BUFFALO RIVER, EASTERN CAPE
4.1.2 Textile effluents
General characteristics
The textile industry in South Africa is recognised as one of the largest water users, and produces the highest volume of industrial effluent (Trivedy and Gudekar, 1987;
Gravelet-Blondin et al., 1997). Large amounts of water are required for wet processing and the vast quantities of wastewater produced are extremely variable in composition and pollution load (Correia et al., 1994). The pollutants in the wastewater arise from the removal of impurities from the raw material and the residual chemical reagents used for processing. The composition of wastewater from textile plants is complex and varies according to the process used at a plant, as well as depending on the fabric and yarns processed. The strong colour of textile wastewater is the most obvious indicator of water pollution; the degree of colouration dependent on the colour or shade dyed and the type of dye used (Steffen et al., 1993; Carliell et al., 1996). The colour, which is a visible source of pollution, is perceived as harmful. If the colour is not properly dealt with, it can interfere with light penetration, thereby inhibiting or impairing biological processes such as photosynthetic action (Samira and Doma, 1989; Buckley, 1992; Meyer et al., 1992; Lin and Lin, 1993; Gravelet-Blondin et al., 1997).
The removal of dyes from textile effluents is problematic as biological treatment processes are not effective in removing colour (Meyer et al., 1992; Correia et al., 1994; Carliell et al., 1996). Reactive dyes are the most difficult to remove due to their
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solubility as they pass through biological sewage treatment systems and enter the receiving water (Lin and Lin, 1993; Carliell et al., 1996). Dyes and surfactants predominate in textile effluents, as they are not fully retained in the final product. Azo dyes such as Orange II (C18H11O4NaSN2) represent the largest group of textile dyes.
The textile effluent contains high but variable concentrations of Biological Oxygen Demand (BOD), and Chemical Oxygen Demand (COD) (Buckley, 1992; Orhon et al., 1992; Correia et al., 1994). Dissolved Oxygen (DO) is almost zero, Total Dissolved Solids (TDS) and Total Suspended Solids (TSS) are usually high in the effluent.
Temperature is relatively high due to hot rinse water. Effluents are highly alkaline with a pH range of 8.2 to 12.2. Chromium is also generated from the chemical material used in the dyeing process (Germirli et al., 1990).
Sodium hydroxide is used extensively in the textile industry, resulting in high concentrations of chloride and sodium in the effluent. The discharge of industrial effluents containing sodium hydroxide is problematic since these effluents contribute to the mineral enrichment and increasing salinisation of the receiving waters.
The manufacturing process (raw material and final product)
The production sequence for textiles is shown in Figure 4.1. There are three main processing stages of fibres: fibre pre-treatment, dyeing and finishing. The fibre pre- treatment prepares yarns and fabrics for dyeing by removing foreign impurities and assuring good wettability, the required whiteness and high dye intake. There are both dry and wet processing stages.
Fibre pre-treatment
1. Blending and spinning (Steps 1 and 2)
First the raw fibre is sorted and cleaned before being blended as required. Raw cotton and polyester staples are then blended together. The fibres are drawn up into yarn and twisted (spinning). From here, the yarn goes for sizing where enzymes are added to provide protection from abrasion during weaving. The yarn is then washed in hot water counter-current washing machines. Effluent is produced at this stage. The yarn
is sent for weaving, which takes place under controlled high-humidity conditions to minimize the breaking of yarn. At this stage, which is usually a dry process, the yarn is converted into fabric. Desizing follows after weaving, where the sizing agent (enzymes) is removed (Correira et al., 1994).
Figure 4.1 A flow chart of the textile production sequence in the textile manufacturing process (Steffen et al., 1993).
2. Scouring (Step 3)
The yarn/cotton is scoured to remove natural waxes, spinning oils and other non- cellulosic compounds, using hot alkaline solutions (NaOH) containing detergents or soaps. Raw wool scouring is the highest-polluting process with large volumes of concentrated wastewater being produced. A typical effluent contains wool grease, dirt (from sand, fibre and vegetable matter), and suint salts (salts produced by natural excretions). The COD of the effluent can be as high as 50 000mg/l (Towsend et al., 1989). Organochlorine compounds and organophosphates, which are used as parasite control agents by sheep farmers, are found in significant quantities in raw wool scouring effluents (Shaw, 1994). The non-biodegradability of many of the impurities
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in scouring wastewater affects the proper operation of biological treatment systems (Correia et al., 1994). The disposal of the effluent is mainly by solar evaporation and direct discharge into water resources. Both these methods have become unacceptable due to their environmental impact (Towsend et al., 1989).
3. Bleaching (Step 4)
Bleaching aims to remove the natural yellowish colouring of cotton fibres thereby increasing its whiteness. Hydrogen peroxide and sodium hypochlorite are used as oxidizing agents. H2SO4, HCl and NaOH are also used during bleaching and final rinsing. After the fabric has been bleached, it is taken for mercerizing where the fabric is treated with NaOH to increase dye-ability and impart sheen. The fabric is then washed with a weak organic acid in order to neutralize the fabric. The effluent produced is highly alkaline, is at high temperatures and has high residual concentrations of sodium hydroxide (Steffen et al., 1993). From here, the fabric is sent for either printing or dyeing.
Yarn processing
From bleaching, the fabric goes for dyeing and/or printing. Dyeing may be carried out in a batch or continuous process. Textile processing involves a wide variety of dyes and the fibre concerned determines the selection of the dye. Effluents from batch dyeing of cotton with reactive dyes are usually high in dissolved solids, as the process demands a high concentration of salts and sufficient alkali to raise the pH to 12 (Correia et al., 1994). Ninety percent of dyes end up in fabric, with the remaining 10%
discharged to waste stream (Porter, 1978 cited by Maguire, 1992). Large quantities of effluent are produced at this stage. Some fabric is sent for printing, where designs are added.
Finishing
Finishing processes involve impregnation of the fabric followed by fixation, heat, and washing to remove residual chemicals. The processes improve the stability of the fabric and impart properties such as stain and shrink resistance, moth and fireproofing.
Although the volume of effluents produced is low, they are extremely variable in
composition and can contain toxic organic substances such as ethylchlorophosphates and pentachlorophenols (Correia et al. 1994). Dyeing-finishing works are heavy water users and their effluents contain synthetic dyes, surfactants and various additives (Timofeeva, 1991).
Generally, the effluent streams discharged from textile mills include wool scouring effluent, textile soaping effluent, effluents from dyeing, bleaching and acrylic emulsion effluent (Townsend et al., 1989). As mentioned in Step 3, effluents from wool scouring contain mainly grease, dirt (sand, fibre and vegetable matter) and suint salts (i.e. salts produced by natural excretions) (Correia et al., 1994). Wool scouring produces an effluent considered to be the most polluting of textile effluents (Townsend et al., 1989; 1992). Textile soaping effluent produced in the cleaning process after dyeing and printing, contains dissolved and colloidal dyestuffs, detergents and some salts. Polyester/viscose effluent contains both soluble and colloidal dyestuff, acetate, alkali, salts and organic auxiliary chemicals (Townsend et al., 1992).
The production of these strong caustic effluents, with the mills not meeting water quality guidelines, resulted in the initiation by the Water Research Commission (WRC) of investigations into development of technologies. The application of these technologies would alleviate discharge problems (Buckely et al., 1990).
The majority of textile effluents are discharged and treated in local sewage treatment works. Treated effluent can be disposed of by irrigation, or discharged directly into a river or sea (DWAF, 1999). Effluent discharged by sewer must meet set standards that comply with the requirements set by the local authority. These requirements in turn must comply with DWAF standards. However, where effluent is discharged directly into the marine environment, the river or discharged for irrigation, the mill must have a license from DWAF as a water user.
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Biological treatment processes such as aerated lagoons and conventional activated sludge processes are frequently used to treat textile effluents and are efficient in the removal of suspended solids and COD, but not colour. Adsorption appears to offer the best prospect for overall treatment and promises also to be effective for the removal of colour (Meyer et al., 1992; Altinbas et al., 1995). Davies and Cottingham (1994) found that the visible colour of the textile effluent was reduced as the effluent passed through wetland beds. Ozonation and chlorine were also found to be highly effective in removing colour (Lin and Lin, 1993; Tünay et al., 1996). Although ozone residuals are toxic to aquatic organisms, they are rapidly reduced in wastewater, hence unlikely to be found in the final discharge. For high strength dyes, ozonation is used to remove colour and reduce turbidity, in combination with chemical coagulants such as aluminium sulphate (Lin and Lin, 1993). Chlorine is more effective at lower pH, but there are concerns about the effects of residuals and by-products on the aquatic environment (Nicolaou and Hadjivassilis, 1992). Trivedy and Gudekar (1987) found the water hyacinth to be very efficient in treating textile wastewater; the treatment is attributed to microbial activity.