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

2. INTRODUCTION
TWO PARAMETERS
Environmental
parameter
Operational
parameter

3. ENVIRONMENTAL FACTORS
pH
Volatile acid concentrations
Alkalinity
Temperature
Nutrient availability
Toxic materials

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

7. pH DEPENDENCE OF METHANOGENIC ACTIVITY

8. EFFECT OF PH CHANGES ON METHANE GENERATION RATE

9. EFFLUENT ALKALINITIES AND pHS AT DIFFERENT INFLUENT SULFATE LEVELS

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

13. RELATIVE GROWTH RATE OF PSYCHROPHILIC, MESOPHILIC AND
THERMOPHILIC METHANOGENS

14. ACTIVITIES OF MESOPHILIC METHANOGENS AT DIFFERENT TEMPERATURES

15. EFFECT OF TEMPERATURE CHANGE ON BIOGAS PRODUCTION RATE

16. EFFECT OF TEMPERATURE ON METHANOGENS

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