The Erode Court Ruling of 2005 now applies to all of Tamil Nadu. Tanneries not discharging to a common effluent treatment plant (CETP) must meet 2,100 ppm TDS or install Reverse Osmosis (RO) at high capital, operating and maintenance costs. By the end of 2007, CETPs must meet 2,100 ppm TDS or have zero effluent discharge which will again mean installation of RO. This means that all TN tanneries or their CETPs require RO. RO concentrates will be solar evaporated.
Tanneries in Erode have installed RO but most of those in wet-blue plants are not yet fully operational. A number of larger TN tanneries processing hides and skins from wet-blue to finished leather have found that the use of low salt permeate from RO for processing provides considerable chemical savings in dyes and other expensive chemicals, due to the low TDS of the water. These savings offset costs.
In wet-blue plants there are no chemical savings and it is likely that there will be problems with RO membrane fouling in many wet-blue tanneries. This is less likely to occur for goat skins when the hair is saved.
An audit has found that most Erode tanneries have adequate solar evaporation systems for waste liquors, including RO reject, and improvement of the systems will be another major expense. No answer has been found for disposal or use of large amounts of the recovered evaporated salts. Other than for application to coconut palms in Kerala, no disposal options are available for these mixed salts; they willl be piled at tanneries and elsewhere.
It is likely that during the monsoon, salt will be dispersed into the environment. In November 2006, three Erode tanneries were closed down on account of pollution problems. At present the main focus seems to be on zero discharge rather than TDS and salt use reduction measures, though both are intertwined. Eventually the government and industry must realise this.
The aim of a joint CLRI-CSIRO-ACIAR project has been to develop and apply industrially viable systems to eliminate or significantly reduce salt use in hide and skin preservation and processing.
The major components of the project are:
* Reduced salt in curing: ongoing industry trials
* Chilling of hides and skins: ready for implementation
* Pickle liquor recycling: implemented
* Direct chrome liquor recycling (DCLR): ready for implementation
Hide and skin preservation: Low salt preservation of goat skins
To reduce TDS, first handlers of Indian goat skins can salt unopened skins as usual with only 20% salt on skin weight. This will reduce salt use three to four-fold and there will be few salt crystals on the skins and little solid waste.
An additive must be used with the salt to achieve good preservation. It is best that the additive is pre-mixed with the salt. It will be most important that wet skins are drained well before salting.
It appears to be advisable not to use boron or zinc compounds or sodium fluoride for the preservation in India. It is unlikely that the Indian Environmental Charter which excludes the use of boron compounds will be changed. The concentration of zinc and boron in Indian tanning centres could lead to a build-up of damaging levels in soils.
A satisfactory additive used with the salt in TN industry trials up until July 2005 was 2% magnesium oxide but there was a problem with the flesh sides of the skin sticking together. In summer this was a major problem because the sticking made opening up of the skins very difficult.
After further CLRI laboratory trials, the tannery achieved a good result with skins preserved with 20% salt and 2% sodium carbonate and held for over three weeks in hot summer conditions. The treated skins were more moist than the control skins but they were well preserved and could easily be turned flesh out and opened.
A trial with 20% salt, 1% sodium carbonate plus 1% magnesium oxide is being undertaken. Skins preserved with low salt will have no excess salt and will not require desalting by machine and disposal of salt. Also, soak liquors will contain less salt.
Chilling
The main advantage of chilling is that salt is not used in the preservation of hides and skins. For processing 1 tonne of hide, it is possible that 500kg less salt is used and discharged and disposed of as solid waste.
Elimination of salt for hide and skin preservation would have the greatest impact on reducing TDS. Chilling in chillers or with ice has been widely used in Australia for many years. Chilled hides are usually only kept for a few days but some are kept for over a week at low temperatures. If Indian hides and skins are available in large numbers in a region within a relatively short drive from a tanning centre, chilling would be feasible.
The CLRI mobile chilling unit has now been used for three industry trials. All the trials in Kerala and Erode have been very successful with wet-blue quality being assessed as no different to usual production. KKSK, the largest hide tannery in Erode, have done two trials and are very impressed with chilling.
However, after major recent capital expenditure, including installing RO and extended solar evaporation, KKSK are meeting TDS requirements and have achieved zero effluent discharge. Kerala coconut farmers take the recovered salts. In spite of this achievement, KKSK hope to also introduce chilling.
In India the system will need:
* Blast chillers with generators and additional chilled storage areas at collection centres
* Insulated, air conditioner trucks for transport
* Chilled storage areas for holding hides at tanneries before processing
* Quick delivery of hides to collection centres
Tanneries usually hire trucks for transporting hides. The cost of refrigerated transport will be about 60% higher than current transport costs.
Pickle liquor recycling
* Production of vegetable tanned leathers with reduced salt use and discharge
* Spent pickle liquors are reconstituted with acid and salt
It is possible to continuously recycle pckle liquors for many cycles without loss in skin quality. Tanneries should initially trial ten continuous cycles and this should be gradually increased.
To prevent bacterial growth and odour, it is important to acidify the spent pickle liquor soon after a pickle cycle is finished. This also allows time for the liquor to cool to room temperature. The liquor is adjusted to pH3.5 or below. It is recommended that the usual amount of 98% sulphuric acid is all added direct to the pickle liquor.
Preparation of pickle liquor
* Collect pickle liquors through screen
* Add acid to pH3.5 to prevent odours. The usual amount of 98% sulphuric acid can be added
* Mix well
* Settl
* Fine calcium sulfate precipitates. It does not cause a problem but can be settled and removed periodically
* Add salt to at least 7°Be
One Dindigul goat skin tannery which used to discharge pickle liquor after each use has successfully adopted the recycling system for regular processing.
Direct chrome liquor recycling
DCLR has been used for many years in Australia and reduces the use of both sodium chloride and chrome powder, which contains up to 30% sodium sulfate. Spent chrome liquors are used for the subsequent pickle after acidification to pH<1. Excess chrome liquors are precipitated and the chrome re-used.
The greatest savings in salt use are made if the spent chrome liquor from the drum is collected undiluted for recycling and diluted liquors are precipitated. Good practice allows indefinite re-use of the chrome liquor. Total chrome precipitation from all spent chrome liquors results in far higher TDS levels.
Preparation of pickle liquor
* The recovered chrome liquor is screened and grease is skimmed if necessary
* The liquor must be acidified before it is re-used for the next pack of delimed hides. The usual amount of 98% sulphuric acid is all added direct to the pickle tank. The pH is <1 and this prevents chrome staining. At this low pH, the chromium species present are of low molecular weight and rapidly penetrate the hide
* When the liquor is acidified, calcium sulfate precipitates and emulsified grease is released. Both can be removed periodically but do not cause a problem in the process
* The acidification should be done early to allow time for the liquor to cool. Cooling systems can be used.
Typical recycle process for wet-blue
Delime wash
Drain well, no float
* 2% salt (it is vital that the SG is sufficient to control swelling to the same degree as in the normal tannery process)
* 1% sodium formate
* Drum 10 minutes
* Add 45% acidified recycle pickle while drumming continuously, check pH, SG/Barko, temperature. Drum as for usual pickle, check pH, SG/Barko, adjust if necessary
* Add 5.5% chrome powder
* Usual drumming, fungicide addition, basification and checks
* Unload with little washing
Successful commercial trials have been undertaken in four Indian tanneries. It has been found that it is critical that the drum is rotating during the addition of the acidified pickle to the drum. Otherwise drawn grain caused by acid burn can occur. Also, the pickle tank should be covered to avoid fumes.
No tanneries have yet implemented the process but now that chrome prices have increased and TDS discharge is more critical, the benefis have increased and tanneries are planning to scale up trials.
Conclusion
The advantage of the four project developments is that salt use is reduced and the problem of recovered salt is greatly reduced. Cleaner processes are better than end-of-pipe treatments. The developments could make zero discharge from some tanneries feasible.
To achieve significant salt use reductions in India, there will need to be considerable uptake of low-salt preservation of skins and chilling of hides. The proposed low-salt preservation will entail little change for first handlers of skins but greater care will be required to ensure even salt application. There will be resistance to change. One possibility that may bring about change: the tanner could pay more for skins with less salt and good preservation and less for skins with excess salt.
The costs associated with chilling will be considerable. To achieve industry implementation, government may need to provide funding and incentives to tanneries, as have been available for RO installations.
Recently the main focus for TN tanneries has been zero dicharge, not salt use reduction measures. Both approaches will be necessary to achieve satisfactory environmental outcomes: low TDS in effluent and less waste salt.
