Generally, factors such as digester temperature, retention time, fermentation pH value, digester pressure, volatile fatty acid, and sublayer composition have been identified to affect the digestion of feedstock in the anaerobic process
4. The pH (Hydrogen ion concentration) range suitable
for gas production is rather narrow (between 6.5 and 7.5)
Can controlled by natural buffering effect of NH4
+ and
HCO3
- ions
pH falls with the production of volatile fatty acids
(VFAs) but attains a more or less constant level once the
reaction progress
HYDROGEN ION CONCENTRATION
5. • The optimum biogas production is achieved when the
pH value of input mixture in the digester is between
6.5 and 7.5
• In the initial period of fermentation, as large amounts
of organic acids are produced by acid forming
bacteria, the pH inside the digester can decrease to
below 5
• This inhibits or even stops the digestion or
fermentation process
• Methanogenic bacteria are very sensitive to pH and
do not thrive below a value of 6.5
6. EFFECTIVE OF pH ON METHANE PRODUCTION
PH
value
From 5 6 7 8 9 10
To 6 6.5 7 7.5 8 8.5
Biogas
yield
12.7 14.8 22.5 24.6 17.8 10.2
10. TEMPERATURE
Three zones of temperature in which biogas is produced by
anaerobic fermentation of organic matter
Mesophillic zone : Optimum temperature : 25-35°C
Thermophillic zone : More than 55°C
Psycrophillic zone : Less than 20°C
In different temperature zones different sets of microbes (bacteria)
especially the mehtanogens remain active; where as the other two
groups of microbes either remain dormant and thus more or less
inactive as far as the anaerobic digestion is concerned or get killed
The rate of fermentation is much faster @ high temperature
11. • The methanogens are inactive in extreme high and low
temperatures
• The optimum temperature is 35 °C
• When the ambient temperature goes down to 10 °C,
gas production virtually stops.
• Satisfactory gas production takes place in the
mesophilic range, between 25 to 30 °C
• Proper insulation of digester helps to increase gas
production in the cold season
TEMPERATURE
12. Temperature Biogas yield(m3/1 ton of
dung/day)
15 0.150
20 0.300
25 0.600
30 1.000
35 2.000
40 0.700
45 0.320
BIOGAS YIELD AT VARIOUS TEMPERATURES
17. CARBON-NITROGEN RATIO
Relationship between the amount of carbon and nitrogen present
in organic materials
Biogas producing raw materials can be divided into two parts:
Nitrogen rich
Nitrogen poor
Nitrogen concentration is considered with respect to carbon
contents of the raw materials
It is termed as CN ratio
In the case of cattle dung the problem of nutrient concentration
does not exist as C/N ratio is usually around 25:1
18. Optimum C/N ratio - 25 to 30 : 1
Very high, C/N ratio- rapid consumption of N2 by
methanogens for protein requirements - no longer
react on the left over carbon content of the material-
results very low gas production
Very low C/N ratio is - ammonia (NH4) - the pH
value increase in the digester
pH > 8.5 will start showing toxic effect on
methanogen population
CARBON-NITROGEN RATIO
19. TOXIC MATERIALS
Mineral ions, heavy metals and the detergents are some of the
toxic materials that inhibit the normal growth of pathogens in the
digester
Small quantity of mineral ions (sodium, potassium, calcium,
magnesium, ammonium and sulphur) also stimulates the growth of
bacteria, while very heavy concentration of these ions will have
toxic effect
Similarly, heavy metals such as copper, nickel, chromium, zinc,
lead, etc in small quantities are essential for the growth of bacteria
but their higher concentration has toxic effects
Likewise, detergents including soap, antibiotics, organic solvents,
etc. inhibit the activities of methane producing bacteria and
addition of these substances in the digester should be avoided
The metal in solution as an ‘ion’ can adversely affect the bacteria.
If it is removed, it will not enter into the bacterial cell, but can
affect the metabolism of an organism
20. Composition of organic substrate
Retention time
Concentration of substrate
Organic loading rate
Degree of mixing
Heating & Heat balance
OPERATIONAL FACTORS
21. • Total solid concentration (TS %) is a measure of the
dilution ratio of the input material
• It’s calculated by dividing the weight of the remaining
portion after drying at temperature of 105°C (to constant
weight) by the original weight
• The TS ratio is another important factor in the production
of the biogas
• when the total solid concentration values exceed the
optimal point; the yield of the biogas also decreases and
the result mixture become too dense to effectively flow
through the digester, the optimum dilution ratio for cattle
manure is 1 part of dung with 1 part of water
TOTAL SOLID CONCENTRATION
22. Total yield(ml) Total solid concentration (%)
2 4 6 8 10 12
25-270c(summer
& autumn)
2915 3500 6295 4090 3960 2510
18-230c(winter &
spring)
1030 1080 1140 1380 2580 1850
INTERRELATED EFFECTS OF TEMPERATURE
AND TOTAL SOLID CONCENTRATION ON THE
BIOGAS
23. Loading rate is the amount of raw materials fed per unit
volume of digester capacity per day
The volumetric organic loading rate, in relation to the
hydraulic retention time of a digester, can induce toxicity by
increasing the concentration of the toxic substance
If the plant is overfed, accumulation of intermediates such as
volatile acids will accumulate and methane production will be
inhibited by creating toxic conditions
Similarly, if the plant is underfed, the gas production will also
be low
LOADING RATE
24. Increasing the number of methane formers with a
digested slurry from the working biogas plant will rich
in methanogens which leads to gas production
But beyond a certain seed concentration, the gas
production will decrease, due to reduction of raw cow
dung fed to the digester
SEEDING
25. Uniform feeding should be done so that the
microorganisms are kept in a relatively constant load
at all times
Digester must be fed at the same time everyday with a
balanced feed of the same quality and quantity
UNIFORM FEEDING
26. Diameter to depth ratio should be between 0.66 and
1 for maximum production
Digester size depends up to the desired rate of
biogas production (m3/day)
DIGESTER SIZE AND SHAPE
27. Addition of certain nutrients like N2, P, S, C, H2
accelerate anaerobic digestion rate
The nutrients are additional materials added to the
slurry in the digester
Human excreta contains phosphorus
NUTRIENTS
28. RETENTION TIME
Retention time (also known as hydraulic retention time or
detention time) is the average period that a given quantity of
input remains in the digester to begin gas production
Most important factor in determining the volume of the
digester which in turn determines the cost of the plant
The larger the retention period, higher the construction cost
It varies as 30, 40, 55 days according to the regions in each
state
A digester should have a volume of 50 to 60 times the slurry
added daily
The retention time is also dependent on the temperature and
up to 35 C, the higher the temperature, the lower the
retention time
29. MIXING AND STIRRING
Proper mixing of manure to form an homogenous slurry
before it is fed in the digester
It is an essential operation for better efficiency of biogas
systems
Proper stirring of digester slurry ensures repeated contact of
microbes with substrate and results in the utilization of total
contents of the digesters
Important function of stirring is the prevention of formation
of scum layer on the upper surface of the digester slurry
which, if formed, reduces the effective digester volume and
restricts the upward flow of gas to the gas storage chamber
Mixing results in premature discharge of some of the input
& a perfectly unmixed system is likely to result in better
reaction rate but for the problem of scum formation
30. HEATING AND HEAT BALANCE
Digester operational temperatures need to be maintained
constant y applying heat to the digest contents
Various practical approaches have been suggested
earlier and use of solar energy offers great potentialities
Internal heating of digesters by circulating hot water
through pipe coils will also involve extra costs
The out flowing slurry has a higher temperature than the
inflowing, which accounts for a loss of the heat from the
digester
31. MIXING OR STIRRING OR AGITATION
OF THE CONTENT OF THE DIGESTER
Slurry is properly mixed and bacteria get their food
supply
Slight mixing improves the fermentation
However a violent slurry agitation retards the
digestion
Mixing is achieved by designing the inlet and outlet
arrangements in a proper manner so that incoming
slurry tends to stir up the contents of the digester
32. All plant and animal wastes
Cow and buffalo dung, human excreta ,poultry
droppings, pig dung, waste materials of plants, cobs etc
can all be used as feed stocks
These feed stocks are combined in proportion
Following proportion has been recommended
Cow-dung plus solid waste 1:1 by weight and forming
about 10% feed content
TYPE OF FEED STOCKS
33. Intermediate products are produced during biodigestion
This causes decrease in pH
These acids can be converted into methane by addition
of neem cake
Acid accumulation problem does not arise in continuous
fermenting systems
It occurs in batch digestion systems
ACID ACCUMULATION INSIDE THE
DIGESTER
34. Temperature ( 35-37 C Mesophilic condition)
C/N ratio ( optimum between 25:1 to 30:1)
pH ( optimally pH between 6.5-7.5)
Solid content (feed material should have
approx.10:1)Should not have toxic material/ harmful
material to bacteria in digester
HRT ( Hydraulic Retention Time – 30, 40, 55 days)
Loading Rate : 10 kg of dung per m3 volume of
digester
Dilution and Consistency of Inputs : TS - 7 to 10 %,
SUMMARY