Indian Journal of Fibre & Textile Research Vol. 29, June 2004, pp. 239-259

Review Article

Colour removal from textile effluents

M Joshi', R Bansal & R Purwar

Department of Textile Technology, Indian Institute of Technology, New Delhi 110 016, India

Received 17 February 2003; accepted JO July 2003

The environmental issues associated with residual colour in textile efnuents have posed a major challenge to environmental scientists as well as the textile colouration processors. The requirements to remove colour from textile efnuent on si te prior to discharge to sewer have been progressively tightened due to increased public complaints about coloured watercourses. Dyes are highly dispersible aesthetic pollutants and are difficult to treat, as most dyes are highly stable molecules made to resist degradation by light, chemical, biological and other treatment or exposure. There has been a lot of research going in the past few decades to develop efficient and cost effective technologies to remove colour from textile efnuent. This paper presents a critical review of the current literature available on various textile wastewater decolourisation techniques being applied and researched to remove colour from textile wastewater.

Keywords: Colour removal, Dye separation, Textile effluent

fPC Code: Int. Cl.7 C08J 11/04; C02F 1/00; C02F 3/00

1 Introduction Wet processing operations during textile chemical

processing, i.e. desizing, scouring, bleaching, dyeing, printing and finishing, are the major causes of water pollution. A major contribution to colour in textile wastewater is usually the dyeing and the washing operation after dyeing during which as much as 50% of the dye might be released into the effluent. During dyeing, most of the dye is exhausted on the fibre, but the unfixed dye goes into wastewater causing deep colour. The wa<;tewater is extremely variable in composition due to large number of dyes and other chemicals used in processing. The problem is further made complex by the thousands of dyestuff commercially available. The use of different dyes depends upon the characteristics of the fibre, the specific colour to be applied and the desired finish required on the fibre. Although dyes constitute a small portion of the total volume of waste discharged in textile processing, colour removal from effluent is a major problem for textile industry because of several reasons: • The presence of even a small fraction of dyes in

water is highly visible due to high tinctorial value of dyes and affects the aesthetic merit of streams and other water resources.

"To whom all the correspondence should be addressed. Phone: 26596623; Fax: +91-11 -26581103; E-mail : man gala @textile.iitd.ernet.in

• Most dyes have complex aromatic structure resistant to light, biological activity, ozone and other degradative environments and hence not readily removed by typical waste treatment processes.

• The possible long-term effects of a few dyes and dye degradation products are becoming of increasing concern. The possible mutagenic, carcinogenic and/or allergenic effects of dyes have been established'·6. Over 90% of some 4000 dyes tested in an ET AD survey had LD50 (Lethal Dose at 50% survival) values greater than 2 x 103

mg/kg. The highest rates of toxicity were found amongst basic and diazo dyes6.

• Unless and otherwise properly treated, dyes can significantly affect photosynthetic activity in aquatic life due to reduced light penetration and may also be toxic to certain forms of aquatic iife due to presence of metals and chlorides in them7

•9

•

There is little evidence that the dyes found in watercourses are toxic to fish and other wild life at the concentrations likely to be present'o. Most of the studies on toxicity of dyes and pigments are concerned with the hazards due to occupational exposure of employees to dyes in the user . d II In ustry .

• Dyes have also been known to interfere with certain municipal wastewater treatment operations7

, i.e. UV disinfection, etc.

240 INDIAN J. FrBRE TEXT. RES., JUNE 2004

2 Textile Dyes and Environmental Concerns Textile dyes are classified under the categories of

anionic, cationic and nonionic types. Anionic dyes mostly include the direct, acid and reactive dyes. Basic dyes are the only class of cationic dyes used in the textile industry. Nonionic dyes refer to disperse dyes, which do not ionize in an aqueous medium.

It is estimated that about 15% of the total world production of colorants are lost in the synthesis and processing of colorant8. The wool and acrylic dyes tend to be in the best situation in this respect since high exhaustion is normal when applying anionic dyes to wool and cationic dyes to acrylics. The biggest problem relates to the dyeing of cotton with reactive and sulphur dyes because of the low level of exhaustion and fixation 12 as shown in Table 1.

Another important problem in the dyeing of cellulose fibres with anionic reactive dyes is the requirement of large amount of inorganic salt to suppress the negative charge at the fibre surface so as to increase its exhaustion level on the fibre. However, various developments have taken place in the recent years to increase the exhaustion level of reactive dyes lJ

-23

. In 1986, the Health and Safety executive issued publications concerning the possible respiratory irritant and sensitization effects from handling the dry powders or from aerosols containing these materials 4 .

The chromophores in anionic and nonionic dyes are mostly azo group or anthraquinone types. The reactive cleavage of azo linkage is responsible for the formation o f toxic amines in the effluent. Anthraquinone based dyes are more resistant to degradation due to their fused aromatic structures and, therefore, remain coloured for a longer period o f time in the wastewater.

The azo dye and pigment manufacturing plants produce a waste which has low pH, high colour, high organic content (high COD) and low amenabil ity to biological degradation (low BOD) and can be defined as a typical dye waste8

. Dyes have a very low rate of removal ratio for BOD to COD (BO~/CO~ is less than O.l) and not removed by typical biological treatment plants. Furthermore, dyes can be detrimental to the microbial population present in such plants and may decrease the efficiency or lead to treatment failures in such plants24

•

'The alkaline reducing systems based on Na2S in some dyeing recipe cause discharges of the effluent containing sulphur, which give foul smell and

Table 1-- Exhaustion range of various dye classes·2

Dye class Fibre Degree of Loss to fixation, % effluent , %

Acid Polyamide 80-95 5-20 Basic Acrylic 95-1 00 0-5 Direct Cellulose 70-95 5-30 Disperse Polyester 90-1 00 0- 10 MetaI-comp\ex Wool 90-98 2-10 Reactive Cellulose 50-90 10-50 Sulphur Cellulose 60-90 10-40 Vat Cellulose 80-95 5-20

contaminate sea water/river water with their toxicity and destroy marine life9

•

The environmental impact of metals in wastewater effluents is also an important issue faced by the dye manufacturing and application industries today. This is an important point because a significant number of dyestuffs are metallized dyes. Commonly employed metals are chromium, cobalt, nickel and copper, all of which are designated as priority pollutants by the US Environmental Protection Agency (EPAi.

3 Dyestuff and Colour Removal from Textile Emuents Colour removal is a pertinent problem for all

categories of textile effluents due to the variety of chemicals used in dyeing and printing of fibre, yam or fabric.

Colour pollution can be most efficiently controlled by good source reduction practices, administrative and engineering controls, process and product design and work practices. The search for dynamic response and improved productivity has served to focus the attention of the colouration industry on right first time (RFf) production techniques2

4-27. A high level of RFf minimizes waste and makes a significant contribution to reduce colour loads in the effluent28

• Improving the exhaustion levels of the various dyes in the dye bath is another area which has received lot of attention from researchers as it will not only improve shade reproducibility and problem of repeat shades but also s. live the colour effluent problems which will then reduce to a matter of controlling and handling spills and clean-up. Dyes showi ng a high level of exhaustion and fixation on the fi bre have been and will continue to be the prime targets of research and development activity.

However, quantitative dye bath exhaustion is not possible with most systems and hence the removal of colour from textile e~f1uents is a major problem faced by environmental scientists and engineers.

JOSHI et al.: COLOUR REMOVAL FROM TEXTILE EFFLUENTS 241

Most current practices for wastewater decolourization treatment fall into following four main classes:

-Physical or physico-chemical techniques, I.e. precIpitation, coagulation/flocculation, ion exchange, adsorption, and membrane separation. These remove or separate the colour physically and result in need for solid waste disposal,

--Chemical techniques, i.e. ozonolysis, chemical oxidation / reduction, etc. These technologies remove the colour from the effluent by breaking down the dye into simpler fragments and destroy the chromopore responsible for colour,

-Biological techniques, i.e. aerobic and anaerobic digestion, whereby decolourization takes place either by adsorption of dyes on activated sludge or by biological degradation of dye molecules, and

-Electrochemical techniques, i.e. electrodialysis / ion oxidation. It combines the oxidation of the dye and the other polluting contaminants by means of the electrolytic process with the physico-chemical precipitation of the sludge.

Each technique has a specific application and distinct advantages and disadvantages. As a result, each method has to be evaluated according to cost, application and performance relative to desired goal. Although no treatment technology has universal application, combination of one or two is generally employed depending on wastewater characteristics.

3.1 Physical or Physico-chemical Techniques 3.1.1 Coagulation / Flocculation

Over the years, coagulation has been the only economically feasible method for colour removal. The primary treatment in a conventional wastewater treatment scheme consists of coagulation and flocculation, which removes the colloidal particles of colour, turbidity and bacteria.

Coagulants such as Fe (III) or AI (III) salts when added in a sufficiently high concentration, rapid precipitation in form of hydroxides takes place. The colloidal particles become enmeshed in the precipitate and settle along with ie8

. Coagulant aids can be used to produce more compact floc and lead to improve settling. Coagulant dosing required depends on concentration of colloidal impurities present in the water. Synthetic organic polymers also work as coagulant by effecting interparticle bridges due to presence of groups, which can get adsorbed onto the colloidal particle. Iron was found to be superior to

aluminum for colour removaf9. Several studies have been reported on the use of different coagulants for textile wastewater treatment30

.36

. It has been found that a particular flocculent is suitable for only certain dyes; for example, alum is unsatisfactory for the removal of colour generated from azoic, reactive, acid and basic dyes, but is good for treating disperse, vat and sulphur dyes4

. Combinations of various chemicals have been used to improve colour removal from effluent containing the more common dye types.

The characteristics of the molecules themselves have an influence on their removal by coagulation. Molecules having acidic functional groups, which are able to coordinate with iron to form relatively hydrophobic complexes can be removed by coagulation. Hence, coagulation cannot treat all kinds of dye32

. Cationic dyes do not coagulate at all, making their removal by this technique impossible. Acid, direct, vat, mordant and reactive dyes usually coagulate, but the resulting floc is of poor quality and does not settle well even after introduction of a flocculent. Sulphur and disperse dyes coagulate well and settle easily . The coagulant dose required to achieve this colour removal depends on the type of effluent in terms of class and concentration of dye and other processing aids used and is an important consideration in evaluating its strength as well as economics of the treatment and recycle. In the presence of surfactants, the dosing of chemicals has to be significantly increased to achieve satisfactory colour removal. The other process parameter, viz mlxmg intensity, duration of mlxmg and configuration of flocculation tanks, also influence the coagulation/flocculation process. Kang et al. 33 did a comparative study on coagulation of textile secondary effluents using the ferrous coagulation (pH 8.0-10.0) vs. Fenton's coagulation (pH 3.0-5.0) and found that with the same ferrous dosage, the ratio of COD removal ranges from 1.4 to 2.3 and colour removal ranges from 1.1 to l.9 respectively .

Major disadvantage of using coagulation and flocculation is the generation of large amount of toxic sludge creating a lot of handling and disposal problems. The reappearance of colour in subsequent steps due to oxidation is also a critical problem faced in these plants. The other major drawback is that th is scheme is not able to reduce the total dissolved solids (TDS) rather the TDS levels are enhanced during treatment and hence poses lots of problems in water recycle and reuse.

242 INDIA N J. FIBRE TEXT. RES., JUNE 2004

3. 1.1.1 Synthetic Organic Coaglliants

These polymer coagulants are highly charged cationic polyelectrolytes and promote precipitation of dye residues , formin g small insoluble colour particles. These small particles are removed using a su itable solid/liquid separation process. These often completely eliminate the need for the large amounts of aluminum or iron based salts traditionally used. The mechani sm of dye removal is electrostatic attraction between oppositely charged soluble dye and polymer molecule (Scheme 1).

3.1.2 Adsorption

Adsorption is the phenomenon by which the molecules of a gas, vapour or liquid spontaneously concentrate at contacting sUlface without undergoing any reaction. It is an effective method for lowering the concentration of dissolved organics in an effluent. The use of any adsorbent, whether ion-exchanger, activated carbon or high surface area inorganic material , for removing species from a liquid stream depends on the equilibrium between the adsorbed and the free species4

.

The various adsorbents reported in literature for the removal of colour from textile effluent are:

3. 1.2.1 Activated Carbon

Activated carbon has been evaluated extensively fo r the treatment of different classes of dyes, i.e. acid, direct, basic, disperse, reactive, etc and is now the most widely used adsorbent for dyes37

-38

. Several pilot plant and commerci al-scale systems using activated carbon adsorption columns have been developed 39

.

The molecular structure of a dye has a significant effect on the extent to which it will be adsorbed with decreasing solubility and polarity of the dye favouring absorbability on carbon. Disperse dyes, vat dyes and pigments have such low solubility in water that their rate of adsorption on carbon is prohibitively slow at room temperature. On the other hand, water-soluble dyes such as acid, basic, direct, metallised mordant and reactive dyes are also not readily adsorbed on carbon . One of the main reasons for the observed poor adsorption is the polar nature of these dyes vs. the non-polar nature of carbon. Hence, the carbon adsorption of dyes is neither efficient nor economical when used alone, However, when used in combination with polymer flocculation, chemical coagulation or biodegradation, it becomes a very useful polishing step for efficient dye removal. Factors such as choice of activated carbon , temperature, pH, contact time and dosage must be taken into consideration for optimum removal of dyes

XXX-R-O- +

Dissolved dye molecule

R , I N-R

1 R

Coagulant molecule

R - ,I

XXX- R- O---- -N - R 1 R

'~ R- N - R , R

: - • 1 XXX- R- (). ---- --N-R

Stronger and enlarge floc (Not yet residual change)

1 R

R - • 1

XXX-R--()' - - - - - -N-R

1

R

Weakly bound Coprccipi tatc

R , I R0

R-N - R t R

High r.m.m flocculant molecule

Scheme l-Coagulation and fl occulation mechanism4

from wastewater. Activated carbon although reasonably effective at removing dyes from aqueous streams needs either regeneration or disposal once it is fully loaded. The other limitations are the high cost and 10- 15% loss of adsorbent on reactivation.

3.1.2.2 Bioadsorbents

In recent years, investigations have been undertaken to evaluate inexpensive alternate materials of biological origin as potential adsorbents for dyes , which include chitin, chitosan4

.40, sawdust4 1.42,

carbonized wool43, activated sludge44 ,wood bark, rice

husk"and cotton waste45.

3.1.2.3 Biomass

Because of its low cost and wide spread avai lability, biomass has been extensively investigated to remove colour and has shown some promising resuIt46

• Biomass here refers to dead plant and animal matters, such as agriculture, forest, fermentation and shellfish byproducts or wastes. Biomass decolourizes textile wastewater by adsorption and ion exchange mechanisms. Unfortunately, without prior chemical modification these materials uniformly have very low adsorption capacities for anionic dyes. There have been several studies reported on chemical modification of cell uloses and ligno-celluloses extracted from cotton waste, sawdust and corn stalks47

.

3.1.2.4 Chitin and Chitosan

Chitin, a polysaccharide, is very similar in structure to cellulose, being composed of poly 2-acetamido-2-dioxy-D-glucose. Chitosan is a well-known derivative

JOSHI et al.: COLOUR REMOVAL FROM TEXTILE EFFLUENTS 243

of chitin produced by the deacetylation of chitin which is a natural biopolymer extracted from the shell of arthropods4. Due to its unique molecular structure, chitosan has an extremely high affinity for many classes of dyes, including disperse, direct, reactive, acid, vat, sulphur and naphthol. The rate of the diffusion of dyes in chitosan is similar to that in cellulose. The only class for which chitosan has a low affinity is basic dyes. There are several studies on the use of chitin and chitosan for the removal of dyes48.51 . Knorr48 was the first one to examine the dye binding properties of chitin and chitosan and found that chitosan had better dye uptake property than chitin. Mckay et al. did extensive work on chitin and reported adsorption equilibrium studies49, batch and column studies5o, kinetic studies and mass transfer models51 for the adsorption of various dyestuffs on chitin.

More recently, Quin52 investigated the possibility of using chitosan fibre, which has amino groups and therefore shows the advantage of more adsorption capacity and much easier desorption. A moderately crosslinked chitosan fibre53.54 allows the fibres to be used at low pH which improves the dye binding capacity without solubilising the chitosan and was found to have an Acid Orange II (a monovalent anion) having the binding capacity of about 4.5 mollkg at pH 3-4.

3.1.2.5 Microbial Biomass

The uptake or accumulation of chemicals by microbial biomass is termed as biosorption. Dead bacteria, yeast and fungi have all been used for the purpose of decolourizing dye containing effluents55.56. Depending on the dye and the species of microorganisms used, different binding rates and capacities were found. The use of biomass has its advantages, especially if the dye containing effluent is very toxic.

Biomass adsorption is effective when conditions are not favorable for the growth of microbial population. Adsorption by biomass occurs by IOn exchange.

Many industrially useful fungi contain chitin and chitosan in their cell walls. Hence, the fungal biomass byproducts of industrial fermentation processes can serve as dye adsorbent. The cell wall of Myrothecium verruca ria was shown to bind azo dyes, including Acid Orange IT and Acid Red 114 (a divalent anion). The dye binding to the fungal material was

moderately slow, requiring 4-6 h to reach the equilibrium.

Hu57 examined the adsorption of reactive dyes to Aeromonas biomass. Dye absorption kinetics was moderately fast with the equilibrium reaching within 2 h.

3.1.2.6 Vnlllodified Lignocellulose Biomass

Mckay et a1. 43.45 examined wood bark, rice husk,

and cotton waste for their ability to bind Congo Red (a divalent anion) and observed negligible amounts of adsorption. Poots et al. 58

.59 showed that the wood

could adsorb acid dyes successfully but long contact period is required to reach equilibrium. Similarly , Mckay et a1. 60

,61 reported wood shavings to have a capacity for Congo Red of 0.001 mol/kg. The binding of Acid Blue 25 to sugarcane bagasse is slow and the capacity is only 0.05 mol/kg. Maize (corn)62 also binds Acid Blue 25 slowly, requiring more than 3 h to reach the equilibrium.

3.1.2.7 Chemically·modified Cellulose and Lignocellulose

Hwang and Chen63.65 reported a series of adsorbents prepared from the reaction of polyamide­epichlorohydrin resin and cellulose. This material, composed of 10-30% cellulose, has a high adsorption capacity for acid, direct and reactive dyes. Unfortunately, the rate of dye adsorption is very slow, requiring 3 days at 30°C to reach the equilibrium. The apparent adsorption capacity of PAE-cellulose is pH dependent (similar to chitosan). The adsorption capacity for Direct Blue 86 (a divalent anion) of the PAE-cellulose (25 % cellulose) material is 1.0 mollkg.

Youssef>6 described the chemical modification of cellulose (cotton) with the N-methylol derivatives of tris- and bis -(2-carbamoylethyl) ethylamine to enhance acid dye adsorption . The dye Acid Bluel3 was bound to the 30% bis derivative to the extent of 0.013 mollkg.The equilibrium with the dye was reached within 30 min, indicating excellent kinetics.

Phosphorylated cellulose as cationic dye adsorbents has been reported by Kammel67. Abo-Shosha et al.68

prepared cellulose/glycidyl methacrylate/acrylic acid cation exchange composite and could remove some basic dyes from textile effluents. The cellulose derived from sugarcane bagasse was derivatized to its carbamoyl derivative and used as direct dye adsorbent69. But, it was observed that the dye binding capacity of the untreated cellulose was higher than its deri vati ve.

244 INDIAN 1. FIBRE TEXT. RES., JUNE 2004

Quaternary ammonium groups can be introduced into cellulose and lignocellulosic materials. The quaternary ammonium group introduces a permanent positive charge to the substrate, making the materials very effective acidic dye adsorbents. Gangneux et l 70,7 1 . a . demonstrated that quatermzed cellulose has an

exchange capacity of 0.6-0.7 eq/kg for acid, direct and reactive dyes. The equilibrium adsorption of the dye was achieved in < 2 h. While quaternized cellulose has most of the desired performance characteristics (high capacity, rapid kinetics, etc), it fails to retain the most important attribute--Iow cost. Presumably, this is due to the cost of preparing pure cellulose, not because of the cost of quaternization. A low-cost adsorbent can be prepared by quaternization of lignocellulosic materials, such as corn cob72

, saw d 73 d b 74 . ust an sugarcane agasse . These matenals have the exchange capacities in the range of 0.35-0.85 eq/kg. The dye adsorption and desorption characteristics of the quaternized lignocelluloses are quite comparable to the cationic materials prepared from pure cellulose.

Thus, relatively inexpensive, moderately high capacity, anionic dye adsorbents could be prepared from lignocellulose biomass. However, none of these adsorbents has any commercial importance. Table 2 summarises the main advantages and disadvantages of different bioadsorbents reported for the removal of dyes from textile wastewater.

3.1.2.8 Inorganic AdsorbenlS

In recent years, investigations have been undertaken to evaluate inexpensive inorganic materials as potential adsorbents for dyes, which include peat?, flyash 75

, bentonite76, calcium

metasilicate77, activated aluminum78

, clay and bauxite79

. The use of bentonite for basic dyes and anthracite charcoal for acid yellows are also known80

.

The use of inorganic adsorbents, such as high surface area silica, cinder ash and clays, has been tried for a range of dyes8 1

.84

• Silica was found to be reasonably effective for treating effluents containing basic dyes . Again the process has little effect on the major inorganic charge of the effluent. The use of cinder ash appears to be a cost effective solution where a readily available supply of the ash is found locally and the effluent does not contain reactive dyes.

3.1.2.91011 /:.xchange Resins

Since most dyes are chemically either anionic or cationic, they could in theory be removed on ion

Table 2- Principle difference between reverse osmosis, nano filtration and ultra filtration 108

System Pressure Cross flow Process Retention Mpa rate tlux

· 1 Im·2h·1 IllS

Reverse 3-6 2-3 5-40 >90%NaCl

osmosis

Nano 2-4 2-3 20-80

>90%Lactose filtration <.SO%NaCl

Ultra 0.5-2.5 3-4 5-200

4-200xlcY filtration MWCO

h . 4885-90 H h exc ange reSInS " . owever, t ey have not been widely used for the treatment of dye containing effluents, mainly due to the fact that 10n excharlge resins cannot accommodate a wide range of dyes i.e. not effective for disperse dyes and have to be regenerated using costly organic solvents once they are saturated. The process is feasible provided an organic solvent such as methanol is used during the regeneration step, which can be recovered later.

In the early 1980s , the Institute Textile de France studied the use of cellulose based ion exchange materials for the treatment of dye house effluents4

.

Because of the synthetic nature of these materials, they could be tailored to have great chemical resistance. A combined process involving adsorption on synthetic polymer and ion exchange has been used to decolour dye wastes85

• The process has several operating and performance advantages over activated carbon adsorption.

3. 1.2.10 Macrosorb / Acrasorb

Macrosorb is the trade name for a range of inorganic particulate adsorbents developed9 I

,92.

Macrosorb is synthetic inorganic clay and the crystal structure consists of parallel layers of clay platelets, which due to their chemical composition carry a net positive charge. Between these layered platelets, there is anion which balances the cationic nature of clay. They are thus engineered for optimum adsorption of the compounds found in dyehouse effluents, which have a larger negatively charged or polar molecule, i.e. dyes, organohalides, pesticides, etc. The adsorbent capacities are very high and can remove these contaminants from process water down to extremely low levels and are then removed by gravity settlement in the form of pumpable sludge. This offers prospect of being able not only to meet any current or envisaged consent limits but also of water reuse. Acid, metal complex, direct and reactive dyes are

JOSHI et al.: COLOUR REMOVAL FROM TEXTILE EFFLUENTS 245

easily adsorbed by Macrosorb. Disperse dyes are only slightly soluble in water but a combination of adsorption and gravity settlement in the overall system gives good removal. In the unchromed state, chrome dyes are adsorbed by Macrosorb. In the chromed state such colouring matters are not in solution but are removed by the accelerated gravity settlement of the treatment.

Similarly Arcasorb D is a proprietary biological adsorbent from Archaeus Technology Group which removes soluble dyes (in particular reactive dyes) and other contaminants from textile waste stream93

.

The main mechanisms involved in the removal of colour and other contaminants from effluent streams by Macrosorb and Arcasorb D are believed to be physical adsorption and anion exchange.

3.1.2.11 Polymer Waste

Polymers are also capable of adsorbing the residual dyestuff from textile effluent. Polyamide, which can be dyed with most of the common dyestuff classes and has reactive groups available, is a primary

candidate in this respect94. The waste polyamjde

fibres or plastics can be used as dye cleaners by suitably degrading the polyamide to bifunctional low molecular fragments with termjnal reactive amino and carboxyl groups important for dye uptake.

3.1.3 Compleximeteric Technique Cucurbiturial is a cyclic polymer of glycoluril and

formaldehyde, so named because its structure is shaped like a pumpkin95 (a member of the plant family cucurbitaceae). Cucurbitile showed extraordinary good sorption capacity for various types of textile dyes96

• It is known to form host guest complexes with aromatic compounds and this may be the mechanism for reactive dye adsorption. To be industrially feasible, it needs to be incorporated into fixed bed sorption filters. High cost is a disadvantage.

3.1.4 Membrane Separation Membrane filtration technology is extensively applied

in process industries to concentrate, purify and improve the final product97

•

Table 3 -Summary of bioadsorbenr.s47

Biomass Adso~tion caQacit:t. mol/kg Major drawback Monovalent dye Divalent dye

Chitin 0.45 0.13 Slow kinetics (Acid Blue 25) (Acid Red I)

Chitosan 0.45 Slow kinetics (Acid Red I)

Crosslinked chitosan 4.5 pH sensitive (Acid orange II)

Fungal biomass 0.05 0. 11 Slow kinetics (Acid orange II) (Acid Red 114)

Bacterial biomass 0.08 (Reactive Yellow)"

Sugarcane bagasse 0.05 Low capacity (Acid orange 25)

Wood shaving 0.001 Low capacity (Congo Red)

Maize cob 0. 1 Low capacity (Acid Blue 25)

Peat moss 0.13 Low capacity (Lanasyn Black)

Rice Hull 0.14 Low capacity (Lanasyn Black)

PAE-cellulose Slow kinetics

Carbamoy Icell u lose 1.0 Low capacity

Quaternized-cellulose 0.6-1.1 b Direct Blue 86) 0.013 Expensive

Quaternized- 0.035-0.85b (Acid Blue 13)

None lignocellulose

"Trivalent anion bCaQacit:t ( eqlkg)

246 INDIAN 1. FIBRE TEXT. RES., JUNE 2004

It can be sutxlivided into four categories, namely reverse osmosis, nano filtration, ultra filtration, and micro filtration. The principal difference between reverse osmosis, nano filtration and ultra filtration are summarized in Table 3. Ultra filtration is used to separate solutes of molecular sizes between O.(X)1/Lm and O.I/Lm. Below Mw - 1000, the osmotic pressure begins to increase significantly and reverse osmosis or nano filtration starts. Nano filtration retains small organic molecules having a molecular weight >300 and even polyvalent ions while allowing smaller ions to pass through the membrane. True reverse osmosis membranes, however, reject even the smallest ions and allow the passage of pure water only. Hence, the operational pressure for reverse osmosis systems is higher in the range of 0.69-6.9 MPa whereas ultra fi ltration systems operate at pressures usually in the range of 0.069-0.69 Mpa (ref. 98).

In textile industry, membrane separation processes have found several applications such as improvement in the quality of finished product, increased yield, saving in raw material or recovery of product from waste and increased dryer capacity. The use of membrane technology to treat liquid effluents from textile uni ts has already been reported97

-1 \0 with great interest.

Reverse osmosis and ultra filtration are very effective for the removal of colour from dye house effluent regardless of the type of dyestuff used. Decolourisation by these procedures is in the range of 95-100%. The various dyes were studied to observe if removal is possible by micro filtration, ultra ftItration or nano filtration modules. A large proportion of disperse and vat dyes are removed by a 0.45 ~m pore micro filtration membrane. The proportion of other classes of dyes removed on these membranes is small or nil. Porter and Gomes lO3 reported that a polypropylene micro ftItration membrane rejects both salt and Direct Red 2 from aqueous solution when the conductivity of the solution is below 500 m~ siemens. Ultra filtration achieves complete colour removal for all classes of dyes except reactive dyes. But, care is needed to avoid membrane clogging, which appears to occur rapidly. Nano filtration membranes allow complete colour removal but with less membrane fouling. Separation of reactive dyes from effluents containing reactive dyes and salts using commercial nano filtration membranes has recently been reported III . An experimental investigation on electric fieJd enhanced nano filtration is reported for a direct dye solution. A 100% dye rejection was obtained for the membranes tested. An electric field was found to be

efficient in reducing fouling for both membranes studieAl 109.

3. 1.4.1 Membrane Materials

Textile effluents have a very wide range of composition in terms of pH, acidity/alkalinity, type of dyes and other contaminants and may be quite hot (50-80°C). Hence, the membrane to be used for such an application should have good chemical as well as thermal resistance. Reverse Osmosis and ultra filtration membranes made out of a variety of polymers, e.g. polyamides ll2

, poly (phthalazine ether sulfone ketone) 113, styrene copolymer I 14,

polyacrylonitrile, polysulfones l15, polycarbonate, and

fluorocarbon based polymers have been used for textile applications I 16. These membranes have excellent thermal, chemical and mechanical stability, allow the system to be operated at high flux rates and are resistant to wide range of pH, temperature and solvents. These membranes are therefore found to be suitable for separation of organic dyes from textile effluents. Ceramic membranes withstand higher mechanical forces and tolerate rough effluent conditions, such as temperature higher than 100°C and pH greater than 12. Generally, the membrane lifetime is longer than for organic membranes but they are fragile and require frequent back pulsing.

3. 1.4.2 Membrane Configuration

Membranes are packaged in modules that control the pressure, feed stream velocity and turbulence to reduce concentration polarization (fouling) effects. There are four basic kinds of modules, namely plate and frame, tubular, spiral wound and hollow fibre modules 117 .

Spiral wound modules provide a relatively high packing density «1000m2/m3) and allow for a compact design, but they are intolerant of particulates. A prefiitration step to remove fibres, grease, etc in early process effluents may be necessary to avoid clogging of the flow channels.

Tubular modules are relatively insensitive to clogging and more effectively cleaned during backwash procedures. However, packing densities are very low C<100m2/m3) and feed flow rates per unit membrane are high. Tubular Membranes have been extensively used for desalting of product dyes (reactive dyes) and it is possible to use the technique to recover sodium chloride solution from dye bath containing reactive dyes for reuse. Tubular membranes also find application for organic dye removal from textile

JOSHI et at.: COLOUR REMOVAL FROM TEXTILE EFFLUENTS 247

effluent l18. Nowaka et al. I19 reported the suitability of

capillary membrane modules (UF and NF) for decolourization of both simulated and industrial dye effluent and found 92-99 % retention coefficient for organic dyes of molecular wt -780.

The main advantage of membrane processes is that concentration is achieved without any input of thermal energy or a change of state, making the process energy efficient. Calabro et at. 120 conducted energy analysis of integrated membrane processes in treatment of solutions simulating textile effluents and did an energy analysis.

Another great advantage of these membrane processes is that the wastewater can be treated successfully to a level required for recycle and reuse l21

.

Reuse options for the nano filtration permeates exist either as dye bath makeup stream or in the following rinsing stages, depending on specification. Reductions of fresh water use of the order of 60 % and energy savings up to 50% can be achieved by integration of membrane technology into continuous washing/rinsing processes. The removal of the dyestuffs from the spent dye bath liquor using membrane technology makes it possible the recycling of water in dye houses and the concentration and reuse of dyestuffs. Thus, it not only helps in pollution control and waste management but also helps in conservation of chemicals and water. Techno-economic viable solutions have been suggested to combat water shortages with innovative use of membranes by Yedavyasan 122

• Compared to other

separation processes their space requirements are low and modular construction and design allow relatively easy expansion.

The major problem faced in application of membrane technology for effluent treatment is the high cost of membrane and other membrane filtration equipment, depending on the size of plant, the operating condition and associated water treatment costs besides the problem of lower productivity with time due to the deposition of precipitated dyestuff, i.e. fouling. Membrane systems are limited in several ways as a consequence of their structure. The use of polymeric membranes is limited to the temperature of 70°C, pressure of < 6-7 MPa and a pH range of 2-12 (maximum). Another problem faced is the disposal of concentrates from the membrane processes. There have been several attempts to solve this difficulty by oxidative degradation of the membrane concentrates by Fenton's reagent l23 and ozonation of membrane concentrated secondary effluent l24

•

3.2 Chemical Techniques 3.2.1 Chemical Oxidation

Many dyes are effectively decolourized using chemical oxidizing agents and found to hold potential for future use in the textile industry. Many studies on usage of different oxidizing agents, i.e. chlorination, chlorine dioxide treatment, ozonation, use of hydrogen peroxide with other salts (Fenton's reagent), permanganate, etc, have been reported in literature8l

, l25 and summarized in Table 4.

Method

Table 4 -Summary of chemical oxidation techniques used for decolourisation 125

Comments

Sodium hypochlorite

Hydrogen peroxide

Fenton's reagent

Ozone

UV irradiation

Gamma irradiation

UV irradiationlhydrogen peroxide

UV irradiation/ozone

Effective on decolorisation, cheaper than other oxidants, and easily applicable (20-40°C, 5-30 min). Risk of halogenated hydrocarbon (AOX) increase and bacterial toxicity. Can only be used with small amounts of wastewater.

Environment-friendly application. Not effective on all dyes as oxidation potential is not very high.

More effective than hydrogen peroxide on different classes of dyes. Wastewater may be reused following this treatment and removes heavy metals. Causes severe sludge problems.

Specially useful in decolorisation of water-soluble dyes. Does not sufficiently decrease COD and turbidity. Acids, aldehydes and ketones are reaction products. Recommended that coagulation and ozone can be used prior to biological treatment.

Photocatalytic reactions of some organic species in aqueous solutions are feasible. Removes heavy metals. Sludge and harmful UV scattering problems. .

New technique.

Increased rate and strength of oxidation, but the cost of producing UV irradiation does not compensate for the increase. Environment-friendly application, apart from some UV scattering.

Increased rate and strength of oxidation, but the cost incurred by the UV irradiation does not compensate for this increase. Environment-friendly, apart from UV and ozone scattering. Wastewater may be reused since reaction products could be carbon dioxide, water, nitrogen, etc.

248 INDIAN J. FIBRE TEXT. RES ., JUNE 2004

3.2.1.1 Chlorination

Chlorination (using chlorine gas or sodium hypochlorite, i.e. NaOCI) has been evaluated l26 for its effectiveness in colour removal. At a chlorine level of 150 mg/L, colour was reduced by 77% but 110 mg/L of total chlorine remained in wastewater. In 1980, Auburn University reported the results of laboratory scale studies concerning the TSI chlorination

127128 ChI .. d b ff ' system ' . onne IS reporte to e more e ectlve for dye decolourization at pH 3.5 than at pH 7.0 or 10.0. Chlorine rapidly decolourized acid and reactive dyes but even large doses of chlorine failed to completely decolourize direct and disperse dyes, rather persistent yellow decomposition products were formed. Recently , an electrolytic process based on chlorine generation was adapted to the wastewater containing textile dyes 129 . In situ production of hypochlorous acid was achieved in an undivided electrolytic cell.

Although, decolourization using sodium hypocholorite is inexpensive and effective but dechlorination of wastewater is necessary in order to prevent toxic effects in the ensuing biological processes. Moreover, chlorine is viewed with increasing disfavor because it has potential for generating toxic chlorinated compounds i.e. AOX (absorbable organohalides) that are harmful to humans and environment.

3.2.1.2 Chlorille Dioxide

Chlorine dioxide is less reactive than chlorine and has been claimed to give rise to fewer side reactions4

• The experimental study shows, however, that it does not decoloUIize dye waste efficiently to consent conditions, as it has no effect on some dye classes, such as vat dyes. Nevertheless, chlorine dioxide is highly effective against reactive, direct, disperse and anionic premetallised dyes. It could be used as a polishing treatment.

3.2.1.3 Ozonation

Ozone is a more powerful oxidant than chlorine and other oxidizing agents, i.e. 0 3, Ch and H20 2 with oxidation potential of 2.07, 1.36 and 1.78 respectively and offers a mechanism for oxidizing dye wastewater without producing harmful chlorinated organics. Ozone reacts with dye molecules in two ways: (i) below pH 5-6, ozone is present mostly as 0 3 and reacts selectively with double bonds in dye molecules, and (ii) at higher pH (above 8), ozone rapidly decomposes forming hydroxyl­free radicals that react non-selectively with organic compounds. Ozone fading of dyes occurs by the oxidative cleavage of the conjugated system of the

Et0-Q-N= N-Q-CH=CHV'N=~OEt

1 NaO,S

0,

EtO--Q-N= N--Q-CHO + OOHC-O--N=~OEt

'"0.' j H20

Scheme 2-Ozone fading of Cry sop he nine G (ref. (35)

molecule (Scheme 2). Ozone is useful for removing many toxic chemicals from wastewater, as it is capable to decompose detergents, chlorinated hydrocarbons, phenols, pesticides and aromatic hydrocarbons.

Ozone treatment has been successfully used to remove colour from dyeing wastewater I 30-135. Some classes of dye respond more readily to oxidation by ozone than others. According to some authors4

• reactive dyes are degraded to greatest extent and ozonation is moderately successful in treating wastewater-containing sulphur, azoic and basic dyes. However, disperse dyes have poor response to ozonation. The dosage applied to the dye containing effluent is dependent on total colour and residual COD to be removed with no residues or sludge C • d . bol ' 136 0 . 10rmatIOl1 an no tOXIC meta Ites . zonatIOn may decrease COD and increase the bio degradability of waste stream but produce little reduction in TOC. Cost and efficiency are barriers associated with ozonation. Another major drawback is its short life (half-life, 20 min), requiring continuous ozonation. Improvement in ozone diffusion by means of membrane contractor (higher gas / liquid contact surface) in order to further reduce operating costs has been recently reported by Ciardelli et at. 137 in their ozonation pilot plant study.

Additionally, since ozone is hazardous, it will require an ozone destruction unit to prevent ozone from escaping and damaging the environment.

3.2. 1.4 Hydrogen Peroxide

Hydrogen peroxide is the main oxidizing agent used for decolourization by chemical means and removes the dye from the dye containing effluent by oxidation resulting in aromatic ring cleavage of the dye molecules. This agent needs to be activated by

JOSHI et al.: COLOUR REMOVAL FROM TEXTILE EFFLUENTS 249

some means, for example ultra violet light, inorganic salts ( Fe+2

) , ozone or ultrasound. Many methods of chemical decolourization vary, depending on the way in which H20 2 is activated.

3.2.1 .5 H20r Fe (1/) Salts (Fenton's Reagel/t)

Fenton's reagent is a suitable chemical means of treating wastewater which is resistant to biological treatment or is poisonous to live biomass. In acid solution, iron (II) as a catalyst, peroxide vigorously forms hydroxide radicals (Scheme 3), which are used to decolourize dye wastes94 .

It is capable of treating soluble dyes, such as reactive, as well as insoluble dyes, such as vat and di sperse, and does achieve consent conditions for both the concentrated and dilute waste investigated4. The vigorous oxidation also reduces the COD of the effl uent. Neutralization of the effluent after treatment causes precipitation of the iron oxide and hydroxide, which removes any remaining insoluble dyes from the effl uent by adsorption and / or flocculation. One of the major disadvantages of this method is sludge generation through flocculation of the reagent and dye molecule. The sludge, which contains the concentrated impurities, still requires disposal and the performance depending on final floc formation and it's settling quality . In a comparative study' 38 on oxidation of di sperse dyes by electrochemical process, ozone, hypochlorite and fenton's reagent, the best results were obtained with Fenton process which under the optimal pH of 3 and H20 2 and FeS doses of 600 and 550 mgm-3 respectively results in a final colourless effluent with low residual COD (100 mg/cm\

3.2.1.6 H20 2 1UV

Decolourization of dyes using UV/H 20 2

photochemical oxidation has already been investigated by several researchers 139- 143 . This method degrades dye molecules to CO2 and H20 by UV treatment in the presence of HzOz (refs 139,140) . The degradation is caused by the production of high concentration of hydroxide radicals. UV light activates the destruction of H20 2 into two hydroxyl radicals (Scheme 4), which causes chemical oxidation of organic material.

The rate of dye removal is influenced by the intensity of UV radiation, pH, dye structure and the dye bath composition 141. This may be set up in a bath or continuous column unit l42. Depending Oil the initi al material, additional by products, such as halides ,

Scheme 3--(:atalytic action of iron on hydrogen perox ide94

Scheme 4--Hydroxyl radical generation through UV radiation 139

metals, inorganic acids, organic aldehydes and organic acids, may be produced 139. There are advantages of photochemical treatment of dye

containing effluent-no sludge is produced and foul odors are greatly reduced.

3.2. 1. 7 H20 ] - Ozone

Advanced techniques using combination of ozone and hydrogen peroxide make it possible to remove odor, colour, COD, TOe and absorbable organo halogens (AOX). The resultant products are often easily biodegradable l

l.143. Decolourization by means

of Hz0 2/03 combination is applicable for direct dyes , metal complex or blue disperse dyes, but there are some problems with decolouration of acid, red d · d d h' . 141 Isperse yes an t elr mIxture .

3. 2.1 .8 H20 r Peroxidase

Peroxidase can also be used as an acti vator for H20 2 for decolourization purposes. Morita et al .144

studied decolourization of acid dye using three types of peroxidases as peroxide activators . The decolouration rate increases with increasi ng peroxidase concentration and temperature of the medium and was the greatest at pH 9.5.

3.2. 1.9 H20 2 -UV-Ultrasound

Fung et al. 145 studied the decolourization and degradation kinetics of reactive dye wastewater by a UV/ultrasonic/peroxide system. It was found that the degradation of the reactive dye follows a pseudo first­order kinetic model at different pH and H20 z concentrations. It was observed that the ultrasound in combination with UV dramatically improves the initial reaction rate and the overall dye removal efficiency. Ultrasound may increase the oxygen uptake and transfer rates which enhance the oxidation processes due to the hydroxyl radicals.

3.2.2 Irradiation

Gamma radiation induced oxidation has also been reported 125.146 in decolouring refractory dyes that withstand many of the conventional textile waste treatment processes. The rate of reaction is controlled

250 INDI AN J. FIBRE TEXT. RES., JUNE 2004

by radi ati on dose and the availability of oxygen in the soluti on. It proved to be an effecti ve technique fo r the removal o f dyes as well as toxic o rganic compo unds, i. e. benzene, to luene and chloropheno ls, by ox idi zing them into more eas ily biodegradable co mpounds . Both COD and AOX were decreased and 90% of colour in the wastewater was e liminated. However, very hi gh cos t is the major di sad vantage.

Wet ai r oxidation (W AO) is another process by which the effluent could be effectively disposed 147.

3.2.3 Chemical Reductioll

For many dyes, pW1icui arly azo dyes, chemical reduction is an effective decolourisation technique. Chemical reduction of azo dyes causes cleavage of azo bond, generating small nonchromophoric aromatic Jm incs that in theolY are more amenable to subsequent aerobic biological treatment than the parent dye structure. Tn addition, publi shed results indicate that certain reacti ve dyes are adsorbed well on to activated carbon when pretreated with a reducing agent l48. Technologies based on bisulphi te catalyzed sodium borohydride a long with a cationic agent have been applied to water-soluble dyes containing azo or other reducible groups and copper based metalli zed dyes 149. Using this process, colour

reduction of >90% was poss ible. Sodium borohydride is one of the strongest water-soluble reducing agents commerciall y available, while bisulphi te is not consumed but acts as a regenerable coreagent or catalyst l49. The other most commonly used chemical reducing agents w·e sodium hydrosulphite, thiourea dioxide, tin (II) chloride, etc. While evaluating the chemical decolouri sation of wastewater using reducing agents, it is impol1ant to investigate the potenti al reversal of the reaction upon exposure to oxygen, s ince colour may reappear upon di schw·ging the wastewater to the envi ronment

3.3 Biological Treatment

Biological digestion involves the aerobic (in presence of O2) or anaerobic (in absence of O2) degradation of organic substances by microorganisms and has been widely researched and rev iewed I50.15 1.

3.3.1 Aerobic Treatmellt

Aerobic biological treatment using acti vated sludge is one of the most commonly used treatment methods for wastewater generated from textile dyeing operations.

onnally , this removes the biodegradable components of the effluent, e.g. carbohydrates, waxes and the readily degradable auxjliwy compounds, al though more complex xenobiotic compounds such as dyes and surfactants remain as it is.

The ineffectiveness of aerobic bio logical system in removing colour has caused aesthetic problems in the recelvll1g waters and encouraged dischw·gers to investigate other altemati ves . Dyes themselves cu·e generally res istant to oxidati ve biodegradation , since one of the most imp0l1ant properti es built in the commercial dyes is res istance to fading caused by chemical and light induced oxidation. The issue of toxicity of the effluent is also a li sing concem. Another problem wi th aerobic bio logical treatment of dye wastewater is the diffi culty in acclimating the microorganisms to the substrate. Acclimation presents a problem in tex tiles due to constant product changes and batch dyeing operations. Shriver and Dague l52 in their study on BOD tests concluded that colour in textile wastewater would be expected to undergo biodegradation J.t s lower rate than that fro m typ ical domesti c wastewater. Several studies on aerobic biological treatment of textile dye

151 154 . d· h I· I b· d I · f· wastewater .. 111 Icate t at Itt e 10 egrac atlon 0

dyes actually occurs and that the primcuy removal mechanism in volves adsorption to acti vated sludge.

The adsorptio n properti es of acti vated sludge are s imilar to th at of acti vated carbon in volving acid, d irec t, reactive, disperse and bas ic dyes and are mainly dependent o n dye properties (mo lecular struc ture and type, number and pOSitIO n of subs tituents in the dye mo lecul e) . Adsorptio n is increased in presence of hydroxy, nitro and azo groups but decreased by sulpho nic acid groupsl 53 . The facto rs inhibiting permeation of the dye through the microbial ce ll membrane (e.g . increased or decreased water solu bility and increased molecular weight) reduce the effic iency of the biologica l system. ET AD stud ies 155 on removal of dyes by aet i vated s ludge process indicate that lo w adsorpti on on s ludge occurs with ac id and reacti ve dyes, high adsorption occurs w ith basic and direct dyes and h igh to mediu m adsorption occurs with di sperse dyes .

CI I I I 156 157 . d L . lU rc 1 ey et a .. · cWTle out a Cllemometnc analys is to find out the level of bioelimination of water soluble dyes. This analysis was conducted to corre late bioelimination with chemical structure/functionali ty. It was found that the level of bioelimination is propol1ional to the size/chw·ge ratio of the dye. Tn case of acid and direct dyes, bioelimination vwi es between 0% and 95% fo r the series of dyes studied, while in case of reacti ve clyes, the level of bioelimination vcu·ies from 0% to only 25%. To maximize the bioelimination of reactive dyes, large and planer triazine based dyes should be used l57.

JOSH! et al.: COLOUR REMOVAL FROM TEXTILE EFFLUENTS 251

In an activated sludge tank, water is mixed and aerated with a suspension of microorganisms. A microbial flocculation forms; oxidizing dissolved and suspended organic matter. Activated sludge systems lack tme contact time between the bacteria of the system and the suspended and dissolved waste present. Large bacterial population creates a secondary waste treatment concern in the form of bio solids or bactelia. These bacteria, often called sludge, must be disposed of and this increases overall waste treatment costs.

Immobilized microbe reactors (fMBRs) address the need of increased contact between microbial population and the waste without concomitant production of excessive bio solids l58. This is done through the use of a solid but porous matrix (beads) to which a tailored microbial consol1ium of organism has been attached. This allows for a greater number of organisms to be available for waste degradation without the need for a suspended population and protection from shocks for bacterial population in the interiors of porous beads.

Although aerobic digestion of textile effluent removes 60-70% of organic waste, the toxicity level was hardly reduced due to the presence of organic matter that could not be degraded, requiring tertiary treatments to remove the toxicity . Efforts have also been reported to tailor the activated sludge to accommodate the textile effluents more effectively by sludoe seedino and sludoe b b b

conditioning. Both methods aim to produce a biased microbial community, dominated by species that are more suited for digesting a specific waste type l59.

The PACT system patented by Du Pont is a combined powdered activated carbon-activated sludge systeml(,(I. It consists of adding powdered activated carbon to an activated sludge system. The carbon dosage depends on the wastewater characteristics and varies from lOppm to 5000 ppm and can be added at any stage to the waste

VIRGIN CARBON STORAGE

VIRGIN POLYELECTROLYTE CARBON STORAGE STORAGE

POLYELECTROLYTE STORAGE

PRIMARY _~-1-r--"". - , EFFlUENr

-S.l UOGE RECYCLING

TO REGENERATION OR DISPOSAL

EFFLUENT

Fig. I-Two-stage PACT system as used at the Du Pont chamber works l 60

stream i.e. before treatment, to the recycled sludge or to aeration tank itself (Fig.!). It can effectively treat effluents with COD in the range of 50-50,000 mg/L. Some of the other advantages of the PACT system are improved organics removal, control of odors, colour removal and metals removal. It is also resistant to shock loads.

3.3.2 Anaerobic Treatment

Anaerobic biological reduction of azo dyes has been investigated from many perspectives, i.e. chemical degradation (treatment) and colour removal. Anaerobic reducing conditions found in the environment include sediments at the bottom of steams of cel1ain sections of landfills where there is no oxygen. Anaerobic bioremediation of azo and other soluble dyes to be decolourized by breaking them into corresponding amines (Scheme 5) has been widely studied l6 1.1 62 and contemplated with great reluctance. The intentional generation of amines that are more toxic than dye itself are not appealing from environmental perspective. The decolourization occurs due to azo reduction but the complete mineralization does not occur.

Azo dyes acts as an oxidizing agent for reduced flavin nucleotides of microbial electron chain and are reduced and decolourized concun'ently with reoxidation of the reduced flavin nuchotides. In order for this to occur, additional carbon is required for decolourization to proceed at a viable rate and later convel1 to methane, H2S and CO2. This additional carbon source may be a limiting factor commercially.

In many conditions, decolourization of reactive azo dyes under anaerobic conditions is due to the action of azo reductase enzyme l63 . Gonclaves et al. l64 studied the

3- Aminobenzane· sui phonic acid

Acid yellow 36

+

N·Phcnyl-I ,4-diaminobenzene

Scheme 5--Anaerobic reduction of Acid Yellow 36 (ref. lSI )

252 INDIAN J. FIBRE T EXT. RES., JUNE 2004

removal of colour from textile effluent using a laboratory 'upflow anaerobic sludge blanket' reactor. Several commercial dyes were selected to study the effect of dye structure on colour removal. The average removal effic iency for acid dyes using this method was between 80 % and 90% and that observed for the direct dye used was 8 1 %. Laboratory experiments using this anaerobic reactor with disperse dyes, such as an anthraquinone­based dye, were unsuccessful even at velY low concentrations.

However, the breakdown of azo dyes to thei r corresponding amines accompli shes two goals (i) decolourization of wastewater, and (ii ) preparation of wastewater for subsequent biological treatment since the aromatic amines released by anaerobic digestion of azo dyes are effectively treated by aerobic treatment processes. Hence, the CUITent literature '65 reveals that an anaerobic - aerobic treatment sequence has been found more effective method fo r textile wastewater treatment.

Anaerobic process usually occupies less space, treats waste up to 30,000 mglL COD, has lower recurring costs and produces less sludge. If complete mineralisation occurs, convers ion of organic contaminants into methane and O2 and then production of biogas are the major attraction because of heat, power and reduced energy costs.

3.3.3 Anaerobic - Aerobic Treatment

The anaerobic - aerobic sequence shows significantly greater colour reduction than aerobic alone (88% vs 28%), in addition to improved colour reduction. TOe reduction (79-90%) also improves when compared to aerobic treatment alone. The reduction in COD and BOD is greater (up to 8%) for anaerobic-aerobic sequence than that fo r aerobic treatment. Table 5 compares the results for colour, COD, BOD and TOe reduction from both treatment schemes.

A similar process, namely AB (adsorption/bio­oxidation) process has been developed in Europe l66

. The

treatment process is a modified two-stage activated sludge design and uses physical, biochemical and biological reaction mechanisms to reduce a vely wide spectrum of organic pollutants. The firs t stage is the adsorption stage (A) with anaerobic conditions with a very high food to microorganism (F/M) ratio fostering growth of bacteria and a sholt retention ti me. The second stage is biological oxidation (B) with aerobic activated sludge system. The microbial ecosystems in the two stages are kept distinctly separate, enabling job sharing and better effici ency of the process. This patented process reduces COD and BOD vely effectively , while being largely resistant to shock loads and pH fl uctuations.

3.3.4 Decolourisatioll. using Cullures of Bacteria, Fungi, Algae and Yeast

Microbial decolourization and degradation of dyes have been found as cost effective and ecofriendly method fo r removing them fro m textile effluen ts. Recent fundamental work has revealed the existence of a wide variety of microorganisms capable of decolourizing equally wide range of dyes. Banat e/ al.150 have given a detailed review of progress in biological decolourization of dyes .

3.].4.1 Baclerial Cullures

Numerous bacteria capable of dye decolOLllization have been repolted '5o. Efforts to isolate bacterial cul tures capable of degrading azo dyes started in 1970's with report s of Bacillus SUblilis l 67

, then Acroillonas ltydriphi l68

followed by Bacillus cereus l69. [solating such microbes

proved to be a difficult task and ex tended periods of adaptation under chemos tat conditions were needed to isolate the strain capable of decolourization. An azo reductase enzyme was responsible for the in itial degradation of the Orange 11 dye by these strains but substituting any of the groups near the azo group in the chemical structure of the dye hindered the degradation.

Table 5-- Per cent reduction from laboratory scale treatment processes involoving Navy 106 using anaerobic-aerobic sequence l5 1

Treatment process Treatment step Colour TOC BOD COD

Anaerobic-aerobic Dilution 40 4 1 13 38 Anaerobic SO 3 -2 I Aerobic -2 38 80 38 Total reduct ion 88 82 9 1 77

Aerobic Diluti on 28 38 IS 39 Aerobic I 36 75 30 Total reduction 29 74 90 69

,',\

JOSHI el al.: COLOUR REMOV AL FROM TEXTILE EFFLUENTS 253

An upsurge in interest in these bacterial cultures took place in 1990's and Haug et al. 17o described a bacterial consOitium capable of mineralizing the sulphonated azo dye- mordant yellow. An alteration from anaerobic to aerobic conditions was required for complete degradation in such mixed bacterial cultures. Several other strains have been repolted which can decololllize Cu- based azo dyes and reactive dyes including anthraquinone and phthaJocyanine through adsorption of dyes to the cell ular b· . I d d' 171 lomass Wit lout any egra atlon .

3.3.4.2 Algae

Few species of algae, namely Chlorella and Oscillatorial 50

, are capable of degrading azo dyes to their aromatic amines through an induced form of azo reductase and further metabolise the aromatic amines to simple organic compounds or CO2, These algae species utilize azo dyes as thei r sole source of carbon and nitrogen. Such algae can be used for stabilization of ponds as they can play a role in aromatic amines removal.

3.3.4.3 FUllgi

Several fungal systems including the white rot fungi (Phanerochaete chrysosporuilll), in pruticular, have been the subject of intense reseru'ch related to the biodegradation of a wide range of recalcitrant xenobiotic

d . I I' d 150 171 17? WhO f' compoun S lIlC UC lIlg azo yes . . -. Ite rot ungl are those organisms that are capable of degrading lignin, the structural component found in woody plants. They have been found to degrade dyes using enzymes, such as lignin peroxidase (Li-P), manganese dependent peroxidases (MnP), other enzymes such as gl ucose-l­oxidase, glucose-2-oxidase laccase and a phenol oxidase enzyme.

Azo dye, the lru'gest class of commercial dyes, is not easily degraded by microorganism but can be degraded by these enzymes. Commercial azo, triruylmethane, anthraquinone and indigoid textile dyes ru'e efficiently decolourized with enzyme prepru'ations from Pleurotus OSlreatus, Schizophyllul1l comm.ence, Neurospora crassa, Polyporus, SclerotiulIl rolfsii, Trametes villosa and Myceliophtora thermoplzila l73.

Mn- peroxidase (Mn-P) ruld Iigninase (Li-P) belong to the class of peroxidases that oxidize their substrates by two consecutive one electron oxidation steps with intermediate cation radical. While Mn-P only attacks phenolic substrates using Mn2+lMn3+ as an intermediate redox couple, Li-P with a higher redox potential prefers non phenol ic methoxy substituted lignin subunits as substrate173

. Laccases have very broad substrate

specificity with respect to the electron donor. They catalyse the removal of a hydrogen atom from the hydroxyl group of oltho and para substituted mono and poly phenolic substrates and from aromatic amines by one electron abstraction to form free radicals capable of undergoing fLllther depolymerization, repolymerization, d h I . . t· . 173 emet y atlon or qUlllone ormation .

White rot fungi (P. chrysoporuim) is also capable of degrading dioxins, polychlorinated biphenyl (PCBs) and other chloro-organics, thus rendering the effluents less toxiC l74

.

The colour removal achieved with these enzymatic treatments range from 40% to 99% , depending on the dye complexity, nitrogen available, activity of the cul ture, presence of other auxiliaries in the effluent, concentration of dye, retention time, etc. The nature of substitutents on the dyes, i.e. benzene rings, influence rhe enzyme activity. Electron donating methyl and methoxy substituents seemed to enhance enzymatic degradation, while electron withdrawing chloro, fluoro and nitro substituents inhibited oxidation by enzyme Iaccase l73 .

The presence of salts and other textile dyeing auxi liaries shows a potential inhibitory effect on enzymatic action. Higher salt concentrations decrease the decolourization efficiency by up to 80% and even lead to partial precipitation of proteins caused by increased surface tension and hydrophobic interactions. Laccase, containing 3 copper binding sites per protein molecule or one copper, one iron and two zi nc atoms per protein molecule, is inactivated by copper and iron.

Chelating agents and anionic detergents seem to prutially denature prote:ns (enzyme) and almost 70% loss of activity is repOited. Thus, knowledge not only about the substrate specifici ty but also about the effect of auxiliru'ies is important for selecting suitable enzymes for dye decolourization under industrial conditions.

Some of the drawbacks of enzymatic decolourization ru'e:

• Substrate specificity of some enzymes and hence the selection of appropriate enzymes fo r continuously varyi ng compOSItion of textile effluent may be an arduous task.

• The rate of reaction, which can be slow unless condition of pH and temperature are optimal.

• Commercial production of these specific enzymes is very diffic1.!1t.

• The presence of salts and other auxiliaries in the effl uent has an inhibitory effect on enzymatic activity .

254 INDIAN J. FfBRE TEXT. RES. , J UNE 2004

3.3.4.4 Yeast

Yeasts, such as Klyveromyces lI1arxialllls , are capable of decolourizing Ramazol Black B dyes by 78-98% (ref. 175). Zissi et al. 176 showed that Bacillus slIbtilis could be used to breakdown p-amino azo benzene, a specific azo dye. Further research using mesophilic and thermophilic microbes has also shown them to degrade and decolourize dyes.

Future Trends

• The advantages of mixed culture are apparent as some strains can collectively CatTY out complex bioremediatation task that no individual strai n can achieve independently. Similar bacteri al cultures have been reported recently 177.

• Future investigation should focus on the immobilization of selected enzymes to form the base for industrial application of enzymatic decoloLlIization. Banat et af. 178 have reported cheap suppOtt media for bio film development for acti ve tex tile dye decolourization using gravel, calcium alginate beads, PS and PU foam chips, nylon webs inett PE chips, porous volcanic rocks, etc.

• One of the routes still to be explored is the use of thermotolerant or thermophilic microorgani sms in decolourization systems. This would be an advantage as many textiles and other dye effluents are produced at relatively high temperatures (50-

60°C), even after cooling or heat exchange step. The literature review thus suggests a great potential for

microbial decolourizing system for achieving total colour removal and occasionally with only hours of exposure. Such biological processes could be adopted as a pretreatment decolourization step combined with the conventional treatment system to reduce the BOD and COD as an effective alternative for use in textil e dyeing industties.

3.4 Electrochemical Decolourization

Elecrochemical ion generation is a proven technology for removing colour, BOD, COD, TOC, solid (suspended and dissolved) and heavy metals such as chromium, copper and zinc from textile mill wastewater l79

.

The system most commonly utilizes an electrochemical cell to generate ferrous hydroxide directly by steel electrodes. The electrochemical cell (Fig. 2) consists of a fibre glass body containing a number of electrodes separated from each other by small gaps. Wastewater flows through the gaps and remains in contact with the electrodes. A direct cun'ent (DC) power supply is connected between the two end electrodes of

E

Cooling water

;. Influent

Emuent:contamillats adsorbed, coprecipitated within iron matrix

Fig. 2 - Electrochemical cell 179

Cathode SS·304

c :

+ Anode PVTI

Cooling jacket

Surge .... essel

pHIR

RIR

Sampling

Fig. 3--Experimental laboratory pilot plant using e lectrochemical cell l80 [C = Electrolytic cell , E =Electromagnetic valve, P = Rec ircul ating pump, TICR = Temperature indicator controller recorder, pHIR = pH indicator, and RIR = Redox indicator]

the cell (Fig. 3). As CUITent flows from one electrode to another through the process water, the positively charged sides of the electrodes (anodes) give off ferrous ions. At negative sides (the cathodes), the water decomposes into hydrogen gas and hydroxyl ions. 11le overall reaction results in the formation of hydrous iron oxide, ferrous hydroxide and felnc oxyhydroxide. The electrodes are slowly consumed as the metal hydroxide is generated. The process water then enters a degassing tank, pH is adjusted at < 7 or> 11 for satisfactory Fe +2 precipitation.

JOSHI el al.: COLOUR REMOVAL FROM TEXTILE EFFLUENTS 255

The water is then pumped from the reactor tank to clarifier where the newly forming solids settle for which a small amount of polymeric flocculent is added to improve flocculation and setting of precipitated solids. The clarifier supernatant flows to a polishing filter before leaving the system. The use of aluminum cell or combined iron- aluminum cells is also reported in the literature. If aluminum is used, the end products are a combination of aluminum hydroxide or oxyhydroxide. Recently , the use of titanium or titanium-platinum­iridium electrode, as anodes has also been reported l80

.

Electroflocculation is the combination of an oxidation, fl occulation and flotation . Oxidation is achieved by means of electrodes, dipped in the effluent to treat where a difference of potenti al is applied. Tlivalent iron and aluminum flocculate oxidized substances because of electrostatic interaction and co-precipitation. When metals are present as soluble inorganic ions, co­precipitation and adsorption are the primary removal mechanisms. Efficiency of the process depends on several parameters i.e. difference of potential, nature of electrodes, pH, salt concentration of solution, number of electrodes, distance between them, stirring and CUlTent . • 18 1 Il1tenslty .

Electrochemical technology is repOlted to remove acid, disperse and metal complex dyes effectivel/ 82

. The removal of dyes from aqueous solution results from adsorption ancl/or degradation of the dyestuff following the interaction wi th iron electrodes. If metal complex dyes are present, dye solubilising and charge are the most important factors for successful removal of heavy metals. pH has to be adjusted to maximize dye insolubili ty. Demmin and Uhrich.183 in their studies found that BOD level is generally reduced by 30-35 % and COD level by 50-70%. Lin and Chen l84 have reported that the addition of small amount of hydrogen peroxide in the magnitude of around 200 mgIL has been found to elevate the effic iency of electrochemical treatment process by as much as 100%. This is due to in situ generation of hydroxide radical generated (Fenton 's reagent Fe+2/H20 2)

when iron electrode is used, leading to rapid ox idation of Fe+2 to Fe+3.

Electroflocculation is, thus, a promising method for producing recyclable process water because it combines oxidation of the polluting content by means of electrolytic process with physico-chemical precipitation of sludge. There is no simultaneous addition of anions such as sulfate or chloride. This is in contrast to conventional chemical precIpItation methods that introduce either chloride or sulphate ions both of which

increase the TDS of the effluent. Moreover, evidence of salt content (sulphate and chloride) reduction was found , which makes the treatment even more interesting for recycl ing from technical point of view.

4 Conclusions The majority o f colour removal techniques work

either by concentrating the colour into sludge or by the partial or complete breakdown of the coloured molecule . In principle, decolourizations are achievable using one or combination of the following methods: adsorption, filtration , precipitation, coagulation , fl occulation, chemical, photo and biodegradati on.

Coagulation/ flocculation is the most widely used method for colour removal because of economic feasib ility, rap id removal of colour and signi ficant reduction o f COD. With rapid changes in dyes and stri cter consent limits, these alone do not give co mpletely satisfactory treatments especiall y with high ly soluble reactive dyes. Moreover, large quantity of sludge generation contalnll1g all the toxic compounds present in the effluent is likely to increase disposal costs substantially and thi s must be considered before choosing a system.

Adsorption processes have also got a lot of attention for colour removal. Activated carbon is used mostly as the final po li shing or tertiary treatment because regeneration costs are very high and high volume of textile effluent involved with dyeing. The size and cost of plant required become disproportionately high and pay back is poor. Low cost bioadsorbents have been employed for treatment of wastes containing dyes. However the specificity to remove certain contaminates, slow kinetics , regeneration and flow problems have limited the commercial success of these adsorbents at the industrial scale. Membrane systems provide a means to recover water and other auxiliaries fo r total treatment or stand alone process for scouring o r dye bath liquor separation. The major limitations are the economic feasibility because of high costs and the treatment of highly concentrated streams.

In the light of an integrated approach to waste treatment, it is likely that destruction technologies will ga in favour at the expense of technologies that j ust transfer the pollutant from liquid phase to sol id phase for disposal or form a liquid concentrate for further treatment. The best established destruction technologi es are based on chemical , bio and

256 INOlAN J. FIBRE TEXT. RES., JUNE 2004

electrochemical oxidation techniques. These lead to either partial oxidation of the dye, destroying the conjugated bonding system in the chromophore and thus, removing colour or total oxidation to form carbon di oxide and inorganic ions which is the most ideal. However, total oxidation using these reagents is not economically viable. The problem associated with parti al oxidation lies in the unknown nature of the products formed and the risk that some of these may even be more harmful to the environment than the initial components of the effl uent. Moreover, nearly al l chemical mechanisms are difficult to control once the reaction starts and therefore control over reacti on may be difficult even in the lab systems and virtuall y impossible in a variable feed stock such as an industria l textile effluent. These mostly reduce carbon loading of the effluent but can lIlcrease the concentration of inorganic content.

Enzymatic microbial decolouri zati on of textile dyeing effluents is also a very promising and innovativc area of investigation. It eliminates the need fo r inorganic oxidizing agents (ozone) or precipitating agents (polymeric f1occul ents), which lead to hi gher tox ici ty in wastewater and generate additional soli d waste. Microbial decolourization using enzymes degrades the aromatic ring structure to carbon diox ide, molecul ar nitrogen and water, thereby causing less toxicity. However specificity of enzymes, slow ratc of reaction unless pH and temperature are optimal and difficulty of commercial production of enzymes may be limitations which need to be overcome by more intense research in the fie ld of biotechnology.

The problem of colour in the effluent can be reduced to an extent by adopting right first time approach, proper work practices and waste minimization programs. The dye manufactures may also help by producing dyes (parti cularly reactive dyes) with a better fixation that can be achieved at present. Although developments in thi s field are ongo ing, in future standards will be enforced with increas ing severity by the regul ators. Hence the need is urgent and immediate and the pressure must be maintained to evolve more effective, widely app li cable and commercially viable techniques of colour removal from textile effluents.

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