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Zero Effluent Discharge System with NA2SO4 Recovery for the Textile Industry

Chemtech Processors Limited, Nallur, Tamilnadu

For wastewater management at its plant in Nallur in TamilNadu,  Chemtech Processors has installed what was the first Zero Liquid Discharge & Salt Recovery System in the Textile Industry in India and probably in the world.

Wastewater management and water recovery has become a necessity in almost all industries. Chemtech Processors, an independent premier textile processing company had similar problems and has effectively tackled the dye house effluent. Chemtech Processors is one of the many companies of the Reliance Dyeing Group, in existence since 1977. it is the manufacturer of a range of world class knits and premium garments.

For its wastewater treatment, Mr. K.L. Palaniswamy, Managing Director of Chemtech Processors, approached Chem Process Systems Pvt. Ltd., Ahmedabad, a name in the Vacuum & Evaporation Technology, for a comprehensive solution for Zero Liquid Discharge. The technical team at Chem Process Systems, after a thorough analysis, concluded that the following were to be addressed simultaneously:

  • Raw Water Treatment System & Distribution
  • Primary Treatment
  • Effluent Stream Segregation System
  • Reject Stream Evaporation System
  • Salt Recovery System
  • Sludge Handling System
  • Solar Pond

The designing of the complete system required careful attention to the overall water balance so that the raw water treatment system can be designed only as a make-up.

The Effluent Treatment System was required to handle various streams from the plant. Due to the difference in the nature of the effluent, some streams required to be segregated for separate treatment. It was analyzed that many streams were quite different in nature as compared to each other.

The treated effluent from the primary effluent treatment system is taken to the two-stage Reverse Osmosis, RO plant. The RO system supplied by Advent Water Treatment comprises of sub sections like polishing for pretreated water, reverse osmosis system and post-treatment system including water degassing. The presence of the high Total Dissolved Solids, TDS, necessitated the use of Reverse Osmosis as a preferred desalination system, for the recovery and reuse of water.

Reverse Osmosis technology was selected as it offered various advantages. Variant ions in TDS levels in the effluent can be handled by the RO. The process has low operational requirements and less operating costs. Besides providing consisting results in most cases, it also has less civil works requirement. However the RO system required a careful pretreatment to avoid fouling of the membranes.

The pretreatment was designed to handle the impurities from biological origins and from colloidal origins. The pretreatment also required handling the variations in the flow rate as well as impurities effectively, and still ensure, that the membrane feed water is not of fouling nature. The pretreatment was achieved by combination of following unit processes:

  • Installation of Aeration System
  • Modification of the chemical and its dosing in the existing ETP
  • Installation of Gas Chlorination System
  • Modification of Dual Media Filtration
  • Change in existing Activated Carbon Filtration
  • Micron Filtration
  • Chemical Conditioning

The water analysis of the inlet to/outlet from the RO Plant:

pH

Mg/ltr

7.08

6.57

Total Suspended Solids, TSS

Mg/ltr

58

2

Total Dissolved Solids, TDS

Mg/ltr

3800

70

Chlorides (as Cl)

Mg/ltr

960

36

Sulphates (as SO4)

Mg/ltr

828

1

BOD – 3 days

Mg/ltr

105

4

COD

Mg/ltr

208

17

The reject water of the RO Plant is taken to the MEEP.

The water analysis of inlet to the MEEP (including dye bath and RO reject):

pH

Mg/ltr

10.60

Total Suspended Solids, TSS

Mg/ltr

2802

Total Dissolved Solids, TDS

Mg/ltr

30000 – 40000

Chlorides (as Cl)

Mg/ltr

16364

Sulphates (as SO4)

Mg/ltr

30704

BOD – 3 days

Mg/ltr

5500

COD

Mg/ltr

6611

A system was designed with the above specifications. Careful selection of the type of evaporator was carried out after considering all the pros and cons of the nature of the effluent from the dye bath vis-à-vis the behavior of various Evaporator types and their configurations along with the capital investments required and operational costs.

Chem has extensive technical know-how from their own development work with numerous designed plants. After thorough study a quadruple effect falling film type evaporation plant with vapour compressor was decided to be employed for the application. The metallurgy selection was stringently done for all the components after the detailed analysis of the effluent characteristics.

The usage of Sodium Sulphate was suggested to bring down the capital cost, increase the life of the equipment and reduce the steam consumption. The usage of Sodium Sulphate also offered the additional and more important advantages of a better dyeing quality and recovery of the salts for reuse.

The water analysis of the condensate from the MEEP

pH

Mg/ltr

7

Total Suspended Solids, TSS

Mg/ltr

2

Total Dissolved Solids, TDS

Mg/ltr

56

Chlorides (as Cl)

Mg/ltr

27

Sulphates (as SO4)

Mg/ltr

1

BOD – 3 days

Mg/ltr

5

COD

Mg/ltr

Nil

Oil and Grease

Mg/ltr

BDL

The concentrated reject from the MEEP is taken to the Crystallization plant for Salt Recovery.

Chem specializes in vacuum technology and hence has mastered the technique of Crystallization under Vacuum. Crystallization under vacuum recovers pure sodium sulphate salts with a very good crystal quality. The concentrate from the evaporator having a density of 1.2 gm/cc is transferred to the Crystallizer for Salt Recovery.

In the Crystallizer, liquid is subjected to flash vaporization under controlled vacuum and temperature. The crystallized sodium sulphate in the form of NA2SO4.10H20 is drained for gravity filtration to separate the salt. The filtered liquid to the tune of 5000 litres/day with all other residual impurities is sent to the Solar Evaporation Pond. The system uses the existing solar pond with impervious layer liners to hold the residual concentrated solution from the salt recycling system. The final evaporation is with the help of solar energy. During the solar evaporation the individual salts reach the concentration limits and crystallize gradually. The crystallized salts are manually removed from the pond and shall be taken for secured landfill or alternate usage.

During the rainy season, when the solar pond is not effective, the zero discharge is achieved by sending residual effluent to the single effect forced circulation evaporator to recover the mixed salts.

The schematic block diagram (attached) of the entire system describes the complex processes. It involves combination of various technologies like wastewater treatment, reverse osmosis, thermal evaporation and crystallization. In order to facilitate the project operation and maintenance the system is provided with ample instrumentation, controls, interlocks and alarms at various stages. The operation and maintenance procedures are chalked out in detail to ensure minimum upsets.

Conclusion

The wastewater recycling system at Chemtech Processors delivers consistent operation thus making the availability factor as more than 99 percent. The system is a zero liquid discharge system in the true sense. It is possibly a trendsetter for a number of more such projects that are likely to come up.

Key Advantages

  • Re-dyeing of fabric down from 25-30% to almost nil
  • Pure water recovery for re-use
  • No moving parts
  • Increase in production
  • Pure glauber salt, NA2SO4 10H20, recovery for re-use
  • Reduction in use of fabric softeners
  • No choking or scale formation
  • High steam economy, Low power consumption
  • Least maintenance, Easy to uprate
  • Even distribution of mixing
  • Balanced capital cost, steam cost & power cost
  • Longer plant life without corrosion
  • Provided with control panel for: safety, alarm, steam shut down during no or low liquor flow, regulated steam & liquor flow control, low & high vacuum alarm, float switches, pump shut-offs in case of dry running, process parameters indication, etc.
  • Can be operated even with low pressure steam of 2-3 kg/cm2

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