Industrial Effluents

7/31/2019 Industrial Effluents

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UNIT 11 INDUSTRIAL EFFLUENTS

Structure

11.1 IntroductionObjectives

11.2 Industrial Effluents Pollution Parameters and Treatment MethodsPollution Parameters

Treatment Methods

11.3 Effluents from Food and Food Processing Industries Dairy WasteSources of Waste

Methods for Reducing Wastewater Quantity

Treatment of Dairy Waste

11.4 Effluents from PetrochemicalsThe Petrochemicals Industry

Waste Characteristics

Waste Disposal Treatment

11.5 Effluents from TextilesThe Textile Industry

Textile Waste Characteristics

Textile Wastewater Problems

Textile Waste Treatment

11.6 Effluents from Pulp and Paper IndustryThe Pulp and Paper Industry

Effluent from Pulp and Paper Industry

Characteristics of Effluent

Suspended Solids Reduction

Sludge Dewatering and Disposal

Methods for the Reduction of Organics

Land Disposal by Irrigation and Seepage

11.7 Effluents from TanneriesThe Leather Industry

Tannery Waste Characteristics

Tannery Waste Treatment

11.8 Hazardous WastesHazardous Waste GenerationHazardous Waste Management

11.9 Summary11.10 Terminal Questions11.11 Answers11.12 Glossary11.13 Suggested Readings

11.1 INTRODUCTION

Pollution of streams, lakes and coastal waters has emerged as a major water pollution

problem. It renders these natural resources unsuitable for human consumption and

even for recreational purposes like swimming, boating etc.Industrial wastes,municipal sewage and domestic wastes are the major sources of stream pollution.

Agricultural activities which discharge both excessive quantities of silt and chemicals

leached or washed from the soil, also contribute to water pollution.

Industrial wastewater is an inevitable result of nearly all manufacturing industries.

Water is used in large quantities for many purposes by industry. Only a small fraction

of the water used is consumed in manufacturing process and its major portion is

discharged from the plant premises. These discharges are inevitably contaminated with


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Pollutants in Soil and

Water

materials used in the plant. The contaminants include raw materials, unwanted

substances accompanying the raw materials, manufactured products, intermediate

products, by-products and other substances used in the processing. All these materials

contribute to pollution of the receiving water bodies.

Objectives

After studying this unit, you should be able to:

define industrial effluents and describe their physico- chemical characteristics, explain the methods of industrial effluents treatment, describe various methods of dairy waste treatment, describe sources of generation of wastewaters from petrochemical industries, understand waste disposal methods for refineries and petrochemical industries, explain sludge disposal methods used in refineries, describe characteristics of effluents from pulp and paper industry and their

treatment methods,

describe the salient features of tanneries and describe characteristics of its wastes, define hazardous wastes, describe its sources and their management and describe treatment and disposal of hazardous wastes.11.2 INDUSTRIAL EFFLUENTS - POLLUTION

PARAMETERS AND TREATMENT METHODS

Industrial effluents are characterized by their differences rather than their similarities.

Each individual plant is an individual problem. But, there are common parameters bywhich pollution can be evaluated. Also, there are general treatment methods applicable

to the plants in any given industry due to their similarities. The common parameters and

treatment methods need to be understood before studying the characteristics and

treatment methodologies for specific industries. In this section, we will discuss the

physico-chemical parameters commonly used in the study of wastewaters.

11.2.1 Pollution Parameters

Natural water is never pure and inevitably carries, in dissolved or suspended form,

every material it has contacted. The presence of one or more substances at a level

which does not produce objectionable characteristics is not pollution. The word

"pollution" is to be applied only when impurities are present in objectionable

concentrations. Quantitative evaluation of stream pollution is based on theconcentration of specific impurities or identifiable groups of impurities in the stream.

These concentrations are related, in turn, to the corresponding concentrations in waste

waters discharged into the stream. Concentrations are usually reported in milligrams

per litre (mg L1) or parts per million (ppm). Flows are usually stated in cubic meter

per second (in short written as cumec). It is often desirable to express pollution in total

quantity rather than in concentration.

(i) Total Suspended Solids (TSS): The total suspended solids or nonfiltrable residue is

the dry weight of residue retained by filter. This is obviously the particulate matter

and not the dissolved impurities; though very fine particulate matter and colloidal solids

may not be retained on the filter. A distinction between fixedand volatile suspended

matter can then be made by ignition. Coarse and floating solids are impossible to

sample in a representative manner; hence they should be avoided in the sample. There

is no reason to permit such wastes in an industrial or municipal effluent. They occur, of


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Industrial Effluentscourse, in almost all the industries, like food processing plants, textile mills, and other

industries, but are readily removed from the waste stream before discharge.

The principal objection to total suspended solids is that they give unesthetic appearance

to the stream. It is, however, no different from silt and other natural suspended matter,

unless the color is unusual. Suspended matter also reduces light penetration into the

stream water and hence affects oxygen regeneration by photosynthesis.

(ii) Settleable Solids : The portion of total suspended matter which can be removed byquiescent settling is particularly significant. In a stream, settling tends to clarify the

water, but at the expense of creating bottom deposits or sludge banks. These deposits,

containing organics, decompose with formation of objectionable odours and also

remove oxygen from the stream. Deliberate removal of settleable matter by

sedimentation, in specially designed equipment, is one of the most useful methods of

waste treatment. It is used to remove settleable matter from the raw waste and that

produced from nonsettleable contaminants during the waste treatment process.

Settleable matter can be determined volumetrically, using an Imhoff cone or similar

vessel. It is more often determined by weight, and is the difference between total

suspended solids in the water before and after laboratory settling.

(iii) Total Dissolved Solids (TDS) : Dissolved solids or filtrable residue, in contrast tototal suspended solids, is the portion of "residue" which passes through the filter in

solution or in fine suspension in the water.

(iv) Turbidity: It is a measure of the effect on light, and hence on appearance of the

wastewater caused by suspended and colloidal matters. It has obvious esthetic

significance, both in the stream itself and in any municipal water supplies drawn from

it. It may render the waters unsuitable for municipal supplies or for certain industrial

uses, notably for food, beverage, and paper manufacture.

(v) Oils: Oils and other floating films are highly objectionable in natural waters, for their

unaesthetic qualities and for actual damage to the water-way. They retard aeration of

the stream, thus killing fish. Heavy oil films have been known to trap and kill waterfowland land wildlife. Oily films coat bridge piers and other structures, leaving unsightly

deposits. They may be difficult to remove. Soluble or colloidal oils are less troublesome,

except as chemical or biological action in the stream may release them as free oils.

(vi) Color : Color occurs naturally in many streams, typically as a yellow or brown

shade from swamp waters, or brown from iron-bearing waters. These and other colorsmay also be imparted by industrial effluents. Acid mine waters and natural drainage in

mining regions often carry heavy iron discoloration, sometimes worsened by tannic acid

and similar natural constituents of the water. Dye wastes from dyestuff manufacture,

paper mills, and textile dyehouses may be spectacular and undesirable. The wastes of

many industries are colored and hence objectionable to the public. Colored waters are

not usable for public water supplies without treatment, which is often elaborate andexpensive. They may not be usable for certain industrial waters under any

circumstances. Color itself may be harmless but it is unwanted, and it may be an

indication of more serious pollution such as a toxic substance.

(vii) Taste : Tastes caused by pollution are significant in potable waters and in waters

used for fishing. In the latter case, fish flesh often acquires and even concentrates

taste-producing contaminants of the water. Many chemicals mainly phenol and its

derivatives cause objectionable tastes. The medicinal taste of phenol is worsened by

chlorination, used in water treatment. Phenols sometimes occur naturally, due to

Imhoff cone is described

in Unit 13 on Municipal

and Domestic Wastes.


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Water

decomposition of tannin compounds in swamp waters. Their presence in water is,

however, generally blamed on industries such as coke and gas manufacture, petroleum

refining, and the production of chemicals and plastics.

(viii) Odor: Odour may be a characteristic of the raw waste, or it may develop in the

waste or the receiving stream by decomposition of other pollutants. Odours are

aesthetically offensive, even at an appreciable distance from their source. Commercial

food preparation and similar operations cannot be conducted in an odourous

environment. Some of the gases associated with odour may be physically damaging tostructures, by causing chemical corrosion or discoloration of paint. Sulphide odours are

the principal cause of complaint.

(ix) Acidity, Alkalinity and pH: One of the most damaging characteristics of many

industrial wastes, particularly from the inorganic industries, is their acid or alkali

content. Either a high or a low pH may kill fish, cause general sterility in natural

streams and inactivate the essential microorganisms in sewage treatment processes.

Wastes of low pH are corrosive to steel and concrete structures in water ways orsewerage systems. The pH is maintained in wastewaters by chemical neutralisation.

Adequate controls are available for this process, and the final effluent can be made

suitable for discharge to either a stream or a sewer.

Both acidity and alkalinity are customarily expressed in milligrams of calcium carbonate

equivalent per litre. The intensity of acid or alkaline quality is expressed by the pH

value, which has a more direct relationship than acidity or alkalinity to the most

pollutional effects of the waste. Fish kills, for example, are caused by low pH ratherthan by a high concentration of titratable acidity. The measurement of pH is relatively

simple, using electrometric technique. At the point of discharge, wastes should be near

neutrality, usually in pH range from 6 to 9, though it may vary with the receiving stream

or sewage.

(x) Chlorides: Sodium chloride brines are waste products from many industries,

particularly crude petroleum production and the manufacture of soda ash and other

chemicals. Concentrations of chloride up to about 500 mg L1 are acceptable in mostnatural waters, with much higher quantities permissible in some, but uncontrolled

discharge of brines would far surpass any reasonable limit in many streams. There is

no practicable treatment to remove chlorides, so the usual solution to the problem is

impoundment and carefully controlled release into streams that provide adequate

dilution. Disposal to the ocean is feasible in some locations and disposal to deep

underground formations in others, but chloride wastes remain one of the majorunsolved problems of industries. Chlorides may be removed by reverse osmosis or

electrodialysis; however, these techniques are not cost effective.

(xi) Hardness: All natural waters contain some degree of hardness, caused primarily

by calcium and magnesium. Hardness, in any degree, is expensive as it causes scale

formation, increases soap consumption and forms precipitates or scums. Industrialeffluents that cause hardness include calcium chloride brines from many chemical

industries and wastes from lime-consuming industries such as water treatment plants,pulp mills and tanneries.

(xii) Surfactants: Surface-active chemicals, including the synthetic detergents cause

problems in sewage treatment plants and in natural streams because of their ability to

form froth. The problem has been worsened because the conventional alkyl benzene

sulfonate surfactants are persistent, and refractory. Despite their organic composition,

they are broken down but slowly by natural stream organisms. The detergent

Acidity or alkalinity is

determined by titration to a

specified pH. More

commonly, the

phenolphthalein and methylorange endpoints,

approximately 8.3 and 4.3,

respectively, are used for

their determination. These

titrations indicate the

quantity of neutralising

material required for

treatment which may pre-

exist in the receiving stream

or may be added as a part of


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Industrial Effluentsmanufacturing industry is shifting to biodegradable types of detergents. Frothing may

continue until the surfactant properties are destroyed in natural stream environments.

Though unaesthetic, froth is not harmful and serves a useful purpose of indicating

domestic pollution.

(xiii) Other Solutes: Almost every chemical or substance used in industry escapes to

some degree with the wastewater. These may include raw materials, intermediates,

final products, by-products and processing substances. The most troublesome being

sulphates, nitrates, ammonium compounds and salts of heavy metals.

(xiv) Radioactivity: Radioactive wastes have great pollutional potential. Radioisotopes

are becoming more widely employed for industrial and other uses and the number of

their discharge points has increased. They are hazardous to health because they emit

high energy radiations. The danger increases when the water is consumed for drinking

or for food preparation. It may worsen if the dispersed radioactive pollutants become

concentrated by the chemical precipitation or the action of living organisms including

microorganisms. In general, radioactivity cannotbe destroyed, but radioactive wastes

can be either disposed of either by dilution to a degree that is safe, or by concentrating

and keeping in isolated, protected locations until they decay naturally. The choice of

technique depends on the volume and concentration of the waste and other factors.

(xv) Dissolved Oxygen (DO): The most generally accepted single criterion of pollution

is the DO content of the stream. Under favorable nonpolluted conditions, it may

approach the saturation value, or may even exceed saturation because of

photosynthesis or oxygen generation in the stream. The DO saturation value depends

on temperature. The reserve of available DO, for bio-oxidation of organic wastes, is

not large and is easily depleted. Of course, oxygen is restored to the stream waters by

photosynthesis, which is intermittent, and by absorption from the atmosphere, which is

limited in rate. An overpolluted stream can thus become totally devoid of DO, a

condition that is readily apparent by dead fish, bad appearance and offensive odours. If

about 2 mg L1

of DO can be maintained in all parts of the stream, nuisance conditions

can be avoided. A higher concentration, perhaps 4-6 mg L1, is necessary for a healthy

population.

(xvi) Biochemical Oxygen Demand (BOD): The major pollutional effect of organic

wastes in a stream is their consumption of DO under the influence of living

microorganisms. The rate and extent of oxygen depletion is customarily evaluated by

the biochemical oxygen demand (BOD) test. This is not a direct measure of organic

content, but is a measure of its most significant pollutional characteristic, the capacity

to consume oxygen. It is reported as milligrams of oxygen per litre.

(xvii) Chemical Oxygen Demand (COD): The BOD test is useful in many ways but

has the disadvantage that it requires long incubation time for completion (5 days at

20C or 3 days at 27C) . Organic matter in wastewater can be evaluated in a fewhours by the chemical oxygen demand or COD test. This does not duplicate the BODtest; it may or may not have a consistent ratio to the BOD. There is no fully acceptable

substitute for the BOD test, but the COD test has been a considerable substitute. Both

tests measure organic matter and certain types of organic compounds are resistant to

each of the two tests but not in the same manner.

After studying the parameters, let us now study the treatment methods.

11.2.2 Treatment Methods

The BOD test involves, first,

a measurement of the DOinitially present in a sample

of water containing the waste

under study. This may be a

sample from the polluted

stream or a batch of special

water to which a known

amount of waste has been

added. Other samples of

mixed water and waste are

then incubated, in the

presence of suitable

microorganisms and nutrients

and in an appropriate

environment, usually for 5days at 20C or 3 days at

27C. The remaining DO is

then measured, the amount of

DO consumed during the test

is computed, and the oxygen

requirements of the waste are

reported as BOD. If all the

DO is consumed during the

incubation period, no

quantitative report can be

made; however, waste can be

diluted to produce usable

data. There can be several

interferences in the BOD test.

The determination of DO

requires many precautions

such as avoiding changes

during handling of the sample.

The analysis must be

performed, or at least carried

to the point of chemical

fixation, at the sampling site.

The actual analysis involvesabsorption of the DO on

freshly precipitated

manganous hydroxide, release

of an equivalent concentration

of free iodine from iodide salt,

and titration of the iodine

with a standard sodium

thiosulphate solution.

Though complex, the whole

procedure has been


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Water

Industrial wastewaters are discharged from the plant premises to some receiving water

bodies which have to some degree capacity for assimilating pollution. The degree of

assimilation depends on the ratio of dilution available, downstream uses of the

waterand characteristics of the waste . It is reasonable and proper to utilise the

stream's assimilative powers to the full, short of impairing its usefulness and esthetic

quality. Such reasonable use for waste transport should not be considered as pollution.

A somewhat similar situation exists when industrial waste is discharged into a

municipal sewer. This is often a convenient means of disposal, but it requiresconsideration of the possible effects. Most municipalities ban industrial wastes that aredeleterious to the sewerage system, by corrosion of the sewers and plant equipment or

by interference with the sewage treatment plant and process. The industrial waste

contaminants may be destroyed by the sewage treatment, they may disrupt the

treatment or they may simply

flow unchanged through the sewerage system. In the last case, they are at least diluted

by the municipal wastewater. If the assimilative capabilities of the stream or of the

sewerage system are not adequate, the industrial waste should not be released in its

original form. Treatment of the waste before discharge is, therefore, essential. Severaltreatment methodologies are available: some are related to the processes of sewage

treatment while others are developed specifically for industrial wastes. These are

discussed below. It is often economical to eliminate or reduce the quantity of waste atits source in the manufacturing plant prior to treatment. Several techniques exist based

on engineering practices and common sense.

(1) In plant Measures

(i) Process Change: It is not unusual to change the manufacturing process in order to

ease the pollution problem. Several instances of this kind exist and it will probably

become more common as antipollution efforts continue. A related change, at great

cost, has been that of the synthetic detergents i.e. from hard detergents to

bio-degradable materials so that domestic laundering wastes shall be less troublesome.

Now cleaner technologies are being developed to minimise pollution.

(ii) Material Recovery: Raw materials and products are valuable to the manufacturer

but some fraction of each tends to be lost into the waste stream. This is an economic

loss and to avoid pollution, their removal also adds to the cost. There is an economic

gain if such substances can be reused in manufacturing or used for some otherpurpose. Several substances can be recovered from waste or intercepted before they

enter the waste stream. The recovery of by-products may also have merit and has

been profitable to some industries.

(iii) Water Reuse: Most industries use water, often in large quantities yet little or none

enters the final product. The most or all of it must be discharged as a pollution-bearing

waste. Water is costly, and water of quality suitable for industry becomes scarcer

every year. Sometimes, with only partial purification, spent water can be reused onceor several times in the manufacturing process. Spent cooling water is particularly

amenable to reuse but even contaminated wastewaters have salvage value. Water

unsuitable for direct reuse may be used for those purposes in which quality

requirements are less strict. Final rinsing of a product may require nearly pure water,

but the spent rinsewater may be usable for primary rinsing, or in a countercurrent

series of rinses without intermediate purification. The concept ofzero discharge has

now been introduced which means complete recycle and reuse of water.

The COD test is based on

treating the wastewater with

a known amount ofdichromate, digesting at an

elevated temperature to

oxidise the organic matter and

titrating the unconsumed

dichromate with standard

ferrous ammonium sulphate

using ferroin indicator. The

oxygen equivalent of the

dichromate destroyed is

reported as the COD.

Chlorides are the most

significant source of error,

due to their reaction with

dichromate however, a

correction for chlorides can be

made or chloride interference

could be eliminated by adding

mercuric sulphate to the


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Industrial Effluents(iv) Wastewater Collection: Usually the first step toward wastewater treatment is itscollection from the several sources in the plant and transportation to a treatment site.

Certain types of wastes should be kept separate until they reach the treatment plant or

even some advanced stage of treatment e.g. acid and cyanide wastes should be

segregated for the sake of safety. Strong and weak wastes, otherwise similar, are

often separated for more economical treatment, perhaps to be blended after they

become more or less similar in quality. On the other hand, the mixing of wastes may

provide partial treatment in itself, as by partial neutralisation between acid and alkaline

wastes. Mixing, too, provides equalisation or leveling of fluctuations in flow or incomposition. This leads to simpler treatment or better control of discharge. In general,

if treatment of discharge is facilitated by separate handling of certain wastes,

separation should be maintained, but only as long as there is real benefit.

(2) Particulate Removal

The removal of particulate or suspended matter from a wastewater is an important part

of its treatment because this is a less expensive operation than the chemical or

biological operations needed for dissolved matter removal. The most common

techniques being settling ,flotation andfiltration .

(i) Settling : Settling or sedimentation, is the least expensive physical treatment. Severalsettling operations may be utilised in the processing of a given wastewater. Heavy grits

are removed by passing the wastes through screens , however, finer suspended matter

originally present and precipitates or sludges formed by chemical or biological

treatment are removed by settling. Settling is accomplished in ordinary lagoons but

more commonly special tanks are built for the purpose. An inevitable by-product of

sedimentation is sludge. Removal of sludge from the settling basin may be intermittent

if the quantity is small or continuous (usually by scraper mechanisms and pumps) when

its volume is greater. Some heavy inorganic settled materials are solid or semisolid in

nature, but most sludges are fluid slurries that require further dewatering prior to

disposal. Disposal is often a problem, principally because of the land area required.

Particles lighter than water of course move upward and are removed by skimmers

rather than scrapers. Many oils fall under this category and are recoverable by settlingand skimming.

(ii) Flotation: Fine solids or particulate matter with a density less than or close to thatof water are not readily removable by simple settling. Their removal can be

accelerated by floating the particles with minute air bubbles. The bubbles and solids (or

oils) rise to the surface and are removed by skimming.

(iii) Filtration: It is generally more expensive than settling but it provides a better

removal. The filtration system takes less space and has certain special applications.

Sludges produced by settling can be further dewatered by filtering either on a sand bedor by mechanical equipment. Filtration of raw wastes is not a common practice

because of the large volume. Treated wastes are filtered as a final treatment step if thedischarge requirements are critical, as for effluents that are to be injected into porous

underground formations. Screening has some mechanical similarity to filtration, and is

almost universally used as an early step in industrial waste treatment. It removes

coarse materials that might interfere with subsequent operations in the treatment.

(3) Physical Treatment Methods

Heat treatment of wastes is expensive and unusual. It is sometimes employed, usually

on special wastes or wastes having small volume for emulsion breaking or oxidation of

organic matter.

Screens range from coarse

bar racks with 10 or 15 cm

openings to fine screens of

40 mesh or smaller. They

must be kept free fromsolids accumulations, lest

their head loss become

excessively high or their

throughput low.

The bubbles are created

either by pressurizing the

wastewater with air and

subsequently releasing the

pressure or by applying

vacuum to air-saturated

Temperature control,

usually by gentle heating,

is necessary in certain

biological treatments such

as anaerobic digestion of

organic sludges and the


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Water Incineration of dewatered sludges is common in municipal sewage treatment, and

may be utilised for organic industrial wastes. It is, commonly used for disposal of trash

and other solid wastes.

Cooling of hot wastes may be necessary before their discharge to a stream or sewer.

It is usually accomplished by spraying, cascading or simple holding. This is a problem of

considerable importance in the power industry. Radioactive wastes may be stored for

decay of radioactivity and above a certain minimum concentration, they requireelaborate cooling equipment to dissipate the heat energy liberated spontaneously.

(4) Adjustment of pH

Wastes of excessively low or high pH require neutralisation before their release. Some

plants, especially in the chemical industries, create both acid and alkali wastes, for

which blending provides partial neutralisation. For final pH adjustme nt, and sometimes

for the whole task of neutralisation, lime, soda ash or mineral acids are used.

Acidic wastes can be neutralised with various basic chemicals. Caustic soda and

sodium carbonate are somewhat expensive but extremely convenient. They are widely

used by small plants or for treatments where small quantities are adequate. Lime ischeaper but somewhat less convenient and is the most widely used alkaline reagent.

Alkaline wastes are less problematic than acid wastes, but nevertheless often require

treatment. If acidic waste streams are not available or are not adequate to neutralise

alkaline wastes, sulphuric acid is commonly employed. In some industries, carbon

dioxide in the form of fuel gas has been used to neutralise strong alkaline wastes.

Neutralisation can be controlled by automatic instruments, and it is often desirable to

record the pH of final effluent as it is discharged. Deliberate acidification wastestreams or alkalisation is sometimes used as a special form of treatment e.g., oil

emulsions may be broken by acidification. The pH must usually be restored close to

neutrality (pH 7) before final discharge.

(5) Oxidation and Reduction

The pollution characteristics of organic wastes and a few inorganic wastes can be

destroyed or minimised by oxidation. Oxidation can be done by one or more oxidising

reagents.

Aeration is the least costly means of oxidation, but has limited effectiveness. Fewindustrial wastes are adequately treated by aeration alone. Aeration is necessary in

some types of biological treatment, and aids chemical treatment with ferrous iron

coagulants. Oxygen is more effective than air, but is rarely used because of its cost.

High temperature high pressure oxidation with air is used in sewage sludge destruction

and for some industrial wastes.

Chlorination is the most common chemical oxidation method in industrial waste

treatment. Since chlorine is costly, it should be used on concentrated rather than dilutewastes, except for final disinfection, if needed. Hypochlorite salts and chlorine dioxide

are used in special circumstances.

Ozone is used for oxidising certain industrial wastes, notably phenols and cyanides.

Permanganates and dichromates also have specific applications. The only significant

application ofchemical reduction to treat an industrial waste is in the conversion of

Aerate: To permeate or

saturate a liquid with air.

Chlorination:

Application of chlorine to

water or wastewater,

generally for the purpose

of disinfection, but

frequently for

accomplishing other


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Industrial Effluentschromate wastes containing hexavalent chromium to trivalent chromium, which is more

easily precipitated.

(6) Chemical Precipitation

Chemicals are often added to an industrial waste for precipitating a particular

component. Metal ions, for example, are removed from solution by precipitation with

alkaline reagents, and then by settling or filtration. Similarly, other components can be

converted to volatile compounds by chemical treatment, and then removed in an air orgas stream.

Chemical coagulation involves the precipitation of chemicals deliberately added to the

wastewater in such a manner that the precipitate removes colloidal and finely dividedcontaminants. Iron salts, for example, are added to the waste, usually with lime, to form

a precipitate of ferric hydroxide. As this precipitates, it agglomerates to large flocculant

particles. On settling, they entrap fine particles that otherwise do not readily settle by

themselves. This technique is used in many industries, especially for organic colloidal

wastes from meat packing, canning and petroleum refining.

(7) Other Chemical Methods

Ion exchange is used to treat certain industrial wastes, especially if the contaminants

are few and noninterfering, and if the recovered material has salvage value. Dilute

rinsewaters from the metal processing industries can, in this manner, be recovered for

their metal values and for reusable water.Adsorption, usually by activated carbon,

often removes trace concentrations of particularly troublesome contaminants. Phenolic

wastes from coke production and plastic manufacture can be treated by activated

carbon to remove even fractions of a milligram per litre. Some adsorbed materials are

recoverable and the adsorbent regenerable.

(8) Aerobic Biological Treatment

Dilute organic wastes, difficult or impossible to treat by physical or chemical methods

alone, are often amenable to attack by microorganisms which may be aerobic (in the

presence of dissolved oxygen) or anaerobic (in the depleted state of dissolved

oxygen). Certain nutrient elements must be present, notably nitrogen and phosphorus.

The organic components are destroyed by metabolic processes, ending as innocuous

materials. In aerobic systems, much of the carbon is removed as carbon dioxide, the

hydrogen as water, and nitrogen and sulphur as nitrate ion or nitrogen gas and sulphate

ion, respectively. Some of the waste is converted to cell tissue in the microorganisms

and the excess appears as sludge. Some commonly used aerobic biological processes

are described below:

(i) Trickling Filters: It is a well-known technique for promoting contact among anindustrial waste, the microorganisms that will consume it and the oxygen needed for

metabolism. This is a bed of crushed rock or other suitable medium through which the

wastewater trickles and the air penetrates by natural or forced ventilation. Biological

growths develop as a film on the medium, from microorganisms which are present in

the wastewater or are added in the form of domestic sewage or other culture. As the

microorganisms grow, the organic wastes are consumed as food and metabolised to

simple and harmless compounds. Fragments of the film, containing dead and living

organisms, slough off from time to time and must be removed from the effluent, usually

by settling.


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Water A few organic wastes are not amenable to microbiological attack, but most compounds

can, in time, develop a biological community that consumes them. Toxic materials,

especially if they occur suddenly in the waste, interfere with or even kill the

microorganisms, resulting in failure of the process. Equalisation of the wastewater

before treatment helps to prevent such shock dosages. Nutrient elements, if not present

in the waste, can be added either as chemicals or as domestic or sanitary sewage.

(ii) Activated Sludge: This process brings the organic waste into contact withmicroorganisms in large aeration tanks, in which the bacteria grow as flocculent

suspended growths. Compressed air is supplied through diffuser plates or tubes as the

source of oxygen. Presettled organic waste flows continuously into the tank, for a

detention and aeration period of several hours. Mixed liquor suspended solids (MLSS)

overflows at the same rate and is settled. A portion of the floc is returned to the

aeration unit as seed.

This process often provides a smooth-running operation with excellent removal of

organic matter from the waste. Many faults, however, can develop in the process.

Shock loads of waste, abrupt variations in pH, temperature, other environmental factors

and the presence of toxic wastes are among the major causes of trouble. Nitrogen and

phosphorus must be present; if they do not occur in the waste, sanitary sewage fromthe plant can be used as a source. Uniform feed and operating conditions encourage

good activated sludge treatment.

(iii) Lagoons: Lagoons, holding ponds or oxidation ponds provide several types of

waste improvement including biological removal of organic wastes. Equalisation,sedimentation and opportunity for self-treatment inevitably occur. Chemicals can be

added, if desired, for further treatment. The biological processes in a lagoon should be

predominantly aerobic, for most satisfactory operation. Anaerobic conditions usually

prevail in the bottom muds, but if the main part of the lagoon becomes anaerobic, bad

odours and a poor degree of treatment result. Shallow lagoons and avoiding of overload

are helpful in maintaining desired level of dissolved oxygen. Sodium nitrate is used as a

substitute or supplement for DO but usually in emergency conditions only, to avoid

odours.

Lagoons are used by many industries if land is available at reasonable cost. They may

be suitable as sole treatment in some plants but are better employed as final or

polishing treatment of the waste before discharge. They require minimum maintenance,

but some control is desirable. For good DO control, they should be shallow i.e., 1-2 m

in depth and excessive plant growths should not be allowed. The banks should be kept

clear of vegetation other than grasses. Any sludge build up is to be removed which is

more common in inorganic settling lagoons. Lagoons are sometimes operated in series,

to minimise short-circuiting of flow.

(9) Anaerobic Biological Treatment

In some cases, anaerobic microorganisms provide an effective treatment. The bottom

layers of aerobic lagoons are usually anaerobic and may serve to decompose much

organic matter even without nuisance. It is used for treating many organic wastes.

Anaerobic treatment is widely used for strong wastes or waste fractions. As a rule of

thumb, a 1% concentration of organic matter has been suggested as the minimum

practical feed to an anaerobic process. This concentration is available in some organic

industry wastes, in most organic sludges and in many solid wastes such as garbage and

food wastes. Raw industrial wastes are sometimes treated by anaerobic fermentationof the whole waste in large holding tanks. The effluent is rarely suitable for discharge

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Industrial Effluentsbecause of bad odour and high residual organics but aerobic treatment may be

employed for polishing it for release.

Settling tank sludges are often further processed by anaerobic digestion . The

reduction in volume from raw waste to sludge makes this feasible. In digestion, much

of the organic matter is gasified to methane and carbon dioxide and the residual solids

or humus are not objectionable in their nature. Temperature and pH controls are

desirable for optimum digestion. The solids, after 1 to 2 months digestion, can be

disposed off by dewatering and incineration or other methods.

(10) Ultimate Disposal

The final disposal of adequately treated wastewater usually presents no special

problem. It may be simply discharged into the municipal sewer or a natural water

course, or into a ditch leading to the water course. Discharge to a river or lake may be

through multiple outlets, to hasten the dilution. In a few instances, river water is to be

pumped into the treatment plant to assure adequate dilution at the moment of

discharge.

Sludges are unavoidable by-products of wastewater conditioning and their disposal may

present more serious problems. Sludges contain most of the pollutants removed bytreatment plus the treatment chemicals. If suitable and adequate land areas are

available, sludges may be disposed off by secured landfill system. Reduction in sludge

volume eases its disposal and is a common practice. It includes dewatering by

thickening, filtration or drying. Further reductions are accomplished by digestion of

organic sludges, wet oxidation under high pressure and temperature, or incineration.

Complete destruction cannot be accomplished but disposal by dumping is usually

possible after volume reduction and removal of putrescibility and other obnoxious

features.

SAQ 1

What are physico-chemical parameters commonly used in the study of wastewater?

SAQ 2

Describe inplant measures to be adopted for pollution control.

SAQ 3

Briefly give various particulate removal methods.


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Water

11.3 EFFLUENTS FROM FOOD AND FOOD

PROCESSING INDUSTRIES DAIRY WASTES

The products processed by dairy include fluid products such as homogenised

pasteurized milkfor immediate consumption or nonfat dry milk powder, cheese and

other less perishable products that may be stored. These products may later be used

by the dairy industry or sold to the baking, candy and other industries for use as

ingredients of their products. Wastes from the dairy industry resemble other food

wastes but have particular problems of their susceptibility to biological attack.

11.3.1 Sources of Waste

Plants handling milk can be classified as receiving, bottling, condensing , dry milk

manufacturing, ice cream manufacturing ,cheese making and butter making.Other types of milk processing operations are of lesser importance.

Wastewaters from dairy plants fall into three categories: industrial waste, domestic

waste and spent uncontaminated waters. The last group includes water used in the

refrigeration system of condensing and precooling various pasteurized products. Itcontains no milk solids.Industrial wastes from dairy plants consist of various dilutions

of milk that enter the drainage system. These may be classified according to their

source as follows:

(i) Rinsewater and washwater from cans, tank trucks, equipment, product pipelines

and floors

(ii) Spillage, freeze-on, overflow and leakage caused by improperly maintained

equipment and poor operating practices

(iii) Entrainment from evaporators

(iv) By-products such as buttermilk, whey, and in some cases skim milk that cannot be

utilised

(v) Spoiled or damaged raw or manufactured products or by-products that cannot besalvaged for other uses

The volume of industrial waste discharged by a dairy plant depends mainly on the

availability of water and the care exercised in managing it. A dry floor plant, using

water conservation and waste saving practices, is the preferred method of operation

now used in most modern plants. By employing such procedures, savings can be

realised in the initial cost of water and in reduced cost of disposing of a smaller volume

of waste.

Let us now study about various methods which can be used to reduce the quantity of

wastewater.

11.3.2 Methods for Reducing Wastewater Quantity

(i) Clear Water Segregation: Much of the water used in a milk plant remains

uncontaminated after use. Condensing water and ammonia compressor jacket water

from the refrigeration system and cooling water from milk coolers, vat pasteurizers,


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Industrial Effluentsand air- conditioning system fall under this category. These waters should be collected

in a separate sewer and discharged directly to the receiving stream. These waters may

also be reused in the plant, sometimes saving steam when used in another process

requiring hot water.

(ii) Waste Prevention: Loss from leakage can be prevented by care in assembly of

equipment and by proper maintenance. Overflow may occur where product flows into

an open or vented piece of equipment. It can be prevented by the use of float operated

or electronic level controls that shut off the flow.

Spillage generally results either by careless handling or improper design/layout of

equipment. This loss occurs primarily where product is dumped from one vessel into

another. Proper care in the dumping process eliminates most of these losses.

Freeze-on may occur wherever product is in contact with a refrigerated surface, as in

a cooler or an ice cream freezer. Proper controls or the use of ice water prevent

freeze-on at coolers. Willful waste includes products such as whey, buttermilk and

separated nonfat milk that often have little or commercial value. These high BOD

products should never be allowed to enter the plant sewer system.

Residual waste is found in all plants. It consists of milk products that cling to thesurface of equipment or pipelines after they have been drained. It may be removed

from equipment and pipelines by draining after standing for a period of time, then

flushing with water and collecting the first of the rinsewater for salvage. Suc h residual

products can be disposed of by pasteurizing and returning to the farm for stock feed.

If care is not exercised in the operation of a vacuum pan, milk solids may become

entrained into the vapour and carried over into the condenser water. This ma y be

corrected by level controls and entrainment separators.

11.3.3 Treatment of Dairy Wastes

Dairy wastes are composed almost wholly of organic matter, and, therefore, are high in

oxygen demand. Milk is nature's most perfect food for bacterial and other microscopic

organisms also. Dairy wastes in a stream are, therefore, consumed at a very rapid rate,

causing depletion of oxygen and in some cases exhaustion resulting in evidence of

serious pollution.

Milk wastes also contain nitrogen and phosphorus, which are excellent plant nutrients.

In disposal by irrigation, this property has value in maintenance of a ground cover crop;

however, in stream the effect is detrimental. Algae and other aquatic plants are caused

to grow profusely and, upon dying the dead plants add taste and odour to the water.

These are often not removed by water treatment processes preparing water for

domestic use.

Settleable solids are not important in dairy wastes since all organic material is in

colloidal or dissolved state. Little or no BOD reduction can be effected by plain

sedimentation. Detention inprimary settling tank causes the rapid conversion of

lactose to lactic acid which precipitates the casein. The acid effluent then becomes

difficult to treat in the secondary process and the casein will not digest due to the high

acidity that has bactericidal qualities.

Although primary sedimentation is of little value in the treatment of dairy wastes, a grit

removal chamber is essential. Sand and other gritty material can damage pumps in the

treatment plant and interfere with the treatment processes.


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Water Another important element in dairy waste treatment is equalisation of flow. Volume

and BOD of the waste vary greatly over a 24-hour period. To apply the waste

uniformly to the treatment system, a balancing tank is necessary. This tank should be

of sufficient capacity to hold surges while the waste is pumped out at a uniform rate. It

is essential that wastes in this tank not become septic, so air is introduced through

diffusers or other means to keep the raw wastes aerobic.

The treatment systems most effective for dairy wastes provide sufficient oxygen tobiochemically oxidise the organic constituents. Treatment systems used extensivelytoday are aeration or modified activated sludge, trickling filters and irrigation.

Lagooning has also met with some success.

(i) Aeration: The heart of aeration treatment is the aeration tank. Aerobic

microorganisms are fed by organic material present in the fresh waste and are supplied

with oxygen taken from air, thus providing suitable environment and conditions for their

growth and reproduction. The diffused air produces constant agitation that prevents

sludge from settling and brings floc particles into intimate contact with the fresh wastes

entering the tank.

Treatment by oxidation has two distinct phases. The assimilation phase occurs as freshmilk wastes enter the aeration tank. During this phase, the bacteria rapidly consume

organic matter and require oxygen supply at a high rate for a short time. In the second

or indigenous phase, the bacteria receive no new food but digest the food ingested

during assimilation. This requires considerably less oxygen over a longer period of time.

Since autodigestion takes place continuously even during the assimilation phase, oxygen

for it must be supplied during the full time of treatment.

Aeration is associated with the following problems:

Foaming results by excess air applied during the indigenous phase where the demand

is only 10% of the assimilation phase. Reduction of the air supply approximately 3

hours after fresh waste addition may correct this condition.

Bulking may be caused by overloading because of insufficient time to complete the

oxidation or insufficient air to take care of the excess organic material. An effective

in-plant waste prevention program may overcome this problem.

An excess ratio of carbohydrate to protein may occur in wastes from ice cream

manufacture due to the added sugar. This becomes evident as a light sludge that is

difficult to settle. Nitrogen supplement helps correct this problem.

(ii) Tricking Filters: It is one of the first methods attempted for the treatment of dairy

wastes. Early experience indicates that strong dairy wastes applied to standard rate

filters causes ponding, thus making the filter inoperative. Dilution is necessary, either by

the addition of cooling and other clear waters, or by use of the high rate filter withrecirculation of effluent from the secondary clarifier to the plant influent.

Problems with trickling filter treatment arise, usually because the lactose rapidly

converts to lactic acid and reduces the pH of the holding or primary tank. This causes

the casein to precipitate and also inhibits biological activity. Thus, the filters clog and

natural biologicalfilm does not develop. This results in odour and poor effluent from the

treatment system. Aeration of the holding tank may improve operation of the filter,

eliminating ponding and odours, and improve over-all treatment plant efficiency.

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Industrial Effluents(iii) Irrigation: Where suitable land is available, disposal of dairy wastes by irrigationis feasible. It is often the most economical and least troublesome method.

Transpiration, evaporation, and, to a small degree, percolation take off the water.

Bacteria, naturally present in the earth, convert the organic constituents to carbon

dioxide, water and other products such as nitrates.

Since the bacteria are mostly of the aerobic type and require oxygen; therefore, it is

necessary to provide a period of rest to allow the soil to take in air. As the liquid

percolates through the soil it draws in air, making oxygen available for the aerobicprocess. If application of waste to the soil is continuous the available oxygen is

consumed, causing anaerobiosis and accompanying malodours. To prevent this

condition, irrigation piping should be moved every day or every second day.

(iv) Lagooning: Stabilisation lagoons are used extensively for treatment of domestic

wastes. These systems have also been used to treat combinations of dome stic and

dairy wastes. Aerobic lagoons or oxidation ponds are designed to have a large surface

area per unit volume. Oxygen is introduced into the liquid by natural or mechanicalaeration and by photosynthesis. BOD loading is important and must be controlled so

that oxygen demand of waste decomposition does not exceed the ability of the

reoxygenation processes to supply the necessary oxygen.

(v) Other Treatment Systems: Chemical precipitation has been used where lime is

used to raise the pH, and ferrous sulphate or other coagulating chemicals to cause

precipitation. The precipitate, however, removes only material in colloidal suspensionand does not alter the composition of the dissolved solids. The cost of chemicals, the

cost of labor to handle the chemicals, and the sludge make this process unattractive.

SAQ 4

What are sources of waste in a dairy plant?

SAQ 5

Describe in short aeration methods for the treatment of dairy waste.

11.4 EFFLUENT FROM PETROCHEMICAL

INDUSTRY

Petroleum industry operations, from production to marketing, result in wastewaters of

various compositions, but all are characterised by their content of oil. Other industries

including metal finishing, vegetable oil production, meat production, and organic

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Water

chemicals manufacture, also produce oil wastes and have utilised techniques developed

for petroleum industry.

11.4.1 The Petrochemicals Industry

The major units of petroleum industry are; (i) production (or mining) (ii) transportation,

(iii) refining and (iv) marketing. The special field of petrochemical manufacture, is alsoadded to these in which petroleum serves as the basic raw material. The petrochemical

products are innumerable comprising pharmaceuticals, polymers, fertilizers etc.

(i) Production: It involves recovery of petroleum from underground sources as crude

oil. Natural gas is obtained from the same or separate wells but its recovery and

purification are relatively free from wastewater problems. Smaller quantities of oil are

produced from oil shales, obtained by hard-rock mining, and recovered by distillation orother processes.

Wastes from production operations include muds lost during drilling operations; oily

losses during drilling and production and brines accompaning the crude oil. If adequate

stream flow is available, brines may be disposed of by dilution, after oil removal.

Discharge underground is a more common practice, and may be used to maintain

pressure on the producing well. Ponding for evaporation is sometimes feasible, but may

cause contamination of potable aquifers.

(ii) Transportation: It includes moving crude oil from production area to refinery and

distribution of finished products to market. Pollution problems arise primarily from

accidents and equipment failure. Accidents can be minimised and procedures for

salvaging or destroying spills should be known to all personnel involved. Water ballast

in empty oil tankers can be reasonably freed from oil before discharge to waterway.

Wastes from the cleaning of transportation equipment also require special treatment.

(iii) Refining : The major products of refining are fuels, lubricants, greases and

semiraw materials for the chemical and petrochemical industries. Modernisation has

improved the effectiveness of wastewater treatments. Expansion of refining capacity

has also reduced over-all water usage.

Water is used in refining operations primarily for indirect cooling and does not come

into contact with hydrocarbons. However, water used in washing stocks and steam

recovered as condensate from processing operations comprise process wastewater

and is usually contaminated.

(iv) Petrochemical Production: The following main processes convert petroleum

hydrocarbons into raw materials for chemical industries: (a) removal of hydrogen to

give unsaturated hydrocarbons (alkenes or aromatics), (b) oxidation and (c)

chlorination. Wastewaters from petrochemical works comprise surface run-off,

process water and cooling water. The problem of water-immiscible oils and solvents,possibly with high BOD and significant solid levels are common to both oil refineries

and petrochemical works.

(v) Marketing: The marketing of petroleum products may cause stream pollution by

spills or other losses during transportation and by minor losses at bulk stations and

individual retailers. The latter are difficult to control because of their small size, large

number, wide distribution and the fact that most discharges are to municipal seweragesystems and are not easily observed. Adequate control of wastewater prevents oily

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Industrial Effluentswastes from entering the sewer, as by collecting used crankcase oil and by providing

oil traps in the sewer to collect oils that enter despite the precautions taken.

11.4.2 Waste Characteristics

Wastewaters usually contain oil which may be separable, emulsified or dissolved

chemicals including acids, alkalis, sulphides, mercaptans, ammonia and phenols andsuspended solids.

The extent of recirculation and reuse of water in refineries depends upon its availability

having satisfactory quality. Increased recirculation and reuse of water results in

decrease in water intake and net effluent discharge, but paradoxically is accompanied

by an increase in actual consumptive use because of the resulting evaporation.

Second to cooling , the major use of water is for boiler feed. The resulting steam is

used for stripping and distillation and because the steam comes in contact with

petroleum products, the process condensate may be contaminated. Process

wastewater is usually defined as any water or steam condensate that has been in

direct contact with oil and can therefore contain oil or chemical contaminants. This

includes spent caustic and acid solutions, product and crude washwater, crude desalting

water condensate from steam stripping, distillation and steam cleaning or regenerationof catalysts and clays.

Separable and emulsified oils are found in wastewater sources. Sulphides occur in

crude desalting water and from gasoline condensate receivers at distillation and

cracking units. Phenolic wastes are found in condensate waters from catalytic

cracking, in gasoline washwaters following caustic treating, and from lube oil and

solvent production processes utilising phenols.

Many oxidisable solutes present in refinery wastewaters contribute to the over-alloxygen demand. Taste and odour in wastewaters are caused by phenolic, naphthenic,

nitrogenous and organic sulphur compounds. They originate from treating operations

for removal of sulphur, nitrogen, and oxygen compounds from crude and distillate

products, decomposition products from distillation and catalytic cracking, barometric

condenser water and crude desalting washwater.

11.4.3 Waste Disposal /Treatment

(i) In-Plant Procedures : They are procedures which include a number of steps related

to waste control. These involve specific operations and specific equipment for waste

control in the most effective and least expensive manner. They are used to prevent

entry to the sewer system of substances that would complicate the final wastetreatment operation. Other in-plant procedures involve recovery of raw materials,

intermediates and final products within a particular unit area; reuse of water in cooling

towers; reuse of a chemical in a series of treatment steps in which initial use requires

the highest concentration and successive uses are met by lower strength; andtreatment procedures that condition a waste for discharge to the sewer or remove it

from the system for special or selective disposition.

Oil is usually collected from a drainage system at each refining unit, in a number ofsumps conveniently located near the unit. Wastewater from the sumps is usually

collected in a closed system that discharges to the refinery main sewers, from which

the oil is collected in master separators.

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Water

Cooling tower is the most important of the reuse systems which may increase the

consumptive uses of water, but reduces the amount of water taken from the raw water

supply. Thus, it reduces the volume of wastewater to be treated. The cooling tower

operates as a closed system; hence is effective in reducing oil losses to refinery sewer.

Caustic soda is used in the treatment of many crude stocks to remove hydrogen sulfide

and mercaptans. Incidental to this treatment is the formation of caustic cresylates.

Other stock treating operations employ caustic cresylates to accomplish sweetening.

There is usually an accumulation of caustic cresylate to be discarded from the system.

(ii) Stripping and Oxidation: In the distillation of crude oil and in catalytic cracking,

sour condensate waters are produced which contain varying concentrations of

sulphides, mercaptans, ammonia and phenolics depending upon the type of crude oil,the cracking conditions, and the amount of steam used. The most common method of

treating sour condensate water is by steam stripping in a tower. Sulphides,

mercaptans, and ammonia leave the tower as overhead gases, and the major part of

the phenolics is drained to the sewer system in the tower bottoms. Distribution of the

components between overhead and bottoms is controlled by design and manner of

operation. Hydrogen sulphide and mercaptans are relatively easy to strip. Complete

ammonia removal is difficult, and requires large amounts of steam. Some refiners use a

combination of fuel, gas and steam for stripping these waters.

Oxidation of sour water in towers is done by leading it into the bottom of the towerwith sufficient air to oxidise the sulphur compounds and ammonia by exothermic

reaction. Usually the sulphur compounds are oxidised to thiosulphate and the ammonia

to water and nitrogen; phenolics are not affected. The degree of oxidation is controlled

by the contact time provided in the tower. Oxidation to sulphate can be done by using

higher pressure and extended contact time. Spent caustic solutions containing sulphides

and mercaptides are also oxidised in towers; in some instances, along with the sour

waters.

Fig 11.1: Steam Distillation Plant

(iii) Combustion: It is used to a limited extent for selected wastes. Hydrofluoric acid

alkylation and spent aluminum chloride sludges are sometimes burned in special

incinerators. Combustion of spent caustic solutions, including sulfide, mercaptide and

cresol types has found limited application because of air pollution, corrosion of the

incinerator and difficulties of handling the slag which is mostly sodium carbonate,

formed in the combustion chamber.

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Industrial EffluentsIn some refineries, where petrochemical manufacture is a part of the operations, spent

or waste solvents are incinerated as a means of disposal. However, chlorinated

hydrocarbon disposal by this means requires absorption towers for removing the

hydrogen chloride formed. The small size of this operation may permit discharge of the

absorbed acid to the sewer system or permit the expense of neutralisation. The burning

of oily sludges and salt- bearing acid sludges in power station furnaces has been

practiced in the past and is feasible where oil or coal is used as fuel. Air pollution

control, however, limits this practice.

(iv) Solvent Extraction: It finds limited use in an oil refinery and is sometimes used on

phenol-bearing wastes such as caustic cresylates and sour water stripper bottoms.

Control of the wastewater pH is required to put the phenolic material in the acid form.

The cresol-bearing solvent is recovered by extraction with strong caustic solution to

produce the marketable product.

(v) Housekeeping : Good housekeeping includes repair of leaks, avoidance of oil

discharge with the necessary elimination of water from process units, proper disposition

of solids and spent chemicals, careful handling of emulsions, an awareness by all

personnel that waste treatment begins at the process unit. Sampling lines should be

installed in a manner to allow collection at the unit of line flushings.

(vi) Terminal Treatment: Oil refineries usually employ separate sewer systems foroily water or process waste, spent cooling water, storm drainage, and sanitary sewage.

Sanitary sewage is usually collected in a separate system and discharged to city

sewers. If municipal facilities are not available, septic tanks are employed, with the

overflow going to drainage fields. If secondary treatment is used for the refinery

effluent, the overflow from sanitary sewage septic tanks is discharged to the nearest oil

water sewer.

(vii) Gravity Separators: Gravity separation is the final treatment step for oil refinery

wastewaters. For process wastewaters or the oily water sewer system an oil-waterseparator designed by American Petroleum Institute and known as API separator is

commonly used. A longitudinal section of API separator is shown in Fig. 11.2.

Fig. 11.2: American Petroleum Institute (API) Separator

This is a gravity separator in which oil accumulates on the surface and heavy solids

settle to the bottom. Oil skimming and bottom sludge removal are required periodically.

Storm drainage and spent cooling water are frequently routed through separators of

this type. Gravity separation may provide the total treatment needed for simple

refineries, however, where there is a complex of operations involving lube oils and

petrochemicals, additional treatment may be required. It may consist ofchemical

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Water

flocculation , air flotation with or without chemicals, or biological oxidation by

means oftrickling filters, activated sludge orponding for an extended time.

(viii) Chemical Flocculation: It consists of the addition of a reagent to a wastewater

to form a precipitate removable by settling. Aluminium sulphate, ferric chloride and

aluminium chloride are the most common reagents used. Sometimes, substancespresent in the wastewater, such as calcium bicarbonate and magnesium carbonate may

be made to form precipitates and serve as flocculation agents. When the pH of the

wastewater is controlled within the proper range, hydrated reaction products of theflocculation agents result; these relatively insoluble compounds, initially present as

colloids, agglomerate as flocs . During agglomeration, they become associated with

other colloidal and suspended matter. As the floc particles grow, their apparent density

increases and they settle, carrying with them whatever insoluble matter may have

become trapped during the growth period. Polyelectrolytes are frequently used to

hasten the flocculation process.

Chemical flocculation is effective in reducing the suspended matter content of

wastewater including insoluble matter in a finely divided state. Oil content can be

reduced to its solubility level. Some BOD is removed but the removal is limited to

oxygen-demanding substances present initially as colloids or other particulates.

(ix) Coalescence: Coalescence de -oilers have been developed to get away from

chemical based wastewater treatment systems. The coalescer works by aphysico-chemical action of drawing tiny oil droplets together, separation of discrete

larger droplets within the coalescing media and production of large oil droplets which

are collected at the top of the vessel. An outline of coalsecnce de -oiler is given in Fig.

11.3. The plant normally works in upflow mode.

Fig. 11.3: Coalesence De-Oiler

(x) Air Flotation: In this process, wastewater is saturated, usually under pressure with

a gas such as air and is then released to a vessel at atmospheric or reduced pressure.

The supersaturation is relieved by formation of tiny bubbles which while forming and

rising through the water attach themselves to particulate matter and take it to the

surface, from where it can be skimmed. Air flotation, with or without chemicals, can

yield a greatly improved effluent in appearance and oil content. It can serve as the finaltreatment for refinery effluent whe re oxygen demand requirements are not important.

(xi) Biological Oxidation: It is used extensively for treating refinery wastewater.

Both trickling filters and activated sludge have been adapted to the treatment of

selected waste streams and to total effluents. Bio-oxidation ponds also are widely used.

Biotreatment has been limited to selected streams which contain phenolic compounds.

With the decline of water usage, refinery effluents become of poorer quality and the

need for treatment is greater, even though the absolute amount of polluting materials

may be less than it was when effluent volumes were greater.

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Industrial EffluentsA clear-cut basis is lacking for selection of a trickling filter or activated sludge unit for

treating the total effluent from a refinery. Both units can give approximately equal

performances. Installation costs are usually greater for a trickling filter but it is

considered to withstand the effects of overloads and toxic materials better than

activated sludge.

(xii) Chemical Oxidation: It has limited use in the treatment of refinery wastes,

however, it is employed in towers of spent caustic solutions containing sulphides and

mercaptides and for sour condensate. The operation is carried out as a continuousprocess, with solution and air fed to the bottom of the tower. The reaction is

exothermic and provides heat once it is started. Sulphur compounds are converted to

thiosulphates. Temperature and pressure vary with composition of the waste and

degree of oxidation desired.

Oxidation by ozonation or chlorination is seldom used because of costs. Use of theseagents on biotreated effluents offers a possible tertiary treatment in extreme conditions.

This could be justified only for taste and odour compounds that are refractory to the

usual biomethods. However, the taste and odour compounds are among the last

components in the mixture to react with these oxidising agents. The relatively large

amount of oxidant consumed in oxidising residual BOD, plus that required to establish

mass action conditions needed to effect oxidation of the taste and odour constituents,

makes the process prohibitively expensive.

(xiii) Final Waste Water Treatment: It involves removal of residual suspended solids

to achieve still lower BOD and suspended solids levels. It is commonly done by using

pressure filter. The problem is more likely to lie with residual COD arising from

dissolved organic compounds which resist biological treatment. These compounds are

removed by using activated carbon as a tertiary treatment method. Powdered activated

carbon is added to the aeration basin in the activated sludge process. The toxic organic

compounds get adsorbed and are removed.

(xiv) Underground Disposal: Deep well disposal is being increasingly used for

selected industrial wastes. It has be en used for brine disposal in oil field regions for

many years. At the refinery, wells can be employed to dispose of sour condensate

streams, spent caustic solutions and spent acids.

Waste solutions charged to the well must be free from suspended matter in order to

avoid plugging the sidewell of the borehole. Fresh water or acid may be injected into

the well initially, to establish a front or barrier between the waste and the original

formation water. This prevents reactions between components of the waste and the

underground water which would give precipitates and plug the formation.

(xv) Dilution: It offers only limited possibilities for meeting the oxygen demands of a

waste and has considerable capacity for accepting a waste containing high dissolved

solids. The characteristics of the waste and the properties of the waters into which the

waste is to be discharged must be carefully weighed when treatment requirements arebeing balanced against dilution potentialities.

(xvi) Evaporation: It can be an important asset in arid regions where ponds without

outlets i.e., seepage and evaporation ponds, are used.

(xvii) Sludge Disposal: It is one of the most difficult problems in the treatment of

refinery wastewaters. Sludges are derived directly from waste treatment and they also

originate in oil processing operations. Segregation of sludges minimises difficulties and

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Water

expenses in ultimate disposition. Sludges can be considered under the following four

general types: oily , oil-free, chemical and biotreatment.

(a) Oily Sludges: These are derived from oil-water separators, tank bottoms or

cleanings, air flotation treatment of wastewaters, and cleaning or dredging from

lagoons or oxidation ponds. They consist of slurries of oily solids in water. If final

disposition is to be in some remote area, thickening may be required to minimise

transportation costs. Centrifuging may be used to accomplish a higher degree of water

removal, after thickening of the sludge to 10-15% oily solids to produce a satisfactorycentrifuge feed. The dewatered and relatively oil-free solid can be disposed of by

incineration or used as landfill.

(b) Oil-free Sludges: Solids from water conditioning may consist of silt, calcium

carbonate, magnesium hydroxide and minor amounts of organic matter along with

precipitated treating agents. The composition varies with the water source. They

should not be discharged to the oily water sewer system because they can promote

emulsification and inhibit oil-water separation. These solids are thickened anddewatered by centrifuging which are then suitable for landfill.

(c) Chemical Sludges: A variety of chemical sludges result from oil refining

operations. Spent aluminum chloride complex, sulphuric acid alkylation sludge, calcium

fluoride from HF alkylation, and acid sludge from the treating of lube oil stocks requirespecial handling. Sludges from lube oil stock treating and sulphuric acid alkylation are

treated for acid recovery in the refinery, or sold to acid manufacturers. Spent aluminum

chloride complex has been disposed of by buried with crushed lime stone or hydrolyingand using as a source of aluminum in chemical flocculation. Where possible, such

sludge is simply drowned to avoid hydrochloric acid liberation and sent to a pit

containing alkaline waste in which the aluminum hydroxide is allowed to accumulate.

Calcium fluoride sludge is an end product from treating HF alkylation plant

wastewater. It is removed from the final settler as a slurry, drained free from water

and buried.

(d) Biotreatment Sludge: Sludges from the treatment of refinery wastes by trickling

filters or the activated sludge process can be digested anaerobically in the samemanner as sanitary sewage sludge. Sludges from oxidation ponds and aeration basins,

however, contain considerable oily matter and respond poorly to anaerobic digestion.

Disposition of these sludges can be accomplished by dewatering, centrifuging and

incinerating; spreading in a suitable area for complete drying or using as landfill.

SAQ 6

What are the major contaminants of wastewater from refineries and petrochemical

industries?

SAQ 7

What is an API oil separator?


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Industrial Effluents

11.5 EFFLUENT FROM TEXTILES INDUSTRY

Textile manufacturing operations are among the major industrial water users. This

industry has the wastes most difficult to treat satisfactorily. Textile wastes are variable

in character which makes their treatment a complex problem. In a modern textile mill,

many compounds produced by other industries are used. Synthetic yarns, dyes and

finishes from the chemical industries and sizes from the food-producing industries are

among these substances. Some of these compounds are added onto the basic fiber and

then partially or wholly washed off as pollutants. These compounds may be organic or

inorganic in nature. Inorganic materials may render water unsuitable for use because

of excess concentrations of soluble salts, and because insoluble salts precipitate anddeposit on stream bottoms, blanketing aquatic life and even clogging streams. The

organic compounds may undergo a gradual chemical or biological change which

removes oxygen from the water, resulting in a septic condition characterised by odours,

gases, floating solids and a generally disagreeable appearance. Metal salts may be

toxic to animals, fish and other aquatic life.

11.5.1 The Textile Industry

All fibers used in the textile industry are either natural or manufactured. The natural

fibers can be further subdivided into animal and vegetable fibers and the

manufactured fibers are divided into regeneratedand synthetic fibers. The principal

fibers of commercial significance are given in Table 11.1.

Table 11.1: Fibers of Commercial Significance

Natural Manufactured

Animal Vegetable Regenerated Synthetic

Wool

Silk

Hair

Cotton

Flax

Hemp

Rayon

Soyabean

Casein

Polyamide

Polyacrylic

Polyester

(i) Animal Fibers

Wool contains large quantities of dirt, grass, burrs, dried sheep perspiration and wool

grease discharged from glands to protect the fiber during growth of the animal. In

extreme cases, it may contain as little as 30% fiber, with 70% foreign matter, and may

be as high as 45% grease. Wastes are contributed by the scouring, carbonising,

bleaching, dyeing andfinishing operations.

The foreign matter may be removed by scouring in warm water with soap and alkali.

The amount of soap present increases the BOD of the waste. Currently, soap has been

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Water

replaced by synthetic detergents with an extremely low BOD. Consequently, BOD of

the discharge can be attributed almost exclusively to the raw wool itself.

Wool scouring wastes would interfere with municipal sewage treatment system

because of their grease content and the broken fibers. When soap and soda ash were

used as the detergent materials, the liquors were treated with concentrated sulphuric

acid to separate the grease. This was drawn off and purified for other uses. With

synthetic detergents, this method is not always feasible and the centrifuge is frequently

used to remove fibers and excess grease. Centrifuging generally produces a betterquality of grease, suitable for use in cosmetics, but the recovery is about one -third ofthe grease present, whereas the acid-cracking method gives almost quantitative yields.

Wool grease has been recovered by treating waste liquor with calcium hypochlorite,

reducing the pH to about 7.5, discarding the clear liquors and treating the scum and

sludge with sulphuric acid. Solvent scouring of wool has been introduced for recovering

the grease and hence reducing the pollution load on the stream.

Scoured wool may be dyed. Wastes from a dyeing and finishing process arecontributed by the spent liquors and subsequent washing of the wool after bleaching,

dyeing or other finishing. It is usually not practical to separate rinsewaters from the

stronger wastes. These wastes vary greatly in strength because of the amount ofwater used and various types of dyes and other chemicals. Waste from wool dyeing

and finishing has been treated by chemical precipitation with iron or aluminum salts.

Usually, it is better to use biological treatment with either the trickling filter or an

aeration process.

(ii) Vegetable Fibers

Cotton has been the mainstay of the vegetable fiber industries and is converted to

finished fibre by the following operations:

(a) Slashing : Before yarn can be woven into fabric, it must be strengthened by loading

with starch or other sizing substances. This operation is called "Slashing" and littleliquid waste results. Substitution of sizes with low BOD values such as carboxymethyl

cellulose in place of starch reduces BOD in the final effluent.

(b) Scouring and Sizing: It is necessary to remove certain natural ingredients and the

sizing materials from slashing operations to prepare cloth for dyeing and finishing. This

may contribute about 50% of the total pollution load in textile processing. Acids and

enzymes are used to hydrolyse the starch. Caustic soda, soda ash, chlorine, peroxides,silicates, sodium bisulphite, acids, detergents and penetrants are used in scouring to

prepare a clean, white cloth for finishing. Scouring may contribute up to 30% of the

total waste load. Natural impurities in cotton contribute about 3% of the total BOD in

the final waste.

(c) Bleaching: Bleaching operations use chlorine or peroxide to remove natural

colouring matters which contributes about 10% to the pollution load.

(d) Mercerizing: It consists of passing cloth through 20% caustic soda solution. It

contributes negligible BOD but a high degree of alkalinity.

(e) Dyeing: It is done in many ways and new dyes and auxiliary chemicals add to the

complexity of the operation. The pollution load may be 20-40% but volume is large and

there is a high degree of color.

The cloth at slashing stage is

"creige," and may be sold as

such. Subsequent operations

are considered as finishing but

include preparatory

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Industrial Effluents

These dye wastes are difficult to treat in biological processes because of high air

requirements to oxidise the sulphur dye molecule and because of the toxicity of

chromate. Chromium removal is, therefore, carried out using ferrous salt to bring down

chromium content below 3 mg L1

in the waste going to conventional biotreatment

plants and below 10 mg L1

in that going to bio-aeration lagoons or prolonged activated

sludge treatment processes.

(f) Finishing : Finishing operations impart the desired feel and appearance, and better

Industrial Effluents

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