Study of the energy recovery of animal by-products and analysis of the energy matrix for the energy self-sufficiency of industrial animal slaughter and processing

2021 S udy o he ene gy eco e y o animal by-p oduc s and analysis o he ene gy ma ix o he ene gy sel -su iciency o indus ial animal slaugh e and p ocessing Mas e 's Deg ee in Renewable Ene gies, Uni e si y o La Laguna José Ay hami Pé ez Remedios Tu o s: Ka ina El i a Rod íguez Espinoza Jose F ancisco Gómez González Abs ac Tene i e is one o he main islands o he Cana y Islands, which, due o i s cha ac e is ics as he ou e mos egion, has a high ene gy dependence as well as a limi a ion on a ailable e i o y; in addi ion o being designa ed as a Remo e Zone, he elimina ion o Animal By-P oduc s (ABPs) in land ills is pe mi ed. This ea men does no con ibu e o he cu en end o a ci cula economy and nega i ely ha ms he en i onmen . The ene gy eco e y o his was e h ough anae obic diges ion o he p oduc ion o biogas would enhance he use o enewable ene gies, con ibu ing he mea indus y o ene gy independence and be e managemen o he was e gene a ed, p omo ing an ene gy ansi ion owa ds cleane ene gies in line wi h he Sus ainable De elopmen Goals, especially SDG 7, "A o dable and non-pollu ing ene gy". The s udy o he po en ial o biome haniza ion o hese by-p oduc s has been ca ied ou bo h sepa a ely and in co-diges ion in sea ch o he bes biogas p oduc ion. O he samples s udied, only biogas was ob ained in he anae obic diges ion o he umen con en , sewage sludge and o he co-diges ion o isce a (ca le, pigs, goa s, sheep and abbi ), aw blood and sewage sludge. Highligh ing, o he la e shows a p oduc ion o 972 mL o biogas / g VS o he mix u e and his would ha e a o al o 499 MWhe o elec ical ene gy o he es ima ed was e o Tene i e du ing he yea 2019. Keywo ds: was e- o-ene gy, Tene i e, anae obic diges ion, animal by-p oduc s, slaugh e house, enewable ene gy Table o con en s 1. In oduc ion ........................................................................................................................... 1 1.1. The case o Cana y Islands ............................................................................................ 3 2. Ma e ials and me hods .......................................................................................................... 4 2.1 Quan i ica ion o animal by-p oduc s in Tene i e ......................................................... 4 2.2 Anae obic diges ion....................................................................................................... 4 2.2.1 Raw ma e ials ........................................................................................................ 4 2.2.1 P epa a ion o biodiges e ..................................................................................... 4 2.2.2 Reac i a ion o biodiges e .................................................................................... 5 2.2.3 Quan i ica ion o gases .......................................................................................... 6 2.2.4 Composi ion o gases ............................................................................................ 6 2.2.5 Analysis o o al, ola ile and ixed solids. ........................................................... 6 3. Resul s and discussion ........................................................................................................... 8 3.1 Quan i ica ion o animal by-p oduc s in Tene i e ......................................................... 8 3.2 Anae obic diges ion....................................................................................................... 9 3.2.1 Gas p oduc ion ...................................................................................................... 9 3.2.2 Gas composi ion .................................................................................................. 15 3.2.3 To al, ola ile and ixed solids ............................................................................ 15 3.3 Ene gy p oduc ion o biogas ....................................................................................... 16 4. Conclusions ......................................................................................................................... 19 5. Acknowledgemen s ............................................................................................................. 20 6. Re e ences ........................................................................................................................... 21 Figu es Figu e 1. G ow h o mea p oduc ion by egion and mea ype in 2029 om 2017 and 2019 a e age da a o he wo ld o each animal. .................................................................................. 1 Figu e 2. Accumula ed gas p oduc ion o umen con en samples. .......................................... 11 Figu e 3. Accumula ed gas p oduc ion o sewage sludge samples. .......................................... 11 Figu e 4. Accumula ed gas p oduc ion o aw blood samples. ................................................. 12 Figu e 5. Accumula ed gas p oduc ion o dehyd a ed blood samples. ....................................... 12 Figu e 6. Accumula ed gas p oduc ion o isce a samples. ...................................................... 13 Figu e 7. Accumula ed gas p oduc ion o isce a and aw blood samples. .............................. 13 Figu e 8. Accumula ed gas p oduc ion o isce a and dehyd a ed blood samples. .................. 14 Figu e 9. Accumula ed gas p oduc ion o isce a, aw blood and sewage sludge samples. ..... 14 Tables Table 1. Pa ame e s used o es ima e quan i y o animal by-p oduc s. ........................................ 4 Table 2. Composi ion o he samples acco ding o he ypes o animal by-p oduc s. .................. 5 Table 3. Samples eac i a ed in his s udy. .................................................................................. 6 Table 4. Es ima ion o he quan i y o animal by-p oduc s, in kilog ams, by each animal p oduced in he slaugh e house. .................................................................................................................... 8 Table 5. To al o animal by-p oduc s in Tene i e in 2019. ........................................................... 9 Table 6. Gas p oduc ion and pe iod o ime by sample. ............................................................. 10 Table 7. A e age and maximum me hane composi ion ob ained o each sample. ..................... 15 Table 8. Solid con en be o e and a e anae obic diges ion. ..................................................... 16 Table 9. Dec ease in ola ile solid. ............................................................................................. 16 Table 10. Ene gy es ima ion esul s ob ained. ............................................................................ 17 Table 11. Biochemical me hane po en ial o each sample s udied. ............................................ 18 1 1. In oduc ion Highe popula ion g ow h, globaliza ion and echnological de elopmen s p oducing an inc easing o ene gy demand, was e gene a ion and an h opogenic emissions [1]. The ene gy p oduc ion scena ios a e e y pollu ing because o majo i y a e p oduced wi h ossil uels, a non- enewable uel. In Spain, only he 37.5% o he elec ic ene gy comes o enewable sou ces [2]. In 2017, o example, 73.9 % o he uel needed o gene a ing p ima y non- enewable ene gy was ob ained om Alge ia, Saudi A abia, Nige ia, Mexico and Pe u, among o he s [3]. On he o he hand, was e is e y impo an because he land ill uses up a la ge amoun o land ha could be used o o he p oduc i e ac i i ies such as ag icul u e. On he o he hand, he Spanish ax on land ill was highe han 50% in 2019 [3], [4]. In addi ion, land ills cause en i onmen al p oblems as bad smells and me hane emissions, one o he mos pollu an gases. Land ills a e no he bes ea men o was e, because hey do no ollow he ci cula economy whe e was e is ans o med in o a esou ce o be used again in he same p ocess o in o he p ocesses. One o he mos widely used ea men s oday is o ans o m his was e in o ene gy, known in he li e a u e as was e- o-ene gy (WTE). The e a e di e en was e- o-ene gy al e na i es o p oduce ene gy om biomass sou ces, g ouped in he mochemical p ocesses and biocon e sion p ocesses [5]. Whe e adequa e echnology depends on he biomass cha ac e is ics and p ope ies. Biocon e sion p ocesses, such as anae obic diges ion, a e mo e app op ia e o biomass sou ces con aining mo e han 50% mois u e, whe eas he mochemical p ocesses, such as biomass combus ion, a e a be e op ion. Al hough, he e a e o he echnologies wi h less ma u i y like py olysis, gasi ica ion, and e men a ion [5]. The high nu i ional alue o mea , i.e. high p o ein, bioa ailable mine als and i amins con en , esul s in an inc easing demand o li es ock p oduc s and consequen ly, in inc easing animal by- p oduc s gene a ion o igina ing in slaugh e houses [6]. Managemen o animal by-p oduc s o ood indus y is e y impo an o ensu e he sa e y o he human and animal ood chain [7]. The animal by-p oduc s (ABPs) in slaugh e houses a e was e om he p ocess o mea indus y. The Figu e 1 shows ha mea p oduc ion is g owing, and acco ding o he Food and Ag icul u e O ganiza ion (FAO), in 2029 he e is going o be an 80% inc ease compa ed o he a e age alues o he 2017-2019 se ies. Fo ha , he amoun o was e p oduced will also ise. Figu e 1. G ow h o mea p oduc ion by egion and mea ype in 2029 om 2017 and 2019 a e age da a o he wo ld o each animal. Sou ce: [8]. Then, ABPs a e necessa y sani a y ules o p e en and minimize isks o public and animal heal h o p ese e he sa e y o he ood and he animal chain [9]. The e a e wo ules, one o his is Regula ion (EC) 1069/2009, which ABPs a e ca ego ises based on hei isk[10] and Regula ion (EC) 142/2011, which es ablishes heal h ules o animal by-p oduc s and de i ed p oduc s no 2 in ended o human consump ion [11], [12]. This egula ion was adap ed o Spanish legisla ion by Royal Dec ee 1528/2012 [13]. ABPs a e classi ied in o h ee g oups (ca ego ies 1, 2 and 3) acco ding o Regula ion (EC) 1069/2011, whe e dec easing o de signi ies an inc ease in dange ousness. Following ha , Regula ion (EC) 142/2011 desc ibes he managemen o hese by-p oduc s, such as he necessa y p e- ea men o use hem in an applica ion, such as ene gy eco e y o animal ood. As ene gy p e- ea men is necessa y o ensu e ood sa e y in he p ocess, a oiding he ans e o possible diseases om animals o people, hese p e- ea men s can be he mal o he mochemical [14]. Nume ous s udies ha e been conduc ed o examine he e ec o p e- ea men on biogas p oduc ion in o de o ind he bes me hod o imp o ing anae obic diges ion pe o mance. Ca e e e al. ha e ca ied ou a s udy o he di e en p e- ea men s, obse ing ha wi h he mochemical p e- ea men s, be e esul s a e ob ained in he p oduc ion o biogas [14]. I is highligh ed ha wi h animal by-p oduc s, he he mochemical p e- ea men s p esen be e esul s on a labo a o y scale, highligh ing he combina ion o he mal p e- ea men and saponi ica ion, due o he high a con en o hese by-p oduc s. Emphasizing ha p e- ea men can imp o e he po en ial o biome haniza ion by up o 50% [15]. The ag icul u e and li es ock was e a e gene a ed in huge quan i ies wo ldwide and possess se ious en i onmen al and heal h isks. The e o e, solu ions ha e been de eloped o his p oblem. Anae obic diges ion is one o he solu ions o con e a was e ha has no alue in o o he by-p oduc s ha do [16]. Cu en ly, mos de eloping coun ies ha e al eady used he anae obic diges ion p ocess as a unique way o implemen was e- o-ene gy echnologies o gene a e clean ene gy [17]. In pa icula , he case o animal by-p oduc s ha a e p oduced om slaugh e houses has a g ea po en ial o he gene a ion o biogas [18], [19]. In his sec o , he e a e nume ous in es iga ions whe e s udies o di e en anae obic diges ion echnologies such as ba ch and semi-ba ch diges ion ha e been ca ied ou [17-20]. In addi ion, he e a e con igu a ions in which he s ages o diges ion a e sepa a ed in o wo eac o s, as is he case wi h he anae obic memb ane diges e [24]. On he o he place, he con ol o anae obic diges ion wi h he egula ion o pa ame e s such as pH and empe a u e is impo an o he op imal e olu ion and g ow h o me hanogenic bac e ia, which p o ide he g ea es composi ion o biogas. Schmid e al. ca y ou a s udy o he e ec o seasonal empe a u e a ia ions on he anae obic diges ion p ocess whe e i is obse ed ha a low ambien empe a u es, hey gene a e a nega i e impac on biogas p oduc ion and p ocess s abiliza ion [25]. Apa om he animal by-p oduc eco e y s udy, he e a e a icles in which ene gy eco e y by anae obic diges ion o slaugh e house sludge is s udied, whe e anae obic up low eac o s ha e been used [26] o s udies o cogene a ion wi h o he ag icul u al elemen s ha allows o be e pe o mance [27]. Expe imen s ha e also been made wi h o he was e such as was e om a poul y indus y [28]. Then, all a e ocused on he op imiza ion and cha ac e iza ion o was e po en ial o in he gene al analysis o he po en ial in a coun y like Indonesia o I an [17], [29]. On he o he hand, he p ocess o anae obic diges ion is a biological p ocess, hen condi ions o how i de elops a e impo an since hey a ec he p oduc ion o biogas by he mic obial communi y. The e o e, wo king condi ions a e unde con inuous in es iga ion as hey depend on each aw ma e ial used in he p ocess. Mao e al. analyse he s udy pa ame e s o anae obic diges ion ha mus be op imized o achie e be e pe o mance, wi h pH, C/N a io, o ganic load and e en ion ime [30]. In addi ion, due o he high ola ile a y acid con en o slaugh e house was e [31] he co-diges ion o hese esidues wi h o he ag o-indus ial was es (such as oli e lea es and o ange peel) and also adjus ing he C/N a io a e s udied [6]. Rod íguez-Abalde e al. s udy he op imiza ion o anae obic diges ion p e iously pas eu ized wi h po k slu y and glyce ine [32]. Ano he al e na i e o co-diges ion has been seen ha i is anae obic diges ion in mul is age whe e p oduc ion is imp o ed. A case is anae obic diges ion in wo s ages uni ied wi h empe a u e a ia ion, he mophilic empe a u es (55 °C) du ing he phase o hyd olysis and acidi ica ion o he ma e ial and mesophilic condi ions subsequen ly (35 °C), ensu ing g ea e s abili y and di e si y o me hanogenic bac e ia in he me hane p oduc ion phase [33]. Ano he way o gi e s abili y o he p ocess is by adding inoculum, which being he al eady diges ed ma e 3 whe e he ecosys em o bac e ia ha eeds on he o he ma e ha is added o i has been c ea ed [18]. 1.1. The case o Cana y Islands The Cana y Islands a e one o he ou e mos egions o Eu opean Union. They ace pe sis en and combined di icul ies ha hinde hei socio-economic de elopmen : g ea emo eness, insula i y, small a ea, complex o og aphy and economic dependence on a small numbe o p oduc s. Fo his eason, Cana y Islands is called Remo e A ea om 1 o Ma ch o 2012 [7] and i has been ex ended se e al imes and nowadays i is un il 15 o June o 2022 [34]. This name was decla ed by he Cana y Islands Adminis a ion because o he special o og aphic condi ions o he Cana y Islands o he ope a ions o wi hd awal, anspo , handling, and s o age o animal by-p oduc s, oge he wi h he emo eness and insula i y o he e i o y. Then, animal by-p oduc s can be aken o he land ill whene e en i onmen al complexes ha e en i onmen al au ho iza ion, and emo al and ans e o such by-p oduc s om gene a ing ac i i ies o en i onmen al complexes we e ca ied ou by companies au ho ized o he pu pose. To his o m o ensu e aceabili y and ele an con ols by means o he co esponding eco ds and documen s [34]. A pa o his, each island ha compose Cana y Islands o ms an isola ed ene gy sys em wi h an ex e nal ene gy dependence o ossil uels and a limi a ion o he e i o y. Each ene gy sys em mus end o ene gy ansi ion owa ds a cleane one, a ou ing he pene a ion o enewables wi h a manageable enewable ene gy like biogas. This is in line wi h he goal o he Sus ainable De elopmen Goal (SDG) numbe 7, “Ensu e access o a o dable, eliable, sus ainable and mode n ene gy o all”. Likewise, complying wi h law published he 21 o May o his yea , Law 7/2021 called “Clima e Change and Ene gy T ansi ion Law”, which highligh s he s eps o a clean ene gy ansi ion. In he p esen s udy, he a ailable da a on was e gene a ed and disposed in he land ill by he slaugh e house on he island o Tene i e is collec ed and analysed. Subsequen ly, i is expe imen ally de e mined wi h samples supplied by he slaugh e house how much ene gy ha is possible o ob ain om he was ed by-p oduc s. This in o ma ion will make i possible o de e mine he sel -gene a ion capaci y a ailable in his ype o indus ial ac i i y using i s own animal by-p oduc s. 4 2. Ma e ials and me hods 2.1 Quan i ica ion o animal by-p oduc s in Tene i e The Cana y Islands Go e nmen has a quan i ica ion o he numbe o animal (head) slaugh e ed in Tene i e, which includes bo ine, po cine, goa and o ine and abbi . This s udy is ca ied ou wi h he da a collec ed o he i s ime du ing he yea 2019. The es ima ion o he quan i y (kg/head) o animal by-p oduc s is es ima ed ollowing published pa ame e s ( o see Table 1). Then, each animal was used a pa ame e o bibliog aphy by head. Fo bo ine and po cine, he pa ame e s o he head a e s udied by Sagas ume e al. [35]. In he case o goa and o ine conside ed as a small uminan s wi h a median weigh o 40 kg, 50% o isce a and 3% o blood [17], [36]. Finally, he abbi s ha e a median weigh o 2.30 kg and a 40% was e a e [37]. Table 1. Pa ame e s used o es ima e quan i y o animal by-p oduc s. Species Visce a (kg/head) Blood (kg/head) Bo ine 91.80 17.20 Po cine 9.22 4.35 Goa -O ine 20.00 1.20 Rabbi 0.92 - 2.2 Anae obic diges ion 2.2.1 Raw ma e ials In Tene i e, o send animal by-p oduc s o he land ill, he slaugh e house has o pays an au ho ized manage o anspo hem o he land ill. On he o he hand, because he was e mus be solid ma e ial o be elimina ed on he land ill, he blood is dehyd a ed using s eam a 180ºC o 40 minu es o send o he land ill. Consequen ly, he e ec o aw and dehyd a ed blood on biogas p oduc ion was in es iga ed in his s udy. These animal by-p oduc s mus be sani ized, acco ding o Regula ion (EU) 142/2011, and he p e- ea men used in his s udy was pas eu isa ion o 24 hou s a 85 ºC o ensu e ha a empe a u e o 70 °C is eached inside he sample o one hou . Be o e ha , animal by-p oduc s mus be cu in o pieces smalle han 12 mm. The animal by-p oduc s used a e he mix o isce a by each animal (bo ine, po cine, goa , o ine and abbi ) and, he blood and sewage sludge o a mix o all animals. The isce a p opo ion was acco ding o he dis ibu ion o isce a quan i y o each animal. 2.2.1 P epa a ion o biodiges e ISO bo le we e used as biodiges e s. In each diges e , 60 g o animal by-p oduc s pas eu ized we e placed, excep he biodiges e wi h umen con en due o i s low densi y only we e placed 15 g. Then, 290 mL o dis illed wa e was added, and he con en was shaken and he pH was measu ed. A e ha , 4.8 g NH4Cl (pu i y 99.5 %, Pan eac) we e added o p o ide he medium wi h a ni ogen sou ce, 540 mg o CaCO3 (pu i y 99.0 %, Pan eac) and 10 mL o bu e solu ion (pH=7.0 (20ºC), Scha lau) we e subsequen ly added o ac as a bu e and pH was measu ed again. In he case o he umen con en was added 2.4 g NH4Cl, 270 mg o CaCO3 and 10 mL o bu e solu ion o 7.0 pH. 4M NaOH solu ion (pu i y 99.0%, Scha lau) was added i necessa y o achie e a pH in he ange o 7.5-8.5. A e ha , biodiges e s we e he me ically sealed wi h a GL45 cap 11 Figu e 2. Accumula ed gas p oduc ion o umen con en samples. 3.2.1.2 Sewage sludge In slaugh e houses, a la ge olume o wa e is used and as esul a la ge amoun o he by-p oduc o wa e sewage sludge is p oduced. In Figu e 3, biogas accumula ed p oduc ion is shown wi h a o al olume o 1,465 and 329 mL/g o VS, samples 3 and 4, espec i ely. In his pe iod was no necessa y he eac i a ion o he samples, bu sample 4 s opped biogas p oduc ion al hough i s inal pH was 8.35. An expe imen al e o could explain his beha iou . 3.2.1.3 Raw blood O he animal by-p oduc s gene a ion is blood and, i s accumula ed gas p oduc ion is shown in Figu e 4. Exponen ial g ow h can be seen in ha samples in he i s week ollowed by a mo e g adual inc ease. The gas p oduc ion was 205 and 296 mL/g o VS, o samples 5 and 6, espec i ely. In he pe iod o 115 days o diges ion, no eac i a ion o samples was equi ed. 0 200 400 600 800 1000 1200 1400 1600 1800 2000 050 100 150 Accumula ed Gas P oduc ion (mL) Time (days) Sample 1 Sample 2 0 2000 4000 6000 8000 10000 12000 14000 050 100 150 Accumula ed Gas P oduc ion (mL) Time (days) Sample 3 Sample 4 Figu e 3. Accumula ed gas p oduc ion o sewage sludge samples. 12 Figu e 4. Accumula ed gas p oduc ion o aw blood samples. 3.2.1.4 Dehyd a ed blood The dehyd a ed blood is he same blood as aw blood bu i is p e- ea ed o emo e mois u e, so ha i s anspo can be op imised, in addi ions o his, liquid was e canno be aken o he land ill. The gas p oduc ion was 51 and 77 mL/g o VS, o samples 7 and 8, espec i ely. The beha iou o hese gas p oduc ion is shown in Figu e 5. I is obse ed ha he gas p oduc ion is highe du ing he i s mon h app oxima ely and hen he gas p oduc ion a e dec eased. A e 75 days bo h samples we e eac i a ed bu quickly inhibi ed and he p oduc ion s opped. Figu e 5. Accumula ed gas p oduc ion o dehyd a ed blood samples. 3.2.1.5 Visce a This sample is a mix o he isce a o bo ine, po cine, goa , o ine and abbi . The accumula ed gas p oduc ion has an exponen ial ini ial beha iou simila o aw blood wi h a e y as ini ial p oduc ion. As shown in Figu e 6, bo h samples a e eac i a ed wice a e 62 and 102 days, esul ing in wo changes in he gas p oduc ion cu e, wi h he i s eac i a ion inc easing gas p oduc ion was highe han in he second. The gas p oduc ion o isce a samples was 59 and 31 mL/g o VS o samples 9 and 10, espec i ely. - 100 200 300 400 500 600 700 800 900 020 40 60 80 100 120 Accumula ed Gas P oduc ion (mL) Time (days) Sample 5 Sample 6 - 100 200 300 400 500 600 700 020 40 60 80 100 120 Accumula ed Gas P oduc ion (mL) Time (days) Sample 7 Sample 8 13 Figu e 6. Accumula ed gas p oduc ion o isce a samples. 3.2.1.6 Visce a and aw blood The co-diges ion o isce a wi h aw blood can show he e ec o he blood gas p oduc ion wi h espec o isce a samples. Add blood o he isce a imp o e diges ion because he gas p oduc ion inc ease, bu he g ow h a e was slowe a he beginning and hen i was highe han he isce a alone. In Figu e 7 shows he beha iou o his mix u es. Bo h samples we e eac i a ed a e 61 days, bu sample 12 emained nea ly cons an hen, ano he agen o he han pH could explain he momen a y inc ease in gas p oduc ion, ha was 122 and 156 mL/g o VS, o samples 11 and 12, espec i ely. Figu e 7. Accumula ed gas p oduc ion o isce a and aw blood samples. 3.2.1.7 Visce a and dehyd a ed blood The beha iou o co-diges ion o isce a wi h dehyd a ed blood is he same as ha o only dehyd a ed blood, and he p e- ea men has a nega i e impac on gas p oduc ion. Because he pH is less han 7, bo h samples should be eac i a ed a e 60 days. Then, gas p oduc ion le elled o . A e ha , he gas p oduc ion did no inc ease as much as he o he samples. - 100 200 300 400 500 600 700 800 050 100 150 Accumula ed Gas P oduc ion (mL) Time (days) Sample 9 Sample 10 - 200 400 600 800 1.000 1.200 050 100 150 Accumula ed Gas P oduc ion (mL) Time (days) Sample 11 Sample 12 14 The gas p oduc ion o hese mix u es was 113 and 131 mL/g o VS o samples 13 and 14, espec i ely. The p oduc ion beha iou is shown in Figu e 8, wi h an exponen ial gas g ow h on he i s wen y days and hen s abilized. Figu e 8. Accumula ed gas p oduc ion o isce a and dehyd a ed blood samples. 3.2.1.8 Visce a, aw blood and sewage sludge In o de o ep esen he diges ion o all animal by-p oduc s gene a ed in Tene i e was analysed his sample o he pu pose o assess he ene gy po en ial allowing o he ene gy alo isa ion o hese animal by-p oduc s. The biogas p oduc ion o hese samples was 972 and 189 mL/g o VS, o sample 15 and sample 16, espec i ely. Sample 15 is he hi d sample wi h mo e biogas p oduc ion and he only one o he co-diges ed samples in which mo e biogas was ob ained. In compa ison o isce a and aw blood mix u e, in his case he gas p oduc ion is slowe and achie es high alues. These samples, howe e , ha e p oduced o a long ime wi hou being eac i a ed. The beha iou o he biogas p oduc ion is shown in he Figu e 9, whe e he e a e some peaks o p oduc ion ha can be ela ed wi h a sel -adjus men o he ecosys em o me hanogenic bac e ia. Figu e 9. Accumula ed gas p oduc ion o isce a, aw blood and sewage sludge samples. - 100 200 300 400 500 600 700 800 900 020 40 60 80 Accumula ed Gas P oduc ion (mL) Time (days) Sample 13 Sample 14 0 500 1000 1500 2000 2500 3000 3500 020 40 60 80 100 Accumula ed Gas P oduc ion (mL) Time (days) Sample 15 Sample 16 15 3.2.2 Gas composi ion Table 7 shows he a e age me hane composi ion ob ained in his s udy. Table 7. A e age and maximum me hane composi ion ob ained o each sample. Sample A e age o me hane composi ion (%) Maximum me hane composi ion (%) Rumen con en 34.13 64.74 Sewage sludge 48.74 68.92 Raw blood 3.66 16.46 Dehyd a ed blood 0.04 0.18 Visce a 0.03 0.32 Visce a and aw blood 13.36 32.29 Visce a and dehyd a ed blood 3.14 28.31 Visce a, aw blood and sewage sludge 42.99 69.19 Only umen con en , sewage sludge, and he co-diges ion o isce a and aw blood wi h sewage sludge p oduced biogas because i s me hane composi ion is highe han 55%. The e o e, hey ha e he po en ial o be used o ob ain elec ical o he mal ene gy, because me hane is a gas wi h a high ene gy po en ial. Cu en ly, he umen con en wi h a high ege al biomass composi ion is used in ae obic diges ion as compos , and hen i is used as a bio e ilize in ag icul u e. Al hough wi h anae obic diges ion he by-p oduc is he same as bio e ilize , bu his p ocess gene a es ene gy oo. O he s, like aw blood, co-diges ion o isce a wi h aw blood o co-diges ion o isce a wi h dehyd a ed blood he me hane concen a ion is lowe han 55% as well. Samples ha lead o lowe me hane, he e o e gas ich in ca bon dioxide can be used o ans o m his CO2 o o he p oduc s wi h highe ene gy alue as dime hyl e he (BioDME), whe e he gas is con e ed o me hanol wi h a ca aly ic p ocess be o e BioDME p oduc ion, addi ionally biome hanol (Bio- MeOH) also can be ob ained. 3.2.3 To al, ola ile and ixed solids Solid analysis is necessa y o de e mining he amoun o deg ada ion o o ganic ma e ials. Table 8 shows he solid analysis be o e and a e diges ion anae obic p ocess wi h o al, ixed and ola ile solids. 16 Table 8. Solid con en be o e and a e anae obic diges ion. Sample TS eed (%) FS eed (%) VS eed (%) TSou (%) FSou (%) VSou (%) Rumen con en 93.5 8.0 92.0 6.6 19.4 80.6 Sewage sludge 97.5 20.4 79.6 14.5 34.6 65.4 Raw blood 22.3 4.2 95.8 2.1 43.8 56.2 Dehyd a ed blood 24.9 0.0 100.0 1.9 50.0 50.0 Visce a 42.7 3.6 96.4 3.3 50.0 50.0 Visce a and aw blood 38.2 2.6 97.4 3.0 33.3 66.7 Visce a and dehyd a ed blood 39.8 3.7 96.3 3.4 30.6 69.4 Visce a, aw blood and sewage sludge 46.8 12.6 87.4 5.4 24.5 75.5 F om hese da a, he dec ease in ola ile solids can be calcula ed, which will gi e an idea o he deg ada ion o he solids. The esul s ob ained a e p esen ed in Table 9. Table 9. Dec ease in ola ile solids. Sample ola ile was e (g /100g diges a e) ΔVS (%) Rumen con en 5.32 93.82 Sewage sludge 9.48 87.78 Raw blood 1.18 94.48 Dehyd a ed blood 0.95 96.18 Visce a 1.65 95.99 Visce a and aw blood 2.00 94.63 Visce a and dehyd a ed blood 2.36 93.84 Visce a, aw blood and sewage sludge 4.08 90.03 Anae obic diges ion is a p ocess in which o ganic ma e ial can be educed, and he esul s show ha he educ ion is be ween 90.0 and 96.2%, been he highe o dehyd a ed blood and he lowe o he mix u e o isce a, aw blood and sewage sludge. Ne e heless Table 9 show ha he e a e s ill a ela i e high p opo ion o ola ile in he diges a e o be able o con inue he diges ion p ocess. The diges a e wi h he highes p opo ion o ola ile solids is he umen con en , al hough he one wi h he highes con en o ola ile solids pe 100 g ams o diges a e is he sewage sludge. Besides, anae obic diges ion by-p oduc s, he diges a e, can be used like e ilize in he ag icul u e in ag eemen p og ess wi h o he ci cula economy concep . 3.3 Ene gy p oduc ion o biogas F om he poin o iew o ene gy eco e y om biogas some o he samples used p oduce a gas wi h a me hane composi ion g ea e han 55%, which can hen be used o ob ain he mal and elec ical ene gy h ough cogene a ion. In he case o slaugh e houses bo h ypes o ene gy a e necessa y and he biogas gene a ed could be used. The ene gy can be es ima ed by he nex equa ion: 17 𝐸𝑏𝑖𝑜𝑔𝑎𝑠 = 𝐶 × 𝐶𝐻4× 𝑃 × 𝜂 (4) whe e Ebiogas is he quan i y o elec ici y o hea ene gy p oduced (kWh/yea ); C ep esen s he lowe calo i ic alue o me hane, which was conside ed as 36 MJ (o 10 kWh) pe cubic me e o me hane (1 m3 CH4 = 36 MJ; 1 kWh = 3.6 MJ; 1 m3 CH4 = 10 kWh); CH4 ep esen s he me hane con en ; P is he amoun o biogas p oduced pe yea o he biogas po en ial as desc ibed in Eq. (4) (m3/yea ); and 𝜂 ep esen s he o e all e iciency o he con e sion o biogas (%). I is no ed ha he 𝜂 alues (𝜂𝑒 o 𝜂𝑐) we e conside ed as 𝜂𝑒 = 25% (e iciency o hea - o-elec ici y con e sion) and 𝜂𝑐 = 80% (e iciency o he combus ion p ocess), espec i ely, o elec ic and he mal ene gies [40]. In his pape , he esul s om each sample we e analysed o es ima e he ene gy p oduc ion using equa ion (4. Table 10 he es ima ion o ene gy p oduc ion wi h co-gene a ion, elec ical and he mal ene gy. The samples mix u e o he p incipal animal by-p oduc s ( isce a, sewage sludge and aw blood) has he highes ene gy p oduc ion ans o ming in a good al e na i e o educe he was e gene a ed in he slaugh e house, allowing he gene a ion o 499 MWh/yea and 5,747 GJ/yea elec ical and he mal ene gy in 2019, espec i ely. Table 10. Ene gy es ima ion esul s ob ained. Samples Elec ical ene gy (kWh/yea ) Elec ical ene gy (MWh/yea ) The mal ene gy (MJ/yea ) The mal ene gy (GJ/yea ) Sewage sludge 333,034 333 3,836,552 3,837 Raw blood 397 0.40 4,571 4.57 Dehyd a ed blood 1,9 1.95E-03 22 0.02 Visce a 16 1.62E-02 186 0.19 Visce a and aw blood 14,063 14 162,002 162 Visce a and dehyd a ed blood 3,166 3 36,477 36 Visce a, aw blood and sewage sludge 498,876 499 5,747,051 5,747 O e all, he biological me hane po en ial (BMP) is usually impo an in he anae obic diges ion analysis o cha ac e ize each was e. In his s udy, Table 11 shows he BMP o each sample in mL g VS−1 whe e he ange o alues is in he ange om 49.5 o 650.9 mLCH4 gVS−1 [41]. 18 Table 11. Biochemical me hane po en ial o each sample s udied. Sample Me hane p oduc ion (mL/g VS) Rumen con en 374 Sewage sludge 714 Raw blood 8 Dehyd a ed blood - Visce a - Visce a and aw blood 16 Visce a and dehyd a ed blood 4 Visce a, aw blood and sewage sludge 418 Cu en ly, he e a e some s udies on ene gy mix o sel -su icien slaugh e house o educe he impac o his indus y on he en i onmen al. A pa o he anae obic diges ion as manageable enewable ene gy due o biogas can be s o aged. Al hough his enewable ene gy educes emissions compa ed o o he ea men s, i s emissions a e no ze o, so al e na i es a e sough o educe he ene gy consump ion needed o keep he eac o empe a u e cons an . Acco ding o Da wesh e . al. he biogas p oduc ion can use sola ene gy as i s main ene gy sou ce o p o ide he mesophilic empe a u es needed o e men a ion [42]. O he s udy used pho o ol aic he mal echnology, Concen a ed Pho o ol aic The mal (CPVT) collec o s, o p oduce elec ici y o upg ading biogas o biome hane and hea o hea ing he biodiges e [43]. A pa o his, in acco dance wi h “P ojec ed Cos s o Gene a ing Elec ici y 2020” he alue o le elized cos o ene gy o pho o ol aic is lowes p ice o low-ca bon echnologies o p oduce ene gy because i has had a huge all in p ices. Then, he mix o pho o ol aic wi h biogas can be a good choice. In ac , acco ding o he Pho o ol aic Geog aphical In o ma ion Sys em (PVGIS) a he loca ion o he main slaugh e houses in Tene i e, he pho o ol aic esou ce is e y good wi h a mon hly a e age global i adia ion pe squa e me e o 172 kWh/m2 wi h an op imal ixed slope o he modules o 23ºC. To know he iabili y o his ene gy gene a ion sys em, i is necessa y o design he sys em ollowing he ene gy demand o he slaugh e houses. I so, his sys em would make he mea indus y mo e sus ainable om an en i onmen al and economic poin o iew. 19 4. Conclusions The animal by-p oduc s ha e a po en ial o ene gy eco e y h ough anae obic diges ion. As a esul , his p ocess is an app oach o he ea men o was e dumped in land ills, a oiding anspo and land ill emissions and inancial expenses o pay an agen o ca y ou he anspo o he goods. In addi ion, he blood does no ha e o submi o a he mal p ocess because i does no imp o e he co-diges ion o ABPs and ene gy would be was ed. A mix u e o he majo i y o animal by-p oduc s gene a ed in Tene i e, wi h an es ima ion o 1,292 onnes o 2019, and a p opo ion analysed in his ex , gene a e a biogas p oduc ion o 580 mL/g SV and a biochemical me hane po en ial o 418 mL CH4 / g SV. A e wa ds, animal p oduc s will gene a e an a e age o 499 MWh/yea and 5,747 GJ/yea o elec ical and he mal ene gy. O e all, wi h his al e na i e o ea men o animal by-p oduc s con e ing his was e in o a esou ce in he ene gy eco e y o supply he mea indus y ene gy demand, u ning his indus y in o a mo e sus ainable and mo e sel -su iciency a ou ing he ci cula economy. 20 5. Acknowledgemen s The au ho s would like o hank Ma ade o Insula de Tene i e S.A. o p o iding samples o he ABPs and helping us o unde s and he p ocess and p o iding some was e gene a ion da a. This esea ch has been co- unded by FEDER unds, INTERREG MAC 2014-2020 p og am, wi hin he ENERMAC p ojec (MAC/1.1a/117) and ACLIEMAC p ojec (MAC2/3.5b/380).