Water Conservation In Textile Wet Processing
Ghulam Abubakar
Objectives
Introduction
What is water conservation?
Why is it necessary?
How to conserve water?
Where to conserve water?
Benefits of water conservation?
Introduction
The average water consumption in a textile mill is 150 to 200 m3 per metric ton of finished goods. Costs for water supply and wastewater treatment account for 4.3% of the total wet processing costs. It is not unusual to find situations where a 10% to 30% reduction in water use can be achieved without major investments. Common Common situations where water is unnecessarily consumed include hoses left running, broken or missing valves, cooling water that is running while machines are not in use, anddefective toilets and water coolers.
Why Water Conservation is necessary?
Water is expensive to buy, treat, and dispose. If your firm does not have a water conservation program, you are pouring money down the drain. The amount of water used in many fabric pretreatment operations is often preset to treat the most difficult cloth processed. As a result, large amounts of water are wasted when cleaner/easier fabric is treated. There is a tendency in the textile industry to use more water than necessary when removing chemicals during rinsing. Similarly there are many operations in the wet processing industry which doesn’t require water of drinking quality but all water used in the industry is of drinking water standard which requires expensive treatments.
Wet Processing
Pre-Treatments
Dyeing/Printing
Finishing
Pre-Treatments
Water usage in different textile wet processing operations Process
Volume in gal/1000 lbs product
Sizing
60 – 940
De-sizing
300 – 1,100
Kiering
310 – 1,700
Scouring
2300 – 5,100
Bleaching
300 – 14,900
Mercerizing
27,900 – 36,950
Dyeing Aniline black
15000 – 23,000
Basic
18000 – 36,000
Vat
1000 – 20,000
Direct
1,700 – 6,400
Napthol
2,300 – 16,800
Water Consumption in Pre-treatments in gal/1000 lbs of Product 38000 35500 33000 30500 28000 25500 23000
De-sizing
20500
Kiering
18000 15500
Scouring
13000
Bleaching
10500
Mercerizing
8000 5500 3000 500
How to conserve water?
The effluent from the desize J-box can be used to makeup the desize mix. Reuse the wash water from the bleach washer in the caustic washer. Wash water from the caustic washer can be reused in the desize washer. The caustic present in this water will enhance the removal of sizing chemicals. Use counter current washing. Batch operation processes do not easily allow for water recycling. When trying to reuse wastewater in batch operations, op erations, storage facilities for the reusable wastewater must be provided. Other problems associated with the reuse of wastewater from batch bleaching and scouring are the noncontinuous character of the waste stream and the higher liquor ratios. Supplying only the needed amount of water to a machine and reducing the number of throughputs can result in significant water w ater savings.
Reuse of Scouring Rinses for De-sizing
The rinse water from the scouring operation is adequate for reuse in other processes such as desizing that do not require water of an extremely high quality. This reuse is particularly true with scouring wastes from synthetic or cotton/synthetic blend fabrics. Scouring rinses may, in certain cases, also be reused to wash floors and equipment.
Reuse of Mercerizing or Bleach Wash Water for Scouring or De-sizing
Mercerizing or bleaching rinse water can be used in scouring and de-sizing operations as long as size recovery is not practiced. Generally, the caustic or bleach stream will degrade many size compounds to an extent that they cannot be recovered
Using Counter Current Flow
In counter current washing, the water flows in the direction opposite to the fabric. This method of water reuse is contrary to the traditional method of providing clean water at every stage of the wash w ash cycle. Water and energy savings are related to the number of boxes provided with the counter flow. As a result, the least contaminated water contacts the cleanest fabric. Table shows that the technique can result in significant savings. Many mills use counter current flow in continuous preparation processes. This technique can also be used for rinsing after dyeing procedures. Counter-current washing can be applied at de-size washers, scour washers, mercerizing washers, bleach washers, dye ranges, and print house soaper ranges.
Reducing Flow of water in a single stage bleach range for processing knit goods in rope form.
The unit consists of a saturator, J-box with wet heel and six si x box washers. Acetic acid is added to the last two washers to neutralize the fabric. The bleach washers operate at a flow of 3040 gallons per minute (gpm) between 170 and 19OoF. The fabric leaving the fourth washer has a caustic concentration of 0.025 % at a flow rate of 40 gpm. Reducing the flow to 15 gpm increases the concentration of caustic to 0.033 %. The concentration of caustic in the fabric’ is not changed when the temperature of the water is lowered from 205°F to 145oF. As a result of the experiments the water temperature is lowered to 140- 160°F and the flow rate to 20-25 GPM. These measures result in significant water and energy savings
Possibilities to reduce water and energy consumption in a open width bleach range.
This preparation train consists of 3 stages: de-sizing, scouring scour ing and peroxide bleaching. The water consumption in these stages can be reduced by flow reduction and counter current flow. Reducing the water consumption will also reduce the energy consumption since the temperature of the water used varies from 150°F to 190°F. The bleach washers can be operated at a temperature of 150°F - 160oF. The desize and caustic washers must be operated at 180oF-190oF. Therefore less energy will be needed if the amount of water is reduced. The results of this work are shown in Table.
Optimize Washing
In a mill caustic scoured cotton yarn packages were batch washed by a 10 minute hot running rinse followed by a 10 minute cold running rinse. Full-scale experiments were performed to optimize the process. The experiments revealed that the same fabric quality was obtained by a 3 minute hot running rinse followed by a 3 minute cold running rinse.
Water and energy savings available by process changes Change
Savings Water gal/hour
Energy lbs.steam/hour
Reduce the temperature of water by 10 F at the bleach stage washers
0
250
Decrease the water flow by 5 gal/min at the caustic washer
300
300
Counter flow from bleach washers to caustic washers
3000
2600
Fully counter flow wash water
6000
5300
Eliminate one stage
3100
3700
Water Savings by Process Change 7000
Water gal/hour 6000 5000 4000 3000 2000 1000 0 Reduce temp. Reduce flow Counter flow Counter flow in Eliminate one Single Stage by 10 F by 5 gal/min from bleach washing range stage Bleaching washers to caustic washers
Energy Savings by process change
7000
Energy lbs.s team/hour team/hour
6000 5000 4000 3000 2000 1000 0 Reduce temp. Reduce flow Counter flow Counter flow Eliminate one Single Stage by 10 F by 5 gal/min from bleach in washing stage Bleaching washers to range caustic
DYEING
Water Consumption doesn’t necessarily depend on the type of machine!!
The liquor ratio is the ratio of the amount of liquor (in pounds) in the exhaust dye bath to the amount of fabric (in pounds). This ratio r atio varies according to the machine type. Low liquor ratio dyeing machines have been developed to save water. But the liquor ratio does not n ot reflect the amount of water used during rinsing. The fact is that the largest quantities of water in dyeing operations are not used in the dye bath but in subsequent rinsing. It is therefore not necessary true that a dye machine with a low liquor ratio has a low overall water use.
Water usage & Liquor ratio in different dyeing machines Dyeing machine
Water consumption gal/lbs
Liquor ratio
Continuous
20
1:1
Beck
28
17:1
Jet
24
12:1
Jig
12
5:1
Beam
20
10:1
Package
22
10:1
Paddle
35
40:1
Stock
20
12:1
The amount of water used during rinsing depends on the dye class, and the type and weight of the fabric. The dye class, fabric and desired effects determine the dye equipment used and thus the amount of water required for rinsing. It is sometimes possible to achieve the same s ame shades with dyes of two different dye classes. Each of the dye classes requires different dye techniques, chemicals, energy and equipment. All these factors and the pollution load of the procedures must be taken into consideration when comparing different techniques.
Max. Water Usage in Different Dye Classes 1000 gal/1000 lbs
38 33 28
Aniline Black Vat Direct Sulphur Naphthol Basic
23 18 13 8 3 -2 Aniline Black
Vat
Dire ct
Sulphur
Naphthol
B as ic
Rapid Inverse Dyeing
Rapid inverse dyeing (RID) is a dye technique that is successfully used in dyeing Polyester/cotton blends using disperse and fiber reactive dyes. In the normal dyeing procedure the polyester is dyed with disperse dyes at elevated temperatures. The fabric is then washed to remove all traces of dye and acetic acid. The machine is refilled with reactive dye solution to dye the cotton portion of the fabric. After completion of the dyeing process the excess dye is removed by dye-extraction and rinsing. In RID the cotton is dyed first with reactive dyes. The acidic disperse dye bath is used as a wash for the fiber reactive dyes. This technique reduces the water and energy consumption. The duration of the dye cycle is also reduced
Reuse of Final Rinse Water From Dyeing for Dye Bath Make-Up
The rinse water from the final rinse in a batch dyeing operation is fairly clean and can be used directly for further rinsing or to make up subsequent dye baths. Several woven fabric and carpet mills use this rinse water for dye bath make-up.
Reuse of Soaper Wastewater
The colored wastewater from the soaping operation can be reused at the back grey washer, which does not require water of a very high quality. Alternatively, the wastewater can be used for cleaning floors and equipment in the print and color shop.
Printing
Water Consumption during Printing
The print range has a high water demand d emand for cleaning. The print paste is thick and difficult to remove from the machinery parts and drums used to make up the different colors. Another source of water consumption is the print blanket. A glue is applied to the blanket at the beginning of the print range. This is to ensure good contact between the blanket and the fabric. This glue contains starch and PVA. Removal of the t he glue from the blanket after the printing operations requires large quantities of water. The screens and other parts are cleaned with a hose until they are almost spotless which consumes a large amount of water.
How to Conserve Water in Printing??
Use of Vacuum extraction to clean the blanket after printing. Use of other water-based adhesives that are available, which might function as well and can be removed more easily. The inside of the screens should be cleaned with a cylinder shaped broom through which water is sprayed. The other parts of the equipment should also be b e cleaned with high pressure cleaning equipment.
Water Conservation During Washing
The effectiveness of washes increases directly with the volume of water being used but it increases with the power of the number of washes.
It is therefore more effective to conduct several washes with a small amount of water than to wash the fabric once with a large volume of water.
Removing all the excess water before the next portion of wash water is added to the fabric will prevent pr event excessive contamination of the wash water.
Water Conservation In Finishing
The function of the wet-out stage is to wet the fabric so that creases can be removed during subsequent drying. The wet out pan is overflowing at a high rate (15-25 gpm). It is necessary to overflow this box to prevent the buildup of solids in the pan. Filtering the water leaving the wet out pan and recycling the water to this pan would result in significant savings.
Miscellaneous Methods
Reusing non-contact Cooling Water
The non-contact cooling water used in finishing and dyeing operations is often discharged to the drain. This water can be use as makeup water for the boiler and as processing water in operations that do not required drinking water quality.
It is also possible to reuse this water as noncontact cooling water after heat exchange .
Reclaimed Water
Reclaimed water, sometimes called recycled water, is former wastewater (sewage) that has
been treated to remove solids and certain ce rtain impurities, and then allowed to recharge the aquifer rather than being discharged to surface water.
Use of Reclaimed Water
The required quality of water used in textile wet processing operations is controversial. The industry uses water of drinking water quality although several processes do not require water of this quality. Harker (1980), reported the successful use of a sand filtered, chlorinated activated sludge effluent in a mill in Yorkshire. This company produces high quality blazers and no differences were observed between fabric processed with water of drinking water quality and fabric prepared with reclaimed water. Inoua and co-workers investigated in 1977 the use of reclaimed water for scouring, bleaching and dyeing of cotton, wool and synthetic fibers. The types of water used in the full-scale experiments were municipal water, water after flocculation, sedimentation and sand filtration of biologically treated wastewater and the same water after activated carbon absorption. Slight differences were observed in the effects of the different water sources on various fibers but these effects were not no t considered significant.
Tworeck (1984) used municipal supply water and water from the Athlone wastewater treatment plant in his experiments. The treatment plant consisted of an activated sludge reactor followed by sandfilters, prechlorination, activated carbon absorption and chlorine disinfection. He compared the effects of sand filtered water and final effluent water on the fabric with water of drinking water quality. The water was used in fabric preparation, dyeing and finishing of polyester, nylon, a cotton/nylon blend, wool and other polyester and nylon blends. The reclaimed water had slight effects on the dyeing of the fabric but they were fully acceptable to the mill. Most wastewater leaving a textile mill needs treatment t reatment before reuse is possible. However, there are ar e waste streams that can be reused directly.
Use of Automatic Shut-Off Valves
An automatic shut-off valve set to time, level, or temperature will control the flow of water into a process unit.
One plant estimated that a reduction in water use of up to 20 percent could be achieved with thermally controlled shut-off valves.
Use of Flow Control Valves
A flow- or pressure-reduction valve can significantly reduce the quantity of water used in a wash or clean-up step.
These valves are particularly useful in cleaning areas where operators are not always aware of the need for water conservation
Good Housekeeping
A program of maintenance, inspection, and evaluation of production practices should be established. Significant reductions in water use can be made by implementing the following: Minimizing leaks and spills, Maintaining production equipment properly, Identifying unnecessary washing of both fabric and equipment, Training employees on the importance of water conservation.
Influence of conservation & recycling on the water & energy consumption in a typical continuous preparation operation Process modification
Water consumption gpd
Average process temp, oF
Energy for heating water 104 Btu/day
No conservation
346,000
165
274
Temp. increase & water reduction
194,000
195
202
Multiple use of water
158,000
195
165
Water recycled after ultra
43,000
195
45
Influence of conservation and recycling on water consumption 400,000
350,000
300,000
250,000
Water Consumption
200,000
150,000
100,000
50,000
0 No No Con Conser serva vati tion on
Tem Temp. incr ncrease ease and and flow reduction
Mul Multipl tiplee use of of wat water er
Using sing rec recyc yclled wat water er
Influence of conservation and recycling on energy consumption 300
250
200
Energy for heating water 104 Btu/day
150
100
50
0 No Con Conserv servati ation
Tem Temp. in increase crease & flow reduction
Multiple use of water
Using recycled water
Benefits of Water Conservation
Loss reduction.
Reduction of chemical, water and energy consumption, thereby resulting in savings and increased production.
Reduced liability for waste produced.
Improved compliance with regulations.
Facts and Figures Reusing non-contact cooling water at the fiberglass f iberglass processing plant will reduce the water consumption by 76% and results in a savings of $99,400 per year. Implementing counter-current flow between bleach washers will save $154,000 $15 4,000 per year due to reduced consumption of water and energy. The savings will be $ 336,000 per year if the existing washers are replaced by more efficient washers. By using non-contact cooling water in the cotton bleaching and dyeing units the estimated savings are $66,000 per year.
Any Questions???
