The textile industry is a major water consumer and producer of effluent waste water. A
comprehensive review of the literature shows an urgent need to improve the treatment of dyeing
and finishing plant effluent. To destroy dyes and purify dye house effluents modem
photochemical oxidation methods, such as the UV /hydrogen peroxide process, provide an
effective treatment technology. Organic contaminants can be completely mineralised by this
method but it is relatively expensive. Photochemical oxidation with subsequent microbiological
treatment appears an attractive and cost effective approach in textile waste water treatment.
This research studied the feasibility of combining UV /hydrogen peroxide photochemical
oxidation with microbiological treatment to destroy organic dyes and to decolourize waste
water. Experiments on the decolourization of a number of mono- and disazo dyes by UV/oxygen
and hydrogen peroxide as a function of initial concentration of dyes, dissolved oxygen and
hydrogen peroxide, alkalinity and temperature of solutions were carried out. UV /hydrogen
peroxide photochemical treatment was shown to rapidly decolourize and decompose organic azo
dyes. Increase in oxidant concentration to give a molar ratio between hydrogen peroxide and dye
concentrations of 200/1 was found to increase the decolourization rate. Changes in pH to acidic
and alkaline also increase dye decolourization rate, the highest decolourization was observed at
pH 10.5. Increase in the temperature from 25°C to 60°C slightly increases decolourization rate.
Chromatographic, spectrophotometric and mass-spectrometric techniques were used to elucidate
breakdown pathways and identify intermediates. Suggested type of photo-degradation is
destructive oxidation involving rapid breakdown of the azo group with the formation of
colourless intermediate substances such as naphthalene (naphthoquinone) and benzene (phenol)
derivatives. These intermediates do not accumulate and are further photo-degraded to lower
molecular weight substances, organic acids.
Work on establishing ecotoxicological effects and biodegradability of dyes and their
photochemical breakdown products, with particularly reference to micro-organisrns, was carried
out with the biological tests using the freshwater green alga Chlorella vulgaris, the bacterium
Pseudomonas putida and activated sludge. Photochemical treatment of dye solutions with
UV/oxygen or UV irradiation alone does not eliminate their toxicity. When the
UV /hydrogen peroxide system was used decolourized solutions were non-toxic to microorganisms.
Activated sludge is slow and inefficient in removing residual colour from the dye
solutions partially decolourized by photo-oxidation, generally removal of residual colour
was in the range of 82 - 85.5 % after 72 hours of biotreatment. An increase in biochemical
treatment time from 24 to 144 hours also did not have an influence on colour removal. With
an increase in preliminary photochemical treatment time susceptibility of the dye solutions
to the biochemical action of activated sludge increases.
The combination of the UV /hydrogen peroxide process with subsequent microbiological
treatment is a satisfactory treatment for azo dye solutions suitable for commercial application.
Further research to extend the application to other dye classes, particularly reactive dyes In
justified
