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.
