WASTEWATER TREATMENT

Dr. Prakash D. Vaidya

V. V. Mariwala Lecturer in Chemical Engineering

Institute of Chemical Technology, Mumbai

Tel.: 022-24145616; Email: pdv@udct.org

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Introduction and basic concepts

Industrial wastewater treatment techniques

Illustrating examples

Lecture Content

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Introduction and Basic Concepts

Part I

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Water, adversely affected in quality by anthropogenic

influence (e.g., sewer outfalls, industrial discharges,

agricultural or urban runoffs), is defined as wastewater

Wastewater Constituents:

Oxygen-demanding wastes, disease-causing agents,

organic compounds, inorganic chemicals and minerals,

plant nutrients, sediments, radioactive susbtances,

thermal discharges and oil

Wastewater

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Wastewater can be classified as

Domestic Wastewater

It is discharged from residential/commercial establishments

Industrial Wastewater

It is discharged from manufacturing plants

Wastewaters are also classified as strong, medium or weak,

depending upon the amounts of physical, chemical and

biological constituents

Wastewater (ctd.)

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Treatment of following industrial wastewaters is essential:

Chemical Engineering

Petrochemical Metallurgy

Textiles Laundry

Steel Agriculture

Paper Making Dairy

Food Processing Tanning

Coke Ovens Industrial Oil Production

Industrial Wastewater

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Major pollutants in industrial wastewaters are:

ORGANIC INORGANIC

Proteins Acids

Carbohydrates Alkalies

Fats Metals

Oils Salts

Dyestuffs Phosphates

Organic acids Nitrates

Phenols Sulfides

Detergents Cyanides

Organo-pesticides Minerals

Industrial Wastewater (ctd.)

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Important wastewater characteristics:

Biochemical Oxygen Demand (BOD)

Chemical Oxygen Demand (COD)

Total Organic carbon (TOC)

Theoretical Oxygen Demand (TOD)

Other important parameters are pH, total solids

(dissolved and suspended), total nitrogen, total

phosphorus, chlorides and total metal content

Wastewater Characteristics

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BOD is the amount of oxygen required by

microorganisms to biologically degrade the waste

It is a direct measure of oxygen requirement and an

indirect measure of biodegradable organic matter

It is expressed in terms of the BOD5 value

Biochemical Oxygen Demand

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Biochemical Oxygen Demand (ctd.)

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BOD5 (mg/L) :

…….d is dilution factor

…….DO is dissolved oxygen

Limitations of BOD Test:

Nitrogenous nutrients may create problems

Toxic materials present in industrial wastewaters may

interfere with the growth of microorganisms

The presence of algae in wastewater may lead to

higher BOD values

Biochemical Oxygen Demand (ctd.)

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COD is the amount of oxygen required to chemically

oxidize the wastes

The oxidizing bacteria of the BOD test are replaced

here by a strong oxidizing agent under acidic conditions

It is a measure of the total oxidizable organic material

in the sample

Chemical Oxygen Demand

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Chemical Oxygen Demand (ctd.)

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CaHbNcOd + (a + b/4 – d/2) O2 CO2 + H2O + N2

Advantages of COD Test:

Useful for quick estimation of oxygen requirements of

industrial wastewaters

Useful when BOD test is not applicable due to the

presence of toxic substances or low rate of oxidation

BOD / COD ratio gives an indication of the degree of

biotreatability of the waste

Chemical Oxygen Demand (ctd.)

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TOC is based on the oxidation of carbon present in

organic matter to CO2, which is measured by a non-

dispersive infrared analyzer

Organic Carbon = Total Carbon – Inorganic Carbon

TOC value can be quickly estimated when compared to

BOD and COD measurements

An empirical correlation between TOC and COD or

BOD can be developed for a specific plant operation

Total Organic Carbon

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Total Organic Carbon (ctd.)

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TOC is related to COD through a carbon-oxygen balance:

When the organic material is resistant to dichromate

oxidation,

COD/TOC = 0

TOD of wastewater is calculated as the oxygen

required to oxidize the organics to end products

TOD test measures organic carbon and unoxidized

nitrogen and sulfur

For most organics (except some aromatics),

COD = TOD

Theoretical Oxygen Demand

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Theoretical Oxygen Demand (ctd.)

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Water Pollution Act (1974)

Water (Prevention and Control of Pollution)

Cess Act (1977)

Environmental (Protection) Act, 1986

Environmental (Protection) Rules, 1986

Water Pollution Laws and Standards

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Industrial Wastewater Treatment Techniques

Part II

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Wastewater Treatment Processes

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Physical

e.g., screening, sedimentation, flotation and filtration

Chemical

e.g., precipitation and coagulation

Biological

e.g., activated sludge process and trickling filters

Air Stripping, Carbon Adsorption, Oxidation and

Reduction, Ion Exchange, and Membrane Processes

are of significance too!

Treatment Processes (ctd.)

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Another classification is as follows:

Primary

Removal of suspended solids and floating matter

Secondary (or Biological)

Removal of soluble or colloidal organic matter

Tertiary (or Advanced)

Removal of soluble non-biodegradable organics

(e.g., surfactants) and dissolved inorganic salts

General Overview

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Primary Secondary Tertiary Polishing

Physical/ Biological Filtration/ Disinfection

Chemical Adsorption

Sludge Treatment

Clarifier

General Overview (ctd.)

Secondary Sludge

Primary Sludge

Clarifier

Raw Wastewater Influent

PRIMARY

DISINFECTION

Biological

Treatment

System

SECONDARY

Clean Wastewater Effluent

Discharge to Receiving Waters

Preliminary Residuals

(i.e., grit, rags, etc.)A

B

C

Wastewater

Treatment

Residuals

Biosolids

Processing

and Disposal

(e.g., attached-growth

Suspended-Growth,

Constructed Wetland, etc.)

Clarifier

PRELIMINARY

Usually to Landfill

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Treatment Processes (ctd.)

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Criteria for selection of a treatment process:

Wastewater characteristics

(e. g., type of pollutant, biodegradability , toxicity)

Required effluent quality

Costs and availability of land

Primary Treatment

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Primary treatment comprises:

Pretreatment

Sedimentation

Flotation

Neutralization

Coagulation

Pretreatment

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Large floating and suspended solids are removed

STEPS:

Screening

Manual or Mechanical

Contaminants removed during screening are

disposed by burial, incineration and grinding

A communitor may be used instead of the screens

Grit removal

Grit chambers remove inorganic grit (e.g., sand, gravel,

cinders, and pebbles)

Pretreatment (ctd.)

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Equalization

• It is done in a mixing basin to level out the hour-to-

hour variations in flows and concentrations

• Equalization basins may be designed to equalize

flow, concentrations or both

• Size and type of basin varies with the quantity of

waste and variability of the wastewater stream

Pretreatment (ctd.)

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Equalization Basins:

Qin = Qout

Qin variable Qout constant

Pretreatment (ctd.)

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Equalization is essential:

To prevent shock loading of biological systems

To provide adequate pH control and minimize chemical

requirement for neutralization

To minimize flow surges to physical-chemical treatment

systems

To distribute waste loads more evenly

Sedimentation

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Settleable solids are removed by gravitational settling

under quiescent conditions

Sludge formed at the bottom of the tank is removed as

underflow, whereas the clear liquid is removed as overflow

Sedimentation may be carried out in rectangular

horizontal flow, circular radial flow or vertical flow basins

Sedimentation (ctd.)

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TYPES:

Discrete settling

Flocculent settling

Zone settling

Flotation may be used instead of sedimentation

Flotation

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TYPES:

Dispersed air flotation

Dissolved air flotation

Without recycle

With recycle

Flotation (ctd.)

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Oil Separation

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• Free oil is floated to the surface of a tank and skimmed

off

• Emulsions of oily materials are broken (e.g., by

acidification or addition of lime) and they can be separated

by gravity, coagulation or air flotation

Neutralization

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Neutralization of industrial wastes containing acidic or

alkaline materials is essential

e.g., for biological treatment, pH between 6.5 and 8.5 is

essential for optimum biological activity

The degree of neutralization required depends upon the

causticity or acidity present in the waste

Neutralization (ctd.)

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TYPES

Mixing acidic and alkaline waste streams

Neutralization of acid wastes through limestone beds

Mixing acid wastes with lime slurries

Neutralization of alkaline wastes using strong acids

Coagulation

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It is used for the removal of suspended and colloidal

solids

Alum is the most popular coagulant used in wastewater

treatment

Wastes containing emulsified oils can be clarified by

coagulation too

Secondary Treatment

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In secondary treatment, organic substrate is converted

by microorganisms into CO2, H2O and new cells

Types of Microorganisms:

Aerobic (requiring free oxygen)

Anaerobic (not requiring free oxygen)

Facultative (growing with or without oxygen)

Anoxic (using bound oxygen, e.g., from NO3 for

denitrification)

Aerobic Processes

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Biodegradation of organic matter is

achieved by aerobic bacteria

TYPES:

Activated Sludge System

Trickling Filters

Rotating Biological Contactors

Activated Sludge Process

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Activated Sludge Process (ctd.)

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System Constituents:

Aeration tank

Clarifier

The process is reliable, suitable for handling large volumes

of wastewater, and provides a high degree of treatment

Activated Sludge Process (ctd.)

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PROCESS MODIFICATIONS:

Conventional system

Tapered aeration

Step aeration

Complete mix system

Contact stabilization

Pure oxygen system

Trickling Filters

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Trickling Filters (ctd.)

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Trickling Filters (ctd.)

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Waste is sprinkled over a bed packing coated with a

biological slime

Microorganisms convert organics into CO2 and NO3

The system has good adaptability to handle peak shock

loads and is easy to operate

Milk processing, paper mill and pharmaceutical wastes

are among those treated by trickling filters

TF vs. ASP

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Trickling filters

• Bacterial growth is fixed on the media

• All solids from the settler are wasted

• Less sensitive to shock loading

• Less effective in removing pathogens

• Low operating costs

Activated sludge system

• Bacterial growth is suspended as a dispersed floc

• Solids from the settler are partially recycled

• More sensitive to shock loadings

• More effective in removing pathogens

• High operating costs

Rotating Biological Contactors

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Rotating Biological Contactors (ctd.)

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Rotating Biological Contactors (ctd.)

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It consists of large-diameter plastic media mounted

on a horizontal shaft in a tank

A 1 to 4 mm layer of slime biomass is developed on

the media

As the contactor rotates, it carries a film of

wastewater through the air, resulting in oxygen and

nutrient transfer

Additional removal occurs as the contactor rotates

through the liquid in the tank

Biological Processes

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Sludge Treatment and Disposal

• Concentration – gravity thickening and flotation

• Digestion – aerobic, anaerobic, sludge lagoons

• Conditioning – chemical addition, heat treatment

• Dewatering – centrifuging, vacuum filtration, pressure

filtration, drying beds, heat drying

• Oxidation – incineration, wet air oxidation

• Ultimate sludge disposal

Anaerobic Processes

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Anaerobic decomposition involves the breakdown of

organic wastes into gas (CH4 and CO2) in the absence

of oxygen

Anaerobic Processes are used in the treatment of:

Meat packing wastewater

Pharmaceutical wastewater

Beet-sugar wastewater

Paper mill wastewater

Dairy wastewater, food-processing and brewery waste

Anaerobic Processes (ctd.)

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Fats Proteins

Carbohydrates

Acetic acid/

Propionic acid

CH4, CO2

H2O

H2O H2O

H2O Methane Bacteria

MECHANISM

Anaerobic Processes (ctd.)

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Types of Anaerobic Processes:

Anaerobic Contact Process

Anaerobic Filter Process

Up-flow Anaerobic Sludge Blanket (UASB)

Fluidized Bed Reactor

ADI-BVF Process

Factors affecting process operation are temperature,

pH and the presence of toxic metals, ions and

compounds

Aerobic vs. Anaerobic Processes

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Anaerobic processes:

Growth rate is slow

Yield of organisms is less

Removal rate of organics is less

Sludge yield is considerably less

Nutrient requirements are less

Tertiary Treatment

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TERTIARY TREATMENT TECHNIQUES

Filtration

• It is usually carried out using beds of porous media such

as sand or coal

• A mixed-media filter, graded coarse to fine in the

direction of water flow, may be used too

• It comprises fine garnet in the bottom layer, silica sand in

the middle layer and coarser coal in the top layer

Tertiary Treatment (ctd.)

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Chemical Oxidation

• Disinfection of wastewater

• Breakpoint Chlorination

Examples of chemical oxidants are chlorine and ozone

Tertiary Treatment

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Solvent Extraction

• e.g., Benzene is used as a solvent for the extraction

of phenol from wastewater

• e.g., Amines are used as extractants for the recovery

of metal cyanides from plating waste streams

Adsorption on Activated Carbon

DuPont’s powdered activated carbon process involves

direct addition of adsorbent into aeration tank of activated

sludge system

Tertiary Treatment (ctd.)

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Ion Exchange

Used for removal of water hardness and recovery of

trace metals from industrial wastes

Membrane Separation

• Reverse Osmosis

Used for desalting, separation of toxic ions from

plating wastes, concentration of radioactive wastes

• Electrodialysis

Advanced Oxidation Processes

• Wet Air Oxidation

• Fenton Oxidation

Wet Air Oxidation

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It involves sub-critical oxidation of organics and some

oxidizable inorganics in aqueous phase at high

temperatures (150 – 300 oC) and pressures (0.5 – 20 MPa)

Organic compounds are oxidized into CO2 and other

innocuous end products; nitrogen is converted into

ammonia, NO3 or elemental nitrogen; halogen and sulfur

are converted into inorganic halides and sulfates

It is suitable for treatment of substances that are resistant

to biological treatment. Energy required for this process is

much less than that required for incineration.

Wet Air Oxidation (ctd.)

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Schematic Diagram

Wet Air Oxidation (ctd.)

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LOPROX Process

Wet Air Oxidation (ctd.)

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Industrial Applications of Wet Oxidation:

• Wet oxidation of municipal sewage sludge

• Wet oxidation of alcohol distillery waste

• Treatment of pulp and paper mill effluent

• Treatment of cyanide, cyanate and nitrile wastewater

• Regeneration of spent carbon and spent earth

• Energy and resource generation

• Wet oxidation of phenol-bearing spent caustic

Fenton Oxidation

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Organic substrate is oxidized by H2O2 in presence of

homogeneous iron catalyst

MECHANISM

Advanced Fenton processes are UV-Fenton,

Photo-Fenton, Fenton-Ozonation and Fenton-

Biological Treatment

Fenton Oxidation (ctd.)

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Illustrating Examples

Part III

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Distillery Waste Treatment Options

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Distillery Waste (ctd.)

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Several processes (e.g., aerobic, anaerobic and

physico-chemical) have been used for treatment

Physico-chemical treatment has met with little

success, whereas anaerobic treatment with biogas

recovery is highly effective

An inverse anaerobic fluidization technology, which

enables 85 % COD reduction, is very attractive

according to Sowmeyan and Swaminathan (2008)

Nitrogenous Organic Pollutants

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Industrial waters polluted by nitrogenous organics:

Production of rubber additives (e.g., aniline)

Synthesis of dyes

TNT production

Acetonitrile production

Nitrogenous Organics (ctd.)

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Treatment of toxic nitrogenous organics (e.g., aniline,

nitrobenzene, nitrophenol and piperazine) by following

advanced oxidation processes is promising:

WET AIR OXIDATION

PHOTO-FENTON

UV

OZONATION

Recent Trends

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Wastewater reclamation

e.g., use of treated wastewater for municipal

purposes, recycle and reuse of treated effluents

Zero effluent discharge

Hybrid processes (e.g., MEMWO, SONIWO)

Membrane bioreactors

Rootzone technology

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