The effects of clay amendment and composting on metal speciation in digested sludge

Sewage sludge usually contains significant heavy metals that may limit its land application. Heavy metals in municipal solid waste have been shown to be less mobile by amendment with bauxite refining residue (red mud) prior to the composting process. In the present research a sequential step extraction was employed to investigate metal speciation (into exchangeable, bound to carbonate, to Mn and Fe oxides, to organic matter and in residue phase) and the effect of red mud on metal speciation in compost of sewage sludge for Cr, Cu, Ni, Pb and Zn. The effect of red mud addition and composting process on metal distribution in sewage sludge compost is significant. Red mud addition generally reduces metal leachability and therefore the potential hazard of releasing metals from sludge compost through adsorption and complexation of the metals on to inorganic components to different extents for the different metals. Red mud cannot desorb, however, metals bound to organic matter in the sludge. The composting process breaks down organic matter in the sludge and may release the heavy metals. The addition of red mud prior to the sludge composting binds the released metals on to the red mud for those not strongly readsorbed by the remaining organics (Cu, Ni and Zn).