Treatment of wastewaters from the textile industry:
When they reach the manufacturer, cotton fibres contain natural waxes and
fats which must be removed before bleaching or colouring matter is introduced
to dyed yarn or fabrics. Scouring or kiering removes most of these.
Effluent treatment – Enzymes in activated sludge 203
Woven cotton fabric or yarn in the raw state is boiled in solutions of alkali,
fats which must be removed before bleaching or colouring matter is introduced
to dyed yarn or fabrics. Scouring or kiering removes most of these.
Effluent treatment – Enzymes in activated sludge 203
Woven cotton fabric or yarn in the raw state is boiled in solutions of alkali,
often under pressure.
Impurities possessed by woven cotton goods include natural impurities
like those mentioned above. Size and starch, or various substitutes, are used
to facilitate weaving. These can be removed by hydrolysis with acids or
enzymes, followed by rinsing, or may be removed during the scouring
process.
Scouring liquors tend to be alkaline and contain large amounts of organic
matter (see Table 6.2), which are both expensive to treat. The standards of
the river authorities, for example the Environment Agency in the UK, are
very high.
After scouring, the cotton is bleached using:
Hypochlorite, HClO Æ H+ + ClOUsually,
potassium or calcium chlorite, salts or peroxide are used
in bleaching. These remove and destroy natural colouring matter. Wastewaters
from this process contain partly spent bleaching solution
and impurities removed from the fibres. After bleaching, cotton is washed
and then immersed in a solution of sodium bicarbonate or weak H2SO4,
which destroys the bleach. Finally, a thorough wash with a soap solution is
necessary.
Mercerisation increases lustre and dye affinity.The process involves treating
the cotton fabric with sodium hydroxide, NaOH, and then washing with
hot water whilst it is under tension. Any residual sodium hydroxide is
neutralised with acid and the cotton material is thoroughly washed.
Dyeing of fabric results in large volumes of liquor being discharged.
These liquors vary in character, depending on the nature and class of dye
used; for example they may be acidic or alkaline and will contain high concentrations
of salt and synthetic surface-active agents. Post-dyeing treat-
204 Textile processing with enzymes
Table 6.2 Typical scouring waste liquor from cotton linters
Properties and constituents Concentration (mg/dm3)
Permanganate value (4 hour) 8850
BOD (5 day) 10650
Ammoniacal nitrogen 52
Total nitrogen 492
Phosphate 35
Potassium 148
Alkalinity (CaCO3) 16750
ments involve the use of a variety of finishes such as starches, gums and
waxes, resins and other materials, depending on the required finish,
although these tend to be low in concentration because of the large volumes
of water that are added during the finishing processes.
Wastewaters from the processing of wool include scour, bleach and dye
liquors, plus associated washing waters. The two main differences from
cotton wastes are the degree of alkalinity and the nature of the organic
matter. Also, waste liquor from wool treatment may be acidic. Scouring
wastewaters contain a high proportion of wool grease (lanolin), in an emulsified
form, soluble organic matter and sand. Lanolin contains cholesterol
and other complex organic substances, which are readily absorbed by the
skin and are used in ointments and cosmetics.
Back washing liquor and yarn scouring liquor contain emulsified oils,
grease and fatty matter derived from soaps. Emulsions must be centrifuged
and then cracked with acid or other chemicals. Cracking is the process of
breaking down into smaller molecules which are more amenable to further
treatment processes.This aids wastewater treatment by making the soluble
components more able to be broken down by biological treatment.
Greasy solids are normally removed by flocculation and filtration prior
to mixing with other wastewater. Mixing and balancing waste liquors evens
out large variations in composition, temperature and flow. Balancing
involves neutralising acid or alkali so that the liquid is in a suitable condition
to go forward for secondary treatment.
Waste liquor from wool treatment is similar to that from cotton dyeing
but may be acidic. It contains large amounts of wetting agents and softeners
plus some salts and potentially toxic materials. These can be inhibitory
to biochemical processes, i.e. they retard or tend to prevent naturally occurring
biological processes.Waste liquor from the treatment of synthetic fibres
contains a variety of organic substances at several hundreds of milligrams
per litre many only slowly degraded by special microorganisms.
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