Friday, July 6, 2012

Leather technology & the environment



Chennai Environmental Company of Tanneries (CEMCOT), Chennai


India has emerged as a major tanning centre of the world, processing about 3,000-tonnes of raw materials per day. The main centres of tanning include Jullunder in the North; Kanpur, Unnao and Kolkata in the East; and Chennai, Ranipet, Ambur, Vaniyambadi, Pernambut, Erode, Dindigul and Trichy in the South. About 45% of country's total tanning capacity is in the South; 18% in Kolkata; 25% in Kanpur; about 7% in Jullunder and the rest scattered in rural areas.

Being a water-intensive process, tanning generates considerable volume of wastewater. On the average, about 35-m3 of wastewater is generated while processing one tonne of raw material. It is roughly assessed that about 100,000-m3 of wastewater is generated per day by the tanneries in the country.

In the process of leather making, a number of chemicals are also used in the tanning and post-tanning processes. It has been observed that the amount of chemicals absorbed by the leather is not more than 20%; the rest 80% being washed away with the process water. The effluent of tanneries thus carries a huge volume of a cocktail of chemicals. Besides, the solid waste generated while processing hides and skins works out to about 65% of the weight of the raw material. This includes hair, fleshings and trimmings of raw-, semi-processed or finished leather, shavings and leather dust, besides the sludge generated by wastewater treatment plants.

The solid and liquid waste generated by the tanning process thus poses a major challenge of waste treatment and management.

Pollutant discharge standards

The pollutant discharge standards have been specified by respective Pollution Control Boards in different states of the country. Whereas Minimum National Standards (MINAS) have been prescribed by the Central Pollution Control Board (CPCB), the state boards have been authorized to introduce more stringent norms according to the local situation. The standards generally applicable refer to pH, BOD, COD, TSS, TDS, Cr and these vary according to the recipient – such as surface discharge, for irrigation, marine discharge or sewer discharge. Generally there is no limit for TDS prescribed if marine discharge is authorised. With regard to sewer discharge too, depending on the dilution potential available, TDS limits may be modulated.

Nature of liquid waste and treatment process

It is desirable to know a little about the nature of pollutants in the liquid waste.

The liquid waste carries both suspended and dissolved solids. The suspended solids consist of dirt and particles of raw hides shaken off the raw material, some traces of dissolved hair, fleshings, leather pieces, leather dust etc. Generally, these suspended solids are either removed by the screening process, by use of fine screens or settled by chemical process and withdrawn as sludge. These are also removed by biological treatment in the aeration tanks or by anaerobic process. In this manner, pH, BOD, COD, TSS and Cr standards are achievable by physio-chemical and biological treatment in the waste treatment plants. The suspended solids are removed as sludge from the effluent treatment plants. It has been estimated that 3-4 kg of dry solid sludge is generated by treating 1-m3 of wastewater. The process adopted is generally referred to as conventional waste treatment system.

The dissolved solids however pose a major challenge. This consists of ions of sodium chloride, sodium sulphate and carbonates. Traces of dyes are also found. Conventional waste treatment systems do not provide for treatment of dissolved solids. In developed countries the treated effluent high in TDS is diluted in sewage treatment plants before discharge. In some locations, marine discharge is practised. Where such options are absent, such as in Tamil Nadu, for dealing with such pollutants, expensive reverse osmosis (RO) technology only can be employed. The reject of RO system has to be evaporated either by natural process using solar energy or through suitable mechanical system of evaporation. The mechanical process is energy intensive and very expensive.

Solid waste management

With regard to solid waste management, there are a variety of end-uses for the same. Typically, the solid wastes generated in tanneries are converted into by-products as shown in Table 1.

It has been observed that the factories in India converting the solid waste into various products are in the micro- and small-scale sector, employing basic technologies. With more efforts and focus, it is possible to increase value realization from such waste by employing superior technologies and producing better quality and variety byproducts. Italy and Spain have modern factories processing different solid waste of tanneries to high value-added products.

Currently, the sludge generated within tanneries as well as in the ETPs and CETPs is deemed hazardous in India and many other countries because these contain chromium, though in its trivalent form. It is therefore required to dump such sludge in secure landfill. In advanced countries like the USA, such sludge is not treated as hazardous because trivalent chromium is not deemed harmful. Be that as it may, many experiments have been done in India to demonstrate that such chromium in the sludge could be immobilized. In India, bricks, both burnt and unburnt, have been made using such sludge with clay and small quantity of cement. Also manure has been made using sludge and vegetable waste. These products displayed immobilization of chromium. At one point of time, CPCB has allowed use or disposal of sludge containing chromium upto 5000-ppm provided it was trivalent, but this notification was withdrawn sometime later. As of date, such sludge is deemed hazardous in India and hence it has to be disposed in safe and secure landfills. Apart from the cost aspect for creating new secure landfills, land is not easily available in the neighborhood of tannery districts.

Liquid waste management

First, let us briefly deal with the end-of-pipe treatment before looking at process technology options.

The discharge standards vary from location to location, with Kolkata opting for marine discharge standards and Kanpur (Jajmau) for sewer standards. But in Tamil Nadu surface discharge standards have been prescribed, which calls for reaching TDS level of 2100-ppm in treated effluent with chloride and sulphate being less than 600-ppm.

The TNPCB has directed that all tanneries in the state should go in for zero liquid discharge (ZLD) system of treatment with the twin objectives of recovery of process water and prevention of contamination of ground water and soil. The calls for employment of RO/mechanical evaporation systems. In terms of capital investment, it works out to more than Rs. 1.5 lakhs additional investment per cubic meter of wastewater treated. With regard to O&M cost, it is about Rs. 120 per cubic meter, about four times the operational cost of conventional treatment systems. It is noteworthy that 65% of the cost of operation of a ZLD system is accounted for by energy and fuel for boiler.

Of the 15 CETPs in the state, 13 have opted for ZLD system. The rest have the option of sewer discharge. Two ZLD systems are operational at Perundurai and Melvisharam; three covering four CETPs are under stabilization at Thuthipet, Maligaithope and Vaniyambadi; five, covering six CETPs at Pallavaram, Ranipet (3) and Pernabut are to be commissioned before end of March 2012. While the one at Dindigul has the option of sewer discharge, yet it is opting to go for ZLD too. Besides, it is reported that about 50 individual tanneries have their own RO systems.

Though it has been established that technologically it would be feasible to establish ZLD systems, its sustainability is a major question. Some advantages of the ZLD system include recovery of almost the entire wastewater for reuse, less consumption of chemicals due to improved process water and prevention of contamination of soil and ground water by high TDS effluent. In due course of time, the land and ground water contaminated earlier will be able to recoup. But, ultimately, the tanneries have to survive in a fiercely competitive global market!

The issue of sustainability of ZLD system has to be viewed from the points of view of:

Improving process technology thus reducing pollution, especially of TDS;

Stabilising process parameters for O&M of the ZLD system; and

Seeking support of government in critical areas for this unique environmental initiative.

While improvements in process technology may help the situation to an extent, unless strong government support is forthcoming, it is quite likely that the industry will face very serious difficulties, which may result in closure or migration of a good number of tanneries from Tamil Nadu. It will be a pity if this laudable initiative is not enabled to succeed.

Improved process technology

It is against this background that the issue of improved tanning technology has to be viewed. With mounting cost of waste treatment, the question naturally arises as to whether the generation of such voluminous polluting wastewater could be controlled.

The Central Leather Research Institute (CLRI) has been constantly looking for ways of introducing new technologies at different stages, which could reduce both volume and pollution intensity of wastewater.

Water conservation

This is a key objective, as tanneries will be charged as per volume of wastewater discharged.

It involves recycling of various streams of wastewater, such as counter-current soaking, liming, reliming, pickling and chrome liquor. The technology is well demonstrated in actual working environment in tanneries, and besides reducing water consumption, helps improve absorption of chemicals. Current uptake of these technologies has been few and far between. One reason is that many tanners being job tanners do not want to adopt any new process that could impact on quality of leather as others provide the raw material.

Reduction of TDS

This is achieved by:

Enzymatic/other methods of curing of hides and skins, eliminating or reducing salt used for preservation (50% reduction in volume of salt used possible);

Mechanical or manual desalting of hides and skins: about 15% of salt could be removed in this process;

Enzymatic unhairing: this helps eliminate or reduce use of sulphide in the process and recover undissolved hair;

Pickle-less tanning, which reduces TDS by about 30% overall;

Carbon dioxide deliming; and

Chrome recovery and reuse.

These technologies have been adopted only by a few. Minimum duration of preservation is 40-45 days and hide dealers are not confident of any preservative other than salt.

Perhaps charging tanners according to volume and intensity of TDS may help them move towards these technologies.

REACH standards - Residual Substances Limit in the European Union

Limits have been prescribed for 44 chemicals, not all relevant for tanneries. More are on the anvil. CLRI constantly keeps tab, and advises industry in advance of alternatives.

New technologies with multiple objectives

Three step tanning

The approach involves, removal of hair, flesh and fibre opening using biocatalysts and sodium hydroxide at pH 8.5 for cow hides. This is followed by a pickle-free chrome tanning, which does not require a basification step. Hence, this tanning technique involves primarily three steps: dehairing, fibre opening and tanning leading to near zero waste tanning.

Integrated wet finishing process

A compact wet finishing process has been developed for making both upper and garment leathers. The process provides leathers having comparable or even better physical and bulk properties to that derived from conventional wet finishing process. The water consumption is reduced significantly by 73% for processing 1-tonne of wet blue shaved leathers which is one of the pioneering achievements. This success story led the researcher to design and develop process for integrating tanning and wet finishing of leather processing.

Colouring leathers naturally; gains importance

An attempt has been made to colour leathers using natural dyes such as Rhine, Rhine M, Indus, Pacific, Caspian, Henna and modified Logwood. Twenty-four shades were developed using combination of seven natural colorants by mordanting with three metal ions. Sixteen developed colours have potential value in the global leather market in the context of environmentally benign leather processing.

Reverse leather processing through fundamental changes

A new greener and cleaner processing could be developed which will revolutionise the leather tanning industry. Reverse leather tanning works backward from the point where conventional tanning ends. The methodology saves time, energy and chemicals, along with reduction in water usage and pollution load.

Eco-efficient leather processing for clean and green leather

The process involves salt-free curing, lime and sulphide-free beam-house process and post-tanning followed by tanning employing a reverse leather processing technique. The functional performance of the leather is found to be on par with that of conventionally processed leathers. The rationalized leather process reduces the usage and discharge of chemicals and also makes a significant reduction in pollution loads.

Zero emission research initiative for leather – a way forward

Water recycle and reuse method based on zero wastewater discharge from beam house has been developed and standardized at semi-technical scales. In the new methodology, water consumption is reduced from 17-litres to 1.7-litres for one kg of hide in raw to wet blue processing. This approach can, in principle, lead to water renovation and recycle in individual tanneries through applications of membrane and other advanced technologies.

While all these new technologies have been developed b the CLRI with a view to help industry cope with the new challenges faced by them in environment management, given the structure of the industry, with SMEs dominating and many working as job tanners, it is a major challenge as to how to make them take to these very useful technologies.

Stabilising process parameters for the O&M of ZLD systems

As indicated elsewhere, the ZLD system using UF/RO/mechanical evaporators for treatment of tannery wastewater has been introduced for the first time in the world in Tamil Nadu. Even suppliers of RO systems/evaporators are not quite aware of the ideal process parameters as they are dealing with this type of wastewater for the first time. Indian Leather Industry Foundation (ILIFO), Chennai, has some experience of monitoring operation of some ZLD systems in ETPs of tanneries, but such ETPs do not have mechanical evaporators and instead resort to accelerated solar evaporation of the RO reject. Though some data is available for operation of ZLD in ETPs, dealing with the CETPs where wastewater is discharged by a number of tanneries producing different types of products, poses a different set of problems.

RO is basically a filter with very minute apertures and through which wastewater is passed at great pressure to filter out the dissolved solids. Physical parameters such as pressure, back washing for periodical cleaning of the membranes etc. can be controlled. But with regard to the impact of specific pollutants that cause blockage, corrosion, scaling, etc. there are no benchmarks yet. Reasonable precautions have been taken to arrest all pollutants, including organics through DMF and organic scavenger prior to applying wastewater on the RO.

Suffice it to say that the O&M operators, contractors and suppliers are jointly working towards stabilizing the process parameters. The longer the life of the UF/RO membranes and the evaporator, the lower will be the O&M cost of the system over a period of time.

Areas of Government support

Support of the government is critical for survival of the industry at this juncture. The industry has no doubt demonstrated its sincerity towards complying with the TNPCB direction regarding ZLD system, despite heavy cost. But, now the government must extend a helping hand to enable the industry remain competitive.

Concessional power tariff

As pointed out earlier, power and fuel for boiler are main contributors to O&M cost of ZLD system – working out to about 65% of the cost of operation. The power tariff for ZLD systems is on usual industrial/commercial rates. It is learnt that for sewage treatment plants operated by municipalities, the power tariff is different. If the sewage treatment systems in the tannery districts were operational, there may have been no need for the ZLD system at all. It is therefore desirable that the concessional tariff extended to sewage treatment plants may be also extended to the ZLD systems operated by the industry. This will provide some relief.

Recovery & usage of salts

The as yet unresolved issue of what to do with the solid salt recovered from the ZLD system needs tackling. Some efforts are underway to segregate these different salts and either use or sell these. After a technology is found, we have to find an investor to invest in a plant to recover different salts.

TNPCB had initiated discussion with the industry and R&D institutions in this regard. It is appropriate that the TNPCB may engage its experts to find other alternative means of use or disposal of the same. The CETPs will have to keep the recovered salt stored in safe condition until a viable alternative emerges.

Crisis fund for CETPs

The members of all CETPs are generally from the SME sector, many of who lead a hand-to-mouth existence. If any upheaval takes place in the marketplace, they would be the first to suffer.In order to ensure that such temporary setbacks do not result in the ZLD systems not being able to collect the O&M cost from members, a way out has to be found. A designated fund may be created to be kept at the disposal of a state agency, to extend interest-free loan to such CETPs as may need it, for meeting such crisis situations. Generally CETPs should be able to overcome such difficulties in a season or two.

Temporary closure of CETPs

From a technical point of view, it has to be realized that the ZLD system, like any other system, is liable to face sudden technical problems necessitating temporary closure for repair etc. But it would de virtually difficult to halt production in tanneries, as they would have commitments to meet. It is therefore for consideration that over a one-year period CETPs be allowed discharge effluent, after secondary and tertiary treatment, or after RO, for a maximum of 20 days, at any rate, not more than 3 days consecutively on any one occasion. Such a provision is necessary to avoid tanneries resorting to subterfuges when faced with a crisis.

Assistance for technology upgradation

The ZLD systems may need upgradation at an interval at an interval of five years or so. When major capital expenditures are required to be made, such investments may be treated as upgradation and the CETPs made eligible to drawn assistance from the Government towards capital expenditure, to extent of 50%. Wherever alternative to ZLD may exist or emerge, such as dilution by sewage or marine disposal, tanneries should be encouraged to avail such alternatives.

Way forward

ZLD systems now demanded in Tamil Nadu may, in future, become the norm in other parts of the country. In fact, some other countries, including Italy, are closely watching the developments in India.

If this is an irreversible situation, it makes sense for tanners to look for ways and means of:

Conserving use of water in the process;

Achieve better absorption of chemicals in leather; and

Reduce the generation of TDS to the maximum extent feasible.

Evaporation of rejects is a very expensive component of treatment cost and therefore, it makes eminent sense to reduce TDS content in effluent to the maximum extent by suitable in-process control measures. Modern and new process technologies can only provide answers. Bio-processing is a promising alternative.

It is necessary for tanners to have an open mind to embrace these new opportunities. It is equally necessary for the government to keep an open and sympathetic mind and extend a helping hand to the industry to overcome teething troubles in the initial years. There may be initial hiccups, but if the objective is clear and the technology provider is confident, there is no reason why the industry cannot move ahead in this direction.

(Lead lecture at LERIG 2012 held at CLRI, Chennai, on 28 January 2012)