Salt - reduce, recycle, reuse or remove?

The following e-mail arrived in the office a while back: 'There is an incredible amount of excess salt produced in the world taking into account the number of sheepskins/lambskins produced on an annual basis which are drum salted/hand salted and, thus, produce excess salt which has had contact with the skins and, therefore, is contaminated in some way and strictly speaking should not be used again for the salting of raw sheepskins/lambskins. It then has to be disposed of or used in some way.' This raises a number of questions: * Where does the excess salt go? * Is it recycled in some way? * If not, can it be recycled in some way? * Who, if anybody, is addressing the issue? 'All of the above is probably an issue that the industry does not really want to know about but it has to be approached at some time and I am sure it would make for an interesting article as well as possibly paving the way for a solution that would be of benefit to all.'¹ Having investigated, I have to admit that there seems to be no direct answer to that specific problem. The options are to reduce, remove, reuse or recycle; the latter two being the favoured options. I suppose if those are implemented, then a reduction in the amount of salt used is an inevitable outcome. Figure 1 shows a material balance of salt applied in curing one tonne of raw hides. Most of the research done by the likes of Unido and Csiro has shown ways of reducing the total dissolved solids (TDS) in effluent, a large part of which is sodium chloride, - more of which later - but research into solid salt removal seems to be limited to Unido trials in India. Table 1 lists the types of salt used in the industry². Partial solid salt elimination is possible by using hand shaking, mechanical brushes or a drum type shaker to eliminate up to 10% of the salt added to hides and skins for preservation. It's assumed that the salt can then be reused, probably after dissolution and removal of other solids, in the pickle processes.³ The method gives a partial answer to the salt pollution, but it doesn't solve the problem if the producers are sending the salted hides and skins to tanneries for processing. In India, Unido has developed a number of ways of recycling salt. A report from Unido: 'Desalting of raw hides/skins and reuse of dusted salt in pickling operation' highlights some of the ways.⁴ The report says that since the scope for green processing is limited in India, sodium chloride is widely used to preserve raw hides and skins. However, it contributes to a high volume of TDS in the soak waste liquor and no commercially viable technology for treating effluent has been developed to date. A large amount of the salt sticking to the hides and skins can be removed by shaking the hides mechanically or manually. The Unido regional programme for pollution control set up a pilot scheme to demonstrate different options for: * desalting hides and skins prior to soaking and * reusing dusted salt in the pickling operation after purifying the salt recovered. They used three different desalting methods. These used a simple wooden frame, a perforated drum and a brush-type machine. The frame was found suitable for desalting skins and small hides and perforated drum for hides. The brush-type machine was found suitable for all types of hides and skins. Though the quantity of salt removed varied with the method of desalting, it is generally in the range of 4-8% on the weight of raw stock. Though the dusted salt from desalting operations contains many impurities, a fairly clear salt solution can be obtained by dissolving the salt recovered in water, simply screening it and then clarifying it using poly-aluminium chloride. Demonstrations showed that the salt solution so obtained could be safely used in pickling operations. Using these physical methods of desalting of raw stock reduces the TDS level in the composite tannery wastewater by about 15%. Though desalting may not directly result in considerable financial benefit, in India there are indirect benefits including more efficient soaking and reduction in the volume of soak liquor discharged which, in turn, reduces the area of land required for the solar evaporation of saline effluent, a mandatory requirement in Tamil Nadu. More importantly it prevents, to an extent, salt entering the groundwater and soil. In the UK, the question of solid salt is usually solved by dilution. Many water authorities discharge into estuaries and, thus, dilution factors mean the concentration of chloride is no real problem. One sheepskin processor I spoke to said that with imported stock, which has been dry salted, the salt is impregnated within the skins and, on subsequent soaking, the level of additional salt required can be reduced to give the degree of Bè required. On the other hand, the UK domestic skins used are broken over with a band knife and the excess salt is removed before processing starts. In this case, the salt goes out with the fleshings to the renderer. In Australia, Darryl Cassingham, technical director of Packer Tanning commented⁵: 'It [salt] enters the general effluent streams and is either taken out with solid organic waste or goes through our biological system. If it goes through the system, approximately 40% of this effluent is recycled for such activities as beamhouse processes, floor washing and running the effluent plant. It is finally discharged to sewer where we pay a set rate for flow and various other items but not for chlorides [currently]. This suggests that chloride isn't currently a problem to our local authority. 'The chloride in the solid waste is not of concern as it is generally bound to the waste, ie doesn't freely leach. Currently, we are working on composting our wastes and are doing leachate tests and experimenting with the addition of gypsum to further tie up any free chloride. We are also running plant growth trials with our compost and, while we had some initial problems with chloride, we have overcome these.' Elsewhere, Csiro have developed a number of ways of reducing the reliance on sodium chloride. In a recent report⁶, they noted that about 50% of Australian hides and skins were not salted before tanning and various methods have been developed to facilitate this green processing. Chemical methods and chilling are used to preserve hides and skins during transportation and storage. However, short-term preservation should be more widely used because green processing, which eliminates the cost of salting and the environmental problems associated with the salt, is only worthwhile if there is no deterioration in the raw material. Short-term preservation is also necessary if there are delays before curing, drying or other processing, such as fellmongering. Table 2 lists the advantages and disadvantages of the various non-salt preservation processes, while table 3 highlights some of the recommended practices for various hides and skins. Of course, another important point to remember is that salt is needed in processing to aid removal of the non-collagenous proteins. If salt is not present, it is difficult to make good leather. However, most research has concentrated on looking for reducing the TDS levels. Aloy commented on the problem of TDS⁷: 'In arid countries, facing the low availability and quality of water, it seems useful to evaluate every scope for reuse of treated effluents being discharged to the natural media. Regardless of the quality of waste treatment, most tannery effluents contain a high level of TDS preventing any possibility of reuse for post tanning processes. Furthermore, in order to protect water recipients, including ground water, many countries have established discharge limits for TDS. Thus, they considered it worthwhile to test the viability of reverse osmosis for this purpose.' Under a Unido supported project, a pilot plant was installed in a tannery in Tamil Nadu, India, to process effluent following full physico-chemical and biological treatment. The first part of the pilot unit consists of pretreatment equipment needed to adapt the quality of the effluent required for the RO process. The main part of the plant is fitted with high pressure pumps and membranes comprising two stages; the first one with nanofiltration characteristics and the second one as a reverse osmosis stage. It was possible to reduce TDS from 5,000-7000mg/l to 300-500mg/l with a recovery rate of 70-75% of the initial volume processed. The cost of treatment was comparable with that of fresh water bought by tanners and brought to the tannery by a tank truck. The treatment and/or disposal of the concentrate (25-30g/l of TDS) and the lasting life of the membrane are the limiting factors for successful application of this technology. So, the answer to the original e-mail seems to be that excess salt is either washed to effluent, the TDS being reduced by dilution with other effluents or, where water usage is restricted, used in pickle liquor and constantly recycled and mended - usually after a treatment to remove impurities. That also answers the second and third questions of recycling. The complete alternative of using something else is only likely to come into being if, or when, the regulatory authorities set limits for chloride levels. Perhaps the leather industry is being too complacent in its use of salt - OK, e-beam or gamma-ray preservation⁸ are viable alternatives if you have the money. They are not for most tanning countries, but simple alternatives are there, as the research by Csiro and Unido has shown. The industry should consider using them, before it is forced to.