DECLARATION - Environmental and Biosystems …ebe.uonbi.ac.ke/sites/default/files/cae/engineering/ebe/beryl final... · 2.3 Biogas from fish wastes ... Gy Gas yield HRT Hydraulic

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.



is merely a surface phenomenon, while the process is molecular for smaller macromolecules


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





matter, a group of methanogenic archea both reduce carbon dioxide using hydrogenas electron


Page 9



aller macromolecules


This is performed by heterotrophic

e enzymes in the vicinity of the particle and

producing microbial

During acidogenesis, amino acids and simple





sing hydrogenas electron


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).

F21/0037/2008 Page 13

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



• The volatile solid content,VS


• 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


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


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






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






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

F21/0037/2008 Page 43

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

Documents

DECLARATION - Environmental and Biosystems …ebe.uonbi.ac.ke/sites/default/files/cae/engineering/ebe/beryl final... · 2.3 Biogas from fish wastes ... Gy Gas yield HRT Hydraulic