SCIENCE OF SOIL
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Why we need to know about soil?
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Soil
A collection of natural bodies developed in the
unconsolidated mineral and organic material on the
immediate surface of the earth that serves as a natural
medium for the growth of land plants and has
properties due to the effects of climate and living
matter acting upon parent material, as conditioned by
topography, over a period of time.
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GENESIS OF SOIL
Rocks are chief sources for the parent material
over which soils are developed
Types of rocks• Igneous
• Sedimentary
• Metamorphic
Genesis includes –weathering of rocks &
formation of soil
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Primary and Secondary Minerals
 Primary Minerals: Minerals that have persisted
with little change in composition since they were
extruded in molten lava(eg. quartz, mica and
feldspars).They are most prominent in sand and
silt fractions.
 Secondary Minerals: Minerals such as the
silicate clays and iron oxides, have been formed
by the breakdown and weathering of less
resistant minerals as soil formation progressed.
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Weathering of rocks
It is physical and chemical disintegration and
decomposition of rocks. Weathering creates
the parent material over which the soil
formation takes place. Later weathering, soil
formation
and
development
proceeds
simultaneously.
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Physical weathering
•
•
•
•
Temperature
Water
Wind
Plants & animals
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Chemical weathering
•
•
•
•
•
•
Solution
Hydration
Hydrolysis
Acidification
Oxidation
Reduction
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Chemical weathering
As soon as physical disintegration of rock and mineral begins,
chemical decomposition starts.
Water and its solution – hydrolysis, hydration, dissolution
KAlSi3O8 + H2O ------> HAlSi3O8 + K+ + OH-
2 HAlSi3O8 + 11 H2O ---- Al2O3 + 6 H4SiO4
Al2O3 + 3H2O ----- Al2O3.3H2O
Acid solution weathering
• CaCO3 + H2CO3 -----> Ca2+ + 2 HCO39
• Oxidation
3 MgFeSiO4 + 2 H2O H4Mg3Si2O9 + SiO2 +
3FeO
• 4 FeO + O2 + 2 H2O -- 4 FeOOH
It is particularly manifest in rocks containing
iron
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Soil formation
The mineral weathering combines with the
associated physical and chemical phenomena
constitute the process of soil formation.
It includes1. The addition of organic & mineral materials
2. The loss of these materials from the soil
3. Translocation of materials from one point to
Another within the soil column
4. Transformation of minerals & organic
substances within the soil
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Two Approaches:
• Pedological
• Edaphological
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The origin of the soil ,its classification, and its
description are examined in pedology (pedon-soil or
earth in greek). Pedology is the study of the soil as a
natural body and does not focus primarily on the soli’s
immediate practical use. A pedologist studies, examines,
and classifies soils as they occur in their natural
environment.
Edaphology (edaphos means soil or ground in greek) is the
study of soil from the stand point of higher plants.
Edaphologists consider the various properties of soils in
relation to plant production. They are practical and have
the production of food and fibre as their ultimate goal.
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Composition of soil
5%
25%
Air
Water
Mineral
45%
Organic
25%
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Soil Profile and its Layers(Horizons)
• Examination of a vertical section of a soil as seen in a
roadside cut or in the walls of a pit dug in the field,
reveals the presence of more or less distinct
horizontal layers. Such a section is called a profile,
and the individual layers are known as horizons
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Topsoil and Subsoil
• When a soil is ploughed and cultivated, the natural
state of the upper 12-18 centimeters(5-7 inches) is
modified. This manipulated part of the soil is referred
to as the surface soil or the topsoil.
• The subsoil is comprised of those soils layers
underneath the top soil.
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Mineral (inorganic) and organic soils
• Mineral soils: Mineral or inorganic in
composition, low in organic matter ranges from 1
-6%.
• Organic soils: 50% organic matter by volume (at
least 20% by weight).
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Soil Texture and Soil Structure
• Soil Texture: Proportions of different sized
particles present in soil.
• Soil Structure: The arrangement of the sand
silt and clay particles within the soil.
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Table: General properties of three major inorganic soil particles
Property
Sand (0.052mm)
Silt (0.002-0.05mm)
Clay(<0.002
mm)
1. Means of observation
Naked eye
Microscopic
Electron
Microscope
2.Dominant minerals
Primary
Primary and
Secondary
Secondary
3.Attraction of particles for each
other
Low
Medium
High
4. Attraction of particles for water
Low
Medium
High
5.Ability to hold chemical nutrients Very low
and supply them to plants
Low
High
6.Consistency properties when wet
Smooth
Sticky,
plastic
Powdery, some
clods
Hard clods
Loose , gritty
7.Consistency properties when dry Very loose,
gritty
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Soil Air
Soil air differs from the atmospheric air in several
respectsFirst ,the composition of soil air is quite dynamic and
varies greatly from place to place within a given
soil.
Second, soil air generally has a higher moisture
content than the atmosphere; the relative humidity
of soil air approaches 100% when the soil moisture
is optimum.
Third, carbon dioxide in soil air is often several times
higher than the 0.03% commonly found in the
atmosphere, Oxygen decreases accordingly and, in
extreme cases 5-10%, or even less, as compared to
about 20% for normal atmosphere.
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Composition of soil air
Particulars
Percentage by
volume
Nitrogen
Oxygen
Carbon dioxide
Atmospheric air 79.00
20.95
0.03
Soil air
79.20
20.60
0.25
Sandy soil air
79.20
19.95
0.30
Loamy soil air
79.20
19.20
0.62
Clay soil air
79.20
19.69
0.66
Manured soil
air
79.20
18.23
1.85
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Soil Organic Matter
 Soil organic matter comprises an accumulation of
partially disintegrated and decomposed plant and animal
residues and other organic compounds synthesized by the
soil microbes as the decay occurs. Such material is
continually being broken down and re-synthesized by soil
microorganisms. Consequently, organic matter is a rather
transitory soil constituent, lasting for a few hours to
several hundred years.
 Organic matter binds mineral particles into granules that
are largely responsible for the loose. easily managed
condition of productive soils and increases the number of
water a soil can hold.
 It is also major soil source of phosphorus and sulfur and
the primary source of nitrogen (3 elements essential for
plant growth)
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• Organic matter, including plant and animal
residues, is the main source of energy for soil
organisms. Without it biochemical activity would
come to a near standstill.
• In addition to the original plant and animal
residues and to their partial breakdown products,
soil organic matter includes complex compounds
that are relatively resistant to decay. These
complex materials, along with some that are
synthesized by the soil microorganisms, are
collectively known as humus. This material is
usually black and brown in colour, is very
fine(colloidal) in nature.
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Soil Water
 Water is hold in the soil for varying degree of tenacity
depending on the amount of water present and the size of
the pores.
 Together with its soluble constituents, including nutrient
elements(eg. Ca, P, N and K), soil water makes up the soil
solution, which is the critical medium for supplying
nutrients to growing plants.
 The movement can be in any direction; downward in
response to gravity, upward as water moves to the soil
surface to replace that lost by evaporation, and in any
direction toward plant roots as they absorb this important
liquid. Although some of the soil moisture is removed by
the growing plants, some remains in the tiny pores and in
thin films around soil particles. The soil solids strongly
attract the soil water and consequently compete for it with
plant roots.
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Soil Solution
 The soil solution contains small but significant
quantities of soluble inorganic and organic
compounds, some of which contain elements
that are essential for plant growth
 Critical property of the soil solution is its
acidity or alkalinity. Many chemical and
biological reactions are dependent on the
levels of hydrogen ions and hydroxide ions in
the soil. These levels influence the solubility,
and in turn the availability to plants, of several
essential nutrient elements such as Fe, Mn, P,
Zn and Mo.
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• The concentration of hydrogen(H+) and hydroxide
ions(OH-) in the soil solution is commonly ascertained by
determining its pH. Technically the pH is the negative
logarithm of the concentration of hydrogen ion in the soil
solution. Thus each unit change in pH represents a
tenfold change in the activity of the H+ and OH- ions.
Acidity
Alkalinity
Very Strong
stro
ng
3-4
4-5
Moderate
3
Slight
Neutr Slight
al
Moderate
Strong
Very
strong
9-10
10-11
4
5-6
6-7
7
7-8
8-9
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Clay and Humus
• The attraction of ions such as Ca2+, Mg2+, and K+ on the
surfaces of colloidal clay and humus is not as exciting as
is the exchange of these ions for other ions in the soil
solution. For example, an H+ ion released to the soil
solution by a plant root exchange readily with a potassium
ion(K+) adsorbed on the colloidal surface .The K+ ion is
then available in the soil solution for uptake by the roots of
crop plants. A simple example of such cation exchange
illustrates this point.
colloid K+ + H+(aq)
colloid H+ + K+(aq)
(adsorbed)
(in soil solution)
(adsorbed)
(in soil solution)
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-ve charge
+ve charge
Al
Ca
Mg
Clay Micelle
K
Na
H
Ionic double layer
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pH-dependent charge
Common in humus, allophane, Fe & Al hydroxides
Negative Charges
Al – OH + OH- = Al – O- + H2O
-CO-OH + OH- = -CO-O- + H2O
No Charge
-ve charge
These reactions are reversible. If the pH increases, more OH ions are
available to force the reaction to the right
Positive charge
Under moderate to extreme acid soil conditions
Al – OH + H+ = Al–OH2+. In some cases,
Al-O- + H+ = ALOH + H+ = Al–OH2+
High pH
Low pH
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Essential nutrient element and their sources
Use in relatively
large amounts
Use in relatively large Use in relatively small
amounts
amounts
Mostly from air and
water
From soil
From soil
Carbon(C )
Nitrogen(N)
Iron(Fe)
Hydrogen(H)
Phosphorus(P)
Manganese(Mn)
Oxygen(O)
Calcium(Ca)
Boron(B)
Magnesium(Mg)
Molybdenum(Mo)
Sulfur(S)
Copper(Cu)
Potassium (K)
Zinc(Zn)
Chlorine(Cl)
Cobalt(Co)
Nickel (Ni)
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Soil Degradation
 Soil degradation is a concept in which the
value of the biophysical environment is
affected by one or more combination of
human-induced processes acting upon the
land. It is viewed as any change or disturbance
to the land perceived to be deleterious or
undesirable. Natural hazards are excluded as a
cause, however human activities can indirectly
affect phenomena such as floods and
bushfires.
It is estimated that up to 40% of the
world's agricultural land is seriously degraded.
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Causes
The major causes include:
 Land clearance, such as clear cutting and
deforestation
 Agricultural depletion of soil nutrients through
poor farming practices
 Overgrazing
 Inappropriate Irrigation and over-drafting
 Urban sprawl and commercial development
 Land pollution including industrial waste
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• Vehicle off-roading
• Quarrying of stone, sand, ore and minerals
Overcutting of vegetation
• Overgrazing
• shifting cultivation without adequate fallow
periods, absence of soil conservation
measures,
• Population pressure
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Effects
 The major stresses on vulnerable land include:
 Accelerated soil erosion by wind and water
 Soil acidification and the formation of acid sulfate soil
resulting in barren soil
 Soil alkalinisation owing to irrigation with water
containing sodium bicarbonate leading to poor soil
structure and reduced crop yields
 Soil salinization in irrigated land requiring soil salinity
control to reclaim the land
 Waterlogging in irrigated land which calls for some
form of subsurface land drainage to remediate the
negative effects
 Destruction of soil structure including loss of organic
matter
 Ultimately results into low vegetation cover, extensive
soil erosion which leads towards desertification
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• Every year 84 billion tonnes of productive
top soil are lost world wide through
degradation.
• Degradation has already affected 1900 m
ha of land globally (De Man et. al. 2007).
• Additionally each year over 14 million acres
of productive lands are oversalted because
of improper water management.
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Soil Erosion
 Soil erosion is the process of detachment of soil
particles from the parent body and transportation of
the detached soil particles by wind or water.
Mechanism of Water Erosion:
a. Detachment
b. Transportation
Causes:
a. Natural
b. Anthropogenic
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Forms of Water Erosion
 Sheet Erosion: uniform removal of top soil in thin layer from the
field, least conspicuous.
 Rill Erosion: channelization begins ,no longer uniform.
 Gully Erosion: unchecked rills result in increased channelization of
runoff.
 Ravines: manifestation of prolonged process of gully erosion.
Deepening & Widening of gullies used to form ravines.
 Landslides: occur in mountain slopes when the slope exceeds 20
per cent and width 6 m.
 Stream-bank Erosion: Seasonal streams or rivulets often change
their course from season to season due to blockage of their
previous course by transported rocks, clods of soil & vegetation
grown during lean periods.
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Gully erosion
Ravine erosion
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Forms of wind erosion
 Suspension- Most spectacular method of transporting soil
particles is by suspension. Dust particles of fine sand ( less
than 0.1 mm dia) are moved parallel to ground surface
and upward. About 5-15 % of wind erosion afftected soil
is transported by this process.
 Saltation- Particles in the range 0.1-0.5 mm diameter are
lifted by the wind, then fall back to the ground, so they
move in a hopping or bouncing fashion. These particles
cause abrasion of the soil surface and as they hit other
particles they break into smaller particles, a process
called attrition. Depending on conditions, this process
may account for 50-70% of the total movement of soil.
 Surface creep- Rolling and sliding of larger particles (more
than 0.5 mm dia) along the surface. Surface creep
account to 5-25% of total movement due to action of
wind.
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Soil Conservation
Definition
Soil conservation is using and managing
land based on the capabilities of the land
itself.
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Soil Conservation Measures
Agronomic Measures
 Contour Cultivation – By ploughing and sowing
across the slope, each ridge of plough furrow and
each row of the crop act as an obstruction to runoff,
providing more opportune time for water to enter into
the soil and reduce soil loss.
 Tillage – Tillage alters soil physical characters like
porosity, bulk density, surface roughness and hardness
of pans. Conventional tillage includes ploughing
twice or thrice followed by some secondary
operations like harrowing and planking that smoothen
and pack the soil in seed-bed and/or control weeds.
 Mulching – Mulches are any material such as straw,
plant residues, leaves, loose soil or plastic film placed
on the soil surface to reduce evaporation, erosion or
to protect plant roots from extremely low or high
temperature.
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Mechanical Measures
 Contour Bunding – Runoff from any given surface is along the line
of greatest slope and the velocity of runoff increases with the
vertical distance through which it is moved. The contour bund
being on the same elevation, assures that the depth of water against
the bund is uniform throughout its length. It ensures uniform
distribution of water above the bunds and therefore, better
cultivation possibilities than any other type of bund. As the bunds
are at regular intervals, they intercept the runoff from attaining
erosive velocity and causing erosion. The velocity of flowing water
is slowed down and water thus held on the field for a longer time,
soaks into the soils.
 Broad Base Terrace - A terrace is a combination of ridge and
channel built across the slope. These terraces have wide base and
low height of ridge and usually formed with machinery. BBTs are
constructed in soils with high clay content which develop deep
cracks in summer (e.g. Black soil).
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Bench Terracing - Bench terracing consists of transforming relatively
steep land into a series of level strips or platforms across the slope of
the land. It reduces the slope length and consequently erosion. The field
is made into a series of benches by excavating the soil from upper part
of the terrace and filling in the lower part. On steeply sloping and
undulated land, farming practices is possible only with bench terracing.
It is usually practiced on slopes ranging from 16 to 33%.
Trenching –Contour trenches are made in non-agricultural land for
providing adequate moisture conditions in order to raise trees or grass
species. The trenches are usually 60 cm × 48 cm in size. The spacing
varies from 10 to 30 m.
Vegetative Barriers – these are closely spaced plantations-usually a few
rows of grasses or shrubs --- grown along contours . They act as barrier
to check the velocity of overland flow and entrapment of silt load behind
them. Khus (Vettiveria zelanica) is the most suitable plant for this
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purpose.
Grassed Waterways – These are drainage channel either
developed by shaping the existing drainage ways or constructed
separately. Suitable perennial grasses that are not edible by cattle,
deep rooted and spreading type are established subsequently for
the stability of the waterway (e.g Panicum repens, Brachiara
mutica, Cynodon dactylon, Paspalum notatum). The objectives
are- 1. to provide drainage, 2. to convert gullies or unstable
channels into stable channels by providing grass cover, and 3. for
leading water at non-erosive velocity into a water body.
Gully Control – The basic approach to gully control involves
reduction of peak flow rates through the gully and provision of
stable channel for the flow that has to be handled. Temporary and
permanent structures such as check dams, drop-spill ways are
constructed.
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Agrostological Measures
 Grasses prevent soil erosion by intercepting rainfall, by
binding the soil particles and by improving soil structure.
A grass-legume association is ideal for soil conservation.
E.g Pennisetum pupureum, Cenchrus ciliaris, Setaria
sphacelata.
Forestry Measure
 Afforestation and re-forestation in wastelands
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AA
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Semi-circular & triangular
contour bunds
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Check dam
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Waste Management
What are Wastes?
Waste (also known as rubbish, trash, refuse, garbage, junk, litter,
and ort) is unwanted or useless materials. In biology, waste is
any of the many unwanted substances or toxins that are
expelled from living organisms, metabolic waste; such as urea
and sweat.
Basel Convention Definition of Wastes
“substances or objects which are disposed of or are intended
to be disposed of or are required to be disposed of by the
provisions of the law”
Disposal means
“any operation which may lead to resource recovery,
recycling, reclamation, direct re-use or alternative uses
(Annex IVB of the Basel convention)”
Basel Convention
• The Basel Convention on the Control of Transboundary Movements of
Hazardous Wastes and Their Disposal, usually known simply as Basel
Convention, is an international treaty that was designed to reduce the
movements of hazardous waste between nations, specially to prevent
transfer of hazardous waste from developed to less developed countries
(LDCs). It does not, however, address the movement of radioactive waste.
The convention is also intended to minimize the amount and toxicity of
wastes generated, to ensure their environmentally sound management as
closely as possible to the source of generation, and to assist LDCs in
environmentally sound management of the hazardous and other wastes
they generate.
• The Convention was opened for signature on 22nd March 1989, and
entered into force on 5 May 1992.
Classification of Wastes according to
their Properties
Bio-degradable
can be degraded (paper, wood, fruits and
others)
Non-biodegradable
cannot be degraded (plastics, bottles, old
machines,cans, styrofoam containers and
others)
Classification of Wastes according to
their Effects on Human Health and the
Environment
• Hazardous wastes
• Substances unsafe to use commercially,
industrially, agriculturally, or economically and
have any of the following properties- ignitability,
corrosivity, reactivity & toxicity.
• Non-hazardous
• Substances safe to use commercially,
industrially, agriculturally, or economically and
do not have any of those properties mentioned
above. These substances usually create disposal
problems.
Classification of wastes according to their origin
and type
•
•
•
•
•
•
•
Municipal Solid wastes: Solid wastes that include household garbage, rubbish,
construction & demolition debris, sanitation residues, packaging materials, trade
refuges etc. are managed by any municipality.
Bio-medical wastes: Solid or liquid wastes including containers, intermediate or
end products generated during diagnosis, treatment & research activities of
medical sciences.
Industrial wastes: Liquid and solid wastes that are generated by manufacturing &
processing units of various industries like chemical, petroleum, coal, metal gas,
sanitary & paper etc.
Agricultural wastes: Wastes generated from farming activities. These substances
are mostly biodegradable.
Fishery wastes: Wastes generated due to fishery activities. These are extensively
found in coastal & estuarine areas.
Radioactive wastes: Waste containing radioactive materials. Usually these are
byproducts of nuclear processes. Sometimes industries that are not directly
involved in nuclear activities, may also produce some radioactive wastes, e.g.
radio-isotopes, chemical sludge etc.
E-wastes: Electronic wastes generated from any modern establishments. They may
be described as discarded electrical or electronic devices. Some electronic scrap
components, such as CRTs, may contain contaminants such as Pb, Cd, Be or
brominated flame retardants.
Waste hierarchy
Waste hierarchy refers to 3 Rs
Reduce, Reuse, Recycle
Waste to Wealth
(A) Waste to compost (i.e.
resource recovery)
 Aerobic/Anaerobic
composting
 Vermicomposting – A
major component of
organic farming.
(B) Waste to energy
 Anaerobic digestion
(Biomethanation)
 Incineration
 Pyrolysis
 Gasification
 Pelletization (Refuse
derived fuel or RDF)
 Landfill gas recovery
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Basic techniques of energy
recovery
Energy can be recovered from the organic
fraction of waste (bio-degradable as well as
non bio-degradable) through two methods:
i. Thermo-chemical conversion: This process
entails thermal decomposition of organic
matter to produce either heat energy or fuel
oil or gas.
67
Contd…
ii. Biochemical conversion: This process is
based on enzymatic decomposition of
organic matter by microbial action to
produce methane gas or alcohol.
Some of the important energy recovery techniques
are discussed under the following heads:
a)
Anaerobic digestion (AD):

Also known as bio-methanation
 Segregating the organic fractions of waste
68
Contd..
 Feeding them into a closed container (biogas
digester) under anaerobic condition
Organic wastes undergone bio-degradation
and produce methane rich biogas and
effluent/ sludge
Biogas produced, 50-150 m3/ton depending
upon waste composition
Fundamentally, anaerobic digestion process
can be divided into three stages with 3 distinct
physiological groups of micro-organisms.
69
Contd..
Stage I: Fermentative bacteria (anaerobic &
facultative micro-organisms) e.g. Bacteroides
succinogens, Clostridium sp.
Complex organic materials, carbohydrates,
proteins and lipids
hydrolyzed &
fermented into fatty acids, alcohol, CO2, H2,
NH3 and sulfides.
Stage II: Acetogenic bacteria consume these
primary products and produce H2, CO2 &
acetic acid (CH3COOH). e.g. Syntrophobactor
wolinii, Syntrophomonas wolfei
70
Contd..
Stage III: Two types of methanogenic bacteria
First one (reduces) CO2 to CH4 (e.g.
Methanosprillium sp.)
Second one (decarboxylates) CH3COO- to CH4 (e.g.
Methanosarcina sp.)
71
Contd..
b) Incineration:
Direct burning of wastes in the presence of
excess air (oxygen)
Liberates heat energy, inert gas and ash
About 65-80% of energy content of organic
matter can be recovered
72
Contd..
c) Pyrolysis:
Also known as destructive distillation or
carbonization
Thermal decomposition or organic matter at
high temperature (about 900oC) in an inert (O2
deficient) atmosphere or vacuum.
Produces a mixture of combustible and noncombustible gases
73
Contd..
c) Gasification:
Thermal decomposition of organic matter at
high temperature in presence of limited
amount of oxygen
Produces mainly a mixture of combustible &
non-combustible gases
Temperature > 1000oC
The gas can be cooled, cleaned and utilized to
generate electricity
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