production of biogas through anaerobic digestion of ...psrcentre.org/images/extraimages/37...

Abstract—Anaerobic digestion is proposed to produce biogas

and enhance the methane production by identifying the best substrate.

This paper reviews the biogas production from anaerobic digestion of

various wastes. Feedstock composition is one of the major factors

that affect the production of biogas. High yields of methane depend

mainly on the substrates used as feeding material. However, the

difference in total methane yield varies based on the type of

interactions between different wastes that interfere with digestibility

of wastes in the system. The rate of digestion of organic wastes

depends mainly on the relative proportion of the component, the

amount of the mixture and other physical variables such as

temperature and pressure. There is limited information on the

optimum conditions that can enhance methane yields and treatment

of residues. It is, therefore, recommended that optimum conditions

for anaerobic co-digestion must be investigated as well as treatment

of sludge to manage the landfill crisis.

Keywords—Anaerobic-digestion, Feedstock composition,

Municipal solid waste, Waste generation

I. INTRODUCTION

REQUENT rises in fuel prices and advanced methods of

refining conventional fuels from crude oils pose a threat to

the environment and calls for a search to find cost effective

and environmentally cautious methods of finding alternative

fuels and improving engine‘s efficiencies in fuel combustion

[1].

A study has shown that Landfill gas (LFG) receives a great

deal of attention due to both negative and positive

environmental impacts, global warming and a green energy

source, respectively. Due to the exhaustion of landfills,

continuous complains from the people living in the vicinity of

landfills, and environmental impact of landfills [2], like all

methods of waste disposal, landfilling imposes both financial

and external cost on society. Financial costs refer to actual

M. Sebola is with the Department of Chemical Engineering Technology,

University of Johannesburg, Doornfontein, Johannesburg 2028 (e-mail:

Sebola [email protected])

H. Tesfagiorgis is with the Department of Chemical Engineering

Technology, University of Johannesburg, Doornfontein, Johannesburg 2028

(e-mail: [email protected])

E. Muzenda is with the Department of Chemical Engineering Technology,

Faculty of Engineering and the Built Environment, University of

Johannesburg, Doornfontein, Johannesburg 2028, Tel: +27115596817, Fax:

+27115596430, (email: [email protected])

financial outlays associated with establishment, operation and

end-of-life management of the landfill site [2]. Hence, the

conversion of biodegradable waste into energy has great

potential of reducing landfills issues while delivering energy,

economic benefits and social stability to the country.

Although the government agencies are making considerable

effort in tackling waste related problems, there are still major

gaps to be filled especially in the solid waste management.

Anaerobic digestion (AD) is one of the promising technologies

for recovering energy from municipal solid waste. It is already

a common alternative method for sewage and manure

treatments. Since food waste has the advantage of high organic

content compared with sewage or manure, AD is now

increasingly considered as a viable alternative for recovering

energy from the organic fraction of municipal solid waste,

which usually has food waste as a main component. Anaerobic

digestion is a biological process performed by many classes of

bacteria and generally consists of four steps: hydrolysis,

acidogenesis, acetogenesis, and methanogenesis, [3]. The main

product this process, methane, can be used as a vehicle fuel or

co-generation of electricity and heat, and thus, can lead to

reductions in greenhouse gas emissions.

Additionally, the transport sector, as one of the major

contributors towards energy deficiencies and greenhouse gas

emissions, is identified as an area that requires urgent

intervention. More efforts are required to address the

envisaged fuel shortage and mitigate the environmental

challenges. This can be achieved through research and

systematic programmes aimed at greening the economy

through a low carbon and resource-productive economy [4].

The transport sector is particularly of great interest due to the

high social cost of transport in South Africa.

As a renewable and sustainable source of energy, several

countries have used biogas as a preferred option [5]. However,

the process of converting bio-waste to vehicular fuel in the

form of compressed biogas (CBG) is a new technology in this

country. In addition, there is not much information concerning

how the efficiency of the energy recovery from the solid waste

can be improved.

The primary objective of this study was to review and

workout an efficient co-digestion strategy that would maximize

methane yield from the complete digestion of selected

industrial sludge.

Production of Biogas through Anaerobic

Digestion of various Waste: Review

Rebecca Sebola, Habtom Tesfagiorgis, and Edison Muzenda

F

Intl' Conf. on Chemical, Integrated Waste Management & Environmental Engineering (ICCIWEE'2014) April 15-16, 2014 Johannesburg

196

mailto:[email protected]

II. CURRENT ADVANCEMENT OF BIOGAS

In the European Union, both the primary production of

biogas and the gross electricity production from biogas have

increased by almost 18 % between 2006 and 2007 [6]. The

greatest share of this growth was achieved in Germany with

biogas companies expanding their business despite rising costs

for substrate, especially in 2008. By far, Germany has a

leading role in Europe with almost 4000 biogas plants, most of

them on farms for cogeneration.

Feedstock composition is one of the major factors that affect

the production of biogas [7]. Therefore, when designing and

operating an anaerobic digester, the quantity and

characteristics of the feedstock are important and need to be

assessed. Germany, Austria and Denmark produce the largest

share of their biogas in agricultural plants using energy crops,

agricultural by-products and manure [8]. Wastes generated

from various industries differ significantly in both their

qualities and quantities and depend on the industrial processes

and products [7]. Since it is not economically feasible to treat

these industrial wastes in separate digesters at each plant, a

centralized treatment facility is recommended [8]. Further

studies have shown that the total methane yield is linked with

the type of interaction between different wastes that interfere

with digestibility of wastes in ad processes [8]. Thus, it is

necessary to separate the negatively interacting sludge pairs

into different batches, as well as keeping all positively

interacting pairs together in AD process to improve the overall

methane yield [9].

The concept of AD whereby energy rich organic waste

material or biogas crops are added to animal manure was

realized in large scale biogas plants about two decades ago,

have shown the state of the art of co-digestion on sewage

sludge, the organic fraction of municipal solid waste

(OFMSW) and energy crops with recent progress in research

on anaerobic digestion [10], [11], [12], [13]. However, the

most used basic substrate in agriculture is pig or cow manure

in co-fermentation with biogas crops [14].

In contrast the United Kingdom, Italy, France and Spain

predominantly use landfill gas [6].

While the biogas sector grows impressively every year, it

hasn‘t received the same attention as for example liquid

biofuels for transportation [15]. The majority of people are not

aware that natural gas powered vehicles have been available

for a long time and that bio-methane could play an important

role in the transportation sector. So far, only Sweden has

established a market for bio-methane-driven cars. Due to its

relatively low prices for electricity, Sweden has traditionally

used biogas for heat production (currently around 50 % of

biogas) and focused less on electricity (8 %). About 25 % of

the produced biogas is upgraded and used as vehicle fuel while

the rest is flared or used for other applications [15].

The use of biogas in China began in 1930s and continued to

develop until today due to improving technology and

management system. Today biogas has become the biggest

biomass energy industry in China [16].

In contrast, South Africa is one of the highest emitters of

greenhouse gases per capita in the world. Carbon emissions

per capita are comparable to developed countries, whilst

human development indices remain far lower. Therefore, there

is an urgent need to decrease the carbon intensity of the South

African economy. Again, organic waste in South Africa

usually ends up in landfill sites which results in loss of a

potential energy resource whilst causing environmentally

negative impacts. To introduce biogas technology as an avenue

for renewable energy in South Africa will demonstrate the use

of available local organic waste in this technology.

Furthermore, South Africa is currently facing a potential

liquid fuels shortage [17]. It is expected that the transport

demand will increase significantly in South Africa, where more

than a doubling is expected within the next 30 years [18]. The

increase in the transport demand will strengthen South Africa‘s

dependency on oil imports which are apparent due to the lack

of own oil resources [17], as long as no alternative energy

carriers are being used extensively. Currently, Coal to Liquid

(CTL) technology is used to satisfy one third of the transport

energy demand, which has about three time‘s higher

greenhouse gas emissions than conventional petrol and diesel

fuels. Therefore, an increasing demand for fossil based

petroleum products goes hand-in-hand with an increase in

energy related GHG emissions. On the other side the use of

biogas as an alternative fuel in South Africa is not easy to

determine due to limited research in this field. Even though

some areas in SA use it for household use, the use for biogas

for vehicular use is not yet explored which can be a possible

solution to the current energy crisis.

III. ADVANTAGES OF USING BIOGAS

Due to the increasing population, access to affordable

energy services is becoming a prerequisite [19]. There is a

strong correlation among energy availability and education,

health, urban migration, empowerment, local employment and

income generation, and an overall improvement in the quality

of life [20]. Understanding and taking into account the current

status of developing nations, biogas technology has implicit

potential in improving waste management, producing clean

energy, and creating employment.

A considerable amount of renewable feedstocks in the form

of animal manure, crop residues, food and food processing

wastes, and OFMSW available in developing countries can be

utilized economically for biogas production and at the same

time reducing landfilling. In addition, resources currently

being used in the management of such wastes can be diverted

for establishing biogas plants and harness clean energy in the

form of biogas.

Mahanty et al [8] studied the effect of AD on methane yield

and observed that the reduction of industrial and municipal

wastes through anaerobic digestion followed by an aerobic

treatment such as composting could be considered as an

environmental friendly methodology [8]. It was further noted

that although some wastes are poorly biodegradable due to


197

their low solubility or suboptimal C/N ratio, satisfactorily

degradation of these substrates can take in certain

combinations [21]. Hence, a proper mixture of waste for co-

digestion can enhance sludge Solubilization, digestion, and

biomethane production by ameliorating the antagonistic and

synergistic effects of different sludges. This approach provides

some practical solutions to treats from diverse industrial

sludges in economic and environmental perspectives [8].

Additionally, AD of animal manure and other biogenic wastes

offers several environmental, agricultural and socio-economic

benefits through the fertilizer value of the digestate,

considerable reduction in odor and in activation of pathogens,

and ultimately biogas as a clean renewable fuel for multiple

end applications. It further offers many possible ecological,

technological and economical benefits [8]. Bioenergy

production in biogas plants could be enhanced by 40-80% by

using organic wastes and by-products as co-substrates [8]. In

addition to being a viable alternative of fuel source, it has the

potential to reduce the green house gases thereby creating new

possibilities of carbon trading in the global market.

Other advantages of AD include: dilution of the toxic

substances coming from any of the substrates involved, an

improved nutrient balance, synergistic effects on

microorganisms, a high digestion rate, and possible

detoxification based on the co-metabolism process [22].

Moreover, the addition of suitable organic waste favours a

more efficient stabilization, enhancing the biogas production

[22]. The dilution of toxic substance can reduce GHG

emission thus improving air quality. Additionally, it can be

produced locally, saving hard currency that are normally used

on imported natural gas and fuel [22].

IV. CHALLENGES THAT HINDER PRODUCTION OF BIOGAS AND

UTILIZATION

Besides the fact that there‘s limited knowledge on the

technology, the initial cost of installation may be high.

Funding for research is also often limited and investors might

not be keen as the biogas technology is very new. Hence the

level of the technology is not advanced to convince funders.

However, the South African National Energy Development

Institute (SANEDI) has taken an encouraging initiative to

support the energy projects. Furthermore, the production of

biogas involves multiple steps which require multi-disciplinary

inputs. For instance, the physical components of the system

requires proper designing and efficient ways of evaluating the

quality and quantity of the product. The process of anaerobic

digestion is mainly performed by diverse microorganisms.

Hence, understanding the microbiological part of the system is

critical for the success of the project. Unfortunately, this is a

rare case in most research groups where teams are set based on

common background rather than interest.

The cost of the gas may be a limiting factor for broad

consumption. For instance, the current price 9kg cylinder

ranges between R190 to R210. Such prices can exclude low

the target groups from buying the gas for different purposes. In

addition, the manner in which the gas is stored can raise a

concern of fire risks. Again health concerns like allergy and

sinuses may arise but unlike firewood, crop residues and dried

cattle dung, biogas provides a clean, smoke-free environment.

Furthermore, there can be some perception on the

environmental pollution. During the production of the gas,

Carbon dioxide is emitted to the atmosphere. However, the

same carbon dioxide released to the atmosphere is the same

released by humans, in that case there are no threat posed to

the environment. This is also supported by the theory that the

technology utilizes the carbon which is already in the

ecosystem, and not through the generation of new carbon.

V. FUTURE PERSPECTIVES

With the introduction of biogas as an alternative energy

source in SA, SA would have taken a step to develop and

implement an integrated energy strategy. This will be a

noticeable and different development path that ensures energy

for all in an equitable and environmentally friendly manner. In

decades to come, SA will be powered by a low carbon

economy with a significant share of green jobs, where citizens

have accessible, affordable, safe, efficient energy services and

the transport system that does not affect the health of people.

The use of biogas will also make SA to focus on clean energy

technology that will promote a visible shift towards low

polluting transport sectors, fuels and vehicles.

Similarly, all disadvantaged communities will also be

provided with effective energy services depending on their

needs. A noticeable reduction of fuel poverty, respiratory

illnesses and safety threats will be observed through the use of

cleaner and safer household fuels.

VI. BIOGAS PRODUCTION FROM VARIOUS WASTE

Anaerobic co-digestion of different organic wastes together

can improve nutrient balance, dilute potentially toxic

compounds such as sulphur-containing substances, and

subsequently increase the processing capacity and biogas yield

[8]. Weiland, 2010 [23] reported that bioenergy production in

biogas plants could be enhanced by 40-80% by using organic

wastes and by-products as co-substrates.

Tewelde et al. [24] investigated the biogas production from

co-digestion of brewery waste (BW) and cow dung (CW).

Total solids (TS), volatile solids (VS), chemical oxygen

demand (COD), methane yield (CH4) and carbon dioxide

(CO2) were measured as shown in Table I. the study reported

74% conversion of organic solids. The maximum methane

yield of 69% was obtained when the ratio CD/BW was 70:30.


198

TABLE I

CHEMICAL COMPOSITION OF BW AND CD AND CORRESPONDING METHANE YIELDS AT VARIOUS RATIOS [24]

Parameters

TS[%] VS[%] pH

COD

[mg/l]

BOD

[mg/l]

Nitrogen

[mg/l]

Phosphorous

[mg/l]

BW 8.2 94 4.8 6000-8100 2800-6100 40-60 30-40

CD 8 83 7.3 6100 4290 30-38 10

Ratios of CD:BW

90:10 80:20 70:30 60:40 40:60 20:80

CH4

[%] 67 67.5 69 66 63.2 59.6

CO2

[%] 30.5 30 29.7 31.5 32.8 33.9

Alvarez, and Liden [25] investigated the semi- continuous

co-digestion of slaughterhouse waste, manure and fruit and

vegetable waste. Their results showed that the co-digestion of

slaughterhouse waste with various co-substrates showed

positive methane production of 80%. This was supported by

other researchers [26] who did an investigation on biogas

production from cow dung, cow pea and cassava pealing and

attained 76% methane yield.

A similar study was done by Ward et al. [27] on

optimization of anaerobic digestion of agricultural resources

showed 82% conversion of volatile solids, proving an increase

in methane production.

In Mahanty et al. [8] investigated the optimization of

various industrial sludges for biogas production. The waste

sludges were collected from waste treatment facilities of paper,

chemicals, automobile, food processing and petrochemicals. A

polynomial model was used to optimize the gas production as

depicted in Fig. 1.

Fig. 1 Optimisation scheme flow diagram for sludge co-digestion

using polynomial model [8]

It was reported that the maximum possible methane yield is

increased from one batch to three batches (specific

combination of sludge) of co-digestion process as shown in

Fig. 2. This was due to the positively interacting pairs together

in co-digestion process described by Abdullah et al, [28], who

studied simplex-centroid mixture formulation for optimized

composting of kitchen waste. Furthermore, they noted that

methane yield was found to decrease under five batches based

optimized co-digestion process. Thus, the digestibility of

various industrial sludges is improved under different batches.

Fig. 2 Methane yield from utilisation of sludges in different co-

digestion process scenarios consisting of one to five independent co-

digestion batches [8].

The biogas production from co-digestion of corn stover

(CS) and chicken manure (CM) was studied by Yegin et al.

[29]. Their tests were carried out in triplicates using 1 litre

bottles with working volumes 0.5L at 37˚C. The Co-digestion

of CS and CM significantly increased methane yield, with

methane yield reaching as much as 218.8 mL/g.

Xiao, Xingbao and Zheng [30] presented a pilot scale

anaerobic co-digestion of municipal biomass waste. The focus

was on methane production and green house gas (GHG)

reduction. It was reported that 78% methane was produced

with. Grisel et al, in [31] also investigated the biogas

production from co-digestion of coffee pulp and cow-dung

under solar radiation. It was found that during the first month

co-digestion at mesophillic conditions, methane content in the

biogas obtained was 50%. The content increased up to 60%

and remained constant for at least eight months of further

digestion. However, Thong et al [32] on the thermophillic

anaerobic co-digestion of oil palm empty fruit bunches with

palm mill effluent for efficient biogas production, showed 98%

biodegradability of the feedstock and 82% methane yield with

the corresponding energy content of 36 MJ per m3. This was


199

found to be feasible in the thermophillic acidogenic hydrolysis

of lignocellulosic in the empty fruit bunches up to mixing ratio

of 2:3:1.

This section of the study review the investigation conducted

by Bernd, Ivo, Gabriel and Vincent in [14]. [14] Investigated

the mesophillic anaerobic co-digestion of cow dung manure

and biogas crops in German biogas plants. In this study the

effect of hydraulic retention time and volatile solid (VS) crop

proportion in the mixture on methane yield was studied.

Methane yield as a function of retention time in storage tank

for varying temperature with zero pressure, time in storage

tank needed to reach a certain degradation of digestate as a

function of temperature and methane yields as a function of

retention time in storage tank for varying temperature with

pressure of 1 are shown in Fig. 3, 4, 5 respectively. Fig. 3

showed that for higher storage tank temperature, maximum

methane production is obtained faster. Long time is required to

reach satisfactory degradation of feed sock as shown in Fig 5.

The study further revealed that co-digestion of organic wastes

depends mainly on the relative proportion of the component,

the amount of the mixture and other physical variables such as

temperature and pressure.

Fig. 3 Methane yield as a function of retention time in storage tank

for varying temperature with zero pressure [14].

Fig. 4 Time in storage tank needed to reach a certain degradation of

digestate as a function of temperature [14].

Fig. 5 Methane yields as a function of retention time in storage tank

for varying temperature with pressure of 1[14].

Despite the well known benefits of co-digestion, such as

optimum humidity, buffering capacity and C/N ratio or

inhibitory substances dilution [9], it is not clear whether some

co-substrates have adverse impact when they are co-digested

with another waste in particular if there is synergisms or

antagonisms among the co-digested substrates and if several

co-substrates of similar biochemical composition can be co-

digested [9]. Therefore, it is critical to obtain an optimal

mixture of the available co-substrates as well as the optimum

operating conditions, which allow high biogas yields without

compromising the stability of the process [9].

Pastor et al. [33] also reviewed the composition effect on

biogas production. It was observed that an adequate mixture

formulation is needed in order to ensure the correct

functioning of the anaerobic digestion process. The following

parameters have been taken into account in order to obtain an

adequate mixture formulation for co-digestion: biogas

production improvement, composition, nutrient balance and

risk of inhibition by long chain fatty acids (LCFA) [33].

VII. CONCLUSION

Conversion of waste into energy is a technology that has the

potential in producing cleaner energy and greener alternative

fuel. Anaerobic digestion technology is considered to be a

practical method to reduce waste. It is not feasible and

economic to treat these industrial wastes in separate digesters

at each plant rather to install a centralized treatment facility for

all combined waste together. Studies determining the

limitations of co-digestion, parameters influencing the

anaerobic process and reactions involved to attain methane

however, optimum conditions to enhance satisfactory methane

yields and treatment of residues have not been reported in

literature. It is therefore, recommended that optimum

conditions for anaerobic co-digestion must be investigated as

well as treatment of sludge to manage the landfill crisis.


200

ACKNOWLEDGMENT

The authors are grateful to the South African National

Energy Development Institute (SANEDI) and the

Department of Chemical Engineering for supporting the

research.

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