TDS - an unsolved problem

The fact that the River Palar in Tamil Nadu, India, has not flowed for three years is of great concern to the tanners of Vaniyambadi. They are concerned because the river not flowing means that the total dissolved solids in their effluent is higher than the limits imposed by the regional government. So, the tanners need to spend Rs250 crore (about US$53.8 million) to transport the wastewater from the CETP, via a pipe, to the sea. The reason is that the Indian regional government have imposed a strict limit on the levels of TDS, one of the first countries to do this. Environmentally, it makes sense. In arid regions, especially in places where the scope for combining tannery effluent with domestic sewage does not exist, TDS is turning into the main adversary. To illustrate this, while most CETPs are able to discharge effluent within the limits set by the authorities, the treated effluent is virtually useless or rather potentially harmful due to its high total dissolved solids level. This is mainly sulfate and chloride. The share of sulfate and chloride as a function of leather processing are shown in the pie chart: the majority of sulfate originates in the tanning process; chloride ions in the soak. The problem is that there seems to be no cost-effective technology that can deal with this problem, anywhere in the world. While those environmental specialists and enforcement agencies look for technical and economically viable options, it is most likely there is no one track solution. The problem has to be tackled all along the leathermaking process. Measures that could be considered include: * Salt free or low salt preservation, processing of green hides and skins, chilling, drying, short term preservation with safe biocides * Manual or mechanical desalting of hides and skins, ammonium free deliming and bating * Modifications of tanyard and wet finishing operations resulting in significant reductions of sulfate and chloride discharges, emphasis on optimum chrome management and various waste float recycling * Concentration and accelerated solar evaporation of (recycled) soak or other concentrated saline liquors * Application of membrane technology for TDS removal either in separate streams or, in some cases, from the treated effluent * Transport and discharge of saline waters into the sea * Reuse of treated effluent for irrigation of selected land plots planted with salinity resistant species The processing of chilled hides is probably impractical in hot climate countries such as India or Africa, but there 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. 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 bactericides1. The hides were inpregnated with the solution while the lorry was going to its destination. One of the obvious ways to reduce TDS is to remove the salt before it enters the float. This is achieved by either desalting the hides and skins by hand or by using a mechanical desalting machine. This is basically a rotating seive 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. A Unido backed scheme at Sura Tannery in Ambur, Tamil Nadu, carries out just this process. The Improved solar evaporation system works by taking the soak liquors, clarifying and pumping the liquor onto an inclined roof, where it runs down to a collecting vessel. The sun heats the roof and water begins to evaporate leading to a concentrated saline solution. This solution is then put into solar pans and the water allowed to evaporate completely, leaving a crusty salt, which can be mixed with new salt and reused in salting hides and skins. The use of a roof to evaporate the water is necessary because the greater surface area achieved compared with just putting the liquor into a solar pan means that the evaporation rate is faster, leading to a quicker turnround time. Dr Bailey came up with an interesting alternative to common salt, which is to use potassium chloride rather than sodium chloride. Potassium chloride has similar physical and chemical properties to common salt, but is very different in one respect. While sodium chloride has a negative effect on the growth of plants when applied to the soil, potassium chloride is required for proper plant growth. Bailey carried out a number of trials involving around 4,000 hides and came to the conclusion that there was virtually no difference in the final leather properties. The problem is the higher cost of the potassium salt and the fact that the pressure is more about getting clean drinkable water than the effects on the soil. Possibly the best way forward will involve the use of membrane technology3. In recent years membrane technolgies have been developing rapidly and their cost, which was a major inhibiting factor, is continuing to reduce while the application possibilities are expanding. The main advantage of a membrane based process is that concentration and separation are achieved without a change of state and without use of chemicals or thermal energy. Thus, the process is energy efficient and ideally suited for recovery applications. Applications have been successfully carried out for the recovery of salty waters from soak liquors for reuse, so the recovery of salt could be achieved using a combination of membrane technology and evaporation. Dissolved solids in waste liquors is a serious concern that could cause the industry, especially in areas where water is scarce, a lot of problems. Given that there is not way of completely cleaning large volumes of salty water, the obvious solution is to reduce the amounts of salt going into the soak in the first place. If the legislators begin to impose strict controls on the levels of TDS, reducing the use of salt will become mandatory rather than advisable.