Page 157 - Basic Principles of Textile Coloration

146 WATER TREATMENT

8.5 DYEHOUSE EFFLUENT AND ITS TREATMENT [2]

8.5.1 Contaminants in dyehouse effluents

For many years, the textile dyeing industry was a major source of water pollution.
Increasing public awareness of this problem has resulted in much stricter
legislation to protect the environment with controls on the types of contaminants
and the amounts released. Governments have also become much more aggressive
in updating legislation and in enforcing it. Water quality criteria are usually
established for drinking water, and for surface and ground waters. The control of
the discharge of effluent has become a major preoccupation for industries in
Europe and North America. Legislation often sets maximum daily limits and a
longer-term average limit for a whole range of contaminants. Controls in
developing countries are much less stringent and, in some cases, almost non-
existent. This has been a significant factor in the recent re-structuring of the
dyestuffs manufacturing and textile finishing industries.

There is little doubt that water quality standards will become increasingly
important. Therefore, effluent treatment before discharge will be required, with
increasing costs to the textile industry. The impact of textile effluent on aquatic
life is an active field of research. There is considerable collaboration through
ETAD (the Ecological and Toxicological Association of the Dyes and Organic
Pigments Manufacturing Industry [3]) and ADMI (the American Dye
Manufacturers Institute). Both have undertaken numerous studies in this area.

Most textile effluent water is discharged into surface waters such as rivers and
lakes, either directly or through municipal sewers. The main problem is the wide
range of chemicals that it contains and the high level of dilution that usually
exists. Table 8.4 lists some of the common pollutants. Of particular concern are
those chemicals that are not degraded by water-borne bacteria, and which
therefore persist and accumulate in the environment. Many such chemicals, such
as polychlorobiphenyls, are extremely toxic and dangerous.

Discharge of effluent to a municipal sewer has the advantage that the sewage
treatment is able to remove many, but not all, of the contaminants. Provided the
nature and approximate amounts of the contaminants are known, sewage
treatment can reduce pollutant levels to the point at which discharge into surface
water is then feasible.

Because of the diversity of chemicals in textile effluent, it is usually
characterised by a number of general criteria rather than in terms of specific
contaminants. These include those described below.