Page 160 - Basic Principles of Textile Coloration

DYEHOUSE EFFLUENT AND ITS TREATMENT 149

(4) process changes and optimisation of the use of problematic chemicals – for
example, for the oxidation of leuco vat dyes, hydrogen peroxide or sodium
perborate can be substituted for sodium dichromate. Replacement of dyes
that require aftertreatment with chromium or copper salts will have a
beneficial impact on the immediate water environment. Glucose can replace
sodium sulphide as a reducing agent for sulphur dyes. The dye manufacturers
now offer ‘greener’ products and processes such as low salt/high fixation
reactive dyes. Many companies are examining the possibilities of recycling
water and chemicals, and of recovering heat from hot effluent.

Various types of processes are used in textile effluent treatment. Dyehouse effluent
has a composition that is highly time-dependent in terms of both the types and
quantities of contaminants. A first stage in treatment is often an equalising
lagoon. Equalisation involves holding the combined process effluents for a given
period to allow stabilisation of pH and BOD, and time for sedimentation of some
solids. This considerably reduces fluctuations in the composition of the water
leaving the dyehouse, which can upset down stream processes such as activated
sludge treatment.

Biological treatment with organisms may be aerobic (with oxygen) or anaerobic
(without oxygen). Many textile mills in urban areas discharge their effluent into
municipal sewers. The sewage may be treated in an activated sludge plant. The
effluent is mixed with micro-organisms, aerated and then the sludge allowed to
settle. The phosphate and nitrogen nutrients needed for bacterial growth are not a
problem if the industrial effluent is mixed with regular sewage. Activated sludge
treatment considerably reduces the BOD by aerobic oxidation, and adsorption or
coagulation of the contaminants. The effects are improved if the sludge is
combined with activated carbon. This also protects the micro-organisms from
heavy metals. They are sensitive to sudden changes in conditions and industrial
effluents in the sewer system should be of relatively constant composition and
concentration. The main problem is that of sludge disposal. After suitable
treatment, it can be used as fertiliser, or for landfill. In a trickle filter plant, the
effluent percolates through a filter bed with the bacteria growing on the surface of
the filter medium.

These aerobic microbial oxidation processes reduce the BOD, COD and TOC.
The effect on colour from dyes, however, is often only marginal. Some dyes are
adsorbed to some extent by the biological treatment. This has only limited effects