Denim Specification Sheet

Effluent treatment in denim industry

Denim Effluent
Characteristics
-Dark Blue Indigo color
-High Dissolved Solids ( Decomposed products of hydro)
-High Chemical Oxygen Demand (COD)
-High pH
-Chlorides and Sulphates of Suspended matter

Characteristics of Effluent
Appearance: Dark Blue
pH: 9-10
Suspended Solids: 250ppm
Dissolved Solids: 3500-5000 ppm
Oil/Grease: Traces
BOD ( 5 days, 20 deg C): 160-350 ppm
COD: 570-1100 ppm
Alkalinity (pH): 400 ppm
,,,,,,,,,,,,,,,,,(MO): 1700 ppm
Total Hardness: 220 ppm
Chlorides: 210-480 ppm
Sulphates ( SO4): 1200 ppm
Calcium: 15 ppm
Magnesium: 45 ppm
Ammonical Nitrogen: 2.5 ppm
Color (pt.Co): 250

Effluent is characterized by “high strength low volume”, as most of the most contaminated (“high strength”) effluent come from comparatively small quantity ( “low volume”) of wash waters used for rinse after yarn dyeing with indigo. Major contaminant is Indigo.

Permissible Limits for Cotton/ Synthetic Textile Industry (India) Effluent
pH= 5.5-9.0
Suspended Solids= 100 ppm
Oil and Grease= 10 ppm
BOD= 30 ppm
COD= 100 ppm
Hexavalent chromium: 0.1 ppm
Total Chromium= 2.0 ppm
Phenolic Compunds= 5 ppm
Sodium absorption ratio= 26
Sulphides= 2.0 ppm
color ( pt.Co.Scale) = 100
Bioassay test= 90% survival of fish after 9 hours in 100% effluent.


Effluent Treatment Scheme

1. Equalization
Equalization tank in two compartments. Retention time of at least 7-8 hours.

2. Flash Mixing
Equalisation Tank flash mixer ( to adjust pH) clarifloculator Unit ( Alum/Poly Aluminium Chloride) for coagulation/segmentation

3. flocculation
( it is a physico chemical process with 35-40% COD removal, 25-30% BOD and 70-80% color removal, also >95% color removal is possible if PAC and polymer dosage increased) overflow rate at CFU < 20 m^3/day
The sludge withdrawal should not be too less or too large ( can take place in lamella unit also )

4. Aeration
Effluent after CFU aeration process ( time > 18 hours) ( New recent aerators use injectors which produce very fine bubble resulting in a large air/water interface. Waste water is used as pressure water fro the operations of injection. Water +air stream are subjected at the bottom of the tower to prevent any possible sedimentation. Gas bubble rise to full height of the tower long resident time. Good utilization of oxygen upto 80% is possible.

5. Clarification
Effluent from aeration  clarifier ( resident time 3-4 hours)  activated sludge recalculated from clarifier to aeration tank sludge thickened  centrifugal decanter filtrate is then discharged to another tank.

Dissolving Oxygen
Clarified Effluent deficient in dissolved oxyen (DO)( for bioassay parameters) DO make uptank ( 2 hours resident time) – the output is expected to meet the criteria.

6. Ultrafilteration
Process for filtration of particles >5 n meters, from feed water made to flow at low pressure through membrane having pore size of 4-5 nm.
Useful for elimination of high molecular weight organic compounds. By using this ( the original indigo concentration in rinse water is 0.05%) fully usable 5% dispersion of indigo dye is obtained.
There are two types of membranes available. 1. Organic 2. Mineral – resistant to pH 0-14, resistant to mechanical and thermal conditions and are unaffected by solvents.

7. Incineration
Burning of waste
Major threat to possible health
Destruction of resources
Expensive
Generate toxins

8. Sludge Disposal
 85% of the waste is biodegradable. Can be used for compost. Lime sludge has agricultural value as it is free from pathogenic microorganisms
Bugs convert dyes into colorless substances
Microorganisms ( Geotrichum Candidum  filament fungus isolated from soil) can decompose 18 different kinds of dyes in to colorless substances. Preferred pH for them is 4-7 at a temperature of 20-30 deg C. Can destroy dye in two days ( at a concentration of 12 g/lit). They can eat indigo also.

Process Control for Effluent
Usually 10% of the applied indigo is washed off in rinses. Indigo fixation of yarn could be improved by:
-Slightly lower pH- can reduce indigo consumption for a given visual depth of shade
-Use of pre reduced indigo and indigo dyeing under nitrogen blanket. Can cut hydro consumption
-Use of prereduced sulpher dye and maintain reduction potential with hydrol ( glucose + other oligomeric reducing agent) instead of sod. Sulphide.

Effluent volume can be reduced through water conservation
-Washing in counter current type
-Decrease size of wash tanks
-Use Na2CO3 (Sod. Bicarbonate) in first rinse tank
-Use Co2 for neutralization of alkali
-Use as many nips as possible during washing to squeeze out alkali to maximum ( squeezed liquor should not drop back into bath)
-Relying more on spray rather than immersion into the bath
-Create enough stir in wash tank for best washing efficiency.

Integrated Finishing for Denim- my Notes

Preshrinking of Textile Fabrics- or compressive shrinkage

Shrinkage can be
- Natural Wash shrinkage- When the fibres swell in the presence of water and tensions induced during Spinning, weaving and processing of fabrics
- Compressive or Mechanical Shrinkage

It can be compared to the action of press. The effectiveness of ironing depends upon:
Temperature of the press
Amount of Moisture present in the fabrics
Amount of Physical Pressure Applied
Time duration of application of press.

In case of pressing, the new memory is set by drying, in compressive shrinking, it is the palmer unit which dries the fabric, thereby setting its new memory

Amount of preshrinkage left in the fabric is adjusted by varying the relative speed of the palmer to that of the rubber belt unit.

Temperature

The temperature affects in the following way the various components of the palmer/ compressive shrinkage unit

- Rubber Belt Cylinder
o Lower Temperature- Fabric appearance is affected- Sharp crease
o Higher temperature- rubber belt life is shortened
- Palmer Cylinder
o If lower temperature: No dry Stuff; Elongation of preshrunk fabric- also stretching of inspection and rolling operation.
o The purpose of palmer cylinder is to dry a fabric to a level of about 4% relative humidity. If there is higher temperature, there is elongation with natural moisture regain.
- Temperature of fabric as it enters rubber belt unit
If properly controlled, then high production. Most fabrics will shrink more easily if heated before entering the rubber belt unit.
Moisture

- 100% cotton denim may require as much as 14% moisture to permit effective pre shrinkage
- Moisture must be uniform thoroughout the length, width, and depth of fabric

How to ensure moisture uniformity
- Use of heated cans
- Apply needed moisture of fabric vial cooling water applied to rubber belt surface. But it also depends upon the condition of the rubber belt. Freshly grinded rubber belt carry more water à water removal roll of the rubber belt unit

Penetration of moisture applied to the fabric is very important. If insufficient moistureà Innermost dry layers of yarn will act like spring and cause the fabric to elongate.

Pressure

Maximum amount of rubber belt compression should not be greater than 25% of the actual belt thickness.

More heavy the fabric, more potential, more compression will it need

d. Duration

If above three factors are maintained and we have a sufficiently large palmer unit, we can compress durably a fabric to its ‘zero’ potential.

Its important to use cooling cans at the exit of the palmer

Fabric Scray: Use of exit scray allows additional time for fabric cooling as it is impossible to roll fabrics without the use of lengthwise tension.

Guider: The function of the guider is to keep the fabric to its full width.

Skyer: It is a sort of time delay device allowing time for moisture to penetrate into the fabric without the need to increase the machine length.

Heated Can: purpose: it is to drive the surface moisture into the fabric and to preheat the fabric.

Palmer

Function

Dry the fabric and set shrinkage
Adjust the shrinkage
To compare incoming and outgoing fabric tension and determine fabric shrinkage

Less dense the felt, greater is the drying capacity

Exit Scary
- To relax and cool;
- To prevent hot stop marks. It increases the rubber belt life
- To facilitate shrink environment
Why Wet Finishing for Denim

- Moisture doesn’t penetrate in the core- in foam finishing
- Its better to shrink fabric with a low moisture content than those which are bone dried

In integrated Machine

Padder- wetting
- squeezing the moisture
- application of heat

Once it is shrunk the fabric is thoroughly dried by palmer

- Hand can be adjusted in padder use of starch, lubricant
- Width can be controlled by adjusting tension between the padder and dry can
- Higher speed
- Even ness of the moisture content- Residual moisture after leaving palmer should be 4%

Drying depends upon the pressure of the steam, m/c speed, size of palmer , construction of felt

Rubber Belts: 36-40 deg. Shore

Harder- crack and lesser shrinkage capacity
Softer- require replacement frequently
Thickness- 67mm

Thicker- more grinding
- cracking
- more wear and tear to machine parts
- energy consumption

Rubber belt: Inside circumference- 3.962m
- Rubber surface width should exceed fabric width by at least 6” and preferably 8”

How to increase the life of the Rubber belt

- Nip pressure used on rubber belt should be optimum
- Over tension in the rubber belt should be avoided
- Belt should be run with sufficient cooling water in its interior and exterior surface
- Belt should be run with lowest possible operating temperature 115 deg- 140deg
- Frequency of grinding of the rubber belt should be optimum ie should be enough and at sufficient depth.
- Grind when density of belt surface has varied by 10% of its original hardness
- Water removal roll pressure adjustment is very important to insure max. belt life. Water acts not only to cool the rubber, but is also a lubricant
- Product machine stops or “hot stops” should be avoided to the maximum extent possible. One of the best ways to eliminate is to install scrays at the entrance and exit of the shrinking machine.
- Foreign objects should be avoided. Knot size for joining fabrics should be smaller.
- Regular cleaning and inspection of rubber belt.
- Use correct belt width
- Be careful during installation and maintenance of rubber belt, avoid use of chemicals.

Function of Felt Palmer

- It is required to maintain the preshrunk fabrics in intimate uniform contact with the surface of the heated cylinder in order to ensure uniform smooth drying of the fabricà new dimensionally stable memory.
- Fabric drying depends upon, palmer cylinder temperature, shrinking machine speed and permeability of the drying felt.
- It helps in precise fabric shrinking adjustment. It acts as a fabric puller to precisely control tension on the fabric
- Provides a pressing and calendaring effect on the preshrunk fabric

Balancing of a spinning line for denim Manufacturing