Renewable electricity generation from biogas of anaerobic co-digestion of sludge, cattle manure and water / Geração de eletricidade renovável a partir do biogás da co-digestão anaeróbica de lodo, esterco de gado e água

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Braz. J. of Develop.,Curitiba, v. 6, n.5, p.23319-23334 may. 2020. ISSN 2525-8761

Renewable electricity generation from biogas of anaerobic co-digestion of

sludge, cattle manure and water

Geração de eletricidade renovável a partir do biogás da co-digestão

anaeróbica de lodo, esterco de gado e água

DOI:10.34117/bjdv6n5-038

Recebimento dos originais: 15/04/2020 Aceitação para publicação: 04/05/2020

Juliana Lobo Paes

Doutora em Engenharia Agrícola pela Universidade Federal de Viçosa Instituição: Universidade Federal Rural do Rio de Janeiro

Endereço: Rodovia BR 467, Km 7, s/n – Campus Universitário, Seropédica, Rio de Janeiro, Brasil

E-mail: juliana.lobop@gmail.com

Priscilla Tojado dos Santos

Bacharel em Engenharia Agrícola e Ambiental pela Universidade Federal Rural do Rio de Janeiro

Instituição: Universidade Federal Rural do Rio de Janeiro

E-mail: priscillatojado@gmail.com

Romulo Cardoso Valadão

Doutor em Ciência & Tecnologia de Alimentos pela Universidade Federal Rural do Rio de Janeiro

E-mail: romulocv@yahoo.com.br

Saulo Emílio Guerrieri Araújo Damm

Mestrando em Desenvolvimento Regional e Meio Ambiente pela Universidade Estadual de Santa Cruz

Instituição: Universidade Estadual de Santa Cruz

Endereço: Rodovia Jorge Amado, Km 16, Bairro Salobrinho, Ilhéus-BA, Brasil E-mail: saulodamm@hotmail.com

Gabriela Ferreira Pagani

Graduanda em Engenharia Agrícola e Ambiental pela Universidade Federal Rural do Rio De Janeiro

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Beatriz Costalonga Vargas

Mestranda em Engenharia Agrícola pela Universidade Federal de Viçosa Instituição: Universidade Federal de Viçosa

Endereço: Avenida Prof. Peter Henry Rolfs, s/n – Campus Universitário, Viçosa, MG, Brasil E-mail: bia11vargas@hotmail.com

ABSTRACT

This study reported the viability of energy supply from the energetic conversion of biogas generated by the anaerobic co-digestion in biodigester of rural properties with different sizes of the herd. Indian model benchtop biodigesters were filled with sewage sludge (SS), cattle manure (CM) and deionized water (DW). The energy conversion calculation was based on the cumulative biogas yield (CBY), amount of cattle manure and one cubic meter equivalence of gas with electricity. In the SS co-digestion biogas production was accelerated and 25:50:25 SS:CM:DW presented higher cumulative production. Experimental data on the cumulative production of this ratio fitted to the exponential model, in which 28.15 L were obtained. The efficiency of anaerobic co-digestion of SS and CM was proved with a higher cumulative biogas production than in the single substrate digestion of SS and the single substrate digestion of CM. High immediate production was only observed with SS. There was a long lag-phase for digestion of only CM. An intermediate production profile to the previous ones was noticed when balancing the three co-digesters. The CBY was 16.56 L kg-1_{, 0.48 L gTS}

added-1 and 0.59

L gVSadded-1to 25:50:25 SS:CM:DW. The implement of biodigesters in rural properties for

electricity generation provided monthly 2600 kWh and economy of US$ 7669.19. It was concluded that the implantation of biodigesters with 25:50:25 SS:CM:DW to produce electric energy from biogas allows energy self-sufficiency of the property, enabling the sustainable development of the activity through the proper disposal of waste and economic gains to the producer.

Keywords: Anaerobic sewage treatment; cattle manure co-digestion; biogas production;

electricity generation.

RESUMO

Este estudo relatou a viabilidade do suprimento de energia a partir da conversão energética do biogás gerado pela co-digestão anaeróbica em biodigestor de propriedades rurais com diferentes tamanhos de rebanho. Os biodigestores de bancada modelo indiano foram preenchidos com lodo de esgoto (ES), esterco bovino (CM) e água desionizada (DW). O cálculo da conversão de energia foi baseado no rendimento acumulado de biogás (CBY), quantidade de esterco bovino e equivalência de um metro cúbico de gás com eletricidade. Na co-digestão com SS, a produção de biogás foi acelerada e 25:50:25 SS: CM: DW apresentaram maior produção cumulativa. Dados experimentais sobre a produção cumulativa dessa relação foram ajustados ao modelo exponencial, no qual foram obtidos 28,15 L. A eficiência da co-digestão anaeróbica de SS e CM foi comprovada com maior produção cumulativa de biogás do que na digestão de substrato único de SS e digestão de substrato único de CM. Alta produção imediata foi observada apenas com SS. Houve uma longa fase de atraso para digestão apenas de CM. Um perfil intermediário de produção foi observado ao equilibrar os três co-digestores. O CBY foi de 16,56 L kg-1, 0,48 L gTSadded-1 e 0,59 L gVSadded-1 a 25:50:25 SS: CM: DW. A implementação de biodigestores nas propriedades rurais para geração de eletricidade forneceu 2600 kWh e economia mensal de US $ 7669,19.

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Braz. J. of Develop.,Curitiba, v. 6, n.5, p.23319-23334 may. 2020. ISSN 2525-8761 se que a implantação de biodigestores com 25:50:25 SS: CM: DW para produzir energia elétrica a partir do biogás permite a auto-suficiência energética da propriedade, possibilitando o desenvolvimento sustentável da atividade por meio do descarte adequado de resíduos e ganhos econômicos para o produtor.

Palavras-chave: Tratamento de esgoto anaeróbico; co-digestão de esterco bovino; produção

de biogás; geração da eletricidade.

1 INTRODUCTION

The intense modernization process in the agriculture and livestock industry leads to an increase in energy demand. Therefore the search for alternative, additional energy sources to reduce the production costs stimulates the development of technological solutions to replace fossil fuel for clean and renewable energy technologies (Adriamanohiarisoamanana et al., 2018; Guimarães et al., 2018; Arango-Osório et al., 2019; Clercq et al., 2019).

Biogas plants have been highlighted as an alternative to solve the energy supply associated with waste management since it enables to treat different kinds of biomass from rural, industrial and urban waste for parallel electricity and heat generation (Bloch-Michalik & Gaworski, 2016; Roubík & Mazancová, 2016; Dubrovskis & Plume, 2017; Guimarães et al., 2018; Krištof & Gaduš, 2018; Donoso-Bravo et al., 2019; Neba et al., 2020). In terms of rural waste, long initial start-up phases biogas production, low methane yield, and reactor efficiency that are typical for anaerobic mono-digestion (AmD) of cattle manure (Adriamanohiarisoamanana et al., 2017; Matos et al., 2017; Adriamanohiarisoamanana et al., 2018; McVoitte & Clark, 2019) can lead to economic losses. In this sense, anaerobic co-digestion (AcoD) process (simultaneous co-digestion multiple biodegradable substrate), is a promising alternative to solve problems related to AmD and to improve the economic viability of anaerobic digestion (AD) plants (Mehryar et al., 2017; Adriamanohiarisoamanana et al., 2017; Adriamanohiarisoamanana et al., 2018; Krištof & Gaduš, 2018; Clercq et al., 2019; Sarpong et al., 2019; González et al., 2020).

The AcoD leads to increased microbial diversity, biodegradability and accelerated hydrolysis process, resulting in an improvement of hydrolysis rate, reduced lad phases and increased biogas recovery (Guimarães et al., 2018; Adriamanohiarisoamanana et al., 2017; Donoso-Bravo et al., 2019; Panigrahi & Dubey, 2019). The addition of sewage sludge (SS) as a co-digestion eases the digestion of aggregates of particles, floating materials and wastes with unfavorable fluid dynamics (Sarpong et al., 2019).

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Braz. J. of Develop.,Curitiba, v. 6, n.5, p.23319-23334 may. 2020. ISSN 2525-8761 However, it is important to choose the ideal proportion of the mixture to favor synergism, to dilute inhibitory and toxic compounds, to optimize biogas and methane production and to promote product quality and digestion stability (Mata-Álvarez et al., 2014; Adriamanohiarisoamanana et al., 2017; Adriamanohiarisoamanana et al., 2018; Panigrahi & Dubey, 2019). Therefore, maintaining adequate amounts of micronutrients and water in the influents submitted to AcoD are essential requirements for the microorganisms involved in the fermentation process within the biodigesters. These microorganisms, especially the methanogenic archaea, guarantee the follow-up of the ideal fermentative route and hydrolysis and methanogenic activity, ensuring greater production of biogas rich in methane (Donoso-Bravo et al., 2019; Panigrahi & Dubey, 2019).

Nowadays, there is the advantage of adopting the Program for the Development of Distributed Generation of Electric Energy (ProGD) in Brazil, in which the supply of electricity generated in loco can be passed on to a power distribution company. The production of electricity generated in the property itself leads to a reduction in dependence on fossil fuels and local concessionaires (Freitas et al., 2019). According to Lopes et al. (2019), biogas system could generate a total of 162.1 GWh for 30 years, and the solar system 15.1 GWh for 25 years of operation, for a Brazilian case study. Therefore, the economic analysis of the use of AD agricultural biogas plants as a source of electric energy is a way of encouraging public policies (Collotta & Tomasoni, 2017; Roubík et al., 2017; Piñas et al., 2019) that aims the implantation of biodigesters in rural areas for the Brazilian scenario.

In this context, the main objective of this study was to evaluate the viability of the energy supply from the energetic conversion of the biogas generated by the AcoD of sewage sludge, cattle manure, and water in biodigester of rural properties.

2 MATERIALS AND METHODS

Sewage sludge (SS) from the Sewage Treatment Plant (WWTP) located in the city of Rio de Janeiro, Brazil, fresh cattle manure (CM) collected from Dairy Cattle Sector of Federal Rural University of Rio de Janeiro (UFRRJ), Seropédica - RJ campus and deionized water (DW) were used as influent in the AD composed by AmD and AcoD processes. SS was collected after the primary decanters of the WWTP, showing respectively, values of average pH, moisture content (MC), total solids content (TS) and total volatile solids content (VS) of 5.10, 96.42%, 3.58%, and 3.18%. Levels of pH, TS, and VS were measured according to

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Braz. J. of Develop.,Curitiba, v. 6, n.5, p.23319-23334 may. 2020. ISSN 2525-8761 standard methods (APHA, 2005). The VS/TS ratio of CM and SS was 0.89 and 0.84, respectively, which indicates a highly biodegradable substrate.

Fresh CM was collected in the morning by scraping with mason shovel, avoiding the removal of foreign materials (soil, pasture, and stone). Dairy cattle under conventional production system, in which the animals were fed Tanzania grass (Panicum maximum) with corn, soybean meal, and wheat meal. The average pH, MC, TS and VS of the CM was 5.15, 83.87%, 16.13%, and 13.48%, respectively. Analyses of pH, TS and VS were determined according to Standard Methods for the Examination of Water and Wastewater (APHA, 2005). All substrates were transferred in sterilized plastic containers to the Laboratory of Rural Electrification and Alternative Energies located in UFRRJ, Seropédica - RJ campus, which geographic coordinates are 22º 45' 48.74" S and 43º 41' 19.01" W. According to Köppen's classification, the climate in this area is classified as Aw and has an average annual temperature of 24.5ºC.

Indian model benchtop biodigesters (2.0 L) were used, which were consisting of a "water seal" containment chamber, fermentation chamber, gasometer and U-tube manometer for the water column, as described by Matos et al. (2017). The biodigesters were placed on a bench top, under room conditions, sheltered from sunlight and rain.

The AmD and AcoD of SS, CM, and DW were conducted in sets of batch experiments, described as following. In experiment I, each substrate was mono-digested at ratio of 100:0:0 and 0:100:0, while in experiment II SS, CM, and DW were co-digested at ratio of 50:25:25 (92.82% MC, 7.18% TS and 5.73% VS), 25:25:50 (96.35% MC, 3.65% TS and 3.03% VS), 33:33:33 (93.78% MC, 6.22 % TS and 5.01% VS) and 25:50:25 (89.97% MC, 10.03% TS, and 8.17% VS). The biodigesters were operated with different influent rates simultaneously.

The supply system was carried out discontinuously, that is, the 1.7 kg of influent was conditioned in the biodigester only at the beginning of the experiment. The supply of the biodigesters with the influent were initialized 24 h after its collection to avoid loss of biogas generated due to the early fermentative process. The hydraulic retention time (HRT) was 82 days (12 weeks). The average ambient temperature was 27.3°C.

The product of the vertical displacement of the gasometer by its internal cross-sectional area determined the volume of biogas produced. After each measuring, the gasometers were zeroed. Biogas volume measurements were carried out each three days at 10:00 h a.m.

The correction of biogas volume for the standard temperature and pressure (101.32 kPa, 293.15 K) was carried out based on the studies proceeded by Matos et al. (2017), in which

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Braz. J. of Develop.,Curitiba, v. 6, n.5, p.23319-23334 may. 2020. ISSN 2525-8761 used the combined gas law (Boyle's Law, Charles' Law, and Gay-Lussac's Law), as describe equation 1.

𝑉₀× 𝑃₀ 𝑇₀ =

𝑉₁ × 𝑃₁

𝑇₁ (1)

where: V0 -corrected biogas volume, L; P0 -corrected biogas pressure, 101.32 kPa; T0 -

corrected biogas temperature, 293.15 K; V1 -biogas volume in the gasometer, L; P1 -biogas

pressure in the gasometer, kPa; T1 - biogas temperature in the gasometer K.

P1 was obtained by the sum between Seropédicas' atmospheric pressures and U-tube manometer. The average pressure was checked by using data from the National Meteorological Institute (INMET), placed at the automatic station of Agricultural Ecology, located in the county of Seropédica, Rio de Janeiro, Brazil. This meteorological station is located 2.73 km away from the UFRRJ.

Generated electric power was based on the highest cumulative biogas yield among SS:CM:DW ratio, influent amount (SS, manure produced on a UFRRJ's Dairy Cattle,andDW) and the equivalent cubic meter of gas with electricity (Equation 2).

EC = CBY × IA × BEQ (2)

where: EC - Energy Conversion, kWh; CBY - cumulative biogas yield, L kg-1_{; IA - influent}

amount, kg; BEQ - Biogas Equivalence, kWh L-1_.

CBY is the relation between accumulated production and the amount of affluent placed in the biodigester (1.7 kg). The proportion of SS and DW was calculated, based on the values of CM produced on the Dairy Cattle Sector of UFRRJ, to determine the influent amount (IA). The dairy cattle sector has 122 animals, from which, 55 cows, 45 dairy cows, 20 calves and 2 steers. For comparison purposes, the amount of influent in the 0:100:0 SS:CM:DW ratio was also calculated. Each animal produces 15 kg of manure daily, and it was considered that one cubic meter of biogas corresponds to 1.43 kWh (Barreira, 2017).

The result was then compared to the energy demand of the Dairy Cattle Sector of UFRRJ. The total energy consumption of the sector was calculated through the survey of the electric pieces of equipment of the rural property, required power, and the daily operating time (Table 1).

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Table 1. Energy demand of the Dairy Cattle Sector of UFRRJ

Equipament Amount Operating time (h) Use frenquency (days mounth-1) Power (W) Monthly consumption (kWh mounth-1₎ Computer 10 8 20 63 10 Notebook 2 4 20 20 2 Minibar 19 24 30 26 19 Freezer 53 24 30 73 53 Water heater 240 4 30 2,000 240 Milking machine 177 4 30 1,471 176 Fan 125 16 30 65 31 Refrigerator 54 24 30 75 54 Drinking fountain 25 8 30 105 25 Coffee machine 7 1 30 218 6 Fluorescent lamp 96 4 30 40 96 Reflector lamp 95 14 30 75 94

Total monthly property consumption (kWh mounth-1₎ ₉₀₁

For the calculation of the energy economy, it was considered 0.70736 R$ kWh-1_or

2.94969 US$ for rural properties located in Rio de Janeiro with consumption above 300 kWh (LIGHT, 2020). The price expressed in US Dollar (US$) was collected in January 2020, where US$ 1.00 = R$ 4.17 (BCB, 2020).

The experimental results of cumulative production as a function of HRT were submitted to regression analysis using the R statistical program. The graphs of cumulative and cumulative biogas yield were made using the Sigma Plot 2001 software, version 7,0.

3 RESULTS AND DISCUSSION

Fig. 1 shows immediate biogas production on the first week of AD to the ratio that were considered, except the 0:100:0 SS:CM:DW. According to Adriamanohiarisoamanana et al. (2017), immediate biogas production indicates a fast response of anaerobic microbial population towards the influent.

The cumulative production for the SS mono-digested was higher than the others until the third week, followed by a tendency towards stabilization, while for the CM it was practically null until the seventh week. Concerning AcoD, there is an increasing cumulative production,

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Braz. J. of Develop.,Curitiba, v. 6, n.5, p.23319-23334 may. 2020. ISSN 2525-8761 being higher than the SS mono-digested from the third, fourth, fifth, and eighth weeks for the 50:25:25, 25:50:25, 25:25:50, and 33:33:33 SS:CM:DW, respectively.

Hydraulic retention time (week)

0 1 2 3 4 5 6 7 8 9 10 11 12 A ccum ul at ed bi og as pro du cti on (L) 0 2 4 6 8 10 12 14 16 18 20 22 24 100:0:0 SS:CM:W 50:25:25 SS:CM:W 25:25:50 SS:CM:W 33:33:33 SS:CM:W 25:50:25 SS:CM:W 0:100:0 SS:CM:W

Figure 1. Cumulative production (L) function of HRT (week).

The difference in biogas production at the beginning of the AD process can be explained by the lag-phase, in which there is the adaptation and multiplication of the microbial population, followed by the log-phase with rapid decomposition of organic matter and high microbial growth and stationary phase (Metcalf & Eddy, 2003). In LE mono-digested, the short lag phase may be due to the existence of an adapted and stabilized microbial population and organic matter resulting from the sewage treatment in decanters at the WWTP. For an influent containing only CM the longest lag-phase, may have occurred due to the absence of liquid medium for the adaptation and multiplication of the microbial population aiming at the breakdown of complex molecules that compose the organic matter and, consequently, start to biogas production. Besides that, CM has greater amounts of fibers (lignocellulosic materials) with recalcitrant characteristics, which hinders the hydrolysis rate (Adriamanohiarisoamanana et al., 2017). The liquid medium is the most important variable in the AD process, once it provides the necessary nutrients to the microorganisms, and it also performs as a conductor of enzymes and macrobiotic metabolites that are important to help the biological degradation process of organic matter (Orrico Jr et al., 2010; Ladino & Silvio, 2014).

In AcoD, it can be seen that in the 50:25:25 and 25:50:25 SS:CM:DW biogas production was maintained throughout the HRT. However, when there is a higher proportion of SS (50% SS) and an equal amount of CM (25% CM), which characterizes the supply of microbial population and organic matter, and DW (25% DW), a factor that favors the hydrolysis phase,

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Braz. J. of Develop.,Curitiba, v. 6, n.5, p.23319-23334 may. 2020. ISSN 2525-8761 observed high biogas production in the initial stage, as well as throughout the HRT. When using a higher proportion of cattle manure (50% CM), and equal amounts of sewage sludge (25% SS) and water (25% DW), biogas production is observed up to the lower fifth week of only 100:0:0 and 50:25:25 SS:CM:DW, followed by greater production of biogas when compared to other rations until the end of the HRT. This result may be linked to the greater availability of organic matter composed of carbohydrates, proteins, and fats, in addition to containing microorganisms necessary to start the process in CM associated with greater ease in breaking down its complexes molecules due to the extra community of adapted microorganisms and liquid medium with the presence of SS and DW. In relations 25:25:50 and 33:33:33 SS:CM:DW, probably the excess of water and equal proportions of SS:CM:DW did not favor AcoD, characterized by the reduced period and low biogas production. Thus, when assessing the profile of cumulative biogas production, which is increasing and characteristic of each relation under study when adopting AcoD, one can infer the symbiotic relationship between the stabilized microbial load originating from SS, organic matter load from CM, and DW that favors the hydrolytic phase.

The equations that describe the behavior of cumulative production during HRT, according to the regression analysis presented in Table 2, corroborate the results obtained in Fig. 1.

Table 2. Regression equations adjusted to the cumulative production data during the AD process and respective determination coefficients Ratio SS:CM:DW Interval Equation Coefficients of determination (r2) 100:0:0 xi< 2.32 ˆy= 8.1909+3.3793xi 0.97 2.32 ≤ x_i≤ 12 ˆy= 8.1909 50:25:25 xi< 6.14 ˆy= 13.1667+2.0557xi 0.97 6.14≤ x_i≤ 12 ˆy= 13.1667 25:25:50 xi< 5.52 ˆy= 9.4386+1.7361xi 0.98 5.52 ≤ xi≤ 12 ˆy= 9.4386 33:33:33 0 ≤ xi≤ 12 ˆy= 2. 3002exp(0.1306xi) 0.98 25:50:25 0 ≤ xi≤ 12 ˆy= 5.2714exp(0.1396xi) 0.82 0:100:0 0 ≤ xi≤ 12 ˆy= 0. 07165exp(0.3947xi) 0.97

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Braz. J. of Develop.,Curitiba, v. 6, n.5, p.23319-23334 may. 2020. ISSN 2525-8761 Experimental data on the cumulative production of the 100:0:0, 50:25:25 and 25:25:50 SS:CM:DW ratio was fitted to the Linear Response Plateau model. According to this regression model, the maximum cumulative production of 8.1909, 13.1667, and 9.4386 L in HRT of 2.32, 6.14, and 5.52 weeks, respectively, to 100:0:0, 50:25:25, and 25:25:50 SS:CM:DW. From this HRT, it is observed that cumulative production achieved its steady state due to the ceasing of biogas generation.

For the 33:33:33, 25:50:25, and 0:100:0 SS:CM:DW ratio, the cumulative production as a function of HRT was represented by the exponential model. By using this regression, it was obtained cumulative biogas production of 11.02, 28.15, and 8.17 L, respectively, at the end of the HRT. The maximum cumulative production tends to be reached after the 12th week of the anaerobic process, as the exponential model indicates. Thus, it can be inferred that these ratios require a longer period for the stabilization of organic matter. However, the absence of liquid medium (SS and DW), delays the AmD process of biogas production for almost seven weeks. This result proves that AcoD proves to be a viable solution to improve the efficiency of biogas production.

The importance of using SS as co-digester can be confirmed when comparing the results obtained by Matos et al. (2017). In the work carried out by these authors, the main peak of biogas production occurred only in the ninth and 14 week when biodigesters were supplied with water and CM originated by animals under organic and conventional production system, respectively.

The CBY in L per kg of influent, kg TS and kg VS during AD of SS, CM and DW are shown in Fig.2. As expected due to the highest cumulative production (28.15 L), the 25:50:25 SS:CM:DW ratio showed the highest CBY (Fig. 2). Its efficiency in converting affluents to biogas, in terms of L per kg of affluent, was 71, 53, 66, 61, and 71% for 100: 0: 0, 50:25:25, 25:25:50, 33: 33:33, and 0: 100: 0 SS: CM: DW (Fig. 2, a), respectively. The CBY per kg TS added and kg VS added was, respectively, 18 and 25%, 35 and 33%, 8 and 10%, 37 and 36%, and 82% for 100:0:0, 50:25:25, 25:25:50, 33:33:33 and 0:100:0 SS:CM:DW (Fig. 2, b).

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Braz. J. of Develop.,Curitiba, v. 6, n.5, p.23319-23334 may. 2020. ISSN 2525-8761 a) C u m u la tive bi o g a s y ie ld (L kg -1 ) 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 100:0:0 SS:CM:DW 50:25:25 SS:CM:DW 25:25:50 SS:CM:DW 33:33:33 SS:CM:DW 25:50:25 SS:CM:DW 0:100:0 SS:CM:DW Influent 4.82 7.75 5.55 6.48 16.56 4.81 b) C u m u la tive bi o g a s y ie ld (L g -1 ) 0,0 0,1 0,2 0,3 0,4 0,5 0,6 100:0:0 SS:CM:DW 50:25:25 SS:CM:DW 25:25:50 SS:CM:DW 33:33:33 SS:CM:DW 25:50:25 SS:CM:DW 0:100:0 SS:CM:DW TSadded VSadded 0.39 0.44 0.39 0.31 0.44 0.53 0.30 0.37 0.48 0.59 0.09 0.10

Figure 2. Cumulative biogas yield L per a) kg of influent and b) g TSadded and g VSadded.

Larger CM ratio in AcoD process causing higher CBY were reported by Paes et al. (2018) when studying CM and SS as co digester and Adriamanohiarisoamanana et al. (2017) for AcoD to CM and meat, and bone meal. This behavior may be attached to the fact that the CM has microbial community diversity and high moisture content, organic matter content and biodegradability (Adriamanohiarisoamanana et al., 2017; Adriamanohiarisoamanana et al., 2018). According to Adriamanohiarisoamanana et al. (2017), Adriamanohiarisoamanana et al. (2018) and McVoitte & Clark (2019), although the CBY obtained from dairy cattle manure in AmD process is lower than that of other substrate, due to its microbial and physico-chemical characteristics is widely used as co-digester.

Regarding the use of dairy cattle manure in the AmD process, McVoitte & Clark (2019) reported that the CBY is close to that found in the present experiment, ranging between 0.01 - 0.12 L gVSadded-1. However, Adriamanohiarisoamanana et al. (2017) found higher potentials,

around 0.3 L of biogas per g of SV. This difference can be explained by the fact that the influent is composed of inoculum and water, whereas in the present work only CM was used. The absence of a liquid medium leads to more time for the microorganisms that promote AD to consume the available organic matter. It is also possible to infer that the use of sewage sludge (organic and microbial stabilized) as co-digester provided the beginning of immediate biogas production (Fig. 1), but does not guarantee greater potentials (Fig. 2).

The highest CBY of 25:50:25 SS:CM:DW was comparable to the literature for another co digester (Fig. 2), indicating that the result reported in this study is consistent with that

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Braz. J. of Develop.,Curitiba, v. 6, n.5, p.23319-23334 may. 2020. ISSN 2525-8761 reported in others study. When evaluating the AcoD between cattle and swine manure, Paes et al. (2018) obtained higher potentials by using the largest quantity of cattle manure in the ratios, being 0.008 m³ kg-1 (equivalent 8 L kg-1) and 0.015 m³ kg-1 (equivalent 15 L kg-1) for treatments 1:4 and 4:1, respectively. Evaluating CM and meat and bone meal as co-digester, Adriamanohiarisoamanana et al. (2017) found CBY of approximately 0.5 L of biogas per g SV for 90% CM and 10% meat and bone meal. Adriamanohiarisoamanana et al. (2018) obtained CBY of 0.59 L gVSadded-1 50:37:13 CM: meat and bone meal:crude glycerol.

The influent amount of 3,660 kg day-1_{was obtained adopting 25:50:25 SS:CM:DW, being}

915 kg day-1 of SS, 1,830 kg day-1 of CM and 915 kg day-1 of DW. Thus, the adoption of biodigesters in the UFRRJ cattle sector would generate 2,600 kWh monthly. In monetary terms, considering the low voltage fare for rural consumption class, it would be a monthly savings of US$ 7,669.19. In the case of the biodigester supplied with only CM, monthly, the electric energy generation would be 377 kWh and savings of US$ 1,112.87. According to Quadros et al. (2010), for an average biogas production of 0.003 m3 kg-1 from the co-digestion process between goat and sheep waste, converted into electricity would result in 505 kWh month-1. According to these authors, for the average monthly consumption of a 200 kWh home, it would be possible to meet the energy demand of 2.6 homes a year.

The biogas produced by AcoD to supply monthly electricity to the UFRRJ's Dairy Cattle (Table 1), points out that its generation in loco can make the sector self-sufficient. The surplus of 1,699 kWh of electric energy can be passed on to the local concessionaire in the compensation system when the ProGD is contracted in Brazil, or even when planning new equipment acquisition. There is also the possibility of using biogas to generate thermal energy for use in water heating and cooking. According to the authors Bloch-Michalik & Gaworski (2016) and Roubík & Mazancová (2016), the thermal energy generated in biogas plants can lead to economic, social and environmental gains when used in the facilities (agroindustries, houses or buildings) surrounding.

Although the adoption of the AmD process does not generate enough electrical energy to meet all energy demand in the UFRRJ's Dairy Cattle (Table 1), the strategy would be to meet the energy demand for devices or sets essential for the management of activities with a power of up to 377 kWh. Thus, it is necessary to have a monthly plan to analyze the energy demand that should be met as a priority. An alternative to increase energy self-sufficiency would be a photothermal heating system for the water used to wash the milking machine, as well as the replacement of fluorescent and reflective lamps with LEDs (Light Emitting Diode). When

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Braz. J. of Develop.,Curitiba, v. 6, n.5, p.23319-23334 may. 2020. ISSN 2525-8761 comparing the monthly consumption of fluorescent and reflector lamps (190 kWh month-1) to LED lamps of 15 W (55 kWh month-1), there is a 71% reduction in monthly consumption. In monetary terms, replacing the water heater and fluorescent and reflector lamps would, save US$ 707.93 and US$ 162.23, totaling US$ 870.16.

4 CONCLUSIONS

Implantation of biodigesters with 25:50:25 SS:CM:DW to produce electric energy from biogas allows energy self-sufficiency, in part, of the property as the number of animals increases, enabling the sustainable development of the activity through the proper disposal of waste and economic gains to the producer.

ACKNOWLEDGEMENTS. The authors wish to thank the Fundação de Amparo à Pesquisa do Estado do Rio de Janeiro (FAPERJ) that provided support for this research project.

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