Reduced environmental loading key to survival

For organisations read tanneries and, specifically, the need to meet the plethora of environmental regulations that come from the regulatory authorities. Many tanners have failed to comply with the new rules and have closed; some have tried but the regulations have just been too tough; others have 'cheated' the system and moved production to areas of the world where the regulations are either less stringent or not policed as well. But, some tanneries have changed their production so that they can comply with the regulations. The first question to ask is where does most of the pollution in a tanning process come from? Table 1 gives figures for the environmental impact of leather production from beamhouse to tanning and it can clearly be seen that the former is the major contributor to pollution, accounting for about 75%. So, the aim of many researchers is how a tannery could operate a beamhouse operation with as little waste as possible; the dream being a zero waste tannery for the beamhouse operation with the sum of the processes utilised being commercially neutral. The challenge is to create savings that could be used to reinvest in new technologies - an iterative process leading to zero pollution. There are technologies around that could ensure 'zero' or reduced pollution loads. First, and one of the more obvious, would be to treat the solid wastes. As Leather International3 highlighted last year, tanneries such as Tanneries Nouvelles Pechdo, the French glove manufacturers, are looking at alternative revenue sources and saving costs. Nouvelles Pechdo produce approximately eight tons of solid waste a day from their fleshing operation. This represents 90% of the total solid waste produced in the tannery. The waste was sent to landfill as a category two material, which was more expensive to dispose of than typical household landfill. However, they installed a fat fuel burner unit. The waste fleshings are separated in fat and water by heating to 70ºC and the solid fat waste is combusted, while the water is directed back to the tannery effluent treatment plant. The burner works as a combustion system and not an incineration method and the latent heat produced is used to heat the water. Secondly, there is the use of fresh hides rather than salted, so reducing the total dissolved solids in wastewater, as Table 1 also shows. This is particularly important in countries where water is at a premium. However, the conundrum is that countries that have a poor water supply generally tend to have a poor infrastructure generally, have higher ambient temperatures and poor collection facilities; all of which are non-conducive to fresh hide/skin production. Preservation The tanner has three options for preservation depending where he is in the world: * alternatives to salt * salt-saving processes * short-term preservation In hot climate countries such as India or most of Africa, drying either in the open air or in sheds is a real possibility. Indeed, many tanneries in India use the outside conditions to dry their wet-blue stock. Interestingly, the majority of the raw material seems to be wet-salted, presumably because the time needed to rehydrate air dried or sun dried stock can be inordinately long. Also, there can be problems of case hardening if the skin is dried too quickly. One of the obvious ways to reduce TDS is to remove the salt before it enters the float. This can be achieved by either desalting the hides and skins by hand or by using a mechanical desalting machine.4,5 This can be as basic as a rotating sieve in which the hides are placed. Rotating the drum expresses the salt through the holes. The salt could be reused, probably after washing and recrystallising. If the hide is not desalted, then the alternative is to collect the soak liquors and evaporate the water and collect the salt for reuse. The reduction in TDS can also be achieved by: * ammonium-free deliming, eg carbon dioxide, and bating * modifications of tanyard and wet finishing operations resulting in significant reductions of sulfate and chloride discharges * the concentration and accelerated solar evaporation of (recycled) soak or other concentrated saline liquors * the application of membrane technology for TDS removal either in separate streams or, in some cases, from the treated effluent * the transport and discharge of saline waters into the sea and the reuse of treated effluent for irrigation of selected land plots planted with salinity resistant species 'That salt, in spite of its inherent impact on the environment, is the most widely used preservative today shows how difficult it has been to find a suitable alternative', David Bailey commented in his 2003 JA Wilson lecture to the American Leather Chemists' Association. He said the only major change in practice in the last few thousand years was using brine curing instead of salt packs and this occurred less than half a century ago. Salt curing presents environmental challenges for the tanner and the packer that become more difficult to overcome as time goes on. Despite all of these shortcomings, no commercially acceptable alternatives have been put into use on a large scale. And Bailey has spent a large part of his career looking for alternatives. In many ways he was successful, but industry felt, and probably still feels, that salt is the easiest, cheapest, and best known preservative for hides. There are, of course, many alternatives and in the US these could include: * using hides fresh, but unless the tannery is right on the doorstep, this is impractical. However, it may be possible for someone like Tyson to process through to the blue, but there are concerns about whether available markets make the monetary outlay realistic * chilling hides, which is expensive, and needs infrastructure, but some of the costs could be offset by other uses of the hides, such as gelatine and collagen cases * the possible use of chemical preservatives such as biocides, sulfite/acetic acid, and bacitracins, which are anti-bacterial peptides. In some small scale trials, these proved very effective against bacteria at room temperature. However, the work has been halted due to possible patent applications, Bailey noted with regret. The chemically-based one he seemed to like best was the mixture of sulfite and acetic acid, which he trialed in the late 60s. In a 30 day trial, hides were kept in a barrel at elevated temperatures and, when tanned, produced perfectly good leather. Bailey suggested the mechanism probably involves the release of sulfur dioxide, which acts as the preservative * on an even simpler level, replacing sodium chloride with the potassium salt is something that could be put in place tomorrow. 'It's (KCl) a perfect copy with none of the environmental concerns.' Bailey cannot understand why this technology hasn't been developed further. He has done large scale trials and shown that KCl has no adverse effect on leather quality. * another favourite, and one which may eventually become an industry standard, is irradiation. Having tried both gamma and electron beam irradiation, Bailey is convinced of the latter. Given that the FDA have approved e-beam irradiation of meat, it is just one step back to the hides. He pointed out that e-beam irradiation is not sterilisation of the hide, but that a small initial offer of bactericide ensures the hide is preserved long enough to transport and process6 * short-term preservation is another option. There were trials in South America to determine whether it was possible to put freshly killed hides that had been washed of blood and flesh into a lorry that had a revolving trailer, much like a cement mixer, containing dilute bactericides.7 The hides were impregnated with the solution while the lorry was going to its destination. Then, there is the application of enzymes in the beamhouse. Again, a huge amount of work has been done to reduce or remove the use of lime and sulfide in the unhairing process. However, the use of enzymes in the industry has been an enigma since the days of Wood and Turney at the beginning of the last century. They seem to offer cleaner production routes but then there are always problems of maintaining control of the reaction, temperature, pH, cost etc, or when they are used. During bating, for example, there is debate as to what they do or, indeed, whether they are needed at all. Leather area is primarily controlled by the structural proteins in the skin and their distribution and alignment within the grain, the corium and the flesh. It is the grain layer that has the most area to gain because it is composed of finer fibres, which are arranged in a convoluted manner. So, the use of alkali stable enzymes, while seemingly benefiting the tanner from the point of view of a cleaner production, must be carefully controlled if the leather is not to become too loose. Hair removal There are essentially two mechanisms of hair removal: breaking down the structure to a sufficient degree that it appears to dissolve (hair burning), or to detach the hair at its anchoring point in the follicle, so that at least part of the structure is apparently removed intact (hair saving). Again there have been many attempts to harness the latter, ranging from the commercially acceptable procedures such as 'Blair Hair' to the old methods of sweating and allowing bacteria to remove the wool from sheep, to more esoteric ideas involving chlorine dioxide and oxidative unhairing.8 It has been shown9, through microscopy studies, that the morphology of hair structure, during typical hair burning with lime and sulfide, with and without the presence of proteolytic enzymes, demonstrated different rates of decomposition of the various components of hair: cuticle, cortex, medulla and root zone. The decomposition mechanism involves the dissolution of the cortex, the main constituent of the internal structure of the hair shaft. Whether or not the hair is cut prior to applying the hair burning agents, the cortex is dissolved, together with the medulla if there is a proteolytic enzyme present. Within the conditions and time period of typical hair burning, the outer part of the hair structure, the cuticle, is not dissolved. It may either remain intact, but collapse inwards, due to the hollowing mechanism, or it may break down into its scaly components: this is the scud of residual hair. The alternative approach with enzymes is to use them to detach the hair at its base, the basis of the hair saving techniques discussed above. However, use of proteolytic enzymes in such a procedure can lead to over opening up. A better idea is to attack the anchoring mechanism, the collagenous proteins of the basement membrane. This is something that was first suggested in the late 80s10 and the idea was updated to use dispase in enzymatic unhairing. Workers at BLC Leathersellers Research Company (BLCLRC) showed how the use of enzymatic removal of hair during beamhouse processing offered a suitable alternative, at least on the non-industrial scale, to destructive sulfide unhairing. One approach to enzymatic hair removal involved the degradation of the basement membrane to bring about epidermal sloughing with concomitant hair loosening. To be successful in this, the enzyme used must be able to degrade the basement membrane, type IV collagen in particular, while leaving the fibrous collagen of the dermis unaffected. The neutral protease dispase has such specificity and was consequently used in a series of studies to assess its effectiveness in hair removal from bovine hides. The enzyme caused loosening of the hair and associated hair loss, without damaging the fibrous collagen of the dermis. The elastin network of the grain layer was significantly affected by enzyme treatment, as were the physical properties of the resultant leather when compared with that of conventionally processed hides. Treating effluent Whatever beamhouse processes are used, the outcome will inevitably be effluent that needs to be disposed of. How this is done is a problem that is still being worked upon, but incineration and gasification are being considered, as was shown at the beginning of this article with the French tannery, Pechdo. Gasification is the conversion of a carbon containing solid fuel with a limited amount of oxidising at an increased temperature to produce a gaseous product consisting of hydrogen, carbon monoxide, methane, water, nitrogen and maybe ethane and propane, which can be used as a fuel to generate electricity and heat. Leather shavings or wastes such as sludge, trimmings, buffing dusts, and other leather waste materials are dried to 90% dry solids in a flash dryer and briquetted into a dense 'fuel'. The dryer heat requirement is fuelled by the syngas from the gasifier, thereby closing the loop and providing a self sustaining system. The dry fuel is pyrolysed or thermally degraded at temperatures above 250°C. So, the environmental impact of the beamhouse can be reduced by the potential use of fresh hides rather than salted, the application of enzymes to unhair, the replacement of ammoniacal salts in deliming by carbon dioxide, elimination of pickle salt etc, etc. The problem is that although individually each change has its merits, putting the whole lot together often leaves the tanner out of pocket, either through poorer quality leathers or extra costs through necessary upgrading of hardware. BLCLRC have an ongoing project to put such technologies together and to build a process capable of commercial exploitation by tanners. Once this process has been established and demonstrates a commercially acceptable quality, they will run a sequence of processes where the chemicals and water from the process are recycled.12