Page 161 - Basic Principles of Textile Coloration

150 WATER TREATMENT

in removing hydrolysed reactive dyes from the effluent. Because of the high levels
of waste, textile auxiliary products should ideally biodegrade rapidly in water,
although this is often not the case. Alkyl benzene sulphonate detergents, with a
branched alkyl chain, such as that derived from propene tetramer, caused
mountains of foam on rivers throughout the industrialised world in the 1960s
because of their low rate of biodegradation (Section 9.3.5).

One of the most common effluent treatment methods is that of precipitation.
This often involves a combination of precipitation of insoluble salts, coagulation of
colloidal material and flocculation. This is similar in principle to the method used
for clarifying water described in Section 8.1. Addition of lime (CaO) to the
effluent is quite common. This neutralises any excess acidity and precipitates
many types of anionic compounds. Treatments with alum or ferric chloride are also
popular. The aluminium or ferric hydroxide, along with precipitated aluminium or
ferric salts, removes colloidal matter and a number of anionic dyes. This reduces
the COD, colour and suspended solids. Sedimentation is assisted by adding a
flocculant such as a polyacrylic acid derivative, or a cationic polymer, the latter
being able to bind hydrolysed reactive dyes. Again, the disposal of the sediment
sludge needs to be considered.

There are a variety of chemical technologies for effluent treatment. Few of
these are used in the textile industry because of their cost. This situation may
change as environmental protection becomes even more socially and politically
acceptable. Chlorination with sodium hypochlorite and acid eliminates much
organic material in waste water but may generate even more toxic organochlorine
compounds. Oxidation by ozone is much safer but the cost of generating ozone by
electrical discharge through oxygen gas is still prohibitive. Other technologies
involve reverse osmosis and membrane filtration, adsorption on active carbon, or
generation of coagulants by electrochemical techniques. The highest standards of
effluent treatment require combinations of different types of treatment. These will
probably become more significant, despite the expense, as regulatory controls are
increasingly enforced.