Page 158 - Basic Principles of Textile Coloration

DYEHOUSE EFFLUENT AND ITS TREATMENT 147

Table 8.4 List of contaminants found in textile finishing effluent

Water pollutants Examples

Heavy metals Chromium, copper, zinc
Organochlorine compounds Moth-proofing agents
Insecticides Aldrin
Sulphides Sodium sulphide from dyeing
Colour Dyes
Surfactants Detergents, dispersing agents
Oils Emulsions from scouring
Carbohydrates Starch sizing
Solvents Degreasing solvents from scouring
Acids and alkalis Dyeing assistants

(1) The volume of effluent.
(2) A measure of the amount of oxygen it will consume for oxidation of the

organic chemicals it contains. This point is important because depletion of
oxygen in water has a negative impact on aquatic life. The biological oxygen
demand (BOD) [1] is the amount of oxygen (mg l–1 or ppm) consumed in 5
days at 20 °C by growth of bacteria from a culture added to the water. The
chemical oxygen demand (COD) [1] is based on a much faster chemical
oxidation of organic compounds with hot sodium dichromate solution. The
two values are often close but not equivalent. Many organic compounds are
readily oxidised by hot dichromate but are resistant to microbial oxidation at
ambient temperature. It is typical of textile effluent that the COD is much
higher than the BOD. The COD is less affected by the usual effluent
treatment processes, and is thus more persistent in the environment. Values
range from 200–3000 mg O2 l–1 for BOD and from 500–5000 mg O2 l–1 for
COD. The total organic carbon (TOC) in the water serves as an alternative
to BOD and COD. All these can be determined by standardised analytical
procedures.
(3) Floating insoluble chemicals, mainly insoluble oils and solvents.
(4) Suspended solid materials. These are quite diverse and include short fibres
and insoluble dyes or compounds that have precipitated in the effluent
because of a change in temperature or pH. Quantities range from 50–
5 0 0 m g l–1. This can be estimated by filtration or by turbidity measurements.
(5) Colour. This is visible pollution. While it may not be toxic, colour does
reduce light transmission into waters and limits photosynthesis. The dyeing
industry discharges about 9% of the dyestuffs it consumes. This corresponds