18. Polluted soils- industrial effluents- Characteristics, reclamation and management
Pollution is a serious problem ever since sewage and industrial effluents are disposed into
water courses and on land. It has increased with the growth of industry as well as of population.
Industries can conveniently be classified in to two groups viz. (i) Dry process industries and (ii)
Wet process industries. Dry process industries (mostly engineering and assembling industries) do
not use water in their process, while wet process industries use water either as raw material or in
their process or for both. Only a little quantity of water is absorbed in the process and the rest
being discharged as effluent. Practically all the volume of water used is calculated as the volume
of effluent. In addition to the process waters, large volumes of cooling water are also discharged
as waste. Boiler blow down is also an effluent contributing significant quantities to the bulk.
These effluents contain a wider variety of materials of both organic and inorganic nature
including toxic substances and are usually discharged with or without treatment into surface
waters such as rivers, streams, lakes or into oceans or on land or into sewers. The solid wastes
produced in these industries are not given so much importance as they are either recycled
dumped or disposed of to remote places. Liquid industrial wastes are of great concern because of
their harmful effects. The effects are described in detail below.
Effects of industrial effluents
Industries need a wide variety of raw materials and chemicals which are later discharged
as effluents. Acids, alkalis, toxic metals, pesticides and other poisonous substances such as
cyanide, dyes, oils, detergents, resins, rubbers are a few to mention. Heated effluents that impart
thermal loading on receiving waters and effluents containing radio active materials are also of
prime concern. Some of the effluents such as from tanning and meat packing may also contain
pathogenic bacteria. The nature and extent of pollution depends on the materials present in the
effluent and on the quantity discharged.
Effects of industrial effluents on water ecosystem
Color : The effluents contain dyes in higher concentrations which impart color to the receiving
streams and they persist for longer distances. Photosynthesis of phytoplankton is affected by
these colors.
,pH value : The extreme alkalinity makes the receiving water unfit for any purpose. Further, it is
deleterious to most of the aquatic life.
Suspended impurities : The colloidal and suspended impurities produce turbidity in the
receiving waters. The turbidity and color along with the oil and scum create an unsightly
appearance.
Depletion of oxygen : Natural substances such as starch and dextrin and inorganic substances
such as sulfide and nitrite present in the effluent exert an immediate oxygen demand. The stream
will then be devoid of oxygen and the aquatic life are affected adversely.
Toxic substances : Chromium, sulfide, chlorine and aniline dyes present in these wastes are
directly toxic to fish and microbial organisms which carryout purification. Thus the self
purification of the water body is affected.
Oils : Various oils (mineral) in the effluent interfere with the oxygenation of stream as they form
a blanket on the surface and prevents the entry of oxygen at air/water interface.
Dissolved minerals : The mineral materials, mostly sodium salts increase the salinity of the
water and consequently it becomes unfit for irrigation.
Effects on land
1. The excess content of sodium (60%) and boron (2 mg/l) are deleterious to crops.
2. The high sodium alkalinity combined with salinity impairs the growth of plants.
3. Texture of the soil is affected by sodium and penetration of roots is prevented.
4. Soil permeability is also affected by sodium and ultimately the soils will loss its productivity.
5. Suspended and colloidal impurities clog the pores and form a mat on the surface of soil
preventing the passage of air, water etc.
, LEATHER INDUSTRY
The leather processing industries are tanneries. Tannery is a major foreign exchange
earning industry in India. But this industry appears to be the major source of pollution, especially
in Tamil Nadu state which contributes more than 70% of this industry functioning in India. High
concentration of salts (NaCl) and the heavy metal namely Chromium (Cr) are the major
pollutants of tannery wastes. While the effluent is rich in NaCl, the sludge on average contained
20000 mg kg-1 of chromium. The improper disposal of tannery wastes lead to extensive
contamination of land and water with salts and chromium, thereby the agricultural productivity
have been drastically affected not only due to low yield but also due to poor quality of produce.
Chemistry of chromium
Chromium exists in both anionic and cationic state and the nature of the species may vary
with charges in the soil characteristics. Among the various forms of chromium, the trivalent form
Cr (III) and hexavalent form Cr V1 are the important transformation stages. Hexavalent species
Cr (V1) is a strong oxidizer and result in oxygenated species, that are pH dependent, highly
soluble and hence it is toxic than trivalent form of Cr (III). The dominant hexavalent species are
HC4O4- (pH of 0 + 0 5.9) and CrO42- (pH of >6). Trivalent Cr (Cr III) is the more stable form and
less affinity for O2, hence it is less toxic.
Remediation of chromium contaminated soils
Traditional techniques for remediating chromium contaminated water involve reduction
of Cr (VI) to CR (III) by chemical(usually Fe2+) or electrochemical means at pH > 5, followed by
precipitation and finally filtration or sedimentation. A rapid developing area of biotechnology is
“Bioremediation” the use of microbes of break down pollutants in the environment, particularly
the soil.
Examples of chromate reducing microorganisms
Aerobic Anaerobic
, Agrobacterium radiobacter Enterobacter cloacae
Bacillus subtilis Escherichia coli
Pseudomonas flourescens Pseudomonas florescencens
Escherichia coli Agrobacterium radiobacter
SUGAR AND DISTILLERY INDUSTRIES
In India, the sugar industry is the second largest agro processing industry after textiles. A
typical sugar factory generates a large quantity of by products like baggasse, pressmud, molasses
and spent wash from distillery. The following Fig. shows the processing of sugarcane in the
sugar factory with its by products.
Sugarcane
Preparatory devises and mills
Cane juice with impurities Baggasse raw material
Lime Heat for paper and pulp
SO2 or CO2
Juice with CaSO3
or
CaCO3
CaSO3 (Sulphitation pressmud cake)
Pollution is a serious problem ever since sewage and industrial effluents are disposed into
water courses and on land. It has increased with the growth of industry as well as of population.
Industries can conveniently be classified in to two groups viz. (i) Dry process industries and (ii)
Wet process industries. Dry process industries (mostly engineering and assembling industries) do
not use water in their process, while wet process industries use water either as raw material or in
their process or for both. Only a little quantity of water is absorbed in the process and the rest
being discharged as effluent. Practically all the volume of water used is calculated as the volume
of effluent. In addition to the process waters, large volumes of cooling water are also discharged
as waste. Boiler blow down is also an effluent contributing significant quantities to the bulk.
These effluents contain a wider variety of materials of both organic and inorganic nature
including toxic substances and are usually discharged with or without treatment into surface
waters such as rivers, streams, lakes or into oceans or on land or into sewers. The solid wastes
produced in these industries are not given so much importance as they are either recycled
dumped or disposed of to remote places. Liquid industrial wastes are of great concern because of
their harmful effects. The effects are described in detail below.
Effects of industrial effluents
Industries need a wide variety of raw materials and chemicals which are later discharged
as effluents. Acids, alkalis, toxic metals, pesticides and other poisonous substances such as
cyanide, dyes, oils, detergents, resins, rubbers are a few to mention. Heated effluents that impart
thermal loading on receiving waters and effluents containing radio active materials are also of
prime concern. Some of the effluents such as from tanning and meat packing may also contain
pathogenic bacteria. The nature and extent of pollution depends on the materials present in the
effluent and on the quantity discharged.
Effects of industrial effluents on water ecosystem
Color : The effluents contain dyes in higher concentrations which impart color to the receiving
streams and they persist for longer distances. Photosynthesis of phytoplankton is affected by
these colors.
,pH value : The extreme alkalinity makes the receiving water unfit for any purpose. Further, it is
deleterious to most of the aquatic life.
Suspended impurities : The colloidal and suspended impurities produce turbidity in the
receiving waters. The turbidity and color along with the oil and scum create an unsightly
appearance.
Depletion of oxygen : Natural substances such as starch and dextrin and inorganic substances
such as sulfide and nitrite present in the effluent exert an immediate oxygen demand. The stream
will then be devoid of oxygen and the aquatic life are affected adversely.
Toxic substances : Chromium, sulfide, chlorine and aniline dyes present in these wastes are
directly toxic to fish and microbial organisms which carryout purification. Thus the self
purification of the water body is affected.
Oils : Various oils (mineral) in the effluent interfere with the oxygenation of stream as they form
a blanket on the surface and prevents the entry of oxygen at air/water interface.
Dissolved minerals : The mineral materials, mostly sodium salts increase the salinity of the
water and consequently it becomes unfit for irrigation.
Effects on land
1. The excess content of sodium (60%) and boron (2 mg/l) are deleterious to crops.
2. The high sodium alkalinity combined with salinity impairs the growth of plants.
3. Texture of the soil is affected by sodium and penetration of roots is prevented.
4. Soil permeability is also affected by sodium and ultimately the soils will loss its productivity.
5. Suspended and colloidal impurities clog the pores and form a mat on the surface of soil
preventing the passage of air, water etc.
, LEATHER INDUSTRY
The leather processing industries are tanneries. Tannery is a major foreign exchange
earning industry in India. But this industry appears to be the major source of pollution, especially
in Tamil Nadu state which contributes more than 70% of this industry functioning in India. High
concentration of salts (NaCl) and the heavy metal namely Chromium (Cr) are the major
pollutants of tannery wastes. While the effluent is rich in NaCl, the sludge on average contained
20000 mg kg-1 of chromium. The improper disposal of tannery wastes lead to extensive
contamination of land and water with salts and chromium, thereby the agricultural productivity
have been drastically affected not only due to low yield but also due to poor quality of produce.
Chemistry of chromium
Chromium exists in both anionic and cationic state and the nature of the species may vary
with charges in the soil characteristics. Among the various forms of chromium, the trivalent form
Cr (III) and hexavalent form Cr V1 are the important transformation stages. Hexavalent species
Cr (V1) is a strong oxidizer and result in oxygenated species, that are pH dependent, highly
soluble and hence it is toxic than trivalent form of Cr (III). The dominant hexavalent species are
HC4O4- (pH of 0 + 0 5.9) and CrO42- (pH of >6). Trivalent Cr (Cr III) is the more stable form and
less affinity for O2, hence it is less toxic.
Remediation of chromium contaminated soils
Traditional techniques for remediating chromium contaminated water involve reduction
of Cr (VI) to CR (III) by chemical(usually Fe2+) or electrochemical means at pH > 5, followed by
precipitation and finally filtration or sedimentation. A rapid developing area of biotechnology is
“Bioremediation” the use of microbes of break down pollutants in the environment, particularly
the soil.
Examples of chromate reducing microorganisms
Aerobic Anaerobic
, Agrobacterium radiobacter Enterobacter cloacae
Bacillus subtilis Escherichia coli
Pseudomonas flourescens Pseudomonas florescencens
Escherichia coli Agrobacterium radiobacter
SUGAR AND DISTILLERY INDUSTRIES
In India, the sugar industry is the second largest agro processing industry after textiles. A
typical sugar factory generates a large quantity of by products like baggasse, pressmud, molasses
and spent wash from distillery. The following Fig. shows the processing of sugarcane in the
sugar factory with its by products.
Sugarcane
Preparatory devises and mills
Cane juice with impurities Baggasse raw material
Lime Heat for paper and pulp
SO2 or CO2
Juice with CaSO3
or
CaCO3
CaSO3 (Sulphitation pressmud cake)
