Pyrolyzed tannery sludge as sorbent of volatile organic compounds from tannery emissions

The Italian tanning industry is the leading European producer of high-quality leathers, with a value equal to 63 % of the continental turnover contributing to 23 % of the global value produced worldwide. However, it has a great impact on the environment, due to its large water consumption with a huge amount of biological and chemical waste generated, and the high emissions of volatile organic compounds, VOCs, from leather finishing cabins. It is estimated that, from every tonof raw hide/skin, between 4 and 50 kg of VOCs are emitted into the air. The wastewater streams produced by the leather industry are commonly conveyed to centralised wastewater treatment plants where the typical treatmentscomprise physical–chemical processes followed by activated sludge-based biological treatment to eliminate the organic matter. In Italy, also a tertiary treatment must be often included such as the Fenton process (oxidation) to meet the COD limit of 160 mg/L set by the Italianlaw for discharge into surface water bodies. The resultant tannery sludge (TS) is a solid waste with high S content and heavy metals due to basic chromium salts, syntans, pigments, dyes, retanning agents, etc. used in the tanning process. In Italy, these sludges are classified as special non -hazardous waste, and their current destination is in controlled landfill. The putrescible organic material and the presence of heavy metalsas Cr3+ leads to the need to stabilize the TS before being disposed of or, preferably, recovered. Thermal treatment as pyrolysis permits to stabilise, preventing the Cr 3+ oxidation, and reduce the volume/mass of waste to be disposed of. In addition, the carbonaceous residue obtained from pyrolysis of TS (PTS) could be used as sorbent depending upon its properties finding a valuable alternative to landfill. In the present work the use of PTS coming from an Italian tannery sludgepyrolysis plant was investigated, after further pyrolysis carried at controlled temperature and time, as adsorbent for the removal of n-butyl acetate, a typical solvent used in the finishing leather formulations, from air emissions.