Journal of Chemical Technology and Biotechnology J Chem Technol Biotechnol 81:1307–1315 (2006) Solidification/stabilization of thermally-treated toxic tannery sludge Somasundaram Swarnalatha, 1 Munusamy Arasakumari, 1 Arumugam Gnanamani 2 and Ganesan Sekaran 1∗ 1 Department of Environmental Technology, Central Leather Research Institute, Adyar, Chennai – 600 020, Tamil Nadu, India 2 CHORD division, Central Leather Research Institute, Adyar, Chennai – 600 020, Tamil Nadu, India Abstract: The high concentration of trivalent chromium along with organic/inorganic compounds in tannery sludge causes severe ground-water contamination in the case of land disposal and chronic air pollution during incineration. In the present investigation the sludge was subjected to starved-air combustion at 800 ◦ C, which prevented the conversion of Cr 3+ to Cr 6+ . The efficiency of starved-air combustion was confirmed through differential thermo-gravimetric analysis (DTG), electron spin resonance (ESR) and Fourier transform infrared (FTIR) analysis. The calcined sludge was solidified/stabilized using fly ash, clay, lime and Portland cement as mixture constituents. The solidified specimens were tested for compressive strength and heavy metal fixation. The compressive strength and metal fixation of the calcined sludge (Cs)–fly ash (F)–cement (C) mortar at a ratio of 41.66% Cs, 41.66% F, 16.66% C were 185 kg cm −2 and 93.84%, respectively. The stabilization of chromium(III) in the cement gel matrix was confirmed with scanning electron microscopy. Leachability studies were carried out to determine the percentage of metal fixation and chemical oxygen demand in the leachate.  2006 Society of Chemical Industry Keywords: tannery sludge; chromium; starved-air combustion; fly ash; Portland cement INTRODUCTION In tanneries, raw skins/hides are transformed into leather by means of a series of chemical and mechanical operations that lead to discharge of hazardous chemicals into effluent treatment plants (ETP). 1 Basic chromium sulfate is the most widely used tanning material for converting putrescible collagen fibers into non-putrescible leather matrix. Only 60% of the chromium salts applied in the tanning process react with the raw materials (these Cr 3+ ions are in co- ordination bonds with the peptide linkages of collagen fibers of skins/hides). The rest of the chromium salts remain in the exhaust tanning bath and are subsequently discharged into the wastewater. 2 The dissolved chromium and other spent chemicals, namely proteins, polyphenolic compounds, surfac- tants, dyes, etc., present in the wastewater are removed through the chemical precipitation technique using lime before the wastewater is allowed to enter the bio- logical treatment process. The precipitated chromium along with the other organic compounds is discharged as sludge. 3 The current techniques used for the disposal of tannery sludge are land-filling, hydrometallurgical methods to recover chromium, incineration and gasification. 4–6 The land-filling scenario faces the highest cost, which is explained by the combination of the overall highly-polluting emissions and the low recovery of energy; the limited capacity of available land-fill sites and the difficulty of new sites becoming authorized are also important in this context. 7 The hydrometallurgical method results in high cost and poor quality chromium due to the presence of organic lipolytic components, metals and other impurities. 2,6 Furthermore, this process is associated with the generation of additional effluent. The investment costs are far higher for gasification; therefore, the total cost is always higher than for incineration and land-filling. The incineration process, however, has only a total cost of about one-sixth of the total cost for the land- filling scenario. 8 So, thermal incineration is considered to be the cheapest alternative and an attractive method for its simultaneous energy production and reduction in the volume of solid waste. Thermal incineration of solid wastes from tanneries needs attention on issues such as the release of toxic chromium(VI), halogenated organic compounds, poly aromatic hydrocarbons, etc., into the environment. 9 Trivalent chromium is a natural compound of low toxicity which is hard to dissolve in water, and which plants find difficult to absorb. It is an essential micro-nutrient for human beings and other mammals. Trivalent chromium compounds usually exist in the soil as Cr(OH) 3 or Cr 2 O 3 . On the other hand, hexavalent chromium is a highly-toxic industrial product, and is soluble in water. It is a carcinogen, which can enter the body via inhalation, ingestion, and contact with the skin. It causes nose bleeding, lung cancer, ulcers, visceral damage, inflamed skin, ∗ Correspondence to: Ganesan Sekaran, Department of Environmental Technology, Central Leather Research Institute, Adyar, Chennai – 600 020, Tamil Nadu, India E-mail: ganesansekaran@hotmail.com (Received 8 December 2005; revised version received 26 December 2005; accepted 28 December 2005) Published online 26 May 2006; DOI: 10.1002/jctb.1539  2006 Society of Chemical Industry. J Chem Technol Biotechnol 0268–2575/2006/$30.00