Table 9 shows the auxiliaries required for different dyes and Table 8 shows the amount of water used and liquor rations of dyeing machines.
Wastewater generation from a typical dyeing facility is estimated at 3800 to 7500 m3 per day. Including the dyeing, post scouring and rinsing processes, approximately 100 to 150 m3 of wastewater per tonne of product are produced for disperse dyeing, while 125 to 170 m3 per tonne is more typical for direct and reactive dyeing.
The primary source of wastewater is spent dyebath and washwater, which contain by-products (hydrolyzed dye), some intact dye, and auxiliary chemicals. In addition to process water and chemicals, a major source of toxic pollutants in wastewater is cleaning solvents used in dyeing and printing machine cleaning, such as oxalic acid, hydrochloric acid and carbon tetrachloride.
With the abundance of individual dyes and the wide range of dyeing equipment in use today, it is difficult to summarize waste stream characteristics. In general, wastewater from batch dyeing is high in volume and pollutant load, and tends to contain heavy metals, aromatics, and halogenated hydrocarbons from the dyebath makeup. All are toxic to aquatic life. In addition to the dyestuff itself, many auxiliary chemicals are used to aid in dye transfer; the majority of these chemicals, including unreacted color, are discharged with the spent bath .
|Dyeing Machine||Water consumption (m3/tonne)||Liquor/goods ratio|
As mentioned earlier, dyes and auxiliaries used in printing are similar to those used in fabric dyeing. However, the colour application techniques are quite different. In the most commonly used technique, pigment printing, the main source of waste is from the cleanup, during which unused printing paste is removed from the screen. Consequently, proper planning of paste use and housekeeping are major issues in minimizing waste in printing operations . Volatile organic compounds from the paste composition are generated. Table 11 gives, according to reference 33, the characteristics of a typical textile fabric printing run, characteristics for different printing equipment and sources of mineral spirit emissions.
|Leveling or retarder agents||x||x||x||x|
|Dye Class||Wool||Cotton||Cellulose Derivatives||Polyamide||Polyester||Acrylics|
X=suitable; (X)=of secondary importance. Pigments may be applied to any substrate by the use of adhesives.
|Roller||Rotary Screen||Flat Screen|
|Characteristics of the run|
|Wet Pickup Rate(1)||Kg/Kg||0.51-0.58||0.56||0.10-1.89||0.58||0.22-0.83||0.35|
|Fabric Weight (2)||Kg/m2||0.116||0.116||0.314|
|Mineral Spirits added to print paste||weight %||0-60||26||0-50||3||23|
|Print Paste used in run (3)||Kg||673-764||741||137-2497||764||787-2975||1254|
|Mineral Spirits used in run (4)||Kg||0-458||193||0-1249||23||181-684||288|
|Source of emissions|
|Wasted mineral spirits (potential water emissions) (5)||Kg||0-28||12||0-77||1||11-42||18|
|Overprinted mineral spirit fugitives (6)||Kg||0-16||7||0-44||1||6-24||10|
|Tray and barrel fugitives (7)||Kg||0-1||1||0-4||0||1-2||1|
|Flashoff fugitives (7)||Kg||0-7||3||0-19||0||3-10||4|
|Dryer emissions (7)||Kg||0-405||170||0-1105||21||160-606||255|
Length of run=10,000 m, fabric width=1.14 m, total fabric area=11,400 m2, line speed=40 m/min, distance, printer to oven = 5 m.
(1) Wet pickup ratio is a method of yield calculation in which mass of print paste consumed is divided by mass of fabric used.
(2) Only average fabric weight is presented.
(3) Print paste used per fabric area multiplied by area of fabric printed.
(4) Print paste used in run multiplied by mineral spirits added to print paste, weight percent.
(5) Estimate provided by industry contact.
(6) Estimated on the basis of 2.5cm of overprint on each side of fabric.
(7) Emission splits calculated from percentages provided by evaporation computations.