Page 163 - Membranes for Industrial Wastewater Recovery and Re-Use

Industrial waters 13 7

Table 3.22 Pollution load from desizing of 50:50 polyesterkotton (PRG, 1983)
U e s iz i n g
~ ~ ~ ~
Enzyme starch 6-8 3078 6155 1583 288
Polyvinyl alcohol 6-8 200 400 4029 192
Carboxymethyl 6-8 3 14 400 4349 751
cellulose

resulting from the alkali and from impurities in the raw cotton material, is
generally high (Table 3.22). Along with desizing, cotton scouring generates very
high BOD concentrations. These two processes thus make by far the greatest
contribution to effluent BOD in the wet phase processing of cotton goods (Nolan,
1972; Smith, 1989). When synthetic sizes are used desizing and scouring are
usually carried out in a single operation. Since synthetic fibres are free from
chemical impurities, 100% synthetic fabrics (woven or knitted) require only
light scouring in order to remove sizes and lubricants. The process is not
normally a significant source of organic or suspended solids pollution. In cases
where desizing, scouring and dyeing are performed simultaneously, effluent with
an increased pollution load results (Table 3.21).
Raw wool scouring is the highest-polluting operation within the textile
industry (Table 3.20). The large volumes of effluent and high levels of
contaminants generated by this operation have made it an area of the industry
of key concern, and much work has been carried out in this area towards
abatement of pollution from this process (BTTG, 1992; Nolan, 1972; OECD,
1981). The pollution load results from impurities present in the raw wool, (wax,
urine, faeces, vegetable and mineral dirt, and parasite-control chemicals)
together with soap, detergent and alkali used during the scouring and washing
processes. The use of some of the more onerous organochlorine chemicals in
sheep dipping has been restricted by legislation in recent years, but there remain
chemicals such as organophosphates that are still used and so arise in raw wool
scouring effluents (Shaw, 1994a,b). Due to their non-biodegradability or
toxicity, many impurities in scouring effluents (Table 3.23), such as antistatic
agents (synthetic fibres), pesticides, cotton waxes and wool grease or wax, can
pose problems in the operation of biological treatment systems. Scouring of
woollen goods is generally duplicated downstream to remove added substances.
These include oils and weaving sizes or lubricants, which are removed using
detergents.

Bleaching
Bleaching removes the natural yellow hue of cotton, increasing its whiteness.
This operation is generally required if the finished fabric is to be white or dyed a
light colour. It is usually carried out by chemical oxidation with sodium
hypochlorite or hydrogen peroxide. Auxiliary chemicals such as sulphuric
acid, hydrochloric acid, caustic soda, sodium bisulphite, surfactants and