Chrome recovery and reuse
The conventional method of chrome tanning, employed in Leather industry, results in very poor exhaustion of chrome, thereby adding to the toxicity of the effluent. One of the cleaner processing options recommended is that the chrome which is let out in the effluent, may be recovered for reuse to ensure not only economy of usage of chrome, but also effective management of this toxic substance.
The methodology involves precipitation of chrome with an alkali, whereby the supernatant liquor is separated, allowing the resulting precipitate to settle down. This precipitate is treated with sulfuric acid again to get the BCS, which can be reused. Commercial polyelectrolytes are used for further facilitation of chrome precipitation.
The technology has been implemented in large number of commercial tanneries. It has been shown that it is possible to recover about 98-99% of chrome. The cost of installation of the plant can be recovered in 12-24 months depending on the scale of operation.
Chrome recovery plants can be established to meet different scales of production starting from individual small-scale tannery to a group of tanneries. Paddle based process for small tanneries, batch type process for medium and large tanneries and semi-continuous plants for common chrome recovery plants have been standardized.
UASB Technology complete with sulfur recovery plant
The Upflow Anaerobic Sludge Blanket (UASB) technology has been well known for treating wastewater primarily because of its low sludge production and less requirement of energy as well as area. The major advantage of this technology is that it provides a potential possibility to generate energy rather than consuming the same while treating the wastewater.
Despite the aforesaid advantages, there is a major constraint of using this technology for treating tannery waste water because of the high sulfate content, which results in generation of methane gas, contaminated with hydrogen sulfide, thereby restricting its use as a source of energy.
An improved system has therefore been developed whereby the sulfate is completely removed as elemental sulfur and the treatment of the tannery waste water is carried out in conventional UASB reactor, thereby generating energy. This system ensures removal of COD and sulfate TDS by 60% and 90% respectively, whereby the sulfur can be recovered. A number of plants have been established based on this CLRI Technology.
Biomethanation for solid waste disposal
Solid wastes generated by the leather processing industry are posing a major challenge. Appropriate technology has been developed for the profitable disposal of these solid wastes.
Tannery fleshings, which are the major solid wastes emanating from the beam house of a tannery, are subjected to biomethanation. It is a process whereby the fleshing is liquefied completely biologically and the resulting liquefied fleshing is treated in anaerobic reactors to produce biogas. Any anaerobic reactor like UASB reactor can be used for this purpose. Depending on the microbes present in the system, the generated gas may contain methane as well. The technology has been implemented in Melvisharam, Tamil Nadu for biomethanation of tannery fleshings.
The same biomethanation technology can be used for the disposal of other solid wastes as well. The technology has been implemented by CLRI in M/S Alkabeer, Hyderabad and M/S Hind Agro, Aligarh , UP for the disposal of slaughter house wastes. One more such plant is being implemented at Chennai for economic disposal of the vegetable wastes emanated from the Koyambedu vegetable market.
Chemo Autotrophic Activated Carbon (CAACO) System for Waste water treatment
It is an integrated system of biological and chemical oxidation carried out in a single hybrid reactor packed with activated carbon based redox resin. The process involves anaerobic treatment of the waste water followed by wet air oxidation using activated carbon which contains Chemo Autotrophic bacteria in immobilized state. Oxygen required for the oxidation of organics is facilitated by supplying air from the bottom of the reactor. Since only the stoichiometric quantity of oxygen is supplied for oxidation, utilization of electrical energy is minimized, leading to a saving of electrical power consumption to an extent of 75%, as compared to the conventional aerobic system. The treatment eliminates about 92% of suspended solids, 98%BOD, 85% COD, 100%sulfide, 100% odour with high degree of performance consistency. Another major advantage of the system is that the pH of the processed liquor lies very close to the neutral range, thereby facilitating easy disposal of the processed water.
The technology has successfully been implemented in several tanneries for treatment of the tannery effluents. It has also been implemented in multiple textile units ( United Bleachers, Mettupalayam; India Dyeing Pvt. Ltd, Erode; Shuttleweaves, Karur; Celebrity, Madras ) Sago units as well as chemical units. The technology has also been found to be suitable for treating domestic sewage and has already been commercially operational.
Removal of TDS (Total Dissolved Solids) in tannery effluents
The issue relating to the Total Dissolved Solids (TDS) in tannery effluents has been of major environmental concern. TDS is generated by the chemicals, which are used in leather processing, but are let out without being fully absorbed in the leather. The effluents having very high TDS affect the ground water adversely and considering this, the Tamil Nadu Pollution Control Board (TNPCB) has set a limit of 2100ppm in treated effluents. But in practice, the treated effluent is found to contain much more TDS than the prescribed limit. Viable solution for reduction of TDS in tannery effluent is therefore of prime importance.
This technology relates to a series of in plant measures involving a combination of different eco-benign technological options like less salt/ salt less curing, enzyme based pre-tanning operations, recycling of pickle liquor, pickle less tanning. Potential for reduction of TDS through these measures has been quantified.