Challenges in the Treatment of Tannery Effluents

The only true leather-building substance in raw hide is corium collagen, everything else (hair, epidermis, fat, some proteinaceous matter etc.) has to be removed for which purpose substantial quantities of both general and specialty chemicals are needed; only some 20% of them is actually retained by leather. Accordingly, cleaner processing methods can only have limited impact. On average one ton of wet salted light hides results in 195 kg (138 m2) of shoe upper leather and 60 kg (60 m2) of split leather. BOD5, COD, TKN, TDS etc. are useful indicators of the effluent pollution load and potential environmental hazard it represents. Agglomerations of traditional tanning units with inadequate investment potential, lack of space and specific skills were conducive for setting up Common Effluent Treatment Plants (CETPs) servicing tannery clusters, the concept also adopted in relocations and/or establishing new leather clusters in developing countries. Due to only gradual increase in volumes of effluents and seasonal fluctuations associated with religious festivals, new CETPs are designed in modular way. Effluent collection and conveyance network is normally a combination of gravity and pressure lines; scale forming, silting and generation of toxic fumes (H2S), especially in deep manholes, are the major issues in their operation and maintenance. On-site, physical–chemical (primary) treatment, including strict segregation of streams, catalytic oxidation of sulphide-bearing spent floats, chrome management as well as equalization, settling, sludge removal and dewatering are by now already well-established technologies. Biological (secondary) treatment, typically using activated sludge method, often in raceways, in addition to high energy consumption, requires experienced personnel and close monitoring, in particular the removal of excess nitrogen (nitrification/denitrification). Advanced (tertiary) treatments such as Fenton process and ozone treatment deal with recalcitrant residual COD whereas Reverse Osmosis (RO) in combination with Multi Effect Evaporation (MEE) seems like a desperate attempt to deal with excess TDS (colloquially salinity) so harmful in arid regions. In addition to being prohibitively expensive, it is still without solution for disposal of chloride/sulphate salt residue, the issue even more challenging than sludge disposal. A long-term solution might be linking CETPs with Municipal Wastewater Works (MWW). The expanding leather products sector (footwear, leather goods, apparel), including significant local consumption, points towards competitive and successful overall leather industry in developing countries. The scope for utilization, possibly even recycling of disposed leather products (apparel, footwear, leather goods, furniture and transport upholstery) might turn vital for the long-term perspective of the sector. A paradox here is that fashion changes contribute to the higher demand but also to the volume of disposed leather items, its longevity another proof in favour of leather’s sustainability. However, it is important for the leather sector to take a proactive role in battling negative publicity and spreading of misleading information by fake green lobbies and pressure groups. Better coordination among various national, regional and global leather organizations, including R & D institutions would make a stronger impact.