Literature Survey

Table of Contents
2.1 Introduction
2.2 Background
2.3 Definition of Biogas
2.4 Advantages of Biogas
2.5 Definition of Composting
2.6 Advantages of Composting
2-7 Comparison between Biogas plants and Composting
2-8 Anaerobic Digestion Definition
2-9 Classification of Anaerobic Digestion:
2-9-1 Single-stage of Anaerobic Digestion
2-9-2 Two-stage of Anaerobic Digestion
2-9-3 Three-stage of Anaerobic Digestion

2-10 Types of digesters for Anaerobic Digestion

2-10-1 Wet digesters

2-10-2 Dry digesters

2-11 Factors affecting the Anaerobic Digestion process

2-11-1 Temperature

2-11-2 pH

2-11-3 Nutrient concentration

2-11-4 Loading

2-11-5 Effect of toxins

2.12 Previous studies of biogas and compost production

References
2.1 Introduction
This chapter discusses the theory and methodology related to our current study
and tries to give an overall idea of previous studies about Biogas and Compost
production.
It discusses (anaerobic digestion, biogas plants and their types , compost plants,
factors affecting biogas production, and advantages of biogas and compost
production) .

2.2 Background
The world is facing a waste crisis as more trash put pressure on the environment.
Greenhouse gases from landfills pollute the air and cause climate change. But
there is a better way to deal with organic waste: composting and anaerobic
digestion. These methods turn waste into valuable resources like biogas and
compost, using microorganisms to break down the organic matter. (Al Zuahiri et
al., 2015).
- Organic waste is rich in organic matter that can be transformed by
microorganisms into renewable energy and soil amendments. The anaerobic
digestion process produces biogas that can be used for heating, cooking, or
electricity. The leftover material, called digestate, can be used as a ertilizer to
improve soil quality. (Wang et al., 2011; Kiran et al., 2014) .
Composting is another way to convert organic waste into compost, a natural
ertilizer that enhances soil fertility and water retention.
- The use of bacteria and fungi for waste treatment has many advantages. They
can increase the rate of degradation and the yield of biogas, they can remove
contaminants from the waste, and they can work at low temperatures, saving
energy and reducing emissions.
This study explores how can microorganisms be applied to the waste treatment
system, especially for the composting and biogas production in the anaerobic
digestion process.

Figure (1): Principles of Anaerobic Digestion

2.3 Definition of Biogas:

Biogas is a renewable energy source produced by the breakdown of organic matter
by certain bacteria under anaerobic conditions.
It is a mixture of gases composed of methane (CH4) 40 – 70 vol.%, carbon dioxide
(CO2) 30 – 60 vol.%, other gases 1 – 5 vol.% including, hydrogen (H2) 0 – 1 vol.%
and hydrogen sulphide (H2S) 0 – 3 vol.% (Sawyerr, Trois et al. 2019).
Biogas can be produced from raw materials such as municipal waste, plant
material, sewage , wastewater, and food waste agricultural waste, manure, green
waste.
Biogas is a renewable fuel that can be used for various purposes, such as in fuel
cells and for heating applications, including cooking. It can also be used in a gas
engine to generate electricity and heat from the gas energy. By removing carbon
dioxide and hydrogen sulfide, biogas can be pressurized similarly to natural gas
and used as a vehicle fuel. Biogas is a sustainable resource because it can be
produced and used continuously (Weiland 2009).

2.4 Advantages of Biogas:

• Environmentally friendly: Biogas does not produce smoke when it burns;
therefore, it does not emit harmful gases such as CO2, CO, NO2, and SO2.
• Minimizes waste: The slurry that remains after the biogas production can
be used as fertilizer in farms. The disposal method is safe and effective and
does not require landfills.
• Low-cost technology: Biogas plants have a very low installation cost and
can operate independently within 3-4 months.
• Creates jobs: Biogas production provides employment opportunities for
many people, especially in rural areas.
• Sustainable energy source: Biogas is a renewable energy source because it
is based on the continuous generation of waste (Khayal 2019).
2.5 Definition of Composting :
Composting is a way of turning organic wastes, such as food scraps, leaves, and
manure, into organic fertilizers, by letting them decompose with the help of oxygen
and microorganisms, such as bacteria and fungi (Meena, Karwal et al. 2021).
Composting is good for the environment because it reduces the amount of waste
that goes to landfills and it improves the quality of the soil and the plants that grow
on it.
Composting is a natural process that recycles carbon and nutrients in the
ecosystem(Argun, Tunç et al. 2017).

2.6 Advantages of Composting :

 Composting reduces the amount of organic waste that goes to landfills,

where it would produce greenhouse gases and leachate(Azim, Soudi et al.
2020) .
 Composting produces organic amendments that improve the soil physico-
chemical properties, such as water retention, nutrient availability, and
structure .
 Composting enhances the soil biological activity, such as microbial diversity
and biomass, and suppresses soil-borne pathogens and pests .
 Composting provides a source of renewable energy, such as biogas, that can
be used for heating, electricity, or transportation .
 Composting is a low-cost and sustainable method for managing organic
wastes and improving soil and crop quality(Meena, Karwal et al. 2021)
2-7 Comparison between Biogas plants and Composting :

Biogas plants and composting plants are two different methods for managing
organic waste and producing valuable products. Biogas plants use anaerobic
digestion to convert organic matter into biogas while Composting plants use
aerobic decomposition to transform organic matter into compost, which is a soil
amendment that can improve the physico-chemical and biological properties of the
soil.

Some of the factors that affect the comparison between biogas plants and
composting plants are:

1. Type and quality of the organic waste: Some wastes are more suitable for
anaerobic digestion, such as food waste, animal manure, and sewage
sludge, while others are more suitable for composting, such as yard waste,
paper, and wood(Siciliano, Limonti et al. 2019).
2. Environmental and economic impacts: Biogas plants can reduce
greenhouse gas emissions and fossil fuel consumption by producing
renewable energy, while composting plants can reduce landfill disposal and
chemical fertilizer use by producing organic fertilizer. However, both methods
have some drawbacks, such as odour, leachate, and pathogens(Argun,
Tunç et al. 2017).
3. Technical and operational aspects: Biogas plants require more complex
and expensive equipment, such as digesters, biogas storage, and gas
utilization systems, than composting plants, which mainly need windrows,
turners, and screening machines(Benyahya, Sadik et al. 2021).
 Biogas plants also need more careful control of the process parameters, such
as temperature, pH, organic loading rate, and carbon to nitrogen ratio, than
composting plants, which are more tolerant to variations.
Therefore, the comparison between biogas plants and composting plants depends
on the specific conditions and objectives of each case.

2-8 Anaerobic Digestion Definition:

Anaerobic digestion is a sequence of processes by which microorganisms

break down biodegradable material in the absence of oxygen.
The process is used for industrial or domestic purposes to manage waste or
to produce fuels. Anaerobic digestion involves different groups of anaerobic
microorganisms with specific growth conditions, physiological properties, and
metabolic activities(Meegoda, Li et al. 2018).
The interactions of anaerobic microorganisms are incredibly complex, and
the performance of anaerobic digestion strongly depends on the balance of
these relationships.
Anaerobic digestion is also influenced by many operational and chemical
factors, such as temperature, pH, organic loading rate, carbon to nitrogen
ratio, and aeration(Amani, Nosrati et al. 2010).
Anaerobic digestion can generate renewable and sustainable energy, such
as biogas, and improve soil and crop quality, such as compost.
 Figure (2): Steps involved in anaerobic digestion process

2-9 Classification of Anaerobic Digestion:

Anaerobic Digestion (AD) can be classified into different types based on the
number of stages and reactors involved in the process. The main types of AD
are:

2-9-1 Single-stage of Anaerobic Digestion:

This type of AD uses only one reactor to perform all the steps of AD,
such as hydrolysis, acidogenesis, acetogenesis, and
 methanogenesis. Single-stage AD is simple, low-cost, and easy to
operate, but it may have low efficiency, stability, and quality due to
the different optimal conditions required by different groups of
microorganisms.(Pham Van, Fujiwara et al. 2019)
2-9-2 Two-stage of Anaerobic Digestion :
This type of AD uses two reactors to separate the acidogenic and
methanogenic phases of AD. Two-stage AD can improve the
performance, stability, and quality of AD by providing more suitable
conditions for each phase and avoiding the inhibition of
methanogens by volatile fatty acids and other intermediates(Amani,
Nosrati et al. 2010).
2-9-3 Three-stage of Anaerobic Digestion :
This type of AD uses three reactors to separate the hydrolytic,
acidogenic, and methanogenic phases of AD. Three-stage AD can
further enhance the performance, stability, and quality of AD by
optimizing the hydrolysis rate, controlling the pH, and reducing the
retention time.
There are also other types of AD, such as multi-stage AD, which uses more than
three reactors, and hybrid AD, which combines different types of reactors in one
system The choice of the type of AD depends on the characteristics of the organic
waste, the environmental and economic impacts, and the technical and operational
aspects(Orhorhoro and Erameh 2019).
2-10 Types of digesters for Anaerobic Digestion :
there are different types of digesters for anaerobic digestion (AD) of organic
waste, which can be classified based on the moisture content of the
feedstock, the number of stages and reactors, and the mode of operation.
The moisture content of the feedstock determines whether the digester is wet
or dry.
 2-10-1 Wet digesters: operate with feedstock that has a total solids
(TS) content of less than 15%, while dry digesters operate with
feedstock that has a TS content of more than 15%1. Wet digesters
require less pre-treatment and have higher biogas production rates, but
they also need more water and energy input and have higher risk of
leachate and odour problems.
 2-10-2 Dry digesters : have lower water and energy consumption and
produce more stable and mature compost, but they also need more
pre-treatment and have lower biogas production rates and higher risk
of inhibition and instability(Saravanan, Alagesan et al. 2017).

The mode of operation determines whether the digester is continuous, semi-

continuous, or batch. Continuous digesters operate with a constant inflow and
outflow of feedstock and biogas. Semi-continuous digesters operate with a
periodic inflow and outflow of feedstock and biogas. Batch digesters operate
with a fixed amount of feedstock and biogas for a certain period of time.
Continuous and semi-continuous digesters have higher biogas production
rates and lower retention times, but they also need more control and
monitoring of the process parameters, such as temperature, pH, organic
loading rate, and carbon to nitrogen ratio. Batch digesters have lower biogas
production rates and higher retention times, but they also need less control
and monitoring of the process parameters(Youssef and M’Barek 2015).

Some examples of digesters based on these classifications are:

 Wet single-stage continuous digester: A complete mix system that uses

a stirred tank reactor to digest slurry feedstock with a TS content of less
than 15%12.
 Dry single-stage semi-continuous digester: A plug flow system that uses
a horizontal or vertical reactor to digest solid feedstock with a TS
content of more than 15%.
 Wet two-stage continuous digester: A system that uses two reactors to
separate the acidogenic and methanogenic phases of AD, such as an
acidogenic reactor followed by an up flow anaerobic sludge blanket
(UASB) reactor.
 Dry two-stage semi-continuous digester: A system that uses two
reactors to separate the hydrolytic and methanogenic phases of AD,
such as a leach bed reactor followed by a fixed film reactor.
 Wet three-stage continuous digester: A system that uses three reactors
to separate the hydrolytic, acidogenic, and methanogenic phases of AD,
such as a hydrolytic reactor followed by an acidogenic reactor and a
UASB reactor.
 Dry three-stage semi-continuous digester: A system that uses three
reactors to separate the hydrolytic, acidogenic, and methanogenic
phases of AD, such as a leach bed reactor followed by an acidogenic
reactor and a fixed film reactor.
  Wet multi-stage continuous digester: A system that uses more than
three reactors to separate different phases of AD, such as a hydrolytic
reactor followed by an acidogenic reactor, an acetogenic reactor, and a
UASB reactor.
 Dry multi-stage semi-continuous digester: A system that uses more
than three reactors to separate different phases of AD, such as a leach
bed reactor followed by an acidogenic reactor, an acetogenic reactor,
and a fixed film reactor.
 Batch digester: A system that operates with a fixed amount of feedstock
and biogas for a certain period of time, such as a floating dome digester
or a fixed dome digester

2-11 Factors affecting the Anaerobic Digestion (AD) process:

2-11-1 Temperature:
A temperature range of about 25 – 40 C (mesophilic) is generally optimal.
It can be achieved without additional heating, thus being very economical.
In some cases, additional energy input is provided to increase temperature to
50 – 60 C (thermophilic range) for greater gas production.
Often digesters are constructed below ground to conserve heat.

2-11-2 PH :
pH close to neutral, i.e., 7, is optimum. At lower Ph values (below 5.5), some
bacteria carrying out the process are inhibited.
Excess loading and presence of toxic materials will lower pH levels to below
6.5 and can cause difficulties.
When pH levels are too low, stopping the loading of the digester and/or use of
time is recommended. The presence of alkalinity between 2500 and 5000
mg/L will provide good buffering against excessive pH changes.

2-11-3 Nutrient concentration:

An ideal C: N ratio of 25:1 is to be maintained in any digester. It is an
important parameter, as anaerobic bacteria need nitrogen compound to grow
and multiply. Too much nitrogen, however, can inhibit methanogenic activity.
If the C: N ratio is high, then gas production can be enhanced by adding
nitrogen, and if the C: N ratio is low, it can be increased by adding carbon,
i.e., adding chicken manure, etc., which reduces the possibility of toxicity.
Anaerobic digestion not only breaks down plant materials into biogas, but
also releases plant nutrients, such as nitrogen (N), potassium (K) and
phosphorous (P), and converts them into a form that can be easily absorbed
by plants.
2-11-4 Loading :
When any digester is designed, the main variable to be defined is the
internal volume. The digester volume is related to, as Fulford (1998)
shows, two other parameters, and these are feed rate (Q, measured in
m3/day) and hydraulic loading or retention time (R, measured in days).
The feed rate (Q) is given by the mass of total solid (m, kg) fed daily,
divided by the proportion of total solid (TS) in the mixed slurry (as
summing the density of feed is 1000 kg/m3).
The retention time (R) of any digester is given by the volume of the digester
pit (V, m3), divided by the volume of the daily feed (Q, m3/day).

The loading rate, r (kg. VS/m3/day) of a digester is defined as the mass of

volatile solids added each day per unit volume of digester. This is related to
mass feed rate:

The typical values for the loading rate are between 0.2 and 2.0 kg
VS/m3/day.

2-11-5 Effect of toxins:

The main cause of biogas plants receiving flak is the presence of toxic
substance. Chlorinated hydrocarbons, such as chloroforms and other organic
solvents, are particularly toxic to biogas digestion.
If the digester has been badly poisoned, it may be difficult to remove the
toxins without removing most of the bacteria.
In that case, the digester must be emptied, cleaned with plenty of water and
refilled with fresh slurry.
2.12 Previous studies of biogas and compost production :
The table below includes an Overview of some important, previous review papers

Reference Remarks Subject

Barber and Reviewed currently available literature on the Anaerobic
Stuckey ABR, focusing on reactor development, baffled reactor
(1999) hydrodynamics, performance, biomass (ABR)
characteristics and retention, modeling, full-
scale operation and a comparison with other
well established alternatives
Salminen and Reviewed potential of AD for material recovery AD solid wastes
Rintala and energy production from poultry
(2002) slaughtering by-products and wastes
Kashyap et Discussed psychrophilic AD studies reported Biomethanation
al. (2003) using different substrates and the effect of
operational parameters (substrate type,
inoculums size, VFAs, HRT and OLR) on
TS/VS, BOD/COD, and biogas yield
Mahmoud et Investigated mechanisms and parameters Solid removal
al. (2003) affecting particle separation from wastewater,
with a focus on up flow reactors and the
interactions of various interrelated parameters
and their relationships to solid removal
Demirel et al. Presented general characteristics of dairy Anaerobic
(2005) waste streams, anaerobic degradation treatment of
mechanisms of main constituents of dairy dairy wastewater
 wastewaters, various anaerobic treatments of
dairy wastewaters in detail and areas where
more attention is required in the near future
Aiyuk et al. Reviewed treatment of domestic sewage Anaerobic
(2006) under hot climatic conditions, using the UASB treatment of
reactor as the core module and feasible pre- domestic
and post-treatment steps to ensure effective sewage
discharge and (or) re-use/recycling
Leita˜o et al. Presented a literature review on the types and Anaerobic
(2006) impacts of several operational and wastewater
environmental variations on the performance treatment
of anaerobic wastewater treatment systems systems
Chen et al. Provided a detailed summary of research AD inhibition
(2008) conducted on the inhibition of anaerobic
processes
Ganidi et al. Provided a detailed review of the current AD AD foaming
(2009) foaming problem and identified gaps in
knowledge regarding the theory of foam
formation in anaerobic digesters
Lesteur et al. Presented and evaluated various strategies Determining
(2009) and analytical methods to predict potential anaerobic
methane production and digestion kinetics biodegradability
while reducing the time needed for this
analysis compared with the standard
biochemical methane potential test
(Rajendra This research focuses on developing and Biogas plant
and Modi, constructing a biogas generation system generation
2014) utilizing food waste as the primary feedstock. design based on
The study encompasses the design and food wastes
implementation of an experimental
(Sunil et al., The project investigates the development of a Smart Biogas
2013) efficient, low-cost, portable biogas plant to Plant
generate energy from discarded kitchen and
food waste
(Khotmanee The proposed strategy aims to substitute An Investigation
and refined petroleum with compressed biogas for into the Potential
Pinsopon, a daily consumption of 4,800 tons. Additionally, of Biogas
2021) the plan entails utilizing biogas derived from Production
waste as the primary source for generating
600 MW of electricity.
(Karaca and The goal of this study was to calculate how using animal
Gurdil, 2019) much biogas can be created from animal manure to
manure in Samsun Province, Turkey, and how produce biogas
much energy can be extracted from it.
(Yaqoob et employing biogas production and utilizing it for producing
al., 2021) electricity generation from biomass waste sustainable
presents a sustainable and environmentally biogas from
friendly renewable energy solution. biomass waste
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