Nativa, Sinop, v. 11, n. 1, p. 108-114, 2023.

Pesquisas Agrárias e Ambientais

DOI: https://doi.org/10.31413/nativa.v11i1.14583

ISSN: 2318-7670

Generation of biogas and thermal energy at the Bolo das Oliveiras

Agroindustry, Pombal, Paraíba, Brazil

José Joaquim de SOUZA NETO1, Bruno Fonsêca FEITOSA2* ,

Roberlucia Araujo CANDEIA3, Mônica Tejo CAVALCANTI3,4 , Adriana Silva LIMA3

1Federal University of Paraíba, João Pessoa, PB, Brazil.

2State University of Campinas, Campinas, SP, Brazil.

3Federal University of Campina Grande, Pombal, PB, Brazil.

4National Institute of the Semiarid Region, Campina Grande, PB, Brazil.

*E-mail: brunofonsecafeitosa@live.com

Submission: 10/31/2022; Accepted on 04/10/2023; Published on 04/13/2023.

ABSTRACT: This study aimed to assess the potential for generation of thermal energy from biogas produced

by a rural biodigester in the Bolo das Oliveiras Agroindustry, Pombal/PB, Brazil. The biodigester was fed every

two days with 0.30 m3 of biomass (mixture of water and bovine manure), retention time of 45 days. Affluent

and effluent samples were collected every 15 days for 75 days. The affluent had a higher (p < 0.05) solids

contents than the effluent. The highest dissolved oxygen concentration (6.67 mg L−1) was observed in the

affluent. The effluent had lower (p < 0.05) total alkalinity than the affluent at all sampling times. CH4 values

were higher than CO2 values throughout the experiment. Biogas also contained trace proportions of H2S and

NH3 (2/3 and 1/3 ppMV, respectively). CH4 emissions were estimated at 10.58 m3 day−1. CH4 was the major

constituent of biogas, as indicated by flame combustion behavior. Generation of biogas and thermal energy at

the Bolo das Oliveiras Agroindustry may be economically feasible, providing a minimum monthly savings of

R$ 1,582.00.

Keywords: biodigester; effluent; methane.

Geração de biogás e energia térmica na agroindústria Bolo das Oliveiras,

Pombal, Paraíba, Brasil

RESUMO: Este estudo teve como objetivo avaliar o potencial de geração de energia térmica a partir do biogás

produzido por um biodigestor rural na Agroindústria Bolo das Oliveiras, Pombal/PB, Brasil. O biodigestor foi

alimentado a cada dois dias com 0,30 m3 de biomassa (mistura de água e esterco bovino), tempo de retenção

hidráulica de 45 dias. Amostras de afluentes e efluentes foram coletadas a cada 15 dias durante 75 dias. O

afluente apresentou teores de sólidos maiores (p < 0,05) do que o efluente. A maior concentração de oxigênio

dissolvido (6,67 mg L−1) foi observada no afluente. O efluente apresentou alcalinidade total menor (p < 0,05)

do que o afluente em todos os tempos de amostragem. Os valores de CH4 foram superiores aos valores de CO2

durante todo o experimento. O biogás também continha traços de H2S e NH3 (2/3 e 1/3 ppMV,

respectivamente). As emissões de CH4 foram estimadas em 10,58 m3 dia−1. O CH4 foi o principal constituinte

do biogás, conforme indicado pelo comportamento da combustão da chama. A geração de biogás e energia

térmica na Agroindústria Bolo das Oliveiras pode ser economicamente viável, proporcionando uma economia

mensal mínima de R$ 1.582,00.

Palavras-chave: biodigestor; efluente; metano.

1. INTRODUCTION

In Brazil, cattle farming is one of the most economically

important agricultural activities. The cattle population of

Paraíba State increased by 1.6% in 2017, reaching about 1.25

million head (BRASIL, 2019). However, Brazilian agricultural

enterprises generate high amounts of organic matter and have

significant consequences on the environment (SAADY;

MASSÉ, 2015).

In recent decades, the scientific community has shown

great interest in the development of sustainable alternatives

to minimize waste generation, promoting changes in the

management of cattle waste (RIOS; KALTSCHMITT, 2016).

As animal manure is mainly composed of carbon, hydrogen,

nitrogen, and oxygen, it holds potential in thermal and

electric energy production (PIÑAS et al., 2018). There are,

however, some drawbacks in the use of animal manure, such

as low biodegradability resulting from its high content of

lignocellulosic fibers

(ANDRIAMANOHIARISOAMANANA et al., 2017).

Organic matter decomposition can be exploited to

produce biogas and electric power, contributing to the

reduction of unit costs in agricultural production. Some

researchers investigated the feasibility of using biogas to meet

daily cooking energy needs (SANTOS et al., 2018). An

example of such a system has been implemented in the Bolo

das Oliveiras Agroindustry, Pombal, Paraíba, Brazil, which

had a monthly demand of 14 liquefied petroleum gas (LPG)

cylinders (13 Kg each). To partially replace the use of LPG,

Souza Neto et al.

Nativa, Sinop, v. 11, n. 1, p. 108-114, 2023.

109

we developed a rural, sertanejo-type biodigester and a biogas

purification system that is fed with cattle waste supplied by

residents of the Várzea Comprida dos Oliveiras community.

The purpose of this study was to assess the potential for

generation of biogas and thermal energy of the biodigester

developed for the Bolo das Oliveiras Agroindustry. We

investigated the physical and chemical characteristics of the

influent and effluent at different sampling times, evaluated

biogas quality in terms of its composition, flame color,

combustion characteristics, and CH4 emission, and estimated

the potential savings obtained from thermal energy

generation.

2. MATERIAL AND METHODS

2.1. Location of the experiment

The research project was developed in the Bolo das

Oliveiras Agroindustry (Figures 1A), located in the rural

community of Várzea Comprida dos Oliveiras, Pombal,

Paraíba, Brazil. According to the 2019 census, Pombal has

32,801 inhabitants and an area of 889 km2 (IBGE, 2019). The

rural community is 9.81 km away from the Center for

Agrofood Sciences and Technology (CCTA) of the Federal

University of Campina Grande (UFCG), Pombal campus,

Paraíba, Brazil, where the laboratory analyses were

performed. Geographical coordinates for the site were

determined using a global positioning system (GPS) device

(Garmin®, model 010-01199-10) and are given in UTM

(Universal Transverse Mercator) format based on the

rectangular coordinate system: 625653 m E and 9252998 m

A biodigester (Figure 1B) and biogas purification system

was installed at the Bolo das Oliveiras Agroindustry.

According to Ribeiro Filho et al. (2017), the region has hot

semi-arid climate.

2.2. Biodigester features

The biodigester has a total volume of 14.8 m3. The system

consists of (I) a 0.25 m3 feed tank, (II) a fermenting chamber

with a total capacity of 14.8 m3, (III) a 5 m3 gas holder, (IV)

a pressure adapter, (V) a primary water filter, and (VI) a 0.38

m3 overflow tank (Figure 2).

2.3. Purification system features

The biogas purification system comprises three treatment

columns (Table 1). The system also includes a compressor

that delivers the gas to an adapted oven through a 38 m long

line, operating at a pressure of about 110 lbf in−2.

Figure 2. Bolo das Oliveiras Agroindustry in Pombal, PB, Brazil (A) and area of implantation of the biodigester in the countryside (B).

Figura 2. Agroindústria Bolo das Oliveiras em Pombal, PB, Brasil (A) e área de implantação do biodigestor no interior (B).

Figure 2. Longitudinal section of the sertanejo-type biodigester and purification system. A, chemical solution; B, hydrogen sulfide (H2S) gas

filter; C, water column; D, gas compressor.

Figura 2. Corte longitudinal do biodigestor tipo sertanejo e sistema de purificação. A, solução química; B, filtro de gás sulfureto de hidrogénio

(H2S); C, coluna de água; D, compressor de gás.

Generation of biogas and thermal energy at the Bolo das Oliveiras Agroindustry, Pombal, Paraíba, Brazil

Nativa, Sinop, v. 11, n. 1, p. 108-114, 2023.

110

Table 1. Description of treatment columns of the biogas

purification system.

Tabela 1. Descrição das colunas de tratamento do sistema de

purificação de biogás.

Column

Description

Contains sodium hydroxide (NaOH) and removes

carbon dioxide (CO

Lined internally with iron filings for removal of

hydrogen sulfide (H

S) from the biogas mixture.

Contains artisanal well water and removes CO

, H

ammonia (NH3), and other gases. A safety seal

prevents the reflux of gas to the biodigester.

2.4. Methods

During the study period, the digestion chamber was fed

every two days with 0.30 m3 of biomass (a mixture of 200 L

of water and 100 Kg of bovine manure). The water used was

from an artesian well and a simple effluent treatment system

installed at the agroindustry. Bovine manure was collected

from a herd of about 40 animals raised in a semi-feedlot

system, most of which were lactating cows. The amount of

manure generated by the cattle is equivalent to that produced

by five feedlot animals (10 Kg of manure per day), according

to data from Sganzerla (1983) adapted by Colatto; Langer

(2012). A hydraulic retention time of 45 days was adopted.

Influent and effluent samples were collected every 15 days

for 75 days.

2.5. Physical and chemical analyses

Physical and chemical analyses were performed in five

replications, according to the recommendations of Baird et

al. (2023). Fixed solids (FS, combustion in a muffle furnace

at 550 °C for 1 h), total solids (TS, combustion in a muffle

furnace at 550 °C for 1 h, followed by oven-drying at 105 °C

for 24 h), and volatile solids (VS) were calculated according

to Eqs. (1), (2), and (3), respectively. Electrical conductivity

(direct electrode reading), dissolved oxygen (measured using

a portable oxygen meter under controlled aerobic conditions

at 20 °C for 5 days), pH (measured using a pH meter

calibrated with pH 4.0, 7.0, and 10.0 buffer solutions), and

total alkalinity (TA, volumetric determination using a

standardized solution of sulfuric acid and calculated using

Eq. 4) were also determined.

FS (mg L) = ()×

 (01)

TS (mg L) = ()×

 (02)

VS (mg L) = ()×

 (03)

TA (mg HCO L ) =  × × 

 (04)

where: W0 is the initial weight; W1 the dry weight; W2 the final

weight; Vs the sample volume; V the volume of base used for

titration; M the base molarity; and 61000 the mass of HCO3

expressed in milligrams.

2.6. Biogas qualification and economy estimate

The proportion and composition of gases (CH4, CO2,

H2S, and NH3) in biogas was determined on-site at the time

of sampling using a kit developed by Kunz; Sulzbach (2007).

Flame color and combustion behavior were analyzed in a

biogas stove and oven. CH4 production, expressed in m3, was

calculated using Eqs. (5) and (6) and applied to estimate the

economic savings of thermal energy generation.

CH4 (m3 day1)=××××

 (05)

d = 

 (06)

where: Mt is the total amount of manure fed to each biodigester unit

(Kg day−1), t is the sampling time (days); Na is the number of animals

producing waste, Pb is the biogas production (Kg biogas Kg−1

manure); C is the CH4 content of biogas; Ve is the specific volume

of CH4 (0.67 Kg m−3); d is the density of CH4 (0.72 Kg m−3;, m is

the mass of CH4, and V is the volume of CH4.

Biogas production was determined using the energy

conversion value for cattle recommended by the National

Biomass Reference Center (CENBIO), as adapted by

Colatto; Langer (2011).

2.7. Statistical analysis

The Shapiro-Wilk Test was applied to assess the

normality of the data. Physical and chemical data were

subjected to analysis of variance in a completely randomized

design using Assistat software version 7.7 beta (SILVA;

AZEVEDO, 2016). Means were compared by Tukey’s test at

the 5% significance level (p < 0.05).

3. RESULTS

3.1. Physical analyses

Table 2 describes the physical characteristics of influent

and effluent from the biodigester after 45, 60, and 75 days of

digestion.

The TS (3407.23–3446.67 mg L−1) and VS (3399.25–

3434.58 mg L−1) of effluents at 45 and 60 days of digestion

did not differ (p > 0.05) from each other, nor did the FS

content (6.96–7.98 mg L−1) of effluents at 60 and 75 days.

Effluent EC remained significantly higher (p < 0.05) than

influent, with no differences (p > 0.05) between effluents

collected at different times.

Table 2. Physical characteristics of the affluent and effluent of a sertanejo-type biodigester after 45, 60, and 75 days of digestion.

Tabela 2. Características físicas do afluente e efluente de um biodigestor do tipo sertanejo após 45, 60 e 75 dias de digestão.

Sample Time (days)

Parameter

TS (mg L

−1

)

FS (mg L

−1

)

VS (mg L

−1

)

EC (mS cm

−1

)

Affluent

3672.17 ± 1.02

14.52 ± 0.28

3657.64 ± 0.74

3.76 ± 0.04

Effluent

3446.67 ± 0.92

12.09 ± 0.81

3434.58 ± 0.82

4.01 ± 0.04

3407.23 ± 0.87

7.98 ± 0.50

3399.25 ± 0.51

3.95 ± 0.10

3190.85 ± 0.80

6.96 ± 0.05

3183.89 ± 0.81

3.96 ± 0.05

CV (%)

1.70

4.82

1.72

1.60

value

<0.0001

0.0059

Values are presented as the mean ± standard deviation. Means in a column followed by the same letter are not significantly different at p < 0.05 by Tukey’s

test. -, relative to all sampling periods; TS, total solids; FS, fixed solids; VS, volatile solids; EC, electrical conductivity; CV, coefficient of variation.

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111

3.2. Chemical analyses

The chemical characteristics of influent and effluents at

45, 60, and 75 days are presented in Table 3. DO values

differed significantly from each other (p < 0.05). As expected,

the influent had the highest DO content (6.67 mg L−1).

Nevertheless, digester effluents had high DO concentrations

at all sampling times.

A relationship was observed between effluent pH values,

which did not differ (p > 0.05) from those of the influent only

on days 45 and 60. Effluents had lower TA than the influent

at all sampling times, with significant differences between

effluent samples (p < 0.05).

Table 3. Chemical characteristics of the affluent and effluent of a sertanejo-type biodigester after 45, 60, and 75 days of digestion.

Tabela 3. Características químicas do afluente e efluente de um biodigestor do tipo sertanejo após 45, 60 e 75 dias de digestão.

Sample Time (days)

Parameter

DO (mg L

−1

)

TA (mg HCO

−1

)

Affluent

6.67 ± 0.06

6.67 ± 0.12

1259.67 ± 0.58

Effluent

4.83 ± 0.06

6.89 ± 0.06

1258.67 ± 0.58

4.00 ± 0.10

6.92 ± 0.06

1250.67 ± 0.58

3.60 ± 0.10

7.12 ± 0.17

1233.67 ± 0.58

CV (%)

1.71

1.58

0.05

value

<0.0001

0.007

<0.0001

Values are presented as the mean ± standard deviation. Means in a column followed by the same letter are not significantly different at p < 0.05 by Tukey’s

test. -, relative to all sampling times; DO, dissolved oxygen; TA, total alkalinity; HCO3, hydrogen carbonate; CV, coefficient of variation.

3.3. Biogas qualification and economy estimate

Figure 3A and B shows the results of biogas quality

testing, indicating the concentrations of CH4 and CO2, as well

as the proportions of H2S and NH3.

CH4 levels were higher than those of CO2 throughout the

experiment (Figure 3A). In the current study, the highest CH4

concentration in biogas (78%) was observed at 60 days after

digestion, not differing from those observed at 45 and 75

days (76%). The proportions of H2S and NH3 remained

stable throughout the experimental period (2/3 H2S and 1/3

NH3 ppMV). Figure 4 show photograph of the biogas flame.

The predominantly light blue color of the biogas flame

indicated a high concentration of CH4. We estimated that

CH4 was produced at a rate of 10.58 m3 day−1 or 317.26 m3

month−1 (30 days) under the current experimental

conditions. A CH4 volume of 317.26 m3 is equivalent, in

terms of energy, to 17–18 LPG cylinders of 13 Kg.

Figure 3. Concentrations of methane (CH4) and carbon dioxide (CO2) (A), and proportions of hydrogen sulfide (H2S) and ammonia (NH3)

(B) in biodigester effluents collected after 45, 60, and 75 days of digestion.

Figura 3. Concentrações de metano (CH4) e dióxido de carbono (CO2) (A), e proporções de sulfeto de hidrogênio (H2S) e amônia (NH3)

(B) em efluentes de biodigestores coletados após 45, 60 e 75 dias de digestão.

Figure 4. Photograph of the biogas flame on a stove top.

Figura 4. Fotografia da chama de biogás em cima de um fogão.

4. DISCUSSION

Solids determination is important, as it provides a

characterization of biodegradable matter, which directly

influences the efficiency of anaerobic digestion. Organic

matter is consumed and transformed into biogas through the

action of microorganisms; thus, the higher the amount of

biodegradable matter, the higher the biogas production

potential (HASSANEEN et al., 2020). The values of TS, FS,

and VS throughout the experimental period can be associated

with substrate degradation (Table 2). The solids contents of

the influent were significantly higher (p < 0.05) than those of

the effluents, as is expected for biogas generation, according

to Xiao et al. (2018).

The biodegradation rate of organic matter and,

consequently, the rate of biogas production depend on

Generation of biogas and thermal energy at the Bolo das Oliveiras Agroindustry, Pombal, Paraíba, Brazil

Nativa, Sinop, v. 11, n. 1, p. 108-114, 2023.

112

substrate composition and nutrient content, biomass/water

ratio, inoculum source, and process conditions, such as pH,

temperature, and hydraulic retention time (MONLAU et al.,

2015). As discussed by Simm et al. (2016), biodegradation

rate can also be influenced by digester configuration, such as

by the use of mechanical stirring and post-fermentation.

It is likely that EC values (Table 2) were associated with

the physicochemical characteristics of water from artesian

wells, as previously observed by Farhat et al. (2018). Artesian

well water was used to dilute bovine manure before feeding

the biodigester, possibly increasing the salt content of the

organic material. No study has investigated the correlation

between substrate salt concentration and biogas production.

McVoitte et al. (2019), however, identified that substrate

pretreatment may influence biogas yield and quality.

Furthermore, Arelli et al. (2018) reported that biofertilizers

with high salt concentrations obtained by digestion may

damage the soil and water bodies if used without

pretreatment.

The DO results (Table 3) suggest the occurrence of

microbial multiplication, indicating the need for post-

treatment to avoid pollution or contamination of water

bodies, soil, and air when using the effluent as an organic

fertilizer (ORRICO et al. 2016). TA, volatile fatty acids, and

pH are crucial to assess the level of substrate stability in the

digester and prevent system souring (GUIMARÃES et al.,

2018; JANKE et al., 2018). All pH values were within the

optimal range suggested for anaerobic digestion (pH 6.0–8.0)

by previous studies (GARDONI; AZEVEDO 2019),

obviating the need for pH correction.

According to Rosli et al. (2016), the TA below the affluent

(Table 3) is indicative of an expressive buffering capacity,

considered positive for anaerobic digestion. TA levels greater

than 1000 mg HCO3 L−1 are recommended to maintain a

neutral pH. Normally, TA ranges from 1000 to 5000 mg

HCO3 L−1 in anaerobic processes (PANYAPING;

MOONTEE, 2017).

Similarly, Simm et al. (2016) found high CH4

concentrations in biogas obtained by anaerobic digestion of

crude glycerin. Campos et al. (2013) reported CH4

concentrations of 48.60 to 68.14% in unpurified biogas

during 86 days of anaerobic digestion of coffee wastewater.

According to Piñas et al. (2018), biogas composition may be

influenced by substrate type and animal diet. Leite et al.

(2015), in studying dense sludge samples, observed that the

proportions of H2S and NH3 vary according to the type of

organic matter used for anaerobic digestion. The light of the

biogas flame (Figure 4) in agreement with the findings of

Mario et al. (2015). Calza et al. (2015) highlighted that the

calorific value of biogas varies according to the amount of

CH4 in the gaseous compound.

Considering that a conventional LPG cylinder of 13 Kg

costs R$ 113.00, we estimated that biogas generation

afforded a monthly savings of R$ 1,582.00 for the Bolo das

Oliveiras Agroindustry. As the agroindustry consumes only

14 cylinders per month, biogas generation provides a surplus

of four cylinders (R$ 452.00). Assuming the installation costs

of the biodigester to be R$ 9500.00, it is estimated that the

time for a return on investment is 6 months. This shows that

the sertanejo-type biodigester can be an economically feasible

investment with a short payback period.

5. CONCLUSIONS

The results of this experiment demonstrate that the solids

contents of biodigester effluents are influenced by organic

matter biodegradation rate. Substrate pretreatment was

recommended for higher biogas production and quality.

Chemical analysis underscored the importance of effluent

post-treatment to minimize environmental impacts. The

biogas had a high CH4 concentration, as evidenced by the

predominantly blue flame. The use of biogas for generation

of thermal energy in the Bolo das Oliveiras Agroindustry

seems to be economically feasible, capable of affording

minimum monthly savings of R$ 1,582.00. Biogas plants are

an efficient solution to improve manure management,

mitigate environmental impacts, and stimulate investment in

renewable energy. However, further studies are needed to

improve the efficiency, monitoring, and savings of biogas

generation.

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Author Contributions:

All authors of this work contributed equally in all

functions in the article, from its conception to the writing. All

authors read and agreed to the published version of the

manuscript.

Funding: No funding.

Institutional Review Board Statement: Not applicable.

Informed Consent Statement: Not applicable.

Data Availability Statement:

Study data can be obtained by request to the corresponding

author or the second author, via e-mail. It is not available on the

website as the research project is still under development.

Conflicts of Interest:

The authors declares no conflict of interest. Supporting entities

had no role in the design of the study; in the collection, analyses, or

interpretation of data; in the writing of the manuscript, or in the

decision to publish the results.