What are process controls for treating wastewater in the textile industry?

Oct. 21, 2014

The textile industry makes woven fabric from either natural or synthetic fiber. Natural fibers are vegetable, such as cotton, or animal, e.g., wool and silk. …

The textile industry makes woven fabric from either natural or synthetic fiber. Natural fibers are vegetable, such as cotton, or animal, e.g., wool and silk. Synthetic fibers include rayon, nylon, acrylic, or polyester.

A primary source of natural-fiber waste is the foreign matter present, such as sand, dirt, grease, and waxes. These are removed by boiling or scouring, both of which themselves generate considerable wastes. Chemicals used in processing synthetic fibers generate waste. The scouring of wool produces the greatest waste load of any single process in the textile industry.

Typically, 1,000 pounds of wool as sheared from the lamb will produce approximately 600 pounds of natural impurities and around 400 pounds of finished wool.

Cotton undergoes “sizing,” which is the strengthening of the thread to make it ready for weaving. The thread is then rinsed or “de-sized” to remove the waste starch of polyvinyl alcohol used in sizing. The cotton is then scoured with caustic to remove natural waxes and other impurities. These two processes, sizing and de-sizing, yield more than 80 percent of the waste load from a typical cotton mill.

Textile mills dealing with synthetics generally have lower water usage and waste generation than those mills dealing with natural fibers. For example, mills dealing with the synthetic fiber nylon typically use more than 15,000 gallons of water per 1,000 pounds of cloth with a BOD loading of 40 to 50 pounds per 1,000 pounds of cloth produced. 

Textile mills dealing with natural fibers such as cotton typically requires well over 20,000 gallons of water per 1,000 pounds of cloth with the BOD loading of 50 to 200 pounds per 1,000 pounds of cloth.

The BOD loading for wool is over 400 pounds per 1,000 pounds of cloth.

Control situations

In the textile industry, lint, fiber and strings clog pipes and equipment and collect on the sides of tanks and channels. The natural grease found on the fibers, and the soaps used in processing the fibers, generate a scum that is capable of producing stains and odors as it flows through each treatment process unit. Foam may develop from the soaps and detergents used in processing the textiles.

The pH of wastes differs widely in the textile industry. Wastes having a pH greater than 7.0 are generally less of a treatment problem because they are more susceptible to biological processes. Wastes having a pH less than 7.0 can be pH neutralized by the addition of lime, caustic soda ash or ammonia. Wastes with a pH below 5.5 should never be allowed to enter a wastewater system because they will cause the corrosion of concrete and other structures in the treatment process.

Another potential problem with textile waste is that of variations in hydraulic flow. Unless the flow is equalized as the wastewater enters the treatment facility, periods of high flow will produce reduced contact or treatment times, potentially decrease consistent quality treatment and increase treatment costs. Periods of low flow may cause odors and other treatment operational problems.

Metals, such as copper, used in textile manufacture of wool, may be toxic to treatment processes.

Product situations

Effective treatment of textile wastes, as with most types of industrial waste, begins in the manufacturing process itself.

To improve wastewater treatment capability, consider flow reduction, water reuse, waste segregation, process or material substitution, and good housekeeping. To treat textile wastes by screening, use either stationary, rotary or vibrating screens to remove lint and prevent clogging. Lint may also collect on the sides of tanks and channels yielding odor and other problems.

Equalization is frequently practiced in the treatment of textile wastes, usually to produce steady levels of Flow and Biochemical Oxygen Demand. Segregated wastes are usually equalized, that is, segregated into those that produce the greatest BOD loading.

In producing cotton, the greatest BOD load includes the de-sizing and scouring wastes. In the manufacture of wool, the highest BOD load comes from scouring and the wash following wastes removal. In polyester production, the knitting-oil scour generates BOD-load waste.

Neutralization of the wastewater pH is also a common treatment technique, frequently accomplished by mixing wastes with high and low pH values. 

Certain pretreatment techniques may include the reduction of BOD associated with physical solids in the wastewater, particularly of segregated wastes. Other chemical pretreatment techniques for specific purposes include removal of metals or coagulation for removal of chemicals from detergents.

Specific wastewater treatment processes that have been used effectively in the textile industry include primary treatment processes using chemical coagulation and secondary treatment processes such as trickling filters, activated sludge and rotating biological contactors.

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