In the woven fabric textile finishing industries, singeing is the first process step where greige fabric is passed through the singeing machine prior to wet pretreatment. Singeing operation is employed to destroy singes and tufts on the surface of the fabric, by its direct exposure to the flame, for a very short time. This process is required to improve the chemical uptake of the fabric, by preventing uneven impregnation.

At most of the continuous woven fabric machines of the textile finishing industries, wet fabric is first dried at drum dryers and then cooled through cooling drums so that it could be handled easily. Figure 1 and Figure 2 show a drum dryer where the last drum is cooled with fresh water for reducing the fabric temperature. These cooling drums are provided with fresh water circulation inside. After getting heat of the hot fabric, this warm water is discharged as wastewater. This is a clean and warm water stream.

In some woven fabric textile finishing industries where mercerization process is carried out extensively, Caustic Soda Recovery Plant (CRP) is installed to recover caustic soda from the mercerization process. The purpose of the CRP is to concentrate dilute caustic soda solution (weak lye) of 5 to 10 oBé concentration discharged from the mercerization machines into 20-25 oBé concentration by evaporating water from the weak lye through evaporators. The vapors from weak lye is condensed into hot water through condensers. This hot water is at a high temperature of about 60 to 80oC with alkaline pH. Generally, this hot alkaline water is wasted in the industry.

The mercerization process is carried out for cotton woven fabric, not necessarily but depending on the final product quality. Mercerization is applied to improve properties such as fiber strength, shrinkage resistance, luster, and dye affinity/uptake. In this process, fabric is treated with a caustic soda (NaOH) solution of high concentration under tension (Figure 1 and Figure 2). Caustic soda reacts with the cellulose, swells it, and imparts the above properties.

Today, woven fabric made of cotton or cotton/polyester blends is usually continuously pre-treated. This pretreatment consists of desizing, scouring, and bleaching with associated washing operations, i.e. wash boxes each after desizing, scouring, and bleaching. Further, depending on the desired quality, the fabric is also mercerized. Many industries already practice counter-current washing which means counter-current washing to wash out sizing agents, and to wash the fabric after scouring and bleaching. An example is given in Figure 7. The consumption of water of this real plant is 16 l/kg whereas each of the three washing steps can be operated with 4 l/kg each, i.e. with a total water consumption of 12 l/kg in total (Textile BREF, 2003).

In the continuous pretreatment machine, the scouring wash water can be reused as desizing washing water by directing discharge pipeline of scouring section wash water into the supply line of desizing wash boxes (See Figure-8). For desizing washing, high quality water is generally not required and scouring wash water can be a good option for such type of washing.

Cold caustic soda solution is used in some of the mercerization processes in the textile finishing industries. The caustic soda solution is cooled through an ammonia chiller (see Figure 1). Cooling water is used at the ammonia chiller’s condenser to condense ammonia gas. This cooling water is a clean water stream that is usually wasted. This water can be collected and reused in the process.

In woven fabric textile finishing industries, continuous pretreatment machines are employed to perform desizing washes, scouring and post-scouring washes, bleaching and post bleaching washes, and subsequent neutralization of the alkaline fabric with acid. In this machine, there are three sections of wash boxes: washes for desizing, post-scouring washes, and post-bleaching washes. After post-bleaching washes, the fabric is treated with acid for neutralization of the fabric and then washed in the post-neutralization wash boxes at about 60oC temperature. The post-neutralization wash water can be reused as pre-wash water by directing the discharge pipeline of the neutralization section wash water into the supply line of pre-wash boxes (see Figure 1). For pre-washing, high-quality water is generally not required and neutralization wash water can be a good option for such type of washing.

Sizing agents are applied to warp yarns in order to protect them during the weaving process. However, sizing agents have to be removed practically completely during textile pre-treatment because they would disturb further processing, especially dyeing and/or printing. In the case of finishing woven fabric, sizing agents represent the major part of COD-load which is 30-70% [UBA, 1994, Schoenberger/Schäfer, 2003)). Sizing agents can be recovered from washing liquor by ultrafiltration. However, this is only possible for water-soluble synthetic sizing agents only, such as polyvinyl alcohol, polyacrylates and carboxymethyl cellulose. Recently, it has been confirmed that modified starch can also be recycled such as carboxymethyl starch. The principle of recovery by ultrafiltration is shown in Figure 1. After sizing and weaving, sizing agents are removed during textile pre-treatment by hot washing with water in a water-efficient continuous washing machine where only 3-4 l/kg are used to wash off sizing agents. For minimizing washing liquor flow, the washing process may need to be optimized. The sizing agent concentration in the washing liquor is about 20-30 g/l. In the ultrafiltration plant, they are concentrated to 150-350 g/l and the concentrate can be reused for sizing again. It is kept at a high temperature (80-85°C) and has not to be heated up again. The permeate can be recycled to the washing machine.

Water is the most important resource of textile finishing industries. All wet processes are based on water. Water consumption is directly or indirectly related with energy and chemicals consumption and wastewater hydraulic load. Excess water consumption increases thermal and electrical energy and chemical consumption in the processes. Water management is very important in the textile finishing industry to reduce resource consumption and sustain textile processes. Water management will only be possible if water consumption in the processes is measured, monitored and controlled. Mostly water consumption in the textile finishing industry is not measured, monitored and managed due to its availability in abundance, particularly where fresh water costs are low. Often, the management and workers do not realize the importance of water and associated resource wastage due to its inappropriate use in the process.

In many cases, raw water, that is usually groundwater, contains high content of dissolved solids. The use of water of high dissolved solids for textile finishing leads to various problems such as quality issues in the fabric and relatively higher consumption of chemicals during wet processing. This water also cannot be used for steam boilers. Therefore, in most of the textile finishing industries, raw water of high dissolved solids is pretreated through reverse osmosis (RO) plant to minimize its total dissolved solids (TDS) and hardness in order to provide process water at required quality both for wet finishing processes and boiler feed water (see Figure 1).

As being a separation technology, the RO produces the so-called permeate, which is about 70% of the total raw water supplied to the RO plant, as well as the concentrate, which is highly concentrated with the salts with a percentage of 30% respectively. The concentrate is also called reject water. This reject water is often wasted.

There are many equipment in the textile finishing industries where cooling water is applied. The purpose of supplying cooling water to this equipment is to keep them cool, to minimize undesired reactions (e.g. of reactive dyestuffs), or to prevent damage to their moving and rotating parts. This cooling water is not in direct contact with any media and remains uncontaminated. Only its temperature is increased. Generally, this warm water stream is ultimately discharged as wastewater. Cooling water is used at the following locations/machines:

In the woven fabric textile finishing industry, continuous pretreatment and pad steam dyeing machines are used for pretreatment (desizing, scouring, bleaching) and dyeing of the fabric respectively. These machines are equipped with steamers (generally at the top) for heating the fabric and giving reaction time for the completion of the process (scouring/bleaching/dyeing). In the pretreatment machine where the scouring and bleaching process is carried out separately, there are two steamers (one after the scouring impregnator and the other after the bleaching impregnator) and for the pretreatment machine where the scouring and bleaching process is carried out simultaneously in the same impregnator, there is only one steamer.

In many cases, there is a continuous supply and wastage of fresh water at the fabric exit point of the steamer. This water is supplied to maintain a water lock so that the steam vapors cannot escape from the steamer along with the fabric into the atmosphere (loss of steam and increase of humidity of the environment) (see Figure 1 as an example). This is relatively a clean water stream that can be collected and reused at the wash boxes of the same machine.

In woven fabric textile finishing industries, the fabric is washed in a number of wash boxes (compartments) in continuous machines. Generally, the wash boxes of the machine vary from two to eight. In the following, processes are mentioned which are carried out in continuous machines where fabric is washed in wash boxes:

- Pretreatment (washing for desizing, scouring, bleaching)

- Mercerization

- Washing after pad steam dyeing

- Soaper machine (soaping and washing after printing)

- Washing machines for washing fabric after cold pad batch dyeing or fabric printed with reactive or vat dyestuffs

In most dyeing methods, water is an essential medium for dyeing. The liquor ratio in an exhaust dyebath is the ratio of the weight of the dry material being dyed to the water weight of the dyebath. For example, a liquor ratio of 1:10 means that 1 kg fibre is dyed in 10 L water.

The liquor ratio can differ greatly between machine types and their mechanisms. Using low-liquor ratio equipment, the weight of water used to dye a given weight of goods can be efficiently reduced. To have an efficient dyeing operation, dyer has the responsibility of controlling the system variables where liquor ratio is a significant factor.

Textile material to liquor ratio has the highest impact on water consumption in exhaust dyeing. If the liquor ratio is reduced from 1:8 to 1:4, it results in a 50% water and a ~30% energy saving. Low liquor ratio exhaust dyeing machines reduce water consumption in dyeing of textile materials and reduce the usage of salt and alkali as these are added in grams per litre of the total liquor. Ensuring low liquor ratio in knitwear dyeing process with modern machines has a daily saving potential of 1900-2000 m3 water for big units.

In literature, different levels of water requirement have been reported to dye one kilogram of textile material, depending upon the fibre type, depth of colour needed, dyestuff chemistry, and machinery used for dyeing.

Exhaust dyeing, mostly used for knit fabric dyeing, are batch process and are most water-consuming technology to impart colour to textile material. In exhaust dyeing, a certain amount of textile material is loaded into a dyeing machine and brought to equilibrium with a solution containing the dye and the auxiliaries over a period of time. The dye liquor, with initial high dye contents, becomes gradually exhausted while the dye shifts from the dyeing liquor to the fiber. At the end of the process, the material is washed or rinsed to remove the unfixed dye to comply with given fastness standards.

Exhaust dyeing techniques, also called batch or discontinuous dyeing, use more than 50% to 200% water compared to the continuous dyeing process to dye a kilogram of fabric. A German machine company, Thies, examined the water use and water consumption of different reactive dyeing processes for cotton in their exhaust dyeing machines used in Pakistan by several dye houses. With a water metre that is part of all the machines, it has been found that water use varies from 15 L to almost 70 L water per batch per kg of cotton fabric due to various reasons.

Denim is mainly attributable to the attractiveness of its blue colour (mainly indigo and some indigo derivatives) and the finishing effects developed by various techniques. The blue colour characteristic of denim fabric is related to cotton warp yarns that are dyed by indigo, the most widely used vat dye.

There are 4 types of denim dyeing methods with Indigo which include the following:

- Rope/Ball warp/Chain dyeing (Figure 1)

- Slasher/Open warp/Sheet dyeing (Figure 2 and 3)

- Loop dyeing

- Beam dyeing

Rotary and flatbed printing machines are employed for the continuous printing of woven fabric. The printer table blanket is washed with forced water jets (see Figure 1), by means of two or three rows of multiple nozzles, placed underneath the table in cross direction, to remove the stains adhered to the blanket during printing operation. Generally, the blanket washing water at the printing machines is kept on running and wasted during machine stoppage (due to maintenance or change over). This water wastage can be avoided by installing automatic water shut off valve at the printing machines. Whenever printing machine is stopped, the blanket washing water will automatically be stopped, avoiding its unnecessary wastage. 

The objective of biological wastewater treatment is to i) biodegrade water-soluble organic biodegradable compounds and insoluble organic matter, ii) transform (i.e. partly oxidize) dissolved and particulate biodegradable constituents into non-hazardous metabolites, iii) capture and incorporate suspended and non-settleable colloidal solids into biomass, and iv) to adsorb non-biodegradable compounds to biomass (activated sludge flocs and biofilm).

The removal of dissolved and suspended organic matter is accomplished biologically by a huge variety of microorganisms, mainly bacteria. Microorganisms are used to biochemically oxidize (i.e. to transform) the dissolved and particulate organic matter into mainly carbon dioxide, water, ammonium, sulphate, phosphate, some, often unknown, biochemical transformation products (metabolites) and additional biomass (sludge) which is the results of microbial growth of (mainly) bacteria.

An equalization tank belongs to the first steps of textile wastewater treatment such as fibre removal, heat recovery, and neutralization. Figure 1 shows the whole possible sequence of textile wastewater treatment of which equalization is an important part of. The purpose of the equalization tank for industrial wastewater treatment is as follows (based on Eckenfelder, 2000):

- Dampening of organic load fluctuations to avoid shock loading on biological treatment system as well as to equalize the concentration of multiple organic and inorganic compounds in order to have a feed to biological treatment as constant as possible

- Minimizing flow surges to physical-chemical treatment systems

- Control pH or to minimize the chemical requirements for neutralization

- Even out the effect of a periodic ‘slug’ discharge from a batch process

- To provide continuous feed to biological systems over periods when the manufacturing plant is not operating

- Prevent peak values of toxic or inhibiting chemical compounds from entering the biological treatment stage.