Review on

Wastewater treatment

for Dyestuff Industry

By Miss Aditi Patil

Vat Dyes:

These are water insoluble and fast dyes applied along with strong reducing agents (sodium hydro

D Y E S T U F F I N D U S T R Y W A S T E W A T E R T R E A T M E N T

The Dyestuff industry constitutes three sub-segments, namely

dyes, pigment and intermediates.

The dye intermediates are petroleum downstream products which are

further processed into finished dyes and pigments.

These are essential inputs in major industries like textiles,

plastics, paints, paper and printing inks.

Direct dyes:

In most of the small dyeing houses, direct dyes are used as they are easy to apply

and no auxillary chemicals such as mordants are needed.

Basic dyes:

This class of dyes give bright colours.

They are applied along with weak organic acids (such as tannic acid)

Sulfur dyes:

For dark colours, these dyes are employed.

These are sulfur compounds applied usually with sodium sulfide.

Effluent from this dyeing consists of considerable amount of sulfide.

Types of Dyes

Introduction

sulfite) and alkali to make the dye soluble. The cloth is then exposed to air for oxidation. The excess alkali remaining on the cloth is neutralized by scouring i.e treatment with dilute solution of acid. This types of dyeing generates more volume of effluents, as at the end of each step, the clothes are warmed

Naphthol dyes :

Beta-naphthol is first applied to the fabric, dried and treated with a developer for coupling and diazotization after which the colour is formed. This is followed be soaping and alkali treatment.

Developing dyes :

In this dyeing, the dyes are applied, dried and treated with sodium nitrite and acid and finally with

beta-naphthol. Effluents from this dyeing contain no. of chemicals.

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Dyes pH BOD Gallon wastes per 1000 lb goods

Aniline Black - 40 - 55 15,000 -23,000

Basic 6 -7.5 100 - 200 18,000 – 36,000

Developed colours 5 -10 75 -200 8,900 -25,000

Direct 6.5 - 7.6 220 - 600 1,700 – 6,400

Indigo 5 -10 90 - 1700 600 – 6,000

Naphthol 5 -10 15 - 675 2,300 – 16,800

Sulfur 8 - 10 125 -1,500

2,900 – 25,600

Vats 5 -10

125 – 1,500

1,000 – 20,000

Sr.No. Characteristics Results

1. Temperature 50º C

2. pH value 10.5

3. Phenolphthalein alkalinity (as CaCO3), mg/l 13600

4. Total alkalinity (as CaCO3), mg/l 16100

5. Total solids, mg/l 40000

6. Total Suspended solids, mg/l 25200

7. Total Dissolved Solids, mg/l 29800

8. Dissolved Fixed Solids, mg/l 24060

9. Permangnate value (4hrs), mg/l 376

10. Chemical Oxygen Demand, mg/l 1490

11. Chlorides (as Cl), mg/l 1800

12. Oils & Grease, mg/l 1800

Technologies/ Current Practices

Requirements

Effluent treatment comprising primary (physico-chemical) and secondary (biological) system is in practice. Some of the units have also provided tertiary treatment and incinerators for non-biodegradable waste.

Possibilities for adaptation of cleaner process options for reducing the water consumption and effluent generation; better management practices for segregation and reuse/recycle of the treated effluent; effective utilization of raw materials; improvement in efficiency of process; and recovery of by-products. The effluent generated from manufacturing of some of the dyes and intermediates such as H-acid is not biodegradable, which requires process sludge.

Gaseous emissions such as SO2, NO2, HCl, and NH3 are generally scrubbed.

Properly designed scrubber with recovery reuse of scrubbed liquid is required.

Gypsum, iron sludge and sludge from ETP are generated as solid waste. The gypsum and iron sludge can be used in the cement and pigment industries, The sludge is either disposed off on land/secured landfill or sent to other user industries.

Cleaner process technologies e.g. catalytic hydrogenation, use of spent acid after nitration for acidification of fusion mass, which can eliminate generation of iron and gypsum sludge.

Screening

Screening is the first treatment station, both for surface and wastewater. It's purpose to:

• Protect the structure downstream against large objects which could create obstructions in some of the facility's units,

• Easily separate and remove large matter carried along by the raw water, which might negatively affect the efficiency of later treatment procedures or make their implementation more difficult.

The efficiency of the screening operation depends of the spacing between screen bars:

• Fine screening, for a spacing under 10 mm • Medium screening, for spacing of 10 to 40 mm • Coarse screening, for spacing of over 40 mm

Usually the fine screening is preceded by a preliminary screening operation for purposes of protection.

Screening is carried to out by a manually cleaned bar screen (large in size, in order to reduce the frequency of screenings collection operations) or, preferably, by an automatically cleaned bar screen (essential in cases of high flow rates of for water with a high solids content). The automatic bar screen is usually protected by a sturdy preliminary bar screen, which should also be provided with an automatic cleaning systems in large facilities and in case of raw water containing a high volume of coarse matter.

To reduce manual operations as much as possible, screening procedures have become increasingly automated, even in small facilities. Automation is essential in situations where

P R I M A R Y T R E A T M E N T

� Screening

� Equalization

� Neutralization

� Chemical coagulation

large amounts of plant matter are carried by the water and arrive all at once at the bar screen, tending to mat the bars and completely clogging the screen in a few minutes. Fine screens must be automated.

The collected refuse is stored in a container of given capacity, calculated according the acceptable frequency of refuse disposal operations.

Usual spacing are:

• For surface waters, between 20 and 40 mm (upstream the strainer) • For municipal wastewater: for raw water, from 15 to 30 mm (but upstream from a

straining and/ or lamellae settling process, fine screening is necessary); for sludge (if necessary), 10 mm or less

• For some industrial effluents, especially agri-food effluents, fine bar screening ( or at times, medium screening followed by straining)

HYDRAULIC SIZING-CLOGGING :

Under normal circumstances, the crossing velocity through the bar screen should be sufficient for matter to attach itself to the screen without producing an excessive loss of head or a complete clogging of the bars, or allowing matter to be carried by the flow; normally acceptable crossing velocities between bars average between 0.6 and 1.0 m.s-1 and 1.2 to 1.4 m.s-1 at the maximum water flow.

These velocities apply to the area of the clogged bar screen that is still clear. The degree screen that is still clear. The degree of clogging depends on the water quality and on the system used to recover waste from the bar screen. For automatic bar screens it can be anywhere between 10% (surface water) and 30% wastewater with a high solids content). For manually cleaned bar screens, the area of immersed bar screen must be larger, so as to avoid frequent cleanings.

AUTOMATIC CONTROL AND THE PROTECTION OF BAR SCREENS

In general, the bar screens cleaning systems works on an intermittent basis. It can be controlled in three ways:

a) by a cyclic system of controllable frequencies ( 1 min to 1 h) and lengths of time ( 1 to 15 min)

b) By a differential head loss indicator

c) by a combination of both systems. When the bar screen is located downstream from a pumping station, the control mechanism can be linked to start-up of the pumps, with a built-in timer to keep the screen in operation for 1 to 30 minutes.

Automatic bar screen must be equipped with torque limiter to prevent equipment damage in case of overloading or blocking.

Normally, reciprocating cleaning bar screens, both curved and straight, include a device to ensure that the rake automatically stops moving at a point outside of the screen area, so as to avoid jamming upon restarting.

DIFFERENT TYPES OF SCREENS :

PARABOLIC SCREEN

CURVED SCREEN

INCLINED BAR SCREEN

Inclined bar screen

In operation, sewage flows though the inclined screen, approaching from the upstream side and after passing through the screen, departing on the downstream side. The screen is periodically raked by a mechanized comb system which is actuated either by level switches mounted in the channel upstream of the screen or by time clock. A doctor blade at the top of the comb travel removes the screenings collected by the moving combs.

The moving combs are suspended between two endless side chains, which are driven through a head shaft, and sprockets. The gear motor and moving comb system is protected from damage caused by jamming by a torque overload coupling with a micro switch.

The screenings removed from the doctor blade drop onto a skid plate, which transports the screening down to a container, belt conveyor or sluice pipe.

CURVED BAR SCREEN

This type 2 bar screens uses hydraulics to ensure a simple kinematical operation, you can see it in the following picture

1. Rigid frame with bar screen rack 2

2. Moving frame

3. Moving frame jack

4. Rake carriage

5. Lifting jack

6. Ejector

7. Electrical cabinet

8. Latticed covers

9. Hydraulic unit

In operation, sewage flows though the curved screen, approaching from the upstream side and after passing through the screen, departing on the downstream side. The screen is periodically raked by a mechanized comb which is actuated either by level switches mounted in the channel upstream of the screen or by time clock. The screenings collected by the moving comb are cleaned from the comb by a doctor blade at the top of the comb travel.

The screenings removed from the doctor blade drop onto a skid plate, which transports the screening down to a container, belt conveyor or sluice pipe.

RADIAL BAR SCREEN

Features and operation are similar to the curved bar screen. The drive mechanism is simpler for small installations and will remove a larger quantity of screenings due to two cleanings per revolution.

STEP SCREEN

Step screens have become popular with some clients due to their ability to remove smaller sized solids than the bar type screens. We have access to several makes of this type of machine.

BRUSH TYPE INCLINED SCREEN

These units are inclined screens with a rotary brush type belt cleansing system. This device is strongly favored by some clients instead of the step screen.

STATIC SCREEN

These devices are for smaller flows and require manual cleansing. They may be of the inclined bar type or the parabolic static sieve bend. This latter type can remove solids down to 0.5mm size.

SCREENINGS HANDLING

As a result of the increased volume of entrained sharp objects such as hypodermic syringes and the like, mechanical screenings handling equipment now has a higher profile in the process designers strategy.

Equipment such as dewatering processes, shaftless screen conveyors and bagging units are available through our company

The following chart shows the Upstream cleaning mechanical bar screens

Type of the bar screen

Cleaning operation

Depht of channel (m)

Widht of channel (m)

Bar spacing (mm)

Bar Thickness (mm)

Height of disposal (m)

Water depht (m)

Curved bar screen GDH

type

Contin. 0.75 to 1.75 0.5 to 1.6 10 to 40 10 0 0.50 to 1.5

Hydraulic staight bar

screen GDH type

Recipr. 0.75 to 2.80 0.6 to 1.2 10 to 40 10 0 to 1.2 0.50 to 1.5

Cable straight bar screen GDC type

Recipr. 2.00 to 10.0 0.1 to 2.6 10 to 40 10 0.65 and 1.2 1.5 to 9.5

Rack and pinion bar

screen

Recipr. 1.50 to 5.00 0.6 to 2.0 12 to 80 0.65 and 1.3

Medium screening

Grab bar screen

Recipr. 2.50 to 10.0 1.5 to 10 12 to 100

Fine curved bar screen GFC Type

Contin. 0.75 to 1.75 0.5 to 1.6 1 to 10 0 0.50 to 1.5

Fine straight bar screen GFD type

Contin. 2.0 to 10.0 1.0 to 2.6 1 to 10 0.85 and 1.2 1.5 to 9.5

Fine screening

Endless moving bar screen

Contin. 0.6 to 15.0 0.3 to 4.0 1 to 15 0 to 1.2 0.4 to 14.5

Flow Equalization

Flow equalization is the process of controlling hydraulic velocity, or flow rate, through a wastewater treatment system. The equalization of flow prevents short term, high volumes of incoming flow, called surges, from forcing solids and organic material out of the treatment process. Flow equalization also controls the flow through each stage of the treatment system, allowing adequate time for the physical, biological and chemical processes to take place.

When flow equalization is incorporated into a residential treatment system, numerous benefits are produced:

1. In the case of a septic tank or pretreatment tank, gravity separation of solids is greatly enhanced. This prevents short-circuiting and eliminates excess solids from being carried downstream into the secondary treatment facility or disposal system.

2. In the case of a secondary biological or chemical treatment system, elimination of hydraulic surges guarantees adequate process retention time and a much higher degree of treatment.

3. Clarifiers following secondary treatment will have greater solids separation and improved effluent quality. If a filtration device is used, solids loading to the filtration device will be reduced, resulting in longer filter life and higher effluent quality.

4. The operation of a downstream sand filter, media filter or constructed wetland is enhanced by more consistent loading, the equalization of surge flows and the removal of excess solids.

5. All types of effluent disposal systems, including tile fields, mounds, irrigation systems, etc., will operate longer and more efficiently because organic and hydraulic surges are eliminated and system overloading is prevented.

These benefits clearly demonstrate the important role flow equalization can play in wastewater treatment. Incorporating flow equalization into residential onsite treatment systems makes any system perform better and prevents premature failure

Neutralization

Dyes increase alkalinity of water which makes it unfit for aquatic life.

So Neutralization process is carried out.

Chemical coagulation

Clarifier haviing chemical coaulation process

Secondary treatment

� Trickling filter � Activated sludge process � Aerated lagoon � Oxidation pond � Oxidation ditch � Aerobic Degradation of Dyes � Anaerobic digestion � Biosorption

� Trickling filter

A trickling filter consists of a fixed bed of rocks, gravel, slag, polyurethane foam, sphagnum peat moss, or plastic media over which sewage or other wastewater flows downward and causes a layer or film of microbial slime to grow, covering the bed of media.filtered effluent is then passed through clarifier.

Aerated lagoon

Prolonged aeration to promote the biological oxidation

Oxidation pond

Main function of oxidation pond is lake aeration which increases oxygen saturation of water. Do is a major contributor to water quality. Fish and other aquatic animals require DO. Aerobic bacteria also needs oxygen to decompose organic matter. Therefore, pond bottoms containing organic matter demands more amount of oxygen.

Oxidation ditch

The oxidation ditch (OD) is a sort of equipment used for a long-term aeration. It consists of a long channel of an elliptical or circular shape equipped with an aeration equipment called a rotor for generating a water flow and stirring water in the channel to supply oxygen. Thought it requires a relatively large area, it has a simple structure and can be easily operated as well as being able to remove nitrogen easily. Thus, it has recently been widely used in relatively small wastewater treating plants.

Aerobic Degradation of Dyes

� Inefficient treatment � Resistance to biological oxidation � Poor adsorption of dyes

� Example : Three anionic dyes i.e CL reactive violet 15, reactive blue 19 and reactive red 5 were neither removed nor biodegraded by activated sewage sludge even after 20 days of incubation.

� Similar findings for sulphonated water soluble dyes.

Role of fungi and bacteria

Various aerobic fungi and bacteria are capable of aerobic oxidation of dyes. Majority of fungi belonging to white rot group degrade variety of dyes. Dye decolorizing activity of these fungi has been correlated to production of ligninolytic enzymes such as laccase, lignin peroxidase, manganese peroxidase and manganese independent peroxidase which show broad substrate specificity.

Examples of fungus

� One of the most studied fungus Phanerochaete chrysosporium has been shown to degrade large spectrum of azo, anthraquinone and triphenylmethane dyes, with decolorization efficiency of more than 90%.

� Other examples of white rot fungi degrading industrially relevant azo dyes are Geotrichum candidum, Trametes versicolor, T.modesia, T. pocas, Pleurotus ostreatus, Bjerkandera adustand.

Examples of Bacteria

� Streptomyces species and Flavobacterium ATCC 39723 � Extracellular peroxidases � Ability to degrade xenobiotic compounds including dyestuffs. � Several other bacteria such as Citrobacter sp., Kurthia sp.,

Corynebaterium and Mycobacterium sp., and mixed culture of Pseudomonas mendocina and P. alcaligenes degrade triphenylmethane dyes.

Controversy about aerobic degradation

� In many reports on the aerobic metabolism of azo dyes, the bacterial strains were grown on complex media aerobically and incubated under static conditions in the presence of azo dyes.

� These static cultures presumably become rapidly oxygen depleted and the reactions observed should be viewed as an anaerobic decolorization of dyes.

Anaerobic degradation of dyes

Under anaerobic conditions, many bacteria have been reported to readily decolorize azo dyes.

Baughman and Weber (1994) showed that in anoxic conditions azo dyes readily undergo biologically mediated reduction to the corresponding amines.

An upflow anaerobic fixed film bioreactor bone char as a support matrix and a cattle dung slurry as a source of anaerobic bacteria for decolorization of reactive dyestuff industrial effluent has been developed.

Average colour removal and COD removal efficiency was found to be 70% and 50% respectively at organic loading rate of 7.88 kg COD/m3/day.

The main advantage of fixed film bioreactor is that it allows the retention of active biomass in form of a biofilm attached to a support without recirculation of biomass or addition of fresh biomass during operation of the reactor, thus offering efficient mass transfer and waste stabilization.

Fed batch processes using Pseudomonas luteola was shown to effectively decolorize reactive 22 dye.

BIOSORPTION

A property of certain types of inactive, dead, microbial biomass to bind and concentrate heavy metals from even very dilute aqueous solutions.

Biomass exhibits this property, acting just as a chemical substance, as an ion exchanger of biological origin.

It is particularly the cell wall structure of certain algae, fungi and bacteria which was found responsible for this phenomenon.

These biomass types can accumulate in excess of 25% of their dry weight in deposited heavy metals: Pb, Cd, U, Cu, Zn, even Cr and others.

Research on biosorption is revealing that it is sometimes a complex phenomenon where the metallic species could be deposited in the solid biosorbent through different sorption processes of ion exchange, complexation, chelation, microprecipitation, etc.

Tertiary treatment

� Reverse osmosis � Electrodialysis � Ultrafiltration � Adsorption on powered activated carbon � Membrane filtration � Nanofiltration

� Reverse Osmosis

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Removal of bacteria, salts, sugars, proteins, particles, dyes, and other constituents that have a molecular weight of greater than 150-250 Daltons. The separation of ions with reverse osmosis is aided by charged particles. This means that dissolved ions that carry a charge, such as salts, are more likely to be rejected by the membrane than those that are not charged, such as organics.

Electrodialysis

� The ionic components (heavy metals) are separated through the use of semi-permeable ionselective membranes.

� Application of an electrical potential between the two electrodes causes a migration of cations and anions towards respective electrodes.

� Because of the alternate spacing of cation and anion permeable membranes, cells of concentrated and dilute salts are formed.

� The disadvantage is the formation of metal hydroxides, which clog the membrane.

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Ultrafiltration

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They are pressure driven membrane operations that use porous membranes for the removal of heavy metals. The main disadvantage of this process is the generation of sludge

Adsorption on powered activated carbon

The most commonly used method of dye removal by adsorption.

It is very effective for adsorbing cationic, mordant and acid dyes, and to a slightly lesser extent, dispersed, direct, vat, pigment and reactive dyes.

Performance is dependent on the type of carbon used and the characteristics of the wastewater.

Disadvantage: activated carbon is expensive; it has to be reactivated, which can result in 10-15% loss of sorbent.

Membrane filtration

Membrane filtration can clarify, concentrate and separate dye continuously from effluent.

Advantages compared to other methods are resistance to temperature, to an adverse chemical environment, and to microbial attack.

Disadvantages – disposal of the residue, high capital cost and the need for membrane replacement.

Nanofiltration

� Nanofiltration membranes are similar to reverse osmosis membranes in several respects except the degree of removal of monovalent ions such as chlorides etc.

� Reverse osmosis membranes provide 90 to 99% removal of ions while nanofiltration membrane are used for the selective removal of ions from 50 % to 90 %.

� It depends upon the material and manufacturing of the membrane. � Treatment of water from many surface supplies like wells, rivers or lakes.

Specific Tolerances for Dyestuff effluents

Sr.No. Characteristics Tolerance limits

1. pH value 5.5 to 9.0

2. Suspended solids,mg/l max. 100

3. Dissolved solids (inorganics), mg/l

2100

4. Zinc (as Zn) mg/l, max. 5

5. Colour Absent

6. Biochemical Oxygen Demand, mg/l, max.(5 days at 20 º C)

30

7. Chemical oxygen demand, mg/l, max

250