Treatment of Effluents of Cotton Industry | Environmental Engineering

In this article we will discuss about the treatment and disposal of effluents of cotton and synthetic industry.

There are about 700 Textile mills in India, with gross annual production of 8000 million metres cloth. A large variety of cotton textiles, textiles made from synthetic fibres and cotton blended with synthetic fibres are produced by the mills.

The manufacturing processes of cotton textiles consist of a series of unit operations which are complex and highly specialise. Their actual description is outside the scope of the guide.

However, they may be broadly divided into two steps namely:

(1) Fabrication of the cloth, and

(2) Processing and finishing.

The details of the various processes are shown in fig 17.3.

Fabrication of Cloth:

Raw cotton passes through the following series of preliminary operations:

(a) Mixing and blending of cotton from different bales,

(b) Opening and loosening of the fibres,

(c) Mechanical cleaning in special machines to form an endless band,

(d) Carding for removal of impurities and very short fibres,

(e) Combing,

(f) Spinning,

(g) Slashing:

In this operation the wrap is strengthened by softening it with sizing materials like starch and dextrin which are applied in the form of a thick paste. Other alternative chemicals used in the sizing carboxymethyl cellulose and polyvinyl alcohol.

(h) Weaving and Knitting:

The yarn is woven into cloth in power loom or plain looms. This is a continuous process. The fabric at this stage is called ‘grey cloth’.

In the case of fabrics made of synthetic fibres only and cotton synthetic blends, the preparatory processes are limited to the following:

(a) Texturisation,

(b) Spinning,

(c) Sizing, and

(d) Weaving and knitting.

Processes of Cloth:

The grey cloth from the weaving section passes through a series of operations in the processing section, such as desizing, kiering, bleaching, dyeing and printing. All these processes are wet processes. The wet processing of fabrics made from synthetic fibres and cotton synthetic blends of desizing, bleaching, dyeing, printing and finishing.

1. Desizing:

The sizing material present in the ‘grey cloth’ is removed by one of the following two methods:

(a) Desizing with Enzymes:

The cloth is steeped in malt or enzyme bath followed by an acid treatment. The enzymes act on starch and other materials to produce readily soluble and washable compounds.

(b) Desizing by Grey Sour:

In this process the cloth is treated with dilute sulphuric acid and is followed by kiers.

2. Kiering:

The kiering process consists of boiling the cloth to remove grease, waxes, natural fats and other impurities by an alkaline solution containing 1 to 3 percent of caustic soda, soda ash, sodium silicate, sodium peroxide and other chemicals for several hours with the aid of stream.

The process is carried out either on a batch basis as in some of the older mills or on continuous basis in special units like J boxes in the modern mills. The spent liquor is blown out followed by washing of the cloth with a large volume of water.

3. Bleaching:

The fabric from the kiers subsequently undergoes bleaching with alkaline hypochlorite solution or peroxide (hydrogen peroxide is used for bleaching of delicate fabrics). In peroxide bleaching, sodium peroxide, sulphuric acid, caustic soda and a soluble oil are used. Sometimes peroxide bleaching is carried out directly in the kier itself or as a second stage after the kier.

The bleaching operation is followed by a wash with plain water and then by souring (acid and sodium bisulphite) to remove the last traces of alkali and chlorine. The fabric is then soaped and treated with ultramarine blue to improve its whiteness. The bleaching of synthetic fabrics is carried out only with mild bleaching agents like sodium chlorite.

4. Mercerisation:

The process consists of treating cloth with cold concentrated solution of caustic soda (200 g/l) and then washing with profuse quantities of water by a counter- current system. The strong caustic is reused. The fabric is then treated with a diluted acid solution to remove the last traces of alkali, followed by a second rinse with fresh water. Mercerization improves the strength elasticity, lustre and affinity for colours.

5. Dyeing:

The dyeing of cloth is carried out with a variety of dyes and dyeing processes which are briefly described below:

(a) Direct Dyes:

These are neutral substances and are applied to the fabric without any mordants. These are easy to apply, fairly fast to light and are stable for washing.

(b) Basic Dyes:

These are neutral substances of colour base and are applied with an acid like tannic acid, as mordant followed by a solution of tartar emetic. These dyes are noted for their bright colours.

(c) Vat Dyes:

Vat dyes belong to the class of fast dyes which are insoluble in water but are applied with the aid of powerful reducing agents such as sodium hydrosulphite and alkali. The cloth is then exposed to oxidation by air or oxidising agents followed by souring to neutralise the excess of alkali. The fabric is washed at end of each operation.

(d) Sulphur Dyes:

These contain sulphur compounds and are applied along with a reducing agent like sodium sulphide followed by oxidation.

(e) Naphthol Dyes:

The colour is developed on the fibre by a two-stage process consisting of:

(1) Treatment with betenaphtol and

(2) Diazotization. This is followed by hot soap and caustic soda.

(f) Developed Dyes:

In this the dyeing is carried out in a reverse manner. The colour is first directly applied to the fabric and then treated with sodium nitrite and acid to convert it into the unstable azo dye. Next the stable form of the colour is developed in the fibre by the action of betanaphtol.

(g) Dyeing of Yarn:

In the mills the yarn is also dyed as such depending upon the requirements. Initial scouring is followed by bleaching and then each lot is dyed by circulating the dye liquor and subsequently finished and reused.

The synthetic fibres do not have natural affinity for dyes and colours and hence the dyeing process is carried out at an elevated temperature and pressure in an acid medium containing acetic acid at a pH range of I to 2 in special machines. This is followed scouring with detergents and soap.

6. Printing:

Printing of cotton textiles is carried out either by means of hand block printing or roller printing in machines. Different techniques are employed, depending upon the dye used. The colour is applied as a thick paste along with starch or gum and mordant. The printing of synthetic textiles and synthetic cotton blends is followed by ageing, steaming and carbonizing.

Water Usage:

The water requirement for textile processing is large and varies from one mill to another and this depends on factors like:

(a) Source of water and its availability;

(b) The quantity and quality of the fabrics produced; and

(c) The types of processing and its sequence.

The water usage for different purposes in a typical cotton textile mill and synthetic processing mill is given in Table 17.8.

The water usage in a group of textile mills surveyed is shown in Table 17.9.

To produce one metre of finished cloth the water consumption is in the range of 12 to 65 litres. The longer the processing sequence, the greater will be the quantity of water used. The processing of yarn also requires equally large volume of water. Bulk of the water consumed is in the washing of the fabric at the end of each process.

Sources, Quantities and Characteristic of Waste Waters:

Sources:

The main sources of waste water in a textile mill are the following:

(a) Preparation of the yarn slashing process waste;

(b) Textile processing-(i) desizing, (ii) kier, (iii) bleaching, (iv) mercerisation and (v) dyeing and printing;

(c) Waste water from the washing operations in each step of textile process;

(d) Miscellaneous waste streams, namely, cooling water and boiler blow down, housekeeping (floor washing and washing of equipment), and spills and leaks; and

(e) Sanitary waste.

Quantities:

The quantity of the wastes discharged from each unit process is very small compared to the volume of waste derived from the washings and rinsing of the cloth. The quantities of the waste are highly variable from one mill to another and depend upon the same factors as mentioned in 3. The waste water discharged by the mills lies in the range of 57 to 800 m³ per 1000 kg of cloth produced, as shown in Table 17.8.

Characteristics:

a. Slashing Wastes:

Contain mostly starch and softeners and the material left over after completion of the slashing operation and washing. The waste volume is small, but has a high biochemical oxygen demand (BOD).

b. Desizing Wastes:

Consist of mostly of washings and contain colloidal and dissolved organic matter derived from the hydrolysis of starch by enzymes. The waste volume is small but has a high BOD which amounts to 35 to 50 percent of the total BOD.

c. Kier Wastes:

The spent kier liquor and the washings are dark brown in colour and are strongly alkaline. They contain caustic soda and suspended solids most of which are fragments of cotton. The wastes have high pH, total dissolved solids, alkalinity and BOD. The BOD of the kier liquor is about 30 to 35 percent of the total waste load.

d. Bleaching and Mercerizing Wastes:

These are small in volume but highly alkaline. The wastes contain organic matter and the chemicals used such a hypochlorite, chlorine, caustic soda and peroxide. The BOD load contributed by the wastes from bleach house is about 5 percent.

The wastes from mercerization is almost negligible both in regard to its volume waste load which amounts of less than 1 percent of the BOD load.

e. Dyeing Wastes:

The dyeing process contributes a large volume of waste water. Its composition and characteristics are highly variable and exert a high oxygen demand due to the organic matter of the dyes, reducing agents like sulphide, hydrosulphite, nitrite, acetic acid and soaps. The BOD contribution from the dye house is about six percent of the total mill waste.

f. Printing Wastes:

The waste water derived from the printing process consists of colours, starch, gum and oil used in making the pastes and soaps. The quantity of waste streams is small but has a fairly high BOD and chemical oxygen demand (COD).

g. Finishing Wastes:

The finishing of the cotton fabrics contributes a negligibly small volume waste water containing traces of starch, tallow and salts etc.

The characteristics of the waste streams from the different processes are governed by factors such as the quantity of and types of process chemicals used, water usage and intensity of the washing process as shown in Table 17.11. Chemical characteristics of the process waste water. The chemical characteristics of the different process waste waters are shown in Table 17.12.

Characteristics of Combined Mill Wastes:

The composite waste is discharged by the mills in large volume and is subject to sharp variation in colour, pH value, alkalinity, total dissolved solids, BOD and COD. It contains various process chemicals used by the different sections of the mills as shown in Table 17.13.

Characteristics of Wastes from Small Textile Processing Units:

Due to limited water usage in small processing units the wastes discharged by them are generally stronger than the combined waste from the larger mills as shown in Table 17.14.

a. Pollution Load:

The pollution load contributed by textile mills is shown in Table 8.

b. Pollution Equivalent:

The pollution equivalent per 1000 kg of cloth provided by the different mills are shown in Table 17.15.

Pollutional Effects:

Waste waters from cotton textile industry are of highly polluting nature and affect the water quality in several ways.

a. pH Value:

The high alkalinity of the wastes causes an increase in pH value. Any increase in pH value of the receiving stream greater than 9.0 will have an adverse effect on aquatic life.

b. Colour:

The soluble dyes and colours present in the wastes will persist in the stream and interfere with penetration of sunlight essential for photosynthesis.

c. Turbidity:

The collodial organic matter in the wastes will increase turbidity of the water and along with the colours, dyes and oily scum will produce an unsightly appearance. The oily scum formed on the surface of water will interfere with the mechanism of oxygen transfer at the air/water interface.

d. Oxygen Depiction:

The most serious effect of textile wastes on the receiving body of water will be depletion of dissolves oxygen. The organic matter in the textile wastes like starch, dextrin and inorganic chemicals like sulphide and hydrosulphite and nitrite will exert an immediate oxygen demand, while dyes and colours will exert long term oxygen demand.

Such changes in the oxygen balance of receiving streams will be deleterious to fish life and will also interfere with self-purification. Toxic chemicals like sulphide, chlorine, chromium and aniline dyes will also affect the aquatic life.

Effects of Textile Wastes on Sewers and Agricultural Land:

a. Effects on Sewers:

Due to high pH value, alkalinity and total dissolved solids; the textile wastes have a tendency for incrustation in the sewers. On the other hand, the sulphur dyes and sulphur compounds present in the wastes will gradually lead to crown corrosion. Hence it is desirable that process waste streams containing sulphur dyes and sulphides are excluded from the waste water discharged from the sewer.

b. Effects on Sewage Treatment Plant:

The plant and machinery of municipal sewage treatment plants will be exposed to corrosive action caused by acids and hydrogen sulphide. The pumps and pipelines may be subjected to incrustation. In any case the cumulative effect to textile wastes on sewage treatment plants will be increased cost of maintenance and repairs.

c. Effect on Agricultural Land Textile:

Wastes has adverse effects on land in many ways as follows:

(a) The suspended and collodial matter may clog the pores of the soil by forming an impervious mat;

(b) The high alkalinity may be harmful to crops and high salinity of the wastes will impair their growth;

(c) Sodium has a deflocculating effect on prolonged application;

(d) Sodium displaces divalent cations like Ca++/Mg++;

(e) Sodium hardens the texture of the soil, prevents penetration of the roots; and

(f) The soil losses it water holding capacity. As a cumulative effect, the soil will lose its productivity.

Methods of Treatment and Disposal:

The general approach to the problem of pollution abatement of textile wastes is based on the following criteria:

(a) Reduction in waste volume,

(b) Recovery and reuse of process chemicals,

(c) Reduction in waste load by substitution of process chemicals,

(d) Treatment of waste waters by primary and secondary treatment, and

(e) Application of advance waste treatment methods.

(a) Reduction in Waste Volume by Segregation and Reuse of Water:

Segregation of the relatively highly concentrated waste streams from the less contaminated streams and reuse of the latter often contribute towards greater economy in water usage and reduction in waste water volume, but the gross waste load in terms of kg/l of BOD and suspended solids will, however, remain the same.

The washing of the cotton fabric at the end of each process in a modern textile mill is the most water intensive operation and accounts for the maximum water usage which is of the magnitude of 90 to 135 litre per minute for continuous washing of the fabric which moves at a fairly fast rate of 10 to 20 kg per minute.

Besides every chemical process requires a minimum of one rinse with adequate volume of water and likewise the bleaching process also heavily depends on the intermediate washing. Attempts to reduce water usage in a textile mill are directed towards reduction in the duration of each of the processing sequences.

Use of counter current system of washing ; systematic control of bleach baths, recirculation of washings and scouring wastes and reuse of dye baths often contribute towards greater economy in water consumption, On an average reuse of water is possible to the extent of 20 to 40 percent in a cotton textile mill.

Good housekeeping and prevention of leaks and spills are important steps towards reduction in the waste volume.

(b) Recovery and Reuse of Process Chemicals:

Recovery of the following chemicals used in processing section contributes towards reduction in waste load:

(1) Recovery of synthetic sizing agents like carboxymethyl cellulose from desizing waste liquor by chemical precipitation with metal lie salts for example, aluminium sulphate, and its conversion into sodium salt by treatment with caustic soda;

(2) Caustic recovery from mercirizing and process waste by concentration and evaporation and from other wastes like kier wastes by dialysis; and

(3) Recovery of Dyes:

In the vat dyeing process only about 65 to 75 percent of the dye is absorbed by the fibre and the rest is drained out. This category of dyes may be recovered and reused in the same dyeing process. Similarly bright coloured lakes, lacquers, etc. may be recovered from basic and reactive dye waste liquors by precipitation of their metallic derivatives.

(c) Reduction of Waste Load:

i. Substitution of Process Chemicals:

Use of carboxymethyl cellulose (CMC) and polyvinyl alcohol (PVA) in the place of starch used in sizing of cloth, use of mineral acids; instead of acetic acid in dyeing process and use of non-biodegradable detergents instead of soap are suggested ways and means for the reduction in BOD load of combined textile wastes to the extent of 50 percent.

ii. Process Modifications:

Modification of certain manufacturing processes in the textile mills such as use of high speed machinery with alternative process chemicals have been reported in the literature to be beneficial in reducing pollutional load.

(d) Treatment of Waste Waters by Primary and Secondary Treatment:

The various methods employed in the treatment of textile mill waste waters are briefly described in the following clauses:

1. Pretreatment of Wastes Equalization:

Equalization of waste streams over a period of time is essential for regulation of waste and prevention of shock loads created by batch dumping of certain strong waste like the kier liquor. The effect of such release is sudden increase in temperature, pH value, alkalinity and BOD. Such waste streams should be segregated and contained in holding tanks of adequate capacity and released gradually at a more uniform rate.

2. Primary Treatment:

This consists of-(a) Screening of wastes for removal of coarse floating and suspended material, and

(b) Grit removal for separation of heavier and readily settle able material.

3. Neutralisation of Wastes:

The optimum pH value range for biological treatment of waste lies between 6.0 to 9.0. Textile wastes are strongly alkaline with high amounts of free alkali and therefore neutralization of the waste for pH adjustment is essential prior to biological treatment. Dilute sulphuric acid, carbon dioxide or flue gas may be used.

4. Chemical Treatment:

Waste streams from bleaching and dyeing processes or composite mill wastes are treated with conventional coagulants like alum, ferrous sulphate, ferric chloride and chlorinated copper as. Chemical coagulation is very effective for removal of colour and colloidal organic matter like starch and gums. More recently, certain coagulant aids like polyelectrolytes are used in combination with chemical coagulants.

Reports on studies carried out by various research organizations in the country and abroad on chemical treatment of textile wastes indicate that alum and ferrous sulphate can be successfully used for colour removal and BOD and COD reduction to a satisfactory level.

The results of one of the research investigations are summarized in Table 17.17. Chemical treatment results in sludge production which needs to be separated out and dried on sand drying beds and disposed off as land fill.

The layout for chemical treatment plant will consist of the following units:

(a) Equalization tank,

(b) Chemical house with dosing equipment,

(c) Flash mixer,

(d) Coagulation tank,

(e) Settling tank which may be either a fill and draw type unit for small textile mills or continuous flow type with mechanical desludging equipment for large waste volume, and

(f) Sludge drying beds.

5. Secondary Treatment:

Biological treatment of textile mill wastes is considered necessary when the wastes are to be discharged into rivers and lakes from which water is derived for community and industrial water supply and for fish culture. Textile wastes contain carbon, nitrogen and phosphorous in the requisite proportions and are amenable to treatment by conventional biological systems.

However, biological treatment in combination with domestic sewage is advantageous from the point of economics of treatment, dilution, availability of supplementary nutrients and seeding with micro-organisms.

The biological methods applied for treatment for the wastes are the following:

(a) Trickling filter,

(b) Activated sludge process,

(c) Oxidation ditch,

(d) Aerated lagoon, and

(e) Oxidation pond.

Since the waste water are almost of the same length as normal sewage in regard to BOD, aerobic methods are preferred for their treatment.

(a) Trickling Filter:

The results of biological treatment of textile wastes by trickling filters, as reported in literature, indicate BOD removal in the range of 90 to 95 percent. Conventional high rate filters and their modification like totally enclosed filters with forced ventilation can also be applied.

(b) Activated Sludge Process:

The conventional activated sludge process has been applied successfully in the treatment of textile mill effluents along with sewage in USA. BOD reductions in the range of 90 to 95 percent are reported.

(c) Oxidation Ditch:

The oxidation ditch which operates on the extended aeration principle is also reported to be quite suitable for secondary treatment of textile mill wastes. So far there is no evidence of such plants being installed by textile mills anywhere in this country.

However, information on the application of oxidation ditch for textile wastes based on laboratory studies is available. The results indicate the feasibility of obtaining BOD removal efficiencies in the range of 86 to 93 percent during an aeration period of 12 hours. The effluents from the oxidation ditch will have a BOD in the range of 30 to 35 mg/l.

(d) Aerated Lagoon:

Literature survey on aerated lagoons indicated that they are quite efficient for treatment of textile wastes together with sewage. They are being installed in large numbers in USA by the textile industry. Data available from research investigations in the country indicates the feasibility of obtaining BOD removal efficiencies in the range of 78 to 95 percent with a hydraulic detention period of 6 to 7 days. The final effluent quality is reflected by the low BOD in the range of30 to 50 mg/l.

(e) Oxidation Pond:

The oxidation pond offers a simple and most economical method for treatment of textiles waste in combination with domestic sewage, especially for warm climates prevailing in this country. Pretreatment of wastes for colour removal will be more advantageous from the point of view of light penetration. Pond depths in the range of 0.9 to 1.2 m with retention period in the range of 10 to 30 days are considered suitable.

BOD loading could be of the same order as that of aerobic ponds treating sewage that is 340 kg/hectare/day BOD removal efficiencies to the extent of 75 to 80 percent may be achieved and the BOD of the final effluent will be in the range of 30 to 50 mg/l.

6. Final Treatment:

The treatment methods described above have the main objectives:

(a) Removal of colour,

(b) PH control,

(c) Removal of settleable and suspended solids, and dissolved mineral constituents and the cations.

Textiles wastes have a high sodium content usually in the range of 89 to 98 percent and the effluents resulting from the aforesaid methods will not meet the requirements of IS: 3307-1977.

Therefore, from the point of view of agricultural utilization, treatment with gypsum is considered essential to reduce the percentage of sodium in the total cation content of the waste.

(e) Application of Advanced Waste Treatment Methods:

Advance waste treatment methods for textile mill waste waters will be applicable for reclamation of water for reuse purposes, as a tertiary treatment stage, the following primary and secondary treatment stages.

The methods which may be found suitable are:

(a) Activated carbon treatment,

(b) Reverse osmosis, and

(c) Electro-dialysis.

Treatment of wastes from small textile processing units:

The methods of pretreatment of the wastes water consisting of chemical and biological treatment systems especially the low cost waste treatment systems described above are also suitable for the treatment of process waste waters from textile units. However, it will be more economical to provide a common treatment facility for a group of such units.

Treatment Practices in India:

The disposal of combined textile mill effluents in the country is as follows:

Disposal into public sewers in cities Ahemdabad, Mumbai, Bangalore, Kolkata, Kanpur, Chennai and Madurai.

Directly on land in unsewered town like Kalol and other textile centres in Gujarat and other States.

Ultimate disposal along with sewage:

(a) Into sea in Mumbai and Chennai and

(b) On land for irrigation in cities like Ahemdabad, etc.

Treatment Method Recommended for Adoption in India.

Reviewing the methods described above, it is recommended that the degree of treatment for waste waters from textile mills in India will be based on the ultimate disposal as follows:

1. For Disposal into Public Sewers:

The suggested minimum treatment will be screen in, grit removal, chemical coagulation, flocculation and sedimentation.

2. For Disposal into Streams:

Primary treatment and secondary treatment for BOD reduction.

3. For Land Disposal:

Primary treatment or secondary treatment or both followed by treatment with gypsum.

4. Treatment Process Flow Sheets:

Separate flow sheets for the treatment processes outlined in 8.2 are shown in Fig. 17.4 to 17.5.

Fig. 2. Show the flow diagram of Treatment and Disposal of Effluents of Cotton and Synthetic Textile Industry.