist on the treatment of piggery wastein a laboratory scale bioreactor [9,10]. However, only a few studiesinvestigate the treatment of piggery waste on a full-scale level andon a national level.

Aggarangsi et al. [11] found that up-flow digesters in Thailandused to treat piggery wastewere able to produce up to an average of0.261 m3/kg of COD removal. Monteiro et al. [12] studied the pro-spective application of animal manure for bio-energy production inPortugal and found that the biogas, thermal and electrical energyproduced from swine manure were 0.015 Gm3/year, 52.563 GWh/year and 26.282 GWh/year, respectively. Kaparaju and Rintala [13]estimated that in typical Finnish farms producing 2000 m3 sowmanure and 2500 m3 of pig manure the energy production wouldbe 21.8 and 47.7 MWh electricity per year and 36.3 and 79.5 MWhof heat per year, respectively in a Combine Heat and Power (CHP)unit. Tsai and Lin [14] in an overview analysis of biogas productionto energy generation in Taiwan found that the potential of methanegeneration from livestock manure management during 1995e2007was between 36 and 56 Gg per year. Thus, for the total swinepopulation a methane reduction of 21.5 Gg per year, electricitygeneration of 7.2 � 107 kWh/year and equivalent carbon dioxidemitigation of 500 Gg/year were achieved.

Several full-scale studies [11,12] have identified the potential ofusing animal waste for biogas production. However, few studiesanalyse the treatment of piggery waste from full-scale AnaerobicDigesters for biogas, electricity and thermal energy production on anational level. Therefore, the aim of this study was to present thecurrent status of AD from piggery waste in Cyprus with particularattention on biogas, heat and electricity production. Moreover, thisstudy suggests methods to estimate biogas from a full-scaleanaerobic digester treating piggery waste. Based on the current78 pig farm units, the potential of AD in Cyprus for biogas, elec-tricity and thermal energy production is estimated.We also suggestways to extend the use of AD for piggery waste in Cyprus as well asto make it more efficient.

2. Methodology

Data on the characteristics of AD units using piggery waste inCyprus as well as the consequent biogas production were obtainedusing a questionnaire. Questionnaires sought information on thefollowing: animal population per unit, volume of waste (m3/day),volume of the digester (m3), retention time (days), COD (g/l), typeof waste digested, operating temperature (�C), pre-treatmentmethod, post-treatment method, % of CH4 in produced biogas,use of produced biogas, input waste quality and output wastequality. The results are presented in Table 1. Additionally, data onthe energy generation by these units were given by CERA for theyear 2011 [1].

It should be noted that calculations of theoretical values werebased only on complete data and that no calculations were donewhere data were incomplete, to avoid misinformation.

3. Current status

The stages of AD are as follows: (A) Reception and pre-treatmentof piggery waste: it should be noted that pig waste passes through ahomogenization tank before entering the anaerobic digester. In the

Table 1Characteristics of the AD units examined.

AD unit No 1 2 3 4 5 6 7 8 9 10

Animal population 700 sows 1500 sows 1779 sows 500 sows (3050pig prod. annually)

1500 sows(3400 pigprod. annually)

1000 sows(10,800 pigprod. annually)

n/a 1429 sows 850 sows 3000 sows

Volume of waste(m3/day) Given

65e150 113 110e120 31 (tons/d) n/a 70 120(tons/day)

100e120 n/a 230

Volume of waste(m3/day)a calculated

111 115 96 33 (tons/d) n/a 364 n/a 135 n/a 235

DigesterVolume (m3) 3040 10,312 2400 1000 1250 2 x 5000 3683 3040 n/a 7060HRT (days) 25e30 >90 20e30 30 n/a 25e30 n/a 20e25 10e60 30COD (g/l) 55e75 55e75 n/a 40 40e50 n/a n/a n/a n/a 35e45

Type of waste Piggery wastes Piggerywastes

Piggerywastes

Piggerywastes

Piggerywastes

Piggery,slaughterhouse,agriculture anddairy wastes

Piggery,slaughterhouse,agriculture anddairy wastes

Piggerywastes

Piggerywastes

Piggerywastes

Operatingtemperature (�C)

35e40 n/a 35e40 n/a 39 35e40 n/a 37.5e38.5 30e35 35e40

Pre-treatment Collection/homogenisationtank

Collection/homogenisationtank

Collection/homogenisationtank

Collection/homogenisationtank

Collection/homogenisationtank

Collection/homogenisationtank

Collection/homogenisationtank

n/a n/a Collection/homogenisationtank

Post-treatment Aerobic treatmentand storage/irrigation

Aerobic treatmentand desalination,storage/irrigation

Aerobic treatmentand evaporation

Separator,aerobictreatment

Separator,aerobictreatment,irrigation

Separator,aerobictreatment,irrigation

Separator,aerobictreatment,irrigation

Separator,aerobic treatment,irrigation/evaporation

n/a Separator,aerobictreatment,irrigation/evaporation

Biogas% CH4 60 60 n/a 60 60 60 n/a n/a 55e80 n/aUse Combined

electricityand heatinggenerator(CHP unit)

Combinedelectricityand heatinggenerator(CHP unit)

Combinedelectricityand heatinggenerator(CHP unit)

Combinedelectricityand heatinggenerator(CHP unit)

Combinedelectricityand heatinggenerator(CHP unit)

Combinedelectricityand heatinggenerator(CHP unit)

Combinedelectricity andheatinggenerator(CHP unit)

Combinedelectricity andheatinggenerator(CHP unit)

Combinedelectricity andheatinggenerator(CHP unit)

Combinedelectricityand heatinggenerator(CHP unit)

m3 biogas/m3

reactor volume day0.027 0.0025 0.031 0.029 e 0.020 e 0.035 e 0.019

Thermal energy(kWhth/yr)

2,277,000 2,513,518 e 2,500,000 2,974,172 2,696,123 2,826,000 e 2,328,501

Electrical energy(kWhel/yr)

1,510,479 2,513,518 1,121,402 3,336,184 2,974,172 2,696,123 2,826,000 e 1,418,722

Input waste qualityBOD5 (g/l) 25 1778 kg/d 15 20e30 20e30 25 (when

only pig)18 (whenonly pig)

25 (whenonly pig)

10e15 n/a

Solids (g/l) 40 SS 6256 kg/d TS,5005 kg/d VS

n/a TS 5e6%dry matter

TS 5e6%dry matter

SS 53 (whenonly pig)

TS 8.33% TS 4e5% TS 55, VS 38

TKN (g/l) 5.5 0.26 n/a 5 0.35e0.7% 3.8 (whenonly pig)

3.8 (whenonly pig)

4 (whenonly pig)

3 460 kg/d

Output waste qualityBOD5 (g/l) 10 782 kg/d n/a n/a n/a 12.5 (when

only pig)9 (whenonly pig)

10 (whenonly pig)

n/a 0.37

Solids (g/l) 20 SS 55 TS, 44 n/a n/a n/a 25 (whenonly pig)

TS 2.65% n/a n/a VS 1.043

TKN (g/l) 5.5 n/a n/a n/a n/a 3.8 (whenonly pig)

3.8(when only pig)

4 (whenonly pig)

n/a n/a

a Calculated using Q ¼ V/RT; where: V is the volume of digester (m3) and RT is the retention time (days).

y = 14.641x + 535.955 = 0.994

0

1000

2000

3000

4000

5000

6000

7000

0 50 100 150 200 250 300 350 400

Biog

as p

rodu

c�on

(m/

day)

Volumetric pig waste treated (Q) (m /day)

Fig. 3. Relation of the volumetric loading rate of waste (m3/day) and the volume ofbiogas produced per day (m3/day).

E. Theofanous et al. / Renewable Energy 71 (2014) 263e270266

tank the waste is flushed throughout the day to ensure uniformconsistency and constant loading rate and then good operatingconditions in anaerobic digesters. The COD, BOD and TS valuesvaried between 34 and 75 g COD/L, 10e25 g BOD/L and 40e83 g TS/L, respectively (Table 1). (B) Anaerobic digestion: the piggery wasteis introduced into the digester and operated under mesophilicconditions (Table 1). At the top of the digester, biogas is obtained; atthe bottom, some sludge is aerobically treated; at the middle, theresidual liquid is transported to the stabilization area where thewater gets purified. (C) Purification of the biogas: after leaving thedigester, the biogas is treated to decrease its content in watervapour and H2S. Several methods are used at different AD plants,such as adsorption in powder activated carbon and creation ofmicro-aerophilic conditions. (D) Generation of heat and electricity:the biogas is introduced to CHP engine where heat and electricityare produced. When the CH4 is burnt, CO2 is obtained. Althoughboth gases contribute to the greenhouse effect, the contribution ofCO2 is 21 times smaller than that of CH4; it is therefore preferable toburn the CH4 and allow the CO2 to escape. (E) Aerobic treatment: it isused to stabilize water flowing out of the digester. A mechanicalseparator is often used in combinationwith an evaporation tank forthe liquid fraction and natural composting for the solid fraction(Table 1).

The current status of the biogas production per AD unit exam-ined is presented in Table 2. As shown, this ranges between 787 and5760 m3/day while, the total biogas production is 20,475 m3/day.The values for volume of waste (m3/day) fluctuate significantly andthus it cannot be stated with accuracy (Table 1). For this reason, thevolume of waste treated (m3/day) is calculated based on the rela-tion between the volumes of the bioreactor divided by the reten-tion time (m3waste/RT). The volumetric loading rate and the biogasproduction per day follow a linear trend (R2 ¼ 0.9939) (Fig. 3). Thefollowing empirical equation is proposed BGP¼ 14.64 (PWT)þ 535to calculate the biogas production per day (BGP in m3/day) pervolume of pig waste treated per day (PWT m3/day). To the best ofour knowledge, this is the first study to find a precise correlationbetween the volume of waste treated per day and the biogas pro-duction per day for pig waste (are correlated precisely) (R2¼ 0.994).The proposed equation can be used by engineers to estimate thepotential of biogas production from pig farms in Cyprus using AD.

The energy generated by the AD units using piggery waste isdivided into thermal and electrical energy. The thermal energygenerated is used to cover internal needs of the units, such asheating of water and space heating. Almost all the generatedelectricity (80%) is supplied to the EAC power distribution grid andonly 20% is used internally in the units. In 2011, the total installedelectric power was 8667 kW while the electricity generated was51,610,196 kWhe (Table 3). It should be noted that in 2011 the in-ternal consumption of electricity was 11,897,882 kWhe and the totalelectricity supplied to the EAC power distribution grid was

Table 2Biogas production (m3/day).

AD Units No Biogas production (m3/day)

1 22602 23703 19004 8755 e

6 57607 7878 24239 e

10 4100Total 20,475

39,712,314 kWhe (35,830,571 kWhe in 2013). Also, the totalinstalled thermal power was 8892 kWand the total thermal energygenerated was 49,733,004 kWhth.

4. Theoretical calculation of biogas production and energygeneration

Several theoretical methods for calculation of the biogas pro-ductionwere compared with the actual biogas values in order to bevalidated. Calculating the theoretical production of biogas andenergy was done with equations proposed by Kythreotou et al.[4] which are presented below. The volume of pig waste treated(m3/day) values used for the theoretical calculation of the biogasproduction and the energy generation are those calculated(Q ¼ V(digester)/RT) due to their increased accuracy in comparisonwith the given ones.

4.1. Theoretical biogas production

In this method the theoretical production of biogas was esti-mated according to (i)Method I: the COD consumed and (ii)MethodII: the mass of digested waste, where in both cases the availablebiomass is assumed to be completely digested. The formulas usedare:

(i) COD

BGwst ¼ Mwst=BDwst � CODwst � GFBG

where BGwst is the volume of biogas produced in m3 from theanaerobic digestion of the piggery waste (taken from Table 1),Mwstis the mass of piggery waste in kg (taken from Table 1), BDwst is thebulk density of the piggery waste in kg/l (for piggery waste is0.973 kg/L, [16]), CODwst is the COD concentration of the piggerywaste in g/L when unknown a value of COD then 40 kg/L is used[17]), and GFBG is the volume of biogas produced in m3 per kg CODconsumed (0.55 m3/kg COD).

Table 3Total installed power and energy generation (thermal and electrical) for 2011.

Total installed electric power (kW) 8667Total electricity produced (kWhe) 51,610,196Internal consumption of electricity (kWhe) 11,897,882Electricity supplied to the EAC power distribution grid (kWhe) 39,712,314Total installed thermal power (kW) 8892Total thermal energy produced (kWhth) 49,733,004

E. Theofanous et al. / Renewable Energy 71 (2014) 263e270 267

(ii) Mass of waste digested

BGwst ¼ Mwst � GFBG

where BG is the volume of biogas produced in m3 from the

wstanaerobic digestion of the piggery waste (taken from Table 1),Mwstis the mass of piggery waste of a particular source in kg (taken fromTable 1) and GFBG is the volume of biogas produced per kg of pig-gery waste (m3/kg), where two different values were used. Morespecifically, the values used were those given by the Department ofEnvironment in Cyprus (36 m3/tonne) [2] and those proposed bythe National Centre for Biorenewable Energy (NNFCC), UK (20 m3/tonne) [15].

As shown in Fig. 4, the value, 20 m3/tonne, proposed by theNNFCC is much more accurate than that proposed, 36 m3/tonne, bythe Department of Environment in Cyprus. These values wouldhave an error of on average 4.9% and 85.7% respectively. The resultsindicate that when COD (Method I) is used to estimate the biogasproduction, a total error is estimated to be about 57.7%. This is dueto the high variation of the COD of pig waste in Cyprus. Severalunits indicated COD values that varied widely (Table 1).

Moreover, the results for AD unit 6 and 7 show that the theo-retical values in both methods are much higher than the actualvalues. Despite these findings, the results from the AD unit 6 are inlinewith the equation BGP¼ 14.64 (PWT)þ 535 (R2¼ 0.994). Thesesuggest that the retention time of the pig waste and the volume ofthe bioreactor PWT¼Volume of Digester/Retention time are thetwo important parameters to estimate biogas produced from pigwaste per day in Cyprus. The AD digester volume is between 1000and 10312m3 (Table 1). Therefore, by alternating the retention timein AD units, the biogas produced per day could be calculatedthrough the equation BGP ¼ 14.641 (PWT) þ 535. However, theretention time cannot be substantially decreased because hydro-lytic microorganisms would not have enough time to break downpig waste. Hydrolysis is an important reaction in AD because itbreaks down complex compounds into smaller and simpler sub-strates. In this process, macromolecules such as carbohydrates,proteins and lipids are initially hydrolysed by extracellular enzymessuch as cellulases, proteases, and lipases into sugars, alcohols,amino acids and fatty acids. The retention time for the AD plans inCyprus varies between 10 and 90 days (Table 1).

Besides the relatively low yield of biogas per tonne of pig waste,compared with other waste reported in the literature [15], can beattributed to the antibiotics present in piggery waste and the highconcentration of free ammonia. Antibiotics are widely used in most

Fig. 4. Actual and theoretical b

pig farms in Cyprus to prevent infections, treat diseases as well as topromote growth. Between 17% and 76% of antibiotics administeredto animals are excreted via urine and faeces in an unaltered form oras metabolites of parent compounds [16]. In a laboratory scalestudy [16] it was found that antibiotics such as oxytetracycline(OTC) and chlortetracycline (CTC) substantially decrease AD per-formance and their IC50 are about 9 mg/L. However, a moredetailed analysis of the fate of antibiotics from piggery waste inCyprus is needed. Mass et al. [17] reported that penicillin decreasedCH4 production from anaerobic digestion of pig manure by 35% atmesophilic temperatures, while, Arikan et al. [18] reported thatterramycin decreased total biogas from anaerobic digestion of cowmanure by 27%, but did not affect CH4 content or the degradationrate of volatile solids or soluble organic. Several other studies alsoreported that CH4 production in anaerobic digesters was negativelyaffected by high concentrations of antibiotics [19,20].

4.2. Theoretical thermal and electrical energy generation

To calculate the theoretical thermal and electrical energy gen-eration, the following assumptions weremade: methane content inbiogas is 60%, efficiency of CHP generator is 50% (thermal) and 35%(electrical), and biogas calorific value is 22 MJ/kg [4]. The formulaeused are:

ETH ¼ BG� EFTH � ENBiogas�3:6

Where ETH is the thermal energy production in kWh, BG the totalbiogas produced according to each method used in m3, EFTH thethermal efficiency of the generator in % and ENbiogas is the energydensity of biogas in MJ/kg.

EEL ¼ BG� EFTH � ENBiogas�3:6

where EEL is the electrical energy production in kWh, BG the totalbiogas produced according to each method used in m3, EFEL theelectrical efficiency of the generator in %, and ENbiogas is the energydensity of biogas in MJ/kg.

As results in Figs. 5 and 6 show, the value of 20 m3/tonne can beused to estimate the thermal and electrical energy more accurately.However, for AD unit 6, the actual value and the estimated valuesdiffer significantly. The biogas produced at unit 6 is in line with thelinear equation so the difference at thermal and electrical energycannot be due to the low biogas but it is likely due to technicaloperation of CHP engine at unit 6.

iogas production (m3/day).

0

20,00,000

40,00,000

60,00,000

80,00,000

1,00,00,000

1,20,00,000

0 1 2 3 4 5 6 7 8 9 10

Elec

tric

al e

nerg

y ge

nera

�on

(KW

h/ye

ar)

AD UnitsActual ELectrical Method I Method II (36m3/tonne) Method II (20m3/tonne)

Fig. 6. Actual and theoretical electrical energy generation (kWhel).

E. Theofanous et al. / Renewable Energy 71 (2014) 263e270268

5. Potential biogas production and energy generation frompig waste

The potential biogas production and energy generation throughthe AD of piggery waste are presented in this section. The total pigpopulation in Cyprus is 463,357 pigs, and numbers and pig type inall pig units are presented (Table 4). Moreover, the total potentialproduction of pig waste per day according to each pig type iscalculated according to the pig waste production per day perdifferent pig type given by Refs. [21,22]. The total potential pro-duction of pig waste was first calculated per unit of pig farm. Theequation BGP ¼ 14.64 (PWT) þ 535 was used to calculate the po-tential biogas for each pig farm. Finally, with the equations ofSection 4.2, the potential thermal and electrical energy are calcu-lated and the results are presented in Table 5.

The results of Table 5 show that the possible AD of waste fromthe entire pig population in Cyprus may be a highly attractive aswell as advantageous objective. Not only could a large amount ofenergy be generated in a renewable manner (90.85 thermal and63.59 electrical GWh/yr) but the problem of managing waste andits consequential problems would be addressed. The potentialamount of electrical energy production is very high, two times thanthat produced by all PV systems, and is one-third of the energyproduced by all wind turbines in Cyprus. The treatment of piggerywaste through AD can increase the contribution of biomass torenewable energy from 11.5% to 20.4%. This percentage can befurther increased if other type of organic waste such as municipalorganic waste, poultry waste and agriculture waste is treatedthrough AD. This study however, focuses on the AD treatment ofpiggery waste only.

6. Future alternatives for AD in Cyprus

6.1. Co-digestion

Co-digestion is the simultaneous digestion of a homogeneousmixture of two or more substrates. It provides improved nutrientbalance from a variety of substrates, which helps to maintain astable and reliable digestion performance and produce a goodquality fertiliser digestate. The addition of co-substrates with ahigh-methane potential increases gas yields [23] and the reductionof GHG emissions from manure and organic wastes [7]. Co-digestion of piggery waste with other waste can provide nutrientand improve the rheological qualities of the substrate [24]. Gele-genis et al. [25] attempted to optimise biogas production fromolive-mill wastewater by co-digesting it with diluted poultrymanure in mesophilic conditions. The results showed that thebiogas production was slightly higher when olive-mill wastewaterwas added to diluted poultry manure up to a critical concentration

0

20,00,000

40,00,000

60,00,000

80,00,000

1,00,00,000

1,20,00,000

1,40,00,000

0 1 2 3 4 5 6 7 8 9 10

Ther

mal

ene

rgy

gene

ra�o

n (K

Wh/

year

)

AD unitActual Thermal Method I Method II (36m3/tonne) Method II (20m3/tonne)

Fig. 5. Actual and theoretical thermal energy generation (kWhth).

(about 40%) after which the production decreased. In a laboratorystudy, Monou et al. [26] investigated the AD of potato processingwastewater and its co-digestion with pig slurry and/or slaughter-house wastewater in small-scale experiments. They found that pigslurry significantly improved the process while the co-digestionwith abattoir wastewater did not improve the digestion processdue to poor buffering and low pH value. Moreover, the high lipidscontained in the slaughterhouse waste can contribute to cloggingproblems, adsorption to biomass (causing mass transfer problems)and microbial inhibition due to the slow degradation of long chainfatty acids (LCFA). Therefore, if pig waste is to be co-digested, theadditional other waste must be carefully selected. The co-digestionsources could range from nearby pig farms to industrial organicwaste such as distillery waste, olive oil waste, brewery waste andother agricultural waste. Moreover, co-digestion of piggery wastewith sewage sludge could be another option in Cyprus although themain concern in wastewater treatment plant is the wastewater it-self, not energy generation. Cultivation of energy crops [27] andtheir use in AD is a practice applied in Sweden but the small area ofCyprus excludes this possibility.

Apart from this, Zhang et al. [28] showed that the anaerobic co-digestion of goat manure with corn stalks and rice straw substan-tially improved biogas production at all carbon-to-nitrogen (C/N)ratios. However, anaerobic co-digestion of goat manure with wheatstraw slightly increased biogas production due to the high lignincontent in wheat straw.

6.2. Centralized anaerobic digestion

In Cyprus, most pig farms are of low to medium capacity(Table 5) and in such farms the installation of anaerobic digestersand CHP engines is not financially viable. The electrical and thermalpower produced from such farms would be low. Therefore,centralized AD for these farms could be a suitable way forward forboth waste management and supplying the energy needs of thesefarms. Moreover, there are benefits in using this cooperativearrangement in terms of substrate supply, economics and sustainedgas supply [23]. However, both the geographic locations and theavailable road connections need to be considered; most pig farmsare situated in the south east of Cyprus.

6.3. Hydrothermal pre-treatment

During hydrothermal pre-treatment, organic matter dissolvesand hydrolyses and parts of solid organic matters are liquefied intolow molecular weight organic matter and therefore, the COD insupernatant increases. In a laboratory study Qiao et al. [29] foundthat after hydrothermal pre-treatment at typical conditions (170 �C

Table 4Analysis of the potential volume of pig waste produced and energy generated according to the type of pigs of each unit.

Number ofpig unitsin Cyprus

Pig units separationbased on thenumber of sows

Average number of pigs per type per unit Volume of pig wasteproduced per unit(m3/day)

Biogasproductionper unit(m3/day)

Electricalenergygeneratedper unit(kWh/year)

Thermal energygenerated perunit (kWh/year)

Sows Averagereplacementpigs

Averageboar

Piglets(<20 kg)

Piglets(20e50 kg)

Fatten pigs(50e120 kg)

18 0e100 16 0 1 554 787 1170 15.3 761 594,333 849,04815 101e300 194 9 8 791 427 900 14.4 746 583,012 832,87419 301e500 408 37 5 1591 898 1688 28.5 953 744,342 1,063,3456 501e700 612 36 17 2222 1477 2082 39.5 1115 870,829 1,244,04112 701e1000 858 105 18 3307 2057 2923 56.3 1360 1,061,949 1,517,0703 1001e1300 1061 260 6 3429 1057 1292 42.0 1151 898,621 1,283,7456 1301> 1432 135 13 6241 3848 5202 99.7 1995 1,558,093 2,225,847

Production of pig waste per day for different pig types according to Refs. [19,20]:� Per Sow: 0.015 m3/day� Per average replacement pig: 0.0108 m3/day� Per average boar: 0.016 m3/day� Per piglet<20 kg: 0.0019m3/day� Per piglet (20e50 kg): 0.006 m3/day� Per fatten pig (50e120 kg): 0.008 m3/day

E. Theofanous et al. / Renewable Energy 71 (2014) 263e270 269

at 1 h), the biogas production of pig manure increased by 7.8%. Theheat energy output generated by CHP engine could be used for thispurpose. Moreover, by using hydrothermal pre-treatment thepathogens microorganisms could be considerably reduced.

6.4. Fuel for fuel cells

Biogas in Cyprus is only used in engine-based CHP plants(Table 1). However, when the cost of fuel cells decreases thistechnology will constitute another future alternative for Cyprus.For a biogas application, suitable fuel cells are that of solid oxidefuel cells (SOFC) and molten carbonate fuel cells (MCFC). A hightemperature MCFC is coupled to a full-scale biogas digester anddelivers electricity and heat to the District of Böblingen in Leonberg(Germany) [30]. The fuel cell has an electric capacity of 245 KW(with approximately 47% conversion) and a heat capacity of170 KW.

6.5. Upgrading and injection in the natural gas

Natural gas has been detected in the south coast of Cyprus, andit is expected that natural gas will be themain resource of energy inCyprus during the next decade. An alternative use of biogas is itsupgrading to natural gas quality (biomethane) and injection intothe natural gas grid. Because biogas cannot always be exploitednearby the production facilities, in farming areas, injecting upgra-ded biogas such as biomethane into the natural gas increases theopportunities to transport and use biogas in areas of high-energyconsumption, i.e. in towns. However, this cannot be implementedwithin the next five years because natural gas facilities need to beinstalled first.

6.6. Upgraded and use as vehicle fuel

Natural gas vehicles can also be powered by biogas providedthat the biogas is upgraded to natural gas quality [31]. The upgrade

Table 5Total potential energy generation through the anaerobic digestion of piggery wastes.

Potential electrical energy generation (kWhel/day) 174,242Potential annual electrical energy generation (GWhel/yr) 63.50Potential thermal energy generation (kWhth/day) 248,917Potential annual thermal energy generation (GWhth/yr) 93.85

is essential when used as transport fuel because high contaminantlevels can destroy a car engine and damage the integrity of thenational gas grid. This has successfully been done in Sweden since1995. Initially, only fossil gas was used, but now biogas is the mainsource of vehicle gas. Sweden expects to achieve its EU-RED targetpartly using biogas as vehicle fuel [27]. In order to do the same inCyprus, the installation of natural gas facilities is required firstly.

7. Conclusions

Based on the analysis of the operating anaerobic digestion unitsin Cyprus we conclude the following.

� The volumetric waste treated per day follows a linear relation-ship (R2¼ 0.994) with the volume of generated biogas per day. Itcan be expressed by the equation BGP¼ 14.64 (PWT)þ 535,where BGP is the volume of the biogas produced m3 per day andPWT is the volume of pig waste treated per day in m3/day.

� The value 20 m3/tonne of pig waste was found to predict moreaccurately the biogas production, compared with the value of36 m3/tonne of pig waste and the method using the COD.

� The potential thermal and electrical energy generated by theCHP engines ranges between 2,277,000e2,974,172 kWh/yearand between 1,121,402e3,336,184 kWh/year, respectively.

� Using the total pig population of Cyprus, it is calculated that thepotential thermal and electrical energy are 90.85 GWhth/yr and63.59 GWhel/yr, respectively. The calculated electrical energyrepresents the 20.4% of the total renewable energy. These resultssuggest that anaerobic digestion could be a very attractive routeto produce energy in a sustainable manner.

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