Reduction of effluent in Salt Freight

In our modern, industrialised society, responsibility for active environmental protection is becoming more and more important. Not surprisingly, environmental awareness has grown immensely in leather production just as it has elsewhere. Tough official regulations governing effluents, waste air and solid waste disposal provide the legislative background to this increased environmental sensitivity. To meet the specified limit values, products and application methods must measure up to stringent requirements. Optimum exhaustion of products, environmental compatibility of the byproducts formed and waste avoidance are very important. Compliance with stricter effluent legislation and landfill regulations are tough challenges to the leather industry and the industry supplying it with chemicals. In a modern leather factory, environmental protection is, therefore, every bit as important as leather quality, productivity and profitability. Salt in the effluent is the most difficult form of pollution to be dealt with by the leather industry. COD and BOD can be dealt with by oxidation and flocculation in the water treatment plant. In addition to its polluting effects, chloride corrodes metal and sulfate damages concrete. Only elaborate methods are available to remove salt from wastewater, all having their pros and their cons (Figure 1). Salt reduction methods If salt in the wastewater is to be reduced, the origin of the salt freight has to be identified. The basic rule of a tannery is that what you put in, also comes out (Figure 2). Salt enters the beamhouse process in various ways. It comes in as common salt used for the conservation of raw hides, dilution salt used to formulate powder products and salts used in the beamhouse processes (Figure 3). The main problematic component is chloride from common salt. The chloride found in the wastewater is brought in mainly by conservation salt and sometimes by pickle salt (Figure 4). To give an idea of quantity, the daily output of common salt from the Montebello water treatment plant in 2000 was 120 tons! There are various methods to reduce common salt freight: using fresh or dried hides; mechanically removing conservation salt (Figure 5); low salt pickling; and the use of salt-free products (Figure 6). Conservation salt Conservation salt is the biggest contributor to the total salt freight in wastewater and increases the solid waste (Figure 7). It has to be noted that by eliminating conservation salt, the total salt freight in the wastewater is reduced by around 200kg NaCl per ton of hides. This is about two-thirds of the total salt freight (Figure 8). Conservation salt functions The conservation effect of salt is a well-known factor known even in ancient times. Basically, the salt dehydrates the hide from being 60-70% water to a level of about 14% where it is difficult for micro-organisms to grow. The direct bacteriostatic effect of the salt on most bacteria can be ignored. An effect of conservation salt that should be mentioned here is the hydrotropic effect. During conservation, the salt releases interfibral protein and, if not dosed in sufficient quantities (less than 20% with a concentration lower than 4%), the salt will even attack the collagen. Eliminating salt for conservation The direct processing of hides is common in cases where the slaughterhouse is close to the tannery and the logistics are perfect. As an intermediate solution, hides coming from further away are treated with a bactericide such as Aracit KL New. In this condition, they can be transported for up to one day to the tannery. Another common practice is chilling the hides with chipped ice, by cooling in a fridge (Figure 9) or by using ice-cold water. The effect can be improved by using Aracit KL New in the water to produce the ice. Drying hides is another alternative but not an advisable way to preserve them. Pickling Pickle salt is the second biggest contributor of the total salt freight in the wastewater (Figure 4). Common salt used in the pickle is a source of chloride (Figure 10). Sulfuric acid is the major source of sulfate, with sulfate masked chrome as the second source. Unfixed chrome also contributes to salt levels in the wastewater. Functions of pickle salt Pickle salt has various tasks to perform (Figure 11). Salt has a hydrotropic effect on the pelt that increases when the pH is under the iso-electric point of 5.5. This effect makes the leather softer. The acid disturbs the equilibrium in favour of the positive charges in the lateral amino sites. If there was no salt in the pickle, this would generate repulsive forces within the structure, which would shift the polypeptide chains away from each other. This would then create empty spaces in which water would penetrate rapidly. The very strong osmotic pressure would then destroy the pelt from within. Salt avoids swelling in the pickle by reversing the osmotic pressure. In a good pickle, these two forces are balanced out. The normal shrinkage temperature of an untanned hide is 62-64°C. Under the influence of acids and salt in a normal pickle with a pH of 2.5-3, this temperature is lowered to about 25°C for sheep and 30°C for bovine. Reducing/eliminating salt for pickling Systems and products for pickling to avoid or reduce salt content can be seen in Figure 12. Sellatan P is a modified aromatic polysulfonic acid (Figure 13). This non-swelling acid replaces mineral acids and allows substantial reduction of the common salt used in the pickle. In addition, it reduces chrome in the wastewater by improving uptake and fixation (Figure 14). Dynamic tanning is a system that basically uses the effect of Sellatan P to reduce the pickling time dramatically (Figure 15). The open chrome recycling system (part of the BMBF project 'Integrated Environmental Protection in the Leather Industry') allows a drastic reduction of the chlorides, sulfates and chrome in the discharged float (Figure 16). This recycling system uses a counter-stream principle (Figure 17). An investment in one or two storage tanks has to be made. Sellatan P plays a major role in this system as well. See also the application recipe in Figure 18. Cromeno XT is an acid-free product that allows a substantial reduction of common salt, the elimination of sulfuric acid and the optimisation of chrome fixation without extra basification (Figures 19, 20 and 21). The product is especially useful for pelts with a low substance. Cromeno XP is an acidic version of Cromeno XT. It can be used on limed pelts with higher substance (Figure 22). Pickle float recycling When the pickle float is kept separate, it can be recycled, saving up to 80% of salt and 20-25% of the normal addition of formic and sulfuric acids. To ensure a consistent result, it is essential to drain off the washing float before pickling. In the subsequent chrome tanning bath, up to 2% salt might be needed to adjust the Bé. Accumulation of natural grease has to be avoided by skimming-off the grease. Deliming and bating Ammonium salt and sodium bisulfite are salts used in deliming and bating. Ammonium salt can be eliminated by using CO2 deliming and ammonium-free bating agents. On thicker pelts, CO2 deliming does not work due to lower penetration. In many cases, even on thinner pelts, CO2 deliming does not work in a consistent way. Dermascal DGS New represents a group of products free of ammonium salts and based on dicarboxylic acids. Blends of dicarboxylic acids with ammonium salt are also available. Depending on the amount of ammonium salts in these products, they can contribute to the reduction of ammonium salts in the wastewater. Dermascal CD is based on cyclic esters and is completely salt-free. All types of pelts can be delimed without ammonium salts and without problems (Figure 23). The product does not allow the pH to drop below 8.5 (Figure 24). This product is ideally combined with a liquid salt-free bating agent such as Oropon ANZ. Salt-free products To formulate powder products, sodium chloride or sodium sulfate are normally used. Since they increase the chloride and sulfate freight in the wastewater, products without these salts are advisable. One way is to use liquid, salt-free formulations and the other is to use concentrates. In most cases, concentrates are ruled out since they often do not allow proper distribution in the system. Bating enzymes are an example of when dilution is necessary in order to impart a homogeneous effect. Furthermore, various chemicals can be combined to make a compact product, thus making it possible to create formulations free of salts. Pellivit IS-B contains all the necessary components to perform a good soak. No formulation salt is added and normal application quantities are used. Conclusion It is easy to achieve a major reduction of salts in beamhouse wastewater by simple means. The best solution from the environmental point of view is to process fresh or chilled hides. Whenever salted or brine-cured hides have to be processed, various measures can be taken: 1. Curing salt - maximum impact * Use fresh hides or hides with a short-term, salt-free preservation; proximity to the slaughterhouse has to be considered * Mechanical removal of the curing salt - a one-time investment in a salt-removal device has to be made 2. Salt-free or low-salt pickling - second largest impact * Reduced pickle systems - cost savings in other chemicals and improved quality compensate for extra costs in special chemicals * Pickle-free system - a one-time investment in one or two storage tanks has to be made - extra chemical costs are to be expected * Recycling of the pickle float - this system requires a one-time investment in a storage tank 3. Ammonium-free or low ammonium salt deliming * For ammonium-free deliming systems, extra chemical costs are to be expected; this is often compensated for by the improved quality and optimised area yield 4. Reduction of sulfide * Hair-saving systems are the most realistic options. They can be combined with the re-use of the liming float but this has an effect on the quality of the leather 5. Salt-free products * Compact powder systems do not require extra investments in special chemicals; often they are more economical * Liquid, salt-free products are available and often at the same price Acknowledgements This publication is based on in-house and external reports, field application and lab tests. The authors wish to thank Dr C Page, Dr A Lauton and C Pfeiffer for their valuable input and discussions. Special thanks to A Lenglart, D Herta and B F Müller who conducted many of the trials and the rest of the TFL team for their involvement.