Textile processing is both chemical intensive and water intensive. The data for water consumption in different categories of the textile industry is given under the following headings describing each in more detail. As can be seen, this data clearly shows a significant variation in water usage even within each plant category, due to the differences in the washing cycles, washing equipment employed and extent of water reuse. According to reference , from which these data were extracted, where a large difference exists between the median and maximum values, this probably reflects instances of indiscriminate water use including bad housekeeping. Sharp differences between the median and minimum values may indicate instances of strict control over water use and better housekeeping, water reuse or the selection of improved washing equipment.
Total chemical use can be 10% to over 100% of the weight of the goods . Due to the range of chemicals used and the variability in processing, it is impossible to give general figures for chemical consumption. The composition of the effluent stream may also vary considerably according to the chemicals used in processing, and most of the attempts to reduce the BOD of the effluent are based on the substitution of low BOD process chemicals for those having high BOD values.
Textile process effluents have a high BOD, high total dissolved solids and a high temperature and are generally grey in color. Natural impurities extracted from the type of fibre being processed along with the chemicals used for processing are the two main sources of pollution. Other factors which determine effluent quantity and quality include the unit operations used and the degree to which water and chemicals are preserved in a particular manufacturing plant . As stated in the publication of UNEP, reference , each fibre process produces effluents with their own distinctive characteristics. For all textiles mills processing the same fibre, effluent characteristics are broadly similar but quantities may vary. Differences also arise between different plants processing the same fibre, due usually to variations in the production technology.
Wool effluents are characterized by high BOD, high solids concentration and a high grease content. Cotton finishing effluents are not as strong as those produced by the wool industry but many have a high colour content due to cotton dyeing operations. They also have a high BOD (although this is much lower than that found in wool scouring effluents), contain no grease and have a relatively low solids content. Synthetic finishing effluents are generally lower in volume than those generated in cotton finishing, but may contain toxic substances, especially from the dyeing streams where chemical dyes with a metallic ion content are used more widely.
Also of particular concern are more specific compounds that are toxic to aquatic life, such as heavy metals, primarily from dyeing and finishing (and water impurities), surfactants (wetting agents), compounds used throughout the wet-processing steps, and other process chemicals. Fabric rinsing and/or washing (using detergent) is usually performed between primary process steps, resulting in large quantities of diluted wastewater in excess of spent chemical baths .
A substantial portion of the waste load from wet textile processing results from non-process sources. It is important to recognize these non-process sources which can contribute up to 50% of total waste loads. Some non-process sources include rinsing of dispensing equipment, cleaning of process equipment (residues as well as cleaning chemicals), dumping unused processing solution, shop chemicals and other non-process sources, such as drum washers, and parts cleaner (e.g. print screen cleaners) .
According to reference , organic priority pollutants expected to be found in textile effluents (at the ppb level) and their origin are detailed below:
According also to the above mentioned reference, the sources of major metal pollutants (i.e. zinc, copper, chromium, etc.) are likely to be the dyes used in wet processing operations.