Tannery and Chrome Chemicals Manufacturing Sector

Tanning is the predominant industrial activity carried out in Tamil Nadu over several decades in clusters. However, it is only since 2002 that the Tamil Nadu Pollution Control Board has been exhorting all tanneries to install membrane technologies with suitable reject management system for the recovery and reuse of permeate water and salt in the process and hence to facilitate zero discharge of effluents, considered a cleaner disposal system.

Implementation of the concept of water conservation in Tamil Nadu has become the need of the hour. It is well known that there is no perennial river flowing in Tamil Nadu. Hence, groundwater is the major source in Tamil Nadu for drinking, irrigation, industrial and domestic purposes. Under these circumstances, discharge of effluent with high TDS into the available water sources either directly or indirectly will increase the TDS level or affect the usage it is intended. Hence, the above situation warrants zero liquid discharge (ZLD) into the water sources, land etc., either directly or indirectly and this could be achieved by installing RO plants and RMS by the tannery units in the state which will facilitate water conservation by recovery and reuse of water / salt in the process.

The details of area wise CETPs in the tannery sector are listed below:

SUCCESS STORY : ZERO DISCHARGE - CETP AT VANIAMBADI

The CETP at Vaniambadi that caters to about 130 tanneries which process raw hide to finished leather. The capacity of the CETP is 4 MLD. The length of the effluent transportation network is about 8 kms. Zero Discharge System of effluent disposal is presently run on a trial basis in the CETP. The individual member tanners are carrying out pre treatment comprising settling tank for non chrome bearing effluent. The chrome bearing effluent is separately treated in the chrome recovery unit in individual tanneries.

To achieve zero discharge, the CETP has installed RO system with reject management system (RMS).

The RO system is needed because in CETP only organic pollutant and chromium are removed but not TDS and hence treated tannery waste water from CETP is not suitable for reuse in the tannery for wet finishing operation due to high TDS. RO system addresses the above issues.

To achieve the above feed characteristics, Membrane Bio Reactor (MBR) has been installed in the CETP. An overview of MBR is given below:

• A suspended growth biological reactor integrated with membranes.
• Membranes replace conventional secondary clarifier.
• UF membranes ensure that no particulate matter greater than 0.1 micron is discharged in the effluent . The membranes are immersed directly in the aeration tank or another tank
• A centrifugal or reversible pump is used to create a vacuum in the header connecting the membranes for withdrawal of treated water.
• Air flows introduced at the bottom of the membrane module via coarse aeration to create a turbulence that scours the external surface of the membrane and keeps them clean. This provides process aeration as well.
• Supplemental oxygen for the biological treatment is provided by a separate diffused aeration system.

The MBR has following advantages:

• Treated effluent from MBR is free of suspended solids and micro organisms
• Especially suitable for treatment of complex waste waters even with high and varying organic load
• Low excess sludge production
• Complete control of sludge age is important to allow the development of slow growing micro-organisms such as nitrifying bacteria
• MBR requires less space and the module based design makes it easy to expand the capacity when needed
• The hydraulic retention time (HRT) is less than 5 hours
• Long sludge retention time (SRT)
• Biomass concentration in MBR can be greater than in conventional systems, which reduces reactor volume
• The membrane can retain soluble material with a high molecular weight, improving its biodegradation in the bioreactor
• Because of the membrane separation, the secondary settling tank and tertiary treatment are not necessary

SUCCESS STORY : MINIMIZATION OF SOLID WASTE GENERATION, DISPOSAL OF SLUDGE IN LAND FILL

Minimization of solid wastes in tanning process: Desalting before soaking

To minimize the amount of salt going into the waste stream, desalting is the most practical solution. In fact, it is not a new proposition since many tanners already practice the manual desalting prior to soaking. However, improvements in the methods can be brought about to increase the efficiency of the process. It has been shown that by resorting to mechanical desalting, it is possible to recover as high a s 39%salt on the weight of the salt used in curing. However, specially made slotted drums with an rpm of about 4-6 are employed for mechanical desalting. It has also been shown that an improvised manual desalting can achieve same efficiency.

Wheel type desalting machines have been designed and used for efficient desalting of skins and calf skins in some tanning centers in Tamil Nadu.

Secured Land Fill (SLF) for disposal of sludge

The sludge generated from CETP are disposed off in secured landfills. The team visited one such landfill near Vaniambadi. The following are advantages of decentralized SLF for the leather sector

• Enhanced waste holding capacity in the available land area.
• Leachate generated from the landfill can be treated in the existing CETP.
• Secure landfill is adjacent / within the premise of treatment plant as integral part of CETPs which will facilitate reuse / re-processing of sludge from leather if cost effective and viable technologies are developed in the future.