Page 159 - Basic Principles of Textile Coloration

148 WATER TREATMENT

to a considerable degree of colour in a dyehouse effluent. Dyes are not easily
biodegraded since, by design, they have good stability towards light and
various chemical treatments. Most dyes are not of high toxicity and are
eventually removed from water by oxidation or adsorption on sediment, but
presence of colour in the water from a dyehouse is undesirable. It is a strong
indicator of the presence of much higher quantities of dyeing assistants,
almost all of which are present in the effluent. Even a simple chemical such
as acetic acid can significantly increase the BOD.
(6) Acidity. The pH may vary from about 4 up to near 12. The acidity of water
affects aquatic life and effluent must be neither too acidic nor too alkaline on
discharge. This also applies for discharge into a municipal sewer because the
micro-organisms used in sewage treatment are equally susceptible.
(7) Toxic chemicals. For the textile industry, the major offenders here are heavy
metals such as chromium and copper, organochlorine compounds from
insecticides or moth-proofing agents, and sulphides from dyeing with sulphur
dyes.

8.5.2 Effluent treatment

Any effluent treatment programme must consider whether the water
contaminants are biodegradable, over what time scale, and – if persistent – how
they will influence conditions down stream. Even the influence of subsequent
water chlorination for drinking purposes on the potential formation of toxic
organochlorine compounds must be considered. This a complex subject requiring
detailed on-site analysis before arriving at specific recommendations and their
implementation.

One very important strategy in reducing water pollution from a textile finishing
plant involves minimising waste and optimising process methods so that fewer
contaminants are discharged in the effluent. The following list gives some
examples of this.
(1) re-use of excess dye solution remaining at the end of a process – this can be

incorporated into other dyeing recipes;
(2) mixing acidic and alkaline effluents before discharge to avoid excessive

acidity or alkalinity;
(3) replacing starch-based sizing materials with polyvinyl alcohol, which can be

recovered by membrane separation techniques and recycled;