Effect of composting on sludge contaminated with heavy metals from three different tannery streams and their bioavailability to plants

This research was conducted to study the influence of composting on tannery sludge contaminated by Cr, Cd, Zn, Cu, and Ni. Composting of sludge streams was performed in three different piles for 60 days. The results showed that the total concentration of heavy metals was increased with composting, except in the TT-3 sample where the concentrations of Cd, Cu, and Ni did not increase. The composting led to a loss of weight, Composting rendered part of the metal insoluble. Ni was the most easily extracted by H2O or CaCI2 and was therefore the most weakly bound to the organic matrix; in contrast, Zn remained strongly bound.