Chapter -1

Introduction

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1.1. Introduction

We cannot imagine life without colour. Colour can be imparted to the particular object by

using various types of dyes. They may be Natural or Synthetic origin. Natural dyes are

dyes or colorants derived from plants, invertebrates or minerals. The majority of natural

dyes are vegetable dyes from plant sources – roots, berries, bark, leaves, and wood — and

other organic sources such as fungi and lichens. Archaeologists have found evidence of

textile dyeing dating back to the Neolithic period. In China, dyeing with plants, barks and

insects has been traced back more than 5,000 years. The essential process of dyeing

changed little over time. Typically, the dye material is put in a pot of water and then the

textiles to be dyed are added to the pot, which is heated and stirred until the colour is

transferred. Textile fiber may be dyed before spinning (dyed in the wool), but most

textiles are yarn-dyed or piece-dyed after weaving. Many natural dyes require the use of

chemicals called mordents to bind the dye to the textile fibers; tannin from oak galls, salt,

natural alum, vinegar, and ammonia from stale urine were used by early dyers.

The discovery of man-made synthetic dyes in the mid-19th century triggered the end of the

large-scale market for natural dyes. Synthetic dyes, which could be produced in large

quantities, quickly superseded natural dyes for the commercial textile production enabled

by the industrial revolution. Unlike natural dyes, synthetic dyes were suitable for the

synthetic fibers.

Dyestuff sector is one of the core chemical industries in India. It is also the second highest

export segment in chemical industry. The Indian dyestuff industry is made up of about

1,000 small scale units and 50 large organized units, who produce around 1,30,000 tonnes

of dyestuff. Maharashtra and Gujarat account for 90% of dyestuff production in India due

to the availability of raw materials and dominance of textile industries in these regions.

The major users of dyes in India are textiles, papers, plastics, printing ink and foodstuffs.

The textiles sector consumes around 80% of the total production due to high demand for

polyester and cotton, globally. During year 2006, India contributes about 6% of the share

in the global market with a Compound Annual Growth Rate (CAGR) of more than 15% in

the last decade of 1990 - 2000. The organized sector contribute about 65% of the total

dyestuff production in the country1. The dyestuff industry has recently seen movement

towards consolidation and as a result, organized players are now poised to take a lead in

the global market. Increased focus is being laid on environmental friendliness and at the

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same time the industry is ensuring greater customer focus through technical services and

marketing capabilities, in order to face global competition.

Most of the industries use dyes and pigments to colour their products2. Among various

industries the biggest consumers of dyes are textile, tannery and paper industries3.

Similarly, dyeing and printing industries uses various types of dyes to impart colour on

fabric. The textile industry utilizes large volumes of water in its wet processing operation,

and thereby, generates substantial quantities of wastewater4,5. While manufacturing dyes

and dye intermediates, wastewater is generated in significant amount. This is important

point, since wastewater is the principle route by which dyestuff enters into the

environment. For example, of the 450,000 tons of dyestuffs produced in 1978, 41,000

tons were lost to waste streams from textile processing operations and 9000 tons

constitute the estimated loss from dye manufacturing facilities6. As far as the cost

parameters is concerned, it is beyond question that avoiding, or at least minimizing, and

reusing wastewater is preferable to wastewater treatment for indirect or direct discharge7,

as direct or indirect discharge of wastewater certainly changes the water quality of the

receiving body.

The term “Water Quality” is widely used and has extremely broad spectrum meaning as

the quality of water depends upon its uses e.g. for steam production, water requires low

hardness due to which low scaling occurs and hence it increase the boiler plate life. Low

hardness is also required for laundry purpose, while for drinking and food purposes it

must be free from pathogenic organisms which can cause diseases and free from minerals

and organic substances which are producing adverse physiological effects. The

temperature of water should be reasonable and such water is termed “potable water.”

This has been confirmed by different groups of experts like bacteriologists, biologists and

toxicologists by using advance techniques in assessment and quantization of water quality

parameters. Water quality characterization must take into consideration the following

aspects8,9:

1) The distribution of chemicals in the aqueous phase.

2) Accumulation and release of chemicals by the aquatic biota.

3) Accumulation and release by bottom deposits.

4) Inputs from the land and atmosphere.

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Water is the most abundant compound on the surface of the earth, without it life on the

earth will not sustain. It is one of nature’s major limits upon the human and other organic

population of the world, being required at the right place, right time and right quality.

Pure water is only that which has two atoms of hydrogen and one atom of oxygen. Water

being a very good solvent, dissolves all kind of impurities (gases, liquids, solids) and

carries with it. Even insoluble matter and disease producing bacteria are carried by it

while flowing on the earth10. This pure water is contaminated while sewage and industrial

wastewater is discharged in to water bodies and by many more anthropological activities.

Among various pollutants contained in industrial wastewater, colour is considered to be

very important from the standpoint of aesthetic and is stated as a 'Visible Pollutant'.

Various industries like dye making, pulp and paper, tanneries, coffee pulping,

pharmaceuticals, food processing, electroplating, distilleries, etc., discharge coloured and

toxic wastewater to water bodies rendering them murky, dirty and unusable for further

use. The discharge of highly coloured wastewater containing dyes can be damaging to the

receiving bodies11 . Due to the non biodegradable nature and long-time persistence in the

environment the toxic waste often accumulates through trophic level causing a deleterious

biological effect and higher incidence of water-borne diseases12 . Of these, textile

wastewaters typically have strong colour due to un-fixed dyes, as well as they are bio-

recalcitrant due to the presence of various auxiliary chemicals such as surfactants, fixation

agents, bleaching agents, etc. The degree of dye fixation to fabrics depends on the fiber,

depth of shade and mode of application and, depending on the dye, 2-50% of unfixed dye

can enter the waste stream13 . Reactive dyes are usually found at relatively high

concentrations in wastewaters due to their low fixation especially to fibers such as cotton

and viscose. Dye molecules often receive the largest attention due to their colour, as well

as the toxicity of some of the raw materials used to synthesize dyes (e.g. certain aromatic

amines), although dyes are often not the largest contributor to the textile wastewater14.

Dyes concentration in wastewaters is usually lower than any other chemical found in

these wastewaters, but due to their strong colour they are visible even at very low

concentrations, thus causing serious aesthetic problems in wastewater disposal15. Colour

acquired by wastewater through discharge of coloured industrial wastewater inhibits

growth of the desirable aquatic biota necessary for self-purification (re-oxygenation) by

reducing penetration of sunlight and consequent reduction in photosynthetic activity and

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primary production and also by D.O. depletion as colour prevents entrance of atmospheric

oxygen into open water resources.

Following Figure 1.1.1, re-presents a schematic of “the industrial waste system,” showing

that raw materials, water, and air enter the system, and as a result of the industrial process

(es), products and by-products exit the system, along with airborne wastes, waterborne

wastes, and solid wastes. Because discharge permits are required for each of the waste-

bearing discharges, treatment systems are required. Each of the treatment systems has an

input, the waste stream, and one or more outputs. The output from any of the treatment

systems could be an air discharge, a waterborne discharge, and/or a solid waste stream.

Figure 1.1.1 The Industrial Waste System

A number of studies have shown that industrial activities have grave consequences on the

quality of the water courses. While there was depletion of oxygen around Trent Fall in the

Humber Estuary of UK16 , there was an increase in algae blooms and heavy aquatic plant

growth in Florida due to nutrient in water from upper St. Jones river17 . Studies also show

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that the dyes are carcinogenic and cause allergy, dermatitis, eye/skin infections, irritation,

cancer, mutation, etc18,19,20,21,22,23,24,25.

Several national and international criteria and guidelines have been established for water

quality standard by several agencies like USEPA, WHO26,27. As the wastewaters from

industries have a great deal of influence on pollution of water body by altering the

physical, chemical and biological nature of receiving water body28; CPCB29 and MoEF

have established wastewater discharge norms for various kinds of industries, to protect the

water resources from being polluted by domestic and industrial activities in India.

Any production activity utilizes fresh water and generates wastewater, which in most of

the cases, does not meet to the disposal standards prescribed by the regulating authorities.

Therefore, industries generally have to provide treatment facilities, which can process the

wastewater generated from processing activities and other sources to bring down the

pollution load to the prescribed norms. To achieve these discharge norms various physical

and chemical methods including adsorption, coagulation, precipitation, flocculation and

oxidation have been used for the treatment of wastewater. In most of the cases, industries

have to install treatment facilities which is having more than one methods described

above. The treatment facility is known as Effluent Treatment Plant (ETP).

1.2. General Wastewater Treatment Process

As not a single wastewater treatment process can be useful for treatment of industrial

wastewater for achieving desired disposal norms, generally, an effluent treatment plant is

established for treatment of wastewater containing following treatment sections:

1. Methods for providing primary treatment to wastewater

2. Biological Methods of wastewater treatment

3. Tertiary or Polishing treatment of wastewater

1.3. Methods For Providing Primary Treatment To Wastewater

The aim of primary treatment is to maintain pH and remove colour from the wastewater

prior to discharge it to an aerobic biological treatment system. Primary treatment involves

equalization, neutralization and sedimentation. Primary treatment reduces the toxicity of

wastewater.

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1.3.1 Equalization And pH Control

Among the most effective waste management procedures is equalization of the waste

stream. The ETP receives wastewater from different manufacturing sections while in case

of CETP, it receives wastewater from different manufacturing units, so it receives

wastewater which has different characteristics.

Hence, uniform treatment is not possible. In order to reduce this problem wastewater

from different sections / units is collected in a big tank, commonly known as equalization

tank, for specific periods of time and then they are subjected to aeration or mechanical

agitation to produce homogeneous and equalized wastewater. Equalization can be of two

types: flow equalization and constituent equalization30.

Flow equalization refers to changing the variations in rate of flow throughout the

processing and clean-up cycles to a more steady flow rate that is more nearly equal to the

average flow rate for that period.

Constituent equalization refers to the concentration of the target pollutants in the waste

stream. Throughout the 24-hour day, the concentrations of individual constituents in a

given industrial waste stream typically vary over wide ranges as processes are started up,

operated, shut down, and clean-up takes place.

Waste treatment systems that are designed for given ranges of concentration of target

pollutants often do not perform well when those constituents are in concentrations

significantly different from the design values. Equalization can be either online, as

diagrammed in Figure 1.3.1(a), or offline, as diagrammed in Figure 1.3.1(b). Online flow

equalization is accomplished by allowing the waste stream to flow into a basin. The waste

is then transferred from the basin to the treatment system at a constant, or more nearly

constant, rate. The basin must be sufficiently large that it never overflows and must

always contain enough waste that it never becomes empty, causing the flow to the

treatment system to stop.

As shown in Figure 1.3.1(b), offline equalization is accomplished by restricting the flow

into the treatment system by means of either a flow-regulating valve or a constant speed,

positive displacement pump. When there is excess waste flow, it is directed to the

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equalization tank. When there is insufficient flow, it is made up from the equalization

tank. With respect to constituent equalization, offline equalization can be used

advantageously when wastes generation at night is significantly less than during the day.

A portion of the strong daytime wastes can be stored in the equalization facility, then

directed to the treatment system at night. The treatment system can be significantly

smaller because it is not required to treat wastes at the high rate that they are generated

during the daytime.

(A) Flow Equalization

Flow into the basin is by gravity and varies as the waste generation rate varies. Flow out

of the basin to the treatment system is made constant by either an appropriate valve or a

positive displacement pump. An aerator and/or a mixer are provided to prevent

undesirable occurrences such as settling out of solids or biological activity that would

result in anaerobic conditions with consequent odor problems or chemical or biological

reactions that would change the nature of the wastes. The principal design parameter of a

flow equalization basin is size. Because the cost of the basin is a direct function of its size,

and because operation of the basin, in terms of maintaining complete mixing and aerobic

conditions, is also a direct function of basin size, the flow equalization basin should be no

larger than necessary to accomplish the required degree of equalization.

Figure 1.3.1 Types Of Flow Equalization

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(B) Constituent Equalization

Some amount of constituent equalization always takes place during flow equalization

itself. In fact, it is standard practice to design for flow equalization, and then operate to

attain the degree of constituent equalization needed to achieve treatment objectives. It can

be completed either by manually, or automatically, decrease rates of flow during periods

when constituent concentrations are high.

1.3.1.1 pH Control

Typically, industrial waste discharge permits require that the pH be within the values of

6.5 and 8.5, and many industrial waste treatment processes require that the pH be held

within a range of plus or minus 0.5 pH units. Some treatment processes require an even

smaller pH range for successful operation. For these reasons, pH control is one of the

most important aspects of industrial wastewater treatment.

The standard procedure used to control pH in industrial waste treatment is using a mixing

chamber to mix acidic and/or basic reagents with the wastewater. pH electrodes are

placed either in the discharge from the mixing chamber or, in some cases, in the chamber

itself. The electrical signal from the pH electrodes is amplified and relayed to a controller,

which activates valves or pumps to regulate the flow of acidic or basic reagent into the

mixing tank. Many pH control applications are simple, straightforward, and require only

that the electrodes be kept clean and well calibrated and the control system well

maintained for success.

1.3.2 Chemical Methods Of Wastewater Treatment

Chemical methods of wastewater treatment take advantage of two types of properties:

(1) The chemical characteristics of the pollutants, regarding their tendency to react with,

or interact with, treatment chemicals, and

(2) The chemical characteristics of the products of reaction between pollutants and

treatment chemicals, regarding their solubility, volatility, or other property that relates to

the inability of the product to remain in water solution or suspension.

In general, six chemical process categories can be used to remove substances from

wastewater:

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1. Reaction to produce an insoluble solid.

2. Reaction to produce an insoluble gas.

3. Reduction of surface charge to produce coagulation of a colloidal suspension.

4. Reaction to produce a biologically degradable substance from a non-

biodegradable substance.

5. Reaction to destroy or otherwise deactivate a chelating agent.

6. Oxidation or reduction to produce a non-objectionable substance or a substance

that can be removed more easily by one of the previous methods

Table 1.3.1 Chemical Treatment Technologies And Appropriate Technologies

Capacity

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1.3.2.1 Reaction To Produce An Insoluble Solid

In industry the standard procedure for removing metals from wastewaters is alkaline

precipitation. Alternative methods include precipitation of the metal as the sulfide,

precipitation as the phosphate, precipitation as the carbonate, or co-precipitation with

another metal hydroxide, sulfide, phosphate, or carbonate.

A step-by-step procedure that can be used to develop an effective, efficient, cost-effective

treatment technology is as follows:

1. Identify one or more insoluble compounds of which the target pollutant is an

ingredient.

2. Identify one or more soluble compounds that are reasonably inexpensive sources

of the remaining substances in the insoluble compound(s).

3. Perform experiments in the laboratory to confirm the technical and financial

feasibility of each promising treatment method.

Table 1.3.2 Common Methods And pH Values For Removal Of Heavy Metals

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1.3.2.2 Reaction To Produce An Insoluble Gas

An example of this treatment technology is the removal of nitrite ion by chlorination.

2NO2- + Cl2 + 8H+ → N2↑ 4H2O + 2Cl-

As chlorine is added in the form of chlorine gas or hypochlorite, or another chlorine

compound that dissolves in water to yield free available chlorine, the nitrite ions are

oxidized to nitrogen gas and water. Nitrogen gas, being only sparingly soluble in water,

automatically removes itself from the chemical system, driving the equilibrium to the

right until the entire nitrite ion has been removed.

For many years, breakpoint chlorination has been used to produce a free chloride reside in

drinking water. The basic process of breakpoint chlorination is that chlorine reacts with

ammonia in four different stages to ultimately produce nitrogen gas, hydrogen ions,

chloride ions, and possibly nitrous oxide and nitrate31 in low concentration.

First, chlorine reacts with water to yield hypochlorous and hydrochloric acids

Cl2 + H2O ↔ HOCl + Cl- + H+

Then, hypochlorous acid reacts with ammonia:

HOCl + NH3 ↔ NH2Cl + H2O

HOCl + NH2Cl ↔ NHCl2 + H2O

HOCI + NHCl2 ↔ NCl3 + H2O

HOCI + NCl3 ↔ N2 + N2O + NO3–

Breakpoint chlorination can be used to convert ammonia to nitrogen gas in wastewater

treatment; however, because many substances that are stronger reducing agents than

ammonia will exert their demand for chlorine, therefore this method is suitable only if

such substances are not present in significant amounts.

1.3.2.3 Reduction Of Surface Charge To Produce Coagulation Of A Colloidal

Suspension

A very high percentage of industrial wastewaters consists of colloidal suspensions. In

fact, it is often possible to destabilize industrial wastewaters by chemical coagulation,

allow separation of the destabilized colloidal material from the water, further treat the

water to improve quality by a polishing step, if necessary, and then recover the coagulant

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from the separated waste substances. The coagulant can be reused, and the waste

substances can be further treated, if necessary. The advantage is that the polishing step

can be significantly more economical than if it were used to treat the raw wastewater, and,

in some cases, the separated colloidal material can be recovered as a by-product.

A colloidal suspension consists of one substance in a fine state of aggregation evenly

dispersed throughout a second. The first phase, which may consist of single polymers or

aggregates of smaller molecules, is called the dispersed or discontinuous phase, and the

second phase is called the dispersing medium or continuous phase. The distinguishing

characteristics of a colloid system are the size of the dispersed particles and the behaviour

of the system, which is governed by surface phenomena rather than the chemical

properties of the components. The size of the dispersed particles ranges between 1 and

100 mµ, placing them between molecules and true particles.

Colloidal systems32 can be further classified into emulsions, gels, or sols. Emulsions

consist of two immiscible liquids, one being finely dispersed throughout the other. They

must be stabilized by a surface-active agent, called an emulsifier, which reduces the

surface tension at the interface between the two phases. Free energy from the increased

surface area otherwise would tend to destabilize the emulsion. When an emulsifier is

present, the repulsive forces caused by like electrostatic charges on each of the dispersed

aggregates exerts the principal stabilizing force. These charges arise by several different

means, depending on the nature of the emulsion. The principal destabilizing forces are the

Brownian movement, which causes the aggregates to come into contact with each other,

and the London–van der Waal forces of attraction, which tend to cause the emulsified

aggregates to coalesce after moving to within a critical distance from each other32.

A gel results when organic colloids of long, thin dimensions are dispersed in a liquid

medium, with the resulting formation of a nonuniform lattice structure, when suitable

groups on the colloidal particles come into contact.

The dispersed phase is like a “brush heap” in this respect, and the gel assumes a semi-

solid texture. Gelatin is a familiar example of a substance that forms a gel. The most

common colloidal system encountered in industrial wastes consists of organic particles or

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polymers, and/or inorganic particles, dispersed in a liquid to yield a fluid system known

as a sol. This differs from an emulsion in that an emulsifying agent is not required.

The particles or polymers belong to one of two classes, depending on whether or not they

have an attraction for the dispersing medium. These classes are lyophobic (solvent-

hating), for those substances that do not have an attraction, and lyophillic (solvent-

loving), for those substances that do. In the case of industrial wastes for which the

dispersion medium is water, the classes are called hydrophobic and hydrophillic.

Lyophobic sols Giant molecules or polymers that have no attraction for a particular liquid,

and thus possess no tendency to form a true solution with the liquid, can be induced to

form a hydrophobic sol by applying sufficient energy to disperse the particles uniformly

throughout the liquid medium. If the dispersed particles contain groups that are ionizable

in the dispersing medium or if certain electrolytes are present in the dispersing medium,

the anion or cation of which is preferentially adsorbed by the dispersed particles; the sol

will be stabilized by mutual repulsion of like electrostatic charges on each of the particles

of the dispersed phase.

1.3.2.3.1 Coagulation Of Colloidal Waste Systems: Lyophobic Colloids

Coagulation, or agglomeration of the particles in a lyophobic colloidal system, can be

brought about by neutralization of the surface charge on each particle to the point where

the repulsive forces will be less than the London– van der Waal forces of attraction.

Because the zeta potential directly indicates the strength of the net charge on each

particle, a reduction of the zeta potential is synonymous with a reduction of the stabilizing

forces of the sol. A zeta potential of zero (the isoelectric point of the system) corresponds

to minimum stability; however, the zeta potential need not be zero for coagulation to take

place. It must be reduced only to within a certain minimum range, referred to as the

“critical zeta potential zone.”

The zeta potential can be reduced by one or a combination of several methods, including

increasing the electrolyte concentration of the sol, reducing the potential on the surface of

the dispersed solid by external manipulation of the pH, and/or by adding multivalent

counterions. The Schulze-Hardy rule states that the sensitivity of a colloid system to

destabilization by addition of counter-ions increases far more rapidly than the increase of

the charge of the ion. That is, in increasing from a divalent charge to a trivalent charge,

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the effectiveness of a coagulant increases far more than a factor of 3/2. Overbeek

illustrated this rule by showing that a negatively charged silver iodide colloid was

coagulated by 140 millimoles per liter of NaNO3, 2.6 millimoles per liter of Mg(NO3)2,

0.067 millimoles per liter of Al(NO3)3 , and 0.013 millimoles per liter of Th(NO3)433.

The distinction between primary stability, imparted by the surface charge, and secondary

stability, pertaining to the effective repulsion of collodial particles because of the

magnitude of the zeta potential, is useful for interpreting the coagulation of these particles

by the addition of multivalent ions counter to the surface charge of the colloid. The

presence of the multivalent counter-ions brings about a condition whereby the charges in

the double layers are so compressed that the counter-ions are eventually contained, for the

most part, within the water layers that originally contained the solvated ions in the double

layer. The primary stability is thus reduced as a consequence of neutralization of the

charge on the surface of the colloid, and the secondary stability is reduced because the

particles can now approach each other to within a distance corresponding to coalescence

of their water sheaths (water solvating the counter-ions and co-ions in the diffuse double

layer) (recall that, if an ion is dissolved, it has, by definition, many water molecules

surrounding it and “bonded” to it by “hydrogen bonding”) without prohibitive repulsion

between their respective diffuse double layers of ions.

1.3.2.3.2 Coagulation Of Colloidal Waste Systems: Lyophillic Colloids

The principal stabilizing factor in the case of lyophillic sols is the solvating force exerted

on the particles (not on the ions in the diffuse double layer) by the suspending medium.

Electrolytic repulsion between the particles, although of lesser importance to the total

stabilizing force, must also be dealt with for the ultimate coagulation of a lyophillic

colloid system. Two methods can be employed to de-solvate lyophillic colloids. In the

first method, a liquid can be added to the system that is both a poor solvent for the

suspended particle and highly miscible with the suspending medium. When this is done

the colloid is no longer able to form bonds with the medium that are stronger than the

internal bonds between elements of the medium, and the principal stabilizing factor is

removed.

This process is called “coacervation.” The other method is to add a substance, such as a

sulfate salt, which can form stronger salvation bonds with the solvent than the solvent can

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with the suspended colloids. When this is done, the added salt effectively pulls the

hydrated water layer from the surface of the colloid, again, destroying the principal

stabilizing factor. This process is known as “salting out.” The actual destabilizing

mechanism as well as the overall effect of both of these methods is the same. In each case,

the dispersing medium is able to form stronger bonds (thus decrease its free energy more)

with the additive than with the dispersed phase. If the previously solvated particles carry

no net charge at this point, that is, have either zero, or very small zeta potential, they will

flocculate and separate from the dispersing phase, once coacervation has been

accomplished.

If they do possess a net charge, resulting in a significant zeta potential and thus repulsive

forces stronger than the London–van der Waal forces of attraction, the colloids will not

coagulate but will remain in suspension as a lyophobic sol. Coagulation must then be

effected according to the methods presented in the section on Lyophobic sols.

Coagulants such as alum, ferric sulfate, and cationic polyelectrolyte all work by

supressing the zeta potential of the colloidal system to a value sufficiently low that the

colloidal particles will collide, and then coalesce to make large particle, under the

influence of slow stirring. Anionic and nonionic polyelectrolytes can greatly aid in

building much larger flocculated particles that will both settle faster and produce a less

turbid wastewater. Used in this manner, the anionic and nonionic polyelectrolytes are

referred to as “coagulant aids.”

When alum Al2(SO4)3.18H2O dissolves in water, some of the aluminum goes into true

solution as the trivalent aluminum ion, Al+++. If some colloidal particles have a negative

surface charge, the trivalent aluminum ions, plus other aluminum species such as

Al(OH)++ and Al(OH)2 +, will be attracted to these negatively charged surfaces and will

suppress the net negative surface charge, which is to say they will suppress the zeta

potential.

Other metal salts that dissolve to yield trivalent ions, such as ferric sulfate, ferric chloride,

and aluminum chloride, coagulate colloidal suspensions with effectiveness similar to that

of alum.

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Salts that dissolve to yield divalent ions, such as calcium chloride or manganous sulfate,

also reduce the zeta potential and eventual coagulation, but with efficiency far less than

the efficiency indicated by the difference in ionic charge.

1.3.2.3.3 Chemical Coagulants Used In The Wastewater Treatment

The chemical coagulants can be successfully used to treat dyes, pigments and other

chemicals that tend to adsorb to biomass or flock and resist biodegradation. Generally,

coagulation and flocculation compounds like lime, alum, ferrous and ferric sulfate and

polyelectrolyte are employed for treatment of dye containing wastewater34,35.

A) Lime:

Generally, Hydrated lime Ca(OH)2 is used in various treatment processes, which is a dry

powder produced by reacting quicklime with a sufficient amount of water to satisfy the

quicklime's natural affinity for moisture. The process converts CaO to Ca(OH)2. The

amount of ater required depends on both the subtle characteristics of the quicklime and

the type of hydrating equipment available. Hydrated lime easily forms as a suspension or

slurry and is often pumped to various process locations within industrial plants. The

resulting solution is strongly alkaline, having a pH of 12.4. Most hydrated limes contain

approximately 75 percent CaO and 25 percent H2O. Hydrated lime is used in a variety of

industrial applications including water treatment, as an anti-stripping agent in asphalt, and

in soil stabilization. Some hydrated limes are sold into the food grade market as well.

- Use Of Lime In Water Treatment

In terms of annual tonnage, lime ranks first among chemicals used in the treatment of

potable and industrial water supplies. Lime is used by many municipalities to improve

water quality, especially for water softening and arsenic removal.The American Water

Works Association has issued standards also, that provide for the use of lime in drinking

water treatment.

- Use Of Lime As Water Softening Agent

In water softening, hydrated lime is used to remove carbonate hardness from the water.

Hardness caused by calcium and magnesium salts, called non-carbonate hardness, is

generally treated by means of the lime-soda process, which entails the precipitation of

magnesium by lime. The co-produced calcium salt reacts with the soda ash to form a

calcium carbonate precipitates. Lime enhanced softening can also be used to remove

arsenic from water.

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- Use Of Lime In pH Adjustment And Coagulation

Hydrated lime is widely used to adjust the pH of water to prepare it for further treatment.

Lime is also used to combat "red water" by neutralizing the acid water, thereby reducing

corrosion of pipes and mains from acid waters. The corrosive waters contain excessive

amounts of carbon dioxide. Lime precipitates the carbon dioxide to form calcium

carbonate, which provides a protective coating on the inside of water mains.

Additionally, Lime is used in conjunction with alum or iron salts for coagulating

suspended solids incident to the removal of turbidity from "raw" water. It serves to

maintain the proper pH for most satisfactory coagulation conditions. In some water

treatment plants, alum sludge is treated with lime to facilitate sludge thickening on

pressure filters.

- Use Of Lime To Prevent Pathogen Growth In Water

The addition of lime to bring pH of water between 10.5 to 11.0 and retaining the water in

contact with lime for 24-72 hours, lime controls the environment required for the growth

of bacteria and certain viruses. This application of lime is utilized where the water is

containing significant amount of Phenolic compounds, because chlorine treatment to such

water tends to produce displeasing water due to the presence of phenol. This process is

known as "excess alkalinity treatment," and this process removes most heavy metals also.

- Use Of Lime For Removal Of Impurities From Water

One of the most common methods of removing silica from water is the use of dolomitic

lime. The magnesium component of this lime is the active constituent in silica removal.

Lime is also used to remove manganese, fluoride, organic tannins and iron from water

supplies.

B) Ferrous Sulfate And Ferric Sulfate

Ferric Sulfate and Ferrous Sulfate are both commonly used for municipal and industrial

wastewater treatment. These salts are used as coagulants or flocculants, for odor control to

minimize hydrogen sulfide release, for phosphorus removal, and as a sludge thickening,

conditioning and dewatering agent.

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Liquid ferric sulfate is a red-brown aqueous solution that is typically sold at 50% and 60%

strengths, on a dry basis. The ferric iron (Fe+3) concentration of the two solutions is 10%

and 12% respectively.

Liquid ferrous sulfate is a clear, blue-green aqueous solution that is commercially

available at 25% strength on a dry basis. The ferrous ion concentration (Fe+2) is typically

5-7% by weight.

Typical addition points differ for the two chemicals. Ferric sulfate can be added in the

wastewater collection system or at the treatment plant headworks, where it also provides

benefits in the subsequent treatment processes. Ferrous sulfate is primarily added in the

collection system and is less effective when added in the plant.

- Use Of Ferrous Sulfate And Ferric Sulfate In Wastewater Treatment

Ferric sulfate enhances clarification by forming a rapidly settling floc, whereas ferrous

sulfate does not form a floc suitable for clarification.

- Ferric sulfate controls hydrogen sulfide (H2S) by binding three sulfide ions with every

two ions of iron. Stoichiometrically, this can be expressed as:

2 Fe+3 + 3 HS- Fe2S3 (solid) + 3 H+

- Ferrous sulfate controls hydrogen sulfide by binding one sulfide ion with each single ion

of iron. Stoichiometrically, this can be expressed as:

Fe+2 + HS- FeS (solid) + H+

Theoretically, ferric sulfate consumes 3 mg/L of HS- for every 2 mg/L of ferric metal,

while ferrous sulfate consumes 1 mg/L of HS- for every 1 mg/L of ferrous metal.

Phosphorus control is an important function of wastewater treatment. Theoretically, ferric

sulfate consumes 1 mg/L of orthophosphate (PO4+3) for every 1 mg/L of ferric metal,

while ferrous sulfate consumes 2 mg/L of PO4+3 for every 3 mg/L of ferrous metal.

Stoichiometrically, these reactions are expressed as:

Fe+3 + PO4+3 FePO4 for ferric sulfate , and

3 Fe+2 + 2 PO4+3 Fe3(PO4)2 for ferrous sulfate.

19

Fats, oils and greases (FOG) commonly build up in collection system pipes and drains.

Both ferric sulfate and ferrous sulfate have proven effective in reducing FOG build-up.

Also, adding ferric sulfate at the wastewater treatment plant minimizes FOG scum on

clarifiers.

Struvite (magnesium ammonium phosphate hexahydrate) causes scaling issues within

wastewater plants. Ferric sulfate and ferrous sulfate both inhibit struvite formation by

reducing the proportion of phosphate. As shown above, ferric sulfate more efficiently

precipitates phosphorus.

Manufacturing and handling are important considerations when choosing iron-based

coagulants. Ferric sulfate is made from virgin iron ore containing low levels of trace metal

contaminants. It is moderately corrosive, but compatible with most commonly used

plastics and rubbers, 316 Stainless Steel, ceramics, glass, Hastelloy C-276 and Alloy 20.

Ferrous sulfate is made as a by-product of titanium dioxide production or from scrap iron,

possibly containing undesirable levels of heavy metal contaminants. It is mildly corrosive

and compatible with most commonly used plastics and rubbers, steel, alloys, ceramics and

glass. Ferric sulfate and ferrous sulfate are both versatile chemicals that have many

benefits for wastewater treatment.

20

Table 1.3.3 Comparison Between Typical Chemical Properties Of Ferric Sulfate And

Ferrous Sulfate Solutions

Sr.

No.

Typical Chemical Properties

50% Ferric

Sulfate

16% Ferrous Sulfate

(FeSO4)

or

28% Ferrous Sulfate

Heptahydrate

(FeSO4.7H2O)

1 Soluble Ferric Iron (Fe+3) 10% Not Applicable

2 Soluble Ferrous Iron (Fe+2) Less than 0.2% 5% (approx.)

3 Free Sulfuric Acid (as H2SO4) Less than 3% Less than 6%

4 Water Insoluble materials Less than 0.1% Less than 0.5%

5 Product Density 11.97 lbs./US

gallon

10.09 lbs./US gallon

6 pH, as is 1.0 (approx.) 1.5 - 4.5 (approx.,

depending on source)

7 pH, 1% Solution 4.8 (approx.) Not Applicable

8 Specific Gravity (at 15.6 °C) 1.435 1.15

8 Freezing Temperatures Less than -50°C -2°C (approx.); will

crystallize below 10°C

9 Boiling Temperatures 100°C (approx.) 105 - 110°C

The use of chemical coagulants in the wastewater treatment plant generates large amount

of sludge, which requires proper disposal, their dosing should be optimized to achieve

good reduction in pollution load and lesser sludge generation at the same time36.

21

C) Alum

Alum is both, a specific chemical compound and a class of chemical compounds. The

specific compound is the Hydrated Potassium Aluminium Sulfate (Potassium Alum) with

the formula KAl(SO4)2·12H2O. More widely, Alums are double Sulfate salts, with the

formula AM(SO4)2·12H2O, where A is a monovalent cation such

as Potassium or Ammonium and M is a trivalent metal ion such as Aluminium

or Chromium(III)37.

Alums are useful for a range of industrial processes38. They are soluble in water; have a

sweetish taste; react acid to litmus; and crystallize in regular octahedra. When heated they

liquefy; and if the heating is continued, the water of crystallization is driven off, the salt

froths and swells, and at last an amorphous powder remains. They are astringent and

acidic.

- Use Of Alum In Water /Wastewater Treatment

Potassium Alum is the common Alum of commerce, although Soda Alum, Ferric Alum,

and Ammonium Alum are manufactured.

Alum has been used at least since Roman times for purification of drinking water and

industrial process water. Between 30 and 40 ppm of alum for household wastewater, often

more for industrial wastewater is added to the water so that the negatively charged

colloidal particles clump together into "flocs", which then float to the top of the

liquid, settle to the bottom of the liquid, or can be more easily filtered from the liquid,

prior to further filtration and disinfection of the water.

Alum is used to clarify water by neutralizing the electrical double layer surrounding very

fine suspended particles, allowing them to flocculate (stick together). After flocculation,

the particles will be large enough to settle and can be removed. Alum may be used to

increase the viscosity of a ceramic glaze suspension; this makes the glaze more readily

adherent and slows its rate of sedimentation. Alum is an ingredient in some recipes for

homemade modelling compounds intended for use by children. (These are often called

"play clay" or "play dough" for their similarity to "Play-Doh", a trademarked product

marketed by American toy manufacturer Hasbro).

22

D) Organic Coagulants

Organic coagulants including poly electrolytes, synthetic polymers and natural polymers

are also used for coagulation process39. Polymer with initial monomers such as carboxyl

or amino are called poly electrolytes. Different types of poly electrolyte are used as

coagulant aid recently. Poly electrolyte has huge molecules with a high molecular weight

which has a high ionization power. It produces a large amount of ions in water and shows

the properties of both polymers and electrolytes. Polyamphotypes, anioninc and cationic

poly electrolyts are some types of poly electrolytes. The most practical benefit of poly

electrolytes is the formation of massive flocs. These massive flocs speed up the flocs

settling velocity, reduce the expense of decolourization and also decrease settled sludge

volume40.

However, Laboratory experimentation is always required to determine the optimum doses

of coagulants and coagulant aids as there is no characteristic, substance, or property of a

wastewater that can be measured, and then used as an indicator of the quantity of

coagulant required.

1.3.2.4 Reaction To Produce A Biologically Degradable Substance From A Non-

Biodegradable Substance

Some substances that are resistant to biodegradation can be chemically altered to yield

material that is biodegradable. Examples are long-chain aliphatic organics made soluble

by attachment of ionizable groups, and cellulose or cellulose derivatives. Hydrolysis,

under either acidic or alkaline conditions, can be used to break up many large organic

molecules into smaller segments that are amenable to biological treatment. Heat may be

required for effective hydrolytic action, and consideration of proper reaction time is very

important. Certain fats and oils are made water soluble by chemically attaching ionizable

groups such as sulfonates or ammonium groups to their long-chain hydrocarbon

structures, for use in leather tanning, conditioning and finishing, and other industrial uses.

These water-soluble fats and oils are found in wastewaters from the industries that

produce them, as well as in the wastes from the industries that use them. They are

characteristically resistant to biodegradation, which is one of the reasons for their

usefulness.

Hydrolysis of these water-soluble fats and oils can break them into small segments that

are still soluble in water and readily degradable by anaerobic microorganisms, aerobic

23

microorganisms, or both. Laboratory experimentation is required to determine the

technological feasibility for a given industrial waste, as well as a cost-effective process.

Substances that are made from cellulose or derivatives of cellulose are resistant to

biological degradation, even though the basic building block for cellulose is glucose, a

substance that is among the most readily biodegraded substances in existence. The reason

for the non-biodegradability of cellulose is that microorganisms found in aerobic

biological treatment systems are not capable of producing an enzyme that can break the

particular linkage structure, known as the “beta link,” that joins the individual glucose

molecules end-to-end to produce the very long chain that ultimately winds around itself to

become cellulose.

Acid hydrolysis, usually requiring some heat, is capable of breaking cellulose into small

segments. Just how small depends on the conditions of the hydrolysis process, as regards

acidity, heat, catalysts, and reaction time. In general, anaerobic microorganisms are

capable of rapidly biodegrading segments of cellulose that are in the size range of a few

hundred glucose units, whereas aerobic microorganisms require that the cellulose be

broken down into much smaller units. A patented process that is based on the principles

described earlier uses hydrolysis to alter the structures of refractive organics that remain

in wasted biological treatment sludge after aerobic or anaerobic digestion has been carried

out to essential completion41. As an alternative to dewatering and disposing of this

digested sludge, which, by definition, is not biodegradable, it can be broken into smaller

organic units by hydrolysis, and then sent back through the industry’s biological treatment

process from when it came. Advantage can be taken of periods of low loading, such as

weekends, holidays, periods of plant shut down for maintenance, or during shifts when

loadings to the treatment plant are low.

1.3.2.5 Reaction To Destroy Or Otherwise Deactivate A Chelating Agent

Often, removal of metals from an industrial wastewater by simple pH adjustment, with or

without addition of sulfide, carbonate, phosphate, or a carbamate, is ineffective because of

the presence of chelating agents. Chelating agents are of various make up and include

organic materials, such as EDTA, or inorganic materials, such as polyphosphates. The

following sections enumerate methods that are candidates for solving this type of

wastewater treatment problem.

24

Organic Chelating Agents

1. Destroy the chelating agent by acid hydrolysis.

2. Destroy the chelating agent by hydroxyl free radical oxidation, using one of the

following technologies, as discussed in the section entitled Oxidation or Reduction

to Produce a Non-objectionable Substance.

a. Fenton’s reagent (H2O2+ Ferrous ions)

b. Hydrogen peroxide + Ultraviolet light

c. Ozone + Hydrogen peroxide

d. Ozone + Ultraviolet light

3. Destroy the chelating agent by adding potassium permanganate and heating.

(The manganese ions will then have to be removed along with the target metals.)

4. Pass the wastewater through granular activated carbon. In some cases, the

chelating agent will adsorb to the carbon. In some of those cases, the chelated

metal will remain chelated and thus be removed on the carbon. In other cases,

adsorption to the carbon effects release of the metals, which can then be

precipitated without interference from the chelating agents.

Inorganic Chelating Agents

1. Add a stronger chelating agent, such as heptonic acid, or EDTA, both of which

are organic. Then, use one of the methods given in step 1 for organic chelating

agents.

2. Add a stronger chelating agent, such as heptonic acid or EDTA. Then it is

sometimes possible to remove the metals by passing the wastewater through a

strong cationic exchange resin generated on the hydrogen (acid) cycle.

3. If the chelating agent is polyphosphate, it may be feasible to hydrolyze it with

acid and heat.

As with most cases involving industrial wastewater treatment, a proper program of

laboratory experimentation, followed by truly representative pilot plant work, must be

conducted to develop a cost-effective treatment scheme.

1.3.2.6 Oxidation Or Reduction To Produce A Non-Objectionable Substance

The removal of chromium from industrial wastewaters by chemically reducing hexavalent

chrome ions, which are soluble in water and highly toxic, to the trivalent state, which is

neither soluble (in the correct pH range) nor toxic, is another example of treatment by

25

chemical oxidation or reduction to produce non-objectionable substances. Reducing

agents that have been found to work well include sulfur dioxide, sodium or potassium

bisulfite, or metabisulfite, and sodium or potassium bisulfite plus hydrazine. The correct

range of pH, usually in the acid range, must be maintained for each of these chemical

reduction processes to proceed successfully.

a) Oxidation Of (Soluble) Ferrous Ions To (Insoluble) Ferric Ions By Oxygen

Iron is soluble in water in the +2 (ferrous) valence state. Ferrous ions are very easily

oxidized to the +3 (ferric) valence state, soluble only in strongly acidic aqueous solutions.

The oxygen content of air is a sufficiently strong oxidizing agent to effect this oxidation.

Aeration, followed by sedimentation, has been used successfully to remove iron from

industrial and other wastewaters, including landfill leachate. The aeration step

accomplishes conversion of the dissolved ferrous ions to the insoluble ferric state. The

ferric ions quickly and readily precipitate from solution as ferric oxide (Fe2O3), which

forms a crystal lattice that builds to particle sizes that settle well under the influence of

gravity. Coagulants and/ or coagulant aids are often used to assist the coagulation and

flocculation process, and thus, produce a higher-quality wastewater.

A benefit that often results from removing iron by oxidation followed by sedimentation is

that other metal ions, all of which are inherently hydrophobic, “automatically” adsorb to

the particles of ferric oxide as they are forming via precipitation. They are thus removed

from solution along with the iron.

1. Alkaline chlorination of cyanide to produce carbon dioxide, nitrogen gas, and

chloride ion.

2. Chemical reduction of hexavalent chromium (toxic) to produce insoluble, less-

toxic trivalent chromium.

3. Oxidation of (soluble) ferrous ions to the insoluble ferric state by exposure to

oxygen (air).

4. Destruction of organic materials such as toxic substances (solvents and

pesticides, for example) and malodorous substances (methyl mercaptan and

dimethyl sulfide, for instance) by oxidation by free radicals.

a. Hydrogen peroxide + Ultraviolet light

26

b. Fenton’s reagent (H2O2 + Ferrous ions)

c. Ozone + Hydrogen peroxide

d. Ozone + Ultraviolet light

5. Oxidation of organics with ozone, which may or may not involve free radicals.

6. Oxidation of organics with hydrogen peroxide, which may or may not involve

free radicals.

7. Destruction of toxic organics by oxidation with heat, acid, and either

dichromate or permanganate. Products are carbon dioxide, water, and some non-

objectionable refractory compounds. Just about any organic compound can be

destroyed by this method.

8. Wet air oxidation of various organics such as phenols, organic sulfur, sulfide

sulfur, -and certain pesticides. This process takes place under pressure, with

oxygen supplied as the oxidizing agent by compressed air.

9. Chlorination of hydrogen sulfide to produce elemental sulfur. Subsequent

neutralization is usually required.

The wastewater in the form of ferrous sulfate or other ferrous salt in an amount sufficient

to co-precipitate other metals.

b) Oxidative Destruction Of Organics By Free Radicals

Free radicals are powerful oxidizers that can convert many organics all the way to carbon

dioxide, water, and fully oxidized states of other atoms that were part of the original

organic pollutants, including sulfates and nitrates. Free radicals can be generated in a

controlled manner to destroy a host of objectionable organic substances, including

pesticides, herbicides, solvents, and chelating agents. As discussed previously, chelating

agents interfere with the mechanisms employed to remove metals (by precipitation, for

instance).

A free radical is an atom, or group of atoms, possessing an odd (unpaired) electron (has

no partner of opposite spin). A free radical has such a powerful tendency to obtain an

electron of opposite spin, and thus attain thermodynamic stability that it will easily extract

one from an organic molecule. When this happens, the organic molecule, or a portion of

it, becomes a free radical itself, goes after another molecule, organic or otherwise,

forming another free radical, and so on. A chain reaction is thus set up and, if managed

27

properly, can be induced to continue until nearly all of the target substance has been

removed.

Management of the chain reaction consists of maintaining the pH of the system within a

range favorable for the reaction and supplying enough of the free radical–generating

substance (discussed as follows) to keep the process going. Side reactions (with reducing

agents) “kill” free radicals. If the rate of “killing” free radicals exceeds the rate of

generation of free radicals for a long enough time, the treatment process terminates.

A free radical, then, has one unpaired electron and has the same number of electrons as

protons. A negatively charged ion, in contrast, has an even number of electrons, each

paired with another electron of opposite spin, and has more electrons than protons. Free

radicals can be generated by the following methods:

1. Adding hydrogen peroxide.

2. Adding hydrogen peroxide to a solution that contains ferrous ions, either

present in the wastewater or added along with the hydrogen peroxide (Fenton’s

reagent).

3. Adding hydrogen peroxide, then irradiating with ultraviolet light.

4. Adding ozone and hydrogen peroxide.

5. Adding ozone and irradiating with ultra violet light.

c) Oxidation With Hydrogen Peroxide

Hydrogen peroxide has the chemical formula H2O2 and is an oxidizing agent that is

similar to oxygen in effect but is significantly stronger. The oxidizing activity of

hydrogen peroxide results from the presence of the extra oxygen atom compared to the

structure of water. This extra oxygen atom is described as “peroxidic oxygen” and is

otherwise known as “active oxygen.” Hydrogen peroxide has the ability to oxidize some

compounds directly; as shown following.

The oxygen — oxygen single bond is relatively weak and is subject to break-up to yield

•OH free radicals:

The two •OH free radicals sometimes simply react with each other to produce an

undesirable result; however, the radical can attack a molecule of organic matter, and in so

28

doing, produce another free radical. This is called a chain-initiating step. As previously

discussed, in free radical oxidation of organics, this process continues and the organics are

broken down all the way to carbon dioxide and water.

d) Hydrogen Peroxide Plus Ferrous Ion (Fenton’s Reagent)

Hydrogen peroxide will react with ferrous ions to produce ferric ions, hydroxide ions, and

hydroxyl free radicals as shown in below equation.

Fe+2 + H2O2 → Fe+3 + OH- + •OH

The hydrogen peroxide thus dissociates into one hydroxide ion (nine protons and ten

electrons [OH-]) and one hydroxyl free radical (nine protons and nine electrons [•OH]), as

shown in below equation.

H2O2+ e− •OH + OH-

In this case, there is only one •OH free radical, as opposed to two •OH free radicals, when

hydrogen peroxide breaks down in the absence of ferrous ions as discussed previously.

The single •OH then attacks a molecule of organic material, also previously discussed,

initiating a chain reaction (chain-initiating step) with the result that the organic material is

eventually oxidized all the way to carbon dioxide and water.

e) Hydrogen Peroxide Plus UV Light

When hydrogen peroxide is added to an aqueous solution, which is simultaneously

irradiated with ultraviolet light (UV), the result is that the hydrogen peroxide breaks down

more readily into •OH free radicals than when the UV light is not present, as illustrated in

below equation, two •OH free radicals are produced.

UV

H2O2+ e− 2 •OH

There are, therefore, significantly more hydroxyl free radicals to enter into chain-initiating

steps, as discussed previously, than in the case without UV light.

Ultraviolet light greatly increases the oxidative power of hydrogen peroxide, in a manner

similar to that of metal activation (Fenton’s reagent). Although it has not been made clear

how the reaction proceeds, it seems likely that the ultraviolet energy enables hydrogen

29

peroxide to either separate into two hydroxyl free radicals, each having nine protons and

nine electrons, as illustrated in below equation,

UV

H2O2+ e− 2 •OH

or to obtain an electron from some source, probably the target organic compounds, and

thus dissociate into one hydroxide ion (nine protons and ten electrons (OH-) and one

hydroxyl free radical (nine protons and nine electrons (•OH), The hydroxyl free radicals

then go on to enter or perpetuate a chain reaction, as shown previously.

H2O2+ e− •OH + OH-

f) Oxidation With Ozone

Ozone, having the chemical formula, O3, is a gas at ambient temperatures. Ozone has

physical characteristics similar to oxygen but is a far stronger oxidizing agent. Ozone

reacts with organic compounds in a manner similar to that of oxygen, adding across

double bonds and oxidizing alcohols, aldehydes, and ketones to acids. Ozone requires less

assistance than oxygen, as from heat, catalysts, enzymes, or direct microbial action.

g) Ozone Plus Hydrogen Peroxide

Addition of both ozone and hydrogen peroxide has the effect of oxidizing to destruction

many organics such more strongly and effectively than by adding either ozone or

hydrogen peroxide alone. When both ozone and hydrogen peroxide are present in water

containing organics, •OH free radicals are formed through a complex set of reactions. The

result of the complex reactions is that two •OH radicals are formed from one hydrogen

peroxide and two ozone molecules.

H2O2+ 2O3 2(•OH) + 3O2

The •OH radicals then react with organics to form carbon dioxide, water, and other

smaller molecules. As an example, •OH radicals react with trichloroethylene and

pentachlorophenol. The products in both cases are carbon dioxide, water, and

hydrochloric acid.

C2HCl3+6•OH 2CO2 + 2H2O +3HCl

C6HCl5O +18•OH 6CO2 + 7H2O +5HCl

30

The ozone plus hydrogen peroxide system has the advantage, compared to, say, Fenton’s

reagent, that ozone itself will react in a first order reaction with organics, resulting in

further reduction of pollutants.

In addition to the formation of •OH radicals, there may also be formation of oxygen free

radicals, as follows:

H2O2 + O3 → 2 (•O) +H2O

Oxygen free radicals may then enter into chain reactions to break up organics as shown in

below equations.

This chain reaction may continue to destroy many organics in addition to those destroyed

by ozone and the •OH radicals.

h) Ozone Plus UV Light

Ozone can be used in combination with ultraviolet light, in some cases, to produce more

rapid and more complete oxidation of undesirable organic matter than with either ozone

or ultraviolet light alone below equation illustrates this alternative process:

Organic matter+ ozone+ UV light→CO2+ H2O+ O2

Here, again, free radicals may or may not be involved.

i) Chlorination Of Hydrogen Sulfide To Produce Elemental Sulfur

Hydrogen sulfide is objectionable for several reasons, including its contribution to crown

corrosion in sewers and its malodorous character. Hydrogen sulfide can be oxidized to

elemental sulfur as shown in below equation.

H2S + Cl2 2HCl + S0

It may then be necessary to neutralize the hydrochloric acid by one of the usual methods.

Additional methods of oxidizing hydrogen sulfide to either sulfate ion or elemental sulfur

(both odor free) include the following:

31

1. Raising the level of dissolved oxygen (beyond the level of saturation at

atmospheric pressure) by adding oxygen under pressure.

2. Adding hydrogen peroxide.

3. Adding potassium permanganate.

In the cases of oxygen under pressure or hydrogen peroxide, oxygen oxidizes (takes

electrons away from) sulfur atoms. Normally, if the pH is in the alkaline range, the sulfur

atoms will be oxidized to elemental sulfur, and 1 gram of hydrogen sulfide will require

about 2.4 grams of H2O2, depending on side reactions. If the pH is in the acid range, the

sulfur will be oxidized all the way to sulfate ion, and each gram of hydrogen sulfide will

require four times more, or about 9.6 grams of hydrogen peroxide as the pure chemical. In

contrast, each gram of hydrogen sulfide will require about 4.2 grams of chlorine (as Cl2)

or 11.8 grams of potassium permanganate (KMnO4) to oxidize the sulfur to elemental

sulfur.

j) Co-Precipitation

Although technically a physical treatment method because adsorption is the mechanism,

co-precipitation is discussed here because it is often brought about as a result of chemical

treatment to produce an insoluble substance of another target pollutant. For instance, both

arsenic and cadmium are effectively co-precipitated with aluminum or iron. If there is

dissolved iron (the soluble form is the ferrous, or +2 state) and arsenic and/or cadmium in

an industrial wastewater, oxygen can be added to form insoluble ferric oxide, which then

builds a rather loose crystal lattice structure. This precipitate exists in the wastewater

medium as a suspension of particulate matter. As these particles are formation takes place,

arsenic and/or cadmium ions, which are relatively hydrophobic, adsorb to them,

effectively removing them from solution.

As the particles settle to the bottom of the reaction vessel, and the resulting “sludge” is

removed, the treatment process is completed; however, the removed sludge must now be

dealt with. As discussed in the section entitled Physical Treatment Methods, the usual

procedure is to separate the water from the particles, using vacuum filtration, sludge

pressing, or another physical separation process, followed by evaporation. The resulting

dry mixed metals can be subjected to metal recovery processes, stored for later metals

recovery, or properly disposed off.

32

k) Thermal Oxidation

Certain highly toxic organics, such as PCBs and dioxin, are best destroyed by thermal

oxidation. Dow Chemical Co., as well as others, constructed incinerators in the 1940s to

safely dispose of wastes from the manufacture of many different chemicals. PCBs, which

are resistant to biodegradation (meaning that they are biodegraded very slowly), are

destroyed completely by incineration, but temperatures of 2300 ºF or more are required.

Other organics are destroyed at lower temperatures, and each application must be tested

and proven.

Thermal oxidation has often been used to remediate contaminated soil. The mechanism

and products of thermal oxidation are the same as for chemical oxidation with oxygen.

The mechanism is electrophyllic attack, and the products are carbon dioxide, water, and

oxidized ions and molecules such as sulfate, sulfur oxides, and nitrogen oxides.

Organic matter + O2 + Heat → CO2 + H2O

When heat is added, the process is called thermal oxidation. Thermal oxidation is

required, as opposed to chemical oxidation, when the energy of activation required for the

reactants shown in above equation is too great for the reaction to proceed without heat,

which supplies the energy of activation needed.

l) Catalytic Oxidation

When the energy of activation required for oxygen, or another oxidizing agent, to react

with a certain organic substance is too great for the reaction to proceed under normal

conditions, two ways to overcome this deficiency are (1) to add heat to supply the energy

of activation, and (2) to add a catalyst, which reduces the energy of activation. When a

catalyst is added, the process is called catalytic oxidation. The principal products are the

same, namely, carbon dioxide and water (when the oxidation process is allowed to go to

completion), and the mechanism of oxidation is basically the same, except for the effect

of the catalyst in reducing the activation energy. Supported nobel metals are commonly

used for catalytic oxidation. For example, Ruthenium catalyst supported on Ce0.85Zr0.15O2

carrier was used to study the removal of organic impurities from food industry

wastewaters42. In many cases, the catalyst is not consumed in the process, as illustrated in

below equation

Organic matter + O2 + Catalyst → CO2 + H2O + Catalyst

33

1.4. Biological Methods Of Wastewater Treatment

All forms of biological metabolism involve the disassembly of organic compounds (the

food) and reassembly into new cell protoplasm (growth) and waste products. Not all of

the food can be converted to new cell protoplasm, however. It takes a certain amount of

energy, in the form of chemical bond energy, to assemble new protoplasm; that is, to

complete the chemical bonds that hold the carbon, hydrogen, and other elemental units

together. The source of that energy is the relatively high-energy bonds in the molecules of

the organic matter that is used as food. For instance, when a molecule of glucose is

disassembled to obtain energy to build protoplasm material, carbon-carbon bonds are

broken, with the consequent release of 60 to 110 kcal/mole of bond energy, depending on

what atoms are bonded to the carbon atom being worked on.

Having obtained the energy from the carbon-carbon bond (and other bonds the carbon

atoms were involved in), the organism has no further use for the carbon atoms. These

“waste” carbon atoms cannot simply be discharged to the environment by themselves,

however. They must be attached to other atoms and discharged to the environment as

simple compounds.

Because this process also requires energy, the “waste” compounds must be of lower bond

energy than those of the food that was disassembled. The balance is then available for

constructing new cell protoplasm.

The atoms to which the “waste” atoms are bonded are known as “ultimate electron

acceptors.” This name arises from the fact that the electrons associated with the waste

carbon atoms are paired with electrons of the “ultimate electron acceptor” in constructing

the molecule of waste substance.

The function of oxygen in cell metabolism (any cell—animal, plant, or bacterial) is that of

ultimate electron acceptor. If the source of oxygen is molecular O2 dissolved in water, the

process is termed “aerobic,” and is depicted in below equation.

Organic matter + dissolved oxygen → carbon dioxide + water

34

If the source of oxygen is one or more dissolved anions, such as nitrate (NO-3), or sulfate

(SO-24) and if there is no (or very little) dissolved molecular oxygen present, the process is

termed “anoxic,” and is depicted in below equation.

Organic matter + nitrate/sulfate anions → carbon dioxide +ammonia / hydrogen sulfide

+ methane

If there is no oxygen present, either in the molecular O2 form or in the form of anions, the

condition is said to be “anaerobic.” Under anaerobic conditions, cell metabolism takes

place as a result of substances other than oxygen; functioning as the ultimate electron

acceptor as mentioned in below equation.

Organic Matter + e- → Reduced organic compounds + CH4

The dissolved and suspended organic matter of the primary treated wastewater is removed

by biological process involving bacterial and other microorganisms in secondary

treatment. These processes may be aerobic or anerobic.

1.4.1 Aerobic Biological Treatment

The primary treated wastewater can be treated for aerobic oxidation in activated sludge

units, aeration lagoons, trickling filters, oxidation ditch or oxidation pond.

1.4.1.1 Conventional Activated Sludge Process

Aerobic biological treatment is given to primarily treated wastewater in the aeration tank,

where the wastewater is degraded by microorganisms like bacteria. In earlier days, the

technical realization, and particularly research into the fundamentals, were supported only

in a few industrialized countries and to a relatively low extent. Only in the past 40 years

has biological wastewater treatment been realized to a greater extent. The pollution of our

rivers and lakes caused by society’s industrialization endangered nature and spoiled the

quality of drinking water. Only then were the fundamentals of the activated sludge

process studied in more detail. However, wastewater containing dyes and dye

intermediates are toxic and difficult to degrade by oxidative degradation.

35

Degradation of monoazo dye by activated sludge treatment was investigated by research

scientist of Environmental Protection Agency43. In a study carried out by Porter and

Snider, they found that 30-days BOD tests demonstrated much greater biodegradability of

dye wastes than the standard 5-days BOD tests44. The average BOD(5 days), using eight

commercial textile dyes that included direct, reactive and vat dyes, was determined to be

38% of the 30-day BOD load.

Majority of wastewater treatment plants employ activated sludge process in the secondary

treatment plant to achieve removal of organics in the most economical way. The aeration

tanks, secondary clarifier and recycling pump together form the activated sludge plant.

In an activated sludge plant a mixed culture of microorganism is treated to survive in a

particular environment and to degrade organic being served to them as food. These

trained microorganism house themselves as flocks of solids, commonly referred to as

MLSS (Mixed Liquor Suspended Solids). The conventional plants are generally designed

to maintain 3000 to 4000 mg/L MLSS in aeration tank.

As the primary treated wastewater enters the aeration tank, equivalent flow is displaced to

the secondary clarifier carrying along with the same concentration of MLSS as

maintained in aeration tank. The microorganisms present in these solid flocks are valuable

and need to be recycled back in to the aeration tank. Since these trained or activated

microbes are recycled back as sludge from clarifier, the process is termed as 'activated

sludge process'. Below figure is showing typical ‘activated sludge process’.

36

Figure 1.4.1 Diagram Showing Typical Activated Sludge Process

(CH2O) + O2 CO2 + H2O + BIOMASS SECONDARY SEDIMENTATION

AIR

WASTES FROM PRIMARY TREATMENT

RETURN SLUDGE

AIRATION

TREATED WASTE WATER

EXCESS SLUDGE WASTE

37

1.4.1.2 Extended Aeration Activated Sludge Process

The extended aeration modification of activated sludge has the design objective of lower

costs for waste sludge handling and disposal, at a sacrifice of higher capital costs because

of larger tankage and more land area per unit of BOD loading. There may be a savings in

O&M costs resulting from a lower level of intensity required from the operators, but there

is an increase in the amount of oxygen required per kilogram of BOD in the influent

because of the increased amount of auto-oxidation (there is some increase in aeration via

the surface of the aeration tank because of the larger surface area). The reduction in

amount of wasted sludge to be disposed off per kilogram of BOD removed results from

the increase in auto-oxidation (use of the contents of the cells of dead microbes for food)

because of a longer sludge age.

Generally, the extended aeration modification of activated sludge process is characterised

by low sludge wasting rate, MLVSS level of 1,600 to 2,200 mg/L and Hydraulic retention

time of 20 hours or more.

1.4.1.3 High-Rate Modification Of Activated Sludge Process

The high-rate modification of the activated sludge process in which the MLVSS

concentration is maintained at a significantly higher concentration than in normal for

conventional processes45, and hydraulic retention time is significantly shorter than

“normal.” The high-rate process can be used to treat readily biodegradable wastes such as

fruit-processing wastewaters, but effective equalization is required to prevent short-term

changes in waste characteristics. Advantages of the high-rate process are that it is

effective in overcoming the problem of the tendency toward bulking caused by

dominance of filamentous organisms in the aeration tank systems, as well as lower

construction costs because of smaller tankage. Disadvantages include relatively intense

operational control requirements, as well as relatively large quantities of waste sludge

because of less auto-oxidation.

1.4.1.4 Contact Stabilization Modification Of Activated Sludge Process

The contact stabilization modification differs from the conventional process in that the

activities that normally take place in the aeration tank adsorption of polluting substances

onto the microorganisms, then metabolism, or conversion of these substances to more

microorganisms plus waste products are separated.

38

The untreated wastewater is mixed with return sludge in a relatively small “contact tank.”

The rapid process of adsorption of pollutants takes place here. The mixture then moves on

to an aeration tank, or “stabilization tank,” where microbial metabolism takes place. The

aeration tank is also relatively small, compared to conventional activated sludge, because

only the sludge, after separating from the bulk liquid in the clarifier, proceeds to the

aeration tank. The bulk liquid is either discharged from the clarifier as final wastewater or

is subjected to further treatment, such as disinfection, sand filtration, or another process.

Because only the sludge (with its load of adsorbed organic material) proceeds to the

aeration tank, energy requirements for mixing are lower.

The principal advantage of contact stabilization is the lower capital cost because of

smaller tankage; however, this modification is useful on only wastes that are rapidly

adsorbed. In general, those wastes are characterized by a high proportion of dissolved,

relatively simple organics.

1.4.1.5 Sequencing Batch Reactor Modification Of The Activated Sludge Process

The sequencing batch reactor (SBR) process is a modification of complete mix activated

sludge.

As diagrammed in below figure, the two principal sequential processes of activated

sludge—aeration and settling—take place in the same tank. Settling occurs, after

sufficient aeration time for treatment has taken place, after turning off all aeration and

mixing. Then, clarified supernatant is decanted, the reactor is refilled with fresh, untreated

wastewater, and the cycle is repeated. Waste sludge is withdrawn immediately after the

react phase has been completed.

An SBR is normally operated in six sequential stages, or phases, as follows:

1. Fill phase. The reactor is filled until the desired F/M ratio has been reached.

2. React phase. The reactor is mixed and aerated. Treatment takes place.

3. Sludge wasting phase. A quantity of mixed liquor that corresponds to the

quantity of solids, on a dry basis, is withdrawn from the completely mixed

contents of the reactor. For instance, if a 10-day sludge age is desired, one-tenth

of the volume of the reactor is withdrawn each day.

39

4. Settle phase. Aeration and mixing are terminated, and the reactor functions as a

clarifier.

5. Decant phase. Clarified, treated wastewater is withdrawn from the top one-

quarter to one-third of the reactor.

6. Idle phase. The system can be mixed and aerated at a low rate for a few days at

a time needed between periods of waste generation.

Figure 1.4.2 Schematic Diagram Of Sequencing Batch Reactor Process

There are several modifications to the basic procedure outlined previously. For instance, it

has been found advantageous in some cases to either mix, aerate, or both, while filling is

taking place. In these cases, the first one or two stages are referred to as the “mixed fill

phase,” followed by the “react fill phase” (stages 1 and 2), or just “the react fill phase”

(phase 1).

Among the several important advantages of the SBR process is its capability of having the

react phase extended for as long as is necessary to achieve the desired degree of

treatment. In this respect, it is good design practice to have at least two parallel SBR units,

each of at least half the design capacity. In the absence of a parallel unit, a collection tank

designed and operated as an equalization basin, can receive and store wastewater until the

SBR unit is able to receive more wastewater.

40

1.4.1.6 Aeration Lagoons

Aerated lagoons usually consist of earthen basins equipped with mechanical or diffused

aeration equipment. There is no secondary clarifier except for a quiescent zone at the

outlet. There is no controlled sludge return from the bottom of this quiescent zone. As an

alternative to the quiescent zone, a separate pond is sometimes used, in which case the

pond is referred to as a polishing pond. In other cases, a mechanical clarifier can be used.

A considerable number of pulp and paper mills, in fact, have installed mechanical

clarifiers as part of aerated lagoon systems. These alternatives are desirable if the design

of the lagoon makes use of complete mix conditions as a method to avoid short-circuiting.

There are two distinct types of aerated lagoon systems: (1) aerobic and (2) partially

mixed, facultative.

An aerobic lagoon must have sufficient mixing to suspend all of the solids and must have

enough aeration capacity to satisfy all of the BOD removal aerobically.

A partially mixed, facultative lagoon requires only enough mixing to keep all of the liquid

in motion. A significant portion of the biological and other solids resides at the lagoon

bottom and undergoes anoxic and anaerobic degradation. Enough aeration is applied to

maintain aerobic conditions in only the upper 2 to 3 feet of liquid

In short, aerated lagoons are cement tanks having depth of 3 to 5 m and are lined with

rubber or polythene. The wastewater from primary treatment processes are collected in

these tanks and are aerated with mechanical devices for around 2 to5 days. During this

time, a healthy flocculent sludge is formed which brings about the oxidation of the

dissolved organic matter.

1.4.1.7 Trickling Filter

The term "Trickling filter" although a misnomer has been used for around a century and

seems destined for continuous use. A trickling filter does not purify the wastewater by any

filtering or straining action. In fact, it is purely biological treatment where the soluble

organic matter in the wastewater is purified by the biological absorption and oxidation.

The trickling filter is an attached or fixed growth biological treatment unit that converts

soluble and colloidal organics to settleable solids to be separated from the wastewater in a

41

post filter clarifier. As the wastewater flows down the filter, a thin layer develops on the

media surface. This thin layer of bio-film is composed of a very large and diverse

population of living organisms including bacteria, protozoa, rotifers, algae, fungi,

crustaceans, worms and insect larvae.

The bed of media can be less than 1 m to more than 10 m deep and the media can be

either natural substances or a manmade material. Various types of media have been used

like rock, stones, bricks, slag-coal, wooden slabs, shell plastic molded random media etc.

1.4.1.8 Oxidation Ditch

Oxidation ditch usually consists of an oval shaped continuous channel about 1 to 2 m

deep and lined with plastic, tar or butyl rubber. Wastewater after primary treatment is

allowed into the oxidation ditch for biological treatment and the wastewater is aerated in

the channel with the help of mechanical rotors. Longer retention times are needed for

better removal of organic matter from the wastewater.

1.4.1.9 Oxidation Pond

In a large shallow pond with 0.5 m to 1.5 m depth. The wastewater enters the pond at one

end and the wastewater removed at the other end. Stabilization of organic matter in the

waste is brought about mostly by bacteria. The oxygen requirement for their metabolism

is provided by algae present in the pond. The waste in the upper part of the pond

undergoes aerobic oxidation to CO2 and H2O. Organic matter is oxidized by anaerobic

bacteria to CH4, CO2 and NH3.

1.4.2 Anaerobic Biological Treatment

Anaerobic wastewater treatment, accomplished through microbiological degradation of

organic substances in the absence of dissolved molecular oxygen, has undergone a

complete change in role since the mid-1980s. Used for decades as a slow-rate process

requiring long retention times and elevated temperatures, it was considered economically

viable for only wastes of high organic strength. Its principal role in wastewater treatment

was for stabilization of waste bio-solids from aerobic treatment processes, or as a

treatment step preceding aerobic treatment in which large, complex molecules were

broken down to more readily biodegradable substances. It is now used routinely, at

ambient temperatures, on industrial wastewaters having organic strengths as low as 2,000-

42

5,000 mg/L COD. In fact, there is certainly the potential to turn the wastewater treatment

world upside down because of the economic attractiveness of treating wastewaters using

anaerobic technology first, then polishing with one of the aerobic technologies.

More recent developments have enabled use of anaerobic treatment at cold temperatures

for wastewaters having COD values as low as 100 to 200 mg/L. Research conducted since

the mid-1970s has shown that, by addressing the fundamental reasons for the apparently

slow treatment capability of anaerobic systems, modifications could be developed to

overcome them. The result has been the development of anaerobic technologies that are

capable of treatment performance comparable to aerobic systems, at significantly lower

overall cost. Additionally, anaerobic systems are capable of treating some substances that

are not readily treated by aerobic systems, such as cellulosic materials, certain aromatic

compounds, and certain chlorinated solvents.

There are two types of anaerobic wastewater treatment systems: suspended growth and

attached growth, as is the case with aerobic wastewater treatment systems. Attached

growth systems are commonly referred to as fixed film (FF) systems.

Suspended growth systems are those in which anaerobic microorganisms feed on the

organic content of wastewater in a vessel or lagoon that contains no managed support

medium to which the microorganisms attach. As microbial growth takes place, it is

retained in the reactor by settling before the treated wastewater is decanted. The microbes

form particles that grow to a size that is dictated by the solids management characteristics

of that particular system. In general, the solids management capability and characteristics

differentiate between the several types of anaerobic treatment systems in common use.

Attached growth systems, otherwise known as fixed film systems, have a support

medium, often called “packing,” to which the anaerobic microorganisms attach as they

grow. The media can be stationary or not. Stationary media include rocks, coal, plastic or

metal discs, and plastic packing. Sand is an example of media that is not stationary.

1.4.2.1 Up-flow Anaerobic Sludge Blanket (UASB)

The UASB system is one of the more technologically advanced high-rate anaerobic

wastewater treatment systems. These systems are capable of removing 80% to 90% of

43

COD from wastewaters having influent COD concentrations as low as 2,000 mg/L with

hydraulic retention times of 8 to 10 hours. The principle components and operational

characteristics of the UASB system are:

An influent distribution system

A sludge “blanket” consisting of beads of active anaerobic (and/or anoxic)

microorganisms, formed as described previously

A gas collection system

An wastewater collection and discharge system that excludes air from the

interior of the reactor.

As influent wastewater enters the reactor via the influent distribution system, it flows up

through the sludge blanket. Depending on the rate of flow, the velocity of the rising

influent causes a certain amount of expansion of the sludge. Furthermore, depending on

the cross-sectional area of the sludge blanket, there is certain variability in the distribution

of the influent wastewater. There is a choice to be made here. For a given volume of

sludge blanket(i.e., quantity of active microorganisms),the smaller the ratio of the cross-

sectional area of sludge blanket to its depth, the more uniform is the distribution of

influent, but the greater the head against which the influent wastewater pump(s) must

pump the wastewater with higher pressure. Recently, design practice has favored deeper

sludge blankets of relatively small cross-section.

1.4.2.2 Mixed, Heated Anaerobic Digester

The mixed, heated anaerobic digester, usually arranged in two stages. It represents an

advanced version of the “old” anaerobic treatment technology, in which only mixing and

temperature elevation were used to reduce required hydraulic retention time. The

principal objective of mixing was to improve contact between active microbes and

organic material, often in solid form. The objective of heating was simply to take

advantage of the fact that almost all microbial metabolism doubles in rate for each 10°C

rise in temperature. As an attending benefit, some organics, more soluble at the elevated

temperature, are more readily metabolized because of their more direct availability to the

microorganisms.

44

Methane harvested from the treatment process itself is normally used to heat the digester

contents. This accounts for the fact that the process is simply not economically feasible if

the organic content is less than that represented by 8,000 to 10,000 mg/L COD.

1.4.2.3 Expanded Bed Reactor

The expanded bed system is among the most technologically advanced anaerobic

wastewater treatment options. This technology was developed with the objectives of

(1) Achieving the maximum possible active microbe-to organic matter ratio,

(2) Optimizing the effectiveness of contact between organic substances and

microbes, and

(3) At the same time, minimizing the energy requirement to expand, or “fluidize,”

the bed as well as to pump wastewater through the system.

The objective of maximizing the F/M ratio has been achieved through use of a “packing

medium” that has a surface-to-volume ratio that is as large as possible. The objective of

maximizing the efficiency of contact between microbes and organic matter was achieved

by utilizing the up flow (fluidized bed) configuration. The objective of minimizing the

energy needed to fluidize the bed (expand the bed, in this case) was achieved by utilizing

material of low specific weight (specific gravity only slightly greater than one) as the

packing medium.

1.4.2.4 Fluidized Bed Reactor

The fluidized bed reactor resembles the expanded bed system in physical appearance, but

differs principally in three respects:

(1) The media used are typically much heavier (sand rather than diatomaceous

earth),

(2) The velocity of wastewater upflow is significantly higher (needed to fluidize

the heavier media), and

(3) The amount of expansion is significantly greater (20% rather than 5%).

1.4.2.5 Packed Bed Reactor

An anaerobic packed bed reactor consists of a vessel (round steel tank, rectangular

concrete basin, earthen lagoon, etc.) that is filled with a stationary solid medium to which

microbes can attach and through which wastewater flows and comes into contact with the

microbes.

45

An anaerobic packed bed reactor can be operated in either the up-flow or down flow

mode, and wastewater recycle can be used to equalize the variations in raw wastewater

flow rate. Many different types of packing can be used, but the most common are stones

and plastic devices of various shapes. The design objectives of the plastic media include

high surface-to-volume ratio, structural strength, and to be nonreactive with any chemical

that might be in the wastewater.

1.5. Tertiary Or Polishing Treatment Of Wastewater

It is the final treatment given to the wastewater. It is also known as 'polishing' treatment.

The major objectives of the treatment are,

a. Removal of bacteria

b. Removal of finely divided suspended solids

c. Removal of dissolved inorganic solids

d. Removal of final traces of organic substances.

Bacteria and traces of organic substance can be removed by treatment of ozone and

chlorination, while suspended solids and dissolved inorganic solids can be removed by

sand filter and carbon filter.

1.6. Various Studies Carried Out For Treatment Of Wastewater- Literature Review

A recent study was carried out for treatment of textile wastewater by coagulation, which

suggested that coagulant like lime eliminated colour and COD effectively. However

ferrous sulfate was chosen as an effective coagulant for colour removal as it required

lowest coagulant dose, minimum settled sludge volume and maximum decolourization. It

was also observed that higher percentage of suspended solids was removed by using a

combination of iron salts with lime at pH 10 to 11 as compared to alum in a very short

time period46. Chemical precipitation in combination with adsorption on dolomite was

studied by Duizbek and Kowal47.

Georgiou and Co-workers have carried out a study on textile wastewater treatment

process aiming at the destruction of the wastewater’s colour by means of

coagulation/flocculation techniques using ferrous sulfate and /or lime48. All the

experiments were run in a pilot plant that simulated an actual industrial wastewater

46

treatment plant. Treatment with lime alone proved to be very effective in removing the

colour (70–90%) and part of the COD (50–60%) from the textile wastewater. Moreover,

the treatment with ferrous sulfate regulating the pH in the range 9.0 + 0.5 using lime was

equally effective. The treatment with lime in the presence of increasing doses of ferrous

sulfate was tested successfully, however; it proved to be very costly mainly due to the

massive production of solids that precipitated.

In addition to coagulation and flocculation alternative treatment methods like ozonation,

chemical oxidation, photo-catalytic oxidation, adsorptions etc were studied for its

effectiveness on dye containing wastewater.

While Zhang and co-workers49 studied the decomposition pathways and reaction

intermediate formation during ozonation process of hydrolyzed azo dye, Liakou and Co-

workers50 have employed ozonation as pre-treatment process for textile wastewater and

found that pre-treatment with ozone is a promising method for oxidation of the dyes to

more degradable compounds. Another study was conducted on wastewater generated

from dye intermediate wastewater. In this research, ozonation of H-acid, a dye

intermediate was studied in a semi batch reactor. The effect of pH and ozone dose on

decolorization and degradation of H-acid was studied. The results showed that pH 11.5

was effective for colour and COD removal. Both colour (98.7%) and COD (87.8%) could

be removed by 30 min of ozonation at an ozone dose of 4.33 mg/l. Oxidation and

cleavage of substituent groups were evidenced by the release of nitrate and sulfate during

ozonation. The study also showed that the O3/Fe2+ion influenced the ozonation efficiency

of H-acid. Treatment with O3/H2O2 did not enhance color and COD removal. Acetic acid

and oxalic acid were identified as the major oxidation products. Increase in the

biodegradability as measured by the BOD5/COD ratio was observed after ozonation. The

study revealed that ozonation of dye intermediate is a viable technique for wastewater

treatment51. Fanchiang and Tseng52 investigated the degradation of anthraquinone reactive

dye C.I. Reactive Blue 19 (RB-19) with initial concentration of 100 mg/L in aqueous

solution by ozone oxidation. The results of UV/VIS and FTIR spectra showed that the

anthraquinone structures, nitrogen linkages and amino groups of RB-19 were destroyed

under direct ozone reaction. Lopez and co-workers53 have carried out laboratory scale

experiments and found that ozonaton can effectively improve the biodegradability of azo

dye intermediate wastewater.

47

Chemical oxidation process is also another effective treatment method for dye containing

wastewater. The development and application of several chemical oxidation processes to

destroy toxic and biologically refractory organic contaminants in aqueous solutions

concentrated significant research in the field of environmental engineering during the last

decades. Among them, the Fenton’s reagent is an interesting solution since it allows high

depuration levels at room temperature and pressure conditions using innocuous and easy

to handle reactants. The inorganic reactions involved in Fenton process are well

established and the process has been used for the treatment of a variety of wastewaters.

The high efficiency of this technique can be explained by the formation of strong

hydroxyl radical (.OH) and oxidation of Fe+2 to Fe+3. Both Fe+2 and Fe+3 ions are

coagulants, so the Fenton process can, therefore, have dual function, namely oxidation

and coagulation54, in the treatment processes. It is essential, though, to investigate and set

the operating conditions that best suits the wastewater that are being treated in order to

achieve high degradation efficiencies. Lin Gu and co-workers55 studied an enhanced

fenton process integrating the multi-staged Fenton reaction with inner circulation was

practiced in a pilot scale. This creative design exhibited a particular advantage in treating

the wastewater with high amount of organic substances. Experiments showed a higher

COD removal efficiency (93%) and oxidation efficiency (62%) when two-staged Fenton

reaction with inner circulation (flow-to-influent ratio of 5) was applied, whereas only

40.9% of COD reduction was obtained when using the single-staged Fenton without any

circulation. It was demonstrated experimentally that the wastewater colour went through a

fast change via different stages of the reaction. The main intermediates generated along

the two-staged reaction have been identified as aromatic and aliphatic acids, but their

concentrations demonstrated regular changes in each stage, indicating an enhanced

oxidation effect of staged Fenton process. A study was also carried out for the treatment

of naphthalene dye intermediate wastewater by combined humic acid adsorption and

enhanced Fenton processes56.

Phtocatalytic degradation of sulfonated azo dyes was studied by Alessandra and Co-

workers57 in this study High-performance liquid chromatography coupled to ultraviolet

diode array detection and electrospray ionization mass spectrometry was applied to

monitor the photocatalytic degradation mediated by TiO2 of three sulfonated monoazo

dyes (Orange I, Orange II, and Ethylorange) present in aqueous solution. Photobleaching,

48

organic carbon, nitrogen and sulfur evolution were also followed during the process.

Delayed carbon mineralization was observed with respect to both dyes disappearance and

photobleaching, due to the formation of transient intermediate compounds which were in

turn completely degraded. Among the intermediates produced during the initial

degradation steps the formation of several hydroxylated derivatives, mostly coloured, was

evidenced. The MS2 spectra allowed one to formulate hypothesis about the OH attack

positions; a peculiar reactivity of the azo moiety was shown by Orange I and Orange II. In

another study58, Photocatalytic degradation of azo dye in aqueous TiO2 suspension was

studied. The aim of the study is to degrade azo dyes in the presence of artificial UV light

and photocatalyst, and to identify degradation products of C.I. Acid Orange 7 (AO7) by

liquid chromatography–mass spectrometry (LC/MS) and ion chromatography (IC).

Thereby, establishing the mechanism of photocatalytic degradation. The substrate was

chosen as a simple model for the study of reactions involving the more complex

commercial products used for the dyeing of textile fibers. ZnO-mediated photocatalysis

process has been successfully applied to degradation of commercial dye, CI Reactive Blue

160 (RB 160) by Bansal and Sud59. While Rauf and co-workers60 studied the

photocatalytic degradation of azo dyes in the presence of TiO2 doped with selective

transition metals. Tanaka and co-workers61 have photocatalytically degraded seven azo

dyes in TiO2 suspension.

Adsorption is an effective method for lowering the concentration of dissolved organics in

an wastewater. Commercial activated carbon62 can be prepared from lignite and

bituminous coal, wood, pulp mill residue, coconut shell and blood, having a surface area

ranging from 500 to 1400 m2/g .Several pilot and commercial scale systems using

activated carbon adsorption columns have been developed63,64,65 and was established that

in general, carbon adsorption of dyes is neither very efficient nor economical. Disperse

dyes, vat dyes and pigments have such low solubility that their rate of adsorption on

carbon is prohibitively slow at room temperature. On other hand, water soluble dyes such

as acid, basic, direct, metallized, mordent and reactive dyes are also not readily adsorbed

on carbon. One of the main reasons for the observed poor adsorption is the polar nature of

these dyes versus non-polar nature of carbon66.

49

In South Gujarat region various industrial zones developed by Gujarat Industrial

Development Corporation (GIDC). Sachin industrial area is one of them. Sachin GIDC

has various dyes and dye intermediates manufacturing units. There are two Common

Effluent Treatment Plants (CETP) one is for treatment of dyes and dye intermediate

wastewater having and another is for treatment of textile dyeing house wastewater67

50

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