Water efficiency in textile processes

Water Consumption in Textiles

Water is used extensively throughout textile processing operations. Almost all dyes, specialty chemicals, and finishing chemicals are appliedto textile substrates from water baths. In addition, most fabric preparation steps, including desizing, scouring, bleaching, and mercerizing, use aqueoussystems.

The amount of water used varies widely in the industry,depending on specific processes operated at the mill, equipment used, and prevailing management philosophy concerning water use.

Reducing water consumption in textile processing is important for furthering pollution prevention efforts, due in part because excesswater use dilutes pollutants and adds to the effluent load.

Mills that currently use excessive quantities ofwater can achieve large gains from pollution prevention. A reduction in wateruse of 10 to 30 percent can be accomplished by taking fairly simple measures. Awalk-through audit can uncover water waste in the form of:

Hoses left running.
Broken or missing valves.
Excessive water use in washing operations.
Leaks from pipes, joints, valves, and pumps.
Cooling water or wash boxes left running when machinery is shut down.
Defective toilets and water coolers.

In addition, many less obvious causes of waterwaste exist. These causes are presented below by subcategory, unit process, and machine type.

Subcategory

Textile operations vary greatly in waterconsumption.

Figure 1 summarizes the water consumption of varioustypes of operations. Wool and felted fabrics processes are more water intensivethan other processing subcategories such as wovens, knits, stock, and carpet.

Water use can vary widely between similaroperations as well. For example, knit mills average 10 gallons of water perpound of production, yet water use ranges from a low of 2.5 gallons to a highof 45.2 gallons.

These data serve as a good benchmark for determiningwhether water use in a particular mill is excessive.

Unit Process

Water consumption varies greatly among unit processes, as indicated in Figure 2. Certain dyeing processes and print after-washing are among the more intensive unit processes. Within the dye category, certain unit processes are particularly low in water consumption (e.g., pad-batch).

Machine Type

Different types of processing machinery use different amounts of water, particularly in relation to the bath ratio in dyeing processes (the ratio of the mass of water in an exhaust dye bath to the mass of fabric).

Washing fabric consumes greater quantities of water than dyeing. Water consumption of a batch processing machine depends on its bath ratio and also on mechanical factors such as agitation, mixing, bath and fabric turnover rate (called contact), turbulence and other mechanical considerations, as well as physical flow characteristics involved in washing operations. These factors all affect washing efficiency.

In general, heating, wash, and dye baths constitute the major portion of energy consumed in dyeing.

Therefore, low bath-ratio dyeing equipment not only conserves water but also saves energy, in addition to reducing steam use and air pollution from boilers. Low-bath-ratio dyeing machines conserve chemicals as well as water and also achieve higher fixation efficiency. But the washing efficiency of some types of low-bath-ratio dyeing machines, such as jigs, is inherently poor; therefore, a correlation between bath ratio and total water use is not always exact.

Process Water Conservation

Washing

Washing and rinsing operations are two of the most common operations in textile manufacturing that have significant potential for pollution prevention.

Many processes involve washing and rinsing stages, and optimizing wash processes can conserve significant amounts of water. In some cases, careful auditing and implementation of controls can achieve wastewater reductions of up to 70 percent. The washing and rinsing stages of preparation typically require more water than the other stages (e.g., bleaching, dyeing). Several typical washing and rinsing processes include:

Drop and fill batch washing.
Overflow batch washing.
Continuous washing (countercurrent, horizontal, or inclined washers).

A report on water consumption for a typical continuous bleach range found that consumption was more than 11,000 gallons per hour, or 270,000 million gallons per day. (See Figure 3.) Washing stages accounted for 9,900 gallons per hour, or 90 percent of the total. The application of the following simple, low-technology methods of water conservation reduced water use:

Properly regulating flows: 300 gallons per hour savings.
Counter flowing bleach to scour: 3,000 gallons per hour savings.
Counter flowing scour to desize: 3,000 gallons per hour savings.

The total water savings without process modification was 150,000 million gallons per day, or 55 percent of water use. A process modification such as a combined one-stage bleach and scour also would save 6,200 gallons of water per hour, or an additional 150,000 million gallons per day, along with energy savings.

Drop-Fill versus Overflow Washing

In the drop/fill method of batch washing, spent wash water is drained and the machine is refilled with a fresh wash bath. The fabric or other substrate in the machine retains much of the previous bath, perhaps as much as 350 percent owg. This percentage can be reduced by mechanical means (e.g., extraction, blow down). Comparison of several methods of washing after bleaching shows the benefits of countercurrent wash methods, see Figure 4.

Methods five and six, which implement countercurrent washing, produce savings of 26 and 53 percent compared with the standard drop/fill method.

These results are based on comparisons of washing processes that would produce the same degree of reduction of fabric impurities using computer models.

Countercurrent washing processes require the addition of holding tanks and pumps. The capital cost of setting up such a reuse system typically is less than $50,000 and generates estimated savings of $95,000 annually. In many cases, reducing wastewater also reduces the need for expensive waste treatment systems.

Reusing Wash Water

Many strategies can be applied for reusing wash water. Three of the most common strategies are countercurrent washing, reducing carryover, and reusing wash water for cleaning purposes.

Countercurrent Washing

The countercurrent washing method is relatively straightforward and inexpensive to use in multi-stage washing processes. Basically, the least contaminated water from the final wash is reused for the next-to-last wash and so on until the water reaches the first wash stage, after which it is discharged. This technique is useful for washing after continuous dyeing, printing, desizing, scouring, or bleaching.

An important variant of the countercurrent principle is horizontal or inclined washers. Horizontal or inclined washing is more efficient because of the inherent countercurrent nature of water flow within the process. The mechanical construction of an inclined or horizontal countercurrent washer has to be much better than a traditional vertical washer, however.

Sloppy roll settings, weak or undersized rolls, unevenness, bends, bows, biases, bearing play, or other misalignments within the machine are much more important in a horizontal or inclined washer because the weight of water pressing down on the fabric can cause it to sag, balloon, or stretch.

If properly constructed and maintained, horizontal or inclined washers can produce high-quality fabrics while saving money and water.

Reducing Carryover

Because the purpose of washing is to reduce the amount of impurities in the substrate, as much water as possible must be removed between sequential washing steps in multistage washing operations.

Water containing contaminants that is not removed is .carried over into the next step, contributing to washing inefficiency.

Proper draining in batch drop/fill washing and proper extraction between steps in the continuous washing process are important. Often, 350 percent owg is carried over in typical drop/fill procedures.

This amount can be reduced in some batch machines (e.g., yarn package dyeing, stock dyeing) by using compressed air or vacuum blow down between washing steps.

In continuous washing operations, squeeze rolls or vacuum extractors typically extract water between steps.

Equipment employing vacuum technology to reduce dragout and carryover of chemical solutions with cloth, stock, or yarn is used to increase washing efficiency in multistage washing operations.

In one case history, a processor installed vacuum slots after each wash box in an existing multistage continuous washing line and was able to reduce the number of boxes from eight to three. Wash boxes with built-in vacuum extractors are available for purchase, as well as washers for prints that combine successive spray and vacuum slots without any bath for the fabric to pass through. Because the fabric is never submerged, bleeding, marking off and staining of grounds is minimized, and water use decreases.

Another washer configuration with internal recycling capabilities is the vertical counter flow washer, which sprays re circulated water onto the fabric and uses rollers to squeeze waste through the fabric into a sump, here it is filtered and re circulated. The filter is unique, consisting of continuous loops of polyester fabric that rotate continuously and are cleaned of filtrate at one end with a spray of clean water. This construction allows for maximum removal of suspended solids from water before discharge or reuse in another process. High-efficiency washing with low water use results. Energy use decreases greatly because less water must be heated.

Reuse for Cleaning Purposes

In many types of operations, wash water can be reused for cleaning purposes. In printing, cleanup activities can be performed with used wash water, including:

Back gray blanket washing
Screen and squeegee cleaning
Color shop cleanup
Equipment and facility cleaning

A typical preparation department may also reuse wash water as follows:

Reuse scour rinses for desizing
Reuse mercerizer wash water for scouring
Reuse bleach wash water for scouring
Reuse water-jet loom wash water for desizing
Recycle kier drains to saturator

Work Practices

Workers can greatly influence water use. Sloppy chemical handling and poor housekeeping can result in excessive cleanup. Poor scheduling and mix planning also can require excessive cleanup and lead to unnecessary cleaning of equipment like machines and mix tanks. Leaks and spills should be reported and repaired promptly.

Equipment maintenance, especially maintenance of washing equipment, is essential.

Inappropriate work practices waste significant amounts of water, and good procedures and training are important. When operations are controlled manually, an operations audit checklist is helpful for operator reference, training, and retraining.

In one case history, a knitting mill experienced excessive water use on beck dyeing machines. A study of operating practices revealed that each operator was filling the machines to a different level. Some operators filled the becks to a depth of 16 inches, others as much as 24 inches. Also, the amount of water used for washing varied. Some operators used an overflow procedure, and others used drop/fill or half baths (repeatedly draining half of the bath, then refilling it).

Inspection of the written procedures showed that the fill step simply saidfill. The wash step simply said wash. Without training and without a specific operating procedure, operators were left to determine water use on their own. This case may seem extreme, but even the best mills, which have well- documented production procedures, often do not have documented cleaning procedures. Cleaning operations that contribute large amounts of pollution to the total waste stream include machine cleaning, screen and squeegee cleaning, and drum washing.

Engineering Controls

Every mill should have moveable water meters that can be installed on individual machines to document water use and evaluate improvements. In practice, mills rarely measure water use but rely on manufacturers claims concerning equipment and water use.

The manufacturers estimates are useful starting points for evaluating water consumption, but the actual performance of equipment depends on the chemical system used and the substrate. Therefore, water use is situation-specific and should be measured on-site for accurate results. The water meters should be regularly maintained and calibrated.

Other important engineering controls, some of which have been discussed in other sections of this chapter, include:

Flow control on washers
Flow control on cooling water (use minimum necessary)
Countercurrent washing
High extraction to reduce dragout
Recycle and reuse
Detection and repair of leaks
Detection and repair of defective toilets and water coolers

Machinery should be inspected and improved where possible to facilitate cleaning and to reduce susceptibility to fouling. Bath ratios sometimes can be reduced by using displacers that result in lower chemical requirements for pH control as well as lower water use.

Process Changes

Pad-Batch Dyeing

In pad-batch dyeing, prepared fabric is padded with a solution of fiber reactive dyestuff and alkali, then stored (or batched) on rolls or in boxes and covered with plastic film to prevent evaporation of water or absorption of carbon dioxide from the air. The fabric then is batched for two to 12 hours.

Washing can be done on whatever equipment is available in the mill.

Pad-batch dyeing offers several significant advantages, primarily cost and waste reduction, simplicity, and speed. Production of between 75 and 150 yards per minute, depending on the construction and weight of the goods involved, is common. Also, pad-batch dyeing is flexible compared with a continuous range. Either wovens or knits can be dyed in many constructions.

Frequent changes of shade present no problems because reactive remain water soluble, making cleanup easy. This method of dyeing is useful when versatility is required. Water use typically decreases from 17 gallons per pound to 1.5 gallons per pound, a reduction of more than 90 percent.

Processing Bath Reuse

Water from many processes can be renovated for reuse by a variety of methods. Several research efforts are underway. In a few operations, up to 50 percent of the treated wastewater is recycled directly back from the effluent to the raw-water intake system with no adverse effects on production.

In some cases, specific types of wastewater can be recycled within a process or department. Examples are dye bath reuse, bleach bath reuse, final rinse reuse as a loading bath for the next lot, wash water reuse, countercurrent washing, and reuse for other purposes.

Bleach Bath Reuse

Cotton and cotton blend preparation (e.g., de-sizing, scouring, bleaching) are performed using continuous or batch processes and usually are the largest water consumers in a mill. Continuous processes are much easier to adapt to wastewater recycling/reuse because the waste stream is continuous, shows fairly constant characteristics, and usually is easy to segregate from other waste streams. Waste-stream reuse in a typical bleach unit for polyester/cotton and 100-percent cotton fabrics would include:

Recycling J-box and kier drain wastewater to saturators
Using countercurrent washing
Recycling continuous scour wash water to batch scouring
Recycling washerwater to back gray blanket washing
Recycling washerwater to screen and squeegee cleaning
Recycling washerwater to color shop cleanup
Recycling washerwater to equipment and facility cleaning
Reusing scour rinses for de-sizing
Reusing mercerizer wash water for scouring

Preparation chemicals (including optical brighteners and tints), however, must be selected in such a way that reuse does not create quality problems such as spotting.

Batch scouring and bleaching are less easy to adapt to recycling of waste streams because streams occur intermittently, drains generally go into pits and are not easily segregated, and batch preparation steps frequently are combined. With appropriate holding tanks, however, bleach bath reuse can be practiced in a similar manner to dye bath reuse, and several pieces of equipment are now available that have the necessary holding tanks. The spent bleach bath contains all of the alkali and heat necessary for the next bleaching operation. Peroxide and chelates must be added to reconstitute the bath.

Like dye bath reuse, the number of reuse cycles in bleach bath reuse is limited by impurity buildup. The main impurities are metals, such as iron, that can interfere with the bleaching reaction.

New types of rope bleaching units for knits featuring six to 12-stage jet transport systems have made continuous bleaching of most knit styles possible.

These units were introduced in the late 1970s and typically produce 40 pounds per minute of knit fabric or more than one million pounds per month based on a three-shift, six-day operation. These machines have become very popular with large knit processors because of their flexibility and ability to conserve energy, water, and chemicals.

They also have complete built-in countercurrent capabilities. These units are being promoted for use in after washing fiber reactive and other types of dyes (e.g., after pad batch dyeing) in addition to use as continuous knit preparation ranges.

Final Rinse Reuse as Loading Bath for Next Lot

One simple technique that saves water and, in some cases, BOD loading is to reuse the final bath from one dyeing cycle to load the next lot.

This technique works well in situations where the same shade is being repeated or where the dyeing machine is fairly clean.

A good example of this technique is acid dyeing of nylon hosiery. The final bath usually contains an emulsified softener that exhausts onto the substrate, leaving the emulsifier in the bath. This technique can serve as the wetting agent for loading the next batch, thus saving the water, heat, and wetting agent and associated BOD.

Disclaimer

This is a publication by the North Carolina Department of Environment and Natural Resources Division of Pollution Prevention and Environmental Assistance. Information contained in this publication is believed to be accurate and reliable.

However, the application of this information as at the readers risk.

Mention of products, services, or vendors in this publication does not constitute an endorsement by the State of North Carolina.