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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.



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.



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.



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



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



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



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



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



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.



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.



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



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



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



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



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



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



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


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.).



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.



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


Stabilization, solids destruction,

pathogen kill

Attached growth Anaerobic packed and

fluidized bed


Stabilization, denitrification

Sludge blanket Upflow anaerobic sludge

blanket (UASB)


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



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



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.



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.



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.



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



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



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–



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;



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-



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



(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,



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



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.



• 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.



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



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.



• 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



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)



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



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



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



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.



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



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



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



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).



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