The Biotreatability of Industrial Dye Wastes Before and
After Ozonation and Hypochlorination-Dechlorination
DR. A. NETZER, Head
MR. H.K. MIYAMOTO, Research Engineer
Water and Wastewater Treatment Research
Canada Centre for Inland Waters
Burlington, Ontario L7R 4A6, Canada
INTRODUCTION
Effluents from the dyehouses of textile mills can create serious environmental problems. Not only are these wastewaters characterized by obnoxious colours and turbidity,
but they are also inevitably high in COD, BOD, and TOC. Due to the liberal use of
Glauber's salt (sodium sulfate), excessive sodium and chloride levels are also typical. A
variety of heavy metals, most notably chromium, copper and zinc, are also frequently
present in these effluents.
These heavy metal pollutants can originate from several sources. Some heavy metals,
such as chromium, copper and cobalt, may form an intrinsic part of the dye molecule in the
so-called "premetallized" dyes. Certain types of dyes are marketed in the form of zinc
chloride salts. "Tramp" metals may also be retained in the dyes as impurities from the
synthesis process of the dyes. Process and auxiliary chemicals may also contain heavy metal
pollutants (1).
Some dyes are suspected carcinogens (2, 3) and several other chemicals constituents of
the dyebath formulation (e.g. surfactants, dispersing agents, levelling compounds, etc.) are
known to possess toxic properties (4, 5).
At present, most dyehouses discharge their effluents into the municipal sewer system
with or without some form of prior treatment. However, most dyes are very resistant to
biological degradation and hence colour will very often persist in the treated discharges.
Some dyehouses practice chlorination to reduce the colour of their wastes. Although
chlorination can often provide a very high degree of decolorization, chlorine itself is toxic to
aquatic flora and fauna and the chlorination of such wastes may produce carcinogenic
compounds (6, 7, 8).
A variety of physical-chemical techniques have been studied in recent years for their
applicability in the treatment of textile mill effluents. Among these methods may be
included coagulation-flocculation, activated carbon adsorption, photochemical
degradation, ultrafiltration, synthetic polymeric resin adsorption and ion-exchange (9, 10).
Conventional biological processes are seldom capable of attaining a high degree of
purification for dyehouse effluents. This may be due to the presence of toxic, inhibitory or
biorefractory compounds.
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