Investigation of Operating Conditions
for Optimum Biogas Production in
Plug Flow Type Reactor
K.U.C. Perera
Master of Science Thesis
KTH School of Industrial Engineering and Management
Energy Technology EGI-2009-2011
Division of xxx
SE-100 44 STOCKHOLM
Master of Science Thesis EGI 2009:2011
Title Investigation of Operating Conditions
for Optimum Biogas Production in Plug Flow
Type Digester
Name K.U.C.Perera
Approved
Date
Examiner
Name
Prof. Torsten Fransson
Supervisors
KTH: Joseph Olwa
OUSL: Dr. Gamini Kulathunga
Contact person
Abstract
Biogas is a sustainable alternative for fossil fuels. This also provides a solution to bio wastes. There are
various designs for the efficient production of biogas. Majority of biogas generation units use animal
wastes as feed material. Plug flow type biogas digester was identified as more suitable for using non
conventional feed stocks like green vegetations, municipal waste for the biogas production. These
digesters have been reported to be more efficient in converting feed stocks with higher total solids
content. The aim of this study was to investigate the variables associated with plug flow biogas digesters
and to determine the operational parameters that will give optimum biogas production. In the study three
feed rates were used. The highest specific methane production of 0.341 m3/kgVS per day was observed at
feeding rate of 1.4kg per day. Highest VS reduction of 91.37% was observed at feeding rate of 1.4 kg per
day. VFA profile and pH profile along the digester length showed the distribution of biogas production
stages along the length of the digester especially at lower feed rates.
Table of Contents
Abstract
ii
Index of Tables ........................................................................................................................................................iii
Index of Figures .......................................................................................................................................................iv
Nomenclature ............................................................................................................................................................v
Chapter 1: Introduction
1
1.1 Brief History ........................................................................................................................................................1
1.2 Biogas generation Process................................................................................................................................. 1
1.3 Parameters affecting anaerobic digestion ....................................................................................................... 2
1.3.1 Total solid content ...................................................................................................................................... 2
1.3.2 Temperature ................................................................................................................................................ 2
1.3.3 Particle size .................................................................................................................................................. 3
1.3.4 Retention time ............................................................................................................................................. 3
1.3.5 Inoculums .................................................................................................................................................... 3
1.3.6 pH ................................................................................................................................................................. 3
1.3.7 Alkalinity ...................................................................................................................................................... 4
1.3.8 Carbon to Nitrogen ratio...........................................................................................................................4
1.3.9 Organic loading rates ................................................................................................................................. 4
1.3.10 Additives and inhibitors ..........................................................................................................................4
1.4 Benefits and uses of biogas...............................................................................................................................5
1.5 Types of processes ............................................................................................................................................. 5
1.5.1 Classification by the way material is fed.................................................................................................. 5
1.5.2 Classification by operating temperature .................................................................................................. 7
1.5.3 Classification by fluid flow pattern .......................................................................................................... 7
1.5.4 Classification by biochemical change of feed stock .............................................................................. 8
1.5.5 Other classifications ................................................................................................................................... 8
1.6 Problem identification ....................................................................................................................................... 9
1.7 Objectives ............................................................................................................................................................9
Chapter 2: Literature review
10
2.1 Problems and difficulties in the existing plug flow digesters ....................................................................11
Chapter 3: Methodology and Experimental Set up
12
3.1 Experimental setup ..........................................................................................................................................12
3.2 Initial preparation .............................................................................................................................................14
3.3 Analytical procedure ........................................................................................................................................15
3.3.1 Total solids content ..................................................................................................................................15
3.3.2 Volatile solids content..............................................................................................................................16
3.3.3 Chemical oxygen demand........................................................................................................................16
3.3.4 Volatile fatty acids.....................................................................................................................................16
3.3.5 PH ...............................................................................................................................................................16
3.3.6 Gas composition .......................................................................................................................................16
3.3.7 Specific methane production ..................................................................................................................17
3.38 Volatile solids reduction ...........................................................................................................................17
Chapter 4: Results and Analysis
18
4.1 Gas volume and composition.........................................................................................................................18
4.2 pH variation of effluent...................................................................................................................................23
4.3 VFA variation along the digester length .......................................................................................................25
4.4 Effluent COD ...................................................................................................................................................28
4.5 Process performance........................................................................................................................................29
Chapter 5: Discussion
31
Chapter 6: Conclusion
33
Bibliography
34
Annexure I Gas collection data
33
Annexure II Gas composition data
39
Annexure III pH Data
40
Annexure IV VFA Data
41
Annexure V COD Data
44
Annexure VI Data for TS and VS Determination
45
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Index of Tables
Table3.1: Feedstock characteristics
15
Table3.2: Feeding plan
15
Table 4.1: Daily average biogas production
18
Table 4.2: Composition variation of biogas for feeding rates of 3kg per day and 1.4kg per day in Digester 1
22
Table 4.3: Composition variation of biogas for feeding rate of 6kg per day in Digester 2
22
Table 4.4: pH Variation of effluent for three feed rates
23
Table 4.5: VFA variation along the digester length for feeding rate of 1.4kg per day in digester 1
25
Table 4.6: VFA variation along the digester length for feeding rate of 3 kg per day in digester 1
25
Table 4.7: VFA variation along the digester length for feeding rate of 6kg per day in digester 2
27
Table 4.8: Effluent COD variation for three feed rates
28
Table 4.9: Process performance
29
Table 5.1: Comparison between similar studies done in plug flow digesters
31
Table 8.1: Gas volume data
38
Table 8.2: Gas composition data
31
Table 8.3: pH data
40
Table 8.4: Titration volume data for 3kg per day
31
Table 8.5: Titration volume data for 1.4kg per day
42
Table 8.6: Titration volume data for 6kg per day
43
Table 8.7: Effluent COD data
44
Table 8.8: Feed stock TS and VS data
45
Table 8.9: Effluent TS and VS data
46
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Index of Figures
Fig. 1.1: Batch type Digester
5
Fig. 1.2: Semicontinuous Digester
6
Fig. 1.3: Continuous Digester
7
Fig. 3.1: Diagram of the Digester
12
Fig.3.2: Digester 1
13
Fig. 3.3: Digester 2
13
Fig.3.4: Inlet of the digester
13
Fig. 3.5: Gas Holder
14
Fig. 4.1: Daily average biogas production in Digestor 1
19
Fig. 4.2. Composition Variation of biogas for feed at 3kg/day and 1.4kg/day feed rates in Digestor 1
21
Fig. 4.3: Composition variation of biogas at feed rate 6kg/day in Digester 2
22
Fig. 4.4: Variation of effluent pH for different feed rates
24
Fig.4.5: VFA variation along the digester length for feeding rate of 1.4kg per day in Digester 1
26
Fig. 4.6: VFA variation along the digester length for feed rate of 3kg/day in Digester 1
27
Fig. 4.7: VFA Variation along the digester length for feed rate of 6kg/day in Digester 2
25
Fig 4.8: COD variation of effluent for different feed rates
29
-iv-
Nomenclature
COD
DBR
GHG
HRT
NERDC
n.d.
OLR
TS
VFA
Vo
VS
VSin
VSout
VSR
W
Chemical Oxygen Demand
Dry Batch Reactor
Green House Gas
Hydraulic Retention Time
National Engineering research and Development Centre
no date
Organic Loading Rate
Total Solids
Volatile Fatty Acids
Average daily methane production
Volatile Solids
Volatile Solids in feedstock
Volatile Solids in effluent
Volatile Solids Reduction
Weight of daily feedstock added
-v-
mg/l
(%)
mg/l
m3/d
(%TS)
(%TS)
(%TS)
(%)
(kg)
Chapter 1: Introduction
With the fast depletion of non-renewable energy sources and high prices, investigation of all possible
alternative energy sources especially the renewable energies such as solar, biomass, and wind has been
increased. In most developing countries biomass waste is abundant source of energy, which can be utilised
to generate energy and to produce manure for agriculture as a by product.
Biogas is generated by anaerobic digestion of organic matter. It is composed of methane and carbon
dioxide (Goswami, n.d.). Organic matter refers to agricultural residues, manure, and garbage and sewage
waste. They originate from wide variety of sources spread throughout the world. Since they can be derived
from relatively recently living material than fossil fuel, they are sustainable.
1.1 Brief History
There are records that biogas was used by Assyrians and Persians for heating water (Lusk, 1998). Marco
Polo in his records mentioned that he had seen covered sewage tanks in China although it is not clear
whether they have utilized the gas (Ding, et al., 2010). Alexander Volta identified the presence of methane
in marsh gas (Wellinger, n.d.). Sir Humphrey Davy identified methane exists in the gas generated by
anaerobic digestion of manure [Lusk, 1998]. In Europe the biogas generation was promoted during First
World War and Second World War times as a solution to the fuel deficits (Lusk, 1998). The biogas
technology started to spread in India by 1950 in rural household applications (Wargert, 2009). The first
monograph on biogas was published in China in1935, which was the first publication on biogas in the
world (Xiaodong, 2009). Following that in 1958 campaigns to promote the multiple uses of biogas helped
the popularization of the technology in China (Lee, 2008). Biogas technology was supported by the
government of China as they have identified importance of the technology for rural development.
According to Sri Lanka Standards Institution (1292:2006), Continuous type digesters are found in Sri
Lanka by 1960.
1.2 Biogas generation Process
Anaerobic digestion is the decomposition of complex organic compounds to simple matter by the microorganisms. This process occurs in three major steps: hydrolysis, acidogenesis, and methanogenisis.
In hydrolysis aerobic micro-organisms convert complex organic compounds to simple forms which are
soluble and can be consumed by the micro-organisms. Polysaccharides are converted into
monosaccharides, lipids to fatty acids, proteins to amino acids and neuclic acids to purines and
pyrimidines.
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In Acidogenisis facultative and obligate anaerobic micro-organisms convert the low molecular weight
products from the first stage to lower molecular weight intermediate compounds of volatile fatty acids,
alcohols, carbon dioxide and hydrogen.
The final step is the methane formation step where the anaerobic methanogen converts the acetic acids in
to methane or reduce carbon dioxide by hydrogen to form methane. The basic chemical equation for
anaerobic digestion of organics can be presented as below (Goswami and Kreith, 2007:
Organic matter+H2O+Neutrients
) (Goswami and Kreith, 2007)
new cells +resistant organic matter+CO2+CH4+NH3+H2S+heat
1.3 Parameters affecting anaerobic digestion
1.3.1 Total solid content
Total Solid content for anaerobic digestion can be divided into three ranges. Low solids content refers to
systems with Total Solids content less than 10% [Monnet, 2003]. Medium solids content refers to Total
Solids content between 15-20% and High solids content refers to Total Solid content in the range of 2240% (Monnet, 2003). Higher Total Solid content requires smaller digester volume due to lower water
content.
1.3.2 Temperature
Anaerobic digestion takes place at two different temperature ranges.

Mesophillic condition 20-45 o C(Monnet,2003)
Mesophillic bacteria have lower metabolic rates. Mesophillic digestion requires longer retention times. But
they are more robust to the changes in temperature. They are able of producing good quality effluent
(Ostrem, 2004)
 Thermophillic condition 50-65 o C(Monnet,2003)
The fermentation is more efficient at the higher temperature process (Chengdu biogas research
institute, 1992). Destruction of pathogens is more efficient at thermophillic temperatures (Goswami, n.d.).
The micro-organisms are sensitive to changes of temperature as smaller as 5oC (Chengdu biogas research
institute, 1992).
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1.3.3 Particle size
Particle size affects the rate of reaction. The smaller the particles size the more the reaction rates due to
increase in surface area. This increases the gas generation rate and reduces the amount of residue, which in
turn reduces the digestion time. The particle size reduction allows the suspension of the particles that
lowers the settling time and subsequently the flow of particles with the fluid.
1.3.4 Retention time
Retention time is the time needed for complete degradation of the organic material. This depends on the
composition of the feedstock, temperature, pH and number of other variables which affect the anaerobic
digestion process. Higher TS content in feed increases the retention time while favourable temperature
ranges decreases the retention time. The smaller the particles size the shorter the retention time due to
high reaction rates.
1.3.5 Inoculums
Micro-organisms produce biogas by digesting the organic compounds. Sufficient quantities of microorganisms are needed for successful biogas production. Amount of micro-organisms in fresh material is
below the required quantity of microbes. Therefore sufficient inoculums must be added at the starting of
the process. The sources of micro-organisms affect the biogas production. Different types of microorganisms have different capabilities in digesting particular type of material. By applying suitable cultures
efficient biogas production can be achieved. Bacterial cultures can be obtained from cattle manure, sewage
waste, and other biogas producing facilities or artificial cultures.
1.3.6 pH
The optimum pH values for the anaerobic digestion are in the range of 6.4 – 7.2. The optimum pH for
methanogens is 6.6 -7.0(Monnet, 2003).Growth rate of methanogenic bacteria is slower than the
acidogenic bacteria. At lower pH values and higher feed rates the growth rate of acidogenic bacteria
increases. Therefore acid formation during acidogenisis reduces the pH of the medium and inhibits the
methanogenisis process.
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1.3.7 Alkalinity
Alkalinity is the ability of the digestion medium to absorb protons or capability in neutralizing the excess
acidic or basic conditions. Calcium carbonate is used as a buffer substance in digestion process and also
used to indicate the alkalinity of the medium.
1.3.8 Carbon to Nitrogen ratio
For effective anaerobic digestion carbon to nitrogen ratio should be maintained between the range 20 30(Chengdu biogas research institute, 1992). Lower C: N ratio causes ammonia accumulation in the
digester and inhibits micro-organism activities. Higher C: N ratio causes lower gas production. Different
types of material are mixed together to maintain the optimum C: N ratio of the feedstock.
1.3.9 Organic loading rates
Organic Loading Rate (ORL) shows the ability of a system to convert the feed stocks to end products.
Exceeding the allowable OLR causes formation of inhibitors in the digester and may cause failures in the
digestion process.OLR is either expressed as Chemical Oxygen Demand or Volatile Solids per unit
volume of reactor. For a given feed stock and a reactor volume COD and VS related with retention time.
1.3.10 Activators and inhibitors
Activators can enhance the biogas production while inhibitors can reduce the biogas production.
Activators or inhibitors are added in small amounts in order to enhance or reduce biogas production in
digesters, respectively. These substances can be enzymes from organic or inorganic compounds. Leaves of
plants, legumes and microbial cultures are examples of additives which are used for enhancing biogas
production. Leucacena leucocephala, Acacia auriculiformis, Dalbergia Sisoo were reported in literature as
enhancing agents of biogas production (Kohil, et al., 2004). Microbial cultures like actinomycetes and
mixed consortia were reported as enhancing biogas production with cattle dung. These cultures stimulate
enzymatic activities which produces biogas. Inorganic compounds that enhance biogas production are
iron salts like FeCl3 and FeSO4, and heavy metals (Goswami, n.d.; Kohil, et al., 2004). Adapted microorganisms can endure the effects of inhibitors and can avoid the effects on biogas production.
Some of the inhibitors which affect the biogas production are light, disinfectants, hydrogen sulphide and
ammonia. These compounds affect the biogas production negatively when presents in higher
concentrations [SEAI, www.seai.ie/Renewables/Bioenergy/Anaerobi./].
-4-
1.4 Benefits and uses of biogas
Biogas is used in the following applications




Heating
Lighting
Electricity Generation
Motor Fuel
The following are the some of the benefits of the biogas







Sustainable energy source
Waste Management
Reduce green house gas emission and other gas emissions which cause climate change
Carbon neutral energy source
Valuable by-product generation
Reduce fossil fuel usage
Rural development
1.5 Types of processes
1.5.1 Classification by the way material is fed
Batch –Fed Digestion
Material fed into the digester at a time and sealed only allowing the gas to exit. When the digestion is
completed the residue is taken out and the next batch can be fed. Biogas generation volume varies with
time for this type of reactor, but it requires less control when the process is implemented correctly.
Gas outlet
Man hole
Slurry
Fig.1.1: Batch type digester (SLSI, 2006)
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Semi- Continuous Digestion
In this process a quarter to half of the total solids of the material inside the digester, during whole
digestion process, is fed at the start-up of the digestion process. During the digestion process fresh
material is fed and some digested material is taken out, intermittently. Fermented material remaining in the
digester, without being removed can be taken out after sometime of operation and feeding can be started
from the beginning. The biogas production is steady in this type of digesters. Start up process and
discharging is labour intensive for this type of digesters. The biogas units mainly used in China is
considered to be semi continuous type. This is used to digest other residues than manure. The straw is fed
when the batch is loaded and the manure can be fed daily. Digested material can be withdrawn once in six
months and the next batch can be loaded (Nijaguna, n.d.).
Inlet pipe
Gas outlet
Man hole
Level
Level
Outlet
Inlet pipe
Slurry
Fig.1.2: Semi continuous digester (SLSI, 2006)
Continuous Fermentation
After starting the operation of the digester regular quantity of material is fed continuously and same
quantity of digested material is discharged. Quantity and quality of the fluid remains stable and the biogas
production is also stable. This technology is suitable for both medium and large scale waste treatment and
large scale biogas production. The floating dome type biogas digesters in India is considered equivalent to
this type due to continuous sludge removal.
-6-
Floating gas
Floating
MixingGas holder
holder
pit
Gas outlet
Outlet
Mixing pit
Level
Inlet pipe
Inlet
pipe
Slurry
Partition
wall
Fig.1.3: Continuous digester (Prabhu and Stalin; 2007)
1.5.2 Classification by operating temperature
Constant Temperature Fermentation
In this type of process the temperature of the working medium is kept constant. Therefore the biogas
yield is stable. Water circulation jackets with heaters, insulation and passive solar heating can be used for
maintaining constant temperature (Ostrem, 2004)
Ordinary Temperature Fermentation
In this process temperature of the working medium changes with the air temperature or earth temperature
and this process does not require control of temperature.
1.5.3 Classification by fluid flow pattern
Unstirred and Stratified Fluid Flow
When the material is fed and not stirred it settles in three layers irrespective of whether the material is
homogeneous or heterogeneous. When the material stratifies micro-organisms can only utilise nutrients in
the near vicinity. Micro-organisms face the difficulty of utilising the nutrients away from them. The
residue at the bottom occupies more of the inside space of the digesters. This will reduce the gas yield. To
replace the residue entire material has to be removed.
-7-
Mixed Flow
In this process material is well mixed by a stirrer. Therefore material and micro-organisms are well mixed
and get contact with the feed. The gas formation is rapid and biogas yield is high. These types of digesters
are used for large scale application where animal wastes and urban sludge is treated.
Plug Flow
Theoretically material does not mix in the vertical direction. Digested material is pushed forward by the
fresh material fed into the digester. This process gives more uniform retention times for the materials fed
to the digester. Actual operation is rather complex and different.
1.5.4 Classification by biochemical change of feed stock
One Phase Digestion
All the phases of fermentation; hydrolysis, acidogenisis and methanogenisis happens inside a single
digester. In this type of digestion biogas yield per reactor volume and per unit mass of the feed stock is
lower than for two phase digestion since acid production can affect the methane production phase.
Two Phase Digestion
Hydrolysis and acidogenis phases are carried out in a separate digester which is well mixed or a plug flow
type digester. Methanogenisis phase is carried out in another type of digester like sludge blanket digester
or anaerobic filter which are highly effective for the methanogenisis process. This gives more biogas yield
per reactor volume and per unit mass of the feed stock.
1.5.5 Other classifications
Some of the other classifications of the digestion process are based on mode of growth of microorganisms, concentration of the material, and number of digesters connected in the process. Digesters
used in practice have a number of above mentioned features integrated in their operation. Therefore they
cannot be categorized into the basic classifications.
-8-
1.6 Problem identification
Great potential exists in the Sri Lanka for biogas generation and it is one of the sustainable solutions for
the waste disposal problem in the country, for rural development of the country and reduction fossil fuel
imports.
First biogas digesters introduced were continuous flow type digesters. Continuous flow biogas reactors
require regular water supply and is preferred for animal and human wastes. Considering these facts a new
biogas digester was developed by National Research and Development Centre of Sri Lanka which is called
as Dry Batch Reactor (DBR). The preferred material for Dry Batch biogas digesters are straw and animal
waste. The moisture content of the input material should be more than 85% while for the continuous type
digester solid content should be 9 to 10% (SLSI, 2006). Disadvantages of the dry batch system are
difficulty in loading and unloading and bio sludge in larger digesters, unstable gas generation rate and the
long operating periods. Several parameters affect the optimum performance of plug flow type biogas
plants. However, no well established information is available for optimum operation of these types of
biogas plants; hence the use of plug flow type has been neglected in compared to other types.
Chanakya, et al. (2004) mentioned the failures in converting other types of biomass into manure like
slurries for biogas production. This reason has led them to introduce two designs for the successful use of
other types of biomass for biogas production. They are plug flow digesters and solid state stratified bed
digesters (Chanakya, et al., 2004).
The plug flow type digester has been used to deal with wastes like green leaves and other floating type
vegetations which are not generally fed to the other continuous type or batch type digesters. Plug flow
reactor is capable of transforming more organic solid waste into biogas. They are capable of converting
feed stocks with Total Solids content of 11-14 % (Natural resources conservation service, 2004). There is
no longitudinal mixing in ideal plug flow digesters. When the new manure is added the previous feed
stocks move in plugs towards the outlet. Operation of plug flow digesters has rather complex behaviour
than described above. Some of feed stock will travel faster than the others, and some will settle in the
digester (Graves, et al., n.d.). There are a number of installations of plug flow reactors in the country with
some having two phase installation design and others using effluent recirculation method. Identification
of issues related to performances of these digesters is important for optimum utilization of feed stock
resources and of the biogas produced. This will guide the path to sustainable use of bio energy.
1.7 Objectives
Objective of this study is to establish the operating conditions for optimum biogas production for
different feed rates for food waste in plug flow type biogas digester.
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Chapter 2: Literature review
Kalia (1988) compared the performance of a plug flow type digester with a Janata biogas digester. Janata
type digester is a unit which operates on semi continuous digestion principle and where the gas is collected
in the digester itself under a fixed dome. He observed that plug flow type produces 16% more biogas than
the Janata type digester. He also studied that gas production rate per unit of effective digester volume in
plug flow reactor was 30% higher than for Janata type reactor. He also identified that plug flow type
digester was less subjective to climate variation and has slightly higher temperature in winter season to
produce more biogas.
In 1998, Anand et al. from Indian Institute of Science Bangalore has studied the suitability of plug flow
type digesters for biogas generation from leaf biomass. Their aim was to develop suitable biogas digesters
for other types of biomass feed stocks like municipal solid waste and leaves, except animal dung. Simple
pre treatment methods for floating biomass types were discussed. They used clay as binder to increase the
bulk density and made into briquettes as a solution to the floatation.
Ghosh and Liu (1998) studied phase separation during anaerobic fermentation of solid substrates in an
innovative plug flow reactor. They stated that they used unmixed plug flow reactor which had zigzagging
parallel channels with floors sloping towards the outlet. Slurries with 3-10% solids content were used as
feed stocks. They conducted steady state runs at hydraulic retention time of 13 to 32 days at organic
loading rates of 6.84 - 0.84 kgVS/m3d. They observed phase separation first when the organic loading rate
was2.05 kgVS/m3d. Under this condition longitudinal variation of pH and Volatile Fatty Acid profiles
showed lowest pH of 6.1 and highest Volatile Fatty Acid content of 1500mg/l. At this loading rate they
observed acidogenic phase within the first 50cm from the inlet while the rest of the reactor was dominated
by methanogenic phase. At an organic loading rate of 6.84 kgVS/m3d acidogenic phase was observed
extending to 83 cm from the inlet. At this condition pH dropped to 4.4 and the Volatile Fatty Acid
content of 9600mg/l.
Chanakya et al. (2004) studied the use of anaerobic digestion technology for other types of biomass feed
stocks as a solution for the limited availability of animal dung. Their solutions were based on the
understanding of the underlying process of biomass fermentation. They proposed two designs, plug flow
digester and solid state stratified bed digesters which are suitable for other types of biomass feed stocks.
They have also discussed the adaptation techniques in order to optimize the uses of biogas technology for
socio economic benefits and the numerous uses of biogas production.
Plug flow digesters have the ability to treat ruminant manure with 11-14% Total Solids and for other
manure types with 8-14% Total Solids (Natural resources conservation service, 2004). The digester
retention time is mentioned as between 15-20 day for manure (Lamb and Nelson, 2002). In Some
literature it is recommended that retention time should be more than 20 days (Natural resources
conservation service, 2004). Length to width ratio of the digester flow path should be in the range of 3.5:1
to 5:1for manure (Natural resources conservation service, 2004). The ratio of the flow path width to
depth should be less than 2.5:1(Xiaodong, 2009). Plug flow digesters are fabricated from concrete, steel,
fibre glass or Poly Vinyl Chloride (Bordas, et al., 1981).
The plug flow digesters have shown their suitability to treat feed stocks with higher solid content. Plug
flow digester is a simple design. It can process material like food waste, municipal solid waste and
agricultural residues. Plug flow digester was shown higher gas production rate per unit of effective digester
-10-
volume than other types of digesters (Kalia, 1988).This design has the ability to take the maximum use of
such resources while contributing to the fulfilment of energy needs of the country. Better understanding
of the process and relation between process parameters in the plug flow digester is required for the
effective operation of the digestion process.
2.1 Problems and difficulties in the existing plug flow
digesters
Issues related to plug flow digesters are as follows
 Leakages
Many of the plug flow digesters are prone to leakages of both material and gas. This can be prevented by
careful construction practices.
 Blockages
Blockages may be due to unsuitable, indigestible material. These materials can block the inlets and outlets
of the digester. The water vapour in the gas condenses and blocks the gas flow. Blockages can be
prevented by paying attention to good operation practices.
 Structural Stability
The digester structure is subjected to pressure from the earth from the outside surface and gas pressure
and hydrostatic pressure from the inside. Plug flow digesters are longer than other digester types.
Therefore special attention should be paid in the construction for structural stability of the plug flow
digester.
 Interruption at Higher Feed Rates
When the amounts of feedstock fed to the digesters are increased the micro-organisms cannot tolerate the
increment in load and cause failures.
 Unexpected substances
The feedstock fed to the digesters may make the fluid acidic, basic or poisonous depending on the
composition of the material. Appropriate measures should be taken to restore the system to normal
condition.
 Value addition to effluent
Effluent from the plug flow digesters has a higher nutrient value as organic fertilizer. In some installations
this effluent is merely disposed to the environment. This has higher economic value and a market
potential.
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Chapter 3: Methodology and Experimental Set up
In order to evaluate the effect of feed rate on biogas production qualitatively and qualitatively, three feed
rates were used. The feed rates used in kg/day were 1.4, 3, and 6.
3.1 Experimental setup
Plastic barrels commonly available in the local market were used for the fabrication of the digester.
Diameter of these barrels is 0.47m which facilitates mounting and space requirement to keep the unit.
Three barrels were fixed together considering the structural stability and the feasibility in supporting. The
length of the digester was 1.58m. The length to width ratio was closer to 3.5:1. This satisfies the length to
width ratio defined for manure based on the plug flow digesters (Natural resources conservation service,
2004).
Hence the dimensions and volume of the digester are:
The diameter of the digester
Length of the digester
The digester volume
Working volume
= 0.47 m
=1.58 m
≈ 0.27 m3
≈ 0.188 m3
The inlet of the digester was fabricated with a PVC pipe of diameter 11cm. The digester had three
windows to observe the inside flow of digestion medium. Three sampling ports were placed in order to
facilitate sample the withdrawal.
Gas Outlet
Outlet
Inlet
01
02
03
Sampling Ports
Gas Collecting Holder
Fig. 3.1: Diagram of the Digester
The gas holder was fabricated with another two plastic barrels both of which have one end opened. One
barrel was filled with water and other which has an inlet and an outlet is dipped in the water. Two plastic
tubes of diameter 12.74 mm are used for inlet and outlet. The gas produced in the digester flows to the
holder and lifts it up. The produced gas amount can be estimated by the diameter of the barrel and the
height of the holder that lifts up. The schematic diagram of the digester is shorn in Fig. 3.1.
Two digesters of the same dimensions were fabricated in order to carry out two parallel tests with two
loading rates simultaneously.
-12-
Inlet
Gas Outlet
2
1
3
Sample Ports
Gas Holder
Manometer
Fig.3.2: Digester 1
Inlet
Gas Outlet
1
2
Gas Holder
3
Sample Ports
Manometer
Fig. 3.3: Digester 2
Inlet
Fig.3.4: Inlet of the digester
-13-
Fig. 3.5: Gas Holder
3.2 Initial preparation
Anand et al. (1998) in his research used leaf biomass 50kg/day for 5m3 digester initially and then increased
to 100kg/day. The digester 50m3 for market garbage treatment designed by Sustainable Energy Authority
which was intended to feed 1000kg/day was able to treat 500kg/day at operating conditions. The plug
flow digesters developed by the NERDC of Sri Lanka has the capacity to treat 10kg/day for one cubic
meter total volume of the digester.
Daily feed stock allowed for the total volume of digester is taken as 10 kg per day per 1m3 total digester
volume.
The digester volume
= 0.27 m3
Therefore daily feed stock
= 10 x 0.27
= 2.7 kg per day
≈ 3.0 kg per day
Therefore, feed rate of 3kg was selected first feed rate. 1.4kg is selected as the lower value from 3kg and
6kg was selected as other higher value from 3 kg.
Operating time
= 20 days
For the bacteria culture to grow initial preparation of the digester should be done. This was done by
feeding cow dung for seven days. The feeding rate of food waste was increased in steps in order to
achieve the expected feeding rate and to avoid the acidification. The feed stocks were ground into smaller
sizes using a domestic grinder to reduce the particle size. Calcium carbonate was used for the pH
adjustment. Temperature variation during day time was between 28oC-31oC
Composition of feed stocks for 1.4 kg per day in digester no 01:
Food waste
350g
Vegetables residue
700g
Fruit waste
50g
Water
300g
Composition of feed stocks for 3kg per day in digester no 01:
Food waste
450g
Vegetables residue
900g
Fruit waste
50g
Water
1600g
-14-
Composition of feed stocks for 6kg per day in digester no 02:
Food waste
900g
Vegetables residue
1800g
Fruit waste
100g
Water
3200g
Table3.1: Feedstock characteristics
Digester 01-1.4 kg
per day
Digester 01- 3kg
per day
Digester 02 – 6kg
per day
Average T S Content (%)
16.1
5.7
5.7
Average V S Content (% VS)
93.3
92.3
92.3
Table3.2: Feeding plan
Digester No.
Loading Rate
(kg/day)
Organic Loading
Rate(kgVS/m3day)
Operating Time
(days)
01
1.4
1.12
20
01
3.0
0.83
20
02
6.0
1.67
20
3.3 Analytical procedure
The experiments were conducted during steady period of operation. Feed stocks were analysed for, TS
content and VS content. The material discharged from the outlet was in liquid form. Effluent was
analysed for COD, pH, TS, VS content. Material obtained from three sample ports along the digester
length was tested for VFA content. Daily produced gas quantity was measure from the height of the gas
holder that lifts up. The gas composition was analysed daily.
3.3.1 Total solids content
Percentage of solids was determined by heating known weight of the sample in a pre weighted crucible in
a convection oven at 105oC for about 12 hours until constant weight is reached. Samples were cooled in a
desiccators and final weight of the sample was measured (Clesceri, et al., 1998).
W0- Weight of dish
W1 – Weight of wet sample +dish
W2 – Weight of dried sample+ dish
% Total Solids =
-15-
3.3.2 Volatile solids content
Samples analysed for total solids content was used for total solids determination. Weighted sample was
heated in a muffle furnace at 550oC for 30 minutes. Samples were cooled in a desiccators and final weight
of the sample was measured (Clesceri, et al., 1998).
W3 - Weight of sample +dish after drying
% Volatile Solids =
3.3.3 Chemical oxygen demand
The effluent samples were digested in Potassium Dichromate in Acidic medium and placed in the digester
at 150oC for 120 minutes. Then the samples cooled to room temperature were analysed in
Spectrophotometer model HACH DR/2010.
3.3.4 Volatile fatty acids
Samples taken from the sample port 01, 02, 03 were centrifuged to separate the liquid. Supernatant was
mixed with Sulphuric acid and distilled .Collected distillate was titrated with Sodium Hydroxide and the
volatile fatty acids content was calculated (Clesceri, et al., 1998).
3.3.5 pH
PH of the effluent was analysed using MI 160 pH meter. pH is an indication of the process stability of
biogas production .
3.3.6 Gas composition
Gas composition of the daily generated gas was analysed by the chromatographic method. CH4, CO2, H2,
O2, N2 content in the biogas was analysed. SHIMADZU GC 2014 model was used for the tests.
-16-
3.3.7 Specific methane production
Specific methane production was calculated as below.
V0 - Average daily methane production (m3/d)
W – Weight of daily feed stock added (kg/d)
% TS – Percentage total solids in feedstock
%VS – Percentage volatile solids in feedstock
Specific methane production
3.3.8 Volatile solids reduction
Volatile solids reduction was calculated as below (Brobst, n.d.).
%VS in = VS in the feedstock
%VS out = VS in the digested effluent
VSR =
-17-
Chapter 4: Results and Analysis
4.1 Gas volume and composition
Table 4.1: Daily average biogas production
Volume of gas generated per day (L/day)
Digester 01
01
Feed rate :
1.4kg per day
99
Feed rate : 3kg
per day
17
Digester 02
Feed rate: 6kg
per day
94
02
139
37
97
03
139
0
73
04
146
48
101
05
101
30
101
06
115
48
101
07
102
49
101
08
106
30
130
09
132
33
147
10
128
46
149
11
153
30
161
12
149
30
175
13
141
60
170
14
135
32
198
15
141
51
158
16
146
0
182
17
75
56
175
18
132
56
167
19
153
48
154
20
133
56
132
Day No.
-18-
Fig. 4.1: Daily average biogas production in Digestor 1
For the 6kg per day and 1.4kg per day higher biogas volume was generated. During the steady state
experiments done at 3kg per day leakages occurred in the gas holder, therefore the generated biogas
volume is lower than that for the other two feeding rates.
-19-
Table 4.2: Composition variation of biogas for feeding rates of 3kg per day and 1.4kg per day in
Digester 1
Feed rate of 1.4 kg per day
Feed rate of 3kg per day
CH4
CO2
(% Volume)
(% Volume)
Time(Day)
CH4
(% Volume)
CO2
(% Volume)
01
53
44
43
52
02
58
40
38
57
03
61
34
46
48
04
64
33
38
58
05
65
31
45
51
06
65
32
46
51
07
61
35
43
53
08
57
37
41
56
09
57
40
45
49
10
54
43
47
48
11
52
45
50
48
12
52
45
50
47
13
52
45
50
47
14
52
45
51
46
15
52
46
51
45
16
52
43
51
46
17
53
42
53
42
18
51
42
55
42
19
53
44
55
39
20
55
42
56
40
-20-
Fig. 4.2: Composition Variation of biogas for feed at 3kg/day and 1.4kg/day feed rates in
Digestor 1
-21-
Table 4.3: Composition variation of biogas for feeding rate of 6kg per day in Digester 2
Time(Day)
01
02
03
04
05
06
07
08
09
10
11
12
13
14
15
16
17
18
19
20
Feed rate of 6kg per day
CH4
CO2
(% Volume)
(% Volume)
41
56
44
52
46
48
46
51
48
45
51
43
53
41
55
40
54
39
51
43
46
49
47
50
49
47
50
47
52
43
51
43
51
44
51
45
50
47
53
42
Fig. 4.3: Composition variation of biogas at feed rate 6kg/day in Digester 2
The composition shows high methane content in biogas for feeding rate of 1.4 kg per day.
-22-
4.2 pH variation of effluent
Table 4.4: pH Variation of effluent for three feed rates
Time(days )
pH out – 1.4 kg/day
pH out – 3 kg/day
pH out – 6 kg/day
01
7.08
6.49
5.96
02
7.08
6.34
6.66
03
6.77
6.43
6.93
04
7.68
6.73
6.77
05
7.66
7.01
6.93
06
7.80
6.82
7.03
07
7.04
6.14
6.85
08
7.44
6.96
6.84
09
7.85
6.62
6.73
10
7.56
6.69
6.52
11
7.32
6.82
6.60
12
7.21
6.82
6.56
13
7.42
6.80
6.76
14
7.51
6.85
6.60
15
7.35
7.09
6.59
16
7.79
7.31
7.01
17
7.18
6.96
6.89
18
7.18
6.78
6.74
19
7.12
6.88
7.07
20
6.76
7.16
6.85
.
-23-
Fig. 4.4: Variation of effluent pH for different feed rates
pH variation of effluent for different feed rates is an indication of process stability. For the three feed rates
effluent pH does not show much larger variation between them. For 3kg /d effluent pH is in the range of
6.14 to 7.31. At 6kg/d pH varies from 5.96 to 7.07. For the three feed rates pH of effluent was usually
within the range optimum range for the methanogenisis reactions. At 1.4kg/d methane composition was
always above 50% with the pH variation in the range of 6.76 to 7.85.
-24-
4.3 VFA variation along the digester length
Table 4.5: VFA variation along the digester length for feeding rate of 1.4 kg per day in digester 1
Time(Day)
01
02
03
04
05
06
07
08
09
10
11
12
13
14
15
16
17
18
19
20
VFA1(mg/l)
9521
13246
10513
10827
12523
10003
9159
20134
8402
9052
9414
15417
8878
8295
8007
10874
7819
9601
10673
10459
VFA2(mg/l)
6854
6794
5849
7203
5367
4281
3987
4978
4603
3169
3451
4509
4281
4791
4402
4576
5166
6754
5059
4389
VFA3(mg/l)
5782
4804
4918
6466
3531
3370
3089
3008
2781
2084
1910
2707
2781
3303
2164
3625
3136
3169
4523
3062
Fig. 4.5: VFA Variation along the digester length for feed rate of 1.4kg/day in Digester 1
-25-
Table 4.6: VFA variation along the digester length for feeding rate of 3kg per day in digester 1
Time(Day)
01
02
03
04
05
06
07
08
09
10
11
12
13
14
15
16
17
18
19
20
VFA1(mg/l)
12214
15363
22312
10017
22037
8717
11410
12462
14827
11196
11558
20201
7685
11357
12844
10499
6559
9615
4241
4020
VFA2(mg/l)
4322
2854
4080
3732
3638
3893
5045
3759
4375
4241
4094
4750
4590
4080
3933
4690
4603
5286
3531
3812
VFA3(mg/l)
4107
4992
2258
2539
2111
3095
2915
2352
13079
355
2673
3102
4040
3504
2928
3089
3116
2915
4858
4174
Fig.4.6: VFA variation along the digester length for feeding rate of 3kg per day in digester 1
-26-
Table 4.7: VFA variation along the digester length for feeding rate of 6kg per day in digester 2
Time(Day)
01
02
03
04
05
06
07
08
09
10
11
12
13
14
15
16
17
18
19
20
VFA1(mg/l)
8127
12791
10030
9521
15806
31759
27491
11035
4389
23799
28992
10017
6894
15765
32127
6017
6305
5126
10593
23176
VFA2(mg/l)
6466
8918
8757
7095
10982
19839
8677
6794
2191
5273
5956
5233
6506
14774
5266
5380
4764
4040
4509
8509
VFA3(mg/l)
2097
422
5709
5796
5970
6171
4616
4670
4764
5729
4389
4509
2499
14023
4000
4938
3745
4415
2794
2660
Fig. 4.7: VFA variation along the digester length for feed rate of 6kg/day in Digester 2
VFA profile along the digester for 1.4kg per day and 3kg per day shows clear variation between the
samples taken from sample port1, 2 and 3. This shows the variation of biogas production stages along the
length for feed rates of 1.4kg per day and 3kg per day. High VFA concentration near sample port 1 is due
to the dominance of acid production stage near the inlet. VFA concentration near the inlet varied from
8000mg/l to 20000mg/l for the OLR of 1.12 kgVS/m3d. VFA concentration near the sample port 01
varied from 4000 mg/l to 22000 mg/l for the OLR of 0.83 kgVS/m3d. Although the VFA profile shows
-27-
similar pattern for feeding rate of 6kg, it shows larger fluctuations in VFA content near sample port 01
and 02. The VFA variation pattern was not prominent as for feeding rates of 1.4kg per day and 3 kg per
day. At the feed rate of 6kg/d high VFA concentrations than for other two feed rates were observed
throughout the reactor. At OLR of 1.67 kgVS/m3d, VFA near the inlet varied from 4000 mg/l to
32000mg/l. This VFA concentration has exceeded the inhibitory limits for methanogenisis reported in the
literature which is around 8000 mg/l (Poprasert, 1996). Ghosh and Liu (1998) observed VFA
concentration of 9600 mg/l at OLR of 6.84 kgVS/m3d at a pH of 4.4.
4.4 Effluent COD
Table 4.8: Effluent COD variation for three feed rates
Effluent COD (mg/l)
Day No.
Feed rate : 1.4 kg/ day
(Digester 1)
Feed rate: 3kg/day
(Digester 1)
Feed rate: 6kg/day
Digester 2
01
6225
7340
14850
02
7425
4370
14100
03
4900
5010
15350
04
5550
4750
17100
05
5900
4930
11350
06
5750
7050
17425
07
6800
7760
10475
08
7000
7750
13775
09
5425
7750
8575
10
5175
8600
11500
11
3050
6220
8850
12
4975
6740
7350
13
4275
6930
11825
14
6925
6660
7250
15
8125
6480
8300
16
5425
6950
7550
17
7425
7210
7825
18
4325
6160
7050
19
6775
7850
7450
20
4050
6260
7350
-28-
Fig 4.8: COD variation of effluent for different feed rates
For feeding rate of 1.4kg per day COD of effluent varies between 3000 mg/l to 8000 mg/l. Effluent COD
level varies between 4000mg/l to 9000mg/l for 3kg feeding rate. For 6kg of feeding rate effluent COD
level varies from 7000mg/l to 17000mg/l. Higher COD level in the effluent was a result of reduced HRT.
Arun, et al. (2005) reported COD reduction from about 40,000-50,000 mg/l down to 1200-3000mg/l.
Higher COD values in the effluent is an indication of undigested organic matter presents in the effluent.
4.5 Process performance
Table 4.9: Process performance
Feed
rate(kg/day)
Average Volatile Solids
Reduction (%)
Average Specific methane
production (m3/kgVS per
day)
1.4
91.37
0.341
3.0
89.36
0.120
6.0
85.92
0.219
The highest average specific methane production was 0.341m3/kgVS per day at OLR of 1.12kgVS/m3 per
day. The average specific methane production is 0.120 m3/kg VS per day for OLR of 0.83 kgVS/m3day.
For OLR of 1.67kgVS/m3day average specific methane production is 0.219 m3/kgVS per day. Relatively
low biogas yield for feed rate of 3kg per day is due to intermittent leakages occurred during the steady
state operating period. Intermittent leakages restrict the successful growth of methanogenisis bacteria
-29-
culture. Low average specific methane production at feed rate of 6kg per day is due to the insufficient
HRT for the larger quantity of feedstock to be digested inside the digester and due to the high VFA
concentration produced at higher feed rate.
OLR of 1.12 kgVS/m3day showed the highest average volatile solids reduction of 91.37%. OLR of 1.67
kgVS/m3day showed the lowest average volatile solids reduction of 85.92%.
-30-
Chapter 5: Discussion
Highest average gas production was observed for the highest OLR, which is also the highest feeding rate.
Low average specific methane production and methane content in biogas was observed at the highest
feeding rate. Insufficient retention time and high VFA concentration due to high organic load reduces
methane production which results in low specific methane production and low methane content in biogas.
Highest average specific methane production was observed for intermediate OLR which is the lowest feed
rate with high TS content. When the OLR is increased total feed rate is reduced by increasing TS content.
At this condition the process is more stable and the hydraulic retention time increases due to lower
feeding rate. High COD concentration in the effluent is also an indication of the insufficient retention
time.
Table5.1: Comparison between similar studies done in plug flow digesters
Waste type
Food Waste
Used cooking grease
with swine manure
Food waste,
Anaerobic sludge,
Digestate, Cow dung
Cassava peel
Specific Methane
Yield (m3/kgVS)
Conditions
0.120
0.83 kgVS/m3day
Temperature :28-31oC
0.219
1.67 kgVS/m3day
Temperature :28-31oC
0.341
1.12 kgVS/m3day
Temperature :28-31oC
0.310
Cooking grease 2.5%,
Temperature: 22-26oC
0.278
Organic loading rate2.5
kgVS/m3day
Temperature: 55oC
0.2259
3.3 kgVS/m3day
Temperature: 55oC
0.146
3.9 kgVS/m3day
Temperature: 55oC
0.377
3.6kgVS/m3day
Temperature :35-39°C
Reference
Present Study
(Botero, et al., 2010)
(Chaudhry ,2008)
(Cuzin, et al.,1992)
Cuzin, et al. (1992) reported specific methane production of 0.377 m3/kgVS for Cassava peel
fermentation in a plug flow digester. Chaudhry (2008) reported 0.278, 0.2259 and 0.146 m3/kg VS
production for municipal solid waste for OLR of 2.5, 3.3 and 3.9 kgVS /m3d respectively.
VFA concentration along the length of the digester shows a large reduction from inlet to the middle span.
This was more prominent at lower feeding rates. Lengthwise VFA variation is an indicator of the biogas
production stages along the length of the digester. VFA results show that acidogenisis phase near the inlet
and mathanogenisis phase near the outlet. High VFA concentration is a reason for low methane content in
the biogas.
-31-
Cuzin, et al. (1992) and group of researchers reported acetate accumulation of 10g/l in the feeding box at
pH of 5 as a result of acidification due to higher loading rates. At this condition they observed 20% less
biogas production than normal biogas production.
VFA concentration exceeded the inhibitory limits mentioned in the literature. This did not cause full
process destruction but resulted in low methane content in the biogas. Distribution of biogas production
stages along the length of the digester reduces negative effect of high VFA concentrations at higher
loading rates.
Highest average VS reduction was observed for intermediate OLR which is the lowest total feed rate with
high TS content. Lowest average VS reduction was observed at highest OLR which is the highest total
feed rate.
High COD concentrations were observed in the effluent. This is due to insufficient HRT.
-32-
Chapter 6: Conclusion
Highest specific methane production and high methane content in biogas was observed at the lowest feed
rate and for feed stock with high TS contents which is 1.4 kg per day. Although the total feed rate was
lowest, OLR was a middle value at the highest specific methane production. This is due to high TS
content of the feed at this feed rate. Highest VS reduction was also observed at the lowest feed rate with
high TS content which is 1.4 kg per day. Insufficient HRT and high VFA content are reasons for lower
specific methane production at the highest feed rate. The VS reduction was also the lowest for the highest
feed rate.
Lowest feed rate with high total solids content showed more stable operation and high specific methane
production although the OLR is high. Increment in HRT at lower feed rate increases specific methane
production. Further studies should be conducted to study the effect of TS content in the feedstock.
Although the VFA concentration near the inlet was much higher than the inhibitory levels it did not lead
to full process destruction. The VFA variation along the digester shows the variation of acidogenisis to
methanogenisis stages along the digester length which is more prominent at lower feed rates. At higher
feed rates high VFA concentrations were observed along the digester length with large fluctuations in
VFA content. Higher VFA content and VFA fluctuations lead to process instability. Co digestion of food
waste with cattle dung, sewage waste or other suitable sources help to control the high VFA
concentrations.
High COD content in the effluent is an indicator of the presence of undigested organic material in the
effluent. The COD levels observed in the effluent for three feed rates are much higher than recommended
levels for discharging effluent. This COD content can be treated in a second digester or the length of the
digester has to be increased to convert COD in the effluent to biogas. Other reason for lower gas quality
is the high VFA production in the digester.
Within the limited time period experiments were done for three different feed rates. Further studies
should be conducted to study the effect of TS content in the feedstock for biogas production. Future
studies should be focused on importance of flow induced by the effect of pressure inside the plug flow
digester on biogas production and its composition.
Effluent nutrient content should be tested to be used it as an organic fertilizer. This source supplies high
quality fertilizer which increases resistance of the plants to diseases and increase the richness of soil.
GHG potential of methane in the biogas is 20% (Wightman, 2005) higher than carbon dioxide. Therefore
it should be carefully trapped and used to avoid leakages.
-33-
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Comparato M.P., Cornacchia G., Lastella G., Sharma V.K., Testa C.; 2000
“Inclined-plug-flow type reactor for anaerobic digestion of semi-solid waste”
Applied Energy, Vol 65, pp 173 – 185
Cuzin N., Farinet J. L., M. Labat, Segretain B C.; 1992
“Methanogenic fermentation of cassava peel using a pilot plug flow digester”
Bio resource technology, Vol 41, pp 259-264
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Ding W., Gao Y., Wang X., Wu Y.; 2010
“Family-size biogas plant using manure and urine mixture at ambient temperature in semi-arid regions of
northwestern China”
World Academy of Science, Engineering and Technology, Vol 65
Fraser K.W.; 2010
“Increased anaerobic digestion efficiency via the use of thermal hydrolysis”
Faculty of the Virginia Polytechnic Institute and State University
Ghosh S., Liu T.; 1997
“Phase separation during anaerobic fermentation of solid substrates in an innovative plug flow reactor”
Water Science and Technology, Vol 36, pp 303-310
Goswami D.Y., Kreith F.; 2007
“Biomass conversion process for energy recovery”
CRC Press, 12-13, ISBN978-1-4200-0348-2
Goswami S.
“Optimization of methane production from solid organic waste”
Graves R.E., Richard T., Topper P.A.
“The fate of nutrients and pathogens during anaerobic digestion of dairy manure”
Agricultural and Biological Engineering, Cooperative Extension, College of Agricultural Sciences
Jones D., lleleji K.E., Jones D.
“Basics of energy production through anaerobic digestion of livestock manure”
PURDUE Extension
Kalia A.K.; 1988
“Development and evaluation of a Fixed Dome Plug Flow Anaerobic Digester”
Biomass, Vol 16, pp225-235
Karve A.D., Karve P., Kulkarni G.; 2005
“A new compact biogas system based on sugary/starchy feedstock”
Energy for Sustainable Development, pp 63-65
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“Enhancement of biogas production from solid substrates using different techniques- a review”
Bioresource Technology, Vol 95, October 2004, pp 1-10
Lamb J., Nelson C.; 2002
“Final Report: Haubenschield farms anaerobic digester”
Minnesota project
Lee A.; 2008
“Renewable energy and agriculture: Promoting biogas in the rural communities of the Lashihai wetland
nature reserve”
Clark University, Yunnan, China
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“Methane recovery from animal manures the current opportunities casebook”
NREL, Colorado, USA
Monnet F.; 2003
“An introduction to anaerobic digestion of organic wastes”
Remade, Scotland
Nijaguna B.T.
“Biogas technology”
New Age International Publishers, New Delhi, ISBN: 81-224-1380-3
Ostrem K.; 2004
“Greening waste: Anaerobic digestion for treating the organic fraction of municipal solid wastes”
Department of Earth and Environmental Engineering, Fu Foundation of School of Engineering and
Applied Science, Columbia University
Pitchel J.; 2005
“Waste Management practices municipal, hazardous and industrial”
CRC Press, ISBN 978-1-4200-3751-7
Poprasert C.; 1996
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John Wiley & sons, ISBN-13, 9780471964827
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-37-
Annexure I Gas Collection Data
Table 8.1: Gas volume data
Height of the gas holder (cm)
01
11.0
57.0
Digester 02
Feed rate: 6kg per
day ***
94.0
02
23.0
80.0
97.0
03
0.0
80.0
73.0
04
30.0
84.0
101.0
05
19.0
58.0
101.0
06
30.0
66.0
101.0
07
31.0
59.0
101.0
08
19.0
61.0
130.0
09
21.0
76.0
147.0
10
29.0
74.0
149.0
11
19.0
88.0
161.0
12
19.0
86.0
175.0
13
38.0
81.0
170.0
14
20.0
78.0
198.0
15
32.0
81.0
158.0
16
0.0
84.0
182.0
17
35.0
43.0
175.0
18
35.0
76.0
167.0
19
30.0
88.0
154.0
20
35.0
76.5
132.0
Day No.
*
**
***
Digester 01
Feed rate : 3kg per
Feed rate : 1.4kg per
day *
day **
Gas holder diameter for 3kg per day is 45 cm
Gas holder diameter for 1.4kg per day is 47 cm
Gas holder diameter for 6kg per day is 47 cm
-38-
Annexure II Gas Composition Data
Table 8.2: Gas composition data
Day
No.
01
02
03
04
05
06
07
08
09
10
11
12
13
14
15
16
17
18
19
20
Feed rate - 3kg per day
(% Volume)
CH4
CO2
CH4
43.260 52.481 43.071
38.172 56.923 38.815
46.235 47.601 46.498
37.992 58.001 38.121
45.023 51.231 45.047
46.047 51.245 45.239
42.621 53.124 42.684
40.633 56.147 41.023
45.243 49.416 45.289
46.992 48.231 47.125
49.852 48.387 50.164
50.216 46.995 50.244
50.375 47.024 50.306
51.278 46.351 51.204
51.421 45.483 51.473
51.416 46.123 51.448
52.587 42.371 52.576
55.046 42.275 55.084
55.417 39.180 55.348
55.953 40.003 56.342
CO2
52.325
57.012
47.536
58.135
50.749
51.324
53.325
55.627
49.423
48.246
48.316
47.012
47.098
46.388
45.467
46.174
42.353
42.194
38.991
40.128
Feed rate - 1.4kg per day
(% Volume)
CH4
CO2
CH4
52.432 44.134 53.421
58.327 40.013 58.412
60.712 34.428 60.423
64.349 32.915 64.106
64.621 31.241 64.821
64.823 31.876 64.513
61.326 34.921 60.974
57.328 37.267 57.381
56.239 40.118 57.013
54.742 43.407 54.219
52.273 45.351 52.246
52.432 45.245 52.503
52.475 45.179 52.436
52.306 45.317 52.273
52.096 45.921 52.264
52.214 43.327 51.965
53.257 42.454 53.392
51.485 42.001 51.479
53.568 44.071 53.382
54.764 42.310 54.853
-39-
CO2
44.236
40.115
34.383
33.076
31.342
32.108
34.857
37.194
39.931
43.398
45.369
45.317
45.204
45.395
46.138
43.291
41.831
42.095
44.136
42.672
Feed rate - 6kg per day
(% Volume)
CH4
CO2
CH4
41.236 56.394 41.309
44.425 52.738 44.361
45.933 47.724 46.052
46.143 51.241 46.212
47.831 45.530 48.079
51.132 43.465 51.117
53.215 41.051 52.947
54.983 40.132 54.824
54.406 39.011 54.343
50.861 42.704 50.695
46.523 49.470 46.278
47.172 50.221 46.957
49.112 47.075 49.215
50.462 47.149 50.334
51.784 43.194 51.907
51.445 43.175 51.319
51.217 44.022 51.261
51.287 45.213 50.865
50.542 47.149 50.188
53.008 42.505 53.117
CO2
56.417
52.256
47.640
51.389
45.423
43.398
40.936
40.328
39.272
42.850
49.381
50.103
47.283
47.268
43.276
42.977
44.008
45.310
47.274
42.42
Annexure III pH Data
Table 8.3: pH data
Time(days )
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
pH out – 3 kg/day
pH out – 6 kg/day
pH out – 1.4 kg/day
Trial 1
Trial 2
Trial 1
Trial 2
Trial 1
Trial 2
6.47
6.37
6.41
6.76
7.02
6.88
6.25
6.95
6.56
6.70
6.83
6.85
6.81
6.89
7.15
7.34
6.93
6.73
6.84
7.18
6.51
6.30
6.44
6.70
6.99
6.76
6.02
6.97
6.68
6.68
6.80
6.79
6.78
6.80
7.03
7.27
6.98
6.82
6.91
7.14
5.94
6.68
6.93
6.75
6.92
7.01
6.84
6.85
6.70
6.51
6.59
6.52
6.79
6.62
6.57
7.04
6.88
6.71
7.03
6.82
5.97
6.63
6.93
6.79
6.94
7.05
6.86
6.83
6.76
6.53
6.61
6.59
6.73
6.57
6.61
6.98
6.90
6.77
7.11
6.88
7.04
7.07
6.76
7.66
7.65
7.81
7.06
7.45
7.84
7.56
7.32
7.21
7.41
7.54
7.34
7.74
7.16
7.12
7.17
6.78
7.11
7.08
6.78
7.70
7.66
7.78
7.01
7.42
7.86
7.55
7.31
7.20
7.43
7.48
7.36
7.84
7.19
7.24
7.06
6.73
-40-
Annexure IV VFA Data
Table 8.4: Titration volume data for 3kg per day
Day No.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
NaOH(ml) –(VFA1)
Trial 1
Trial 2
92.4
105.2
157.4
76.5
148.7
59.7
68.6
89.3
113.4
73.4
89.8
172.2
72.5
80.1
116.2
71.6
42.1
75.5
33.9
32.6
89.9
124.1
175.6
73
180.2
70.4
101.7
96.7
107.9
93.7
82.7
129.3
42.2
89.4
75.5
85.1
55.8
68
29.4
27.4
NaOH(ml) – (VFA2)
Trial 1
Trial 2
35.6
31.1
40.2
23.9
23.5
33.3
45.4
19.2
25.7
35.5
25.9
49.3
35.6
33.7
32.4
22.7
38.1
42.3
22
27.1
28.9
11.5
20.7
31.8
30.8
24.8
29.9
36.9
39.6
27.8
35.2
21.6
32.9
27.2
26.3
47.3
30.6
36.6
30.7
29.8
-41-
NaOH (ml)- (VFA3)
Trial 1
Trial 2
27.1
35
13.4
21.6
11.3
24.5
18.4
20.8
100.2
2.4
22.3
22
27.8
25.9
19.7
24.6
25.1
15.6
34.4
33.5
34.2
39.5
20.3
16.3
20.2
21.7
25.1
14.3
95
2.9
17.6
24.3
32.5
26.4
24
21.5
21.4
27.9
38.1
28.8
Table 8.5: Titration volume data for 1.4 kg per day
Day No.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
NaOH(ml) –(VFA1)
Trial 1
Trial 2
69.3
100.2
74.3
77.9
102.8
78.2
71.4
129.1
65.8
61.9
74.5
97.9
63.4
70.1
58.2
86.8
52.6
73.4
81.5
71.9
72.8
97.5
82.6
83.7
84.1
71.1
65.3
171.4
59.6
73.2
66.0
132.2
69.1
53.7
61.3
75.5
64.1
69.9
77.8
84.2
NaOH(ml) – (VFA2)
Trial 1
Trial 2
51.8
53.1
39.1
55.9
43.4
34.7
23.1
36.2
32.5
24.8
28.7
31.0
35.7
38.5
37.6
28.1
37.8
49.6
45.7
33.1
50.5
48.3
48.2
51.6
36.7
29.2
36.4
38.1
36.2
22.5
22.8
36.3
28.2
33
28.1
40.2
39.3
51.2
29.8
32.4
-42-
NaOH (ml)- (VFA3)
Trial 1
Trial 2
55.8
38.4
31.1
49.5
30.6
26.8
18.6
20.2
17.3
24.0
12.8
20.8
21.4
20.8
14.1
32.5
25.0
20.9
40.6
28.0
30.5
33.3
42.3
47.0
22.1
23.5
27.5
24.7
24.2
7.1
15.7
19.6
20.1
28.5
18.2
21.6
21.8
26.4
26.9
17.7
Table 8.6: Titration volume data for 6kg per day
Day No.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
NaOH(ml) –(VFA1)
Trial 1
Trial 2
70.1
100.3
65.9
67.4
112
254.8
215.8
71.5
34.6
162.4
220.6
56.3
66.1
132.2
246.2
53.6
39.3
44.1
71.2
195.7
51.2
90.6
83.8
74.7
123.9
219.2
194.5
93.2
30.9
192.8
212.1
93.2
36.8
103.1
233.3
36.2
54.8
32.4
86.9
150.2
NaOH(ml) – (VFA2)
Trial 1
Trial 2
44.1
74.3
58.2
48.9
89.3
170.2
71.1
48.5
18.5
36.7
42.2
41.2
54.8
125.1
41.4
45.8
29.7
27.8
36
55.6
52.4
58.8
72.5
57
74.6
125.9
58.4
52.9
14.2
42
46.7
36.9
42.3
95.4
37.2
34.5
41.4
32.5
31.3
71.4
-43-
NaOH (ml)- (VFA3)
Trial 1
Trial 2
18.2
2
44.9
39.7
49.3
49.2
32.5
31.2
39.1
46.4
27.3
27.9
19.1
85.4
31.6
39.7
25.3
36.8
16
16.5
13.1
4.3
40.3
46.8
39.8
42.9
36.4
38.5
32
39.1
38.2
39.4
18.2
123.9
28.1
34
30.6
29.1
25.7
23.2
Annexure V COD Data
Table 8.7: Effluent COD data
Day No.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
COD - 3kg / day*
(mg/l)
COD - 1.4kg / day **
(mg/l)
COD - 6kg /day**
(mg/l)
Trial 1
Trial 2
Trial 1
Trial 2
Trial 1
Trial 2
367
190
250
234
249
344
371
404
381
429
301
356
339
325
311
348
389
292
422
307
367
247
251
241
244
361
405
371
394
431
321
318
354
341
337
347
332
324
363
319
132
139
104
97
108
126
118
142
94
81
63
59
95
126
152
127
176
85
144
81
117
158
92
125
128
104
154
138
123
126
59
140
76
151
173
90
121
88
127
81
286
286
308
351
229
314
193
280
171
234
175
149
235
168
153
128
149
166
121
154
308
278
306
333
225
383
226
271
172
226
179
145
238
122
179
174
164
116
177
140
* Dilution factor for feeding rate of 3kg/day is 20
** Dilution factor for feeding rate 6kg/day and 1.4kg/day is 50
-44-
Annexure VI Data for TS and VS Determination
Table 8.8: Feed stock TS and VS data
Day No
Feed rates: 3kg/day, 6kg/day
W0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
34.2448
39.0271
40.2132
43.2009
32.6570
38.3652
44.3494
44.7718
36.8400
39.9746
34.1900
35.7414
45.1755
34.1954
32.5817
40.2367
W1
84.1330
86.5893
88.5198
89.4815
80.5304
84.8354
91.5714
103.3205
88.2136
85.3427
90.6220
83.8133
91.3552
101.7835
86.4629
92.6792
W2
37.1947
41.9091
42.6934
46.3706
35.3379
41.3333
46.7859
47.6063
38.8532
42.0546
37.2527
38.8154
48.0272
38.7793
35.6931
43.2334
Feed rate: 1.4kg/day
W3
34.4357
39.1891
40.3623
43.5328
32.7923
38.5625
44.4229
45.0606
37.0018
40.1462
34.5636
35.8412
45.3437
34.7549
32.8426
40.5890
-45-
W0
90.8382
47.2047
45.8054
45.1767
44.3417
42.4974
43.5025
44.3098
45.6091
44.3021
47.2237
47.1747
48.2378
43.1691
45.2131
48.2384
W1
W2
W3
180.3087
92.5328
88.6455
83.7521
87.2135
84.8800
84.3457
79.4315
81.2413
86.1047
89.6891
86.9842
91.0534
84.0043
88.2619
90.7834
104.6108
55.0427
53.1769
51.1379
52.0671
49.5242
51.0719
51.0090
49.3021
51.3705
52.2791
52.9952
55.8218
49.7113
52.2513
54.8425
92.0148
47.6532
46.1153
45.5209
44.7876
42.7574
44.0412
44.6370
46.0585
45.1216
47.3445
47.5306
48.5762
43.6631
45.8379
48.8361
Table 8.9: Effluent TS and VS data
Day
No
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
Feed rate: 3kg/day
Feed rate: 6kg/day
Feed rate: 1.4kg/day
W0
W1
W2
W3
W0
W1
W2
W3
W0
W1
W2
W3
73.1704
74.7183
47.2055
45.2125
73.1688
45.8065
36.2167
34.2007
43.2025
38.3651
44.3427
42.5190
34.0243
42.5197
36.1668
45.8061
121.8926
122.8116
96.0465
93.9252
121.2645
94.257
84.5248
82.1305
90.4537
94.6759
100.6103
91.3163
82.7902
88.0042
83.4012
93.9164
73.4687
74.9872
47.5049
45.5086
74.4489
46.0958
36.4711
34.776
43.4789
38.6835
44.6578
42.7983
34.3098
42.7814
36.4508
46.0848
73.2933
74.8290
47.3317
45.3587
73.5891
45.9489
36.2849
34.5278
43.3163
38.4959
44.4821
42.6148
34.1490
42.6461
36.3054
45.9430
34.3465
42.5194
36.4131
39.9753
35.9614
40.2363
45.8112
47.2233
31.5945
43.1643
46.4939
33.7623
38.9636
42.4970
41.1238
43.1682
81.1129
90.8632
84.8783
87.0472
81.7207
90.4330
90.6410
94.2612
80.7109
86.1694
91.9606
87.2311
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