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F21/0037/2008 Page i
DECLARATION
I declare that this project report is my work and has not been submitted for a degree award in
any other university.
Signature ......................................... Date...................................
AJWANG’.B.A
This report has been submitted for examination with my approval as university supervisor.
Signature ...................................... Date.......................................
Dr.DUNCAN MBUGE.
F21/0037/2008 Page ii
DEDICATION
I dedicate this project to my family and my friends for their unfailing love and unending
support in my life.
F21/0037/2008 Page iii
ACKNOWLEDGEMENT
My sincere gratitude to my supervisor Dr.Dancun Mbuge for his great
support,sacrifice,guidance and consultation he continually offered to me through this project
process.
I would also like to recognize the effort of our inspiring chairman of the department Eng.Dr
Ayub Gitau in making our projects a success and to the entire staff of the department of
Environmental and Biosystems Engineering for their unfailling support.
Finally I want to thank all my classmates for their healthy critique and the moral support
given during this project.
F21/0037/2008 Page iv
ABSTRACT
Commercial fish catch in Kenya is dominated by Nile Perch. Of the fish that is processed for
human consumption, 30-40% is wasted. Currently, these wastes are not fully utilized; they
are sold off at low price, converted to low valued products or left to decompose leading to
environmental pollution and wastage of bioresource. This biomass has however a potential to
generate considerable revenue and can be turned into a commercially viable business through
generation of Biogas.
Capital fish (k) limited is one of the companies that deal with Nile Perch processing for both
local consumption as well as for export.They produce between 23-56 tonnes of fillets daily
but do not have an excellent systems for treating their wastes before being released into the
sewerage system while the solid wastes sold off at low costs and the rest are dumped at Rodi
Kopany.A biogas plant for the company will thus help in waaste management as well as
produce an alternative source of power which will reduce the expenditure on electricity.
The main objective of this project was to design a biogas digester for wastes from fish
proceessing plant.To achieve this the amount of waste generated by the company was assesed
and their compostion determined.the projected biogas production was calculated based on the
volatile solids.the plant units were then sized beginning with the collecting tank whose
volume was dependent on the amount of waste generated daily.The mixer volume was
calculated from the feed rate,since the volume should be between 1.5-2 of the daily feed
rate.the digester volume was determined from the formula :hydraulic retention time×slurrry
input/day.the gas holder was dimensioned based on the amount of gas produced daily and the
reationship between digester volume and gasholder volume.Thegas projected gas production
was at 158m3 daily,the mixer volume was 68.94m3 the digester volume 834m3,the collecting
tank volume 196.664m3.
F21/0037/2008 Page v
TABLE OF CONTENTS DECLARATION ........................................................................................................................................... i
DEDICATION .........................................................................................................................................ii
ACKNOWLEDGEMENT ............................................................................................................................. iii
ABSTRACT ................................................................................................................................................ iv
LIST OF TABLES ....................................................................................................................................... vii
LIST OF FIGURES .................................................................................................................................... viii
LIST OF ACRONYMS AND ABBREVIATIONS .............................................................................................. ix
INTRODUCTION ....................................................................................................................................... 1
1.1 Overview ........................................................................................................................................ 1
1.2 PROBLEM STATEMENT ........................................................................................................... 2
1.3 SITE ANALYSIS .......................................................................................................................... 2
1.4 OBJECTIVES ............................................................................................................................... 3
1.4.1 overall objective ..................................................................................................................... 3
1.4.2 Specific objectives .................................................................................................................. 3
1.5 SCOPE ............................................................................................................................................ 3
LITERATURE REVIEW ............................................................................................................................... 4
2.1. History of anaerobic digestion ................................................................................................. 4
2.2 Biogas in developing countries .................................................................................................. 4
2.3 Biogas from fish wastes ............................................................................................................. 5
2.4 Biogas in Kenya .......................................................................................................................... 6
THEORETICAL FRAMEWORK .................................................................................................................... 7
3.1 What is biogas?.......................................................................................................................... 7
3.2 ANAEROBIC DIGESTION ............................................................................................................. 8
3.4 BIOGAS PLANTS ....................................................................................................................... 13
3.5 PARTS OF A BIOGAS PLANT ............................................................................................. 14
3.6 MATHEMATICAL RELATIONS FOR PLANT DESIGN ....................................................... 22
3.6.1 SIZING A BIOGAS PLANT ....................................................................................................... 22
3.6.2 VOLUME OF INFLUENT COLLECTING TANK .......................................................................... 22
3.6.3 MIXING TANK VOLUME ........................................................................................................ 22
3.6.4 INLET PIPE ............................................................................................................................ 22
3.6.5 SIZING THE DIGESTER ........................................................................................................... 23
3.6.6 CALCULATING THE DAILY GAS PRODUCTION,G ................................................................... 23
3.6.7 DIGESTER LOADING .............................................................................................................. 24
F21/0037/2008 Page vi
3.6.8 SIZING THE GAS HOLDER ...................................................................................................... 24
METHODOLOGY ..................................................................................................................................... 27
4.1.Determining amount of waste generated daily and their characteristics. ................................. 27
4.2. Determining the amount of gas produced daily ........................................................................ 27
4.3.Design specification of the plant units: ....................................................................................... 27
4.3.1 Sizing of the collecting tank ................................................................................................. 27
4.3.2 Sizing of the mixing tank....................................................................................................... 28
4.3.3. Sizing of the digester ........................................................................................................... 28
4.3.4 Sizing the gasholder .............................................................................................................. 28
4.3.5 Pipes ..................................................................................................................................... 29
RESULTS AND ANALYSIS ........................................................................................................................ 30
5.1AMOUNT OF WASTE GENERATED ....................................................................................... 30
5.2 .SIZING THE BIOGAS PLANT ................................................................................................ 31
5.2.1.determination of the volume of influent collecting tank .................................................... 31
5.2.2 Calculating the daily gas production,G ................................................................................. 31
5.2.3.Sizing of the mixing tank ...................................................................................................... 32
5.2.4.Sizing the digester ................................................................................................................ 32
5.2.6. Calculating the gasholder size ............................................................................................. 33
5.3 .DISCUSSION ............................................................................................................................ 39
CONCLUSION AND RECOMMENDATIONS ............................................................................................. 41
REFERENCES .......................................................................................................................................... 42
APPENDICES ........................................................................................................................................... 45
F21/0037/2008 Page vii
LIST OF TABLES
Table 1: Typical composition of biogas
Table 2:Waste generated from Nile Perch processing
Table 3:Amount of solid waste generated by capital fish company daily
Table 4.:Ratios of digester to gasholder volume
Table 5::Operating capacities of fish industries in kenya
Table 6:Characterization of the waste water from Nile Perch processing
Table 7:Characterization of solid wastes from Nile Perch processing.
Table 8: Temperature ranges for anaerobic fermentation
Table 9:Biogas production from different substrates.
Table 10: Cost and Income estimate for Nile Perch factory 2008.
F21/0037/2008 Page viii
LIST OF FIGURES
Figure 1:Kinematics of Anaerobic digestion
Figure 2:Conical portion of the gasholder
Figure 3:Cylindrical portion of the gasholder
Figure 4:Schematic diagram of the plant process.
Figure 5: Sketch of the plant units
Figure 6 : Plant layout
F21/0037/2008 Page ix
LIST OF ACRONYMS AND ABBREVIATIONS
AD Anaerobic Digestion
BOD Biochemical Oxygen Demand
CHP Combined Heat and Power
COD Chemical Oxygen Demand
FPW Fish Processing Water
FW Fish Wastes
GHG Green House Gases
Gy Gas yield
HRT Hydraulic Retention Time
LCFA Long Chain Fatty Acids
OLR Organic Loading Rate
RT Retention Time
Sd Daily supply of sludge
SS Suspended Solids
TCOD Total Chemical Oxygen Demand
TSS Total Suspended Solids
VD Digester volume
VFA Volatile Fatty Acids
VG Gasholder volume
VSS Volatile SuspendedSolids
F21/0037/2008 Page 1
1
INTRODUCTION
1.1 Overview
The current global energy supply is highly dependent on fossil sources (crude oil, lignite, hard
coal, natural gas). These are fossilised remains of dead plants and animals, which have been
exposed to heat and pressure in the Earth's crust over hundreds of millions of years. For this
reason, fossil fuels are non-renewable resources which reserves are being depleted much
faster than new ones are being formed. Utilisation of fossil fuels such as lignite, hard coal,
crude oil and natural gas converts carbon, stored for millions of years in the Earth’s crust, and
releases it as carbon dioxide (CO2) into the atmosphere. An increase of the current CO2
concentration in the atmosphere causes global warming as carbon dioxide is a greenhouse gas
(GHG). The combustion of biogas also releases CO2. However, the main difference, when
compared to fossil fuels, is that the carbon in biogas was recently up taken from the
atmosphere, by photosynthetic activity of the plants. The carbon cycle of biogas is thus closed
within a very short time (between one and several years). Biogas production by AD reduces
also emissions of methane (CH4) and nitrous oxide (N2O) from storage and utilisation of
untreated animal manure as fertiliser.( www.interscience.wiley.com)
Biogas is a flexible energy carrier, suitable for many different applications. One of the
simplest applications of biogas is the direct use for cooking and lighting, but in many
countries biogas is used nowadays for combined heat and power generation (CHP) or it is
upgraded and fed into natural gas grids, used as vehicle fuel or in fuel cells. One of the main
environmental problems of today in the society is the continuously increasing production of
organic wastes. In many countries, sustainable waste management as well as waste prevention
and reduction have become major political priorities, representing an important share of the
common efforts to reduce pollution and greenhouse gas emissions and to mitigate global
climate changes. Uncontrolled waste dumping is no longer acceptable today and even
controlled landfill disposal and incineration of organic wastes are not considered optimal
practices, as environmental standards hereof are increasingly stricter and energy recovery and
recycling of nutrients and organic matter is aimed. Production of biogas through anaerobic
digestion (AD) of animal manure and slurries as well as of a wide range of digestible organic
wastes, converts these substrates into renewable energy and offers a natural fertiliser for
F21/0037/2008 Page 2
agriculture. At the same time, it removes the organic fraction from the overall waste streams,
increasing this way the efficiency of energy conversion by incineration of the remaining
wastes and the biochemical stability of landfill sites.(Biogas Handbook)
1.2PROBLEM STATEMENT
Capital Fish Limited comprise an important segment of the economy,however the nature of
fish processing wastewater suggests that they have high Biological Oxygen Demand (BOD)
together with inorganic compounds from detergents and disinfecttants used in this factory.The
factory produces about 7 tonnes os solid wastes daily which includes skins
viscera,bones,rejects,gills and heads.These wastes are not sufficiently treated.Besides,the
factory is located in area with no adequate land for building conventional centralized
wastewater treatment systems like stabilisation ponds or wetland. Currently, these wastes are
not fully utilized; they are sold off at low price, converted to low valued products or left to
decompose leading to environmental pollution and wastage of bioresource. This biomass has
however a potential to generate considerable revenue and can be turned into a commercially
viable business.
According to UN-Habitat report of the Homabay Town intergrated Solid Waste Managent
Baseline Survey,2005, Industrial wastes mainly originated from the Capital Fish (K)
LTD.Biogas technology in which biogas is derived through anaerobic digestion of biomass,
such as agricultural wastes, municipal and Industrial waste (water) is an appropriate and
economically feasible technology that combine solid waste and wastewater treatment and
energy production and can simultaneously protect the surrounding water resources and
enhance energy availability.
1.3SITE ANALYSIS
Capital Fish Kenya Limited is a fish processing company based Homa Bay in Kenya, East
Africa. Was established in the early 1990’s, and have built their heritage of quality and
world-class processing technologies, and are now one of the leading processors of Nile Perch
along the shores of Lake Victoria.
LOCATION
Homa Bay is located on the shores of Lake Victoria, Nyanza province, western Kenya. It lies
in longitude 34.300 E and latitude 0.300 S of the Equator.
F21/0037/2008 Page 3
CLIMATE
Inland equatorial, registering a minimum temperature of 17.50C and a maximum of 34.80C.It
has two rainfall seasons of between 250-700mm.
1.4 OBJECTIVES
1.4.1 overall objective
Design a biogas digester which converts wastes from fish processing industries into
resourceful source of biogas.
1.4.2 Specific objectives
� Determine the amount waste generated daily
� Determine amount of gas produced daily from the wastes.
� Design specification for each unit of the plant
1.5 SCOPE
The scope involved:
Determination of amount of waste generated daily and using the volume to predict the amount
of gas to be generated daily and in designing the plant components: collecting tank, digester,
pipes the mixer, the gas holder.
F21/0037/2008 Page 4
2
LITERATURE REVIEW
2.1.History of anaerobic digestion
Historical evidence indicates that the Anaerobic Digestion(AD) process is one of the oldest
technologies.However,the industrialization of AD began in 1859 with the first digestion plant
in Bombay.By 1895,biogas was recovered from the sewage treatment facility and used to fuel
street lamps in Exter,England.Research led by Buswell and others in the 1930s identified
anaerobic bacteria and the conditions that promote methane production.As the understanding
of the AD process and its benefits improved,more sophisticated equipments and operational
techniques emerged.the result was the use of closed tank and heating and mixing the systems
to optimize AD.Regardless of improvements,AD suffered from the development of anaerobic
treatment and low costs of coal or petroleum.While AD was used only for the treatment of
waste water sludge digestion,developing countries such as india and china embraced the
technology.small scale AD systems were mostly used for enrgy and sanitation
purposes.Numerous failures were reported.Nevertheless,technical improvements and
increasing energy prices have led to the diversification of the waste treated and the large size
AD plants.(Fabien Monner,2003).
2.2Biogas in developing countries
Biogas is used extensively througout rural China and where waste water treatment and
industry coincide,the biogas support program in Nepal has installed over 150,000 biogas
plants in rural areas and in 2005 won as Ashden award for their work.
Vietnams Biogas programme for animal husbandry sector has led to the installation of over
20000 plants throughout that country.
Biogas is also used in use in rural Coasta Rica in Colombia experiments with diesel engines-
generators sets partially fuelled biogas demonstrated that biogas could be used for power
generation,reducing electricity costs by 40% compared with purchase from the regional utiliy.
In most villages in India,manure is in regular supply.Gober gas(also spelled as `gobar’ gas by
some) is biogas generated out of cow dung.in india,Gober gas is generated at the countless
number of micro plants(an estimated more than 2 million)attached to households.
F21/0037/2008 Page 5
In parkistan the concept is quickly growing.The government of Parkistan provides 50% funds
for the construction of moveable gas chamber biogas plants.
In Rwanda,the Kigali Institute of Sciene and Technology has developed and installed large
scale biogas plants at the prisons to treat sewage and provide gas for
cooking.(www.interscience.wiley.com)
2.3Biogas from fish wastes
Thereport, `Biogas production from the waste of the Shrimp manufacture in Sisimiut’-2009,
describes the study of the viability of implementing a biogas plant in Royal Greenland
Sisimiut plant, using its shrimp waste as the substrate of the digestion. The studyconducted
bySara Sorribas Roca and Verónica Martínez Sánchezwas divided in two parts, the first one
focusing on the shrimp waste experimental analysis and the second part was about the future
biogas plant characterization.
From the experimental work was concluded a potential biogas around 200 Nm3 CH4/Tn VS;
due to some incoherence in the experimental results, its value of biogas potential was no used
to characterize the biogas plant.
Lanari & Franci (1998) examined the potential of biogas production by fish farm effluents in
a small-scale close system with partially recirculated water. The system consisted of two fish
tanks with a recirculation rate of 60% and a rainbow trout daily feeding allowance of 1%,
1.5% and 2% of live weight, an up flow anaerobic digester connected with a sedimentation
column and equipped with an aerobic filter run at psychrophilic conditions (24–25 C) and
with hydraulic retention time (HRT) 22–38 days, a zeolite column for final treatment of
effluents, a gas flow meter and a methane analyzer. Biogas and methane production amounted
to 49.8–144.2 L day) 1 and 39.8–115.4 L day) respectively.
The highest biogas and methane production was reported at the highest feeding allowance,
while the biogas methane content at 2% feeding allowance was higher than 80%. A
remarkable reduction of volatile solids (92–97%), suspended solids (96–99%) and total
ammonia nitrogen content (59–70%) in the anaerobic digester was reported; while the zeolite
ion-exchange column improved water quality of effluent produced by the digester, as the
chemical oxygen demand (COD) was reduced up to 45%.
B. Salam, M. Islam and M. T. Rahman in (2009) investigated the production ability of biogas
from anaerobic digestion of fish waste. They conducted a research in laboratory scale to
produce bio gas from fish waste and cow dung. Five laboratory scale digesters were made to
F21/0037/2008 Page 6
co-digest fish waste (FW) and cow dung (CD) in various proportions. The digesters were
made of plastic container of four litre capacity each. Fish wastes were used 200 gm. and 250
gm., and cow dung were used 0 - 300 gm. to make fish waste to cow dung ratios in the range
(wt.) of 1:0 to 1:1.5. The digester were fed on batch basis and operated at ambient
temperatures for 15 days. Total solid contents of 8% were used for all the experiments. The
highest gas yield was obtained about 2 L/kg waste from a fish waste and cow dung ratio of
1:1.2. It was observed that when only fish waste was used, gas yield obtained was 150 ml/kg
waste and it took 10 days to start bio gas generation. Whereas when cow dung was mixed
with fish waste, gas production starting time reduced to 7 days.
In order to overcome the limitation of anaerobic digestion of wastes from Nile Perch wastes
in East Africa resulting from the inhibition by high levels of lipids in the waste and ammonia
intoxication the effects of co-digestion, physical andbiological pretreatments on extent of
methane yield were investigated. At a loading ratio of 1:1(inoculum to substrate) with raw
FPW, a methane yield of 0.56 m3/kgVS was obtained. Co-digestion ofthe residue with 10%
gVS of brewery wastewater enhanced methane yield to a highest increment of66%. Long
chain fatty acids (LCFA) removal prior AD enhanced methane yield to an increment of 52%
atLCFA removal of 8%. Pretreatment of FPW with aerobic microbial cultures isolated from a
fish wastestabilization pond enhanced methane yield to an increment of 60% after 18 h, 68%
after 15 h and 76.0%after 12 h of incubation, respectively, for strains CBR 11, BR 10 and a
mixture of the two (CBR 11 + BR10).(Gumisiriza et al,2009)
2.4Biogas in Kenya
In KenyaBiogas production has been done mainly by farmers at a small scale level. This
includes digesters for manure from cattle or chicken and pig which produces gas mainly for
cooking and lighting. However, currently there is the Nyongara Slaughterhouse Biogas Pilot
plant which is located in Dagorreti.This plant uses the wastes from the slaughterhouse to
produce biogas which is further converted to electricity for lighting. The daily production is
about 40m3 which helps in reducing the general expenditure on electricity by almost half. The
slaughterhouse produces four tonnes of cow dung daily but only 300kg is used in actual
production.
F21/0037/2008 Page 7
3
THEORETICAL FRAMEWORK
3.1What is biogas?
Biogas is a flammable gas produced by microbes when organic materials are fermented in a
certain range of temperatures, moisture contents, and acidities, under airtight conditions. The
chief component of biogas is methane. In ponds, marshes and manure pits where there is a
high content of rotting organic materials, you can often see bubbles coming up to the surface
and if you lit them you would see a blue flame. Because this sort of gas is often seen in ponds
and marshes it is also known as marshgas.Biogas can then be used for cooking and lighting
and in internal combustion engines.
Biogas is a mixture of methane (60-70%), carbon dioxide (CO2), and small quantities of
hydrogen sulphide (H2S), nitrogen (N2), hydrogen (H2), and carbon monoxide (CO), and
several other hydrocarbon compounds. Methane itself is odourless, colourless and tasteless,
but the other gases contained in biogas give it a slight smell ot garlic or rotten eggs.
Table1:Typical composition of biogas
Methane is an important raw material for chemical industries; it can be used in the production
of monochloromethane, dichloromethane, chloroform (which is also a chief source of carbon
tetrachloride), acetylene, methanol etc.(Chinese Hand Book)
MATTER %
Methane CH4 50-75
Carbon dioxideCO2 25-50
Nitrogen N2 0-10
Hydrogen H2 0-1
Hydrogen sulphide H2S 0-3
Oxygen O2 0-2
F21/0037/2008 Page 8
The energy content of biogas from AD is chemically bounded in methane. The composition
and properties of biogas varies to some degree depending on feedstock types, digestion
systems, temperature, retention time.
3.2ANAEROBIC DIGESTION
AD is a microbiological process of decomposition of organic matter, in the absence of
oxygen, common to many natural environments and largely applied today to produce biogas
in airproof reactor tanks, commonly named digesters. A wide range of micro-organisms are
involved in the anaerobic process which has two main end products: biogas and digestate
The digestion can be carried out by different types of methanogenic bacteria according to the
temperature in the digester:
- Phychrophilic bacteria degrade at low temperature, between 10 to 20 0C
- Mesophilic bacteria work at medium temperature, between 20 to 35 0C
- Thermophilic digestion takes place when the temperature of the digester is in the rank 50 –
60 0C.
The methane content of the formed biogas is highly dependent on the digestion temperature,
low one produces biogas with high methane content, but low quantity of biogas because the
bacteria need longer time to digest.
Advantages of anaerobic digestion
• AD Contributes to reducing the greenhouse gases.A well managed AD system will
aim to maximise methane production but not release any gases to the aim thereby
reducing overall emissions.
3.2.1 Kinematics of anaerobic digestion
The conversion of organic matter to biogas by the means of anaerobic digestion is made up by
several steps,
F21/0037/2008
Figure 1:kinematics of anaerobic digestion.
Hydrolysis
In anaerobic digestion (AD) the term hydrolysis is used to describe degradation of a defined
particulate or macromolecular substrate to its soluble monomers. For particulates, hydrolysis
is merely a surface phenomenon, while the process is molecular for sm
(biopolymers). During hydrolysis, proteins are hydrolysed to amino acids, polysaccharide to
simple sugars and lipids to long chain fatty acids (LCFA)
microorganisms that attached to particles, produc
benefit from soluble products released by the enzymatic reaction
Acidogenesis
Acidogenesis (fermentation) is generally defined as an anaerobic acid
process without an additional electron acc
sugars (products of hydrolysis), which are relatively small soluble compounds, are taken up
by heterotrophic bacterial cells through the cell membrane and subsequently fermented or an
aerobically oxidized.
Methanogenesis
Methanogenic bacteria accomplish the final stage in the overall anaerobic conversion
oforganic matter to methane and carbon dioxide. During this last stage of AD oforganic
matter, a group of methanogenic archea both reduce carbon dioxide u
donor (autotrophic methanogens) and decarboxylate acetate to form CH4 andCO2
Figure 1:kinematics of anaerobic digestion.
In anaerobic digestion (AD) the term hydrolysis is used to describe degradation of a defined
particulate or macromolecular substrate to its soluble monomers. For particulates, hydrolysis
is merely a surface phenomenon, while the process is molecular for smaller macromolecules
(biopolymers). During hydrolysis, proteins are hydrolysed to amino acids, polysaccharide to
simple sugars and lipids to long chain fatty acids (LCFA) .This is performed by heterotrophic
microorganisms that attached to particles, produce enzymes in the vicinity of the particle and
benefit from soluble products released by the enzymatic reaction
Acidogenesis (fermentation) is generally defined as an anaerobic acid‐producing microbial
process without an additional electron acceptor.During acidogenesis, amino acids and simple
sugars (products of hydrolysis), which are relatively small soluble compounds, are taken up
by heterotrophic bacterial cells through the cell membrane and subsequently fermented or an
Methanogenic bacteria accomplish the final stage in the overall anaerobic conversion
oforganic matter to methane and carbon dioxide. During this last stage of AD oforganic
matter, a group of methanogenic archea both reduce carbon dioxide using hydrogenas electron
donor (autotrophic methanogens) and decarboxylate acetate to form CH4 andCO2
Page 9
In anaerobic digestion (AD) the term hydrolysis is used to describe degradation of a defined
particulate or macromolecular substrate to its soluble monomers. For particulates, hydrolysis
aller macromolecules
(biopolymers). During hydrolysis, proteins are hydrolysed to amino acids, polysaccharide to
This is performed by heterotrophic
e enzymes in the vicinity of the particle and
producing microbial
During acidogenesis, amino acids and simple
sugars (products of hydrolysis), which are relatively small soluble compounds, are taken up
by heterotrophic bacterial cells through the cell membrane and subsequently fermented or an
Methanogenic bacteria accomplish the final stage in the overall anaerobic conversion
oforganic matter to methane and carbon dioxide. During this last stage of AD oforganic
sing hydrogenas electron
donor (autotrophic methanogens) and decarboxylate acetate to form CH4 andCO2
F21/0037/2008 Page 10
(heterotrophic methanogens). It is only in this stage, when the influent COD isconverted to a
gaseous form that COD leaves the liquid phase of the reactor system .
3.2.2Key affected factors on the CH4 production
There are several key factors affecting CH4 production:
Temperature
Higher temperature can be beneficial for the AD. There are three temperature ranges for the
AD process: psychrophilic (0-20 ºC), mesophilic (30-42 ºC) and thermophilic (43-55 ºC).
Higher temperature can promote the degree of degradation of organic matter and the growth
of bacteria. Hence, the shorter retention time or higher organic load rate can be set in the AD
process when the temperature is in the higher range .In addition, increasing the temperature
can also destroy pathogens (Seadi et al, 2008). However, there are some weaknesses within
the higher temperature, such as lower solubility of gases (H2, NH3 and CO2) leading to the
inhibition of the methanogenesis process and higher energy consumption (Appels et al, 2008).
pH
The pH value is an indicator of acidity or alkalinity of a solution. The microorganisms are
sensitive to pH, so a significant and improper change of this value in the solution will affect
the growth of microorganisms. The effective monitoring and adjustment of pH value in the
suitable steady range is necessary to AD process. Generally speaking, the optimal pH width of
the whole AD is 5.5 to 8.5. In the other word, if the pH value exceeds both boundaries, the
methanogenesis process will be inhibited. The result of pH change is mainly from the
concentration of VFA and ammonia. More VFA accumulation can lead pH level drop. On the
contrary, too much production of ammonia when decomposing protein or too high
concentration of this compound in the substrate can cause an increase in the pH value.
Solid contents
The type of substrate used for digestion directly impacts on biogas production. The easy
degradable fraction is suitable for the digestion such as food residue and grass. On the
contrary, the materials like stone, glass or metal should be screened before digestion
otherwise those will damage the equipment. In addition, the refractory volatile solid like
F21/0037/2008 Page 11
lignocellulosic organic matter is not easy to be degraded in AD process either. So it should be
avoided (Verma, 2002).
Organic loading rate(OLR)
OLR is controlled to meet the buffered circumstances and adapt to the growth rate of bacteria.
Too high OLR could not produce many biogases due to the inhibition of much acid
productions. Besides, as to the CSTR, it may lead to failure of the digestion due to
overloading. Furthermore, if the composition of feedstock is changed in the CSTR, it must be
done progressively to give bacteria enough time to adapt to the new environment. Therefore,
using optimal OLR not only produces high quantity of biogas production, but also improves
the economy of the process (Verma, 2002; Seadi et al, 2008).
Retention time
Retention time is one of the most significant operational parameters. It depends on the volume
of the digester and the substrate fed per day. There are two ways to describe the retention
time, i.e. HRT and SRT, which mean the average times of liquid and solid are kept in the
digester respectively. The duration of retention time directly impacts on the decomposition
rate of organic materials and quantity of the bacteria left in the digester especially
methanogenic bacteria which is of the slowest duplication rate among all types of bacteria in
the AD process (Seadi et al, 2008). Normally, the digester is at the unstable condition when
SRT is less than 5 days due to more VFA accumulation and larger amount of methanogenic
bacteria washed out. During 5-8 days, the content of VFA is still increased, and some organic
compounds are hardly degraded, like lipids. So it is not the best moment to remove the
digestate either.
Stirring
Mixing is another significant factor in the AD process which can blend the feed substrate with
inoculums amply. It can also prevent the production of scum in the surface and sedimentation
of substrate at the bottom of the reactor. In addition, it can create the homogeneous condition
to avoid temperature stratification in the digester, as well as increase the duplication rate of
bacteria by gaining nutrients sufficiently. However, the immoderate stirring will destroy the
microbes. So proper or slow stirring as the auxiliary mixing is the good for the AD.
F21/0037/2008 Page 12
3.2.3 Inhibition substances
Inhibition substances contain VFA, ammonia, other nutrients and toxicity and some gases,
which has potential risk on the AD process. Therefore, the specific principle of inhibition
process of those substances and the solution to reduce the degress of inhibition are introduced
in this section.
Volatile fatty acids(VFA )
VFA is produced from the acidogenesis process, which contains longer carbon chains than
acetate. It could be degraded by acetogenic bacteria. Higher concentration of VFA
accumulation could inhibit the methano-genesis process. One reason of VFA accumulation
can be the presence of macromolecular organic material that is hard to be decomposed
directly in the feedstock. The other reason for this inhibition is low efficiency of VFA
decomposition by acetogenic bacteria (Mshandete et al, 2004). If there is an excessive
accumulation of VFA leading to an abrupt decrease of pH, a certain amount of alkaline could
be added to neutralize the condition and reduce the risk of inhibition to methano-genic
archaea. (Appels et al, 2008).
Ammonia
Ammonia is the by-product in the AD process which is mainly from proteins and other
nitrogen-containing organic materials. There are two forms of ammonia that can be
discovered in the AD, i.e. free ammonia gas (NH3) and ammonia ion (NH4+). Both of them
might bring harmful impact on the methanogenic bacteria according to the study of
Kayhanian (1999) .
The inhibition process of NH3
Kayhanian (1999) assumes that there is a change of pH in the methanogenic cell when NH3
might diffuse into it passively. It could cause that NH3 is converted to NH4+ by adsorbing
protons from the outside of the cell. The cost of it is a potassium antiporter to provide energy
for proton balance. The potassium deficiency or proton imbalance inside of the cell might be
the consequence of NH3 inhibition.
The inhibition process of NH4+
The way of the inhibition of NH4+ is totally different from the inhibition of NH3 which stops
the methane synthesizing enzyme system so as to inhibit the CH4 production from the
reaction of H2 and CO2 (Kayhanian, 1999).
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Other nutrients and toxicity
The necessary elements for the appropriate growth of microorganisms are not only organic
matter including carbon, nitrogen, phosphorous mainly, but also some trace elements such as
iron, nickel and cobalt, to provide enough nutrients and energy. Inadequate nutrients or too
high level of nutrients in the digestate both will inhibit the growth of bacteria.
H2 and CO2
A high pressure of H2 restrains the metabolism of acetogenic bacteria causing VFA and
alcohols accumulation. The reason for high pressure of H2 might be high temperature that can
decrease the solubility of H2 or inhibition of hydrogenotrophic methanogens, which can not
consume the H2 to adjust hydrogen partial pressure.
The superfluous content of CO2 will lead to a decrease in the pH level and destroy the
methanogenesis process. A high concentration of CO2 comes from high temperature or clog
of air outlet.(CHEN SHI)
3.4BIOGAS PLANTS
The two most familiar types in developing countries are the fixed -dome plants and the floating-
drum plants.
3.4.1 Selection of appropriate design
In developing countries, the design selection is determined largely be the prevailing design in
the region, which, in turn takes the climatic, economic and substrate specific conditions into
consideration. Large plants are designed on a case-to-case basis.
Typical design criteria are:
Space: determines mainly the decision if the fermenter is above-ground or underground, if it
is to be constructed as an upright cylinder or as a horizontal plant.
Existing structures may be used like a liquid manure tank, an empty hall or a steel container.
To reduce costs, the planner may need to adjust the design to theses existing structures.
Minimizing costs can be an important design parameter, especially when the monetary
benefits are expected to be low. In this case a flexible cover of the digester is usually the
cheapest solution. Minimizing costs is often opposed to maximizing gas yield.
F21/0037/2008 Page 14
Available substrate determines not only the size and shape of mixing pit but the digester
volume (retention time!), the heating and agitation devices. Agitation through gas injection is
only feasible with homogenous substrate and a dry matter content below 5%. Mechanical
agitation becomes problematic above 10% dry matter.
3.5PARTS OF A BIOGAS PLANT
3.5.1Influent collecting tank
Fresh substrate is usually gathered in an influent collecting tank prior to being fed into the
digester. Depending on the type of system, the tank should hold one to two days’ substrate.
An influent collecting tank can also be used to homogenize the various substrates and to set
up the required consistency, e.g. by adding water to dilute the mixture of vegetable solids
(straw, grass, etc.), or by adding more solids in order to increase the bio-mass.
3.5.2Inlet and outlet
The inlet (feed) and outlet (discharge) pipes lead straight into the digester at a steep angle. For
liquid substrate, the pipe diameter should be 10-15 cm, while fibrous substrate requires a
diameter of 20-30 cm. The inlet and the outlet pipe mostly consist of plastic or concrete.
3.5.3 Digester types and designs
The core of a biogas plant is the digester - an air proof reactor tank, where the
decompositionof feedstock takes place, in absence of oxygen, and where biogas is produced.
Digesters can be made of concrete, steel, brick or plastic, shaped like silos, troughs, basins
orponds, and they may be placed underground or on the surface.
The size of digestersdetermine the scale of biogas plants and varies from few cubic meters in
the case of smallhousehold installations to several thousands of cubic meters, like in the case
of largecommercial plants, often with several digesters. The design of a biogas plant and the
type of digestion are determined by the dry matter content of the digested substrate.
As mentioned before, AD operates with two basic digestionsystems: wet digestion, when the
average dry matter content (DM) of the substrate is lowerthan 15 % and dry digestion, when
the DM content of the substrate is above this value, usually between 20-40 %. These
definitions and their limit values have some regionalvariations or they can be differentiated by
legislation and support schemes .Wet digestion involves feedstock like manure and sewage
sludge, while dry digestion is applied to biogas production from solid animal manure, with
F21/0037/2008 Page 15
high straw content, household waste and solid municipal biowaste, green cuttings and grass
from landscape maintenance or energy crops (fresh or ensiled).
From the point of view of feedstock input and output, there are two basic digester types:
batch and continuous.
Batch-type digesters
The specific operation of batch digesters is that they are loaded with a portion (batch) of fresh
feedstock, which is allowed to digest and then is completely removed. The digester is fed with
a new portion and the process is repeated. Batch-type digesters are the simplest to build and
are usually used for dry digestion.
Compared to other systems, batch digestion has the advantage of low operation costs and
costs of the mechanical technology behind it and the disadvantage of high process energy
consumption and maintenance costs.
Batch digesters are also used for combined dry and wet digestion, in case of
stackablefeedstock types, where additional waste water or percolation liquid is used in larger
quantities
for flooding or percolation.
Continuous-type digesters
In a continuous-type digester, feedstock is are constantly fed into the digester. The
materialmoves through the digester either mechanically or by the pressure of the newly feed
substrate, pushing out the digested material. Unlike batch-type digesters, continuous
digestersproduce biogas without interruption for loading new feedstock and unloading the
digestedeffluent. Biogas production is constant and predictable.Continuous digesters can be
vertical, horizontal or multiple tank systems. Depending on thesolution chosen for stirring the
substrate, continuous digesters can be completely mixeddigesters and plug flow digesters.
Completely mixed digesters are typicallyvertical digesters while plug-flow digesters are
horizontal.
F21/0037/2008 Page 16
Vertical digesters
In practice, most digesters are vertical digesters. Vertical digesters are generally built on-site.
Round tanks of steel or reinforced concrete, often with a conic bottom, for easystirring and
removal of sand sediments. They are air proof, insulated, heated and outfittedwith stirrers or
pumps. The digesters are covered by a roof of concrete, steel or gas proofmembrane and the
produced biogas is piped and stored in an external storage facility, close tothe digester or
under the gas proof membrane. The membrane is inflated by the producedbiogas or it can be
fastened to a central mast .
Horizontal digesters
Horizontal digesters have a horizontal axis and acylindrical shape. This type of digesters are
usually manufactured and transported to thebiogas plant site in one piece, so they are limited
in size and volume. The standard type forsmall scale solutions is a horizontal steel tank of 50-
150 m3, which is used as the maindigester for smaller biogas plants or as pre-digesters for
larger plants. There is also analternative of concrete, the channel type digester, which allows a
larger digester volume of upto 1 000 m3.
Horizontal digesters can also run in parallel, in order to achieve larger throughput
quantities.Because of their shape, the plug-flow stream is automatically used. The feedstock
flowsslowly from the entry side to the discharge side, forming a plug-flow, streaming through
thedigester. The risk of discharging un-decomposed substrate is minimised through a
minimumguaranteed retention time (MGRT) of the substrate inside the digester. Horizontal
continuousflow digesters are usually used for feedstock like chicken manure, grass, maize
silage ormanure with a high straw content.The insulated digester is equipped with a heating
system, gas dome, manure pipes and stirrer.
Removal of sediments in the digester
Sediments of heavy materials such as sand and other non-digestible materials can
accumulateinside continuous-type digesters. Most of these materials can be removed during
pre-storageor during the feeding process. However, sand can be very strongly attached to
organic matter,thus difficult to separate prior to digestion. A large portion of this sand is
released during theAD process in the digester. Animal manure (pig slurry, chicken dung), but
also other types ofbiomass can contain various amounts of sand. Accumulation of sand inside
the tanks anddigesters reduces their active volume. The presence of sand in the biomass flow
F21/0037/2008 Page 17
is heavilyloading the stirring systems, the pumps and the heat exchangers, causing
fouling,obstructions and heavy wear. If not removed periodically, sediment layers can become
hardand can only be removed with heavy equipment. Continuous removal of sediment
layersfrom digesters can be done using floor rakes or a floor drain. Sediment formation and
the problems caused by it can be minimised by some basicmeasures:
· Regularly emptying of pre-storage and storage tanks
· Establishing sufficient pre-storage capacity
· Applying adequate stirring method
· Adequate placement of the pumping pipe stubs, in order to avoid sand circulation
· Avoiding feedstock types with high sand content
· Utilisation of specially developed methods of sand evacuation from the digesters
Measures against foam layers
Forming of foam and swimming layers can be a sign of process imbalance and theirformation
is often caused by the types of feedstock supplied. The presence of foam andswimming layers
on the surface of biomass, inside the digester, can cause clogging of gaslines. To prevent this,
gas lines should be installed as high as possible inside the digester.Foam traps can prevent
penetration of foam in the feedstock pipes and to the post digester orstorage basins. A foam
sensor can be installed in the gas area of the digester, to startautomatically
Requirements
No matter which design is chosen, the digester (fermentation tank) must meet the following
requirements:
• Water/gas tightness – water tightness in order to prevent seepage and the resultant
threat to soil and groundwater quality; gas tightness in order to ensure proper
containment of the entire biogas yield and to prevent air entering into the digester
(which could result in the formation of an explosive mixture).
• Insulation - if and to which extent depends on the required process temperature, the
local climate and the financial means; heat loss should be minimized if outside
F21/0037/2008 Page 18
temperatures are low, warming up of the digester should be facilitated when outside
temperatures are high.
• Minimum surface area - keeps cost of construction to a minimum and reduces heat
losses through the vessel walls. A spherical structure has the best ratio of volume
and surface area. For practical construction, a hemispherical construction with a
conical floor is close to the optimum.
• Structural stability - sufficient to withstand all static and dynamic loads, durable and
resistant to corrosion.
3.5.4 Internal and external forces
Two relevant forces act on the digester. The external active earth pressure causes compressive
forces within the masonry. The internal hydrostatic and gas pressures cause tensile stress in
the masonry. Thus, the external pressure applied by the surrounding earth must be greater at
all points than the internal forces. Round and spherical shapes are able to accept the highest
forces and distribute them uniformly. Edges and corners lead to peak tensile stresses which
can result in cracking.
3.5.5 Gasholders
Basically, there are different designs of construction for gasholders used in simple biogas
plants:
Floating-drum gasholders
Most floating-drum gas-holders are made of 2-4 mm thick sheet steel, with the sides made of
thicker material than the top in order to compensate for the higher degree of corrosive attack.
Structural stability is provided by L-bar bracing that also serves to break up surface scum
when the drum is rotated. A guide frame stabilizes the gas drum and prevents it from tilting
and rubbing against the masonry
Fixed-dome gasholders
A fixed-dome gas-holder can be either the upper part of a hemispherical digester
(CAMARTEC design) or a conical top of a cylindrical digester (e.g. Chinese fixed-dome
plant). In a fixed-dome plant the gas collecting in the upper part of the dome displaces a
corresponding volume of digested slurry.
The following structural measures are recommended to avoid cracks in the gas-holder:
F21/0037/2008 Page 19
• The foot of the dome (gas-holder) should be stabilized by letting the foundation slab
project out enough to allow for an outer ring of mortar.
• A rated break/pivot ring should be provided at a point located between 1/2 and 2/3 of
the minimum slurry level. This in order to limit the occurrence or propagation of
cracks in the vicinity of the dome foot and to displace forces through its
stiffening/articulating effect such that tensile forces are reduced around the gas space.
Alternatively, the lowest point of the gas-holder should be reinforced by a steel ring or
the whole gas-holder be reinforced with chicken mesh wire.
Plastic gas-holders
Gas-holders made of plastic sheeting serve as integrated gas-holders, as separate balloon/bag-
type gas-holders and as integrated gas-transport/storage elements. For plastic (sheet) gas-
holders, the structural details are of less immediate interest than the question of which
materials can be used.
3.5.6 Gas pipe, valves and accessories
Biogas piping
At least 60% of all non-functional biogas units are attributable to defect gas piping. Utmost
care has to be taken, therefore, for proper installation. For the sake of standardization, it is
advisable to select a single size for all pipes, valves and accessories.
The requirements for biogas piping, valves and accessories are essentially the same as for
other gas installations. However, biogas is 100% saturated with water vapor and contains
hydrogen-sulfide. Consequently, no piping, valves or accessories that contain any amounts of
ferrous metals may be used for biogas piping, because they would be destroyed by corrosion
within a short time.
The gas lines may consist of standard galvanized steel pipes. Also suitable (and inexpensive)
is plastic tubing made of rigid PVC or rigid PE. Flexible gas pipes laid in the open must be
UV-resistant.
3.5.7 Stirring facilities
A minimum stirring of biomass inside the digester takes place by passive stirring. Thisoccurs
by insertion of fresh feedstock and the subsequent thermal convection streams as wellas by
the up-flow of gas bubbles. As passive stirring is not sufficient for optimal operation ofthe
F21/0037/2008 Page 20
digester, active stirring must be implemented, using mechanical, hydraulic or
pneumaticequipment.Stirrers can run continuously or in sequences. Experience shows that
stirring sequences canbe empirically optimised and adapted to a specific biogas plant (tank
size, feedstock quality,tendency to form floating layers etc.).
Mechanical stirring
According to their rotation speed, mechanical stirrers can be intensive fast running
stirrers,medium running stirrers and slow running stirrers. They are completely immersed in
the feedstock and usually have two or three winged, geometrically optimised propellers. Due
to their guiding tubing system, consisting of gibbet, cable winch and lead profile, the stirrers
can usually be adjusted to height, tilt and to the side.
Paddle stirrers have a horizontal, vertical or diagonal axis .The motor is positioned outside the
digester. Junctions, where the shaft passes the digester ceiling, membrane roof or the digester
wall, have to be tight. Another possibility for mechanical mixing is axial stirrers. They are
often operatedcontinuously. Axial stirrers are usually mounted on shafts that are centrally
installed on thedigester ceiling. The speed of the engine, which is placed outside of the
digester, is reduced to several revolutions per minute, using a transmission. They should
create a steady stream in the digester that flows from the bottom, up to the walls.
In horizontal digesters, the slow running paddle-reel stirrers are usually used, but they can
also be installed in vertical digesters. Paddles are fixed on the horizontal stirring axis, which is
mixing and pressing forward (plug-flow) the feedstock. The stirring effect should only
provide vertical mixing of the feedstock.
Pneumatic stirring
Pneumatic stirring uses the produced biogas, which is blown from the bottom of the
digesterthrough the mass of the feedstock. The bubbles of rising gas cause a vertical
movement and
stir the feedstock. This system has the advantage that the necessary equipment is
placedoutside the digester (pumps and compressors), so the wear is lower. Pneumatic stirring
notfrequently used in agricultural biogas plants, as the technology is not appropriate
fordestruction of floating layers. Pneumatic stirring can only be used for thin liquid
feedstock,with low tendency of forming floating layers.
F21/0037/2008 Page 21
Hydraulic stirring
If stirred hydraulically, the feedstock is pressed by pumps and, horizontal or additionalvertical
pivoted vents, in the digester. The suction and discharging of the feedstock must bedesigned
in such a way that the digester content is stirred as thoroughly as possible.Hydraulically
stirred systems have the advantage that the mechanical parts of the stirrers areplaced outside
the digester, subject to lower wear and can be easily maintained. Hydraulicmixing is only
occasionally appropriate for destruction of floating layers and, like thepneumatic stirring, only
used for thin liquid feedstock, with low tendency of forming floatinglayers.
3.5.8Optional Parts of Biogas Plants
Heating system
Normally, because of the rather high involved costs, small-scale biogas plants are built
without heating systems. But even for small scale plants, it is of advantage for the bio-
methanation process to warm up the influent substrate to its proper process temperature before
it is fed into the digester. In the following, a number of different ways to get the required
amount of thermal energy into the substrate are described. In principle, one can differentiate
between:
• direct heating in the form of steam or hot water, and
• Indirect heating via heat exchanger, whereby the heating medium, usually hot water,
imparts heat while not mixing with the substrate.
Pumps
Pumps become necessary parts of a biogas unit, when the amounts of substrate require fast
movement and when gravity cannot be used for reasons of topography or substrate
characteristics. Pumps transport the substrate from the point of delivery through all the stages
of fermentation. Therefore, several pumps and types of pumps may be needed. Pumps are
usually found in large scale biogas units.
Weak ring
Position of the weak ring
F21/0037/2008 Page 22
The weak/strong ring improves the gas-tightness of fixed-dome plants. The weak ring
separates the lower part of the hemispherical digester, (filled with digesting substrate), from
the upper part (where the gas is stored).
Vertical cracks, moving upwards from the bottom of the digester, are diverted in this ring of
lean mortar into horizontal cracks.
3.6 MATHEMATICAL RELATIONS FOR PLANT DESIGN
3.6.1 SIZING A BIOGAS PLANT
The size of the biogas plant depends on the quantity,quality,and the kind of available biomass
and on the digesting temperature.the folloing points should be considered:
3.6.2 VOLUME OF INFLUENT COLLECTING TANK
V=Vs+Vw
Where VS=Volume of the solids
Vw=Volume of the waste water
3.6.3MIXING TANK VOLUME
A round,cylindrical shape is the cheapest and best for the mixing tank.an inlet,grit and stones
settles at the bottom of the mixing tank,for this reason,the inlet pipe (p) should be 3-5cm
higher than the tank bottom.
V=πd2h/4
Where; V=Mixing tank volume,m3
D=Diameter of mixing tank,m
h=Depth of tank,m
3.6.4 INLET PIPE
The inlet pipe must be wide and straight enough for the feed material to flow easily into the
digester.
A= πD2/4
F21/0037/2008 Page 23
Where;
A=area of the inlet pipe,m2 D=diameter of the inlet pipe
3.6.5SIZING THE DIGESTER
The size of the digester,i,e the digester volume vd,is determined on the basis of the chosen
retention time RT and the daily inputvquantity Sd.
Vd=Sd×RT[Vd=m3/day×number of days]
The retention time,in turn,is determined by the chosen/given digesting temperature.for an
unheated biogas plant,the temperature prevailing in the digester can be assumed as 1-2Kelvin
above the soil temperatures.seasonal variation must be given due considerations,however,i.e
the digester must be sized for the least favorable season of the year.for a plant of simple
design,th retention time should amount atleast 40 days.on the other hand,extra-long retention
times can increase the gas yield by as much as 40%.
The substrate input depends on how much water has been added to the substrate in order to
arrive at the solid content of 4-8%.
Substrate input(Sd)=biomass(B)+water(W)[m3/day]
3.6.6 CALCULATING THE DAILY GAS PRODUCTION,G
The amount of biogas each day G [m3 gas/day],is calulated on the basis of specific gas yield
Gy of the substrate and the daily substrate input Sd.
The calculation can bebased on:
I. The volatile solid content,VS
G=VS×Gy(solids)[m3/day=kg×m3/(d×kg)]
II. Weight of moist massB
G=B×Gy(moist mass)[m3/day=kg×m3/(d×kg)]
The temperature dependency is given by:
Gy(T,RT)=mGy×f(T,RT)
Where;
F21/0037/2008 Page 24
Gy(T,RT)=gas yield as a function of digester temperature and retention time
mGy=average specific gas yield,egl/kg volatile solids
f(T,RT)=multiplier for the gas yield as a function of digester temperature T and retention
time RT.
As a rule,it is advisable to calculate according to the several different methods, since the
available basic data are usually very imprecise so that a higher degree of sizing certainity can
be achieved by comparing and averaging the results.
3.6.7 DIGESTER LOADING
The digester loading Ld is calculated from the daily total solids input TS/d or the daily
volatile solids input VS/d and the digester volume Vd:
LdT=TS/d÷Vd[kg/(m3 d)]
LdV=VS/d÷Vd[kg/(m3 d)]
Then a calculated parameter should be checked against data from comparable plans in the
region or from pertinent literature.
3.6.8 SIZING THE GAS HOLDER
The size of the gas holder,i,e the gas holder volume Vg depends on the relative rates of gas
generation and gas consumption.the gas holder must be designed to:
1. Cover the peak consumption rate
2. Hold the gas produced during thr longest zero-consumption period
It is determined from:
I. Daily gas production,m3
II. Gas consumption,m3
III. Percentage to cater for fluctuations in production
Digester/gasholder ratio
F21/0037/2008 Page 25
The form of a biogas plant is determined by the size ratio between the digester and the
gasholder.the ratio of the digester volume(VD) and gasholder volume(VG) substantially
determines the shape and the design of a biogas plant.
The differences in digester/gasholder ratios result sorely from the differing retention times
(RT).
The digester/gasholder ratio depends primarily on:
-Retention time(RT)
-Specific gas production(Gy)
-Gasholder capacity(C)
F21/0037/2008 Page 26
F21/0037/2008 Page 27
4
METHODOLOGY
4.1.Determining amount of waste generated daily and their characteristics.
This involved collection of data from the factory as well as from existing
books,journals,internet and rsearch papers.further,the waste characteristics were obtained
from existing recods and the volume of waste generated per day was determined.
4.2.Determining the amount of gas produced daily
The amount of biogas each day G [m3 gas/day],iscalulated on the basis of specific gas yield
Gy of the substrate and the daily substrate input Sd.
The calculation can bebased on:
• The volatile solid content,VS
G=VS×Gy(solids)[m3/day=kg×m3/(d×kg)]
• Weight of moist mass B
G=B×Gy(moist mass)[m3/day=kg×m3/(d×kg)]
The calculation was based on the amount of volatile solids content input daily into the
digester.this was obtained from the amount of waste injected daily.
4.3.Design specification of the plant units:
4.3.1Sizing of the collecting tank
Since the factory operates for 24 hours each day a collecting tank was sized for the same
based on the amount of waste water generated and the solid wastes from the factory.the
volume was given by :
Total solids wastes+waste water generated.
Considering a cylindical tank,the dimensions were determined from the formula
V=���
�h
F21/0037/2008 Page 28
4.3.2Sizing of the mixing tank
The substrate input depends on how much water has been added to the substrate in order to
arrsssssive at the solid content of 4-10% which is the optimu slurry compostion.Accordind to
Fannin and Biljentina(1987) the useful volume of the mixing pit should amount to 1.5-2 times
the daily input quantity.
The daily input quantity was determined by consudering a slurry content of 9% with the
volatile solids percentage of 42 to determine the amount of water .The total sluury was
calculated from adding the solid wastes and the water component.
The mixing tank was considered to be a cylinder and the formulae used:
V=���
�h
4.3.3.Sizing of the digester
This was based on the amount of slurry aailable.the volume was calculated from the formula;
Vd=Sd×RT[m3=m3/day×number of days]
Vd=digester volume
RT=retention time
Sd=amount of slurry supplied daily
4.3.4Sizing the gasholder
The gasholder volume (VG) depends on the volume of gas production and the volume of gas
drawn off.
Gas production depends;
• on the amount and nature of the fermentation of slurry
• digester temperature
• retention time
The gas holder must be able to accept the entire volume of gas consumed at a
time.furthermore,the gasholder must be able to compensate for daily flactuations in gas
production.these flactuations range from 75%-125% of calculated gas production.
F21/0037/2008 Page 29
4.3.5Pipes
They were selected according to project requirements.based on the availability of the
material,cost implications,high strength to weight ratio,resistance to abrasion,resistance to
corrossion and serviceability the steel pipes were adopted as the most economical material for
use because they meet all the necessary mechanical properties.
F21/0037/2008 Page 30
5
RESULTS AND ANALYSIS
5.1AMOUNT OF WASTE GENERATED
The waste generated from Nile Perch processing include:
Receiving section
Filleting section
trimming
By-product
Headed and gutted
Grading and packaging
Caucus/fish rejects,wash-off water
Skin,flame/bony skeleton,bloody water,caucus
Chips,fats,fillet rejects,pieces of bones
Viscera,fats roes/eggs,head,breast
Scales,viscera,bloody water,fins
Fillet rejects,deteriorated fillets
Table 2: Wastes generated from Nile Perch processing.
The size of the biogas plant depends on the quantity,quality,and the kind of available biomass.
Fish Industriesalong Lake Victoria have an average water usage of 20 m3 per tonne of fillet
produced andthe theoretical fillet production per unit weight of fish is 42% (FAO, 2002) thus
the amount of fillet produced daily will be given as
42/100 ×20 tonnes=8.4tonnes
The amount ofsolid waste generated daily by the company is tabulated as follows;
TYPES OF WASTE AVERAGE(KG/DAY)
Fish bones 6300
Skin wastes 300
Fats 203
Fish flesh 100
Eggs 36
Fillet rejects 210
Fish intestines 12
Table 5:Solid wastes generated.
F21/0037/2008 Page 31
The total is 7161kg of solid waste daily, whichis an equivalent to7.161 tonne
The amount of waste water generated per day is estimated as
20m3 tonnes×8.4tonnes=168m3
5.2 .SIZING THE BIOGAS PLANT
5.2.1.determination of the volume of influent collecting tank
V=(Vs+Vw)(S)+d
= Πd2h/4
Where; V=Storage chamber volume,m3
VS=tonnage of solid waste generated per day S=Storage days
Vw=waste water, d=rainwater
D=tank diameter,m h=tank depth,m
Assuming a solid waste density of 1000kg/m3
7.161 ×1000kg into volume=7161/1000
=7.161m3
V=(7.161m3×4)+168m3+0.02m3
=196.664m3/day
Assuming the ratio of 1:1 for the height and diameter of tank,
V=196.664m3 D=6m
D2= (196.664×4)/6π
H=6.5m
5.2.2 Calculating the daily gas production,G
The substrate input depends on how much water has been added to the substrate in order to
arrive at the solid content of 4-10% which is the optimu slurry compostion.
Cosidering 9% of volatile solid Percentage which is 42%
F21/0037/2008 Page 32
Mass of water=42/9×(7.161)-7.161tonnes=6tonnes
=26.26m3
Thus the total slurry=7.161+26.26tonnes=33.42tonnes/day
The feed per hour=33.42tonnes/24=1.39tonnes/h
The amount of biogas each day G [m3 gas/day],is calulated on the basis of specific gas yield
Gy of the substrate and the daily substrate input Sd.
The calculation can bebased on:
I. The volatile solid content,VS
G=VS×Gy(solids)[m3/day=kg×m3/(d×kg)]
Gy=0.042m3 VS=(0.42×7161kg)
G=0.042×3007=126.3m3. allowing for 25% flactuations the adjusted capacity would be
157.9m3.
5.2.3.Sizing of the mixing tank
Accordind to Fannin and Biljentina(1987) the useful volume of the mixing pit should amount
to 1.5-2 times the daily input quantity.Thus the mixing tank volume will be given as
33.42×2=66.84m3.
Allowing for 1.5hrs of mixing,the Volume of the mixing tank=66.84m3+1.5=68.34m3
Considering a diameter of 3m,height =(68.94×4)/π×32=3.1m
5.2.4.Sizing the digester
The size of the digester,i,e the digester volume vd,is determined on the basis of the chosen
retention time RT and the daily inputvquantity Sd.
Vd=Sd×RT[m3=m3/day×number of days]
The retention time for waste treated in a mesophilic digester ranges from 15 to 30
days.settling on a RT of 20 days,the volume of the digester is given as:
Vd=1.39m3/hr×24hrs/day×20days=667.2m3
F21/0037/2008 Page 33
Adding an extra 20 % of the previous volume for theinoculums and 5% as a safety measure,
the volume above isincremented one 25%, thus the final volume of the digestershould be:
667.2×1.25=834m3
Let D=6m H=(834×4)/(π×62)=5.43m
5.2.6. Calculating the gasholder size
The amount of biogas each day G [m3 gas/day],is calulated on the basis of specific gas yield
Gy of the substrate and the daily substrate input Sd.
The calculation can bebased on:
I. The volatile solid content,VS
G=VS×Gy(solids)[m3/day=kg×m3/(d×kg)]
Gy=0.042m3 VS=(0.42×7161kg)
G=0.042×3007=126.3m3. allowing for 25% flactuations the adjusted capacity would be
157.9m3.
The digester volume to gasholder volume ratio=VD:VG is given by
834m3:157.9m3=1:5
The table below shows preffered size for various ratios of gasholder to digester
volumes(sasse-1988).
VG:VD 1:4 1:5 1:6
R √0.32��
√0.34��
√0.35��
Table 4: Ratios of gasholder to digester volume.
VG=Gasholder volume
VD=Digester volume
Using the available data for a ratio of 1:5 the various dimensions were calculated as follows;
R=√0.34��
VD=834m3
R=6.57m
F21/0037/2008 Page 34
Diameter=2×6.57m=13.14m
The depth therefore was;
Volume=base area*depth=πr2.h
Base area= πr2=6.572*π=135.6m2
Depth then equals,
������
�������� =
���
���.� =6.2m
For the gas holder an allowance of 50cm is allowed on either side to avoid contact with the
walls.
Therefore dimensions of gasholder equaled;
Diameter=13.14m-(0.5m*2)=12.14m
The top of the digester should be slanting to avoid collection of rainwater on the surface.The
slanting height is prefferable set at 1.02 times of the radius of the gasholder.
Therefore the slanting height equals;
1.02*6.07=6.12m
Total volume=volume of slanting+volume of cylindrical
=�
�h1r
2+π.r2 h2
L=6.2m
Radius=6.07m Figure 2 Cone portion of the gasholder
h1
F21/0037/2008 Page 35
h1=√(6.2! − 6.07!)=1.26m
slanting volume=π×6.072×1.26/3=48.62m3
cylinrical volume=total volume-slanting volume
157.9-48.62m3=109.28m3
Cylindrical volume=π.r2 h2
Radius=6.07m
H2
Figure 3.
Cylindrical portion of the gasholder
109.28= π*6.072*h2
H2=�%&.!�
��.%'�=0.94m
F21/0037/2008 Page 36
Pipes
Biomass pipelines should have a diameter of 300 mm. Back flow of substrate, from digester
into storage tanks, is prevented through appropriate pipeline layout. When installing the
pipes, an incline of 1-2% should be maintained, in order to allow complete
clearance(biogashandbook).
Gas treatment
1.H2S removal
Hydrogen sulphide mus be removed in order to avoid corrosion. the most common methods
used includes;
-Ion oxide -Iron chloride dosing in the digester slurry
-Activated carbon -NAOH srubbing
2.CO2 removal
Removal of carbobn dioxide enhaces of the energy of gas.it can be achieved through
-Water scrubbing -Polyethylene glycol scrubbing
-Carbon molecular sieves -Membrane seperation
F21/0037/2008 Page 37
Solid wastes
inoculums+CBR11
from collecting tank
Solid wastes
Grit
Figure 4:Schematic diagram of the plant process.
Screenin
g of
wastes
Crushing of the
solid wastes
Mixing tank
Anaerobic digester
Removal of
H2S
Removal
of CO2
Removal of
moisture
Gas storage Treatment of
the sludge
FERTILIZER
F21/0037/2008 Page 38
Figure 5: Sketch of the plant units
F21/0037/2008 Page 39
5.3 .DISCUSSION
The pH of Nile perch fish processing wastewater was about 7 (see appendices, Table 6 and 7).
These value is close to the ones reported for Trout-processing wastewater (Hwang and
Hansen, 1998), and close to 6.9; the pH value reported for slaughter house wastewater
(Masse and Masse, 2000). This suggested that there was less use of chemicals such as
detergents, which alter the effluent pH. Therefore, the waste was amenable to value addition
through bioconversion. Furthermore, fish processing wastewater contained high solid content
of which more than 80% were volatile. These values also tallied with the ones reported for
trout-processing waste-water by Hwang and Hansen (1998). This indicated that most solids
in the fish processing wastewater were of organic origin and have high energy production
potential if efficiently biodegraded through anaerobic digestion.
The relationship between the amount of carbon and nitrogen present in organic materials by
the C/N ratio.From Table 7(see appendices) it is evident enough that the C/N ratio(17) of
fish is lower than the optimum which is between 20-30(Mshandete et al 2009).As a result it
may lead to production of more ammonia which may inhibit the digestion.This however can
be offset by codigesting using other substrates which have a high C/N ratio such as straw or
cowdung or vegetable wastes.
Nile perch fish processing wastewater contains high concentrations of lipids and
proteins, which have high methane yield potential. However, anaerobic digestion (AD) of
Fish Waste Water for methane production is limited due to process inhibition by lipids
and Ammonia intoxication. To overcome these limitations, the waste water can undergo
physical and biological pretreatments before they are codigested with other materials in the
digester.
Gumisiriza et al. (2009),reportedenhancement of AD of Nile perch processing wastewater by
co-digestion, physical and biological pretreatments. It was found that co-digestion of fish
processing wastewater (FPW) with 10% gVS of brewery wastewater enhanced methane yield
to a highest increment of 66%. LCFAs removal prior AD enhanced methane yield to an
increment of 52% at LCFAs removal of 8%. Furthermore, pretreatment of Fish Processing
Waste water with aerobic microbialcultures isolated from a fish waste stabilization pond
enhanced methane yield to an increment of 60% after 18 h, 68% after 15 h and 76 % after 12
h of incubation, respectively, for strains CBR-11, BR 10 and a mixture of the two (CBR-11
+ BR10).
F21/0037/2008 Page 40
Concentrated slurries and waste with a high lipidconcentration such as fish wastes should
preferably be treated in a one-stage digester for two reasons. (1) Lipids will not be hydrolysed
in the absence of methanogenic activity. (2) The possible decrease of the lipid-water interface
in the first stage of a two-stage sludge digester can result in a longer SRT in the second stage.
Moreover hydrolysis and acidification of proteins and carbohydrates are not promoted by
acidogenic conditions
Since the dry matter in the waste is more than 15%, i.e 21% then the type of digestion
undertaken in a Continous plug flow digester is the dry digestion.Since the process is
mesophyllic then the digester has to be maintained constantly at 35-400C.This can be
achieved byheat exchangers or by using hot steam or recycling the energy produce when
biogas is produced.
A centrifugal pump using a rotating impeller can be used to increase the velocity of a fluid
where the gradient does not allow for flow by gravity. The fluid enters the pump impeller
along or near the rotating axis and is accelerated by the impeller, flowing radially outward
into a diffuser or volute chamber, from where it exits into the downstream piping system.
Centrifugal pumps are commonly used to move liquids through a piping system and are
therefore frequently used for handling liquid manure and slurries.
F21/0037/2008 Page 41
6
CONCLUSION AND RECOMMENDATIONS
The wastewater effluents has a characteristically high solid content and nutrient levels. The
solids are organic with high COD values 12,400 ± 140 mg/l (see appendix,Table 6).These
shows that the wastes are biodegradable through anaerobic digestion,and with the need for
renewable energy generation then they can be used in biogas production.
From the analysis it was established that the plant could produce upto 159m3 of gas daily at a
feed rate of 1.39 tonne/hour,the digester volume was 834m3,the mixer volume was 68.94m3
and their dimensions determined. In addition to cutting down on the expenditure on energy,
the plant will reduce the burden on the municipal to deal with the industrial wastes as well as
reduce the environmental impacts due to dumping.
Further investigations on the effect of co-digestion of the wastewater previously
pretreatedwith mixed microbial culture needs to be pursued. Additional studies on these
pretreatment methods at scaled-up level such as continuous stirred tank reactor (CSTR)
need to be conducted before pilot scale operations.
Further research could be done to find out the best substrate to be used in the codigestion
which is locally available yet can produce higher yields of biogas.
F21/0037/2008 Page 42
7
REFERENCES
1. Appels, L., Baeyens, J., Degrève, J. & Dewil, R., (2008) Principles and potential of the
anaerobic digestion of waste-activated sludge. Progress in Energy and Combustion Science.
34: 755-781.
2. Biogas Hand Book by Teodorita Al Seadi, Dominik Rutz, Heinz Prassl, Michael Köttner,
Tobias Finsterwalder,2008.ISBN 978-87-992962-0-0.
3. B. Salam, M. Islam and M. T. Rahman, Proceedings of the International Conference on
Mechanical Engineering 2009 (ICME2009) 26- 28 December 2009, Dhaka, Bangladesh
4. Chen shi,2012, Potential biogas production from fish waste and sludge,Degree Project for
the master program in Water Systems Technology Water, Sewage and Waste Technology
Department of Land and Water Resources Engineering Royal Institute of Technology (KTH)
SE-100 44 STOCKHOLM, Sweden.
5. Cninese HandBook,1997.ISBN 090 3031 655.
6.Dhanqi zhu,2010, codigestion of different wastes for enhanced methane
production,Graduate Program in Food, Agricultural and Biological Engineering,The Ohio
State University.
7. El-Mashad, H. M., and Zhang, R. (2007). Co-digestion of food waste and dairy manure
for biogas production. Transaction of the ASABE 50: 1815-1821
8. Fannin.K.F. and Bilijentina.R.1987.Reactor design in Anaerobic Digestion of biomass,141-
170
9. FAO (2002). FAO fisheries statistical year book 2002. Food and Agricultural
Organization of the United Nations, Rome pp. 27-39
10. Hwang S, Hansen CL (1998). Formation of organic acids and ammonia during
Acidogenesis of trout-processing wastewater. ASAE. 41:151-156.
11. Johns MR (1995). Developments in wastewater treatment in the meat processing industry:
a
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12. Kivaisi AK, Mukisa J (2000). Composition and anaerobic digestion ofsingle and
combined organic fractions of municipal solid waste of Dar es Salaam. Tanz. J. Sci. 26: 67-
78.
13. Kivaisi AK (2002). Pretreatment of robusta coffee hulls and co-digestion with cow-dung
for enhanced anaerobic digestion. Tanz. J. Sci. 28: 1-10.
14. Kayhanian, M., (1999) Ammonia inhibition in high-solids biogasification: an overview
and practical solutions. Environmental Technology. 20 (4): 355-365.
15. Lanari, D. & Franci, C. (1998). Biogas production from solid wastes removed from fish
farm effluents. Aquatic Living Resources, 11, 289–295.
16. Ludwig Sasse,1998-Panning and Construction of Biogas plants.
17. Masse’ DI, Masse L (2000). Characterisation wastewater from hog slaughter houses in
Eastern Canada and evaluation of their in-plant wastewater treatment systems. Canadian
Agric. Eng. 42: 38-46.
18. Mshandete A.M, Kassuwi.S.A.A, and Kivaisi.A.k,2012,Anaerobic co-digestion of
biological and pretreated fish solid wasre with fraction of market solid waste.APRN Journal
of Agricultural and biological science. review. Biores. Technol. 54:203-216.
19. Robert Gumisiriza, Anthony Manoni Mshandete, Mugassa Steven
ThomasRubindamayugi, Frank Kansiime and Amelia Kajumulo Kivaisi,2009, Nile Perch
Fish processing waste along Lake Victoria in East Africa: Auditing and Characterization.
African Journal of Environmental Science and Technology vol. 3(1), pp. 013-020.
20. Robert Gumisiriza, Anthony Manoni Mshandete, Mugassa Steven
ThomasRubindamayugi, Frank Kansiime and Amelia Kajumulo Kivaisi,2009,Enhancement of
anaerobic digestion of nile perch fish processing waste water, African Journal of
Biotechnology vol 8(2),pp328-333.
21. Seadi, T.A., Rutz, D., Prassl, H., Köttner, M., Finsterwalder, T., Volk, S. & Janssen, R.,
(2008) Biogas Handbook. University of Southern Denmark Esbjerg, Esbjerg. 125p.
22. Verma, S., (2002) Anaerobic digestion of biodegradable organics in municipal solid
wastes. Master Thesis in the Department of Earth & Environmental Engineering, Columbia
University.
F21/0037/2008 Page 44
23. Verónica Martínez Sánchez,Sara Sorribas Roca .Biogas Production from the waste of the
shrimp manufacture in Sisimiut.Arctic Technology 11427 – Spring 2009
24. www.interscience.wiley.com
F21/0037/2008
APPENDICES
Table 5:Operating capacities of fish processing companies in kenya
8
APPENDICES
:Operating capacities of fish processing companies in kenya
Page 45
F21/0037/2008 Page 46
Table 6:The characterization of waste water was done as follows(Gumisiral et al,2009)
PARAMETER MAGNITUDE
PH 6.9±0.9
EC(µS) 1180±500
SOLID CONTENT
Total Sol ids(mg/l) 5580±790
Volatile Solids(% TS) 95.4±2.5
Suspended Solids(mg/l) 4500±640
Volati le SS(% of SS) 95.6±3.8
ORGANIC CONTENT
Organic Carbon(%) 52.5±6.4
Organic Nitriogen as TKN(mg/l) 340±50
Protein(mg/l) 2020±290
Total COD(mg/l) 12400±140
Lipids(mg/l) 6160±140
Total Sugars(mg/l) 0
NUTRIENT LEVEL(mg/l)
Ammonium-Nitrogen 61±21
Reactive Phosphorous 9.2±2.4
Total Phosphorous 20±6
F21/0037/2008 Page 47
Table 7:Characterization of solid wastes
Source :Mshandete et al,2009
PARAMETER MAGNITUDE
PH 7.1±0.02
TS(%WW) 37.4±0.03
VS(%TS) 82.37±0.28
MC(%) 62.6±0.45
TOC(%TS) 48.26±0.26
TKN(%WW) 2.78±0.12
NH4-N(mg/l) 8.86±0.25
C:N 17.16
ALKALINITY 5.23±0.3
VFA(g/l) 121.06±0.21
SCOD(gO2/l) 31.19±6.15
SCOD(gO2/gTS) 83.39
TOTAL LIPIDS(%WW) 20.09±O.24
Table 8:Temperature ranges for anaerobic fermentation(source:DEKOTOP,compiled from
various sources)
Fermentation Minimum Optimum maximum Retention time
Psycrophilic 4-100C 15-180C 25-300C Over 100 days
Mesophilic 15-200C 28-330C 35-450C 30-60 days
Thermophylic 25-450C 50-600C 75-800C 10-16 days
F21/0037/2008 Page 48
SUBBSTRATE TS BIOGAS PRODUCTION METHANE
CONCENTRATION
% M3/tons TS M3/ton wet
weight
[%]
Sludge from
waste water
treatment plants
5 300 15 65
Fish waste 42 1279 537 71
Straw 78 265 207 70
Sorted food
wastes
33 618 204 63
Liquid cattle
manure
9 244 22 65
Potato hauler 15 453 68 56
Slaughterhouse
waste
16 575 92 63
Liquid pig
slurry
8 325 26 65
Substrate Handbook for Biogas production(SGC,2009).
Table 9:Biogas production from different substrates.
Estimated income(million
US$)
Estimated costs
(million US $)
Fish fillet export 6.4(94.1%)
Sale of products 0.4(5.9%)
Workers wages 0.32(7%)
Cost of electricity 0.14(3%)
Cost of water and sewarage 0.32(7%)
Fish export fee 3.68(81%)
Export certificates 0.03(0.6%)
Local authority charges 0.01(0.03)
Cost of raw product(fish) 0.02(0.04%)
Table 10:Cost and Income estimate for Nile Perch fsactory 2008.
F21/0037/2008 Page 49
Figure 6 : Dimensioned plant units and their arrangements
F21/0037/2008 Page 50
Figure 7 : Different views of the plant
F21/0037/2008 Page 51
Figure 8: location of the digester
Recommended