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Chapter 1
Introduction
Chapter 1: Introduction
Hydrodynamic Cavitation based degradation of Bio-Refractory pollutants Page 1
1.1 Water a precious resource
Water covers about 71% of the Earth's surface, and is vital for all known
forms of life. On Earth, 97 % of the planet's water is found in oceans, 1.7% in
groundwater, 1.7% in glaciers and the ice caps of Antarctica and Greenland, a small
fraction in other large water bodies. Only 2.5% of the Earth's water is fresh water, and
98.8% of that water is in ice and groundwater. Less than 0.3% of all freshwater is in
rivers, lakes, and the atmosphere.
Safe drinking water is essential to humans and other living life. Water is the
key to life: a crucial resource for humanity and the rest of the living world. Everyone
needs it and not just for drinking. Society uses water to generate and sustained
economic growth and prosperity, through activities such as farming, commercial
fishing, energy production, manufacturing, transport and tourism. The most
challenging problem in today’s world is managing the supply and availability of safe
drinking water for all human and living creatures on this earth. Water scarcity has
emerged as a prominent issue for communities across the country. Nearly every
region of the country has experienced water shortages in the last five years. Water
supplies have decreased due to the drying up of streams, the decline of groundwater
levels because of over pumping, contamination of water resources, and an increase in
drought conditions caused by climate change. The increase in human population,
urbanization and ever-increasing industrialization causes depletion and contamination
of our precious water resources. The society is less concerned about the conservation
and protection of our water body from being polluted. Each year more than five
million people die from water-related disease and around one billion people do not
have accesses to safe drinking water and still we are deteriorating this precious natural
resource. Only 1% of the total fresh water on the earth is available for drinking
purpose through the different water bodies such as river, lakes, pond etc.. Despite this
fact that the world’s population is growing by roughly 80 million people each year
and demand for freshwater is increasing by 64 billion cubic meters a year we are
polluting this only available (1%) form of water by contaminating this with waste.
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Hydrodynamic Cavitation based degradation of Bio-Refractory pollutants Page 2
1.2 Water availability and use in India
India accounts for 2.45% of land area and 4% of water resources of the world
but represents 16% of the world population. Total utilizable water resource in the
country has been estimated to be about 1123 billion cubic meter (BCM) (690 BCM
from surface and 433 BCM from ground), which is just 28% of the water derived
from precipitation. About 85% (688 BCM) of water usage is being diverted for
irrigation, which may increase to 1072 BCM by 2050. Major source for irrigation is
groundwater. Annual groundwater recharge is about 433 BCM of which 212.5 BCM
is used for irrigation and 18.1 BCM for domestic and industrial use (CGWB, 2011).
By 2025, demand for domestic and industrial water usage may increase to 96 BCM
from current demand of 68 BCM. With the present population growth-rate (1.9% per
year), the population is expected to cross the 1.5 billion mark by 2050. Due to
increasing population and all round development in the country, the per capita
average annual freshwater availability has been reducing since 1951 from 5177 m3 to
1869 m3, in 2001 and 1588 m3, in 2010. It is expected to further reduce to 1341 m3 in
2025 and1140 m3 in 2050. Hence, there is an urgent need for efficient water resource
management through enhanced water use efficiency and waste water recycling.
1.3 Water pollution
When toxic substances enter lakes, streams, rivers, oceans, and other water
bodies, they get dissolved or remain suspended in water or get deposited on the bed.
This results in the pollution of water whereby the quality of the water deteriorates,
affecting aquatic ecosystems. Pollutants can also seep down and affect the
groundwater deposits.
Water pollution has many sources. The most polluting of them are the city
sewage and industrial waste discharged into the rivers. The effects of water pollution
are not only devastating to people but also to animals, fish, and birds. Polluted water
is unsuitable for drinking, recreation, agriculture, and industry. It diminishes the
aesthetic quality of lakes and rivers. More seriously, contaminated water destroys
aquatic life and reduces its reproductive ability. Eventually, it is a hazard to human
health. Nobody can escape the effects of water pollution.
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Hydrodynamic Cavitation based degradation of Bio-Refractory pollutants Page 3
1.4 Wastewater generation in India
It is estimated that about 38,254 million litres per day (mld) of wastewater is
generated in urban centres comprising Class I cities and Class II towns having
population of more than 50,000 (accounting for more than 70 per cent of the total
urban population). The municipal wastewater treatment capacity developed so far is
about 11,787 mld, that is about 31 per cent of the wastewater generation in these two
classes of urban centres (CPCB, 2009). Apart from domestic sewage, about 13468
MLD of wastewater is generated by industries of which only 60% is treated. In case
of small scale industries that may not afford cost of waste water treatment plant,
Common Effluent Treatment Plants (CETP) has been set-up for cluster of small scale
industries (CPCB, 2005a). The status of wastewater generation and treatment capacity
developed over the decades in urban centres (Class I and Class II) is presented in
Table 1.1.
In view of the population increase, demand of fresh water for all uses will
become unmanageable. It is estimated that the projected wastewater from urban
centres may cross 120,000 mld by 2051 and that rural India will also generate not less
than 50,000 mld in view of water supply designs for community supplies in rural
areas. However, wastewater management plans do not address this increasing pace of
wastewater generation. Central Pollution Control Board (CPCB) studies depict that
there are 269 sewage treatment plants (STPs) in India, of which only 231 are
operational, thus, the existing treatment capacity is just 21 per cent of the present
sewage generation (CPCB, 2007a). The remaining untreated sewage is the main cause
of pollution of rivers and lakes. Around 12410 mld waste water is generated in the
Ganga basin out of which treatment facilities are available only for 4869 mld of
wastewater (CPCB, 2005b). The rest of 7541 mld untreated wastewater is directly
discharged into the river, which is responsible for making it among the world’s top 10
dirtiest (polluted) rivers. The large numbers of STPs created under Central Funding
schemes such as the Ganga Action Plan and Yamuna Action Plan of National River
Action Plan are not fully operated.
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Hydrodynamic Cavitation based degradation of Bio-Refractory pollutants Page 4
Table1.1: Wastewater generation and treatment capacity in urban India Parameters Class I cities Class II towns
1978-
9
1989-
90
1994-
5
2003-
4
2009 1978-
9
1989-
90
1994-
5
2003-
4
2009
Number 142 212 299 414 498 190 241 345 489 410
Population(millions) 60 102 128 178 187 12.8 20.7 23.6 37.5 30
WaterSupply(mld) 8638 15,191 20,607 29,782 44,769 1533 1622 1936 3035 3324
Wastewater
Generated(mld)
7007 12,145 16,662 23,826 35,558 1226 1280 1650 2428 2696
Wastewatertreated
(mld)(percent)
2756
(39)
2485
(20.5)
4037
(24)
6955
(29)
11,553
(32.5)
67
(5.44)
27
(2.12)
62
(3.73)
89
(3.67)
234
(8.65)
Wastewateruntreated
(mld)(percent)
4251
(61)
9660
(79.5)
12,625
(76)
16,871
(71)
24,004
(67.5)
1160
(94.56)
1252
(97.88)
1588
(96.27)
2339
(96.33)
2463
(91.35)
The conventional wastewater treatment processes are expensive and require
complex operations and maintenance. It is estimated that the total cost for establishing
treatment system for the entire domestic wastewater is around Rs. 7,560 crores
(CPCB, 2007), which is about 10 times the amount which the Indian government
plans to spend. The sludge removal, treatment and handling have been observed to be
the most neglected areas in the operation of the sewage treatment plants (STPs) in
India. Due to improper design, poor maintenance, frequent electricity break downs
and lack of technical man power, the facilities constructed to treat wastewater do not
function properly and remain closed most of the time (CPCB, 2007a). Utilization of
biogas generated from UASB reactors or sludge digesters is also not adequate in most
of the cases. In some cases the gas generated is being flared and not being utilized.
One of the major problems with waste water treatment methods is that none of the
available technologies has a direct economic return. Due to no economic return, local
authorities are generally not interested in taking up waste water treatment. A
performance evaluation of STPs carried out by CPCB in selected cities has indicated
that out of 84 STPs studied, the overall performance of 45 STPs has been found to be
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Hydrodynamic Cavitation based degradation of Bio-Refractory pollutants Page 5
poor or very poor. Out of 84, performance of only 8 STPs has been rated good, while
that of 30 of these have been rated satisfactory. 45 STPs had not met prescribed
standards in respect to BOD thereby making these waters unsuitable for household
purpose. As a result, though the waste water treatment capacity in the country has
increased by about 5 times since 1978-79 yet hardly 10% of the sewage generated is
treated effectively, while the rest finds its way into the natural ecosystems and is
responsible for large-scale pollution of rivers and ground waters. Also, the industrial
effluent coming out from chemical process industries such as textile, pharmaceutical,
fertilizers and petrochemical industries contains high level of hazardous chemicals
which are hard to degrade by conventional processes, further reduce the efficiency of
wastewater treatment plants. These pollutants require high degree of treatment, which
further increase the treatment cost and load on conventional process.
1.5 Wastewater Treatment Technologies
Wastewater Treatment Plant is a facility designed to receive the waste from
domestic, commercial, and industrial sources and to remove materials that damage
water quality and compromise public health and safety when discharged into water
receiving systems. The principal objective of wastewater treatment is generally to
allow domestic and industrial effluents to be disposed off without danger to human
health or unacceptable damage to the natural environment.
Methods of treatment in which the application of physical forces predominate
are known as unit operations. Method of treatment in which the removal of
contaminants is brought about by chemical or biological reactions are known as a unit
processes. At the present time, unit operations and processes are grouped together to
provide various levels of treatment known as preliminary, primary, advanced primary,
secondary (without or with nutrient removal) and advanced (or tertiary) treatment (see
Table 1.2). In preliminary treatment, gross solids such as large objective, rags and grit
are removed that may damage equipment. In primary treatment, a physical operation,
usually sedimentation is used to remove the floating and settleable materials found in
wastewater. For advanced primary treatment, chemicals are added to enhance the
removal of suspended solids and to a lesser extent dissolved solids. In secondary
treatment, biological and chemical processes are used to remove most of the organic
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Hydrodynamic Cavitation based degradation of Bio-Refractory pollutants Page 6
matter. In advanced treatment, additional combination of unit operations and
processes are used to remove residual suspended solids and other constituents that are
not reduced significantly by conventional secondary treatment (Metcalf & Eddy,
4thed.). Figure 1.1 shows a schematic flow diagram of wastewater treatment plant.
Table 1.2: Level of wastewater treatment
Treatment level Description
Preliminary
Removal of wastewater constituents such
as rags, sticks, floatables, grit, and grease
that may cause maintenance or
operational problems with the treatment
operations, processes, and ancillary
systems.
Primary
Removal of a portion of the suspended
solids and organic matter from the
wastewater.
Advanced primary
Enhanced removal of suspended solids
and organic matter from the wastewater.
Typically accomplished by chemical
addition or filtration.
Secondary
Removal of biodegradable organic matter
(in solution or suspension) and suspended
solids. Disinfection is also typically
included in the definition of conventional
secondary treatment.
Secondary with nutrient removal
Removal of biodegradable organics,
suspended solids, and nutrients (nitrogen,
phosphorus, or both nitrogen and
phosphorus).
Tertiary Removal of residual suspended solids
(after secondary treatment), usually by
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Hydrodynamic Cavitation based degradation of Bio-Refractory pollutants Page 7
granular medium filtration or micro
screens. Disinfection is also typically a
part of tertiary treatment. Nutrient
removal is often included in this
definition.
Advanced
Removal of dissolved and suspended
materials remaining after normal
biological treatment when required for
various water reuse applications.
Figure 1.1: Wastewater treatment plant flow diagram
The important constituents of concern in wastewater treatment are listed in
Table 1.3. Secondary treatment standards for wastewater are concerned with the
removal of biodegradable organics, total suspended solids, and pathogens. Many of
the more stringent standards that have been developed recently deal with the removal
influent
Screens and comminution
Grit removal
Primary settling
Biological process
Recycled biocides
Thickener return flow
Thickened biocides
Waste biosolidsthickening
Waste biocides
Secondary settling
Effluent filtration
Waste backwash water storage
Waste backwash water
off -line flow equalization (used to dampen peak flows)
To solids and biosolidsprocessing facilities
Chlorine
Chlorine mixing Chlorine
contact basin
Advanced treatment
Water reuse
Effluent for discharge
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Hydrodynamic Cavitation based degradation of Bio-Refractory pollutants Page 8
of nutrients, heavy metal, and priority pollutants. When wastewater is to be reused,
standards normally include additional requirement for the removal of refractory
organics, heavy metals, and in some case, dissolved inorganic solid. Technologies that
are suitable for water reuse applications include membrane (pressure driven,
electrically driven, and membrane bioreactors), carbon adsorption, advanced
oxidation, ion exchange, and air stripping.
Table 1.3: Principal constituents concerned in wastewater treatment
Constituent Need for Treatment
Suspended solids
Suspended solids can lead to the
development of sludge deposits and
anaerobic conditions when untreated
wastewater is discharged in the aquatic
environment.
Biodegradable organics
Composed principally of proteins,
carbohydrates, and fats, biodegradable
organics are measured most commonly in
terms of BOD (biochemical oxygen
demand) and COD (chemical oxygen
demand). If discharged untreated to the
environment, their biological stabilization
can lead to the depletion of natural
oxygen resources and to the development
of septic conditions.
Pathogens
Communicable diseases can be
transmitted by the pathogenic organisms
that may be present in wastewater.
Nutrients
Both nitrogen and phosphorus, along with
carbon, are essential nutrients for growth.
When discharged to the aquatic
environment, these nutrients can lead to
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Hydrodynamic Cavitation based degradation of Bio-Refractory pollutants Page 9
the growth of undesirable aquatic life.
When discharged in excessive amounts
on land, they can also lead to the
pollution of groundwater.
Priority pollutants
Organic and inorganic compounds
selected on the basis of their known or
suspected carcinogenicity, mutagenicity,
teratogenicity, or high acute toxicity.
Many of these compounds are found in
wastewater.
Refractory organics
These organics lead to resist conventional
methods of wastewater treatment. Typical
examples include surfactants, phenols,
and agricultural pesticides.
Heavy metals
Heavy metals are usually added to
wastewater from commercial and
industrial activities and may have to be
removed if the wastewater is to be reused.
Dissolved inorganics
Inorganic constituents such as calcium,
sodium, and sulfate are added to the
original domestic water supply as a result
of water use and may have to be removed
if the wastewater is to be reused.
Wastewater treatment is a series of steps. Each of the steps can be
accomplished using one or more treatment processes or types of equipment. The
major categories of treatment steps are described below.
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1.5.1 Preliminary and primary Treatment
The objective of preliminary treatment is the removal of coarse solids and
other large materials often found in raw wastewater. Removal of these materials is
necessary to enhance the operation and maintenance (O&M) of subsequent treatment
units. Preliminary treatment operations typically include coarse screening, grit
removal, and in some cases, communication of large objects.
The purpose of primary treatment (primary sedimentation or primary
clarification) is to remove settleable organic and floating solids. Normally, each
primary clarification unit can be expected to remove 90 to 95% settleable solids, 40 to
60% TSS, and 25 to 35% BOD. Primary treatment reduces the organic loading on
downstream treatment processes by removing a large amount of settleable, suspended,
and floating materials.
The unit operations most commonly used in the preliminary and primary
stages of wastewater treatment include (I) screening, (2)coarse solids reduction
(comminution, maceration, and screenings grinding), (3) flow equalization, (4)
mixing and flocculation, (5) grit removal, (6) sedimentation, (7) high-rate
clarification, (8) accelerated gravity separation (vortex separators), (9) flotation, (10)
oxygen transfer,(11) aeration, and (12) volatilization and stripping of volatile organic
compound (VOCs) (Metcalf & Eddy, 4th ed.). Some of the most widely used primary
treatment unit operations are described below.
Screening
The first unit operation generally encountered in wastewater-treatment plants
is screening. A screen is a device with openings, generally of uniform size, that is
used to retain solids found in the influent wastewater to the treatment plant or in
combined wastewater collection systems subject to overflows, especially from storm
water. The principal role of screening is to remove coarse materials from the flow
stream that could (1) damage subsequent process equipment, (2) reduce overall
treatment process reliability and effectiveness, or (3) contaminate waterways.
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Hydrodynamic Cavitation based degradation of Bio-Refractory pollutants Page 11
Two general types of screens, coarse screens and fine screens are used in
preliminary treatment of wastewater. Coarse screens have clear openings ranging
from 6 to150 mm (0.25 to 6 in); fine screens have clear openings less than 6 mm (0.25
in). Microscreens, which generally have screen openings smaller than 50 µm are used
principally in removing fine solids from treated effluents. The screening element may
consist of parallel bars, rods or wires, grating, wire mesh, or perforated plate, and the
openings may be of any shape but generally are circular or rectangular slots.
Grit Removal
Removal of grit from wastewater may be accomplished in grit chamber or by
the centrifugal separation of solid. Grit chambers are designed to remove grit,
consisting of sand, gravel, cinders, or other heavy solid materials that have subsiding
velocities or specific gravity substantially greater than those of the organic putrescible
solids (biodegradable organics) in wastewater. Grit chambers are most commonly
located after the bar screens and before the primary sedimentation tanks. In some
installations, grit chamber precede the screening facilities. Generally, the installation
of screening facilities ahead of the grit chambers makes the operation and
maintenance of the grit removal facilities easier.
Grit chambers are provided to (l) protect moving mechanical equipment from
abrasion and accompanying abnormal wear; (2)reduce formation of heavy deposit in
pipelines, channels, and conduit; and (3) reduce the frequency of digester cleaning
caused by excessive accumulation of grit. The removal of grit is essential ahead of
centrifuges, heat exchangers, and high-pressure diaphragm pump. There are three
general types of grit chambers: horizontal flow, of either a rectangular or a square
configuration; aerated; or vortex type.
Coagulation and Flocculation
The term "chemical coagulation" includes all of the reactions and mechanisms
involved in the chemical destabilization of particles and in the formation of larger
particles through perikinetic flocculation (aggregation of particles in the size range
from 0.0 l to1µm). In general, a coagulant is the chemical that is added to destabilize
the colloidal particles in wastewater so that floc formation can result. A flocculent is a
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chemical, typically organic, added to enhance the flocculation process. Typical
coagulant and flocculants include natural and synthetic organic polymers, metal salts
such as alum or ferric sulfate, and prehydrolized metal salts such as
polyaluminumchloride and polyiron chloride. Flocculants, especially organic
polymers, are also used to enhance the performance of granular medium filters and in
the dewatering of digested biosolids. In these applications the flocculent chemicals are
often identified as filter aids.
The term ''flocculation” is used to describe the process whereby the size of
particles increases as a result of particle collisions. There are two types of
flocculation: (1) microflocculation (also known as perikinetic flocculation), in which
particle aggregation is brought about by the random thermal motion of fluid
molecules known as Brownian motion or movement and (2) macroflocculation (also
known as orthokinetic flocculation). In which particle aggregation is brought about by
inducing velocity gradients and mixing in the fluid containing the particles to be
flocculated. Another form of macroflocculation is brought about by differential
settling in which large particles overtake small particles to form larger particles. The
purpose of flocculation is to produce particle, by means of aggregation, that can be
removed by inexpensive particle-separation procedures such as gravity sedimentation
and filtration.
Sedimentation
The objective of treatment by sedimentation is to remove readily settleable
solids and floating material and thus reduce the suspended solids load. Primary
sedimentation is used as a preliminary step in the further processing of the
wastewater. Normally, sedimentation is used after the mixing of coagulants and
flocculants to enhance the efficiency of sedimentation process. In many cases,
especially for industrial wastewater a combination of flocculator-clarifiers is often
used especially in cases where enhanced settling, such as for industrial wastewater
treatment or for biosolids concentration is required. Inorganic chemicals or polymers
can be added to improve flocculation. Circular clarifiers are ideally suited for
incorporation of an inner, cylindrical flocculation compartment. Wastewater enters
through a center shaft or well and flows into the flocculation compartment, which is
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Hydrodynamic Cavitation based degradation of Bio-Refractory pollutants Page 13
generally equipped with a paddle type or low speed mixer. The gentle stirring causes
flocculent particles to form. From the flocculation compartment, flow then enters the
clarification zone by passing down and radially outward. Settled solids and scum are
collected in the same way as in a conventional clarifier. Efficiently designed and
operated primary sedimentation tanks should remove from 50 to 70 percent of the
suspended solids and from 25 to 40 percent of the BOD. The purpose of
sedimentation is to remove a substantial portion of the organic solids that otherwise
would be discharged directly to the receiving waters.
Flotation
Flotation is a unit operation used to separate solid or liquid particles from a
liquid phase. Separation is brought about by introducing fine gas (usually air) bubbles
into the liquid phase. The bubbles attached to the particulate matter, and the buoyant
force of the combined particle and gas bubbles is great enough to cause the particle to
rise to the surface. Particles that have a higher density than the liquid can thus be
made to rise. The rising of particles with lower density than the liquid can also be
facilitated (e.g., oil suspension in water).
In wastewater treatment, flotation is used principally to remove suspended
matter and to concentrate biosolids. The principal advantages of flotation over
sedimentation are that, very small or high particles that settle slowly can be removed
more completely and in a shorter time. Once the particles have been floated to the
surface, they can be collected by skimming operation.
1.5.2 Secondary Treatment
The objective of secondary treatment is the further treatment of the effluent
from primary treatment to remove the residual organics and suspended solids. In most
cases, secondary treatment follows primary treatment and involves the removal of
biodegradable dissolved and colloidal organic matter using biological treatment
processes.
The overall objective of biological treatment of industrial and domestic
wastewater are to (1) transform (i.e., oxidize) dissolved and particulate biodegradable
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Hydrodynamic Cavitation based degradation of Bio-Refractory pollutants Page 14
constituents into acceptable end products, (2) capture and incorporate suspended and
non-settleable colloidal solid into a biological floc or biofilm, (3) Transform or
remove nutrients such as nitrogen and phosphorus, and (4) To remove or reduce the
concentration of organic and inorganic compounds. Because some of the constituents
and compounds found in industrial wastewater are toxic to microorganisms,
pretreatment may be required before the industrial wastewater can be discharged to a
municipal collection system. The removal of dissolved and particulate carbonaceous
BOD and the stabilization of organic matter found in the wastewater is accomplished
biologically using a variety of microorganisms, principally bacteria. Microorganisms
are used to oxidize (i.e., convert) the dissolved and particulate carbonaceous organic
matter into simple endproducts and additional biomass.
Depending on the metabolic function of microorganisms, biological processes
can be classified as aerobic processes, anaerobic processes, anoxic processes,
facultative processes, and combined processes. Further biological treatment processes
can be classified based on their treatment processes such as suspended growth
processes, attached growth processes, and combination thereof. Table 1.4 shows the
different type of biological treatment processes.
Table 1.4: Types of biological treatment processes
Term Definition
Metabolic process
Aerobic processes Biological treatment processes that
occur in the presence of oxygen
Anaerobic processes Biological treatment processes that
occur in the absence of oxygen
Anoxic processes
The process by which nitrate nitrogen is
converted biologically to nitrogen gas in
the absence of oxygen. This process is
also known as denitrification
Facultative processes Biological treatment processes in which
the organisms can function in the
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Hydrodynamic Cavitation based degradation of Bio-Refractory pollutants Page 15
presence or absence of molecular
oxygen
Combined aerobic/anoxic/anaerobic
processes
Various combinations of aerobic,
anoxic, and anaerobic processes grouped
together to achieve a specific treatment
objective
Treatment processes
Suspended-growth processes
Biological treatment processes in which
the microorganisms responsible for the
conversion of the organic matter or other
constituents in the wastewater to gases
and cell tissue are maintained in
suspension within the liquid
Attached-growth processes
Biological treatment processes in which
the microorganisms responsible for the
conversion of the organic matter or other
constituents in the wastewater to gases
and cell tissue are attached to some inert
medium, such as rocks, slag, or specially
designed ceramic or plastic materials.
Attached growth treatment processes
are also known as fixed-film processes
Combined processes Term used to describe combined
processes (e.g., combined suspended and
attached growth processes
Lagoon processes
A generic term applied to treatment
processes that take place in ponds or
lagoons with various aspect ratios and
depths
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Aerobic oxidation
The removal of BOD can be accomplished in a number of aerobic suspended
growths or attached (fixed film) growth treatment processes. Both require sufficient
contact time between the wastewater and heterotrophic microorganisms, and
sufficient oxygen and nutrients. During the initial biological uptake of the organic
material, more than half of it is oxidized and the remainder is as assimilated as new
biomass, which may be further oxidized by endogenous respiration. For both
suspended and attached growth processes. The excess biomass produced each day is
removed and processed to maintain proper operation and performance. The biomass is
separated from the treated effluent by gravity separation and more recent designs
using membrane separation are finding applications.
In aerobic oxidation, the conversion of organic matter is carried out by mixed
bacterial cultures in general accordance with the stoichiometry shown below (Metcalf
& Eddy, 4th ed.).
Oxidation and synthesis:
Endogenous respiration:
COHNS is used to represent the organic matter in wastewater, which serves as the
electron donor while the oxygen serves as the electron acceptor.
Table 1.5 shows the major aerobic treatment processes used for wastewater
treatment. The aerobic process is uneconomical for wastes which are highly
concentrated (high COD or BOD and low BOD/COD ratio). However, aerobic
process is more versatile, less sensitive to load fluctuations and has its importance as a
pre or post treatment in combination with other processes.
COHNS + O2 + Nutrients CO2 + NH3 + C5H7NO2 + Other end productsBiomass
New cellsCells
C5H7NO2 + 5O2 5CO2 + 2H2O + NH3 + Energy Cells
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Hydrodynamic Cavitation based degradation of Bio-Refractory pollutants Page 17
Table 1.5: Aerobic treatment processes used for waste water treatment process
Type Treatment process Use
Suspended growth
Activated sludge process Carbonaceous BOD removal,
nitrification
Aerated lagoons Carbonaceous BOD removal,
nitrification
Aerobic digestion Stabilization, Carbonaceous
BOD removal
Attached growth
Trickling filters Carbonaceous BOD removal,
nitrification
Rotating biological
contactors
Carbonaceous BOD removal,
nitrification
Packed bed reactors Carbonaceous BOD removal,
nitrification
Hybrid (combined)
suspended and attached
growth process
Trickling filter/ activated
sludge
Carbonaceous BOD removal,
nitrification
Anaerobic processes
Anaerobic fermentation and oxidation processes are used primarily for the
treatment of waste sludge and high strength organic waste. Anaerobic fermentation
processes are advantageous because of the lower biomass yields and because energy
in the form of methane, can be recovered from the biological conversion of organic
substrates. For treating high strength industrial wastewaters, anaerobic treatment has
been shown to provide a very cost effective alternate to aerobic processes with
savings in energy and nutrient addition. Because the effluent quality is not as good as
that obtained with aerobic treatment, anaerobic treatments commonly used as a
pretreatment prior to discharge to a municipal collection system or is followed by an
aerobic process. Table 1.6 shows the major anaerobic treatment processes used for
wastewater treatment (Metcalf & Eddy, 4th ed.).
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Hydrodynamic Cavitation based degradation of Bio-Refractory pollutants Page 18
Process description
The microbial action during anaerobic treatment involves action of a
consortium of anaerobic bacteria upon a solid or semi solid substrate in a carrying
medium containing the nutrients. The substrate or the macromolecules present in the
spent wash amenable to anaerobic digestion are cellulose, carbohydrates, sugars,
proteins and lipids. Three basic steps are involved in the overall anaerobic oxidation
of a waste: (1) hydrolysis, (2) fermentation (also known as acidogenesis), and (3)
methanogenesis.
Hydrolysis: The first step for most fermentation processes, in which particulate
material is converted to soluble compounds that can then be hydrolyzed further to
simple monomers that are used by bacteria that perform fermentation, is termed
hydrolysis. For some industrial wastewater, fermentation maybe the first step in the
anaerobic process.
Fermentation: The second step is fermentation (also referred to as acidogenesis). In
the fermentation process amino acids, sugars, and some fatty acids are degraded
further. Organic substrates serve as both the electron donors and acceptors. The
principal products of fermentation are acetate, hydrogen, CO2, and propionate and
butyrate (butyric acid). The propionate and butyrate are fermented further to also
produce hydrogen, CO2 and acetate. Thus, the final products of fermentation (acetate,
hydrogen, and CO2) are the precursors of methane formation (methanogenesis).
Methanogenesis: The third step, methanogenesis, is carried out by a group of
organisms known collectively as methanogens. Two groups of methanogenic
organisms are involved in methane production. One group, termed acetoclastic
methanogens split acetate into methane and carbon dioxide. The second group, termed
hydrogen-utilizing methanogens, uses hydrogen as the electron donor andCO2as the
electron acceptor to produce methane. Bacteria within anaerobic processes, termed
acetogens are also able to use CO2 to oxidize hydrogen and form acetic acid.
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Hydrodynamic Cavitation based degradation of Bio-Refractory pollutants Page 19
Table 1.6: Anaerobic treatment processes used for wastewater treatment Type Treatment process Use
Suspended growth
Anaerobic contact process Carbonaceous BOD removal,
nitrification
Anaerobic digestion
Carbonaceous BOD removal,
Stabilization, solids destruction,
pathogen kill
Attached growth Anaerobic packed and
fluidized bed
Carbonaceous BOD removal,
Stabilization, denitrification
Sludge blanket Upflow anaerobic sludge
blanket (UASB)
Carbonaceous BOD removal,
especially high strength wastes
Hybrid Upflow-sludge
blanket/attached growth
Carbonaceous BOD removal
1.5.3 Tertiary Treatment
Tertiary wastewater treatment also termed as advanced treatment processes is
employed when specific wastewater constituents which cannot be removed by
secondary treatment must be removed.
Advanced wastewater treatment is defined as the additional treatment needed
to remove suspended colloidal and dissolved constituents remaining after
conventional secondary treatment. Dissolved constituents may range from relatively
simple inorganic ions, such as calcium, potassium, sulfate, nitrate and phosphate to an
ever-increasing number of highly complex synthetic organic compounds. In recent
years, the effects of many of these substances on the environment have become
understood more clearly. Research is ongoing to determine (1) the environmental
effects of potential toxic and biologically active substances found in wastewater and
(2) how these substances can be removed by both conventional and advanced
wastewater treatment processes. As a result, wastewater treatment requirements are
becoming more stringent in terms of both limiting concentration of many of these
substances in the treated plant effluent and establishing whole effluent toxicity limits
under dischargeable standards. To meet these new requirements, many of the existing
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Hydrodynamic Cavitation based degradation of Bio-Refractory pollutants Page 20
secondary treatment facilities will have to be retrofitted and new advanced wastewater
treatment facilities will have to be developed. Some of the advanced treatment
processes used to remove some of the specific dissolved organics includes (1) carbon
adsorption, (2) membrane filtration, (3) reverse osmosis (4) chemical precipitation,
(5) chemical oxidation, (6) advanced chemical oxidation, (7) electrodialysis, and (8)
distillation.
Membrane filtration
Filtration, involves the separation (removal) of particulate and colloidal matter
from a liquid. In membrane filtration the range of particle sizes is extended to include
dissolved constituents (typically 0.0001 to 1.0 µm). The role of the membrane is to
serve as a selective barrier that will allow the passage of certain constituents and will
retain other constituents found in the liquid. Membrane filtration are used for the
removal of TSS, turbidity, bacteria and viruses, colloids, macromolecule (0.005 to 0.2
µm), small and very small molecules( 0.005 to 0.0001 µm), hardness, and ionic
solutes present in water (sulfate, nitrate, sodium and other ions).
Membrane processes include microfiltration (MF), ultrafiltration (UF),
nanofiltrtion (NF), reverse osmosis (RO), dialysis, and electrodialysis (ED).
Membrane processes can be classified in a number of different ways including (l) the
type of material from which the membrane is made (2) the nature of the driving force,
(3) the separation mechanism, and (4) the nominal size of the separation achieved.
The distinguishing characteristic of the first four membrane processes (MF,
UF, NF, and RO) is the application of hydraulic pressure to bring about the desired
separation. Dialysis involves the transport of constituents through a semipermeable
membrane on the basis of concentration differences. Electrodialysis involves the use
of an electromotive force and ion selective membranes to accomplish the separation
of charged ionic species. The separation of particles in MF and UF is accomplished
primarily by straining (sieving). In NF and RO, small particles arc rejected by the
water layer adsorbed on the surface of the membrane which is known as a dense
membrane. Ionic species are transported across the membrane by diffusion through
the pores of the macromolecule comprising the membrane. Typically NF can be used
to reject constituents as small as 0.001 µm whereas RO can reject particles as small as
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Hydrodynamic Cavitation based degradation of Bio-Refractory pollutants Page 21
0.0001 µm. Straining is also important in NF membranes, especially at the larger pore
size openings.
Adsorption
Adsorption is the process of accumulating substances that are in solution on a
suitable interface. Adsorption is a mass transfer operation in that a constituent in the
liquid phase is transferred to the solid phase. The adsorption process has not been
used extensively in wastewater treatment, but demands for a better quality of treated
wastewater effluent, including toxicity reduction, have led to an intensive examination
and use of the process of adsorption on activated carbon. Activated carbon treatment
of wastewater is usually thought of as a polishing process for water that has already
received normal biological treatment. The carbon in this case is used to remove a
portion of the remaining dissolved organic matter. The adsorption process, takes place
in four more or less definable steps: (l) bulk solution transport, (2) film diffusion
transport, (3) pore transport, and (4) adsorption (or sorption).
The principal types of adsorbents include activated carbon, synthetic
polymeric, and silica based adsorbents, although synthetic polymeric and silica based
adsorbents are seldom used for wastewater treatment because of their high cost,
activated carbon is most widely used adsorbent for the wastewater and water
purification unit. Activated carbon based wastewater treatment can be effectively used
for the removal of various organic pollutants including, aromatic solvents (benzene,
toluene, nitrobenzene), chlorinated aromatics (PCBs, chlorophenol), polynuclear
aromatics (acenaphthene, benzopyrenes), pesticides and herbicides (DDT, aldrin,
chlordane, atrazine), chlorinated nonaromatics (carbon tetrachloride, trichloroethane,
chloroform), high molecular weight hydrocarbons (dyes, gasoline, amines) which are
hard to degrade by conventional biological processes. The major problem with the
activated carbon based treatment process is the regeneration and reactivation of the
carbon after its adsorptive capacity has been reached, which makes this process
costly. More effective methods for the regeneration and reactivation of carbon need to
be developed for the economical application of this process.
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Hydrodynamic Cavitation based degradation of Bio-Refractory pollutants Page 22
Ion exchange
Ion exchange is a unit process in which ions of a given species are displaced
from an insoluble exchange material by ions of a different species in solution. The
most widespread use of this process is in domestic water softening, where sodium
ions from a cationic exchange resin replace the calcium and magnesium ions in the
treated water, thus reducing the hardness. Ion exchange has been used in wastewater
applications for the removal of nitrogen, heavy metals and total dissolved solids.
Ion exchange processes can be operated in a batch or continuous mode. In a
batch process, the resin is stirred with the water to be treated in a reactor until the
reaction is complete. The spent resin is removed by settling and subsequently is
regenerated and reused. In a continuous process, the exchange material is placed in a
bed or a packed column, and the water to be treated is passed through it. Continuous
ion exchangers are usually of the down flow, packed bed column type. Wastewater
enters the top of the column under pressure, passes downward through the resin bed,
and is removed at the bottom. When the resin capacity is exhausted, the column is
backwashed to remove trapped solids and is then regenerated.
Naturally occurring ion-exchange materials, known as zeolites, are used for
water softening and ammonium ion removal. Zeolites used for water softening are
complex aluminosilicates with sodium as the mobile ion. Ammonium exchange is
accomplished using a naturally occurring zeolite clinoptilolite. Synthetic
aluminosilicates are manufactured but most synthetic ion exchange materials are
resins or phenolic polymers. Five types of synthetic ion-exchange resins are in use:
(1) strong-acid cation, (2) Weak-acid cation, (3) strong-base anion, (4) weak- base
anion, and (5) heavy-metal selective chelating resins. Most synthetic ion-exchange
resins are manufactured by a process in which styrene and divinylbenzene are
copolymerized. The styrene serves as the basic matrix of the resin, and
divinylbenzene is used to crosslink the polymers to produce an insoluble tough resin.
Important properties of ion-exchange resins include exchange capacity, particle size,
and stability. The exchange capacity of a resin is defined as the quantity of an
exchangeable ion that can be taken up.
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Hydrodynamic Cavitation based degradation of Bio-Refractory pollutants Page 23
1.6 Need for the development of advanced treatment processes
Though, all of the above mentioned advanced treatment methods are effective
in reducing the pollutant concentration to a desired level (reusable water for some
application), but these methods are not much cost effective owing to their high
fabrication and maintenance cost. Most of these advanced treatment processes (such
as membrane separation, adsorption and ion exchange) are separative processes,
which separate pollutant molecule from the wastewater stream and these processes do
not degrade (mineralize) pollutant molecule into the end products (CO2, H2O, etc.).
Therefore in these processes, further separation and disposal of pollutant molecule is
required which is again a rigorous processes and also regeneration and reactivation of
separation media is required such as regeneration and reactivation of activated carbon
and ion exchange resins are required for their reuse. These processes are mainly used
after the secondary treatment for getting the desired reduction in pollutant level and
do not serve as a pretreatment option for the conventional biological treatment
processes for their efficiency improvement. Such limitation makes these processes
very costly and therefore there is a need for the development of new techniques which
can overcome these limitations. In the last two decades lot of research work has been
carried out for the development of new technologies especially in the area of
advanced oxidation technologies for the degradation of complex biorefractory
pollutants.
1.7 Advanced oxidation processes (AOPs)
Advanced oxidation processes (AOPs) arc used to oxidize complex organic
constituents found in wastewaters that are difficult to degrade biologically into
simpler end products. When chemical oxidation is used, it may not be necessary to
oxidize completely a given compound or group of compounds. In many cases, partial
oxidation is sufficient to render specific compounds more amenable to subsequent
biological treatment or to reduce their toxicity. The oxidation of specific compounds
may be characterized by the extent of degradation of the final oxidation products as
follows:
1. Primary degradation: A structural change in the parent compound.
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Hydrodynamic Cavitation based degradation of Bio-Refractory pollutants Page 24
2. Acceptable degradation (defusing): A structural change in the parent
compound to the extent that toxicity is reduced.
3. Ultimate degradation (mineralization): Conversion of organic carbon to
inorganic CO2.
4. Unacceptable degradation (fusing): A structural change in the parent
compound resulting in increased toxicity.
Theory of Advanced Oxidation
Advanced oxidation processes typically involve the generation and use of the
hydroxyl free radical (OH•) as a strong oxidant to destroy compounds that cannot be
oxidized by conventional oxidants such as oxygen, ozone, and chlorine. The relative
oxidizing power of the hydroxyl radical along with other common oxidant is
summarized in Table 1.7 (Metcalf & Eddy, 4th ed.). As shown, with the exception of
fluorine, the hydroxyl radical is one of the most active oxidants known. The hydroxyl
radical reacts with the dissolved constituents, initiating a series of oxidation reactions
until the constituents are completely mineralized. Nonselective in their mode of attack
and able to operate at normal temperature and pressures, hydroxyl radicals are capable
of oxidizing almost all reduced materials present without restriction to specific classes
or groups of compounds as compared to other oxidants.
Table 1.7: Comparison of oxidizing potential of various oxidizing agents
Oxidizing agent Electrochemical oxidation
potential (EOP), V
Fluorine 3.06
Hydroxyl radical 2.80
Oxygen (atomic) 2.42
Ozone 2.08
Hydrogen peroxide 1.78
Hypochlorite 1.49
Chlorine 1.36
Chlorine dioxide 1.27
Oxygen (molecular) 1.23
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Hydrodynamic Cavitation based degradation of Bio-Refractory pollutants Page 25
Advanced oxidation processes differ from the other treatment processes discussed
(such as ion exchange, membrane filtration and adsorption) because wastewater
compounds are degraded rather than concentrated or transferred into a different phase.
Because secondary waste materials are not generated, there is no need to dispose of or
regenerate materials.
In the last two decades different AOPs are developed and tested for the
degradation of different water pollutants (inorganic and organic compounds), these
processes includes cavitation (generated either by means of ultrasonic irradiation or
using constrictions such as valves, orifice, venturi, etc. in the hydraulic devices)
(Adewuyi, 2001; Weavers et al., 1998; Hua and Hoffmann, 1997; Gogate and Pandit,
2000; Senthilkumar et al., 2000; Sivakumar and Pandit, 2002; Vichare et al., 2000;
Pang et al., 2011; Braeutigm et al., 2012), photocatalytic oxidation (using ultraviolet
radiation/near UV light/ Sun light in the presence of semiconductor catalyst) (Choi et
al., 2000; Adewuyi, 2005; Konstantinou and Albanis, 2004; Cao et al., 2006; Zabar et
al., 2012; Yang et al., 2009; Lin et al., 2012) and Fenton chemistry (using reaction
between Fe ions and hydrogen peroxide, i.e. Fenton’s reagent) (Kusic et al., 2006;
Bigda, 1996; Pera-Titus et al., 2004; Ma et al., 2005; Xue et al., 2009; Santos et al.,
2011; Karci et al., 2012). These AOPs are also used in combinations named as hybrid
methods such as sonochemical, sonophotocatalytic, photolytic, Photo-Fenton,
photochemical, etc. to get the enhanced oxidation efficiency (Cheng et al., 2012;
Wang et al., 2011; Pera-Titus et al., 2004).
1.7.1 Cavitation
Cavitation is defined as the phenomena of the formation, growth and
subsequent collapse of micro bubbles or cavities occurring in extremely small interval
of time (milliseconds) and at multiple lactations in the reactor, releasing large
magnitudes of energy. The effects of cavity collapse are, creation of hot spots,
releasing highly reactive free radicals, cleaning of solid surfaces, and enhancement in
mass transfer rates. The collapse of bubbles generates localized ‘‘hot spots’’ with
transient temperature of about 10,000 K, pressures of about 2000 atm (Didenko et al.,
1999). The consequences of these extreme conditions are the cleavage of water
molecules (into H• atoms and OH• radicals) and dissolved oxygen molecules. From
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Hydrodynamic Cavitation based degradation of Bio-Refractory pollutants Page 26
the reactions of these entities (O•, H•, OH•) with each other and with H2O and O2
during the rapid cooling phase, HO•2 radicals and H2O2 are formed. These radicals
(OH•, O• and HOO•) then diffuse into the bulk liquid medium where they react with
organic pollutants and oxidize them. The following are the possible reactions
occurring as the result of cavity collapse.
H20 + ))) HO• + H•
O2+ ))) 2O•
O• + H2O 2HO•
HO• + H• H2O
2HO• O• + H2O
H• +O2 HOO•
2HO• H2O2
2HOO• H2O2 + O2
The two main mechanisms for the destruction of organic pollutants using
cavitation are the thermal decomposition/pyrolysis of the volatile pollutant molecule
entrapped inside the cavity and secondly, the reaction of OH radicals with the
pollutants.
Cavitation is classified into four types based on the mode of generation viz.
Acoustic, Hydrodynamic, Optic and Particle, but only acoustic and hydrodynamic
cavitation have been found to be efficient in bringing about the desired chemical
changes whereas optic and particle cavitation are typically used for single bubble
cavitation, which fails to induce chemical change in the bulk solution.
Acoustic cavitation
In the case of acoustic cavitation, cavitation is produced using the high
frequency sound waves, usually ultrasound, with frequencies in the range of 16 kHz–
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Hydrodynamic Cavitation based degradation of Bio-Refractory pollutants Page 27
100 MHz. Alternate compression and rarefaction cycles of the sound waves results in
various phases of cavitation such as generation of the bubble/cavity, growth phase and
finally collapse. Acoustic cavitations (tiny micro bubbles) are created when it reaches
rarefaction cycle where a negative acoustic pressure is sufficiently large to pull the
water molecules from each other (the critical molecular distance, R for water
molecules is 10-8 cm) (Lorimer and Mason, 1987). As a result, ‘voids’ are created in
the liquid. On the other hand, the acoustic pressure is positive during compression
cycle of ultrasonic wave to push molecules together. Cavitation bubbles will grow
over a few cycles by entrapping most of the vapor from the medium to reach a critical
size before the implosion of the bubbles occurs during compression cycle producing
high local temperature (up to 10000K) and pressure (up to 2000atm).
Different types of acoustic cavitational reactors are being used including
ultrasonic horn, ultrasonic bath and multiple frequency cells. Since 1990, there has
been an increasing interest in the use of ultrasound to destroy organic contaminants
present in wastewater (Weavers et al., 1998; Nagata et al., 2000; Fındık and Gündüz,
2007; Hamdaoui and Naffrechoux, 2008; Wang et al., 2007; Francony and Petrier,
1996; Wang et al., 2006; Golash and Gogate, 2012; Bagal and Gogate, 2012). Many
researchers have reported that ultrasonic irradiation process was capable of degrading
various recalcitrant organic compounds such as phenol compounds, chloroaromatic
compounds, aqueous carbon tetrachloride, pesticides, herbicides, benzene compounds,
polycyclic aromatic hydrocarbons and organic dyes. The frequency of ultrasound,
irradiating surface, intensity of sound wave, calorimetric efficiency of ultrasonic
equipment (power dissipated into the system per unit power supplied),
physicochemical properties of the liquid medium and the presence of air and solid
particles are the important parameters which affects the cavitational efficiency of
acoustic cavitational reactor.
Hydrodynamic cavitation
One of the alternative techniques for the generation of cavitation is the use of
hydraulic devices where cavitation is generated by the passage of the liquid through a
constriction such as valve, orifice plate, venturi etc. (Gogate and Pandit, 2000;
SenthilKumar and Pandit, 1999; Moholkar et al., 1999; Gogate and Pandit, 2005;
Chapter 1: Introduction
Hydrodynamic Cavitation based degradation of Bio-Refractory pollutants Page 28
Moholkar and Pandit, 2001). There are not many reports depicting the use of these
equipments for wastewater treatment, but these offer higher energy efficiencies, more
flexibility and higher potential for scale-up as compared to their acoustic counterparts
(Sivakumar and Pandit, 2002; SenthilKumar et al., 2000; Kalumuck and Chahine,
2000; Franke et al., 2011; Wang and Zhang, 2009; Wang et al., 2011; Braeutigm et
al., 2012). In the case of hydrodynamic cavitation (HC) the intensity of the cavity
collapses (final collapse pressure) and hence the cavitational yield is very much
dependent on the surrounding pressure field (turbulent pressure field). The intensity of
turbulence depends on the magnitude of the pressure drop and the rate of pressure
recovery, which, in turn, depends on the geometry of the constriction and the flow
conditions of the liquid, i.e., the scale of turbulence. The intensity of turbulence has a
profound effect on cavitation intensity. Thus, by controlling the geometric and
operating conditions of the reactor, the required intensity of the cavitation for the
desired physical or chemical change can be generated with maximum energy
efficiency.
1.7.2 Photocatalysis
The photo-activated chemical reactions are characterized by a free radical
mechanism initiated by the interaction of photons of a proper energy level with the
molecules of chemical species present in the solution, with or without the presence of
the catalyst. The radicals can be easily produced using UV radiation by the
homogenous photochemical degradation of oxidizing compounds like hydrogen
peroxide and ozone. An alternative way to obtain free radicals is the photocatalytic
mechanism occurring at the surface of semiconductors (like titanium dioxide) and this
indeed substantially enhances the rate of generation of free radicals and hence the
rates of degradation (Mazzarino et al., 1999). A major advantage of the photocatalytic
oxidation based processes is the possibility to effective use of sun light or near UV
light for irradiation, which should result in considerable economic savings especially
for large-scale operations.
Many researchers have carried out photocatalytic based degradation of various
recalcitrant organic compounds such as haloalkanes/haloalkenes (chloroform,
trichloroethylene, carbon tetrachloride), aliphatic alcohols (methanol, ethanol, 1-
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Hydrodynamic Cavitation based degradation of Bio-Refractory pollutants Page 29
octanol, etc.), amines (alkylamines, alkanolamines), aromatic and phenolic
compounds (toluene, phenol, chlorophenol), surfactants (polyethylene glycol, sodium
dodecyl benzene sulfonate, trimethyl phosphate), herbicides (atrazine, S-trizine,
bentazone), pesticides, dyes (methylene blue, rhodamine-B, methyl orange, etc.), and
many other organic compounds have been successfully degraded using this technique
(Cao et al., 2006; Daneshvar et al., 2003; Choi et al., 2000; Konstantinou and Albanis,
2004; Adewuyi, 2005; Peternel et al., 2007; Wu and Chang, 2006; Song et al., 2006;
Dai et al., 2008; Akpan and Hameed, 2009; Yao et al., 2010; Patil et al., 2011; Zabar
et al., 2012).
Homogeneous photocatalysis
In homogeneous photocatalysis, the reactants and the photocatalysts exist in
the same phase. The most commonly used homogeneous photocatalysts include
ozone, transition metal oxide and Photo-Fenton systems (Fe2+ and Fe2+ /H2O2). The
reactive species is the •OH which is used for oxidation of organic pollutants. The two
main homogeneous photocatalysis process used for the degradation of organic
pollutants are (a) UV/O3 process and (b) Photo-Fenton process.
(a) UV/O3 process
Combining O3 with UV improves the efficiency and gives higher
mineralization rate of organic pollutants due to direct and indirect production of
hydroxyl radicals following O3 decomposition and H2O2 formation, respectively. The
mechanism of hydroxyl radical production by ozone under the effect of UV light
follows the following path.
O3 + hν → O2 + O(1D)
O(1D) + H2O → •OH + •OH
O(1D) + H2O → H2O2
H2O2 + hν → •OH + •OH
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Hydrodynamic Cavitation based degradation of Bio-Refractory pollutants Page 30
(b) Photo-Fenton process
Similarly, the efficiency of Fenton process can be improved by using UV
light, which causes increase in the formation of hydroxyl radicals through the
dissociation of H2O2 by UV light and through the conversion of ferric ion into ferrous
ion. The photo-Fenton system produces hydroxyl radicals by the two mechanism one
by the Fenton chemistry and another through the attack of UV rays on H2O2 and
ferric ion.
Fe2+ + H2O2→ HO• + Fe3+ + OH−
Fe3+ + H2O2→ Fe2+ + HOO• + H+
Fe2+ + HO• → Fe3+ + OH−
In Photo-Fenton type processes, additional sources of OH radicals should be
considered through photolysis of H2O2, and through reduction of Fe3+ ions under UV
light:
H2O2 + hν → HO• + HO•
Fe3+ + H2O + hν → Fe2+ + HO• + H+
The efficiency of Photo-Fenton type processes is influenced by several
operating parameters like concentration of hydrogen peroxide, pH and intensity of
UV. The main advantage of this process is the ability of using sunlight with light
sensitivity up to 450 nm, thus avoiding the high costs of UV lamps and electrical
energy. These reactions have been proven more efficient than the ozonation and
Fenton chemistry but the disadvantages of the process are the low pH values which
are required, since iron precipitates at higher pH values and the fact that iron has to be
removed after treatment.
Heterogeneous photocatalysis
Heterogeneous catalysis has the catalyst in a different phase from the
reactants. Most common heterogeneous photocatalyts are semiconductors, which have
unique characteristics. Unlike the metals which have a continuum of electronic states,
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Hydrodynamic Cavitation based degradation of Bio-Refractory pollutants Page 31
semiconductors possess a void energy region where no energy levels are available to
promote recombination of an electron and hole produced by photoactivation in the
solid. The void region, which extends from the top of the filled valence band to the
bottom of the vacant conduction band, is called the band gap. When light falls on
these semiconductors, the electron present in the valence band jumps to the
conduction band, a result of which is the generation of a positive hole. The
recombination of the electron and the hole must be prevented as much as possible if a
photocatalyzed reaction is to be favored.
Due to the generation of positive holes and electrons, oxidation-reduction
reactions take place at the surface of semiconductors. The photo generated electrons
could reduce the organic molecule or react with electron acceptors such as O2
adsorbed on the catalyst surface or dissolved in water, reducing it to super oxide
radical anion O2•–. The photogenerated holes can oxidize the organic molecule to
form R+, or react with OH− or H2O oxidizing them into OH• radicals.
Oxidative reactions due to photocatalytic effect:
UV + MO → MO (h+ + e−)
Here MO stands for metal oxide
h+ + H2O → H+ + •OH
2 h+ + 2 H2O → 2 H+ + H2O2
H2O2 → HO• + •OH
The reductive reaction due to photocatalytic effect:
e− + O2 → •O2–
•O2– + H+ → HO2
•
•O2– + HO2
• + H+ → H2O2 + O2
HOOH → HO• + •OH
Chapter 1: Introduction
Hydrodynamic Cavitation based degradation of Bio-Refractory pollutants Page 32
Ultimately, the hydroxyl radicals are generated in both the reactions. These hydroxyl
radicals are very oxidative in nature and they react with organic pollutant and oxidize
them.
Organic molecule + OH•→ degradation products
Various chalcogenides (oxides such as TiO2, ZnO,ZrO2, CeO2 etc. or sulfides
such as CdS, ZnS etc.) have been used as photo-catalysts so far in different studies
reported in the literature. The surface area and the number of active sites offered by
the catalyst (thus nature of catalyst, i.e. crystalline or amorphous is important) for the
adsorption of pollutants plays an important role in deciding the overall rates of
degradation as usually the adsorption step is the rate controlling step. The important
parameters which affects the overall efficiency of photocatalytic processes includes
amount and type of catalyst, wavelength of the irradiation, intensity of the radiation,
concentration of the pollutants, medium pH, and presence of ionic species. For
efficient treatment of wastewater using photocatalytic processes requires a complete
understanding of the effect of these parameters.
There are many advantages of photocatalytic processes which make this AOP
as a useful technique for the treatment of complex biorefractory pollutants:
• Operation at conditions of room temperature and pressure.
• Use of natural resources, i.e. sunlight, which should result in considerable
economic savings
• Chemical stability of TiO2 in aqueous media over a larger range of pH (0-14).
• Low cost of titania.
• Total mineralization achieved of many organic pollutants.
However, there are some drawbacks, which hamper successful application of
photocatalytic oxidation on industrial scale operation for wastewater treatment.
• Engineering design and operation strategies are lacking for efficient use of
reactors at large-scale operation.
• Fouling of the photocatalyst with continuous use, results in lowering the rates
of degradation as time Progresses.
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Hydrodynamic Cavitation based degradation of Bio-Refractory pollutants Page 33
• For reactors with catalyst in the suspended form, ultrafine separation is an
inconvenient, time consuming and expensive process and in addition, the
depth of penetration is lower due to the blockage of the incident light by
catalyst particles (shadow effect) as well as absorption by the dissolved
organic species.
1.7.3 Fenton chemistry
Generally, Fenton process involves application of iron salts and hydrogen
peroxide to produce hydroxyl radicals. Ferrous ion is oxidized by hydrogen peroxide
to ferric ion, a hydroxyl radical and a hydroxyl anion. Ferric ion is then reduced back
to ferrous ion, peroxide radical and a proton by the same hydrogen peroxide. The
Fenton’s reaction generally occurs in acidic medium between pH 2 and 4 and involves
the following steps (Masomboon et al., 2009; Rodriguez et al., 2003; Utset et al.,
2000).
Fe2+ + H2O2 •OH + OH– + Fe3+ (1)
Fe3+ + H2O2 Fe2+ + H+ + HOO• (2)
Fe3+ + HOO• Fe2+ + H+ + O2 (3)
Fe2+ + •OH Fe3+ + OH– (4)
•OH + H2O2 H2O + HOO• (5)
Fe2+ + HOO• HOO– + Fe3+ (6)
•OH + •OH H2O2 (7)
•OH + organics products + CO2 + H2O (8)
The rate constant of reaction 1 is around 63 M-1 s-1, while the rate of reaction 2
is only 0.01-0.02 M-1 s-1 (Kang et al., 2002; Martinez et al., 2003). This indicates that
ferrous ions are consumed more rapidly than they are produced. The hydroxyl radicals
will degrade organic compounds through reaction 8 and hydrogen peroxide can also
react with Fe3+ via reaction 2.
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Hydrodynamic Cavitation based degradation of Bio-Refractory pollutants Page 34
Many researchers have studied Fenton chemistry for the oxidation of different
organic pollutants including, aromatic and phenolic compounds, pesticides,
herbicides, and organic dyes organic dyes (Bigada, 1996; Kusic et al., 2006; Pera-
Titus et al., 2004; Ma et al., 2005; Segura et al., 2013; Chu et al., 2012; Sun et al.,
2007; Sun et al., 2009; Zazo et al., 2005; Lu et al., 1999).
In the Fenton reagent driven oxidation of organic pollutant the important
parameters which needs to be consider to get the optimized results includes, ratio of
H2O2 to ferrous ion concentration, operating pH and concentration of reactant.
Though successful on laboratory scale this process finds lesser application on
industrial scale due to its ineffectiveness in reducing certain refractory pollutants such
as acetic acid, acetone, carbon tetrachloride, methylene chloride, n-paraffins, maleic
acid, malonic acid, oxalic acid, trichloro ethane etc. and high cost of chemical reagent
used in this process.
1.8 Need for the development of cost effective technology
Most of these AOPs have been tested for the wastewater treatment on the
laboratory scale and mainly for model organic component. There are lots of issues
arising before their successful implementation on the industrial scale. The economic
considerations and effectiveness of these processes in treating a real industrial effluent
on the larger scale are the major challenges to be overcome for the successful
implementation of these technologies. As most of these AOPs are tested for the
mineralization of water solution contacting only single organic pollutants, raises the
question against the capability of these processes in treating the real industrial effluent
having multiple pollutants. The real wastewater obtained from different chemical
processing industries contains a lot of compounds, both organic and inorganic. Thus,
it is important to check the interference between two or more reactants, which may
also result in the formation of variety of intermediates. The high cost of fabrication
and maintenance is another drawback of these technologies. As, in the case of
acoustic cavitation the material and fabrication cost for the ultrasonic horn and
transducers is very high, making it an uneconomical operation to be tried on industrial
scale. In the case of photo catalytic processes the engineering design and fabrication
consideration for providing uniform distribution of UV radiation throughout the
Chapter 1: Introduction
Hydrodynamic Cavitation based degradation of Bio-Refractory pollutants Page 35
reactor adds cost to the process and further the maintenance cost for catalyst
regeneration and UV lamp life makes this process even more costly. Similarly, for the
Fenton process the cost of chemical reagent (ferric chloride and H2O2) are very high
and further the separation of alum formed due to the addition of ferric chloride and
the presence of unreacted H2O2 in the discharged stream rendering it unsuitable for
subsequent biological treatment, makes it uneconomical process for large scale
operation.
There is a strong need for the development of new cost effective advanced
oxidation technologies for the treatment of biorefractory pollutants, which can be used
as an individual process or can be used in combination with the other conventional
treatment processes for the effective degradation of refractory pollutants. These can
be used as a pretreatment option for the conventional (especially biological processes)
treatment processes so that the efficiency of conventional treatment processes can be
improved, thereby reducing the operating cost of such treatment operations. Among
the above explained AOPs, hydrodynamic cavitation has an advantage over other
processes in terms of its application on industrial scale, easy scaleup and cost
effective process.
The major advantages of hydrodynamic cavitation are:
• It is one of the cheapest and most energy efficient method of generating
cavitation.
• The equipment used for generating cavitation is simple.
• Maintenance of such reactors is very low.
• The scale-up of the above process is relatively easy.
• Independent of the wastewater composition, wastewater having high COD can
be treated more effectively.
• can be used at multiple location i.e. before and after the biological treatment
process and can serve multiple application such as complete oxidation of
refractory pollutant, breakdown of complex molecule into smaller molecule
which can be further degraded by conventional processes, hence increase the
efficiency of conventional processes, and can also used for the disinfection,
thus reducing the chemical usages for disinfection.
Chapter 1: Introduction
Hydrodynamic Cavitation based degradation of Bio-Refractory pollutants Page 36
• Very less or no use of additional oxidizing agent.
1.9 Hydrodynamic cavitation
Hydrodynamic cavitation (HC) can simply be generated by the passage of the
liquid through a constriction such as an orifice plate. When the liquid passes through
the orifice, the kinetic energy/velocity of the liquid increases at the expense of the
pressure. If the throttling is sufficient to cause the pressure around the point of vena
contracta to fall below the threshold pressure for cavitation (usually vapor pressure of
the medium at the operating temperature), millions of cavities are generated.
Subsequently as the liquid jet expands, the pressure recovers and this results in the
collapse of the cavities. During the passage of the liquid through the constriction,
boundary layer separation occurs and a substantial amount of energy is lost in the
form of a permanent pressure drop. Very high intensity turbulence occurs on the
downstream side of the constriction; its intensity depends on the magnitude of the
pressure drop, which, in turn, depends on the geometry of the constriction and the
flow conditions of the liquid. The intensity of turbulence has a profound effect on the
cavitation intensity (Moholkar and Pandit, 1997). Thus, by controlling the geometric
and operating conditions of the reactor, one can produce the required intensity of the
cavitation so as to bring about the desired change with maximum efficiency. Also the
collapse temperatures and pressures generated during the cavitation phenomena are a
strong function of the operating and geometric parameters (Gogate and Pandit, 2000).
Figure 1.2 shows a typical setup to generate cavities hydrodynamically. The
pressure-velocity relationship of the flowing fluid as explained by Bernoulli’s
equation can be exploited to achieve this effect. The flowing liquid, when it passes
through a mechanical constriction, say an orifice or a partially throttled valve, venturi
or an orifice (part a in Figure 1.2), its velocity increases accompanied by increase in
kinetic energy and corresponding decrease in the local pressure (part b in Figure 1.2).
If the throttling is sufficient to reduce the absolute local pressure below the vapor
pressure (at the operating temperature), spontaneous vaporization of the medium in
the form of micro-bubbles (nucleation) occurs. With continued lowering of the
pressure, the cavity continues to grow by further vaporization or desorption of gases
(if some gas is dissolved in the medium) reaching its maximum size at the lowest
Chapter 1: Introduction
Hydrodynamic Cavitation based degradation of Bio-Refractory pollutants Page 37
pressure. Subsequent increasing (pressure recovery) of the pressure compresses this
from fully grown cavity and is made to collapse in adiabatic phase, thus generating
the kind of extreme condition of pressure and temperature.
Figure 1.2: Fluid flow & Pressure variation in hydrodynamic cavitation set-up
A dimensionless number known as cavitation number is used to relate the flow
conditions with the cavitation intensity. Cavitation number is given by the following
equation:
−
=2
2
21
o
vV
v
ppC
ρ
Where, p2 is the fully recovered downstream pressure, pv is the vapor pressure of the
liquid, vo is the velocity at the throat of the cavitating constriction.
The cavitation number at which the inception of cavitation occurs is known as
the cavitation inception number Cvi. Ideally speaking, the cavitation inception occurs
at Cvi equal to 1 and there are significant cavitational effects at Cv values of less than
Flow
Orifice plate
Vena contracta
PV
P2
P1
Distance downstream to orifice
Pres
sure
(a)
(b)
Chapter 1: Introduction
Hydrodynamic Cavitation based degradation of Bio-Refractory pollutants Page 38
1. In the earlier work by Gogate and Pandit (2000), it has been shown that the cavities
oscillate under the influence of fluctuating pressure field and the magnitudes of
pressure pulses generated are much less, insignificant to bring about a desired
chemical change for the case where Cv values are greater than 1. However, cavitation
has been found to occur at a higher cavitation number also, possibly due to the
presence of dissolved gases or some impurities in the liquid medium (Harrison and
Pandit, 1992). Yan and Thorpe (1990) have studied the effect of geometry of
cavitating device (orifice plates) on the inception of cavitation. They observed that for
a given size orifice, the cavitation inception number remains constant within an
experimental error for a specified liquid. The cavitation inception number does not
change with the liquid velocity and is a constant for a given orifice size and is found
to be increasing with an increasing size and dimension of the orifice. Moholkar and
Pandit (1997) have discussed these observations in terms of increased turbulent
fluctuating velocity magnitude and its variation with the orifice dimensions.
In the hydrodynamic cavitation, the cavitational yield (for e.g. amount of
pollutant reduced per unit energy dissipated) depends on the intensity of cavity
collapse which in turn depends on the several parameters such as number of
cavitational events present, the maximum size of the cavity reached before its collapse
and the surrounding pressure field. In hydrodynamic cavitation all these parameters
depends on the geometry of cavitational device and the operating pressure. The
important parameters which decide the efficiency and the overall cavitational yield
are:
• Inlet pressure and the cavitation number
• Physicochemical properties of liquid and initial radius of the nuclei;
• Size and shape of the throat and divergent section (in the case of venturi)
• Percentage free area offered for the flow
The effect of the various design and operating parameters mentioned above
has been studied extensively in terms of the collapse pressures on the basis of the
numerical simulations using bubble dynamics equations (Senthilkumar and Pandit,
1999; Moholkar and Pandit, 1997; Gogate and Pandit, 2000; Moholkar et al., 1999;
Moholkar and Pandit, 2001; Bashir et al., 2011) and also on the basis of experiments
Chapter 1: Introduction
Hydrodynamic Cavitation based degradation of Bio-Refractory pollutants Page 39
done in different reactors (Suslick et al., 1997; Senthilkumar et al., 2000; Vichare et
al., 2000; Sivakumar and Pandit, 2002; Pradhan and Gogate, 2010).
Vichare et al. (2000) have carried out optimization of hydrodynamic cavitation
using decomposition of potassium iodide as a model reaction. They have studied the
effect of various parameters (inlet pressure, flow geometry of orifice plates) on the
iodine liberation rate. They have concluded that in hydrodynamic cavitation, altering
flow geometry or increasing turbulence frequency (ƒT) and the fraction of the flow
area occupied by the shear layer can enhance the cavitational yield. The optimum
frequency of turbulence can be achieved by manipulating the flow conditions and
geometry of the cavitation device. for the plates having the same flow area, it is
advisable to use a plate with a smaller hole size opening, thereby increasing the
number of holes in order to achieve a larger area of the shear layer. Because, for
smaller hole sizes, the value of ƒT increases, leading to a more efficient collapse. On
the contrary, for larger hole sizes the frequency of turbulence (ƒT) is likely to be much
lower than the natural oscillation frequency of the generated cavity, resulting in a
lower collapse intensity. Also, if there is a choice on the magnitude of the flow area,
lower percentage area should be chosen, as with a decrease in flow area, the intensity
of cavitation increases. They have also stated that the rate of iodine liberation
increases with an increase in the inlet pressure. Similar observations have also been
made by the Sivakumar and Pandit (2002) in which they carried out degradation of
rhodamine-B using multiple hole orifice plates. The results of the numerical
simulation in hydrodynamic cavitation carried out by Senthilkumar and Pandit (1999)
were also consistent with the experimental observations made by Vichare et al.
(2000).
Bashir et al. (2011) have carried out optimization of the important geometrical
parameters of a cavitating venturi. They have found that the ratio of the perimeter of
the venturi to the cross sectional area of its constriction quantifies the possible
location of the inception of the cavity. The ratio of the throat length to its height (in
the case of a slit venturi) controls the maximum size of the cavity and the angle of the
divergence section controls the rate of collapse of a cavity. Based on the numerical
study, it was concluded that a slit venturi (α = 2.7) with the slit length equal to its
Chapter 1: Introduction
Hydrodynamic Cavitation based degradation of Bio-Refractory pollutants Page 40
height (1:1) and a half angle of divergence section of 5.5 degrees is an optimum
geometry for best cavitational activity.
Senthilkumar et al. (2000) have studied the effect of different operating
parameters (inlet pressure and cavitation number) on the cavitation yield using KI
degradation. They have found out that the rate of iodine liberation increases with an
increase in inlet pressure, reaches a maximum and then decreases, similarly the rate
increases with a decrease in cavitation number, reaches a maximum and then drops.
Gogate and Pandit (2000) have also found the similar observation using the bubble
dynamic simulation for the hydrodynamic cavitation devices.
All of the above studies depicted that, in hydrodynamic cavitation, the
cavitational yield (efficiency of hydrodynamic cavitation in bringing about the desired
changes physical and chemical changes) depends on the geometrical parameters as
well as on operating parameters (operating pressure and cavitation number). Till date
most of the studies were carried out using the single and multiple hole orifice plate
having circular hole only, and hence there is a huge scope in the field of the design of
different hydrodynamically cavitating devices including different types of venturi
having different size and shape such as circular and noncircular shape (rectangular,
square, elliptical, etc.), and orifice plates having throat of different shapes.
1.9.1 Applications of hydrodynamic cavitation
Ability of cavitation to deliver energy, in concentrated and desired form and
on length and time scales similar to that of transformation, makes it an attractive tool
to be utilized to bring about the transformations in an energy efficient manner. As a
result of this, cavitation is applied for several applications which utilize the primary
and secondary effects to bring about the transformations. Primary effects are the ones
which are direct result of volumetric oscillations or the collapse of cavity, while
secondary effects are those which occur as the result of primary effects. Primary
effects include extremely high pressure temperature (~ 10000 K), high pressure
(~ 2000 atm) and high velocity liquid microjets (~ 100 – 300 m/s) (Suslick et al.,
1997). It is because of these primary effects that cavitation is capable to bring about
intensification of processes. Some secondary effects which are the key benefits of
Chapter 1: Introduction
Hydrodynamic Cavitation based degradation of Bio-Refractory pollutants Page 41
cavitation include free radical generation, enhancement of mass transfer rates, and
increase in interfacial area. A few applications of cavitation are listed below.
• Waste water treatment
• Water disinfection
• Biological cell disruptions
• Hydrolysis of fatty oils
• Pulp and paper digestion
• Preparation of nano particle
• Mixing and uniform dispersion
• Chemical synthesis
1.9.2 Applications of hydrodynamic cavitation to wastewater treatment
As explained earlier, the collapse of cavities releases large magnitude of
energy with transient temperature of 10000 K and pressure of about 2000 atm. Under
these extreme conditions (high temperature and pressure) water and other dissolved
gases can dissociate into free radicals (for e.g. water molecules dissociate into H• and
OH• radicals). These hydroxyl radicals thus generated reacts with the pollutant
molecules trapped inside the cavities and also these OH• radicals diffuses into the bulk
liquid medium where they react with the pollutant molecules and oxidize them. The
other mechanism which causes destruction of organic pollutant is the thermal
pyrolysis of pollutant molecules trapped inside the cavities or present near the cavity
surface during cavity collapse.
There are not many reports indicating the applications of the hydrodynamic
cavitation reactors in wastewater treatment schemes until now. Kalumuck and
Chahine (1998) have studied the destruction of p-nitrophenol in recirculating flow
loops using a variety of cavitating jet configurations and operating conditions and
have shown that, hydrodynamic cavitation can effectively degrade p-nitrophenol.
Submerged cavitating liquid jets were found to generate a two orders of magnitude
increase in energy efficiency compared to the ultrasonic method. The ultrasonic
destruction was studied in an ultrasonic horn.
Chapter 1: Introduction
Hydrodynamic Cavitation based degradation of Bio-Refractory pollutants Page 42
Sivakumar and Pandit (2002) have reported that hydrodynamic cavitation
having orifice plate with multiple holes can be used for the destruction of the
rhodamine B complex in an efficient way as compared to acoustic cavitation.
Acoustic cavitation was studied using an ultrasonic horn (Operating frequency: 22.7
kHz; power input: 240 W; and capacity: 50 ml) an ultrasonic bath (Operating
frequency: 22 kHz; power input: 120 W; and capacity: 0.75 l) as well as a dual
frequency flow cell. They have found that the cavitational yield (grams of rhodamine
B degraded per unit energy supplied) for the hydrodynamic cavitation set-up was
approximately two times higher as compared to the best in the sonochemical reactors
(dual frequency flow cell, operating frequency: combination of 25 and 40 kHz; power
input: 240 W; and capacity: 1.5 l) Moreover, the hydrodynamic cavitation set-up is
able to degrade approximately 50 l of effluent under a single operation as compared to
a few milliliters in the case of the ultrasonic horn and bath and 1.5 l for the ultrasonic
flowcell.
Vichare et al. (2000) have studied the degradation of potassium iodide using
hydrodynamic cavitation. They have concluded that the intensity and number of
cavitation events can be effectively controlled by using different plates differing in
number and diameter of holes. They have found that the flow geometry of the orifice
plates considerably affects the rate of the iodine liberation. They have recommended
that for the plates having the same flow area, it is advisable to use a plate with a
smaller hole size, thereby increasing the number of holes (higher α, the ratio of total
perimeter of holes to the total area of the opening) to get the maximum cavitational
effects.
Bremner et al. (2008) have carried out mineralization of 2,4-
dichlorophenoxyacetic acid by acoustic and hydrodynamic cavitation in conjunction
with the advanced Fenton process. They have compared the efficacies of acoustic and
hydrodynamic cavitation in enhancing the degradation process. It was observed that in
20 min of treatment time(beyond this time, the increase in the TOC removal is only
marginal), the combination of acoustic cavitation and the advanced Fenton process
gives around 60% TOC removal, whereas 70% TOC removal is observed with
hydrodynamic cavitation combined with the advanced Fenton process. They have
concluded that the use of zero-valent iron and hydrogen peroxide in conjunction with
Chapter 1: Introduction
Hydrodynamic Cavitation based degradation of Bio-Refractory pollutants Page 43
acoustic or hydrodynamic cavitation is a very effective means of destroying high
concentrations of 2,4-dichlorophenoxyacetic acid. A combination of advanced Fenton
process and cavitation has been observed to intensify the degradation process by way
of turbulence and generation of additional free radicals. The results achieved using the
hydrodynamic cavitation are particularly good in that this unit operates in a
continuous mode and hence large volumes of contaminated water might be treated
very cost-effectively particularly with low levels of polluted water, at equivalent
energy dissipation levels.
Chakinala et al. (2009) have studied a combination of hydrodynamic
cavitation and heterogeneous advanced Fenton process (AFP) based on the use of zero
valent iron as the catalyst has been investigated for the treatment of real industrial
wastewater. The effect of various operating parameters such as inlet pressure,
temperature, and the presence of copper windings on the extent of mineralization as
measured by total organic carbon (TOC) content have been studied. They have
observed that increased pressures, higher operating temperature and the absence of
copper windings are more favorable for a rapid TOC mineralization. They have
concluded that higher inlet pressures result in greater cavitational activity contributing
to the enhanced hydroxyl radical generation and hence increased TOC mineralization
of the effluent. Around 60% mineralization can be achieved at 1500 psi inlet pressure
as compared to 50% at 500 psi inlet pressure. They have observed that the addition of
copper has a negative impact on the mineralization of organic pollutants present in
wastewater. About 60% of TOC was removed in the presence of iron pieces alone and
only 40% of TOC was removed with copper windings on iron pieces after 150 min of
treatment. This was explained on the basis of relative rates of hydroxyl radical
generation due to the presence of iron and copper. It is well accepted that the rate of
hydroxyl radical generation and hence the extent of TOC mineralization is much
higher in the presence of iron as compared to copper metal.
Wang and Zhang (2009) have studied the degradation of alachlor aqueous
solution by using hydrodynamic cavitation. They have found that alachlor in aqueous
solution can be effectively decomposed with swirling jet-induced cavitation. The
effects of operating parameters such as fluid pressure, solution temperature, initial
concentration of alachlor and medium pH on the degradation rates of alachlor were
Chapter 1: Introduction
Hydrodynamic Cavitation based degradation of Bio-Refractory pollutants Page 44
also discussed. The results showed that the degradation rates of alachlor increased
with increasing pressure and decreased with increasing initial concentration. An
optimum temperature of 40ºC existed for the degradation rate of alachlor and the
degradation rate was also found to be slightly depend on medium pH.
Wang et al. (2011) have studied the degradation of reactive brilliant red K-
2BP (K-2BP) in aqueous solution using swirling jet-induced cavitation combined with
H2O2. They have observed a synergetic effect between hydrodynamic cavitation and
H2O2. The degradation of K-2BP by hydrodynamic cavitation combined with H2O2
follows pseudo first-order kinetics. A variety of experimental conditions were
investigated for the degradation of K-2BP by swirling jet-induced cavitation
combined with H2O2. It was found that lower pH and higher temperature of medium,
higher pressure of fluid, more addition of H2O2 and lower dye initial concentration
are favorable for the degradation of K-2BP using hydrodynamic cavitation.
Recently, Joshi and Goagate (2012) have investigated degradation of an
aqueous solution of dichlorvos using hydrodynamic cavitation reactor. They have
studied the effect of various additives such as hydrogen peroxide, carbon
tetrachloride, and Fenton’s reagent on the degradation rate with an aim of intensifying
the degradation rate of dichlorvos using HC. They have observed that use of hydrogen
peroxide and carbon tetrachloride resulted in the enhancement of the extent of
degradation at optimized conditions but significant enhancement was obtained with
the combined use of hydrodynamic cavitation and Fenton’s chemistry. The maximum
extent of degradation as obtained by using a combination of hydrodynamic cavitation
and Fenton’s chemistry was 91.5% in 1 h of treatment time.
The above works depict that hydrodynamic cavitation has great scope in the
area of wastewater treatment because of its effectiveness in reducing the organic
pollutant and real industrial wastewater to a desirable level, cost effective method as
compared to other advanced oxidation technique and easy to scaleup on an industrial
scale. Though hydrodynamic cavitation offers immense potential and also higher
energy efficiency and cavitational yields, use of these reactors is perhaps lacking on
larger scales. More work is indeed required both on theoretical front as well as on the
Chapter 1: Introduction
Hydrodynamic Cavitation based degradation of Bio-Refractory pollutants Page 45
experimental front for better understanding of the phenomena and subsequent design
methodology.
1.10 Aim and scope of the present thesis
In chapter 2, the hydrodynamic characteristics of a cavitating device (circular
venturi) and cavity dynamics (cavity generation, growth and collapse) inside a
hydrodynamically cavitating device are discussed using the photographic evaluation.
The optimization of cavitating device in terms of inlet pressure and cavitation number
to get the maximum degradation rate is presented for the degradation of Reactive Red
120 dye and the effect of solution pH on the degradation rate is discussed.
Chapter 3 presents the comparative study of hydrodynamic cavitation and
acoustic cavitation for the degradation of Acid Red 88dye. The effect of various
operating parameters such as inlet pressure, initial concentration of dye, pH of
solution, addition of H2O2 and a catalyst (Fe-TiO2) on the extent of decolorisation
and mineralization is discussed.
Chapter 4 presents geometric optimization of different cavitating devices (viz.
orifice plate, circular venturi and slit venture) using degradation of orange-G dye
[OG] as a model pollutant. The cavitational yield of all cavitating devices in terms of
energy efficiency is discussed. The efficacy of all three cavitating device for the
treatment of real industrial wastewater and the scale up aspects are discussed.
Chapter 5 discusses the efficacy of hydrodynamic cavitation in enhancing the
biodegradability of complex wastewater (biomethanated distillery wastewater) along
with reduced toxicity (lower COD/TOC and color).
Chapter 1: Introduction
Hydrodynamic Cavitation based degradation of Bio-Refractory pollutants Page 46
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