EFFECTS OF DYEING ON THE ENVIRONMENT
The major cause of the release of dyestuffs into the environment is by the
discharge of aqueous effluents from industrial dyehouses rather than during
the domestic dyeing process. Indeed, the amount of dye found in receiving
waters (rivers, sewers etc) attributable to domestic dyes is probably
insignificant compared to that from other domestic sources e.g. toilet flush
blocks and fabric softeners. However, the small amount of dye attributable to
domestic dyeing still has an environmental effect.
Below are some relevant points :-
1. The high level of colourfastness of modern dyestuffs to fabrics is such that
losses to the environment after the actual dyeing process are negligible.
2. Dyestuffs have generally low toxicity to fish and other aquatic lifeforms and
do not tend to bioaccumulate over a period of time.
3. The majority of dyestuffs have low toxicity to micro-organisms and do not
generally interfere with the bacterial processes necessary, for instance,
at water treatment plants.
4. A recent paper has shown that any contamination of agricultural land with
dyes has a negligible effect on crops.
5. Biodegradability - To be of use, dyes must have a high degree of stability
to chemical processes and light. Therefore, it is not surprising that they
give poor results in short term tests for aerobic biodegradability. As a
general guideline, no more than 20% of the dye is biodegradable in a
standard OECD Aerobic test.
Dyes are, however, prone to anaerobic degradation. The substantive nature
of dyes is such that when they are discharged into receiving waters they
are likely to be adsorbed onto suspended solids, which find their way into
aquatic sediment. The anaerobic nature of the sediment is such that the
dye is likely to be destroyed by reductive processes.
The chemicals produced as a result of these reduction processes will
themselves degrade easily to harmless substances.
In summary, the likely fate of dyes released into the environment is
adsorption onto solid sediment and destruction by anaerobic processes
either in a land fill site or on the river bed. The products of this process
are harmless to the environment.