Reduction of ETP Load through Wastewater Segregation

Reduction of ETP Load through Wastewater Segregation

Md. Milan Hossain & Md. Belal Hossain
Department of Textile Engineering
Dhaka University of Engineering & Technology (DUET)
Gazipur -1700, Bangladesh


The aim of this study is to reduce operating cost of effluent treatment plant by segregating wastewater of textile dyeing mills. Two types of wastewater produce from the textile dyeing mills such as highly contaminated wastewater & low contaminated wastewater. Data has been collected from dyeing process and characteristics of wastewater has been analyzed interms of chemicals used to categories the drained wastewater from each single step. Environmental load by wastewater has been evaluated carefully through investigation of some key wastewater parameters. The samples have been collected instantly from the drain line of each machine over a period of one year. The experimental data has been statistically analyzed and it has been found that about 35-40% of total ETP load can be minimized through wastewater segregation.


1.1 Introduction
Textile industry is a very diverse sector in terms of raw materials, processes, products and equipment and has very complicated industrial chain. Textile wet processing covers the bleaching, dyeing, printing and stiffening of textile products in the various processing stages (fiber, yarn, fabric, knits, finished items). The purpose of processing is in every instance the improvement of the serviceability and adaptation of the products to meet the ever-changing demands of fashion and function. The impacts on the environment by textile industry have been recognized for some time, both in terms of the discharge of pollutants and of the consumption of water and energy. Finishing processes can be categorized into purely mechanical and wet processes. The liquid phase for the latter type is primarily water, and – to a lesser extent -solvents and liquefied ammonia gas. Another important medium is steam. To achieve the desired effects, a range of chemicals, dyes and chemical auxiliaries are used. Environmental problems of the textile industry are mainly caused by discharges of wastewater. The textile sector has a high water demand. Its biggest impact on the environment is related to primary water consumption and waste water discharge.

Textile processing employs a variety of chemicals, depending on the nature of the raw material and product. Main pollution factors in textile wastewater come from dyeing and finishing processes. These processes require the use of a wide range of chemicals and dyestuffs, which generally are organic compounds of complex structure. As all of these are not contained in the final product, these are drained with as waste and caused disposal problems. Major pollution factors in textile waste waters are high suspended solids, chemical oxygen demand, heat, color, acidity, and other soluble substances. To remove these pollutants, operating cost of ETP is very high. The effluents resulting from these processes differ greatly in composition, due to differences in processes, used fabrics and machinery. Some of them are highly contaminated and some are nearly harmless. Scope of current project is aimed to analyze the direct discharge of low contaminated wastewater as the ETP operating cost can be minimized.

1.2 Objectives
The overall objective of this study was to reduce ETP load by segregating wastewater of dyeing cycle. Specific objectives of this study were:

  1. To reduce overall ETP load.
  2. To determine different parameters of dyeing wastewater.
  3. Comparing the quality parameter of selected wastewater with the standard values.
  4. To determine the safe steps for direct discharge.
  5. To save the Environment.

1.3 Processes and chemicals used in dyeing industries
Dyeing is the process in which a dye molecule gets thoroughly dissolved and dispersed in the Solvent. It can be dissolved in water or some other Solvent also, but it must be able to penetrate and color the textile materials in the process. In the textile dyeing process the dyeing is carried out at different stages like polymer, fiber, yarn, fabric and garments or even at the product stage. Textiles are dyed using a wide range of chemicals and dyestuffs. Dyes are normally synthetic molecules and are sold as powders, granules, pastes, and liquid dispersions. Dyeing can be performed using batch or continuous processes. In batch dyeing, a quantity of the textile is loaded into a dyeing machine and put in contact with the dye liquor. Auxiliary chemicals and bath conditions are used to accelerate the dyeing action. The dye is then fixed using heat and/ or chemicals, and a wash removes unfixed dyes and chemicals from the textile fiber or fabric.

For a dye house to turn grey goods into a finished product, several sequential steps must be occurred, including fabric preparation, dyeing, printing and finishing. Of all the fabric finishing unit processes; scouring, bleaching, dyeing and finishing are the most water-intensive and the object of present study is to reduce ETP load.

Table 1.1 Production Process and Effluent Generation

Types of Pollutants present in the typical effluent of Textile dyeing and Finishing
SL. No.Processing UnitsPossible pollutants in the waste waterWaste water volumeNature of waste water
01.ScouringNaOH, Waxes, Grease, Na2CO3, Na2O, SiO2 and fragments of cloth.Small 10 L/Kg of fabricStrongly alkaline, dark Color, High BOD (30% of total)
02.BleachingNaOCl, Cl2, NaOH, H2O2, Acids etc.Mostly WashingAlkaline constitutes, approx 5% of BOD.
03.DyeingVarious dyes, salts, alkalies, acids, Na2S, Na2S2O4 and soap etc.LargeUnfixed dye (5-40%), salts, fairly BOD (6% of total), high COD, Dissolved solids, low suspended solids, heavy metal alkali, Oxidizing and reducing agents , Organic acids and Cationic fixing agents etc.
04.FinishingTraces of starch, tallow & different finishing agents.Very smallSlightly alkaline and Low BOD

1.3.1 Fabric preparation
Fabric preparation is a series of treatment steps to remove impurities that may interfere with the subsequent dyeing, printing and finishing processes. The preparation treatments usually include desizing, scouring and bleaching, but may also include singeing (a dry process) and mercerizing. The four wet processes are discussed here.

1.3.2 Scouring
Scouring is performed to remove any impurities present in the fabric. The impurities (i.e. oil & wax, lubricants, dirt, surfactants, residual tints) are removed using an alkaline solution, typically sodium hydroxide, at high temperatures to breakdown or emulsify and saponify impurities. The specific scouring procedures vary with the type of fiber or cloth construction. Because soaps and detergents used during scouring may precipitate in hard water, process water is usually softened prior to the start of the scouring process.

1.3.3 Bleaching
Bleaching is the removal of unwanted color from the textile fibers and typically involves the use of one of three main bleaching agents: hydrogen peroxide, sodium hypochlorite or sodium chlorite. Hydrogen peroxide bleaching is performed under alkaline conditions and, as a result, may be combined with the scouring process. The bleaching process includes three main steps: (1) saturating the fabric with the bleaching agent and other necessary chemicals; (2) raising the temperature to the recommended level for the particular textile and maintaining that temperature for a set period of time; and (3) thoroughly washing and drying the fabric.

1.3.4 Neutralization
Neutralization is the process which is performed after every highly alkaline process (Scouring & bleaching, Mercerization and Dyeing) to reduce the alkalinity for smooth fault free processing the textile goods. Generally chemical used for neutralization is acetic acid (CH3COOH). Acid reacts with alkali to minimize the alkalinity of solution and produce a salt and water. This salt remains as a contamination in the wastewater which may create problem for environment.

1.3.5 Mercerizing
Mercerization is a chemical process used to increase the dye ability, strength and appearance of cotton and cotton fabrics. The process requires passing the fabric through a solution of sodium hydroxide. The fabric is then stretched and sprayed with hot water to removed most of the caustic solution. Further removal of the sodium hydroxide solution occurs during subsequent washes while the fabric is held in tension. An acid treatment and several more washes may follow for further neutralization of any remaining hydroxide.

1.3.6 Dyeing
Dyeing adds color to fabrics through the use of several chemicals and dyestuffs, depending on the fabric and processes used. Dyeing is performed in either continuous or batch modes. In the continuous dyeing process, the fabric is passed through a dye bath of sufficient length. The dye is fixed to the fabric using chemicals or steam, and then washed to remove any excess dyes and chemicals (Hendricks, 1995). The batch dying process is similar, though the dye application stage occurs in a dyeing machine where the textile and dye solution are brought to equilibrium. The use of chemicals and/or heat optimizes the batch process. Washing also follows the batch dye application stage.

Common methods of batch dyeing include jig, jet, beam and beck processing. Each dyeing process requires a different dye bath ratio, or the amount of dye needed per unit of fabric. The dye bath ratio ranges from 5 to 50 depending on the type of dye, dyeing system and fabric type (EPA, 1998).

The dyeing process can take place at different stages of the fabric development. Stock dyeing is used to dye textile fibers prior to their incorporation into yarn. Yarn dyeing, including stock, package and skein dyeing, occurs once the fibers have been spun into yarn but prior to the construction of the fabric. Piece dyeing, dyeing of assembled fabric, is the most common dyeing method because it gives the manufacturer maximum flexibility with the color of the fabric. The largest volume of piece dyeing uses the continuous methods of beck and jig dyeing. In beck dyeing, the fabric is passed in rope form through the dye bath until the desired color intensity is reached. Jig dyeing uses the same process, though the fabric is passed through the dye bath at full width rather than in rope form. Other piece dyeing methods include jet dyeing, where fabric is placed in a heated tube or column where jets of dye solution penetrate the fabric, or pad dyeing, used mostly to dye carpet.

1.3.7 Enzyme wash/ bio-polishing of cotton
After scouring and bleaching the cellulosic goods (usually knit fabric) are treated with acetic acid for neutralization; then the fabric is treated with a suitable enzyme within a recommended conditions (temperature, time, pH etc) for polishing the fabric surface by removing the short and immature fibers from the substrate. This process can be applied before or after dyeing (normally before dyeing) operation.

1.3.8 Printing
While dyeing is used to apply solid colors to fabrics, more intricate patterns and designs are added to fabrics during the printing step. During printing, colors are added by applying paste like dyes or pigments. Pigments are used for 75 to 85 percent of all printing operations. As discussed in Section 2.2.2, pigments remain insoluble during application, which results in no washing steps being required and very little water waste being generated. Resin binders are used to attach pigments to fabric fibers and evaporative solvents are used to transport the pigment to the fibers.

Various techniques and machinery are used for printing. The most common technique is rotary screen printing. In rotary screen printing, the fabric passes under a series of cylindrical screens each printing a different color. Once printed, the fabric passes to a drying oven where the patterns are set. Other printing techniques are direct, discharge, resist, ink-jet and heat transfer.

1.3.9 Finishing
Textile finishing
is performed to improve the appearance, texture or performance of a fabric. Qualities such as softness, luster, durability and sometimes water repelling and flame resistance of fabrics are increased with finishing processes. Both chemical and physical methods are used to finish fabrics. Chemical methods usually involve contact of the fabric with a finishing solution after which the fabric is washed and/or dried. Chemical treatments include optical finishes, absorbent and soil release finishes, softeners and abrasion-resistant finishes and physical stabilization and crease-resistant finishes. Physical finishing involves brushing, ironing or other physical means of altering fabric. Physical treatments include heat setting, brushing and napping, softening, optical finishing, shearing and compacting.

1.4 Chemicals
Following chemicals are commonly used in dyeing processes.

  • Hydrogen peroxide 50%
  • Stabilizer
  • Sequestering agent
  • Ant creasing agent
  • Caustic soda
  • Common salt
  • Hydrous
  • Acetic acid
  • Cationic softener & anionic softener
  • Detergent
  • Enzyme
  • Silicon softener
  • Soaping agent

Mainly three dyes are used in dyeing industry.

  • Reactive dyes
  • Sulfur dyes
  • Disperse dyes

1.4.1 Reactive dyes
Fiber reactive dyes derive their name from the fact that they form covalent bonds with the fiber molecules to be dyed. Molecules of fiber reactive dyes are much smaller than the complex molecules of direct dyes. Fiber reactive dyes are unique in that they become an integral part of the textile fiber that is dyed. Although more expensive than direct dyes, advantages of reactive dyes are excellent shade reproducibility and good leveling properties. These dyes can be subdivided into either “hot” or “cold” dyeing groups, based on the temperature of application. Although silk and nylons can be dyed with fiber reactive dyes, the chief fibers dyed are cellulosic and wool. These dyes are also popular for printing textiles, since even the brightest colors are wet fast.

1.4.2 Sulfur dyes
Sulfur dyes are used primarily for cotton and rayon. The application of sulfur dyes requires carefully planned transformations between the water-soluble reduced states of the dye and the insoluble oxidized form. Sulfur dyes can be applied in both batch and continuous processes; continuous applications are preferred because of the lower volume of dye required. These dyes generally have a poor resistance to chlorine. In general, sulfur blacks are the most commercially important colors and are used where good color fastness is more important than shade brightness. Sulfur dyes are not applicable to wool or silk because the fibers are chemically damaged by the dyeing process.

1.4.3 Disperse dyes
Disperse dyes are colloidal and have very low water solubility. Most of these dyes are used for polyester, nylon, acetate, and triacetate fibers. They are usually applied from a dye bath as dispersions by direct colloidal absorption. Dye bath conditions (temperature, use of carrier) are varied based on the degree of difficulty encountered by the dyes in penetrating the fiber being dyed. They are sometimes applied dry at high temperatures by means of a sublimation process followed by colloidal absorption. High temperature sublimes the dye and, once it is inside the fiber, the dye condenses to a solid colloidal state and is absorbed on the fiber.

1.5 Wastewater from dyeing industry
The textile industry, from its beginnings, is responsible for it’s requirement of the large volumes of water required for the preparation and dyeing of cloth. More recently, water consumption and waste generation have become considerable concerns for textile manufacturers and finishers. Textile industry wastewater is characterized primarily by measurements of BOD, COD, color, heavy metals and total dissolved and suspended solids. Color and turbidity both cause an aesthetic and real hazard to the environment. The aesthetic value considers the discoloration of recreational streams and waterways. The real hazards caused by color and solids in waste are dye toxicity and the ability of the coloring agents to interfere with the transmission of light through the water, thus hindering photosynthesis in aquatic plants.

Heavy metals, typically chromium and copper, are very hazardous to human and aquatic life at relatively low concentrations. Heavy metals are introduced into the wastewater of textile manufacturing through the use of pre-metalized dyes and heavy metal after washes, which are used to increase the light fastness of the finished product. Dyeing processes require a large number of water mainly for washing after dyeing and before dyeing. Wastewater from dyeing may contain color pigments, halogens, metals (e.g. cadmium, chromium, copper, and zinc), amines in spent dyes, and other chemicals used as auxiliaries in dye formulation and in the dyeing process (e.g. alkalis, salts, and reducing/oxidizing agents). Dyeing process effluents are characterized by relatively high BOD and COD values. Salt concentration (e.g. from reactive dye use) may range between 2,000 and 3,000 PPM.[source:EPA,1998]

Industry-specific wastewater effluents are related to wet operations, which are conducted during different parts of the textile manufacturing process. Process wastewater from dyeing industry is typically alkaline and has high BOD and COD loads. Pollutants in dyeing effluents include suspended solids and other organic compounds, including phenols. Effluent streams from dyeing processes are typically hot and colored and may contain significant concentrations of heavy metals (e.g. chromium, cadmium, copper, zinc). The most difficult environmental issue for the textile dyeing and finishing industry is the generation of wastewaters. Effluent quality limits can be difficult for companies to meet and are likely to become more stringent, requiring textile dyeing and finishing operations to employ waste minimization, to avoid resorting to expensive on-site treatment.

Environmental issues associated with textile industry effluents include:

  • Residual dyestuffs – toxicity, color, biodegradability
  • halogenated organic compounds
  •  heavy metal contaminants (Cr, Cu, Zn)
  • surfactants and synergistic relationship with toxicants
  • salts in effluent which is to be reused for land application
  • auxiliary agents for dyeing – toxicity and biodegradability
  • finishes – toxicity and biodegradability
  • elevated temperatures
  • high levels of total oxidized sulfur (TOS)
  • high BOD levels
Cotton Fabric production and associated water pollutants
Figure 1.1 Cotton Fabric production and associated water pollutants


2.1 Categorization of waste generated in textile industry
Textile waste is broadly classified into four categories, each of having characteristics that demand different pollution prevention and treatment approaches. Such categories are discussed in the following sections:

2.1.1 Hard to treat wastes
This category of waste includes those that are persistent, resist treatment, or interfere with the operation of waste treatment facilities. Non-biodegradable organic or inorganic materials are the chief sources of wastes, which contain colour, metals, phenols, certain surfactants, toxic organic compounds, pesticides and phosphates.

The chief sources are:

  • Colour & dyeing operation
  • metal preparatory
  • Phosphates processes and dyeing
  • Non-biodegradable organic materials surfactants

Since these types of textile wastes are difficult to treat, the identification and elimination of their sources are the best possible ways to tackle the problem. Some of the methods of prevention are chemical or process substitution, process control and optimization, recycle/reuse and better work practices.

2.1.2 Hazardous or Toxic Wastes
These wastes are a subgroup of hard to treat wastes. But, owing to their substantial impact on the environment, they are treated as a separate class. In textiles, hazardous or toxic wastes include metals, chlorinated solvents, non-biodegradable or volatile organic materials. Some of these materials often are used for non-process applications such as machine cleaning.

2.1.3 High Volume Wastes
Large volume of wastes is sometimes a problem for the textile processing units. Most common large volume wastes include:

  • High volume of waste water
  • Wash water from preparation and continuous dyeing processes and alkaline wastes from preparatory processes
  • Batch dye waste containing large amounts of salt, acid or alkali
  • These wastes sometimes can be reduced by recycle or reuse as well as by process and equipment modification.

2.1.4 Dispersible Wastes
The following operations in textile industry generate highly dispersible  waste:

  • Waste stream from continuous operation (e.g. preparatory, dyeing, printing and finishing)
  • Print paste (printing screen, squeeze and drum cleaning)
  • Lint (preparatory, dyeing and washing operations)
  • Foam from coating operations
  • Solvents from machine cleaning
  • Still bottoms from solvent recovery (dry cleaning operation)
  • Batch dumps of unused processing (finishing mixes)

2.2 Classification of waste water treatment process

Classification of waste water treatment process
Table 2.1 Classification of waste water treatment process

2.2.1 Primary Treatment
After the removal of gross solids, gritty materials and excessive quantities of oil and grease, the next step is to remove the remaining suspended solids as much as possible. This step is aimed at reducing the strength of the waste water and also to facilitate secondary treatment. Screening
Coarse suspended matters such as rags, pieces of fabric, fibers, yarns and lint are removed. Bar screens and mechanically cleaned fine screens remove most of the fibers.

The suspended fibers have to be removed prior to secondary biological treatment; otherwise they may affect the secondary treatment system. They are reported to clog trickling filters, seals or carbon beads. Sedimentation
The suspended matter in textile effluent can be removed efficiently and economically by sedimentation. This process is particularly useful for treatment of wastes containing high percentage of settable solids or when the waste is subjected to combined treatment with sewage. The sedimentation tanks are designed to enable smaller and lighter particles to settle under gravity. The most common equipment used includes horizontal flow sedimentation tanks and centre-feed circular clarifiers. The settled sludge is removed from the sedimentation tanks by mechanical scrapping into hoppers and pumping it out subsequently. Equalization
Effluent streams are collected into sump pit. Sometimes mixed effluents are stirred by rotating agitators or by blowing compressed air from below. The pit has a conical bottom for enhancing the settling of solid particles. Neutralization
Normally, pH values of cotton finishing effluents are on the alkaline side. Hence, pH value of equalized effluent should be adjusted. Use of dilute sulphuric acid and boiler flue gas rich in carbon dioxide are not uncommon. Since most of the secondary biological treatments are effective in the pH 5 to 9, neutralization step is an important process to facilitate. Chemical coagulation and Mechanical flocculation
Finely divided suspended solids and colloidal particles cannot be efficiently removed by simple sedimentation by gravity. In such cases, mechanical flocculation or chemical coagulation is employed. In mechanical flocculation, the textile waste water is passed through a tank under gentle stirring; the finely divided suspended solids coalesce into larger particles and settle out. Specialized equipment such as clariflocculator is also available, wherein flocculation chamber is a part of a sedimentation tank. In order to alter the physical state of colloidal and suspended particles and to facilitate their removal by sedimentation, chemical coagulants are used. It is a controlled process, which forms a flock (flocculent precipitate) and results in obtaining a clear effluent free from matter in suspension or in the colloidal state. The degree of clarification obtained also depends on the quantity of chemicals used. In this method, 80-90% of the total suspended matter, 40-70% of BOD, 5days, 30-60% of the COD and 80-90% of the bacteria can be removed. However, in plain sedimentation, only 50-70% of the total suspended matter and 30-40% of the organic matter settles out. Most commonly used chemicals for chemical coagulation are alum, ferric chloride, ferric sulfate, ferrous sulfate and lime.

2.2.2 Secondary Treatment
The main purpose of secondary treatment is to provide BOD removal beyond what is achievable by simple sedimentation. It also removes appreciable amounts of oil and phenol. In secondary treatment, the dissolved and colloidal organic compounds and color present in waste water is removed or reduced and to stabilize the organic matter. This is achieved biologically using bacteria and other microorganisms. Textile processing effluents are amenable for biological treatments. These processes may be aerobic or anaerobic. In aerobic processes, bacteria and other microorganisms consume organic matter as food. They bring about the following sequential changes:

  1. Coagulation and flocculation of colloidal matter
  2. Oxidation of dissolved organic matter to carbon dioxide
  3. Degradation of nitrogenous organic matter to ammonia, which is then converted into nitrite and eventually to nitrate.

Anaerobic treatment is mainly employed for the digestion of sludge. The efficiency of this process depends upon pH, temperature, waste loading, absence of oxygen and toxic materials. Some of the commonly used biological treatment processes are described below: Aerated lagoons
These are large holding tanks or ponds having a depth of 3-5 m and are lined with cement, polythene or rubber. The effluents from primary treatment processes are collected in these tanks and are aerated with mechanical devices, such as floating aerators, for about 2 to 6 days. During this time, a healthy flocculent sludge is formed which brings about oxidation of the dissolved organic matter. BOD removal to the extent of 99% could be achieved with efficient operation. The major disadvantages are the large space requirements and the bacterial contamination of the lagoon effluent, which necessitates further biological purification. Trickling filters
The trickling filters usually consists of circular or rectangular beds, 1 m to 3 m deep, made of well-graded media (such as broken stone, PVC, Coal, Synthetic resins, Gravel or Clinkers) of size 40 mm to 150 mm, over which wastewater is sprinkled uniformly on the entire bed with the help of a slowly rotating distributor (such as rotary sprinkler) equipped with orifices or nozzles. Thus, the waste water trickles through the media. The filter is arranged in such a fashion that air can enter at the bottom; counter current to the effluent flow and a natural draft is produced. A gelatinous film, comprising of bacteria and aerobic micro-organisms known as Zoo lea, is formed on the surface of the filter medium, which thrive on the nutrients supplied by the waste water. The organic impurities in the waste water are adsorbed on the gelatinous film during its passage and then are oxidized by the bacteria. Activated sludge process
This is the most versatile biological oxidation method employed for the treatment of waste water containing dissolved solids, colloids and coarse solid organic matter. In this process, the waste water is aerated in a reaction tank in which some microbial flock is suspended. The aerobic bacterial flora bring about biological degradation of the waste into carbon dioxide and water molecule, while consuming some organic matter for synthesizing bacteria. The bacteria flora grows and remains suspended in the form of a flock, which is called Activated Sludge. The effluent from the reaction tank is separated from the sludge by settling and discharged. A part of the sludge is recycled to the same tank to provide an effective microbial population for a fresh treatment cycle. The surplus sludge is digested in a sludge digester, along with the primary sludge obtained from primary sedimentation. An efficient aeration for 5 to 24 hours is required for industrial wastes. BOD removal to the extent of 90-95% can be achieved in this process. Oxidation ditch
This can be considered as a modification of the conventional Activated Sludge process. Waste water, after screening in allowed into the oxidation ditch. The mixed liquor containing the sludge solids is aerated in the channel with the help of a mechanical rotor. The usual hydraulic retention time is 12 to 24 hrs and for solids, it is 20-30 days. Most of the sludge formed is recycled for the subsequent treatment cycle. The surplus sludge can be dried without odour on sand drying beds. Oxidation pond
An oxidation pond is a large shallow pond where in stabilization of organic matter in the waste is brought about mostly by bacteria and to some extent by protozoa. The oxygen requirement for their metabolism is provided by algae present in the pond. The algae, in turn, utilize the CO2 released by the bacteria for their photosynthesis. Oxidation ponds are also called waste stabilization ponds. Anaerobic digestion
Sludge is the watery residue from the primary sedimentation tank and humus tank (from secondary treatment). The constituents of the sludge undergo slow fermentation or digestion by anaerobic bacteria in a sludge digester, wherein the sludge is maintained at a temperature of 35oC at pH 7-8 for about 30 days. CH4, CO2 and some NH3 are liberated as the end products.

2.2.3 Tertiary Treatment Processes
It is worthwhile to mention that the textile waste contains significant quantities of non-biodegradable chemical polymers. Since the conventional treatment methods are inadequate, there is the need for efficient tertiary treatment process. Oxidation techniques
A variety of oxidizing agents can be used to decolorize wastes. Sodium hypochlorite decolourizes dye bath efficiently. Though it is a low cost technique, but it forms absorbable toxic organic halides (AOX). Ozone on decomposition generates oxygen and free radicals and the later combines with colouring agents of effluent resulting in the destruction of Colour. The main disadvantage of these techniques is it requires an effective sludge producing pretreatment. Electrolytic precipitation & foam fractionation:
Electrolytic precipitation of concentrated dye wastes by reduction in the cathode space of an electrolytic bath been reported although extremely long contact times were required. Foam fractionation is experimental method based on the phenomena that surface-active solutes collect at gas-liquid interfaces. However, the chemical costs make this treatment method too expensive. Membrane technologies:
Reverse osmosis and electro dialysis are the important examples of membrane process. The TDS from waste water can be removed by reverse osmosis. Reverse osmosis is suitable for removing ions and larger species from dye bath effluents with high efficiency (up to > 90%), clogging of the membrane by dyes after long usage and high capital cost are the main drawbacks of this process. Dyeing process requires use of electrolytes along with the dyes. Neutral electrolyte like NaCl is required to have high exhaustion of the dye. For instance, in cotton dyeing, NaCl concentration in the dyeing bath is in the range of 25-30 g/l for deep tone and about 15 g/l for light tone, but can be as high as 50 g/l in exceptional cases. The exhaustion stage in reactive dyeing on cotton also requires sufficient quantity of salt. Reverse osmosis membrane process is suitable for removing high salt concentrations so that the treated effluent can be re-used again in the processing. The presence of electrolytes in the washing water causes an increase in the hydrolyzed dye affinity (for reactive dyeing on cotton) making it difficult to extract. In electro dialysis, the dissolved salts (ionic in nature) can also be removed by impressing an electrical potential across the water, resulting in the migration of cat ions and anions to respective electrodes via anionic and cationic permeable membranes. To avoid membrane fouling it is essential that turbidity, suspended solids, colloids and trace organics be removed prior to electro dialysis. Electro chemical processes
They have lower temperature requirement than those of other equivalent non-electrochemical treatment and there is no need for additional chemical. It also can prevent the production of unwanted side products. But, if suspended or colloidal solids were high concentration in the waste water, they impede the electrochemical reaction. Therefore, those materials need to be sufficiently removed. Ion exchange method
This is used for the removal of undesirable anions and cat ions from waste water. It involves the passage of waste water through the beds of ion exchange resins where some undesirable cat ions or anions of waste water get exchanged for sodium or hydrogen ions of the resin. Most ion exchange resins now in use are synthetic polymeric materials containing ion groups such as sulphonyl, quarternary ammonium group etc. Photo catalytic degradation
An advanced method to decolorize a wide range of dyes depending upon their molecular structure. In this process, photoactive catalyst illuminates with UV light, generates highly reactive radical, which can decompose organic compounds. Adsorption
It is the exchange of material at the interface between two immiscible phases in contact with one another. Adsorption appears to have considerable potential for the removal of colour from industrial effluents. Owen (1978) after surveying 13 textile industries has reported that adsorption using granular activated carbon has emerged as a practical and economical process for the removal of colour from textile effluents. Thermal evaporation
The use of sodium per sulfate has better oxidizing potential than NaOCl in the thermal evaporator. The process is ecofriendly since there is no sludge formation and no emission of the toxic chlorine fumes during evaporation. Oxidative depolarization of reactive dye by per-sulfate due to the formation of free radicals has been reported in the literature.

2.3 PH
PH is a measure of the acidic or alkaline condition of water. It is a way of expressing the hydrogen ion concentration, or more preciously, the hydrogen ion activity. PH is defined as follows:

pH = -log [H+] ………………………(1)

Where {H+} is the concentration of hydrogen ion(proton) in moles per liter(M).

Water dissociates to form hydrogen ion (H+) and hydroxyl ion (OH-) according to the following equation:

H2O =H+ +OH   …………………..(2)

At equilibrium, we can write,

Kw = [H+] [OH]/ [H2O] …………..(3)

But, since concentration of water is extremely large (Approximately 55.5 mol/ L) and is diminished very little by the slight degree of ionization, it may be considered as a constant and its activity is taken as 1.0 Thus Eq. 3 may be written as:

Kw = [H+] [OH ] …………………….(4)

Where Kw = equilibrium Constant

For pure water at 25oC, Kw = 10-7 × 10-7 = 10-14. This is known as the ion product of water or ionization constant for water. In other words, water (de- ionized or distilled water) at 25oC dissociates  to yield 10-7 mol /L of hydrogen ion  and 10-7 mol /L of hydroxyl  ion. Hence, according to Eq. 1 pH of de-ionized water is equal to 7.0

The pH is usually represented by a scale ranging from zero to 14, with 7 being neutral. Groundwater is often found to be slightly acidic due to the presence of excess carbon- di-oxide. Aeration removes carbon dioxide and hence causes a rise in pH value. Some natural waters are sometimes found to be slightly alkaline sue to the presence of bicarbonate and, less often, carbonate. Water with pH outside the desirable neutral range may exhibit sour taste and accelerate the corrosion of metallic plumbing fittings and hot water services.

2.3.1 Environmental significance of pH
A controlled value of pH is desired in water supplies, sewage treatment and chemical process plants. In water supply pH is important for coagulation, disinfection, water softening and corrosion control. In biological treatment of wastewater, pH is an important parameter, since organisms involved on treatment plants are operative within a certain pH range. According to Bangladesh Environment Conservation Rule (1997). Drinking water standard for PH is 6.5- 8.5

Table 2.2: Limiting pH Values

3.810.0Fish eggs could be hatched, but deformed young were often produced
4.010.1Limits for the most resistant fish species
4.19.5Range tolerated by trout
4.3Carp died in five days
4.59.0Trout eggs and larvae develop normally
4.69.5Limits for perch
5.0Limits for stickleback fish
5.09.0Tolerable range for most fish
8.7Upper limit for good fishing waters
5.411.4Fish avoided waters beyond these limits
6.07.2Optimum (best) range for fish eggs
1.0Mosquito larvae were destroyed at this pH value
3.34.7Mosquito larvae lived within this range
7.58.4Best range for the growth of algae

2.4 Hardness:
Hard waters are generally considered to be those waters that require considerable amounts of soap to produce foam or lather and that also produce scale in hot-water pipes, heaters, boilers, and other units in which the temperature of water is increased substantially. The hardness of water varies considerably from place to place. In general, surface water is softer than groundwater. The hardness of water reflects the nature of the geological formations with which it has been in contact. Hardness is caused by multivalent metallic cations.  Such cations are capable of reacting with soap to form precipitates and with certain anions present in water to form scale. The principal hardness causing cat ions are the divalent calcium, magnesium, strontium, ferrous iron, and manganous ions. These cations and the important anions with they are associated are in Table 1 in the order of their relative abundance in natural waters. Aluminum and ferric ions are sometimes considered as contributing to the hardness of water. However, their solubility is so limited at pH values of natural waters that ionic concentrations are negligible. The hardness of water is derived largely from contact with the soul and rock formations.

Table 2.3 Classification of Water according to the degree of hardness

Classification of waterHardness
Softless than 50 mg/l as CaCO3
Moderately hard50-150 mg/l as CaCO3
Hard150-300 mg/l as CaCO3
Very hardabove 300 mg/l as CaCO3

2.4.1 Environmental significance of hardness
Hard waters are as satisfactory for human consumption as soft waters. Because of their adverse action with soap, however, their use for cleaning purpose is quite unsatisfactory, unless soap costs are disregarded. Soap consumption by hard water represents an economic loss to the water user. Sodium soaps react with multivalent metallic cations to form a precipitate, thereby losing their surfactant properties. Boiler scale, the result of the carbonate hardness precipitation, may cause considerable economic loss through fouling of water heater and hot water pipes. Changes in pH in the water distribution systems may also result in deposits of precipitates. Bicarbonates begin to convert to the less soluble carbonates at pH values above 9.0.

2.5 Total Solids (TS)
The term “total solids” refers to matter suspended or dissolved in water or wastewater, and is related to both specific conductance and turbidity. Total solids (also referred to as total residue) is the term used for material left in a container after evaporation and drying of a water sample. Total Solids includes both total suspended solids, the portion of total solids retained by a filter and total dissolved solids, the portion that passes through a filter. Total solids can be measured by evaporating a water sample in a weighed dish, and then drying the residue in an oven at 103 to 105°C. The increase in weight of the dish represents the total solids. Instead of total solids, laboratories often measure total suspended solids and/or total dissolved solids.

2.6 Total Dissolved Solids (TDS)
Total Dissolved Solids (TDS) are solids in water that can pass through a filter (usually with a pore size of 0.45 micrometers). TDS is a measure of the amount of material dissolved in water. This material can include carbonate, bicarbonate, chloride, sulfate, phosphate, nitrate, calcium, magnesium, sodium, organic ions, and other ions. A certain level of these ions in water is necessary for aquatic life. Changes in TDS concentrations can be harmful because the density of the water determines the flow of water into and out of an organism’s cells (Mitchell and Stapp, 1992).

However, if TDS concentrations are too high or too low, the growth of many aquatic lives can be limited, and death may occur.

2.7 Total Suspended Solid (TSS)
Total Suspended Solids (TSS) is solids in water that can be trapped by a filter. TSS can include a wide variety of material, such as silt, decaying plant and animal matter, industrial wastes, and sewage. High concentrations of suspended solids can cause many problems for stream health and aquatic life.

High TSS can block light from reaching submerged vegetation. As the amount of light passing through the water is reduced, photosynthesis slows down. Reduced rates of photosynthesis causes less dissolved oxygen to be released into the water by plants. If light is completely blocked from bottom dwelling plants, the plants will stop producing oxygen and will die. As the plants are decomposed, bacteria will use up even more oxygen from the water. Low dissolved oxygen can lead to fish kills. High TSS can also cause an increase in surface water temperature, because the suspended particles absorb heat from sunlight. This can cause dissolved oxygen levels to fall even further (because warmer waters can hold less DO), and can harm aquatic life in many other ways, as discussed in the temperature section.

The decrease in water clarity caused by TSS can affect the ability of fish to see and catch food. Suspended sediment can also clog fish gills, reduce growth rates, decrease resistance to disease, and prevent egg and larval development. When suspended solids settle to the bottom of a water body, they can smother the eggs of fish and aquatic insects, as well as suffocate newly hatched insect larvae. Settling sediments can fill in spaces between rocks which could have been used by aquatic organisms for homes.

High TSS in a water body can often mean higher concentrations of bacteria, nutrients, pesticides, and metals in the water. These pollutants may attach to sediment particles on the land and be carried into water bodies with storm water. In the water, the pollutants may be released from the sediment or travel farther downstream (Federal Interagency Stream Restoration Working Group, 1998). High TSS can cause problems for industrial use, because the solids may clog or scour pipes and machinery.

2.8 Dissolved Oxygen (DO)
Dissolved oxygen (DO) is the amount of oxygen that is dissolved in water and is essential to healthy streams and lakes. The dissolved oxygen level can be an indication of how polluted the water is and how well the water can support aquatic plant and animal life. Generally, a higher dissolved oxygen level indicates better water quality. If dissolved oxygen levels are too low, some fish and other organisms may not be able to survive. Much of the dissolved oxygen in water comes from oxygen in the air that has dissolved in the water. Some of the dissolved oxygen in the water is a result of photosynthesis of aquatic plants. Other factors also affect DO levels such as on sunny days high DO levels occur in areas of dense algae or plants due to photosynthesis. Stream turbulence may also increase DO levels because air is trapped under rapidly moving water and the oxygen from the air will dissolve in the water.

In addition, the amount of oxygen that can dissolve in water (DO) depends on temperature. Colder water can hold more oxygen in it than warmer water. A Generally, a higher dissolved oxygen level indicates better water quality. If dissolved oxygen levels are too low, some fish and other organisms may not be able to survive. Much of the dissolved oxygen in water comes from oxygen in the air that has dissolved in the water. Some of the dissolved oxygen in the water is a result of photosynthesis of aquatic plants. Other factors also affect DO levels such as on sunny days high DO levels occur in areas of dense algae or plants due to photosynthesis. Stream turbulence may also increase DO levels because air is trapped under rapidly moving water and the oxygen from the air will dissolve in the water. In addition, the amount of oxygen that can dissolve in water (DO) depends on temperature. Colder water can hold more oxygen in it than warmer water. A difference in DO levels may be detected at the test site if tested early in the morning when the water is cool and then later in the afternoon on a sunny day when the water temperature has risen. A difference in DO levels may also be seen between winter water temperatures and summer water temperatures. Similarly, a difference in DO levels may be apparent at different depths of the water if there is a significant change in water temperature. Dissolved oxygen levels typically can vary from 0 – 18 parts per million (PPM) although most rivers and streams require a minimum of 5 – 6 PPM to support a diverse aquatic life. Additionally, DO levels are sometimes given in terms of Percent Saturation.

2.9 Biochemical Oxygen Demand (BOD)
When biodegradable organic matter/waste (the most common category of pollutant affecting surface water) is released into a water body, micro-organisms (especially bacteria) feed on the wastes, breaking it down to simpler organic and inorganic substances. When this decomposition takes place in an aerobic environment, it produces non- objectionable, stable end products (e.g., CO2, SO4, PO4 and NO3) and in the process draws down the dissolved oxygen (DO) content of water.

Organic matter + O2 = CO2 + H2O + New cells + Stable products exists, higher lice forms are killed or driven off. Noxious condition, including floating sludge, bubbling, odorous gases and slimy fungus growth prevails.

2.10 Chemical Oxygen Demand (COD)
Chemical oxygen demand (COD) is a measure of the ability of chemical reactions to oxidize matter in an aqueous system. The chemical oxygen demand (COD) test is widely used as a means of measuring the organic strength of domestic and industrial wastes. This test allows measurement of a waste in terms of the total quantity of oxygen required for oxidation to carbon dioxide and water. The test is based on the fact that all organic compounds, with a few exceptions can be oxidized by the action of strong oxidizing agents under acid conditions. During the determination of COD, Organic matter is converted to carbon dioxide and water regardless of the biological assimilability of the substance. As a result, COD values are greater than BOD values, especially when biologically resistant organic matter (e.g., lignin) is present.


3.1 Materials and apparatus

3.1.1 Sample: Untreated Industrial wastewater.

3.1.2 Chemicals/Reagents:
The following are the chemical used in this study.

All the chemicals are of analytical reagent grade.

  1. Potassium chromate indicator
  2. Standard  02N sulfuric acid
  3. Concentrated sulfuric acid
  4. Manganese sulfate solution
  5. Alkaline potassium iodide solution
  6. Standard  025N sodium thiosulfate solution(N/40)
  7. Fleshly prepared starch solution
  8. EDTA solution(N/40)
  9. Ammonium chloride-Ammonium hydroxide  buffer
  10. Potassium iodide
  11. potassium permanganate

3.1.3 Apparatus:
The following are the Apparatus which are used in this study.

  1. pH meter
  2. TDS meter
  3. Hardness Test kit
  4. Incubator
  5. Dryer
  6. Electronic Balance

3.2 Selection of factory
Factories are selected on the base of dyeing process and region for this study because of there are some differences between knit dyeing & yarn dyeing process and quality of water varies from region to region. The following are the selected factory from where wastewater is collected

A) Knit dyeing factory:


B) Yarn dyeing factory:


3.3 Common process of yarn and knit fabric dyeing in textile industries

3.3.1 Common Process of Dyeing of Cotton Knit Fabric using in most of the conventional and modern knit dyeing industries:


  1. Demineralization—–600C x 15min
  2. Scouring & Bleaching—–950C x 45min
  3. Hot wash with H2O2 Killer —–800C x 15min
  4. Rinse for 5 to 10 min
  5. Neutralization
  6. Enzyme Treatment —–550C x 60-90min
  7. Hot wash at 600 for 15min
  8. Rinse for 5 to 10 min
  9. Dyeing —–600C x 60 min
  10. Rinse 5 to 15 min
  11. Hot wash at required temperature
  12. Soaping
  13. Hot wash
  14. Rinse for 5 to 10 min
  15. Finishing

Generally Water of the above Process no. 4, 5, 7, 8 10 & 14 is considered as low contaminated wastewater because of no chemical is required to carryout these process except process no.5.

3.3.2 Common Process of Dyeing of Cotton Yarn using in most of the conventional and modern Yarn dyeing industries:


  1. Demineralization—–600C x 15min
  2. Scouring & Bleaching—–950C x 45min
  3. Hot wash with H2O2 Killer —–800C x 15min
  4. Rinse for 5 to 10 min
  5. Neutralization
  6. Rinse for 5 to 10 min
  7. Dyeing —–600C x 60 min
  8. Rinse 5 to 15 min
  9. Hot wash at required temperature
  10. Soaping
  11. Hot wash
  12. Rinse for 5 to 10 min
  13. Finishing

Generally Water of the above process no. 4, 5, 6, 8 & 11 is considered as low contaminated wastewater because of no chemical is required to carryout these process except process no.5.

3.4 Testing of wastewater parameter

Waste Quality Standards for Discharge Point of Industrial Units and Projects
Table 3.1 Waste Quality Standards for Discharge Point of Industrial Units and Projects [source:ECR-1997]
Among the above mentioned waste qualities PH, Hardness, TDS, TSS, BOD and COD are most important water quality for textile dyeing wastewater to discharge into environment. So those waste qualities are measured by the following procedures

3.4.1 Determination of PH of Water

Reagent: Standard pH solution for calibration of pH Meter


  • Perform calibration of the pH meter using standard pH The calibration procedure would depend on the pH range of interest.
  • Take about 100ml of the sample in a beaker. Make sure not to agitate the sample in order to avoid exchange of gases between sample and atmosphere.
  • Insert pH meter in the sample. Allow sometime for attainment of equilibrium. Turn on the pH meter and take reading.

3.4.2 Determination of Hardness of Water


  1. Hardness buffer
  2. Calmagite Indicator
  3. HI 3812-0 EDTA Solution


FOR HIGH RANGE – 0 to 300 mg/L CaCO3:

  • Remove the cap from the small plastic beaker. Rinse the plastic beaker with the water sample, fill to the 5 ml mark and replace the cap.Hardness Buffer
  • Add 5 drops of Hardness Buffer through the cap port and mix carefully swirling the beaker in tight circles.
  • Add 1 drop of Calmagite Indicator through the cap port and mix as described above. The solution becomes a red violet color.
  • Take the titration syringe and push the plunger completely into the syringe. Insert tip into HI 3812-0 EDTA Solution and pull the plunger out until the lower edge of the seal is on the 0 ML mark of the syringe.
  • Place the syringe tip into the cap port of the plastic beaker and slowly add the titration solution drop wise, swirling to mix after each drop.
  • Continue adding the titration solution until the solution becomes purple, then mix for 15 seconds after each additional drop until the solution turns blue.
  • Read off the milliliters of titration solution from the syringe scale and multiply by 300 to obtain mg/L (PPM) CaCO3.

titration syringe

FOR LOW RANGE – 0.0 to 30.0 mg/L CaCO3:
If result is lower than 30 mg/L, the precision of the test can be improved by following the procedure below.

  • Remove the cap from the large plastic beaker. Rinse it with the water sample, fill to the 50 ml mark and replace the cap.
  • Proceed with the titration as for the high range test.
  • Read off the milliliters of titration solution from the syringe scale and Multiply by 30 to obtain mg/L (PPM) CaCO3.

titration solution

3.4.3 Determination of Total Solids, Dissolved Solids and Suspended Solids


For Total Solids:

  • Take a clear dry glass beaker (which was kept at 1030C in an oven for 1 hour) of 150 ml capacity and put appropriate identification mark on it. Weight the beaker and note the weight.
  • Pour 100 ml of the thoroughly mixed sample, measured by the measuring cylinder beaker.
  • Place the beaker in an oven maintained at 1030C for 24 hours. After 24 hours, cool the beaker and weight. Find out the weight of solids in the beaker by subtracting the weight of the clean beaker determined in steps (1).
  • Calculate total solids (TS) as follows:

Total solids, TS (mg/l) = mg of solids in the beaker ×1000/ Volume of sample

For Dissolved Solids:

  • Same as above ( step 1of total solids)
  • Take a 100 ml of sample and filter it through a double layered filter paper and collect the filtrate in a beaker.
  • Then repeat the same procedure as in steps (3) and (4) of total solids determination and determine the dissolved solids content as follows:

Total Dissolved Solids, TDS (mg/l) = mg of solids in the beaker × 1000/ Volume of sample

For Suspended Solids:

  • Total Suspended Solids, TSS (mg/l) = TS (mg/l) – TDS (mg/l)

3.4.4 Determination of Biochemical Oxygen Demand:


  1. Manganous sulfate solution
  2. Alkaline potassium iodide solution
  3. 025N sodium thiosulfate
  4. Starch solution
  5. Concentrated sulfuric acid.

Procedure: (for determination of DO)
The two BOD bottles with sample (or diluted sample); the bottles should be completely filled. Determine initial DO (DOi) in one bottle immediately after filling with sample (or diluted sample). Keep the other bottle in dark at 200C and after particular days (usually 5- days) determine DO (DOf) in the sample (or diluted sample). Dissolved oxygen (DO) is determined according to the following procedure:

  1. Add 1 mol of manganous sulfate solution to the BOD bottle by means of pipette, dipping in end of the pipette just below the surface of the water.
  2. Add 21 ml of alkaline potassium iodide solution to the BOD bottle in a similar manner.
  3. Insert the stopper and mix by inverting the bottle several times.
  4. Allow the precipitates to settle halfway.
  5. Add 1 ml of concentrated sulfuric acid. Immediately insert the stopper and mix as before.
  6. Aloe the solution to stand at least 5 minutes.
  7. Withdraw 100 ml of solution into an Erlenmeyer flask and immediately add 0.025N sodium thiosulfate drop by drop from a burette until the yellow color almost disappears.
  8. Add about 1 ml of starch solution and continue the addition of the thiosulfate solution until the glue color just disappears. Record the ml of thiosulfate solution used (disregard any return of the blue color).

Dissolved oxygen, DO (mg/l) = ml of 0.025n sodium thiosulfate added × 2

The five- days BOD of a diluted sample is given by
BOD5 = ( DOi – DOf) × D.F

Where D.F = Dilution Factor = (Vol. of wastewater + dilution water)/ (Vol. of wastewater)

3.4.5 Determination of chemical Oxygen Demand:


  1. Dilute sulfuric acid
  2. Standard potassium permanganate
  3. Standard Ammonium Oxalate

Procedure: (For water)

  • Pipette 100 ml of the sample into a 250 ml Erlenmeyer flask.
  • Add 10 ml. diluted sulfuric acid and 10 ml of standard potassium permanganate.
  • Heat the flask in a boiling water bath for exactly 30 minutes, keeping the water in the bath above the level of the solution in the flask .The heating enhances the rate of oxidation reaction in the Flask.
  • If the solution becomes faintly colored, it means that most of the potassium permanganate has been utilized in the oxidation of organic matter. In such a case, repeat the above using a smaller sample diluted to 100 ml with distilled eater.
  • After 30 minutes in the water bath, add 10 ml of standard ammonium oxalate into the flask. This 10 ml ammonium oxalate, which is a reducing agent, is just equivalent to the 10 ml potassium permanganate (oxidizing agent) added earlier. The excess of reducing agent (ammonium oxalate) now remaining in the flask is just equivalent to the amount of the oxidizing agent (potassium permanganate) used in the oxidation of organic matter.
  • The quantity of ammonium oxalate remaining in the flask is now determined by titration with standard potassium permanganate. Titrate the content of the flask while hot, with standard potassium permanganate to the first pink coloration. Record the ml of potassium permanganate used.


COD (mg /l) = [ml of MnO4 used in step (6) × (100)/ ml of sample used]


4.1 Segregation of Waste.
In a large industrial plant there may be several classes of waste varying in volume, strength, and toxicity. Some of them are highly contaminated and some are very low contaminated which may be discharged directly in the environment that will not affect the environmental condition i.e. not so harmful to environment. One may be readily amenable to biological treatment with domestic sewage, another may be quite inimical to such processes, and another may have value for by-product recovery. Careful study of their characteristics may indicate that each waste should be handled and treated separately.  It is especially necessary that any toxic wastes be segregated. All such segregation requires careful planning of separate plant sewer systems.

Table 4.1: Standards for Wastewater Discharge from Industrial Units and Projects according to ECR, 1997.

Sl. No.ParameterUnitPlace for Determination of Standards
2Hardness (as CaCO3)mg/L200-500200-500200-500

Note: ISW-Inland surface water, PS- Public sewerage, IL- Irrigated land,

ECR-Environmental Conservation Rules

Table 4.2: Wastewater Discharge from a one ton capacity dyeing machine.

Amount of water discharge before segregation
ProcessM:l ratioAmount of material (kg)Amount of water (l)Remarks
Scouring & Bleaching1:810008000Highly contaminated
Hot Wash1:810008000Highly contaminated
Rinse (5-10΄)1:18100018,000Low contaminated
Neutralization by CH3COOH1:810008000Low contaminated
Bio polishing1:810008000Highly contaminated
Hot Wash1:810008000Low contaminated
Rinse (10-15΄)1:18100018,000Low contaminated
Acid for Controlling pH of dyeing1:810008000Highly contaminated
Rinse (15-20΄)1:20100020,000Highly contaminated
Neutralization1:810008000Highly contaminated
Soap Wash1:810008000Highly contaminated
Hot Wash1:810008000Highly contaminated
Rinse (5-10΄)1:18100018,000Low contaminated
Softening1:1810008000Highly contaminated

Total amount of discharge water from 1000 kg capacity dyeing machine is 154,000 L

4.2 Wastewater quality parameter analysis

pH of different wet processing stages wastewater
Figure 4.1: pH of different wet processing stages wastewater.

Maximum pH value of wastewater for discharge is 6-9 according to ECR, 1997.

In the above chart, it has been found that after dyeing rinse (ADR) wastewater has exceeded the maximum limit of pH value and rest of all the wastewaters are within the control limit. So it will not be discharged into environment without treatment in ETP.

Hardness of different wet processing stages wastewater
Figure 4.2: Hardness of different wet processing stages wastewater

Maximum hardness value of wastewater for discharge is 200-500 PPM according to ECR, 1997. In the above chart, it has been found that hardness value of all the wastewaters is within the control limit.

TSS of different wet processing stages wastewater
Figure 4.3: TSS of different wet processing stages wastewater

Maximum TSS value of wastewater for discharge is 200 ml/L according to ECR, 1997. In the above chart it has been found that after enzyme hot (AEH) water has exceeded the maximum limit of TSS value and rest the wastewaters are within the control limit. So AEH will not be discharged into environment without treatment in ETP.

TDS of different wet processing stages wastewater
Figure 4.4: TDS of different wet processing stages wastewater

Maximum TDS value of wastewater for discharge is 2100 ml/L according to ECR, 1997. In the above chart, it has been found that TDS value of all the wastewaters is within the control limit.

BOD of different wet processing stages wastewater
Figure 4.5: BOD of different wet processing stages wastewater

Maximum BOD value of wastewater for discharge is 50 ml/L according to ECR, 1997. In the above chart it has been found that acid neutralization (AN) water has exceeded the maximum limit of BOD value and after dyeing rinse (ADR) water is to the nearest of the maximum limit of BOD value. As a result they will not be discharged into environment without treatment in ETP. But rest the wastewaters are within the control limit.

COD of different wet processing stages wastewater
Figure 4.6: COD of different wet processing stages wastewater

According to ECR, 1997 maximum COD value of wastewater for discharge is 200 ml/L. In the above chart it has been found that after enzyme hot (AEH) water has exceeded the maximum limit of COD value and after dyeing rinse (ADR) water is to the nearest of the maximum limit of BOD value. For that reason they will not be discharged into environment without treatment in ETP. But rest all the wastewaters are within the control limit.

Therefore, from the above experimental data analysis, it has been found that ASBR, AN, BDR and BFR wastewater have fulfilled the wastewater standards to discharge into environment without treating in ETP. But AEH and ADR have not fulfilled the wastewater standards. Consequently they are required to treat in ETP for discharging into environment.

Table 4.3 Directly dischargeable wastewater from entire dyeing process .

ProcessM:L RatioAmount of material (KG)Amount of water (L)
After scouring & bleaching rinse (5-10΄)1:18100018000
Before dyeing rinse1:18100018000
Before finishing rinse (5-10΄)1:18100018000

Total amount of segregated wastewater from 1000 kg capacity dyeing machine is 62,000L

Hence about 35-40% of ETP load may be decreased by segregating low contaminated wastewaters i.e. ASBR, AN, BDR & BFR from the highly contaminated wastewaters through suitable way.

4.3 ETP costing

a. Chemical Costing:

Table 4.4 Costing of chemical for processing wastewater in ETP

Chemical NameChemical gm/LtrSupply Ltrs /HrChem. Kg/HrPrice/Kg in TKTotal Chemical Cost/Hr in TK
De-Coloring agent21670.334100.0033.40
Total TK/Hr =2056.9

B) Manpower Cost (Per Hour):

  1. Operator = 2×6000 = Tk. 12,000
  2. Helper = 3×3000 = Tk. 9000
  3. In charge = 1×8000 = Tk. 8000

Total Cost = Tk. 29,000
Cost per hour = Tk. 40.27

C) Electricity Cost per Hour:
The calculated Unit Electricity Cost is– TK 1.68.

Supplied Amp for ETP is 80,

Then, Total KW for an Hour,

Total KW for an Hour
Electric Bill Per Hour = 48.77×1.68 = 81.94 = TK.82 (approx.)

D) Cost required for an hour to run ETP:
Cost Per Hour = (Chemical Cost + Manpower Cost + Electric Cost)
= 2056.9 + 40.27 + 82
= Tk. 2179

E) Treatment Cost (Tk. / m3) = Tk. 54.47

F) Treatment Cost (Tk. / L) =TK. 0.05447

G) Total amount of processing cost of ETP before segregation
= (Total amount of wastewater in liter × Treatment cost per liter in taka)
= 154000×0.05447
= 8388.38TK. /day

Therefore, ETP processing cost for a 25 tons capacity dyeing factory before segregation
= 8388.38×25
=209709.5 TK./day

H) Total amount of water have to be treated after segregation
= (Total amount of wastewater –amount of segregated wastewater)
= 154,000-62,000
= 92000 L

Therefore, ETP processing cost after segregation = 92000×0.05447
= 5011.24×25
= 125,281 TK./day

I) Total saving of ETP processing cost by segregating wastewater
= (Total amount of processing cost of ETP before segregation – ETP processing cost after segregation)
= 209709.5 -125,281
= 84000TK. /day

Thus, from the above calculation it is obvious that about 40% of total ETP processing cost can be reduced by segregating low contaminated wastewater.

Table 4.5 Outcome of the study.

TitleAmount Of Waste water (L)ETP Processing Cost (TK./L)Total Cost (TK./day)
Before Segregation154,0000.05447209,709.5
After Segregation92,0000.05447125,281


The major objective of this study was to reduce processing cost of ETP through segregation of wastewater of textile dyeing factory, for this purpose primarily wastewater was divided into two groups such as highly contaminated wastewater and low contaminated wastewater and analysis was performed on only low contaminated wastewater. In order to assess the quality of low contaminated wastewater produced from different stages and to detect possible stages which may be discharged into environment without treating in ETP, wastewater samples from the four numbers of dyeing factories were collected and analyzed for detailed characterization.

Major conclusions derived from the analysis of these results are summarized below:

  • pH values satisfy the limit of Bangladesh standards (DoE Standard) and ECR guideline values.
  • Total solids and Total dissolved solids concentration in all selected segregated wastewater satisfy the limit of Bangladesh standards and ECR guideline values for discharging wastewater and gradually decreases with increasing the time of rinsing.
  • Hardness in all hot washes wastewater has exceeded the Bangladesh standards for discharging wastewater. Hardness concentration of all rinse wastewaters is less than DoE standard for discharging wastewater in the Environment without treating in ETP.
  • BOD5 values satisfy the limit of Bangladesh standards and ECR guideline values.
  • The color concentration in some wastewater has exceeded the Bangladesh standards and ECR guideline values for discharging wastewater in the Environment without treating in ETP.
  • In some wastewater samples, color, TDS, TSS, pH concentrations have exceeded the ECR, 97 guideline values.

The major focus of this study was to determine or find out the stages or steps of dyeing process to discharge the wastewater of those steps without treating in ETP to reduce the load of ETP i.e. reducing processing cost of ETP and encourage the industrialists or manufacturers to use the ETP at low operating cost which will save the environment from the water pollution. From the results of this study, it appears that some stages waste water of dyeing industry is suitable for discharge directly and some are not suitable. From this study it is clear that if Textile dyeing factories discharge the wastewater of the above selected steps then, they will be able to save about 40% of ETP processing cost without hampering the environment.

Major recommendations for continuation of the present work in the future are summarized below:

  • To determine the less contaminated wastewater a large number of tests (pH, Hardness, TDS, TSS, BOD and COD) should be performed carefully.
  • Oil and Grease concentration could not be found out due to lake of lab facilities. It is to be considered for further study.
  • Colour concentration could not be found out due to lake of lab facilities. It is to be considered for further study.
  • All Initial BOD, BOD5 could not be found out and determined due to lake of lab facilities. It is to be considered for further study.
  • Results from this study suggest that the dyeing effluents have high pollution potential and must be treated before disposal.


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  2. Babu,,B. Ramesh, Parande, A.K., Raghu,S. and T. Prem Kumar, Cotton Textile Processing: Waste Generation and Effluent Treatment,2007.
  3. Eswaramoorthi ,S. Dhanapal k. and Chauhan d.s. Advances in textile waste water treatment: the case for uv-ozonation and membrane bioreactor for common effluent treatment plants in tirupur, tamil nadu, india, Environmental technology awareness series. Page 4-15.
  4. Abdessemed, D. and G. Nezzal. 2002. Treatment of primary effluent by coagulation adsorption-ultrafiltration for reuse. Desalination 152:367-373.
  5. DOE (1992), Dhaka Waste Water Management Study under National Environment
  6.  Pollution Control Project, Government of Peoples Republic of Bangladesh, Department of Environment, Dhaka.
  7. ECR(1997), Environmental Quality and Discharge Standards, Environmental Conservation Rules. 1997. Dhaka.
  9. ADB (2004), Country Environmental Analysis Bangladesh, 3rd Draft, July 2004
  10. Gohl, E.P.G., Vilensky, L.D., (1999), Textile Science. 2nd edition. CBS publications. New Delhi, India.
  11. Hendricks, I. and Boardman, G. (1995), “Pollution Prevention Studies in the Textile Wet Processing Industry.” Department of Environmental Quality. Office of Pollution Prevention. May 1995. Available from:
  12. Buschle-Diller, G. AU (leader), Radhakrishnaiah, R. and Freeman, H, Environmentally Benign Preparatory Processes – Introducing a Closed-Loop

Experimental Data of Wastewater quality parameters

Table: 1 Wastewater characteristics of After Scouring & Bleaching Rinse (ASBR).

CharacteristicsSample NoAvg. Value
TDS (mg/l)10001100120015801700150013001400200018001458
TSS (mg/L)4550354030544952474244.4
COD (mg/L)150134122148152130142128136140138.2
BOD (mg/L)4451384642383634403840. 7

Table: 2 Wastewater characteristics of Acid Neutralization (AN).

CharacteristicsSample NoAvg. value

Table: 3 Wastewater characteristics of After Enzyme Hot (AEH).

CharacteristicsSample NoAvg. Value

Table: 4 Wastewater characteristics of Before Dyeing Rinse (BDR).

CharacteristicsSample NoAvg. Value
BOD(mg/L) 7.67.8

Table: 5 Wastewater characteristics of After Dyeing Rinse (ADR).

CharacteristicsSample NoAvg. Value

Table: 6 Wastewater characteristics of Before Finishing Rinse (BFR)

CharacteristicsSample NoAvg. Value

Table: 7 Average Value of Experimental Data of Selected Wastewater

PHHardness (PPM)TDS (mg/L)TSS (mg/L)BOD (mg/L)COD (mg/L)


  • ETP: Effluent Treatment Plant
  • COD: Chemical Oxygen Demand
  • BOD: Biological Oxygen Demand
  • TSS: Total Suspended Solid
  • TDS: Total Dissolved Solid
  • ECR: Bangladesh Environmental Conservation Rules
  • DOE: Department of Environment
  • ASBR: After Scouring & Bleaching Rinse
  • AN: Acid Neutralization
  • AEH: After Enzyme Hot
  • BDR: Before Dyeing Rinse
  • ADR: After Dyeing Rinse
  • BFR: Before Finishing Rinse

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